Evidence of prolonged wet-dry cycling on early Mars.

Mars has an extensive sedimentary record, dating back at least 4.3 billion years, which shows that early Mars had a very difficult climate from today, with permanently wet environments, and possibly even the conditions for life. Whether-or-not this early climate also included areas with episodic or periodic wet intervals has to date, however, been unclear. Such fluctuations in hydrodynamic conditions leave distinctive traces in the sedimentary record, such as cracks, however they are also easily eroded away, and models of the early Martian climate have been ambivalent about the existence of such conditions.

In a paper published in the jornal Nature on 9 August 2023, William Rapin of the Institut de Recherche en Astrophysique et Planétologie at the Université de Toulouse 3, Gilles Dromart of the Laboratoire de Géologie de Lyon Terre, Planètes, Environnement at the École normale supérieure de Lyon, Ben Clark of the Space Science Institute, Juergen Schieber of Indiana University, Edwin Kite of the University of Chicago, Linda Kah of the University of Tennessee, Knoxville, Lucy Thompson of the University of New Brunswick, Olivier Gasnault, Jeremie Lasue, and Pierre-Yves Meslin, also of the Institut de Recherche en Astrophysique et Planétologie at the Université de Toulouse 3, and Patrick Gasda and Nina Lanza of the Los Alamos National Laboratory, report the presence of a well-preserved polygonal mud-crack pattern on strata dated to about 3.6 billion years ago (dating to the Hesperian Eon of Mars), which they believe to be evidence of a wet-dry cycling system, and therefore to provide useful new insight into the early climate of Mars.

NASA's Curiosity Rover has documented hundreds of metres of sediments deposited in lakes., rivers, intermittent lakes, and lake-margin settings within Gale Crater on Mars. The vast majority of these have been smectite (silicone and aluminium rich clay) mudstones, but the rover recently encountered a sulphate-bearing unit, apparently marking a major environmental transition foumd in stratified terrains across Mars. At this horizon the rover found an apparent sulphate evaporite deposit.

Context of observations in Gale crater, Mars. Stratigraphic context (left) of the lower portion of Mount Sharp and map (right) showing Curiosity Rover traverse (white) on the High Resolution Imaging Science Experiment (HiRISE) base map overlaid with Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) S-index, which tracks sulfates (shaded yellow). Red rectangle shows the location of close-up map and detailed stratigraphy. Rapin et al. (2023).

The unit at the base of the sulphate-bearing strata has widespread centimetre-scale polygonal patterns, formed of straight lines intersecting at triple junctions. All observations of this were made within an 18 m interval, with some variations apparently caused by subsequent alteration of the same basic pattern. These polygons appear to persist for tens of centimetres, and can be seen on stepped blocks of sediment. Where visible on bedding plains the polygons are made of raised ridges are about 1 cm high, with the polygons having an average diameter of 4 cm. Junctions have an average angle of 120°. The ridges are made up of aligned nodules, of variable size, and juxtaposed in a variety of ways, and apparently made of calcium sulphate and magnesium sulphate minerals. The sediment in which these ridges are emplaced is generally sulphate-poor, though with some patches with raised calcium sulphate levels.

In situ observations of polygonal ridges. (a) General view of bedrock surrounding the rover on sols 3154 to 3156 showing widespread polygonal ridges. (b) Close-up showing ‘stepped’ exposure of polygons within large bedrock blocks. (c) View of bedrock with polygons and locations of ChemCam analysis on ridge (red rectangle) and Alpha Particle X-Ray Spectrometer analysis on smooth host bedrock (dotted circle). (d) Remote micro-image of cemented ridge with spots analysed by ChemCam (reticles 1 to 5), highlighting details of nodular texture. (e), (f) Bedrock with polygonal pattern (e) and interpretative overlay (f) that shows prominent ridges (solid red lines), less certain ridges (dotted red lines) and cross-cutting later-stage calcium-sulphate-filled veins (white areas). Rapin et al. (2023).

On Earth, polygonal ridge patterns can form on evaporite deposits as a result of subsurface salinity convection, but Rapin et al. believe this to be unlikely, as this generally only occurs on pure salt crusts, and produces polygon patterns with much larger diameters (which would presumably be larger still if they formed under Mars's low gravity conditions). They instead suggest that the more likely explanation is that the ridges formed originally as cracks within a drying sediment, which was then infilled by salt-rich water, which evaporated leaving the nodules, which are more resilient to erosion than the surrounding sediment, leading to raised ridges as this is eroded away. Newly formed desiccation cracks generally have T-junctions between blocks, but where they undergo repeated cycles of wetting and drying, these tend to reform into more even 120° Y-junctions, with about 10 cycles of wetting and frying typically needed on Earth for 120° to be reached, giving a pattern of even hexagons.

Formation model for sulphate-enriched polygonal ridges. (a)–(c) Repeated cycles of desiccation (a), recharge (b) and flooding (c) form a vertically propagating hexagonal pattern of sulphate enrichment. (a) Evaporation (grey arrows) desiccates and cracks near-surface sediment, triggering salt crystallization (red) at and near cracks where the subsurface brine (purple) concentrates. (b) Water recharge heals cracks by sediment hydration. (c) Flooding dissolves excess salts at the surface but subsurface brine and intrasediment sulphate salts are preserved and siliciclastic sediment is deposited on top. (d) Sediment is buried with saturated brine in pore spaces and sulphates are mostly preserved. (e) Later diagenesis partially dissolves intrasediment sulphate salts and late diagenetic fractures are filled with calcium-sulphate (white). (f) Sulphate-cemented polygonal ridges become visible during exhumation as the softer host bedrock is preferentially removed during weathering. Rapin et al. (2023).

The hexagons are located in a sulphate-poor sediment overlaying a sulphate-rich nodular bedrock, making the precipitation of sulphate minerals plausible, although it is unlikely that they originally formed in their current configuration. Most likely, the salts were precipitated in a slurry with sediment particles in original cracks with a T-junction formation, but that then redissolved with each cycle of wetting and re-precipitated with each cycle of evaporation, eventually forming the resilient hexagon-shaped patterns seen today.

Larger colour image of bedrock with polygonal ridges for context. MastCam image and close-ups (a), (b) and (c) with rectangle locations of close-up view. Close-ups (b), (c) show bedrock 10 to 20 meters away where regularly spaced ridges and nodules can be observed supporting lateral extension of the same polygonal pattern although camera resolution prevents detailed geometrical analysis at this distance. Rapin et al. (2023).

The water that caused these patterns was probably brought in by periodic flooding and groundwater recharge, which would have added sediment to the deposits (leading to the depth of strata we see today), while dissolving any surface salts, so that each drying event is likely to have precipitated more salt than its predecessor. This dessication-water recharge cycle is likely to have affected only the upper few centimetres of sediment at any one time, with the hexagons therefore able to move up through the sediment column with repeated cycles. The consistency of the size and shape of the polygons implies a repeated cycle of regular intensity, while the variable size and shape of the nodules within the ridges implies multiple generations of growth.

The repetitive nature of the floods and the limited amount of depth penetration makes it likely that these cycles were seasonal, although a shorter-term cycle cannot be ruled out. The time period over which this cycle occurred is unclear, although the ridges have been identified at multiple points within an 18 m succession, which on a typical Earth floodplain with a sedimentation rate of about 0.01 mm per year, would imply a period of thousands to millions of years, although this might not have been a continuous interval of seasonal flooding; possibly the occurrence of seasonal interludes was itself part of a larger cycle. Individual blocks of polygon propagation are often more than 2 m thick, with no visible boundaries to other polygon-bearing or polygon-free strata, which suggests that if there were longer dry intervals without sedimentation, then little erosion took place in these intervals either. 

The mature hexagon shapes of the polygons indicate they were formed by repeated wet-dry cycles, and the thickness of the strata in which they are found implies that this cycle occurred at least episodically for a very long period of time. Mud crack polygons have also been seen in the underlying Murray Formation, but these typically have a T-junction structure, suggestive of a single drying event, whereas the polygons reported by Rapin et al. appear to be indicative of a repeated wet-dry cycle, which occurred for a sustained period of time on the early surface of Mars. This agrees with models of the early climate of the planet which have suggested that a single event, such as a meteor impact or supervolcano, forming all the water-related features on Mars (one a popular hypothesis) is unlikely. Instead, Mars appears to have had a longer period of Earth-like climate with seasonal flooding events, and evaporite deposits forming in seasonal lakes. 

A climate with wet and dry cycles is considered to be conducive to, and perhaps essential for, the type of prebiotic chemical evolution needed to form the precursors for life. Dessication lowers the amount of water available, thereby increasing the proportion of soluble ingredients within the remaining liquid, raising the rate at which reactions can occur. In particular, nucleotides form more readily from nucleobases in a concentration, and also polymerise to form larger molecules such as DNA or RNA more readily, and amino acids more readily form proteins under such conditions. Dioctahedral smectites, which appear to be ubiquitous clay minerals on the surface of Mars, are capable of tightly adsorbing nucleotides through cation exchange, and have been suggested as having been vital for the formation of the first pre-biotic organic polymers. Thus, seasonal pools on the surface of Mars could have reasonably formed the 'warm little ponds' proposed by Darwin as a location for the appearance of life.

Sediments in Gale Crater have been shown to contain about 500 g of organic material per cubic metre, as well as a variety of other soluble elements. The discovery that this site also once underwent seasonally wet and dry cycles supports the idea that this area was once suitable for prebiotic chemistry, but it is highly unlikely to have been the only place on Mars where such conditions were found, although the discovery does re-enforce the importance of Gale Crater as a site of global importance for understanding the early history of Mars.

The discovery of evidence of wet-dry cycling from a time when organics and volatiles are known to have been accumulating on Mars for over a billion years supports the idea that conditions on Mars during the Noachian–Hesperian transition period may have been favourable for the emergence of life on Mars, possibly more so than the earlier, and apparently wetter, Noachian Eon.

See also...

Follow Sciency Thoughts on Facebook.

Follow Sciency Thoughts on Twitter.

Examining the impact of the Palaeocene–Eocene Thermal Maximum on sedimentation in the Gulf of Mexico.

Roughly 56 million years ago, global temperatures abruptly rose by 5-9°, leading to profound environmental changes across the planet, an event known as the Palaeocene–Eocene Thermal Maximum. This event was marked (and probably caused by) a sharp rise in atmospheric carbon dioxide, something marked in the rock record by a 3.0‰ negative carbon isotope excursion (three parts per thousand drop in the proportion of carbon¹³ to total carbon), which developed over a period of less than 5000 years. Three main stages to this negative carbon isotope extension have been detected; the onset, during which the proportion of carbon¹³ dropped from the pre-excursion level to the excursion level; the body, during which the proportion of carbon¹³ remained steady at the new, lower level; and the recovery, during which the proportion of carbon¹³ returned to the pre-excursion level.

This interval was also marked by a dramatic increase in the prevalence of the Dinoflagellate cyst Apectodinium spp., and a widespread dissolution of carbonates (a sign that the sea had become slightly acidic due to the higher atmospheric carbon dioxide levels). The negative carbon isotope excursion has been detected in a wide range of sedimentary setting, from continental interiors to ocean basins, although its cause is still debated. It is generally accepted that the rise in atmospheric carbon dioxide, combined with the drop in the proportion of carbon¹³, is indicative of the atmosphere receiving a sudden, and very large, input of carbon¹³-depleted carbon dioxide, with the most popular explanations for this being a volcanic source or a sudden increase in the proportion of carbon dioxide being released from land Plants and soils (this could be response to heating, leading to a feed-back loop in which the released carbon dioxide causes a rise in temperature, leading to further carbon dioxide being released, something which concerns climate scientists studying current rising global temperatures). It has also been suggested that the initial pulse of heating might have been caused by an increase in the proportion of biogenic methane (another potent greenhouse gas).

The Gulf of Mexico forms an enclosed basin within the area bounded by the southern coast of the United States, the east coasts of northern Mexico, and the Yucatan and Florida Peninsula. This basin formed by sea-flood spreading during the Jurassic and Early Cretaceous, with deposits of clasitic and carbonate sediments building up along its northern margin during the Cretaceous and Palaeocene. This sedimentation increased rapidly during the Palaeocene–Eocene Thermal Maximum, leading to a prograding  (movement of shoreline towards the sea) of the fluvio-deltaic Wilcox Group. During this time, most of what is now the southern United States formed a single catchment area, driven by the Laramide Orogeny as the Rocky Mountains began to form. The sedimentary material formed by erosion within this catchment was carried into the Gulf of Mexico, forming the deltas of the Houston, Mississippi, and Rio Grande rivers. These sediments served as a trap for hydrocarbons derived from organic material swept into these deltas, which has led to extensive hydrocarbon exploration of the basin in the twentieth and twenty first centuries. This data has enabled geologists to build up a good picture of sedimentation rates within the Gulf of Mexico throughout the Cainozoic, with a distinct increase on sedimentation rates visible at the Palaeocene–Eocene Thermal Maximum.

The Wilcox Group is a succession of fluvial, deltaic, and shallow marine sediments, which outcrops in parts of Alabama and Texas, where it is targeted by numerous onshore oil wells. The group progresses offshore, where its outer margins contain turbidite deposits, which are drilled by offshore oil rigs. The Wilcox Group can be divided into Lower, Middle, and Upper units, which the base of the Upper Unit marked by the Yoakum Shale, which is thought to mark the onset of the Palaeocene-Eocene boundary. The carbon isotope excursion associated with the Palaeocene–Eocene Thermal Maximum has been detected at several locations within the Wilcox Group, although principally within the onshore fluvial and deltaic deposits and the plains of the Gulf of Mexico. Within the distal part of the submarine fan, the Palaeocene–Eocene Thermal Maximum has been detected biostratigraphically, but not through the detection of the carbon isotope excursion. There is localized evidence of environmental change within the delta, recorded by prograding of sediments over an area of thousands of kilometers, with material from river drainages reaching to the deep ocean floor. 

The ability to connect a prograding deep sea fan to a well understood river catchment system provides a unique opportunity to study enviromental changes across an entire sedimentary system from the source to the outer part of the marine basin.

In a paper published in the journal Geology on 9 February 2023, Lucas Vimpere of the Department of Earth Sciences at the University of Geneva, Jorge Spangenberg of the Institute of Earth Surface Dynamics at the University of Lausanne, Marta Roige of the Departament de Geologia at the Universitat Autònoma de Barcelona, Thierry Adatte of the Institute of Earth Sciences at the University of Lausanne, Eric De Kaenel of DeKaenel Paleo-Research, Andrea Fildani of the Deep Time Institute, Julian Clark and Swapan Sahoo of Equinor, Andrew Bowman of the Louisiana Geological Survey, Pietro Sternai of the Dipartimento di Scienze dell’Ambiente e della Terra at the  Università degli Studi di Milano-Bicocca, and Sébastien Castelltort, also of the Department of Earth Sciences at the University of Geneva, present the results of a study that located the isotopic signal of the Palaeocene–Eocene Thermal Maximum within marine sediments in the northern part of the Gulf of Mexico, use this data to place a chronostratigraphic data-point within the strata, and examine the relationship between sedimentation rates and climate change as recorded within the sediments of the Gulf of Mexico.

Vimpere et al. obtained a 543 m thick section from the Logan-1 ultra-deep-water wildcat well, which was sunk in 2011 on Walker Ridge Block, which includes the outer part of the Wilcox Group, about 400 km to the southeast of New Orleans. This well excavated a core beneath 2364 m of water, to a depth of 8351 m beneath sea level. One hundred and seventy eight samples were taken from this section, at three meter intervals, then subjected to bulk and clay X-ray diffraction, Rock-Eval pyrolysis, granulometric, organic carbon isotope, palynological, and calcareous nannofossil analyses.

Topobathymetric elevation model of North America showing the Logan-1 well location (drilled in 2011 on Walker Ridge Block 969, ID WR 969 ST0 #1) and present main geographic features. Depositional context during the Paleocene-Eocene transition is represented by Wilcox Group thickness and key tectono-stratigraphic events in the Gulf of Mexico sediment routing system. PETM; Palaeocene–Eocene Thermal Maximum. Vimpere et al. (2023).

Examination of palynomorphs and calcareous nanofossils identified the Palaeocene–Eocene Thermal Maximum interval as being present between  8181 and 8001 m within the Logan-1 core, and the  Palaeocene-Eocene boundary as lying between the NP9 and NP10-0 horizons of the calcareous nannofossil assemblage. The carbon-isotope excursion can also be identified within the core, at 8196–8001 m, with an onset 15 m below the Palaeocene-Eocene boundary, and no hiatus in sediment deposition. This pattern has been observed at a variety of locations, and suggests a link between the onset of the Palaeocene–Eocene Thermal Maximum and late Palaeocene volcanism on the e North Atlantic volcanic province, the Caribbean, and mid-ocean ridge areas. The main body interval of the carbon-isotope excursion is found between  8196 and 8108 m, and the recovery phase between 8108 and 8101 m. This gives a Palaeocene–Eocene Thermal Maximum deposit with a thickness of 195 m, making it the thickest Palaeocene–Eocene Thermal Maximum deposit yet discovered. This contrasts with other well cores sunk in the Gulf of Mexico, in which the Palaeocene–Eocene Thermal Maximum sequence has been truncated. A marked increase in the abundance of Dinoflagellate cyst Apectodinium spp. was observed at 8169 m, while glauconite concentrations increased at 8172 m. Both of these are thought to represent sediments having become condensed, and a shift in the shoreline to landward, caused by deepening sealevels associated with the global temperature rise. 

Carbon isotope, glauconite concentration, chronostratigraphic, and lithostratigraphic data and correlations with Gulf of Mexico standard stratigraphy for the section studied in the Logan-1 well (drilled in 2011 on Walker Ridge Block 969, ID WR 969 ST0 #1). δ13Corg measurements and three-point averages are illustrated by the circles and the curve, respectively. GR, gamma ray; Sh, shale; Slt, silt; Snd, sand; YS, Yoakum Shale; CIE, carbon isotope excursion; PETM, Palaeocene–Eocene Thermal Maximum. Nannofossils: Bomolithus aquilus, Discoaster araneus, Discoaster mahmoudii, Discoaster diastypus, Fasciculithus tympaniformis, Rhomboaster cuspis, Rhomboaster bitrifida, Tribrachiatus bramlettei, Discoaster mahmoudii, Coccolithus bownii, Bomolithus supremus, Tribrachiatus bramlettei, Thomsonipollis, Fasciculithus richardii, Caycedoae megastypus, Discoaster multiradiatus, Fasciculithus lillianiae, Fasciculithus richardii, Discoaster acutus. Vimpere et al. (2023).

These results suggest that, in this part of the Gulf of Mexico, sedimentation rates were significantly increased during the Palaeocene–Eocene Thermal Maximum. If the Palaeocene–Eocene Thermal Maximum is assumed to have lasted 170 000 years, then this part of the Gulf of Mexico apparently had an average sedimentation rate of 1.15 m per 1000 years during this interval. The main body of the event comprises 88 m of sediment, thought to have been laid down in 80 000 years, giving a sedimentation rate of 1.1 m per 1000 years, while the recovery period is represented by 107 m of sediment laid down in 118 000 years, giving a sedimentation rate of 1.18 m per 1000 years, although distinguishing the main body from the recovery period is difficult, leading to a substantial margin of error in these calculations.

The Yoakum Shale is considered to represent a maximum flooding surface, created when the Palaeocene–Eocene Thermal Maximum caused the shoreline to retreat by 150 m. In the submarine deposits of the Gulf Coastal Plain this corresponds with a drop in the amount of terrestrial sedimentary material arriving, and the formation of an number of submarine canyons, most notably the Yoakum Canyon off the coast of Texas. These canyons tended to funnel sediments down into the ocean basin, bypassing much of the continental shelf, which became starved of sediment. The sediments of the shelf show a higher proportion of marine palynomorphs (which settle out of the water column) than terrestrial palynomorphs (which are carried out to sea with sediment) during this interval, and are also enriched in glauconite (which only forms in marine settings) relative to the rest of the sediment column. 

Palaeographic map of the northern Gulf of Mexico showing evolution of the depositional systems throughout the Paleocene–Eocene Thermal Maximum. Vimpere et al. (2023).

It could be presumed that the heating and increase in sealevel associated with the Palaeocene–Eocene Thermal Maximum led to the transgression onto the shores of the Gulf of Mexicoby itself, however Vimpere et al.'s findings suggest that this was at least in part due to subsidance of the coastal margins associated with the formation of the submarine canyons, although there is not sufficient data to make an absolute assessment of the influence of the two phenomena.

During the Early Eocene, uplift associated with the second pulse of the Laramide Orgeny forced the waterways carrying sediments into the Gulf of Mexico to shift towards the southwest. This is recorded in the Upper Wilcox deposits, where several major fluvio-deltaic systems are rejuvinated. This in turn led to stabilization of the system, with less wandering by channels, enabling sediments to build up and prograde out over the shelf margin. This prograding of the delta sediments is matched by the development of a sandy apron in the deep sea basin, probably formed as the prograding sediments reached the head of the submarine canyons.

Schematic representation of the evolution of the sediment-routing system of North America throughout the Palaeocene–Eocene Thermal Maximum. Increased channel mobility and floodplain reworking led to preferential transport of clays into the basin (i.e., Yoakum Shale) through bypass of the shelf within submarine canyons. Upper Wilcox corresponds to resuming of preferential transport of coarse material into basin-floor aprons due to progradation of deltaic sands onto the shelf and the mud removal by waves. Vimpere et al. (2023).

Within the Logan-1 drill core the Yoakum Shale is overlain by a series of sandy beds which reach from the top of the Yoakum at 8120 m up to 8007 m. This is thought to be linked to the development of a more extreme climate, which switched periodically between intense drought phases and intervals of heavy precipitation. This created periodic heavy flows within the river basins, washing out to see accumulated sands, derived from rocks uplifted by the Laramide Orogeny. The inshore environment is also likely to have suffered an increase in storm and wave action, washing sediments from the delta lobes down into the deep ocean basin. 

Vimpere et al. were able to use a multi-disciplinary approach to locate the Palaeocene-Eocene boundary, Palaeocene–Eocene Thermal Maximum, and the associated carbon isotope excursion, in sediments about 400 km away from the nearest coast. The carbon isotope excursion here is 195 m thick, and confirmed to represent the Palaeocene–Eocene Thermal Maximum by palynological and microfossil analysis, making it the longest Palaeocene–Eocene Thermal Maximum section known. This implies that sedimentation rates in this part of the basin were extremely high during this interval, which in turn implies a strong sedimentological response to the changing hydrological conditions associated with the Palaeocene–Eocene Thermal Maximum. Since other fan deposits of equivalent age are known at many locations around the world, it is reasonable to assume that this was a global, rather than a regional, response to the Palaeocene–Eocene Thermal Maximum.

See also...

Follow Sciency Thoughts on Facebook.

Follow Sciency Thoughts on Twitter.

Understanding the ancient greenhouse climate of Mars.

The surface of Mars today is cold, arid, and heavily oxidised, but covered in surface features that tell of a very different past. The planet is home to hundreds of dry lakes, channels, and other features which must have been laid down in warm, wet conditions. Exactly how this came to be is not completely clear; Mars has always been further from the Sun than the Earth, and the ancient Sun is believed to have been cooler and dimmer than it is today. The most likely explanation is that Mars once had a dense atmosphere, rich in greenhouse gasses such as hydrogen, methane, and carbon dioxide. However, beyond this simple assumption, we know little about the ancient climate of Mars. We do not know whether the features seen on the surface of the planet represent a single warm phase, or whether Mars went through several cycles of warming and cooling, as Earth has done throughout it's history. 

The weathering of minerals on the surface of Mars potentially offer some insight into the ancient climate of the planet. The way in which minerals weather is related to the presence of oxidising chemicals and/or acids at the planet's surface, and on Earth this has been used to interpret ancient climatic conditions. Loosely speaking, the elements calcium, magnesium, sodium, potassium, and manganese are mobile, and tend to be removed from minerals by weathering processes, while the elements titanium, aluminium, and zirconium, are immobile. Over time mobile elements are removed from mineral formations by weathering, while the immobile ones are concentrated by the same processes, forming clays rich in these elements. Many sites on Mars have been shown to have a surface layer of aluminium-rich clays, overlaying a sub-surface layer of iron and magnesium smectites, something which has been interpreted as an weathering-profile, with the iron having been removed from the upper layers. Importantly, iron is usually only a mobile element under reducing conditions, when it becomes soluble in water, potentially providing insights into the ancient climate of Mars. The leaching of iron implies wet, reducing conditions.

The pattern of aluminium-rich clays overlaying iron-rich smectites can be seen at many locations in the Southern Highlands of Mars, although these have not been studied in any detail. Potentially, studies of these deposits could enlighten us about the history of Mars in a number of different ways. These would include determining if the conditions which produced them were global in extent; whether they record a single, or multiple events; and are they all of the same age? 

In a paper published in the journal Communications Earth and Environment on 4 November 2022, Binlong Ye and Joseph Michalski of the Department of Earth Sciences at the University of Hong Kong, Hong Kong, examine 203 exposures, distributed across the Martian surface, at which aluminium- and silicon-rich minerals could be observed overlaying iron- and magnesium-rich minerals, in order to build up a more comprehensive picture of the ancient climate of Mars.

Of the 203 sites Ye and Michalski examined, 54 were new sites, detected during the study. The majority of the sites were in the Mawrth Vallis, Eridania northern basin, Valles Marineris, Nili Fossae, Simois colles/Gorgonum chaos, Noachis Terra and Hellas Basin, at latitudes ranging from 30° north to 40° south. Sites outside this range may exist and not have been observed, or may have been obliterated by polar processes. Other features on Mars which have been associated with an ancient wet climate, such as lake basins or networks of valleys, are also restricted to the range 30° north to 40° south, which might reflect the distribution of ancient rainfall, and are found at altitudes ranging from 3000 m below to 6000 m above the Martian average surface level (lacking sees, Mars has no sealevel). Almost 88% of these exposures are on terrains interpreted as Noachian in age. Of the 203 sites examined, 65 are in units thought to be Early Noachian in age, 85 in units thought to be Middle Noachian in Age, 28 in units thought to be Late Noachian in age, and 18 are in units attributed to the Late Noachian/Early Hesperian transition. Dating based upon the density of craters suggests these sites are between 3970 and 3180 million years old

The relationship between compositional stratigraphy and valley networks and open basin lakes. The green dot represent the presence of weathering profiles in the study. The blue tone indicates the occurrence of the valley network, and the yellow circle represents the location of the open basin lake. Ye & Michalski (2022).

The sites were observed by remote sensing in a wide range of contexts, including impact crater floors, crater rims, crater walls, inter-crater plains and basins, within valley networks, and the knobby terrain of the Eridania deep basin deposits. This method has inherent biases, and is better able to detect steep exposures in dust free regions; it is therefore highly likely that many other similar exposures exist but have been overlooked.

Ye and Michalski combined data on 154 sites studied using the HiRISE  and CRISM instruments on the Mars Reconnaissance Orbiter, and the OMEGA instrument on the Mars Express spacecraft. This data was combined as the HiRISE instrument observes better in the part of the spectrum at which iron minerals are visible, while the CRISM and OMEGA instruments observe better in the part of the spectrum at which aluminium minerals are best detected.

Characteristics of representative geological contacts in martian weathering profiles. HiRISE IRB data (infrared, red, and blue-green) reveal sub-metre compositional differences of geological contacts of weathering profiles in false colour. (a) Mawrth Vallis; (b) Northern Hellas Basin region; (c) Eridania northern basin; (d) Noachis Terra; (e) Nili Fossae region; (f) Terra Tyrrhen; (g) Valls Marineris; (h) Simois colles. Ye & Michalski (2022).

The HiRISE image show contacts between white and red (units in many locations, with the white units always overlaying the red. The contact between the two layers is typically not sharp, and does not follow bedding planes, with the general texture and fabric of the rock being continuous between the two units. Ye and Michalski take this as indicative of iron (the main contributor of red colouration to rocks) having been leached from the upper layers.

On Earth, the crust can be divided into two distinct types, felsic crust, dominated by silicon and aluminium minerals, which makes up the continents, and mafic crust, dominated by iron and magnesium minerals, which makes up the ocean basin. On Mars this differentiation does not exist, with the entire surface of the planet being covered by mafic crust. However, while the long-term differentiation of material that has created the Earth's continental crusts never occurred on Mars, some felsic terrains have been produced in upland areas. Thus, the vast majority of the exposures studied are in mafic terrains, but such exposures within felsic terrains out also to be detectable. 

Such a weathering profiles within felsic terrains were found by Ye and Michalski in the massifs surrounding the Hellas Basin. These massifs are thought to have been formed by uplift or exhumation of the crust following the event which formed the Hellas Basin (actually a giant impact crater) about four billion years ago. The rocks here are very mixed, with a wide variety of lithologies present, but one distinctive rock-type present appears to be anorthasite (calcium-rich plagioclase feldspar) with a high iron content. Where present, this rock-type has a massive structure without bedding planes, probably indicating a plutonic origin. The majority of these rocks appear to be iron-rich silicates, but areas at the top of the massif have absorbance spectra implying the presence of aluminium hydroxides compounds, most likely kaolinite clay, a mineral which on Earth typically forms in hot, moist climates. Similar associations, with felsic terrains having aluminium-clay deposits in their uplands, were also observed in the Xanthe Terra and Noachis Terra regions. 

Example of possible precipitation-driven chemical alteration of felsic materials. (a) The geology context of compositional stratigraphy on the massif of northern Hellas Planitia (66.32 2°E, 25.21°S). MOLA elevation data are draped over THEMIS daytime infrared data (warm colors are at higher elevations and cool colors are at lower elevations). (b) Close-up view of the massif indicates the location of compositional stratigraphy (white arrow). (c) Ratioed CRISM I/F spectra contain aluminium clay minerals, iron/magnesium smectites, and felsic materials. (d) CRISM mineral map shows the distribution of diverse altered minerals: iron/magnesium smectites in red, felsic materials in green and aluminium clay minerals in blue. The different colour arrows show the locations of spectra acquired. Ye & Michalski (2022).

Ye and Michalski considered three possible origin scenarios for these deposits. Firstly, they could all have formed during a single, geologically brief, warm wet episode on ancient Mars. Secondly, Noachian Mars may have had an overall cold and dry climate, but with repeated intervals of warmer, wetter conditions. Such a scenario should, in theory produce some layered deposits in which aluminium rich and iron rich deposits alternate. These would be rarer and harder to find than on the Earth, where sedimentary rock processes are dominated by daily and seasonal water processes, are still likely to be present on Mars even if these events were separated by tens of thousands of years. A third possibility is essentially the same as the second, but in this version the most recent episode will have overwritten earlier events, with available clay always being washed downwards, so that older, lower deposits will contain iron washed down from above.

Hypotheses for the pattern within weathering profiles for aluminium/silicon materials and iron/magnessium clays. A scenario of a single climate transition (left) and another case of multiple repeated climate transitions spread out over geologic time (centre panel). A third hypothesis is that multiple events occur, but the latest event chemically overwrites older weathering profiles as iron migrates downward in the section. The blue tone unit refers to aluminium/silicon (iron-poor) materials, and the warm brown colour indicates the occurrence of iron/magnesium smectites. Ye & Michalski (2022).

Of the 203 profiles studied by Ye and Michalski, 201 showed only a single aluminium-rich layer overlaying iron-rich deposits. However, at the remaining two locations, one on Meridiani Planum and the other in the southern part of Coprates Chasma, iron-rich smectite deposits could be observed overlaying aluminium-rich clay deposits. 

Evidence of multiple pedogenic events in southern Meridiani Planum. (a) The geologic context of weathering profiles on an interfluve in southern Meridiani Planum. MOLA elevation data draped over THEMIS daytime infrared data (warm colours are higher elevation and cool colours are lower elevations). (b) CRISM data extracted from regions of interest are shown as offset ratio spectra compared to laboratory spectra of relevant minerals. (c) A CRISM mineral parameter map shows the distribution of iron/magnesium smectites and aluminium clay minerals (iron/magnesium smectites in red and aluminium clay minerals in blue). (d) 3D view of weathering outcrops is shown in the rectangle of (c) with 5 times vertical exaggeration. The different colour arrows show locations of spectra acquired. (e) Close-up view of HiRISE image shows the morphology and contact of clay-rich outcrops. (f) The subset of HiRISE images exhibits pervasive boxwork veins on the aluminium clay minerals unit. Ye & Michalski (2022).

The example on Meridiani Planum is particularly clear, and close to the area explored by the Opportunity rover. Here, there is an area of intense erosion and impact cratering, where high-standing 'knobs' of material have been left by erosive action. On these knobs two geological units can be observed. The upper of these is a relatively dark unit about 10 m thick, which is spectrally consistent with an iron/magnesium smectite. The lower unit is also massive, but with polygonal fractures (indicative of drying sediment, usually clay) and box-work veins (a feature produced by water percolating through a deposit, dissolving and redepositing minerals), possibly of some sulphate material. This unit is about 80 m thick, and is again comprised principally of iron/magnesium smectite, but with the upper portion apparently being an aluminium-rich clay. This appears to imply at least two phases of deposition, with the lower smectite layer being deposited first, having the soluble metal ions washed out of its upper portion, and then a second smectite layer deposited above it.

The vast majority of the exposures detected show only a single climate transition. However, this does not preclude there having been two or more such transitions, due to the possibility of such events having been over-written by subsequent events. Furthermore, most of the exposures are of a limited size; ideally, to detect repeated climate cycles in the rock record, geologists would look for exposures hundreds or even thousands of metres deep. 

The small size of the majority of the exposures detected also makes it hard to date these exposures using impact crater counting. However, the majority of the profiles are associated with cap units (i.e. the uppermost units in any succession), which gives them wider horizontal exposure, and makes it possible to establish a minimum age by this method.

Using computational stratigraphy to date cap units in the Martian Southern Highlands gives ages of between 3.8 and 3.6 billion years, consistent with previous results. 

The youngest example found by Ye and Michalson is in the floor of the Orson Welles Crater in Xanthe Terra, which is is breached by a series of fissures and graben to the southwest and by the Shalbatana outflow channel to the northeast, and which has been dated to 3.57 billion years before the present, based upon crater density analysis of the rim and ejecta deposits associated with the main crater, or 3.18 billion years, based upon similar analysis of sediments within the crater. The chaotic terrain associated with this crater has several outcrops with aluminium clays overlaying iron/magnesium smectites; notably, the upper layer also appears to contain opaline silica or allophane/imogolite, features that form by the weathering of volcanic ashes in cool environments with a limited water supply. 

The youngest known example of compositional stratigraphy on Mars. (a) The geology context of compositional stratigraphy on the chaotic materials in the Orson Welles crater. MOLA elevation data draped over THEMIS daytime infrared data (warm colours are higher elevations and cool colours are lower elevations). The white arrows indicate the presence of impact ejecta. (b) The CTX shows the overview of layered light-toned compositional stratigraphy. (c) Close-up HiRISE image shows the morphology and texture of compositional stratigraphy. (d) CRISM spectra extracted from regions of interest are shown as offset ratio spectra compared to laboratory spectra of relevant minerals. (e) The CRISM parameter map shows the distribution of alteration minerals, iron/magnessium phyllosilicate in red/yellow and aluminium phyllosilicate in blue. The different colour arrows show locations of spectra acquired. Ye & Michalski (2022).

Ye and Michalski are careful to point out that the age of a deposit and the age of weathering on that deposit are not necessarily the same; thus if a 3.18 billion-year-old deposit shows signs of weathering, then it can be said that the weathering is not older than 3.18 billion-years-old, but no minimum age can be assumed.

The oldest examples in Ye and Michalski's study are apparently rain-weathered pyroclastic-sediments on the flanks of two volcanoes in Thaumasia Planum. These volcanoes have been dated to 3.97 and 3.83 billion-years-old, respectively, and show signs of remnant crustal magnatization, which would indicate a pre-Noachian or early Noachian origin. Again, it is impossible to determine if the weathering is as old as the deposits, but these are the oldest sediments in which this sort of weathering can be seen.

The Orson Welles Crater and Thaumasia Planum deposits provide a time bracket for the deposition of sediments altered by chemical weathering driven by precipitation on Mars. These span the whole of the Noachian and Hesperian periods on Mars.

Exactly how long wet conditions persisted on Mars has been a source of speculation among scientists for many years. A number of methods have been used to approach this, including geomorphic analyses, numerical climate modeling, and  chemical alteration models. Ye and Michalski's results add further information to this debate.

The presence of thick clay-bearing pedogenic profiles is indicative of the presence of water and therefore a climate warm enough to allow liquid water. Ye and Michalski have used remote sensing to detect outcrops of such clays tens of metres thick (smaller deposits would be undetectable using available methods). Such deposits would be considered substantial on Earth, where similar profiles are common, but generally less than a metre thick, although it is quite possible that ancient Earth would have been host to similar massive pedogenic profiles, which have been overlaid by subsequent events.

Clays form on Earth with at a typical rate of about 0.01 mm per year, with the fastest known examples exceeding 0.05 mm per year. Assuming similar rates of formation on Mars, a deposit 120 m thick containing about 15% clay would require less than 40 million years, or 10% of the duration of the Noachian, to form. As few of the clay-profiles discovered exceed 100 m in thickness, and most are in the region of 50-60 m, the time needed to form these deposits is potentially much shorter, perhaps 1-10 million years, only 1-2% of the time-frame within which these deposits are thought to have formed.

The thickness of the deposits and estimated proportion of clay present can provide only a very rough estimate of the duration of weathering events. Rain-driven chemical erosion is caused by the dissolution of cations such as iron or magnesium in water draining through the rock. However, water cannot absorb an infinite amount of such cations; the amount that can be absorbed is determined by the diffusion rate, which in turn relates to temperature. Loosely speaking, the colder it is, the less cations can be absorbed. Another problem is physical erosion; chemically weathered sediments are more prone to disaggregating. These add significant complications to the estimated timeline for the production of weathered clay profiles on Mars, though Ye and Michalski estimate that shorter, warmer, climate excursions would produce thicker clay-deposits than longer-lived, but cooler, excursions.

The rate at which rocks are eroded by water is also dependent on their lithology, permeability, and porosity, which are hard to determine on Mars, although it is likely that many of these deposits were a mixture of volcanic ashes and impact glass. Such volcanic deposits are known to be more prone to chemical alteration by water than other rock-types, which may shorten the time in which these Martian deposits could have formed.

Some of the profiles seen are close to other features associated with running water, such as cross bedding, layered sediments, and channels, which imply they formed as part of a complex pattern of water-rock interactions. The presence of water-deposited sediments is likely to also be indicative of water-related erosion of the parent rocks from which these sediments are derived.

Evidence of sedimentary process on or within weathering profiles on Mars. Layer structures of weathering profiles (a)–(d). Inverted craters (e), (f) and inverted channels on weathering outcrops (g), (h). Ye & Michalski (2022).

Ye and Michalski's Martian global survey has produced over 200 examples of ancient chemical weathering on Mars. The youngest of these appear to be Hesperian in age, although the majority are Noachian, i.e. more than 3.7 billion years old. These profiles range widely in altitude, from 11 km above the Martian average surface level, to 5-6 km below, which appears to support the idea that their formation was driven by top-down water transportation, and was global in extent. 

Almost all of the profiles show only a single transition between climate-states, however, examples of more than one transition are present, and the lack of more widespread multiple transitions can be explained by chemical 're-setting' of the rocks during wet phases. These deposits occur in a wide-variety of mineralogical contexts, suggesting a complex history of water-mineral interactions on Mars.

The most probable origin of these deposits are a series of geologically short-lived (1-10 million years) warm climate excursions driven by reducing greenhouse gasses, such as hydrogen or methane. A wet anoxic environment is ideal for the mobilisation of iron ions in water, while leaving aluminium ions in place, creating the the profiles found by Ye and Michalski. 

The time needed to form the observed profiles is much shorter than the duration of the Noachian, making a single climate transition unlikely. This being the case, the chemical-resetting scenario is the most plausible for the origin of these deposits, which means that each example probably records the last wet interlude in the area where it is found.

Many of the weathering profiles are associated with water-generated sedimentary structures such as degraded impact craters, valley networks, closed-basin lakes, and open-basin lakes. In some cases it appears possible that the weathered profiles have themselves been cut through by water features, although the most plausible scenario is that the features are contemporary in nature, and that most of the weathering profiles are associated with a pulse of valley-network formation in the Late Noachian-Early Hesperian. 

The channels and lakes could have been formed in as little as about a thousand to about a million years, while the weathering profiles are thought to have taken 1-10 million years to form. Thus the weathering profiles widen the interval for the potential presence of water on Mars. Furthermore, although the interval for the formation of each weathering feature is relatively short, the total time in which they could have formed is very long. The presence of top-down weathering profiles in a wide range of geological contexts, apparently formed over a long period of time, suggests multiple formation events over an extended period of Martian history, driven by repeated spikes in the reducing greenhouse gas content of the ancient Martian atmosphere.

See also...

Follow Sciency Thoughts on Facebook.

Follow Sciency Thoughts on Twitter.

Understanding ocean chemistry in the Western Interior Seaway during the Cenomanian–Turonian Extinction Event.

The boundary between the Cenomanian and Turonian stages of the Cretaceous Period is marked by a mass extinction event that saw the demise of a quarter of the marine invertebrates present at the onset of the crisis, combined with carbon, oxygen, and sulphur isotope levels, the deposition of a thick (up to 3 m in places), organic-rich, black shale in many ocean basins, and the onset of a greenhouse climate, known as the Cretaceous Climatic Maximum, which peaked in the early Turonian, then gradually cooled off over the remaining 24 million years of the Cretaceous. Numerous causes have been proposed for the Cenomanian–Turonian Extinction, but the most likely is thought to be massive volcanic emplacements, possibly in the Caribbean Large Igneous Province, which injected large amounts of carbon dioxide, hydrogen sulphide, and sulphur dioxide, as well as a variety of metal compounds, into the ocean-atmosphere system. Increasing atmospheric carbon dioxide would have led to higher global temperatures and higher precipitation on land, which in turn would have led to higher erosion on land, more nutrients being washed into the oceans, and vast Algal Blooms, which would be recorded as a higher burial rate for organic carbon, causing the global carbon isotope excursion, which can be observed from both black shales and carbonate rocks spanning the Cenomanian–Turonian boundary. At the time much of the world's ocean system was dominated by shallow, epicontinental seas (i.e. seaways covering continents in the already warm Cretaceous world), which would have quickly become stagnant when these Algal Blooms were combined with a combination of an injection of oxygen consuming metals and a break-down in ocean circulation caused by the rising temperatures, resulting in large portions of the global ocean becoming anoxic and hostile to multicellular life. The widespread occurrence of black shales at the Cenomanian–Turonian boundary is thought to be a reflection of this. Curiously, however, these phenomena are not recorded in all sequences spanning the Cenomanian–Turonian boundary, with many shallow marine environments (which would be predicted to be the most severely impacted by such events) seemingly unaffected. This variability, with the event leaving a strong signal in some sequences, a light one in others, and being totally absent in some places, leads to the conclusion that the 'global event' may in fact have been a series of overlapping local occurrences, driven by multiple factors rather than a single change in global atmospheric composition.

In a paper published in the journal Scientific Reports on 30 June 2021, Rob Forkner of the Deep Time Institute, Jeremy Dahl of Biomarker Technologies, Inc., and the Stanford University Institute for Materials and Energy Sciences, Andrea Fildani, also of the Deep Time Institute, Silvana Barbanti, also of Biomarker Technologies, Inc., Inessa Yurchenko of the Department of Geological Sciences at Stanford University, and Mike Moldowan, again of the Deep Time Institute, present the results of a study of the USGS Portland-1 core, which was drilled in Colorado, and which includes a section of the Greenhorn Formation including the Cenomanian–Turonian boundary.

 

Palaeogeographic map of North America during Oceanic Anoxic Event 2. The location of the Portland-1 core as well as active volcanic centres are shown. Forkner et al. (2021).

Forkner et al. sampled the core through the Cenomanian–Turonian boundary interval (as determined by the isotope excursion), as well as on either side, for organic geochemical analyses. They initially targeted layers with high organic carbon which were thick enough to determine if reworking or bioturbation had occurred, although this severely limited the number of suitable layers, with the effect that samples were taken at intervals of between 3 and 12 cm across the boundary interval, and 20 cm or more outside this interval. The samples were round segments 2-3 cm in diameter and 1 cm thick taken from the larger core, which were first tested for rock richness and maturity (the extent to which rocks have been heated, altering organic molecules preserved within them), before the most suitable samples were selected for analysis by gas chromatography–mass spectrometry.

Molecular fossils, or biomarkers, are recognisable fragments of molecules synthesised by biological organisms, which can be used to determine the presence and abundance of groups of organisms. Forkner et al. analysed biomarkers from the Portland-1 core across the Cenomanian–Turonian boundary, thereby obtaining a series of snapshots of the water column ecology, which were used to develop a new molecular stratigraphy for the boundary, thereby deriving a wealth of new information with regard to the biota, depositional environment, and the multiple drastic environmental changes that occurred before, during, and after the Cenomanian–Turonian Extinction Event.

A geological examination was used to establish a lithological sequence of events (i.e. changes in the rock type being laid down over time, which would have related to local environmental conditions), using photographs to cover those sections of the core which have previously been heavily sampled by previous workers. This enabled the comparison of similar facies ( specified characteristics, which can be any observable attribute of rocks), in order to correlate changes in the biota in intervals with similar climatic and environmental conditions. This selection process meant that effectively only the finest grained, dark mudrocks were sampled, as these gave the greatest opportunity to detect changes in the water column biota uninfluenced by sedimentary conditions.

The data obtained from the Portland-1 core indicates that, in this area of the Cretaceous Western Interior Seaway at least, conditions in the water column during the Cenomanian–Turonian boundary event (sometimes known as the Cenomanian–Turonian Ocean Anoxic Event) conditions do not appear to have been particularly anoxic. In fact, the sediments laid down across the boundary appear to have been laid down in a more oxygenated environment than wither the sediments above the boundary or those below it, something which has been noted at other locations in the Western Interior Seaway, and coeval shallow water deposits from the Tethys Ocean. The sediments laid down before the boundary layers are predomanenty finely laminated, whereas those across the boundary interval are mostly heavily bioturbated, suggesting a thriving benthic community living within them.

 

USGS Portland-1 core lithologic section, carbon isotope profile and RockEval data. Facies Explanation: (1) Peloidal/foraminiferal, packstone/grainstone; (2) Bioturbated peloidal packstone; (3) Bioturbated peloidal wackestone; (4) Skeletal grainstone; (5) Rippled mudstone; (6) Silty laminated mudstone; (7) Diffusely laminated mudstone; (8) Massive mudstone; (9) Bentonite. Samples were limited to facies (7) and (8). The occurrence of bioturbated peloidal carbonates during the Oceanic Anoxic Event positive carbon isotope excursion indicates that the environment at the time of deposition was oxygenated and supported a diversity of tropical marine life. Note that the core is measured in imperial units as the Portland-1 core and core photos are curated with imperial measurements. This reference is preserved here in the case that the reader wishes to cross-reference these results to the Portland-1 core. Radio-isotopic measurements from bentonites A, B, and C, along with biostratigraphy and correlation of depositional cycles to orbital timescales have produced an average sedimentation rate of 0.93 cm/per thousand years during the Oceanic Anoxic Event positive carbon isotope excursion. The interval of samples with the greatest flux in measured biomarker concentration occurs from about 473 feet (144 m) to about 479 feet (146 m), in the central portion of the Oceanic Anoxic Event positive carbon isotope excursion. Sample spacing in this interval is somewhat irregular in order to stay within the same depositional facies, but varies between 3 and 12 cm indicating that rapid flux in organic geochemical composition of analysed sediments over periods approximately 3–15 thousand years. The carbon isotopic excursion (CIE) that defines the Oceanic Anoxic Event is shown on the carbon¹³ as a proportion of total carbon (δ¹³C) track and highlighted in blue on all compound tracks. Hydrogen Index (HI) is generally negatively correlated with depositional environment oxygen concentrations, thus supporting the trend of Oceanic Anoxic Event positive carbon isotope excursion oxygenation. Oxygen Index (OI) generally correlates positively with depositional environment oxygen concentrations, and again provides evidence for oxygenation during the Oceanic Anoxic Event positive carbon isotope excursion. Forkner et al. (2021).

The geochemical analysis of samples extracted from the core supports the geological analysis. The 'Hydrogen Index', which derives from the proportion of total organic carbon made up of hydrocarbons, is generally negatively correlated with the oxygen concentration in the depositional environment, i.e. the Hydrogen Index tends to go up when there is less oxygen and down when there is more oxygen. In the Portland-1 core the Hydrogen Index above the Cenomanian–Turonian boundary layer averages at 509, during the boundary the average fell to 177, and below the boundary the average rose again, to 423, supporting the idea that oxygen levels in the water column rose rather than fell during the boundary interval. The 'Oxygen Index', derived from the purporting of carbon dioxide to total organic carbon, is positively correlated with the level of oxygen in depositional environment (i.e. the Oxygen Index goes up when the amount of oxygen present in the depositional environment goes up). In the Portland-1 core the Oxygen Index above the boundary layer averages 16, in the boundary layer averages 28, and below the boundary layer averages 15, again suggesting a rise in oxygen levels across the boundary interval.

A number of biomarkers also strongly imply a rise in oxygen levels during the Cenomanian–Turonian boundary interval. The Gammacerane Index is derived from the ratio of the biomarker gammacerane (derived from bacterivorous Ciliates) to hopane (derived from Bacteria), is associated with stratification in the water column, with high levels of gammacerane typically indicating highly saline or reducing conditions. In the Portland-1 core the Gammacerane Index drops to its lowest level during the Cenomanian–Turonian boundary interval, implying conditions became less reducing (generally a sign of higher oxygen levels). The Homohopane Index is derived from the proportion of C₃₅ hopanes (hopane molecules with 35 carbon atoms) to the total C₃₁-C₃₅ hopanes (hopanes with between 31 and 35 carbon atoms). This idex also tends to rise with reducing conditions, and again has its lowest valuse in the Cenomanian–Turonian boundary interval in the Portland-1 core, again suggesting that the enviroment became less reducing during this interval. The proportions of A-oleanane relative to oleanane and 17α-diahopane relative to 17α-hopane are also thought to be indicative of higher oxygen levels, since both A-oleanane and 17a-diahopane require oxygen for their producers (Bacteria and Flowering Plants, respectively), to form them from their precursors, oleanene and hopane. The levels of A-oleanane and 17a-diahopane remain constant throughout the section, but the levels of oleanene and hopane fall during the Cenomanian–Turonian boundary layers, so that the proportion of A-oleanane and 17a-diahopane rise, presumably indicative of a rise in oxygen. 

 

Compound tracks through the Cenomanian–Turonian boundary layers relating to oxygenation before, during, and after the event. Gammacerane and Homohopane Indexes, which are affected by sediment redox conditions, show a significant decrease and the ratios related to 17α-Diahopane exhibit an increase with striking fluctuations within the Cenomanian–Turonian boundary layers. These broad scale changes reflect an overall increase in oxygenation during the Cenomanian–Turonian boundary interval, with periods of reducing conditions punctuated through the event. The relative preservation of des-A-oleanane revealed by the des-Aoleanane/oleanane ratio could be a function of oxidation. Forkner et al. (2021).

Given the generally healthy ecosystem recorded in the sediments of the Portland-1 core across the Cenomanian–Turonian boundary, and the geochemical evidence for a healthy, well-oxygenated water column, biomarkers associated with primary Algal production would be expected to increase across the Cenomanian–Turonian boundary layer. However, a range of such biomarkers, including cholestanes, ergosteranes and C₂₇, C₂₈, and C₃₀ steranes, undergo fluctuations in the boundary layer, with a general decrease in levels. This would appear to represent a deteriorating environment, with repeated stressful intervals.

While this decrease in steranes implies a drop in Algal productivity across the Cenomanian–Turonian boundary, levels of hopanes, indicative of Bacterial productivity, remain relatively high, although again they undergo some severe fluctuations during the boundary interval, suggesting that environmental conditions were fluctuating in the water column. 

Fluctuating biomarker ratios are highly indicative of an unstable environment, with fluctuating primary productivity. In order to further explore this, Forkner et al. examined three further examples. The hopane/sterane ratio reflects the levels of both heterotrophic and photosynthetic Bacteria to primary producers including marine Algae and terrestrial Plants. The 3β-methylhopane/dinosteranes ratio compares the ratio of 3β-methylhopane, derived from aerobic methanotrophs and fermentative Bacteria, to the ratio of dinosteranes, which are almost exclusively derived from Dinoflagellates. The 3β-methyl-24-ethylcholestane/4α-methyl-24-ethylcholestane ratio compares a reworked sterane to one produced by marine Algae. All of these ratios record a significant drop in primary production across the Cenomanian–Turonian interval, but with rapid fluctuations which appear to be unrelated to any change in the lithology, and which Forkner et al. suggest might be related to short-term anoxia events not recorded in the rock record.

 

Compound tracks through the Cenomanian–Turonian boundary interval relating to productivity before, during, and after the event. For the main the Cenomanian–Turonian boundary interval section, the concentration of biomarkers derived from algae such as C₂₇, C₂₈, and C₃₀steranes (24-n-propylcholestanes), and 4α-methyl-24-ethylcholestane 20R decreases significantly relative to concentrations of those derived from bacteria, which increase moderately with variations. This suggests that the record of productivity variability we interpret is reliable and is not simply a record of poor preservation of the organic fraction. Of note is the interval in all track from about 473 feet (144 m) to about 479 feet (146 m) where organic geochemical measurements return the most erratic results. By applying the most recently published timescales through this interval, it is possible to calculate that the productivity cycles of biomarker decline and recovery can vary from about 26 thousand years at the shortest to about 130 thousand years at the longest. These productivity cycles occur within individual lithofacies internal to single lithocycles. Fornkner et al. (2021).

Isoprenoids can be derived from chlorophyll side chains produced by photosynthetic Algae, although other organisms do produce them, including Cyanobacteria and some Archaeans. Notably, head-to-head isoprenoids such as biphytane are produced by marine Archaea. Unfortunately, isoprenoids are found at very low concentrations throughout the core, although some fluctuations can be observed during the Cenomanian–Turonian boundary layers, and biphytane and its diagenetic products are only ever found at trace levels, and often drop below detectability levels. Studies of the Cenomanian–Turonian layer in other sections have suggested that there might have been blooms of opportunistic Archaea during this interval, but this cannot be detected in the Portland-1 core.

A number of biomarkers remain relatively constant on either side of the Cenomanian–Turonian boundary layers, but fluctuate greatly during the boundary interval. Previous work on the Portland-1 core and other cores from the same area have been studied extensively to determine sedimentation rates. These studies have led to a calculated sediment accumulation rate of 0.93 cm per thousand years in the upper part of the Cenomanian–Turonian boundary, using calculations that include radiometric dates from bentonite layers, and orbital time scales. Calibrating this with the observed fluctuations in biomarker ratios suggests that productivity fluctuations were occuring on cyclical scales of between 26 and 90 thousand years, with individual biological collapses happening in as little as 3200 years.

One possible cause of influxes into the shallow, enclosed, Western Interior Seaway during the Cenomanian–Turonian boundary period that has been previously suggested is sea level rises, Evidence for such changes has been found in the Tethys Ocean, where repeated cycles of carbonate platforms being replaced by deeper sediments have been observed. Such an increase in water volume would provide relief from the effects of stagnation, but cannot explain the drastic changes in water column ecology observed by Forkner et al., which appear to happen on a much finer scale. Forkner et al. were unable to find any references to previous examples of such rapid changes in organic composition of a rock sequence, particularly one one independent of changes in the lithology, and conclude that the drivers these changes were clearly independent of the drivers of lithology changes.

Studies of the lithology and astronomical forcing cycles recorded in the Portland-1 core have previously concluded that the carbonate-mudstone cycle here (which would have been driven by changes in sealevel) would have lasted about 100 000 years. The biological productivity cycle, however, is clearly working on a much shorter, more erratic, and independent timescale. Forkner et al. cannot rule out the possibility that some shorter Milankovitch-band processes was occurring in the Western Interior Seaway, but the irregular nature of the cycles makes this unlikely, as does the fact that they are not seen outside the Cenomanian–Turonian boundary interval.

Forkner et al. suggest that these biomarker cycles may have been driven by changes in primary productivity (i.e. photosynthesis), and the subsequent decay of organic matter. During the intervals on either side of the boundary, more reducing conditions prevailed, probably due to higher rates of organic decay. During times of extreme stress within the boundary period, the amount of primary production dropped, and there was a drop in the amount of organic matter in the water column, and therefore the amount of decay that could occur. However, the driver of this stress, and the cause of the erratic cycle it was following, remain unclear.

A cause of environmental stress unrelated to sealevel changes or orbital forcing and operating on rapid and erratic timescales could plausibly be volcanism. Volcanism has the potentially to disrupr Algal productivity rapidly, and would not cause any change in the depositional environment (which changes in sealevel related to Milankovitch forcing would do). A number of other cores from the Western Interior Seaway (including Eagle Ford, Boquillas, and Bouldin Flags) contain numerous bentonite layers (caused by volcanic ash falling on water then settling to the bottom), which have been used to determine the age of the sediments and the rate at which they were accumulating. Volcanism can impact Algal productivity in a number of ways, from lowering light levels to altering the pH of the water. While this cannot be proven from Forkner et al.'s current work, it would potentially have left detectable signs which could be revealed by future investigations. Either way, these findings appear to support the idea that the Cenomanian–Turonian Extinction Event was caused by a prolonged breakdown in environmental conditions rather than a single catastrophic event.

The Cenomanian–Turonian boundary reflects a profound change in environmental conditions on a global level, although the record of this varies from location to location. Geochemical examination of organic molecular fossils preserved in the Portland-1 core, a nearly-continuous core of sediment recovered from the Western Interior Seaway of North America, shows a record of extreme environmental variability not previously observed, with a generally higher level of oxygenation than before the crisis, prior to previous assumptions about a single, massive, anoxia event driving the crisis. Instead, the environment seems to have undergone a series of smaller, but still significant, crises, operating on an irregular cycle divorced from Milancovich forcing and sealevel changes. Forkner et al. hypothesise that this could have been driven by volcanic episodes, which tend to be erratic in their timing. A single, massive, volcanic event has previously been suggested as a possible cause of the Cenomanian–Turonian Extinction Event, injecting vast amounts of material into the atmosphere and oceans, and leading to significant changes in seawater pH, global temperature, and the hydrological cycle, but Forkner et al.'s evidence points towards a more prolonged period of change, with periods of high organic productivity punctuated by sudden collapse. This repeated stressing of the marine environment would have challenged the biota of these ecosystems, potentially causing the observed extinctions.

See also...

Online courses in Palaeontology.  

Follow Sciency Thoughts on Facebook.

Follow Sciency Thoughts on Twitter.