A Reappraisal of Anoxia and Richness of Organic Material, with Emphasis on the Cretaceous North Atlantic

Abstract

Accumulation of anoxic sediments in the North Atlantic from Barremian through Turanian time (Early to middle Cretaceous) was apparently controlled mainly by local conditions, although global climatic and oceanographic factors may have played a supporting role. Evidence does not support the popular hypothesis developed over the past few years of global or ocean-wide anoxia. The data contradicting this hypothesis are based on a reevaluation of criteria for identifying sediments that accumulated in anoxic water, a critical reappraisal of reported world-wide occurrences of anoxic sediments of these ages, and an examination of the spatial and temporal distribution of anoxic sediments in the North Atlantic and elsewhere. A model is proposed in which the earliest anoxic sediments of Cretaceous age in the North Atlantic were deposited in deep restricted basins. By middle Cenomanian time (about 97 m.y. ago), however, sluggish circulation had led to a gradual expansion of the oxygen-minimum layer, permitting deposition of anoxic sediments in nonbasinal settings. The expansion of the oxygen-minimum layer caused the calcite compensation depth to rise, promoting oxidation of organic carbon and causing contraction of the oxygen-minimum layer. The development of anoxia was thus self-damping. Anoxia effectively disappeared by early Senonian time (88 m.y. ago) when improved circulation created oceans more like those of the present. Some evidence indicates that enhanced productivity and upwelling may have played a local role in fostering anoxia, but most data suggest that worldwide marine productivity was generally not high during Early and middle Cretaceous time.

Synthesis of Organic Geochemical Studies, Deep Sea Drilling Project Leg 93, North American Continental Margin

“…Anoxic bottom waters have been postulated to have enabled enhanced preservation of organic matter in the Cretaceous North Atlantic Arthur and Schlanger, 1979;Jenkyns, 1980;Summerhayes and Masran, 1983;Bralower and Thierstein, 1984; as examples). The abundant presence of burrowed, oxidized sediments above, below, and sometimes within the black shales in this ocean argues against such bottom water anoxia being extensive in either volume or duration (Katz and Pheifer, 1982;Waples, 1983). Nonetheless, the high concentrations of organic carbon found in Cenomanian black shales over most of the North Atlantic Ocean are compelling evidence in favor of periods of widespread bottom water anoxia.…”

“…Nonetheless, the overall productivity of the western North Atlantic was probably low to moderate, on the basis of sediment compositions (Waples, 1983;de Graciansky et al, 1984;Bralower and Thierstein, 1984). Indeed, Demaison and Moore (1980) comment on the lack of correlation in modern oceans between productivity and preservation of marine organic matter.…”

“…Another scenario leading to enhanced preservation of organic matter calls for the midwater oxygen minimum zone to become intensified and perhaps expanded through sluggish circulation or enhanced influx of organic matter (e.g., Demaison and Moore, 1980;Waples, 1983). Where a midwater anoxic layer intercepts the ocean bottom, sediments rich in organic matter can accumulate.…”

Phanerozoic environments of black shale deposition and the Wilson Cycle

“…The coeval occurrence of sedimentologically unrelated lithologies deposited in disparate sedimentary environments is potentially one of the reasons why conflicting interpretations of data, due to different estimates of the extent of "black shale" deposition and of ocean anoxia, continue to generate much discussion in the literature (e.g. Ryan and Cita, 1997;Demaison and Moore, 1980;Waples, 1983;Degens et al, 1986;Pedersen and Calvert, 1990;Piper and Calvert, 2009). …”

“…2) deposited in a broad swathe of sedimentary environments from lakes to the abyssal plains of the ocean (Arthur, 1979;Jenkyns, 1980;Waples, 1983;Bohacs et al, 2000;Tyson, 2005). Depending on sedimentary environment, black shales can be deposited by any one or combination of processes that control fine-grained sedimentation: pelagic settling, hemipelagic deposition (including lateral advection of sediment), hemiturbiditic and Extinction of marine genera and major OAEs Raup & Sepkoski, 1986 87 Petroleum reserves generated by source rocks Klemme & Ulmishek, 1991 Oil Gas Climate mode Frakes et al, 1992 G r e e n h o u s e G r e e n h o u s e G r e e n h o u s e I c e h o u s e (Gradstein et al, 2004), climate mode (Frakes et al, 1992), major orogenic intervals, magnetic field reversals (Ogg et al, 2008), oceanic crust production (Stanley, 1999) and LIP aereal extent (Kidder and Worsley, 2010), carbon isotope curve (Prokoph et al, 2008), strontium isotope curve (McArthur, 2010), RCO 2 and CO 2 proxy record (Berner, 2006;Royer, 2006), global temperature (Frakes et al, 1992), eustatic sea level stand (Exxon curve; Haq et al, 1987;Haq and Al-Qahtani, 2005;Miller et al, 2005), continental glaciations (Ridgwell, 2005), extinction of marine genera and major oceanic anoxic events (OAEs) (Raup and Sepkoski, 1986), petroleum reserves generated by source rocks (Klemme and Ulmishek, 1991), and passive margin extension (Bradley, 2008).…”