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Taphonomy of Biosignatures in Microbial Mats on Little Ambergris Cay, Turks and Caicos Islands

Gomes, Maya L. and Riedman, Leigh Anne and O'Reilly, Shane and Lingappa, Usha and Metcalfe, Kyle and Fike, David A. and Grotzinger, John P. and Fischer, Woodward W. and Knoll, Andrew H. (2020) Taphonomy of Biosignatures in Microbial Mats on Little Ambergris Cay, Turks and Caicos Islands. Frontiers in Earth Science, 8 . Art. No. 576712. ISSN 2296-6463. doi:10.3389/feart.2020.576712.

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Microbial mats are taxonomically and metabolically diverse microbial ecosystems, with a characteristic layering that reflects vertical gradients in light and oxygen availability. Silicified microbial mats in Proterozoic carbonate successions are generally interpreted in terms of the surficial, mat building community. However, information about biodiversity in the once-surface-layer can be lost through decay as the mats accrete. To better understand how information about surface microbial communities is impacted by processes of decay within the mat, we studied microbial mats from Little Ambergris Cay, Turks and Caicos Islands. We used molecular techniques, microscopy and geochemistry to investigate microbial mat taphonomy – how processes of degradation affect biological signatures in sedimentary rocks, including fossils, molecular fossils and isotopic records. The top < 1 cm of these mats host cyanobacteria-rich communities overlying and admixed with diverse bacterial and eukaryotic taxa. Lower layers contain abundant, often empty, sheaths of large filamentous cyanobacteria, preserving their record as key mat-builders. Morphological remains and free lipid biomarkers of several bacterial groups, as well as diatoms, arthropods, and other eukaryotes also persist in lower mat layers, although at lower abundances than in surface layers. Carbon isotope signatures of organic matter were consistent with the majority of the biomass being sourced from CO2-limited cyanobacteria. Porewater sulfide sulfur isotope values were lower than seawater sulfate sulfur isotope values by ∼45–50‰, consistent with microbial sulfate reduction under sulfate-replete conditions. Our findings provide insight into how processes of degradation and decay bias biosignatures in the geological record of microbial mats, especially mats that formed widely during the Proterozoic (2,500–541 million years ago) Eon. Cyanobacteria were the key mat-builders, their robust and cohesive fabric retained at depth. Additionally, eukaryotic remains and eukaryotic biosignatures were preserved at depth, which suggests that microbial mats are not inherently biased against eukaryote preservation, either today or in the past.

Item Type:Article
Related URLs:
URLURL TypeDescription
Riedman, Leigh Anne0000-0002-0959-1674
O'Reilly, Shane0000-0001-6303-4329
Lingappa, Usha0000-0001-5691-6788
Metcalfe, Kyle0000-0002-2963-765X
Fike, David A.0000-0003-2848-0328
Grotzinger, John P.0000-0001-9324-1257
Fischer, Woodward W.0000-0002-8836-3054
Knoll, Andrew H.0000-0003-1308-8585
Additional Information:© 2020 Gomes, Riedman, O’Reilly, Lingappa, Metcalfe, Fike, Grotzinger, Fischer and Knoll. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms. Received: 26 June 2020; Accepted: 14 August 2020; Published: 04 September 2020. We thank the Agouron Institute for support of this project and the Department of Environment & Coastal Resources (DECR) of Turks and Caicos Island government for granting a research permit to perform this work to JG. We also thank Roger Tarika, Bevo Tarika, Paul Mahoney, James Seymour, and the support staff on Big Ambergris Cay for logistical support. We are also grateful to Lizzy Trower (CU Boulder), Daven Quinn (Caltech), Nathan Stein (Caltech), Marjorie Cantine (MIT), Justin Strauss (Dartmouth), Hannah Grotzinger (MIT), Emily Orzechowski (UC Berkeley), and Sophia Roosth (Harvard) for help with field and lab work and Elizabeth Sibert (Harvard) for help with statistical comparisons of microscopy data. We also thank Teresa Avila, Stephanie Moore, and Jennifer Houghton (Washington University) for laboratory and technical assistance, Alexandra Freibott (USDA, Forest Service) for help with microfauna identification, and David Johnston (Harvard) and Roger Summons (MIT) for access to facilities. Funding was provided by the Agouron Institute. Author Contributions MG, LR, SO’R, UL, KM, WF, JG, and AK conducted the field work. UL and KM collected and analyzed 16S and 18S rRNA gene data. LR performed microscopy and analyzed the microscopy data. SO’R collected and analyzed organic geochemistry data. MG collected and analyzed carbon and sulfur isotope data. MG drafted the manuscript. All authors interpreted the data as a whole and developed the conclusions, contributed to the final manuscript, and design of the study. Data Availability Statement. The datasets presented in this study can be found in online repositories. The names of the repository/repositories and accession number(s) can be found in the article/Supplementary Material. Edited by: Stefano Dominici, University of Florence, Italy. Reviewed by: Barbara Cavalazzi, University of Bologna, Italy; Frances Westall, Centre National de la Recherche Scientifique (CNRS), France; Dylan Wilmeth, UMR 7154 Institut de Physique du Globe de Paris (IPGP), France.
Funding AgencyGrant Number
Agouron InstituteUNSPECIFIED
Subject Keywords:taphonomy, microbial mats, cyanobacteria, eukaryotes, biosignatures, stable isotope geochemistry, organic geochemistry
Record Number:CaltechAUTHORS:20201016-155212280
Persistent URL:
Official Citation:Gomes ML, Riedman LA, O’Reilly S, Lingappa U, Metcalfe K, Fike DA, Grotzinger JP, Fischer WW and Knoll AH (2020) Taphonomy of Biosignatures in Microbial Mats on Little Ambergris Cay, Turks and Caicos Islands. Front. Earth Sci. 8:576712. doi: 10.3389/feart.2020.576712
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:106128
Deposited By: George Porter
Deposited On:19 Oct 2020 13:59
Last Modified:16 Nov 2021 18:50

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