Role of the North Pacific Ocean in Glacial-Interglacial Carbon Cycling

Evidence from Antarctic ice cores documents a persistent 80-100 ppm oscillation in atmospheric CO2 betwen glacial and interglacial periods over the last 800,000 years. Climate models currently have difficulty in accurately simulating this oscillation, suggesting that there may be fundamental gaps in our understanding of forcings and feedbacks within the Earth's carbon cycle. Given that the oceanic carbon reservoir is the largest global source of exchangeable carbon, it is likely that some change in the physical, chemical and/or biological processes within the ocean was responsible for driving the recorded oscillations in CO2. At present, the North Pacific is a source of atmospheric CO2, despite the ocean being a net sink of carbon dioxide. It is therefore essential to future climate projections to understand the role of the North Pacific in the glacial storage and interglacial release of carbon to the atmosphere.

Funded by the NERC and together with collaborators from the University of South Florida, University of Washington, Lawrence Livermore National Laboratory, University of Michigan and the University of Nottingham, we are currently undertaking a 3-year, multi-proxy (foraminiferal stable isotopes, radiocarbon, diatom δ30Si, redox trace metals in marine sediments) depth transect study of five marine sediment cores from the NE Pacific continental margin within the subpolar-subtropical transition zone in order to determine:

                                1). the ventilation history of the subarctic NE Pacific water column over the last deglaciation (~ 10-20 Ka)
                                2). the role of nutrients in driving changes in the glacial-interglacial productivity signal

We hope that this research will provide a means of assessing the effects of changing ocean circulation, nutrient availability/utilisation and marine biological productivity on atmospheric CO2 and enable a better understanding of the role of the North Pacific in regulating atmospheric CO2 during major climatic transitions.

Low-latitude monsoonal circulation in East Africa and the Arabian Sea during the Pliocene

Superimposed upon a long-term trend of aridification, the Late Cenozoic climate history of Africa was punctuated by episodes of extreme climate variability, characterised by the precessionally-forced appearance and disappearance of large lake systems within the East African Rift Valley. In order to investigate the nature of low-latitude climate variability during the Late Pliocene and Early Pleistocene, my PhD research combined the high-resolution analysis of diatomite deposits from one of the lake phases in the Central Kenyan Rift with the reconstruction of long-term changes in the transport of wind-borne terrigenous dust to the Arabian Sea. Climate in both regions is strongly influenced by relative changes in the strength of the Indian Ocean summer and winter monsoons, which determine rainfall distribution in equatorial East Africa and generate the low-level winds which transport dust offshore from the Arabian Peninsula.

In the Baringo-Bogoria basin in the Central Kenyan Rift, a well-dated package of fluvio-lacustrine sediments and diatomite units (the Chemeron Formation) documents a major humid phase between 2.7 and 2.55 million years ago (Ma), coincident with the intensification of glaciation in the Northern Hemisphere. This humid phase is marked by five lacustrine diatomites which are exposed within the tributaries of the Barsemoi and Ndau rivers in the Tugen Hills. As part of the work conducted by the Baringo Paleontological Research Project (directed by Dr. Andrew Hill, Yale University), my doctoral research sought to investigate the nature of palaeoclimate changes recorded within these precessionally-forced lake deposits in order to understand the nature of fluctuations in monsoonal circulation capable of driving such extreme climate variability. Stable oxygen isotope measurements of diatom silica, combined with the XRF analysis of whole-sample geochemistry, reveal that the deep lake phase was characterised by wet-dry cycles lasting, on average, 1,400 years.

Over longer timescales, variations in the aeolian delivery of lithogenic matter to the Arabian Sea, reflected in normalised flux of titanium, show that monsoonal circulation prior to 2.6 Ma was highly variable and primarily driven by orbitally-forced changes in tropical summer insolation, modulated by the 400,000 year cycle of orbital eccentricity. Millennial-scale fluctuations in the dust record also support the evidence of abrupt wet-dry cycles in East Africa. Such high-resolution cycles are rarely found in older records, thus giving a valuable insight to the nature of short-term fluctuations in Plio-Pleistocene climate.