We present new, detailed carbon-isotope records for bulk carbonate, total organic carbon (TOC) and phytane from three key sections spanning the Cenomanian-Turonian boundary interval (Eastbourne, England; Gubbio, Italy; Tarfaya, Morocco), with the purpose of establishing a common chemostratigraphic framework for Oceanic Anoxic Event (OAE) 2. Isotope curves from all localities are characterized by a positive carbon-isotope excursion of c. 4‰ for TOC and phytane and c. 2.5‰ for carbonate, although diagenetic overprinting appears to have obliterated the primary carbonate carbon-isotope signal in at least part of the Tarfaya section. Stratigraphically, peak ä 13 C values for all components are followed by intervals of high, near-constant ä 13 C in the form of an isotopic plateau. Recognition of an unambiguous return to background ä 13 C values above the plateau is, however, contentious in all sections, hence no firm chemostratigraphic marker for the end-point of the positive isotopic excursion can be established. The stratigraphically consistent first appearance of the calcareous nannofossil Quadrum gartneri at or near the Cenomanian-Turonian boundary as established by ammonite stratigraphy, in conjunction with the end of the ä 13 C maximum characteristic of the isotopic plateau, provides a potentially powerful tool for delimiting the stratigraphic extent and duration of OAE 2. This Oceanic Anoxic Event is demonstrated to be largely, if not wholly, confined to the latest part of the Cenomanian stage.

[1] High-resolution geochemical records from a depth transect through the Cenomanian/Turonian (C/T) Tarfaya Basin (northwest African Shelf) reveal high-amplitude fluctuations in accumulation rates of organic carbon (OC), redox-sensitive and sulphide-forming trace metals, and biomarkers indicative of photic zone euxinia. These fluctuations are in general coeval and thus imply a strong relationship of OC burial and water column redox conditions. The pacing and regularity of the records and the absence of a prominent continental signature suggest a dynamic depositional setting linked to orbital and higher-frequency forcing. Determining the dominant frequency depends on the definition of the most pronounced oceanic anoxic event (OAE2) and its duration. We propose that eccentricity is the main forcing factor at Tarfaya and controlled fluctuations in wind-driven upwelling of nutrient-rich, oxygen-depleted intermediate waters from the adjacent Atlantic and the periodic development of photic zone and bottom water euxinia on the midCretaceous northwest African shelf. Accumulation records clearly identify the basin center as the primary site of sediment deposition with highest temporal variability and an up to six-fold increase in OC burial from $2 g/m 2 Á yr prior to the OAE2 to $12 g/m 2 Á yr during the OAE2. Photic zone and bottom water euxinia alternated with periods of greater oxygenation of the water column in response to climate forcing. Mass balance calculations imply that $2% of the overall global excess OC burial associated with the OAE2 was deposited in the Tarfaya Basin, an area that represented only $0.05% of the total global C/T ocean floor. In fact, the lateral extent of similar black shales along the African continental margin indicates that this part of the ocean contributed significantly to the global increase in organic carbon burial during the OAE2.Citation: Kolonic, S., et al. (2005), Black shale deposition on the northwest African Shelf during the Cenomanian/Turonian oceanic anoxic event: Climate coupling and global organic carbon burial, Paleoceanography, 20, PA1006,

Time series of terrigenous source elements (Al, K, Ti, Zr) from core GeoB4901-8 recovered from the deep-sea fan of the Niger River record variations in riverine sediment discharge over the past 245,000 yr. Although the flux rates of all the elements depend on physical erosion, which is mainly controlled by the extent of vegetation coverage in central Africa, element/Al ratios reflect conditions for chemical weathering in the river basin. Maximum sediment input to the ocean occurs during cold and arid periods, when precipitation intensity and associated freshwater runoff are reduced. High carbonate contents during the same periods indicate that the sediment supply has a positive effect on river-induced marine productivity. In general, variations in the terrestrial signals contain a strong precessional component in tune with changes in low-latitude solar radiation. However, the terrestrial signal lags the insolation signal by several thousand years. K/Al, Ti/Al, and Zr/Al records reveal that African monsoonal precipitation depends on high-latitude forcing. We attribute the shift between insolation cycle and river discharge to the frequently reported nonlinear response of African climate to primary orbital configurations, which may be caused by a complex interaction of the secondary control parameters, such as surface albedo and/or thermohaline circulation.

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In many organic‐rich, low‐carbonate hemipelagic shales, there is a stable and close correlation between the uranium and TOC contents. In this paper, we present a number of case studies using our own data and that from previous publications to investigate black shales with (1) good, (2) fair‐to‐good and (3) poor U/TOC correlations. U/TOC ratios in the different black shale units are compared to each other, and possible reasons for the observed variations are discussed. In general, the U/TOC ratio in a black shale is controlled by a number of factors which include for example the primary uranium content of the water body, the carbonate content and the sedimentation rate. The development of a stable U/TOC ratio may be inhibited by the presence of phosphate, by a high carbonate or sand content, by dissolution (“burn‐down”) of uranium during intermittent oxic periods, and by large‐scale diagenetic remobilisation of uranium. In suitable black shale systems, vertical variations in organic richness can be approximated by measuring the uranium content using spectral gamma‐ray measurements. This may be especially important in outcrop studies because gamma‐ray logging is a straightforward field technique. Before the uranium content can be used as a proxy for TOC content in a black shale system, however, a thorough calibration of uranium and TOC is necessary, in order to determine the stratigraphic and regional limits of the derived U/TOC ratios and to establish the presence of a stable U/TOC correlation.

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1During the Cretaceous greenhouse, episodes of widespread ocean deoxygenation were 2 associated with globally occurring events of black shale deposition. Possibly the most 3 pronounced of these oceanic anoxic events (OAE s was the Cenomanian-Turonian OAE2 4 (~94 Ma). However, although certain redox sensitive trace metals tend to be preferentially 5 sequestered in sediments deposited under anoxic conditions, with Mo drawdown being 6 specifically prone to euxinic settings, these elements are generally somewhat depleted in 7 sediments deposited during OAE2. To understand the driving factors responsible for this 8 depleted trace metal drawdown, we have studied a low latitude section from the proto-North 9Atlantic Ocean (Tarfaya S57), where existing biomarker and iron-sulphur data point to a 10 dominantly euxinic water column, with periodic transitions to ferruginous (Fe-rich) water 11 column conditions. We utilise a variety of redox proxies (Fe-speciation, redox sensitive trace 12 metals and Mo isotopes), which, in combination, allows us to evaluate the detailed nature of