The impact crater at Chicxulub (Yucatán Peninsula, México) is the only terrestrial crater on Earth with a well-preserved peak ring. The asteroid impact is linked to the end-Cretaceous mass extinction event, which wiped out 76% of all species worldwide, along with a near-global loss of vegetation. A collapse in Phytoplankton productivity in the world’s oceans occurred due to the sudden decline in photosynthesis as atmospheric particulates lowered light levels for years after the impact. In 2016, the peak ring of the Chicxulub crater was cored by the International Ocean Discovery Program and International Continental Scientific Drilling Program Expedition 364. A 130 m thick interval of impact melt rock and upward-fining suevite, which overlies fractured basement rock, was deposited immediately after impact. The lower suevite, rich in impact melt rock, is directly overlain by material transported via ocean resurge and then by seiches and a tsunami deposit. The overlying 0.75 m- hick, fine-grained, brown micritic limestone (the 'transitional unit'), deposited in days to years after the impact by continuing seiches and tsunami, contains microfossils of calcareous plankton and trace fossils of burrowing organisms. The transitional unit is overlain by a thin green marlstone, followed by the deposition of 'white' micritic limestone (616.55–616.24 m below seafloor) within 30–200 thousand years, representing the base of the succeeding pelagic-hemipelagic limestone deposit.
In a paper published in the journal Geology on 17 January 2020, Bettina Schaefer, Kliti Grice, and Marco Coolen of the Western Australian Organic and Isotope Geochemistry Centre at Curtin University, Roger Summons and Xingqian Cui of the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology, Thorsten Bauersachs of the Organic Geochemistry Unit at Christian-Albrechts-University, Kiel, Lorenz Schwark, also of the Western Australian Organic and Isotope Geochemistry Centre at Curtin University, and the Organic Geochemistry Unit at Christian-Albrechts-University, Michael Böttcher of the Geochemistry & Isotope Biogeochemistry Group at the Leibniz Institute for Baltic Sea Research, Marine Geochemistry at the University of Greifswald, and the Department of Maritime Systems at the University of Rostock, Timothy Bralower, Shelby Lyons, and Katherine Freeman of the Department of Geosciences at Pennsylvania State University, Charles Cockell of the School of Physics and Astronomy at the University of Edinburgh, Sean Gulick of the Institute for Geophysics at the University of Texas at Austin, Joanna Morgan of the Department of Earth Sciences and Engineering at Imperial College, London, Michael Whalen of the Department of Geosciences at the University of Alaska Fairbanks, Christopher Lowery, also of the Institute for Geophysics at the University of Texas at Austin, and Vivi Vajda of the Department of Palaeobiology at the Swedish Museum of Natural History, present the results of a study of biomarkers within the sediments above the Chicxulub suevites, with the aim of understanding the microbial communities that developed in the post-impact crater.
Map showing International Ocean Discovery Program (IODP) Site M0077A (21.45°N, 89.95°W) at the Chicxulub crater, Mexico. Schaefer et al. (2020).
Evidence of ancient life is generally preserved in sediments as morphological fossils, trace fossils, and molecular fossils (biomarkers). Biomarkers are often well preserved in sediments even where visible mineralized fossils are absent, representing valuable signs of past life, especially microbial life. For example, in the Fiskeler Member in the end-Cretaceous boundary layer at Kulstirenden, Denmark, biomarkers showed that marine productivity recovered within a century following the Chicxulub impact. Schaefer et al. present biomarker distributions and sulfur isotopes of pyrite between 619 mbsf and 608 mbsf at International Ocean Discovery Program Site M0077A. Their aim was to use biomarkers to reconstruct the origin, recovery, and development of microbial life and to determine the paleoenvironmental conditions in the crater from the time of impact to up to about 4 million years after the impact.
Despite low organic matter content and low abundances of biomarkers, the record provided insights into the evolution of microbial communities in this exotic habitat.
The uppermost suevite, found 619.31-617.33 meters below seafloor, was deposited by a tsunami within the first day after impact. This tsunami transported reworked organic matter from outside the crater, as evidenced by the abundance and distribution of perylene and charcoal.Reworked marine inputs shown by biomarkers included long-chain alkanes, indicative of Algae or Cyanobacteria. Further, abundant long-chain steranes from Green Algae and/or land Plants, reflect a mixture of marine and terrigenous inputs. This interval also contains biomarkers derived from anoxygenic photosynthetic Sulfur Bacteria (i.e., isorenieratane, β-isorenieratane, and traces of chlorobactane and okenane). In addition, Cyanobacterial biomarkers in the form of 2α-methylhopanes and heterocyst glycolipids were observed. The latter, diagnostic for nitrogen-fixing Cyanobacteria, represent the oldest reported intact heterocyst glycolipids. From the presence of terrestrial signatures and the depositional regime, we infer that all the organic signatures are reworked materials, likely derived from carbonate platforms and coastal environments close to the site. The biomarkers listed above were also identified in overlying sediments (617.33–608.48 meters below seafloor), where they represent organisms living within the nascent crater. Here, we evaluate the oceanographic and redox conditions in the impact basin as inferred from the biological origins of these compounds.
The 200 000 years after impact is represented by the transitional unit (617.33–616.58 meters below seafloor) of fine-grained brown micrite and overlying green marlstone, and it is likely to contain the first record of microbial life after the impact. The succeeding 'white micrite' is possibly a result of calcite formed photosynthetically by Cyanobacteria that replaced the Calcareous Nanoplankton and other Algae across the Cretaceous-Palaeogene boundary.
Schaefer et al.'s study provides the first evidence of Cyanobacteria 30 000 years. after impact at 617.33–616.58 meters below seafloor, from abundant long chain hopanes. 2α-methylhopane, considered to be a biomarker for Cyanobacterial activity, is found at levels similar to those reported for the Fiskeler Member boundary layer in the Højerup section at Stevns Klint, Denmark, are typical of marine conditions. However, this is significantly lower than those reported in Permian-Triassic and Triassic-Jurassic boundary sections.
Sterane levels were found to be low, showing low Algal inputs relative to Bacteria, particularly Cyanobacteria. In the Fiskeler Member boundary layer in Denmark, the low sterane level was assigned to a decreased Algal input, followed by an immediate increase, suggesting a rapid resurgence of Algae when solar irradiance returned to pre-impact levels. In the transitional unit, sterane level changed within multiple intervals, suggesting that the organic matter in the crater was a mixture of transported and autochthonous material, distinct from other Cretaceous-Palaeogene sites. A similar trend was observed for 2α-methylhopane and homohopane (a biomarker for Bacterial activity), consistent with anoxic-euxinic (low oxygen-high sulphide) conditions. The homohopane to hopane ratio fluctuates considerably, which is thought to have been caused by the increased preservatin of long chain hopanes under euxinic conditions through reduction and cross-linking with reduced sulphur species. The shifts in high to low sterane levels suggest that sedimentation was influenced by water movement, most likely seiches and resuspension.
Heterocyst glycolipids were observed to be low in abundance in this interval, and exclusively consisted of types of long chain heterocyst glycolipid diol and heterocyst glycolipid keto-ol, identified in coastal microbial mats, brackish-marine environments, and in axenic (monospecific) cultures of Nostocalean Cyanobacteria such as Anabaena spp. or Nodularia spp. However, long chain glycolipid diols and triols have been reported in free-living marine Cyanobacteria. It is therefore plausible that the heterocyst glycolipids found here are also derived from a marine source. The low abundance of both components, however, suggests only low productivity of nitrogen-fixing heterocystous Cyanobacteria in the first 200 000 years. after the impact. An increased influx of terrigenous nutrients would have helped to sustain Phytoplankton, as shown by the paired increase in the abundance of long-chain waxy n-alkanes and steranes.from Plants and Green Algae. 3β-methylhopanes show an increase at the top of the transitional unit (616.62–616.58 meters below seafloor) and in the white micrite, indicating the presence of Methanotrophs.
A substantial shift in the microbial community was found in the middle and upper parts of the hemipelagic limestone horizon, found between 616.62 and 616.58 meters below seafloor, and considered to have been laid down between 200 000 and 4 million years after the impact. The heterocyst glycolipid distribution patterns and abundances showed considerable changes indicating shifts in the Cyanobacterial community and an increase in Cyanobacterial productivity by two orders of magnitude compared to the transitional unit, with maximum concentrations at 613.45 meters below seafloor.
In contrast, 2α-methylhopane levels remained constant, with a slight increase at 613.45 meters below seafloor, whereas the homohopane levels increased again between 613.45 and 610.72 meters below seafloor. This increase in (Cyano)bacterial biomarkers and the concomitant rise in the abundance of nitrohen-fixing heterocystous Cyanobacteria suggest a shift toward a nitrogen-limited environment, perhaps triggered by water column stratification. Another possibility is that these organisms were allochthonous (found elsewhere) and were transported into the crater from microbial mats living in relatively shallow waters. The limestone interval between 613.45 and 610.72 meters below seafloor (roughly 64.4–63.1 million years ago) indeed indicated anoxic conditions during deposition, depicted by low pristane/phytane ratios abundant β-carotane from autotrophs, and highly characteristic photic zone euxinia biomarkers from green-green and brown-green pigmented Chlorobiaceae (e.g., chlorobactane and isorenieratane), and purple pigmented Chromatiaceae (okenane). Chlorobiaceae and Chromatiaceae are anaerobic photoautotrophs that use hydrogen sulphide (generated by sulphate-reducing Bacteria) as an electron donor and biosynthesise specific bacteriochlorophyll and accessory carotenoid pigments to capture longer wavelengths of light energy to fix carbon dioxide.Such organisms flourish in benthic mats and as plankton concentrated at the chemocline of lakes or restricted marine basins where sulphide concentrations are high within the photic zone; hence, they are indicative of photic zone euxinia conditions. In this limestone interval, total reduced inorganic sulphur was abundant, consistent with nonlimiting sulphate concentrations, water-column photic zone euxinia, and enhanced pyrite burial. Similar values have been reported for reduced sulphur in Cretaceous black shales. Diagenetic pyrite in shell fillings and sediment matrix indicates recrystallization of primary framboids. The pronounced inorganic sulphur depletion compared to the estimated value of contemporaneous seawater signifies that microbial sulphate reduction probably took place in the water column.
Associated with compelling indicators that periodic photic zone euxinia was prevalent in the Chicxulub crater from about. 64.4 to 63.1 million years ago, the molecular evidence indicates that oxygenated waters overlay the anoxic and sulphidic lower layers of the water column During this time interval, methane from anoxic sediments underlying a sulphidic water column likely migrated upward until it was oxidized by microaerophilic methanotrophic Bacteria at the chemocline, as evidenced by the presence of 3β-methylhopanes. An alternative scenario is the possibility of an oxygen minimum zone existing in the crater water.
Schaefer et al. propose a scenario where, in the initial days after the asteroid impact, debris from microbial mats containing nitrogen-fixing heterocystous Cyanobacteria and photosynthetic Sulfur Bacteria was eroded from adjacent carbonate platforms and transported by ocean resurge or tsunamis into the crater. Microbial ecosystem communities were in a constant state of dynamic flux during the early evolution of the crater. Diminution of sunlight following the impact led to a dramatic decline in cyanobacterial productivity in the crater waters. However, rapid recovery of phytoplankton occurred in the first 200 000 years, and marine primary production was fueled by an influx of terrigenous nutrients. Phytoplankton communities continued to experience rapid changes over the following 4 milllon years. The nascent Chicxulub crater basin was accompanied by major transitions in nutrient and oxygen supplies (periods of euxinia) that shaped the recovery of microbial life.
Evolution of microbial communities in the Chicxulub crater, Yucatán Peninsula, Mexico. Schaefer et al. (2020).
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