Current Research

Agulhas Current system dynamics over the past 5 milion years (IODP Expedition 361)

My research goals align squarely within the scientific objectives of IODP Expedition 361 as laid out in the Scientific Prospectus. Specifically, I aim to assess the variability of the Agulhas Current and Agulhas Leakage, to reconstruct the dynamics of the Agulhas Current system, and to examine the relationship between Agulhas Leakage and deep ocean circulation under contrasting climatic regimes over the past 5 million years.

I will be measuring major and trace element concentrations plus radiogenic isotopes from the fine grained terrigenous sediment, as well as Th and He based flux estimates on samples from the CAPE-01B Site (U1479) to assess Agulhas Leakage variability at ~3 kyr resolution over the Mid-Pleistocene transition, and lower resolution over the last 5 million years.

I will compare my data with the same proxies measured over the same time intervals by my collaborators for Site NV-02B (U1474), under the direct influence of the Agulhas Current and Site and APT-01B (U1475), under the influence of the modern Agulhas Return Current and Subtropical Front. I also plan to collaborate with other shipboard scientists who are using different surface water proxies at these sites, in order to get a better sense for what physical oceanographic changes have occurred in the Agulhas Current region. Comparing these three sites will allow us to reconstruct the dynamics of the Agulhas Current system, and better constrain the magnitude of Agulhas Leakage under contrasting climatic regimes over the past 5 million years. Finally, I will compare proxies for Agulhas Leakage with those for deep water circulation to evaluate the interplay between the surface and deep arms of the global thermohaline circulation during the Mid-Pleistocene transition and over the entire Plio-Pleistocene.

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Neodymium isotope and REE associations in deep-sea sediments and pore waters (IODP Expedition 361)

I'm a scientific participant on International Ocean Discover Program (IODP) Expedition 316 Southern African Climates (SAFARI). We sailed around southern Africa in February and March 2016, taking sediment cores from beneath the Agulhas Current. In addition to the paleoclimate objectives, Expedition 361 involved an intensive interstitial fluids program at four of the sites, at water depths ranging from ~500 to ~3000 meters.

Neodymium isotopes (εNd) measured in authigenic oxides and marine micro-fossils have become increasingly recognized as a useful proxy to reconstruct ocean circulation. The oceanic residence time of Nd is short enough that water masses have distinct εNd based on where they originated. The εNd water mass tracer is especially useful at locations near an interface between two water masses, where the measured εNd reflects the relative mixing proportions of the end-member water masses. Recently, a great deal of attention has been given to the association of neodymium in various sedimentary phases, and the source of their Nd isotope signature (surface ocean, deep ocean, or pore water). Furthermore, it has become clear that the fidelity of foraminiferal Nd isotopes as paleoceanographic tracers of ocean circulation is site specific, and that each location must be tested independently.

I plan to measure neodymium isotope ratios and rare earth element (REE) concentrations in pore waters and various components of the associated sediments at each of the four sites, in order to better understand how diagenesis affects the Nd isotope ratio of those sediments, and to quantify the pore water imprint on the sedimentary signal.

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Understanding the role of the Agulhas Leakage in millennial-scale climate change: Evaluating leads and lags

One hypothesis for the cause of glacial terminations (i.e. the abrupt climate change from cold glacial to warm interglacial conditions) involves a rapid resumption of North Atlantic Deep Water (NADW) formation, possibly triggered by increased salt input from the Agulhas Leakage. Paleoceanographic studies have found significant glacial-interglacial variability in the Agulhas Leakage, with maxima occurring during, or just prior to glacial terminations. There are currently no records of Agulhas Leakage with high enough resolution to resolve millennial scale variability. I am using terrigenous strontium isotope ratios to track changes in the Agulhas Leakage at high resolution during Marine Isotope Stage (MIS) 3, from about 35,000 to 55,000 years ago. I am also measuring deep ocean circulation proxies on the same samples, to directly compare the Agulhas Leakage with NADW changes and evaluate leads and lags. If the terrigenous strontium isotope changes lead changes in NADW proxies, it would support the Agulhas Leakage as a trigger for changes in global ocean circulation. If the strontium isotopes lag behind changes in NADW, this would support a more passive role for the Agulhas Current System, being triggered by upstream changes in the global THC.

RC11-83 climate records

Figure modified from Piotroski et al. (2005) Figure. 1(A). Climate records from the core RC11-83 in the Agulhas Leakage region compared with the Greenland (GISP) and Antarctic Vostok) ice core data for reference.

The red box highlights the time period selected for study.

Strontium isotope ratios measured on the terrigenous sediment from Rutberg et al. (2005) reflect variable amounts of Agulhas Leakage in comparison to other sedimentary sources.

Nd isotope ratios from Piotroski et al. (2005) represent NADW strength through time.

Benthic carbon isotopes from Charles and Fairbanks (1992) and Charles et al. (1996)

YD =Younger Dryas

ACR = Antarctic Cold Reversal

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Did the Southern Hemisphere Westerly Wind Belt Shift Northwards during the LGM?

The Southern Hemisphere (SH) westerly winds may be the most important driver for changes in the Agulhas Current system. In addition, the Southern Hemisphere Westerlies are known to be important for climate due to their effects on the global carbon cycle and on the global thermohaline circulation (THC). Many proxy records suggest that the strength and position of the Southern Hemisphere westerly winds have changed significantly since the Last Glacial Maximum (LGM) at ~21,000 years BP. However, after a recent review of Southern Hemisphere westerly wind changes during the Last Glacial Maximum (LGM) “their strength and position in colder and warmer climates relative to today remain a wide open question” (Kohfeld et al., 2013).

I am using sediment provenance to confirm whether the SH westerlies were north of their modern location during the LGM. In the central South Atlantic, dust can be delivered from South American via the westerlies or from Africa via the trade winds. The continental sources of South America and Africa have very different geochemical signatures, making it possible to distinguish between eolian transport via the westerlies vs. the trade winds using sediment provenance. Far from the coast, atop the mid-ocean ridge, the only significant source of sediment can be wind-blown dust. At high elevations on the ridge, the sediment is not affected by deep ocean currents, and should provide a record of its eolian source. I am therefore making provenance measurements on glacial-aged and Holocene sediment samples from the Mid-Atlantic Ridge to reconstruct the pattern of the SH wind belts.

I presented preliminary results at the 2015 AGU Fall Meeting. Click on the poster below for a full sized image.

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