Skip to main content

Eocene and Cretaceous Oceanography: Disentangling the roles of geography and temperature on deep ocean circulation in past greenhouse climates

Periodic Reporting for period 1 - ECO (Eocene and Cretaceous Oceanography: Disentangling the roles of geography and temperature on deep ocean circulation in past greenhouse climates)

Reporting period: 2020-12-01 to 2022-11-30

Ocean circulation plays a dominant role in distributing heat over the planet and in providing oxygen for life in the deep sea. The thermohaline circulation will likely be affected by global warming, in particular the current areas of deep water formation at high latitudes.
The thermal maxima of the Cretaceous (94 million years ago) and the Eocene (51 million years ago) are the two most important greenhouse climate phases of the last 100 million years and are seen as analogues to current climate change. For the Cretaceous and Eocene, mechanisms of deep-water formation and the role of ocean circulation on heat transport are poorly understood. This research aims to disentangle the controls of geography and temperature on deep circulation in past greenhouse worlds to identify ocean circulation dynamics fundamental to climates warmer than the present-day.

This research has a three-fold approach:
1) To generate neodymium isotope signatures from a range of sites in the Southern Ocean to identify and track deep-water masses under two different circulation regimes: - the opening of gateways in the Eocene and - an episode of sudden warming in the mid-Cretaceous, which led to a widespread lack of oxygen in the world's oceans.
2) To identify the source regions of deep water formation, the geochemical signatures (neodymium isotopes, rare earth elements, mineralogy) of past seawater and detrital sediment contributions are compared and contrasted to reconstructions of paleotopography.
3) To test scenarios of modelled ocean circulation in past greenhouse worlds, the Nd-isotope data are integrated with coupled ocean-atmosphere climate models.

Due to the shorter duration of the project, work has focussed on the preparation of samples for geochemical analyses. The researcher has developed new competencies in geochemical techniques, broadening her scientific skills base, whilst training transferable skills and reaching professional maturity, placing her in a good position to draw the disciplines of oceanography, sedimentology and climate modelling together.
In preparation for Nd-isotope analyses, 46 samples have been selected from International Ocean Discovery Program (IODP) Expedition 369 Sites U1514 and U1516 in the Mentelle Basin, southwest of Australia. Ferromanganese coatings of 23 samples from the mid-Cretaceous interval of Site U1516 have been leached from bulk sediment. Fish debris has been picked from 23 samples from the Eocene interval of Site U1514 and attacked with acid. For each sample, 90% of the sample volume has been purified through column chemistry in metal-free laboratories, resulting in 46 samples ready for measurement of the Nd-isotope composition by Multi-Collector Inductively Coupled Plasma Mass Spectrometry. For all 46 samples an aliquot (10%) has been prepared for the analyses of rare earth elements of seawater archives (fish debris and ferromanganese oxide coatings), to evaluate the veracity of the seawater signal. For 23 samples, the detrital fraction has been isolated through repeated leaching to remove ferromanganese oxide coatings.

To better constrain ages of the Cretaceous samples, high resolution elemental data obtained through XRF core-scanning have been statistically analysed for the potential imprint of orbital forcing. Preliminary time series analyses results reveal the role of eccentricity-modulated precession on the mid-Cretaceous interval of Site U1516, with a sharp drop in sedimentation rate during the climatic perturbation of Oceanic Anoxic Event 2 (OAE 2). These initial results have been compared to biostratigraphy and chemostratigraphic information as part of an integrated stratigraphic approach.
Due to the shorter duration of the project, no new geochemical results have been generated, but materials have been prepared for analysis. Stratigraphic results have been disseminated through submission of a manuscript on which the researcher is co-author, and submission of abstracts international conferences.
Flowchart of the project