Reconstructing variability of the ‘cold water route’ through the mid to late Pleistocene

Overview

The Atlantic meridional overturning circulation (AMOC) plays a fundamental role in regulating the Earth’s climate. Past reductions in the strength of this overturning circulation have been associated with dramatic cooling in the northern hemisphere. A recent trend of cooling in the subpolar Atlantic Ocean and warming in the mid-latitudes is indicative of a weakening of the AMOC. General Circulation Models are typically biased toward a stable AMOC and unable to replicate observed AMOC sensitivity to climate forcing. This bias in AMOC stability within models suggests that certain parameters are inaccurately represented, compromising predictions of future AMOC change. One element of the AMOC that remains poorly understood is the role of Antarctic Intermediate Water (AAIW). AAIW is one of the primary water masses characterizing the northward-flowing upper limb of the AMOC. Despite a recent surge in research effort, a consensus on the role of AAIW in contributing to AMOC variability has not been achieved.
Today, AAIW is primarily formed in the SE Pacific and SW Atlantic Oceans. AAIW and its precursor, Subantarctic Mode Water (SAMW), are advected eastward through Drake Passage along the Subantarctic Front (SAF). This injection of cold, low salinity Pacific water into the South Atlantic Ocean is commonly referred to as the ‘cold water route’ (Figure 1).
Variability in the connectivity between the Pacific and Atlantic Oceans via this cold water route may modify the physical properties of AAIW forming in the SW Atlantic Ocean, in turn affecting the heat and salinity budget of the AMOC. It has been shown that during the last glacial period the SAF was likely in a more northerly position relative to today (Roberts et al., 2017). It has been hypothesised that the southward retreat of the SAF on the deglaciation could have been important for reinvigoration of the AMOC, but further investigation is required to constrain the sensitivity of AMOC to Pacific-Atlantic connectivity through the Drake Passage.
Marine sediments recovered at IODP Site U1534 on Expedition 382 present the first opportunity to constrain variability in endmember physical and geochemical properties of AAIW as it enters the upper limb of the AMOC over multiple glacial-interglacial cycles. This studentship will test the hypothesis that an opening of the ‘cold water route’ during glacial terminations, coincident with reinvigoration of the AMOC, was common to glacial-interglacial cycles during the mid to late Pleistocene.

Click on an image to expand

Image Captions

Figure 1. A. The position of IODP Site U1534 collected on Expedition 382 and B. vertical transect (grey dotted line in A) through the water column showing temperature (colour and black contours) and density (white dashed contours). Blue arrow indicates the ‘cold water route’ associated with the SAF. Site U1534 is bathed by end-member AAIW as it begins its northwards penetration into the AMOC

Figure 2. Correlation of shipboard natural gamma radiation from the upper 83 m below sea floor at U1534 with the LR04 benthic 18O stack.

Methodology

The student will generate a millennial-scale record of sea surface temperature from U1534 spanning the last 700 kyrs (Figure 2) using the UK’37 index. As Roberts et al (2017) showed, UK’37-derived sea surface temperatures can infer migration of the SAF, in-turn opening and closing the cold water route. The student will have opportunities to work with existing international collaborators to augment the temperature record with other proxies, including benthic foraminifera geochemistry (stable isotope stratigraphy and Mg/Ca) and Nd isotopes within fish teeth as a water mass tracer, ice rafted debris flux, and other proxies as appropriate. This will be the first paleoceanographic record beyond the last two glacials from the SW Atlantic. There will also be opportunities to test how the last 700 kyr climate record compares to the earlier, late Pliocene, intensification of Northern Hemisphere glaciation. The student will compare their proxy record with published records of comparable resolution from the SE Pacific, the ‘warm water route’ of the SE Atlantic, the North Atlantic and further afield. Constraining the timing of changes in AAIW entering the upper cell of the AMOC with oceanic and climatic shifts will be essential to determining the cause and effect of cold water route variability.

Project Timeline

Year 1

Review of existing literature relating to the cold water route, AMOC, AAIW and the use of various proxies to reconstruct past oceanographic parameters.
Training in sediment processing and methods involved in generating UK’37 data (biomarker extraction, sample preparation, and analysis).
Initial sample selection, subsampling and preparation.

Year 2

Main focus will be sample preparation and analysis for UK’37 to reconstruct sea surface temperatures.
Age model refinement where necessary using benthic foraminifera oxygen isotope stratigraphy (see further information).
Interpretation of the new dataset and exploration of other proxies to augment the UK’37 record, or identification of intervals where increased temporal resolution would be beneficial.
Presentation at the 14th International Conference on Paleoceanography in Bergen (Sept. 2022)

Year 3

Interpretation of data, including comparison with relevant records.
Presentation at EGU (April/May 2023) and the UK Paleoclimate Society meeting (anticipated to be September 2023)

Year 3.5

Finalising interpretation and writing up thesis chapters and papers for peer-review journals.

Training
& Skills

The student will be trained in sediment sampling and preparation for alkenone analysis (UK’37). This training will include: biomarker extraction and the organic geochemical steps required to isolate alkenones prior to analysis by gas chromatography mass spectrometry (GC-MS). The student will be trained in using the GC-MS and learn how to critically assess their data and perform data quality controls. The student will calibrate their UK’37 record to reconstruct sea surface temperatures.
The student will be supported to pursue foraminifera-based studies, including benthic and planktonic foraminifera assemblage and geochemistry, to augment and aid interpretation of the UK’37 data.
It is anticipated that the student will spend extended visits working in the biomarker laboratory at Durham University during their second year to complete the UK’37 analysis.  Other laboratory and microscope work , interpretation of data and writing will be done at BAS.

Every effort will be made to offer the student shipboard experience of marine sediment collection within the period of their PhD but cannot be guaranteed.

References & further reading

Liu W, Xie S-P, Liu Z, Zhu J (2017) Overlooked possibility of a collapsed Atlantic Meridional overturning circulation in warming climate. Sci Adv 3:e1601666

McClymont, E.L., Elmore, A.C., Kender, S., Leng, M.J., Greaves, M. & Elderfield, H. 2016. Pliocene-Pleistocene evolution of sea surface and intermediate water temperatures from the southwest Pacific. Paleoceanography, 31, 895-913. doi:10.1002/2016pa002954.

Roberts, J., McCave, I.N., McClymont, E.L., Kender, S., Hillenbrand, C.D., Matano, R., Hodell, D.A., and Peck, V.L., 2017. Deglacial changes in flow and frontal structure through the Drake Passage. Earth and Planetary Science Letters, 474:397-408. doi:org/10.1016/j.epsl.2017.07.004.

Rühs, S., Schwarzkopf, F. U., Speich, S., and Biastoch, A.: Cold vs. warm water route – sources for the upper limb of the Atlantic Meridional Overturning Circulation revisited in a high-resolution ocean model, Ocean Sci., 15, 489-512, https://doi.org/10.5194/os-15-489-2019, 2019.

https://iodp.tamu.edu/scienceops/expeditions/iceberg_alley_paleoceanography.html

Further Information

Benthic foraminifera oxygen isotope stratigraphy will provide the age model for U1534. Processing the samples and picking the foraminifera is currently in process, but the student may wish to increase the resolution and/or add a planktic foraminifera isotope record, or apply the epifaunal-infaunal 13C approach to determine benthic ventilation changes. Costs for these analyses are covered by other grants.

Marcus Gutjahr (GEOMAR, Germany) will be measuring the isotopic composition of Nd in fish teeth picked at the same time as the foraminifera. Nd will be used to infer the Pacific-sourced water contribution of AAIW.

Shubham Tripathi (NCPOR) will be measuring the Mg/Ca on paired benthic foraminifera stable isotope samples to estimate past temperatures of AAIW.

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