Indian-Atlantic Ocean connections through the Pliocene and Pleistocene

Overview

An important component of the global thermohaline conveyor is the input of warm and salty Indian Ocean waters to the South-east Atlantic Ocean, via the Agulhas leakage (Figure 1). This is the ‘warm water return route’ of the thermohaline conveyor, driven by 2-20 Sv of subtropical waters entering the Atlantic Ocean [1]. In the late Pleistocene, variability in the strength of Agulhas leakage has been reconstructed on both glacial-interglacial and millennial timescales, and linked to fluctuations in Atlantic Meridional Overturning Circulation (AMOC) intensity [2-4]. However, the exact controls over the strength of Agulhas leakage, and its wider climate impacts, are not resolved. For example, was the Agulhas leakage stronger or weaker during the globally warmer Pliocene epoch (~3-5 Ma)?

This study will provide the first assessment of Agulhas leakage strength spanning the Pliocene, the onset and intensification of glacial cycles in the early Pleistocene, and their further development during the mid-Pleistocene climate transition ~1 million years ago. The project benefits from the recent drilling by the International Ocean Discovery Program (IODP) of a long and continuous marine sediment record from the Cape Basin, to the south-west of South Africa (Figure 1). Previous work at the nearby ODP Site 1087 has indicated that the dominant wind systems of the region may have intensified and/or migrated northward since the Pliocene [3,5], but the impact on Agulhas Leakage remains uncertain.

The key research questions for this project are:
• Was Agulhas leakage stronger during the Pliocene?
• Did Agulhas leakage change with the onset and intensification of glaciation cycles?
• What is the relationship between variability in Agulhas leakage and variability in AMOC?
The PhD student will benefit from close collaboration between the supervisory team and other members of the IODP Expedition 361 science party, who are studying the evolution of southern African climate and the links to circulation in the Indian Ocean and close to the Subtropical Front (Figure 1).

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Image Captions

Figure 1: Site U1479 (this study). Basemap of SSTs (colours) and geostrophic flow velocities (arrows). Other Expedition 361 sites are shown in white. Adapted from Proceedings of IODP Expedition 361 (http://publications.iodp.org/proceedings/361/101/361_101.html).

Methodology

This project will analyse the recently collected, high-resolution marine sediment sequence recovered at Site U1479 during IODP Expedition 361. The extensive sediment sequences which were recovered present outstanding opportunities to generate multi-proxy assessments of regional oceanographic changes at a range of time scales (from thousands to millions of years).

The primary methodology will be the application of established organic geochemistry proxies using the biomarker laboratories at Durham University, to reconstruct:
• Sea surface temperature (SST: UK37 index)
• Sea surface salinity (alkenone stable isotopes)
The student will also have opportunities to learn and apply foraminifera-based proxies including stable isotopes and trace metal analysis, with collaborators Professor Ian Hall (Cardiff) and Dr Thibaut Caley (Bordeaux). These techniques provide complementary but different approaches to reconstruct temperature and salinity, and also ensure the development of a robust age control for the palaeoceanography data.

Project Timeline

Year 1

Literature review; receive training in Durham (biomarker) laboratory techniques; sample preparation for pilot biomarker analyses.

Year 2

Reconstruct SST for the Pliocene and across the mid-Pleistocene climate transition at orbital and sub-orbital timescales; receive training in stable carbon isotope analysis for foraminifera (BGS); identify and prepare selected samples for stable isotope analysis, and submit a grant application for analysis at BGS; visit collaborators at Cardiff and/or Bordeaux for training in foraminifera trace metal analysis; present first data-sets at a national conference and develop first publication.

Year 3

Complete data sets: analyse selected alkenone stable isotope samples to reconstruct salinity changes; finalise foraminifera data sets; develop drafts of manuscripts for publication; draft and receive feedback on the thesis; present final data-sets at an international conference; develop second publication.

Year 3.5

Completion and submission of thesis, finalisation of research papers.

Training
& Skills

The student will receive specialist training and support in the following techniques:
– Biomarker extraction, clean-up and characterisation
– Biomarker isotope analysis (“compound specific isotope analysis”)
– Foraminifera stable isotope mass spectrometry
– Sediment stratigraphy
– Time series analysis and other appropriate statistical techniques to ensure the resulting data sets can be explored to their full extent.

The supervisory team has the necessary expertise to train the student in these specialist skills, all supported by a dedicated team of technicians in Durham Geography. In addition to receiving regular supervisory meetings and support at Durham (in partnership with BGS, Cardiff and Bordeaux), the student will also be enrolled in a graduate training programme at Durham University and through IAPETUS-specific training, gaining a range of transferable skills relevant to completion of the PhD and developing a career path, including writing research proposals and giving oral presentations. S/he will attend national and international conferences, networking events and outreach activities, developing an important network for feedback and future employment. The student will attend and contribute to the programme of regular departmental seminars and discussion groups, to support broad career development.

References & further reading

References cited in the Outline:[1] Richardson, P.L. (2007) Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters. Deep Sea Research Part I. 54, 1361-1389.[2] Caley, T., et al. (2012) Agulhas leakage as a key process in the modes of Quaternary climate changes. Proc. National Academy of Sciences, 109, 6835-6839.[3] Petrick, B.F., McClymont, E.L. et al. (2015) Changing surface water conditions for the last 500 ka in the Southeast Atlantic … Paleoceanography, 30, 1153-1167.[4] Martínez-Méndez, G., Hall, I.R. et al. (2010) Contrasting multiproxy reconstructions of surface ocean hydrography in the Agulhas Corridor and implications for the Agulhas Leakage during the last 345,000 years. Paleoceanography, 25, PA4227.[5] Petrick, B., McClymont, E.L. et al. (2015) Late Pliocene upwelling in the Southern Benguela region. Palaeogeography, Palaeoclimatology, Palaeoecology, 429, 62-71.

Further Information

Professor Erin McClymont, Durham University, 00 44 191 334 3498, erin.mcclymont@durham.ac.uk.

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