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Title: Mantle serpentinisation, carbon and life
Author: Mateeva, T. Y.
ISNI:       0000 0004 7658 7477
Awarding Body: University of Liverpool
Current Institution: University of Liverpool
Date of Award: 2018
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Serpentinisation occurs at hydrothermal vents at slow-spreading ocean ridges and at magma-poor rifted continental margins. Serpentinisation and concomitant reduction of CO2 to methane at modern hydrothermal vents supports hydrogen-driven microbial environments including methanotrophic biosystems. An important question is: "Are such bio-systems locally restricted to hydrothermal vents or are they more pervasive, being linked with the exhumation of serpentinised mantle at the seafloor?" Such a relationship is important for understanding the hidden sub-surface bio-systems and the fate of methane. The ocean-continent transition (OCT) of magma-poor rifted continental margins provides an opportunity to investigate this. Serpentinite, intrusive magmatic rocks and the overlying sediments sampled in remnants of the fossil Tethyan OCTs exposed in the Alps or samples collected as part of the Ocean Drilling Program (ODP) from the OCT of the Iberian Newfoundland margins, were analysed for the presence or absence of methanotrophy within serpentinised exhumed mantle. Sampling of km scale exposures of exhumed serpentinised mantle in the Alps allows precise and extensive sampling which is not possible at ocean ridges. This provides an opportunity to investigate the organic matter in the ophiolite sequences relative to the seafloor. Samples from the Totalp unit, Tasna nappe and Platta unit of the Eastern Swiss Alps and Chenaillet in the Western Alps, all originating from the Alpine Tethyan magma-poor OCT, were selected for analysis. The fact that all these units were little affected by Alpine deformation and underwent only low-grade Alpine metamorphism makes them the world's best field analogue for magma-poor rifted margins. All the Alpine Tethys lithologies sampled show the presence of hydrocarbons such as n-alkanes, low molecular weight polynuclear aromatic hydrocarbons (PAHs, indicating mixed petrogenic and pyrogenic sources), hopanes, steranes (indicating marine origin), and branched alkanes (pristane and phytane, non-specific marine origin). The identifiable biomarkers and the stable isotopic carbon data are consistent with a marine origin and do not indicate a methanotrophic bio-system. The organic matter (OM) does not originate from the exhumed mantle rocks but corresponds to an autochthonous OM probably deriving from the sediment cover and overlying water. The OM indicates a depositional environment of marine organic matter dominated by planktonic algae and bacteria. It is noteworthy that mantle rocks in the Alpine Tethys OCT still contain marine organic matter 160 My after their exhumation, despite having experienced an Alpine reactivation. The analysed ODP cores from the Iberia-Newfoundland margins show cyclic and branched alkanes in addition to the hydrocarbons present in the Alpine Tethyan OCT samples. The hydrocarbons detected in the Iberia-Newfoundland samples may originate from plankton and algae from a marine open-ocean source with no biological and isotopic evidence of a methanotrophic biosystem. However, it should be noted that these observations are based on the analysis of a very small number of ODP core samples. The magmatic and metamorphic ODP core samples show evidence of contamination from drilling mud.
Supervisor: Kusznir, Nick ; Wolff, George ; Wheeler, John ; Manatschal, Gianreto Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral