Emmanuelle Pucéat

Fish tooth δ18O revising Late Cretaceous meridional upper ocean water temperature gradients

The oxygen isotope composition of fossil fi sh teeth, a paleo– upper ocean temperature proxy exceptionally resistant to diagenetic alteration, provides new insight on the evolution of the low- to middlelatitude thermal gradient between the middle Cretaceous climatic optimum and the cooler latest Cretaceous period. The new middle Cretaceous low to middle latitude thermal gradient agrees with that previously inferred from planktonic foraminifera δ 18O recovered from Deep Sea Drilling Project and Ocean Drilling Program drilling sites, although the isotopic temperatures derived from δ 18O of fish teeth are uniformly higher by ~3–4 °C. In contrast, our new latest Cretaceous thermal gradient is markedly steeper than those previously published for this period. Fish tooth δ18O data demonstrate that low- to middle-latitude thermal gradients of the middle Cretaceous climatic optimum and of the cooler latest Cretaceous are similar to the modern one, despite a cooling of 7 °C between the two periods. Our new results imply that no drastic changes in meridional heat transport are required to explain the Late Cretaceous climate. Based on climate models, such a cooling without any change in the low to middle latitude thermal gradient supports an atmospheric CO2 decrease as the primary driver of the climatic evolution recorded during the Late Cretaceous.

Modeling the early Paleozoic long-term climatic trend.

The early Paleozoic climate has been described as warm and equable. However, recent data based on conodont oxygen isotopic composition reveal a large, long, cooling trend through the Ordovician, followed by an abrupt cooling during the Late Ordovician glaciation. This long-term climate change is associated with a major radiation in the Earth life history. Nonetheless, the driving mechanisms for this cooling trend remain unknown. Carbon dioxide consumption by the weathering of fresh rocks from volcanic arcs has recently been suggested as a possible driver for this climate change. However, the impact of the plate motion context has not been explored yet, although it might have a major impact on atmospheric CO 2 levels. Simulations with a climate model coupled to a biogeochemical model (GEOCLIM) show that the atmospheric CO 2 decreased from more than 20 PAL (∼5600 ppmv) in the Furongian down to approximately 10 PAL (∼2800 ppmv) in the Llandovery before rising again in the Early Devonian. We suggest that changes in geography and exposure of fresh volcanic rocks on continents are required to explain the large CO 2 drawdown that led to the onset of cooler to glacial conditions from the Middle Ordovician to the Llandovery. The weathering of fresh volcanic rocks is itself responsible for 33% of the Late Ordovician atmospheric CO 2 decrease; the rest being related to the continent motion through the intertropical convergence zone (ITCZ). Mean annual continental temperature falls by 3°C in the Early Ordovician, reaching 13.5°C during the glacial interval, and rises to 16°C in the Early Devonian.

A better-ventilated ocean triggered by Late Cretaceous changes in continental configuration

Oceanic anoxic events (OAEs) are large-scale events of oxygen depletion in the deep ocean that happened during pre-Cenozoic periods of extreme warmth. Here, to assess the role of major continental configuration changes occurring during the Late Cretaceous on oceanic circulation modes, which in turn influence the oxygenation level of the deep ocean, we use a coupled ocean atmosphere climate model. We simulate ocean dynamics during two different time slices and compare these with existing neodymium isotope data (ɛNd). Although deep-water production in the North Pacific is continuous, the simulations at 94 and 71 Ma show a shift in southern deep-water production sites from South Pacific to South Atlantic and Indian Ocean locations. Our modelling results support the hypothesis that an intensification of southern Atlantic deep-water production and a reversal of deep-water fluxes through the Caribbean Seaway were the main causes of the decrease in ɛNd values recorded in the Atlantic and Indian deep waters during the Late Cretaceous.

Multi-proxy orbital chronology in the aftermath of the Aptian Oceanic Anoxic Event 1a: Palaeoceanographic implications (Serre Chaitieu section, Vocontian Basin, SE France)

In the early Aptian, the Oceanic Anoxic Event (OAE) 1a is well defined by a negative ?13C excursion followed by a positive δ13C excursion, spanning the Deshayesites deshayesi and Dufrenoya furcata ammonite biozones. A cyclostratigraphic approach is performed in the Vocontian Basin, France, to estimate the time required for the carbon cycle recovery following the major disturbance associated to OAE1a and to provide durations of ammonite and foraminifer biozones. The Serre Chaitieu section, which consists of hemipelagic blue-grey marls with occasional marker limestone horizons and encompassing the Deshayesites deshayesi Zone to the end of the Epicheloniceras martini Zone, was used as a reference section in the Vocontian Basin. Using field Spectral Gamma Ray (SGR), 450 measurements were performed throughout the section, and a sample of each measured sediment was collected to further perform calcimetry, clay mineralogy, and magnetic susceptibility (MS) measurements. Detrital clay mineral assemblages consist of illite, illite/smectite mixed-layers (I-S), kaolinite and chlorite. Fluctuations of clay minerals are mainly driven by climate change, progradation/drowning of peri-vocontian platforms and sea-level changes. The proportions of illite and kaolinite covary and fluctuate in opposition with I-S. Cyclic fluctuations of relative proportions of clay minerals are particularly well recorded by the kaolinite/chlorite ratio (K/C). Spectral analyses, using the multi-taper and the amplitude spectrogram methods, were performed on SGR, MS, CaCO3 and K/C signals to detect sedimentary cycles related to an orbital forcing throughout the series. The geochronometer 405-kyr eccentricity cycle well expressed and significant (up to 99% confidence level) is used to provide a robust temporal framework. More than five 405-kyr eccentricity cycles are recognised, providing a total duration of at least 2.49 myr for the whole sedimentary succession. The minimum duration of the D. furcata Zone is assessed at 0.42 myr, and the duration of the E. martini Zone at 1.52 myr. Amplitude spectrograms show a strengthened signal of obliquity during the D. furcata Zone, which is coherent with the global cooling that has been depicted for this interval, and which could have favored the development of lowextension polar ice and thus the lowering of the sea level. Durations of C-isotope zones, worldwide correlated, are also calculated. From these results, the duration of the return to equilibrium in the carbon cycle in the aftermath of OAE1a could be calculated at 1.35 myr.

Cool episode and platform demise in the Early Aptian: New insights on the links between climate and carbonate production

The Early Aptian encountered several crises in neritic and pelagic carbonate production, major perturbations in the carbon cycle, and an oceanic anoxic event (OAE1a). Yet the causal links between these perturbations and climate changes remain poorly understood, partly because temperature records spanning the Early Aptian interval are still scant. We present new δ18O data from well-preserved bivalves from a carbonate platform of the Galve subbasin (Spain) that document a major cooling event postdating most of OAE1a. Our data show that cooling postdates the global platform demise and cannot have triggered this event that occurred during the warmest interval. The warmest temperatures coincide with the time equivalent of OAE1a and with platform biotic assemblages dominated by microbialites at Aliaga as well as on other Tethyan platforms. Coral-dominated assemblages then replace microbialites during the subsequent cooling. Nannoconids are absent during most of the time equivalent of the OAE1a, probably related to the well-known crisis affecting this group. Yet they present a transient recovery in the upper part of this interval with an increase in both size and abundance during the cool interval portion that postdates OAE1a. An evolution toward cooler and drier climatic conditions may have induced the regional change from microbial to coral assemblages as well as nannoconids size and abundance increase by limiting continent-derived input of nutrients.

Orbital Chronology of the Lower–Middle Aptian: Palaeoenvironmental Implications (Serre Chaitieu Section, Vocontian Basin)

A detailed cyclostratigraphic study conducted on several proxies including spectral gamma ray, magnetic susceptibility, clay minerals, and carbonate content suggests that the return to equilibrium in the carbon cycle after the major disturbance linked with Oceanic Anoxic Event 1a occurred in about 2 Myr. The minimum duration of the D. furcata zone is estimated at 0.46 Myr and that of the E. subnodosocostatum zone at 1.45 Myr. Strengthening of the obliquity record in the furcata zone confirms the cooling that characterizes this period.

Carbon and oxygen isotope signals from the Callovian-Oxfordian in French sedimentary basins.

High-resolution carbon and oxygen isotope data from the Paris Basin and the Subalpine Basin (France) are available in a precise biostratigraphic framework for the Callovian–Oxfordian stages. A biostratigraphically well-constrained δ13C curve, derived from bulk carbonates in the Paris Basin and the Subalpine Basin, is provided in order to document carbon-cycle evolution and to serve as a chemostratigraphic reference for the Callovian–Oxfordian in the Tethyan domain. Sea-temperature reconstructions, using diagenetically screened belemnite and oyster data, reveal major climate perturbations at the Middle–Late Jurassic transition.