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.
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.
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.
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.
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.
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