Preserved Organic Matter in the Alpine Tethyan Ocean Continental Transition (Totalp unit, Eastern Swiss Alps)

Abstract:

Observations at hydrothermal systems in modern ocean settings suggest that methane produced by serpentinization can support methanotrophic bio-systems. An important question is whether such bio-systems are localised or are more pervasive in their association with serpentinized mantle in the subsurface. This has implications for the global importance of the hidden sub-surface bio-systems, the fate of methane and the carbon cycle.

The Totalp unit, a remnant of a former Ocean Continent Transition (OCT) exposed in Alps of Eastern Switzerland, has been chosen to investigate the presence or absence of methanotrophic biosystems within serpentinized exhumed mantle in the Alpine Tethyan margin. The Totalp unit is made of serpentinized mantle and ophicalcites overlain by Upper Jurassic to Lower Cretaceous post-rift sediments. The Totalp unit has undergone little Alpine deformation and only a low-grade metamorphic overprint (<200°C).

Totalp samples are characterized by total carbon contents of 0.02% to 12.90% and organic carbon contents of 1x10^-4 % to 8%. This large range of values reflects the large lithological diversity of this area. The serpentinized peridotite, ophicalcite and post-rift sediments contain hydrocarbons in the form of n-alkanes in the range C₂₀ – C₄₀; isoprenoids, for example pristane and phytane are present in sediments. The organic biological marker distribution is consistent with the temperature history of the OCT (i.e.lower maximum temperature than 200°C).

First results from Totalp show evidence for preservation of marine organic matter in the serpentinized mantle and overlying sediments, although there is no evidence that any organic matter is generated from methanotrophic bio-systems. Nevertheless, focussing on Tethyan hydrothermal systems and preserved hydrocarbons will be critical in understanding whether methanotrophic biomarkers can be preserved and if so whether the methane originated from serpentenization.