Mathieu Moiroud PhD thesis

Thumbnail imageEvolution of the oceanic deep circulation during the Cretaceous – Insight from the neodymium isotopes

Funding: research grants from Burgundy Regional Council and CEA-Saclay

Supervisor: Jean-François Deconinck (HDR), university of Burgundy

Co-supervisors: Emmanuelle Pucéat (university of Burgundy) and Yannick Donnadieu (laboratoire des sciences du climat et de l'environnement, CEA Saclay)

Start date: 15 novembre 2010

defended the 2 july 2014

 

Abstract

The Cretaceous is depicted as the warmest period of the last 300 Ma. The Late Cretaceous is characterized by a long-term cooling, the cessation of oceanic anoxic events with deposits rich in organic matter, and major changes in the configuration of the continents. Though the oceanic circulation modes are essential to understand the role of oceans in climate evolution, the oceanic circulation and location of source zones of deep-waters in the basins remain unclear for the Cretaceous.

The neodymium (Nd) isotopes are used to track oceanic circulation and exchanges between water masses, in both past and modern oceans. The Nd isotope composition (eNd) in the ocean derives more or less directly from that of adjacent continental landmasses. In deep-waters, the isotopic signature is related to the composition of the continents surrounding the area of water sinking and current transport, thus every oceanic basin acquires a singular eNd. Unequivocal interpretations of the Cretaceous seawater eNd values stem from the insufficient spatial and temporal cover of the available data. In particular, data from continental margins remain very scarce, yet they are essential to identify the neritic regions of deep-water production by comparing their eNd with basinal bottom waters.

This PhD thesis firstly intents to collect the Nd signature of continental margins and in regions without data for the Cretaceous, with special attention given to the potential source zones of deep-water production. Fossil fish teeth, oxide coatings on foraminifera tests and detrital fraction from Late Cretaceous sediments are analyzed for their eNd. The results of this work are compared to a compilation of eNd values available in the literature for the Cretaceous and the Paleogene, in order to propose hypotheses for the location of the potential deep-water source zones and their evolution throughout the Cretaceous.

These hypotheses are then confronted to a coupled ocean-atmosphere circulation model. The results show that the eNd values seem to follow the long-term temperature variations during the Late Cretaceous and the Paleocene. However the numerical simulations suggest that deep oceanic circulation is not significantly altered by global climate. Instead, the tectonic evolution may be the primary cause of fluctuations in the continental eNd exported to the oceans, as well as the changes in oceanic circulation and climate, through modifications of the basinal configurations and the eroded landmasses.

 

Keywords

Cretaceous, neodymium isotopes, rare earth elements, oceanic circulation, climate model

 

PhD thesis committee members

Emmanuelle Vennin (HDR), laboratoire Biogéosciences, university of Burgundy

Frédéric Fluteau (HDR), Institut de physique du globe de Paris, Laboratoire de paléomagnétisme, Paris