Methane seepage in a Cretaceous greenhouse world recorded by an unusual carbonate deposit from the Tarfaya Basin, Morocco

Abstract

During the Cretaceous major episodes of oceanic anoxic conditions triggered large scale deposition of marine black shales rich in organic carbon. Several oceanic anoxic events (OAEs) have been documented including the Cenomanian to Turonian OAE 2, which is among the best studied examples to date. This study reports on a large limestone body that occurs within a black shale succession exposed in a coastal section of the Tarfaya Basin, Morocco. The black shales were deposited in the aftermath of OAE 2 in a shallow continental sea. To decipher the mode and causes of carbonate formation in black shales, a combination of element geochemistry, palaeontology, thin section petrography, carbon and oxygen stable isotope geochemistry and lipid biomarkers are used. The 13 Cdepleted biphytanic diacids reveal that the carbonate deposit resulted, at least in part, from microbially mediated anaerobic oxidation of methane in the shallow subseafloor at a hydrocarbon seep. The lowest obtained d 13 C carbonate values of À23Á5& are not low enough to exclude other carbon sources than methane apart from admixed marine carbonate, indicating a potential contribution from in situ remineralization of organic matter contained in the black shales. Nannofossil and trace metal inventories of the black shales and the macrofaunal assemblage of the carbonate body reveal that environmental conditions became less reducing during the deposition of the background shales that enclose the carbonate body, but the palaeoenvironment was overall mostly characterized by high productivity and episodically euxinic bottom waters. This study reconstructs the evolution of a hydrocarbon seep that was situated within a shallow continental sea in the aftermath of OAE 2, and sheds light on how these environmental factors influenced carbonate formation and the ecology at the seep site.

Fossiliferous methane‐seep deposits from the Cenozoic Talara Basin in northern Peru

“…In contrast to biphytane, monocyclic biphytanic diacid in the Cerro La Salina sample is only slightly less 13 C-depleted than PMI. This confirms the earlier finding that biphytanic diacids are commonly good recorders of past seepage (Birgel et al 2008a) and are apparently not affected by input from other sources than ANMEs, even though some exceptions have been reported (Smrzka et al 2017). The Peruvian seep deposits not only contain biomarkers of ANME, but also yielded molecular fossils of sulphate-reducing bacteria involved in AOM.…”

Sterol preservation in hypersaline microbial mats

“…On the other hand, α,ω-diacids may also be forming in situ, for instance via terminal oxidation of monoacids or other aliphatic moieties such as n-alkanes (Ishiwatari and Hanya, 1975;Johns and Onder, 1975). Interestingly, α,ω-diacids were also reported in a recent study on Cretaceous hydrocarbon seep limestones, where they were only detected after the dissolution of the authigenic carbonate minerals (Smrzka et al, 2017). Likewise, α,ω-diacids were reported to be remarkably more abundant in carbonate concretions than in their clastic host rocks (Thiel and Hoppert, 2018).…”

“…Consequently, carbonate precipitation often occurs in deeper and older mat layers in which decomposing EPSs gradually release previously bound Ca 2+ , thus facilitating carbonate supersaturation (Arp et al, 1999;Dupraz et al, 2009;Ionescu et al, 2015). Previous studies indicate that early sequestration into a mineral matrix may promote the preservation of organic compounds (Summons et al, 2013;Smrzka et al, 2017;Thiel et al, 1999). Hence, microbial mats possibly provide an enhanced chance for OM to survive in the geosphere if carbonate or other mineral precipitation occurs therein.…”

Geochemistry, mineralogy and depositional environment of black shales of the Duwi Formation, Qusseir area, Red Sea coast, Egypt

“…Consequently, the organic-rich sediments may be enriched in redox-sensitive metals, in U, Cu and Ni and insoluble sulfides such as Mo, V, Cd, Zn and occasionally Co (Brumsack 2006;Marz 2007;Hetzel et al 2009). The black shales usually contain high concentrations of redox-sensitive trace elements (Brumsack 2006;Loukola-Ruskeeniemi and Lahtinen 2013;Smrzka et al 2017 andEl-Shafeiy et al 2017).…”