Kerogen-bound glycerol dialkyl tetraether lipids released by hydropyrolysis of marine sediments: A bias against incorporation of sedimentary organisms?

Over the past 10 years, the membrane lipids of archaea, particular glycerol dialkyl glycerol tetraethers (GDGTs) have been recognised as significant components of marine sediments and as potentially powerful tools in palaeoclimate reconstruction. However, relatively little is known about their preservation and, in particular, their incorporation into macromolecular organic matter fractions. Here, we examine abundances, distributions and stable carbon isotope ratio (δ13C) values of GDGTs and the diether archaeol in sediments from the Benguela Upwelling Region [Ocean Drilling Program (ODP) Site 1084] and a Mediterranean sapropel (S5). These are compared with distributions of biphytanes released from the kerogen (solvent-extracted residue) fraction using hydropyrolysis (Hypy). GDGTs, likely derived from pelagic crenarchaeota, are amongst the most abundant extractable components in both Benguela and S5 sediments, but are an order of magnitude more abundant in the former. Moreover, the Benguela sediments have far greater concentrations of the acyclic GDGT than expected; consistent with the presence of methane in core 1084, co-occurring archaeol and variable GDGT δ13C values, the excess acyclic GDGT probably arises from a contribution from sedimentary methanogens. Hypy-released biphytanes have similar abundances in Benguela and sapropel sediments, and represent a significant proportion of the total tetraether lipid pool at the latter site; however, the distributions of Hypy-released biphytanes, especially in Benguela sediments, do not always match those of the free GDGTs. At both sites, Hypy-released biphytane δ13C values are relatively invariant, consistent with a single source, again likely to be pelagic crenarchaeota. These observations suggest that GDGTs can be incorporated into sedimentary geomacromolecules relatively rapidly, with those derived from pelagic crenarchaeota more likely to be incorporated than those derived from sedimentary methanogens. It is likely that this bias arises from a lack of reactive substrates, i.e. oxygen radicals, in deeper anoxic sediments, consistent with an important role for oxidative cross linking in kerogen formation.