Astrid Forster

Carbon-isotope stratigraphy recorded by the Cenomanian–Turonian Oceanic Anoxic Event: correlation and implications based on three key localities

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 δ13C values for all components are followed by intervals of high, near-constant δ13C in the form of an isotopic plateau. Recognition of an unambiguous return to background δ13C 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 δ13C 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.

High temperatures in the Late Cretaceous Arctic Ocean

To understand the climate dynamics of the warm, equable greenhouse world of the Late Cretaceous period, it is important to determine polar palaeotemperatures. The early palaeoceanographic history of the Arctic Ocean has, however, remained largely unknown, because the sea floor and underlying deposits are usually inaccessible beneath a cover of floating ice. A shallow piston core taken from a drifting ice island in 1970 fortuitously retrieved unconsolidated Upper Cretaceous organic-rich sediment from Alpha ridge, a submarine elevated feature of probable oceanic origin. A lack of carbonate in the sediments from this core has prevented the use of traditional oxygen-isotope palaeothermometry. Here we determine Arctic palaeotemperatures from these Upper Cretaceous deposits using TEX86, a new palaeothermometer that is based on the composition of membrane lipids derived from a ubiquitous component of marine plankton, Crenarchaeota. From these analyses we infer an average sea surface temperature of ∼15 °C for the Arctic Ocean about 70 million years ago. This calibration point implies an Equator-to-pole gradient in sea surface temperatures of ∼15 °C during this interval and, by extrapolation, we suggest that polar waters were generally warmer than 20 °C during the middle Cretaceous (∼ 90 million years ago).

Extremely high sea-surface temperatures at low latitudes during the middle Cretaceous as revealed by archaeal membrane lipids

The middle Cretaceous (125-88 Ma) greenhouse world was characterized by high atmospheric CO 2 levels, the general absence of polar ice caps, and much higher global temperatures than at present. Both δ 1 8 O-based and model-based temperature reconstructions indicate extremely high sea-surface temperatures (SSTs) at high latitudes. However, there are a number of uncertainties with SST reconstructions based on δ 1 8 O isotope data of foraminifera due to diagenetic overprinting effects and tenuous assumptions with respect to the δ 1 8 O value of Cretaceous seawater, the paleoecology of middle Cretaceous marine organisms and seawater pH. Here we applied a novel SST proxy (i.e., TEXT [tetraether index of 86 carbon atoms], based on the membrane lipids of marine crenarchaeota) derived from middle Cretaceous sedimentary rocks deposited at low latitudes. The TEXT proxy indicates that tropical SSTs in the proto-North Atlantic were at 32-36 °C during the early Albian and late Cenomanian-early Turonian. This finding agrees with SST estimates based on δ 1 8 O paleothermometry of well-preserved foraminifera as well as global circulation model calculations. The TEXT proxy indicates cooler SSTs (27-32 °C) for the equatorial Pacific during the early Aptian, which is in agreement with SST estimates based on δ 1 8 O paleothermometry.

Tropical warming and intermittent cooling during the Cenomanian/Turonian oceanic anoxic event 2: Sea surface temperature records from the equatorial Atlantic

Oceanic anoxic event 2 (OAE-2) occurring during the Cenomanian/Turonian (C/T) transition is evident from a globally recognized positive stable carbon isotopic excursion and is thought to represent one of the most extreme carbon cycle perturbations of the last 100 Myr. However, the impact of this major perturbation on and interaction with global climate remains unclear. Here we report new high-resolution records of sea surface temperature (SST) based on TEX86 and ?18O of excellently preserved planktic foraminifera and stable organic carbon isotopes across the C/T transition from black shales located offshore Suriname/French Guiana (Demerara Rise, Ocean Drilling Program Leg 207 Site 1260) and offshore Senegal (Cape Verde Basin, Deep Sea Drilling Project Leg 41 Site 367). At Site 1260, where both SST proxy records can be determined, a good match between conservative SST estimates from TEX86 and ?18O is observed. We find that late Cenomanian SSTs in the equatorial Atlantic Ocean (?33°C) were substantially warmer than today (~27°–29°C) and that the onset of OAE-2 coincided with a rapid shift to an even warmer (~35°–36°C) regime. Within the early stages of the OAE a marked (~4°C) cooling to temperatures lower than pre-OAE conditions is observed. However, well before the termination of OAE-2 the warm regime was reestablished and persisted into the Turonian. Our findings corroborate the view that the C/T transition represents the onset of the interval of peak Cretaceous warmth. More importantly, they are consistent with the hypotheses that mid-Cretaceous warmth can be attributed to high levels of atmospheric carbon dioxide (CO2) and that major OAEs were capable of triggering global cooling through the negative feedback effect of organic carbon-burial-led CO2 sequestration. Evidently, however, the factors that gave rise to the observed shift to a warmer climate regime at the onset of OAE-2 were sufficiently powerful that they were only briefly counterbalanced by the high rates of carbon burial attained during even the most extreme interval of organic carbon burial in the last 100 Myr.

Black shale deposition on the northwest African Shelf during the Cenomanian/Turonian oceanic anoxic event: Climate coupling and global organic carbon burial

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 mid-Cretaceous 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 similar to2 g/m(2) . yr prior to the OAE2 to similar to12 g/m(2) . 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 similar to2% of the overall global excess OC burial associated with the OAE2 was deposited in the Tarfaya Basin, an area that represented only similar to0.05% 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.

Mid-Cretaceous (Albian–Santonian) sea surface temperature record of the tropical Atlantic Ocean

Paleoclimate records of geologic time periods characterized by extreme global warmth such as the mid-Cretaceous are important for a better understanding of the Earth9s climate system operating in an exceptionally warm mode. Here we applied an organic geochemical proxy (TEX 86 ) on organic matter–rich Albian–Santonian sediments, recovered from Ocean Drilling Program Leg 207 Sites 1258 and 1259 on Demerara Rise, to reconstruct sea surface temperatures (SSTs) in the western equatorial Atlantic. Preceded by a stepwise Cenomanian warming trend (∼31–35 °C), the onset of the Cretaceous thermal maximum coincided here with the Cenomanian-Turonian boundary event. Once established, this extreme warm climate regime, characterized by averaged tropical SSTs close to 35 °C, lasted up to the Turonian-Coniacian transition. Two pronounced cooler intervals (∼2–3 °C) interrupt this otherwise remarkably stable record, providing the first δ 18 O independent evidence for middle Turonian cooling that previously has been attributed to glacioeustatic sea-level lowering. Coniacian SSTs decline stepwise, reaching a minimum in the Santonian (∼32–33 °C), where cooling is most pronounced, presumably concomitant with the first progressive opening of a deep-water passage through the equatorial Atlantic gateway.