Abstract
Stromatolites have been a major focus in the search for ancient microbial life, however, the organic carbon biosignatures of dolomitized stromatolites have not yet been fully characterized or correlated with their dolomitizing conditions. Although dolomitization rarely preserves microbial morphology, the presence of organic carbon can provide valuable information for characterization of fossils' biogenicity, syngenicity, and indigeneity to their host rock. The Cambrian Allentown Formation in New Jersey, USA, is an excellent example of dolomitized stromatolites and thrombolites containing diagenetically modified microbial biosignatures. Based on XRD and EPMA data, the dolomite composition is typically stoichiometric, with varying degrees of cationic ordering. The outcrop underwent early dolomitization in a marginal-marine setting and later burial diagenesis resulting in multi-generational dolomite formation: (1) microspar dolomite formed by early diagenetic replacement at or near the surface, (2) zoned dolomite formed penecontemporaneously with the microspar phase as rhombohedral crystals by infilling primary pore spaces within the microspar matrix. The rhombic crystals continued to grow outward in alternating stages of Fe-enriched and -depleted fluids, which were preserved in zoned rims and revealed by cathodoluminescence, and (3) saddle dolomite formed during late stage deep burial with Fe- and Mn-rich fluids, and occurs as a void-filling, high-temperature phase. Organic carbon, characterized using confocal Raman microscopy, has an exclusive distribution within the microspar dolomite, and the D and G bands' characteristics reveal similar thermal alteration to the host rock, indicating that the mapped organic carbon is indigenous and syngenetic with the Cambrian carbonates. The findings presented in this study reveal organic matter found within microspar of various dolomitized facies deriving from different source pools of organic carbon. This study sheds light on biosignatures in secondary dolostones and may aid biosignature detection in older carbonate rocks on Earth and Mars.
Highlights
IntroductionStromatolites are microbially mediated sedimentary structures that record the oldest forms of life on Earth (Barghoorn and Tyler, 1965; Grotzinger and Knoll, 1999; Allwood et al, 2006)
Stromatolites are microbially mediated sedimentary structures that record the oldest forms of life on Earth (Barghoorn and Tyler, 1965; Grotzinger and Knoll, 1999; Allwood et al, 2006).These ancient carbonate structures have drawn a significant focus of geobiology and astrobiology research because of their ability to archive the interactions of biological, physical, and chemical processes (e.g., Hoffman, 2013), and for that reason stromatolites provide an invaluable reference to Earth’s past
A complication in the reconstruction of these structures derives from the fact that, as any other rock and fossil, stromatolites undergo diagenesis over time, which alters original biological signatures, including chemical and physical evidence, making reconstruction of original materials of even the best-preserved stromatolites debatable
Summary
Stromatolites are microbially mediated sedimentary structures that record the oldest forms of life on Earth (Barghoorn and Tyler, 1965; Grotzinger and Knoll, 1999; Allwood et al, 2006). These ancient carbonate structures have drawn a significant focus of geobiology and astrobiology research because of their ability to archive the interactions of biological, physical, and chemical processes (e.g., Hoffman, 2013), and for that reason stromatolites provide an invaluable reference to Earth’s past. The fate of organic matter preserved under such a wide variety of conditions is yet to be properly evaluated
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
