Friday 14 April 2023

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 GenevaJorge Spangenberg of the Institute of Earth Surface Dynamics at the University of LausanneMarta Roige of the Departament de Geologia at the Universitat Autònoma de BarcelonaThierry 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 SurveyPietro 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.

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