Abstract
Abstract. Global warming, changes in the hydrological cycle and enhanced marine primary productivity all have been invoked as having contributed to the occurrence of widespread ocean anoxia during the Cenomanian–Turonian oceanic anoxic event (OAE2; ~94 Ma), but disentangling these factors on a regional scale has remained problematic. In an attempt to separate these forcing factors, we generated palynological and organic geochemical records using a core spanning the OAE2 from Wunstorf, Lower Saxony Basin (LSB; northern Germany), which exhibits cyclic black shale–marl alternations related to the orbital precession cycle. Despite the widely varying depositional conditions complicating the interpretation of the obtained records, TEX86H indicates that sea-surface temperature (SST) evolution in the LSB during OAE2 resembles that of previously studied sites throughout the proto-North Atlantic. Cooling during the so-called Plenus Cold Event interrupted black shale deposition during the early stages of OAE2. However, TEX86 does not vary significantly across black shale–marl alternations, suggesting that temperature variations did not force the formation of the cyclic black shale horizons. Relative (i.e., with respect to marine palynomorphs) and absolute abundances of pollen and spores are elevated during phases of black shale deposition, indicative of enhanced precipitation and run-off. High abundances of cysts from inferred heterotrophic and euryhaline dinoflagellates supports high run-off, which likely introduced additional nutrients to the epicontinental shelf resulting in elevated marine primary productivity. We conclude that orbitally forced enhanced precipitation and run-off, in tandem with elevated marine primary productivity, were critical in cyclic black shale formation on the northern European epicontinental shelf and potentially for other OAE2 sections in the proto-Atlantic and Western Interior Seaway at similar latitudes as well.
Highlights
Dark, often laminated marine sediments that are usually devoid of fossil traces of benthic life and exhibit a total organic carbon (TOC) content of > 1 %
In an attempt to separate these forcing factors, we generated palynological and organic geochemical records using a core spanning the OAE2 from Wunstorf, Lower Saxony Basin (LSB; northern Germany), which exhibits cyclic black shale–marl alternations related to the orbital precession cycle
Despite differences in the preservation of organic matter throughout the OAE2 interval at Wunstorf, the general trend in reconstructed sea-surface temperature (SST), including the cooling phase associated with the Plenus Cold Event, is consistent with the results of previous studies that focused on the proto-North Atlantic
Summary
Often laminated marine sediments that are usually devoid of fossil traces of benthic life and exhibit a total organic carbon (TOC) content of > 1 % 1993) were episodically deposited during the Jurassic and Cretaceous The deposition of these sediments, generally referred to as black shales, has typically been taken to indicate ancient episodes of dysoxic or anoxic bottom water conditions (Schlanger and Jenkyns, 1976). The widespread enhanced organic carbon burial in marine sediments during OAE2 is expressed by a worldwide documented > 2 ‰ positive carbon isotopic excursion (CIE) of carbonate (δ13Ccarb) and organic matter (δ13Corg), with an estimated duration of 450–600 kyr (e.g., Voigt et al, 2008; Meyers et al, 2012) This CIE likely resulted from enhanced burial of δ13C-depleted organic matter (Arthur et al, 1988; Tsikos et al, 2004) and provides a C-isotopic signature of the global exogenic carbon pool, making it a proper tool to confidently correlate OAE2 sections
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