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David Thornally

During the late Quaternary, the Earth’s climate system has undergone climate oscillations on a range of timescales, from multi-decadal processes to glacial-interglacial cycles occurring every ~100,000 years. Changes in ocean circulation played a significant role in these climate events by altering the global redistribution of heat, dissolved nutrients and carbon. My research focuses on constraining how the circulation of the ocean changed in the past and the mechanisms by which these changes affected the global climate system.

I use a wide range of tools including: faunal and elemental ratio (e.g., Mg/Ca, B/Ca, Cd/Ca) analysis of foraminifera to reconstruct water mass properties; geochemical proxies of circulation such as measurement of Pa/Th ratios, Nd isotopes and radiocarbon concentrations; examination of the detrital components of sediment such as ice-rafted detritus and geochemical analysis of tephra for improving stratigraphy; and sediment grain size analysis (e.g. sortable silt analysis) to reconstruct relative changes in paleo-current strength.

Ongoing projects whilst at WHOI include:

The response of the Western Boundary Undercurrent to past abrupt climate change

Deep water produced in the high latitude North Atlantic forms a deep western boundary current (locally termed the Western Boundary Undercurrent, WBUC) that flows southward, at depth, along the eastern margin of North America. The WBUC plays an important role in rapidly transmitting climate signals into the ocean interior and helping ventilate the world’s ocean. I am using grain size analysis to examine how the flow speed structure of the WBUC altered between warm and cold climate intervals during the last glacial period (Figure 1)

Holocene changes in the strength of the Nordic Seas Overflows

The overflow of cold, dense water from the Nordic Seas into the North Atlantic plays a critical role in the global thermohaline circulation, and the compensating inflow of Atlantic surface waters helps warm NW Europe. Using grain size data from cores taken south of Iceland, I am investigating the strength of the eastern Nordic Seas overflow throughout the Holocene (~0-11,000 years ago), examining likely controls and effects.

 

Deglacial changes in the circulation of the Northeast Atlantic

The termination of the last Ice Age was accompanied by abrupt changes in ocean circulation. The Northeast Atlantic in particularly was subject to dramatic reorganisations that are thought to have had an impact on global climate evolution through this period. Continuing on from my PhD work, I am calibrating and using elemental ratios in benthic foraminifera to constrain changes in the physical and chemical properties of the Northeast Atlantic and Nordic Seas.

Subpolar gyre dynamics during the Holocene

The strength of the North Atlantic’s subpolar gyre varies on annual to  millennial timescales and it is likely involved in feedback mechanisms that impact atmospheric circulation and the overturning circulation of the ocean. In collaboration with scientists from Cardiff University, we are investigating how the circulation of subpolar gyre changed on decadal timescales throughout the last 2,000 years, as well as investigating millennial scale changes throughout the Holocene.

Last updated: January 16, 2014