IAP2-22-352

Indian-Atlantic Ocean circulation connections during climate changes of the past

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 warmth of the Pliocene through the development and intensification of glacial cycles in the Pleistocene (~the last 5 million years). 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?

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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

The student will receive full training in all of the methods propose here.

This project will exploit the new, high resolution marine sediment sequence recovered at Site U1479 during IODP Expedition 361. The extensive sediment sequences 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 student will focus on reconstructing organic geochemical proxies of:

• Sea surface temperature (SST)
• Sea surface salinity

The student will also have opportunities to learn and apply foraminifera-based proxies including stable isotopes and trace metal analysis, with external collaborators Professor Hall (Cardiff) and Dr 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 laboratory techniques; sample preparation for biomarker analyses.

Year 2

Reconstruct SST for the Pliocene and Pleistocene at orbital and sub-orbital timescales; receive training in stable carbon isotope analysis for foraminifera (Durham, BGS) and for alkenones (Durham); identify and prepare selected samples for stable isotope analysis, and submit a grant application for analysis at BGS; present first data-sets at a national conference.

Year 3

Complete data sets: analyse selected alkenone stable isotope samples to reconstruct salinity changes, and finalise foraminifera data sets; develop drafts of manuscripts for publication; resent final data-sets at an international conference.

Year 3.5

Draft and receive feedback on the thesis; finalise papers for publication.

Training
& Skills

The student will receive specialist training and support in the following techniques, which are regularly used outside of academia in the analysis of environmental samples:

• Microwave assisted solvent extraction of lipids
• Liquid and gas chromatography (LC, GC)
• Mass spectrometry
• 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 student will be enrolled in a graduate training programme (Durham University) to gain a range of study skills and transferable skills. They will attend and contribute to the regular departmental seminars and discussion groups. They will attend national and international conferences, networking events and outreach activities, developing an important network for feedback and future employment.

References & further reading

[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.[6] Petrick, B., McClymont, E.L., et al. (2018) Oceanographic and climatic evolution of the southeastern subtropical Atlantic over the last 3.5 Ma. Earth and Planetary Science Letters, 492,12-21.

Details of the target core site and broader expedition aims and achievements can be found in the following Proceedings volume:
Hall, I.R., Hemming, S.R., LeVay, L.J., and the Expedition 361 Scientists, 2017. South African Climates (Agulhas LGM Density Profile). Proceedings of the International Ocean Discovery Program, 361: College Station, TX (International Ocean Discovery Program). http://dx.doi.org/10.14379/iodp.proc.361.2017

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