Organic matter in an Ocean Continent Transitions of present-day magma-poor rifted margins: the examples of Iberia and Newfoundland

Abstract:

Serpentinisation and concomitant reduction of CO₂ to methane at modern hydrothermal vents has been observed to support methanotrophic biosystems at mid-ocean ridges. Exhumation of mantle within ocean regions and its subsequent serpentinisation is a common occurrence. It is possible that methanotrophy is not restricted to hydrothermal vents systems alone and could be pervasive within serpentinised exhumed oceanic mantle.

The ocean-continent transition (OCT) of magma-poor rifted continental margins provides an opportunity to investigate this. Serpentinite, intrusive magmatic rocks and overlying sediments collected as part of the Ocean Drilling Program (ODP) from the Iberian and Newfoundland margin, were analysed to find evidence of methanotrophic archaea related to the serpentinisation process.

 Samples from the OCT of Newfoundland (ODP Leg 210) contain n-alkanes, PAHs and isoprenoids (pristane, phytane) while those from the OCT of Iberian (ODP Legs 149 and 173) also contained detectable steranes and hopanes. However, the distribution and composition of hydrocarbons suggest that the OM may not be indigenous to the rocks.

The only clear evidence of Archaea was provided from GDGTs (0 – 3), crenarchaeol and isomers from one sediment sample at the Iberian Margin and these were likely derived from overlying oceanic waters. Hydrocarbon fingerprinting using polar plots suggests that any organic compounds in the igneous and metamorphic rocks were probably introduced from the drilling mud.