Archaeal remains dominate marine organic matter from the early Albian oceanic anoxic event 1b

The sources for both soluble and insoluble organic matter of the early Albian (∼112 Myr) oceanic anoxic event (OAE) 1b black shales of the Ocean Drilling Program (ODP) site 1049C (North Atlantic Ocean off the coast of Florida) and the Ravel section of the Southeast France Basin (SEFB) were determined using optical, chemical, stable carbon and nitrogen isotopic analyses. Archaea-derived isoprenoidal tetraether membrane lipids and free and macromolecularly bound isoprenoid alkanes are abundant in these black shales. More specifically, the presence of certain ether lipids (bi/tricyclic biphytane tetraethers) indicates an important contribution of representatives of marine planktonic archaea. The large difference (up to 12‰) in 13C/12C ratios between algal biomarkers and the much more abundant planktonic archaea-derived biomarkers indicates that the latter were living chemoautotrophically. This offset in 13C/12C ratios was used to estimate that up to ∼40% of the organic matter of the SEFB and up to ∼80% of the organic matter of ODP site 1049C preserved in the black shales is derived from archaea. Furthermore, it is shown that, even though there are apparent similarities (high organic carbon (OC) content, distinct lamination, 13C-enrichment of OC) between the black shales of OAE1b and the Cenomanian/Turonian (∼94 Myr) OAE, the origin of the organic matter (archaeal versus phytoplanktonic) and causes for 13C-enrichment of OC are completely different.