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Abstract:
Cretaceous black shales deposited under severe oxygen-depletion are carriers of proxy signals for paleoenvironmental conditions. Using high-resolution patterns of iron and sulfur speciation, stable sulfur isotope discrimination, and trace element enrichment from black shale sequences of Sites 1258 and 1260 we identified alterations of the depositional environment during the Cenomanian/Turonian boundary Event (OAE 2) in the southern North Atlantic (ODP Leg 207, Demerara Rise).
Changes in redox-conditions are suggested by high ratios of reactive to total iron which indicate that pyrite was formed both in the water column and within the sediment. This corresponds to euxinic paleoenvironmental conditions with at least temporarily free dissolved sulfide in the water column, a situation similar to the modern deep Black Sea. In addition, besides fixation of sulfide as iron sulfide, organic matter acted as an important sulfur trap during early diagenesis. Stable sulfur isotope fractionation went through a minimum within the OAE 2 interval indicating enhanced sulfur isotope discrimination during highest burial of organic matter (OM) potentially due to lower burial efficiency of reduced sulfur and/or a higher contributions from the oxidative part of the sulfur cycle (e.g., in the water column or the surface sediments). Elevated Fe/Al and Co/Al values within the Cenomanian/Turonian interval confirm euxinic conditions but, at the same time, require a zone where reducing but non-sulfidic conditions prevail, allowing reductive Fe and Co mobilization in oxygen-depleted nearshore sediments. The existence of an expanded oxygen-minimum-zone (OMZ) is demonstrated by extremely low Mn/Al ratios.
A change in the trace metal (TM) inventory of seawater is postulated from a decline in seawater derived TM enrichment. Because hypoxic or even euxinic environments form an important sink for TM, the enlargement of euxinic depositional areas at the global onset of black shale deposition during OAE 2 have likely led to a drawdown of the seawater TM reservoir.
