Sulphur and carbon isotopes as tracers of past sub-seafloor microbial activity

Abstract

Microbial life below the seafloor has changed over geological time, but these changes are often not obvious, as they are not recorded in the sediment. Sulphur (S) isotope values in pyrite extracted from a Plio- to Holocene sequence of the Peru Margin (Ocean Drilling Program, ODP, Site 1229) show a down-core pattern that correlates with the pattern of carbon (C) isotopes in diagenetic dolomite. Early formation of the pyrite is indicated by the mineralogical composition of iron, showing a high degree of pyritization throughout the sedimentary sequence. Hence, the S-record could not have been substantially overprinted by later pyrite formation. The S- and C-isotope profiles show, thus, evidence for two episodes of enhanced microbial methane production with a very shallow sulphate-methane transition zone. The events of high activity are correlated with zones of elevated organic C content in the stratigraphic sequence. Our results demonstrate how isotopic signatures preserved in diagenetic mineral phases provide information on changes of past biogeochemical activity in a dynamic sub-seafloor biosphere.

Factors controlling the carbon isotope composition of dissolved inorganic carbon and methane in marine porewater: An evaluation by reaction-transport modelling

“…It remains unclear how both positive and negative carbon isotope compositions are incorporated into diagenetic carbonates, commonly showing a large range of values (Murata et al, 1967;Pisciotto and Mahoney, 1981;Kelts and McKenzie, 1984;Rodriguez et al, 2000;Moore et al, 2004). Temporal and spatial Variations in the C-isotope composition preserved in the diagenetic carbonate record were suggested to indicate changes in microbial activity in a dynamic sub-seafloor biosphere (Malone et al, 2002;Meister et al, 2007Meister et al, , 2008Meister et al, , 2019Contreras et al, 2013;Meister, 2015;Wehrmann et al, 2016). However, further studies using reaction-transport models will be necessary to precisely interpret these carbon isotope signatures with respect to past dynamics of the deep biosphre.…”

The Carbon-Isotope Record of the Sub-Seafloor Biosphere

“…Carbon, in its reduced form, is not only the essential building material of life, but due to its large isotope variations, it can also serve as a tracer of biogeochemical processes in the environment and as an indicator of the state of the global carbon cycle. During assimilation of CO 2 to organic matter in the water column, carbon is depleted in 13 C by 20-30% ; however, variations in δ 13 C in ocean and atmosphere are usually in the few-permil range. These variations are essentially balanced by input to and output from the ocean and atmosphere, and they only change upon variations in rates of primary production and burial of organic carbon relative to inorganic carbon (Broeker, 1970) [1].…”

“…The deep biosphere is organized as zones of different metabolic activity in the sequence of downward decreasing redox potential (Froelich et al, 1979) [3], whereby the presence and extension of these zones vary considerably in different regions of the ocean (D'Hondt et al, 2004) [4]. Sulphate reduction represents the most abundant anaerobic respiration process, and, although dissimilatory degradation of organic matter generally exhibits small fractionation effects (e.g., Hayes et al, 1989) [5], it delivers dissolved inorganic carbon (DIC) strongly depleted in 13 C to the porewater. Furthermore, large fractionation effects are observed during fermentation reactions where the pools of CO 2 and CH 4 are involved.…”

“…Biogenic methane often shows δ 13 C values as negative as −100% relative to the Vienna Peedee Belemnite (VPDB) standard ( Claypool and Kaplan, 1974) [6], while positive values of up to +35% have been observed in dissolved inorganic carbon (DIC) related to methane production (Heuer et al, 2009) [7]. In turn, strongly 13 C-depleted DIC is produced as a result of anaerobic oxidation of methane (AOM). Ultimately, the different metabolic processes result in a large variability in inorganic carbon, especially in proximity to the sulphate-methane transition zone.…”

A New Frontier for Palaeobiology: Earth's Vast Deep Biosphere

“…To answer these questions, substantial effort is now required to locate, verify, analyze, and integrate geological records of the deep biosphere in deep time. Recent work has begun to reveal geologically ancient chemical and isotopic signatures of microbial metabolism in the subsurface; fluctuations in these signals through sedimentary rock successions can in principle reveal changes in deep microbial activity over timescales of at least millions of years . Here, we focus instead on the traditional subject matter of palaeobiology: body fossils—the mineralized or organically preserved remnants of organismal structures.…”