Abstract
Preference for certain stable isotopes (isotope fractionation) during enzyme-mediated reactions is a universal aspect of life. For instance, carbon isotopes are fractionated during anabolic (e.g., photosynthate production) and catabolic (e.g., methanogenesis) reactions. These biological processes exert a major control on ambient micro-scale chemical conditions as well as the large-scale exogenic carbon reservoir. Combined with the ubiquity of bio-mediated carbonate mineral nucleation and obligate enzymatic skeletonization, these biochemical reactions and their control on the exogenic carbon pool are known to leave distinct imprints on carbonate minerals which accumulate as sediments throughout Earth’s history. Here, we study the evolution of the marine carbonate-carbon isotope record based on database compilations from the Precambrian and the Phanerozoic. By looking at the frequency distribution of the amplitude of stratigraphic variation at various temporal resolutions, we assess trends in the carbonate-carbon isotope variability. Part of this variation can only be explained by authigenic and diagenetic carbonate mineral additions, which carry metabolic carbon isotope signatures created in the vicinity of cells and secluded (sub-)seafloor micro-environments. It can be envisioned that compartmentalization (membrane enclosed regions), the accumulation of extracellular polymeric substances (biofilms), and restricted fluid exchange in the early diagenetic environment can create sharp isotope gradients that lead to a high-order of micro-scale carbon isotope variability being imprinted in carbonate rock. The frequency of the high-amplitude variation diminishes with the development of more complex life (metazoan-dominated biosphere); presumably through the dispersing action of bioturbation (eradicating these micro-environments), increased grazing pressure and the advent of obligate biomineralization. On the other hand, stark chemical gradients in a world dominated by unicellular life (prokaryotes and to a lesser extent eukaryotes) are thought to leave a distinctly more variable C isotope signature in carbonate rock. An enhanced understanding of the biogenicity of carbonate carbon isotope signatures at multiple spatial and temporal scales provides a baseline that is usable in the search for signs of (past) extraterrestrial life.
Highlights
Global marine deposits of carbonate minerals constitute the largest archive of past and present activity by life on Earth (Skinner and Jahren, 2003)
The main interest of this study is the sample-to-sample δ13Ccarb differences from studies on a single stratigraphic sequence, 13Cbed−to−bed = δ13Cupper−bed − δ13Clower−bed where the 13Cbed−to−bed denotes the δ13Ccarb compositional difference of a limestone bed relative to the underlying bed. By application of this approach, we attempt to filter-out δ13Ccarb oscillations driven by the carbon cycle from published carbon isotope data-sets, with the notion that metabolic carbon isotope signatures are superimposed on these long-term trends as stratigraphic-confined C isotope shifts
Proximal sites of the Ediacaran display the largest scatter in 13Cbed−to−bed on stratigraphic most-confined scale (d < 25 cm), whereas the reverse is observed for 13Cbed−to−bed variation at large sample spacing (d > 100 cm)
Summary
Global marine deposits of carbonate minerals constitute the largest archive of past and present activity by life on Earth (Skinner and Jahren, 2003). Many microbes (bacteria and archaea) may induce precipitation of carbonate minerals in the immediate vicinity of their cells as a side product of their metabolic activity (Pentecost and Spiro, 1990; Merz, 1992; McConnaughey and Whelan, 1997; Dupraz and Visscher, 2005; Visscher and Stolz, 2005) These bio-mediated carbonate precipitates can form within the water column, on the seafloor, and potentially deep underground within pore spaces of buried sediments; and they accumulate in marine sedimentary stacks that form in a broad spectrum of environments, ranging from the deep sea to the intertidal zone (Webb, 1996; Riding, 2000; Mackenzie and Andersson, 2011). Laboratory experiments on cultures of cyanobacteria as well as members of aerobic and anaerobic bacteria have generated several species-specific models of bio-mediated carbonate precipitation, which implicate the combined action of metal ion adsorption (Ca2+, Mg2+, and Fe2+) on microbial cell surfaces (and EPS) and alkalinization of the extracellular fluid by metabolic activity, and thereby locally change the saturation state in favor of carbonate mineral precipitation (Sánchez-Román et al, 2008, 2014; Bundeleva et al, 2014)
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