Enhanced productivity led to increased organic carbon burial in the euxinic North Atlantic basin during the late Cenomanian oceanic anoxic event

Abstract

[1] Three Cenomanian/Turonian (C/T, $93.5 Ma) black shale sections along a northeast-southwest transect in the southern part of the proto-North Atlantic Ocean were correlated by stable carbon isotope stratigraphy using the characteristic excursion in d 13C values of both bulk organic matter (OM) and molecular fossils of algal chlorophyll and steroids. All three sites show an increase in marine organic carbon (OC) accumulation rates during the C/T Oceanic Anoxic Event (OAE). The occurrence of molecular fossils of anoxygenic photosynthetic green sulfur bacteria, lack of bioturbation, and high abundance of redox sensitive trace metals indicate sulfidic conditions, periodically reaching up into the photic zone before as well as during the C/T OAE. During the C/T OAE, there was a significant rise of the chemocline as indicated by the increase in concentrations of molecular fossils of green sulfur bacteria and Mo/Al ratios. The presence of molecular fossils of the green strain of green sulfur bacteria indicates that euxinic conditions periodically even occurred at very shallow water depths of 15 m or less during the C/T OAE. However, bottom water conditions did not dramatically change as indicated by more or less constant V/Al and Zn/Al ratios at site 367. This suggests that the increase in OC burial rates resulted from enhanced primary productivity rather than increased anoxia, which is supported by stable carbon isotopic evidence and a large increase in Ba/Al ratios during the C/T OAE. The occurrence of the productivity event during a period of globally enhanced organic carbon burial rates (i.e., the C/T OAE) points to a common cause possibly related to the formation of a deep water connection between North and South Atlantic basins.

Evidence for photic‐zone euxinia in the Early Albian Santana Formation (Araripe Basin, NE Brazil)

“…4B). Isorenieratane concentrations from the Araripe Basin are in the same range as those reported for C ⁄ T OAE pelagic sediments (generally <100 lg g )1 TOC; Kuypers et al, 2002b). Vertical variations in carotenoid concentration may reflect changes in anoxygenic productivity.…”

“…Vertical variations in carotenoid concentration may reflect changes in anoxygenic productivity. Similar fluctuations in the abundances of isorenieratene and its diagenetic products have been reported from Holocene, Cretaceous and Jurassic deposits (Sinninghe Damste´et al, 1993;Kuypers et al, 2002b;Schwark and Frimmel, 2004). In these studies, fluctuating concentrations have been interpreted to reflect changes in the position of the chemocline within the water column, which in turn has a strong effect on the productivity of the phototrophic sulphur bacteria.…”

“…The episodic occurrence of argillaceous, organic-rich sediments devoid of or impoverished in benthic fauna, and their global distribution, resulted in the concept of oceanic anoxic events or OAEs (Schlanger and Jenkyns, 1976). There is no doubt that OAEs were accompanied by peculiar palaeoceanographic conditions, which included the establishment of anoxic bottom waters and at times oceanwide stratification with sulphidic conditions reaching up into the photic zone (Sinninghe Damste´and Ko¨ster, 1998;Kuypers et al, 2002b;Pancost et al, 2004;Wagner et al, 2004). Although the geochemical record of OAEs has been studied intensively in oceanic pelagic and hemipelagic settings, the expression of these episodic events in interior marine environments has received less attention.…”

“…The aromatic carotenoid isorenieratene is exclusively synthesized by the browncoloured strain of green sulphur bacteria (Chlorobiaceae). As this type of Kuypers et al, 2002b) as well as during the Coniacian-Santonian OAE 3 (Wagner et al, 2004). Derivatives of isorenieratene are found throughout the Romualdo black shale and include C 40 , C 33 and C 32 diaryl isoprenoids and short-chain aryl isoprenoids with additional aromatic rings (Fig.…”

Secular variation in Late Cretaceous carbon isotopes: a new δ13C carbonate reference curve for the Cenomanian–Campanian (99.6–70.6 Ma)

“…3) is one of the largest and best documented carbon-isotope events in the geological record (Jenkyns, 1980(Jenkyns, , 1985Scholle & Arthur, 1980;Schlanger et al 1987). This event is conventionally interpreted as being related to accelerated burial of marine organic matter, particularly in the Atlantic, but also in other ocean basins (Herbin et al 1986;Arthur et al 1990;Kuypers et al 2002Kuypers et al , 2004. The phenomenon is widely referred to as the Cenomanian/Turonian Boundary Event (CTBE), or Oceanic Anoxic Event 2 (OAE2; Arthur et al 1990).…”