Abstract
Evidence from ultraslow spreading mid-ocean ridges and both fossil and present-day Ocean–Continent Transitions (OCT) demonstrates that mantle serpentinization resulting from the interaction of mantle rock and water during tectonic exhumation is widespread. Observations at white smokers in modern ocean settings suggest that methane produced by serpentinization can support methanotrophic bio-systems, which use methane as the only source of carbon. An important question is whether such bio-systems are more generally pervasive in their association with serpentinized mantle in the subsurface. In this study, we examined whether there is evidence for such a methanotrophic system in exhumed serpentinized mantle at a magma-poor rifted continental margin, by probing for characteristic biological markers in these and associated sedimentary rocks in the Totalp unit of SE Switzerland. This unit represents a remnant of the former OCT of the southern Alpine Tethyan margin and was chosen because of its mild Alpine tectonic and low-grade metamorphic overprint during Alpine orogeny, hence giving potential for the preservation of indigenous organic matter (OM). Totalp samples are characterized by low organic carbon contents of 11–647 ppm. The majority of the samples contain hydrocarbons in the form of n-alkanes in the range C17–C36. Some sediments contain isoprenoids, for example pristane and phytane and a suite of steranes that are consistent with a marine origin for the OM preserved in the rocks. Traces of marine planktonic and bacterial OM are preserved in the serpentinized mantle and overlying sediments of this ancient Tethyan OCT, but there is no evidence that the OM has been generated from methanotrophic bio-systems.
Highlights
Serpentinization is an important metamorphic exothermic hydration process potentially contributing chemical energy for anaerobic life, as well as thermal energy at oceanic hydrothermal vents (e.g. Shock et al 2002; Jamtveit and Hammer 2012)
Evidence from ultraslow spreading mid-ocean ridges and both fossil and present-day Ocean–Continent Transitions (OCT) demonstrates that mantle serpentinization resulting from the interaction of mantle rock and water during tectonic exhumation is widespread
We examined whether there is evidence for such a methanotrophic system in exhumed serpentinized mantle at a magma-poor rifted continental margin, by probing for characteristic biological markers in these and associated sedimentary rocks in the Totalp unit of SE Switzerland
Summary
Serpentinization is an important metamorphic exothermic hydration process potentially contributing chemical energy for anaerobic life, as well as thermal energy at oceanic hydrothermal vents (e.g. Shock et al 2002; Jamtveit and Hammer 2012). Serpentinization is an important metamorphic exothermic hydration process potentially contributing chemical energy for anaerobic life, as well as thermal energy at oceanic hydrothermal vents Serpentinized mantle rocks exposed along slow to ultraslow-spreading Mid-Ocean Ridges (MOR) show positive anomalies of methane and hydrogen in the overlying water column above active tectonic zones (Rona et al 1987; Charlou et al 1988; Rona et al 1992; Bougault et al 1993; Charlou and Donval 1993; Charlou et al 1998; Gracia et al 2000; Kelley and Shank 2010). Sulphate reducers are one of the dominant bacterial populations at hydrothermal vents (McCollom and Shock 1997). A wider suite of biomarkers was used to determine source and thermal maturity of OM preserved in the rocks
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