Emeritus Woodwardian Professor of Geology I. Nicholas McCave

As a geological oceanographer my research aims to understand how the modern deep ocean circulation shapes the sea bed and controls the distribution of sediment types, grain-sizes and bedforms. I then apply that understanding to interpretation of the geological record of the changing deep-sea circulation in piston and gravity cores for the late Pleistocene, and Ocean Drilling Project cores for the Neogene and earlier Pleistocene. Insight developed over the last 30 years concerning mechanics of fine sediment erosion, transport, aggregation and deposition shows that sediments become more cohesive, and all form aggregates, below 10 µm grainsize, but above that size they increasingly behave non-cohesively and are sorted by prevailing currents. This 'sortable silt' (10-63 µm) has a mean size that provides a proxy for depositional current speed and allows insight into changes in deep circulation vigour. We have calibrated the proxy with current meter data which allows quantitative assessment of ocean flow speed changes in the geological record on scales of years to many Ma.

The deeper objective is to understand the impact of climate change on the deep sea (and vice versa). Part of the meridional heat flux on earth is carried by warm surface ocean currents with a corresponding cold deep return flow. We look for good monitoring points for these deep flows and have studied the inflow to the North Atlantic south of Iceland, and the deep inflows to the Pacific east of New Zealand and Indian ocean east of Madagascar. Among our recent striking results are the discovery of a Holocene ~1500-year variability in the flow south of Iceland, matching the Mediaeval Warm Period - Little Ice Age climate changes, flow changes at the inception of the Antarctic circum-polar current, and Southern Ocean control of glacial deep N. Atlantic flow. 

Current work is focussed on records of relatively shallow flows in the sub-Arctic through the Canadian Arctic Archipelago and around Greenland and Iceland as well as the Southern Ocean. Here the problem of disentangling the effects of sediment delivery by ice rafting and current-controlled transport and deposition has been a necessary precursor to examination of late Glacial and Holocene sediment data revealing Atlantic and Pacific (Bering throughflow) influences on the E. Greenland and Labrador Currents. The size of wind-blown dust is very similar to deep-sea 'sortable silt’ which may open a route to estimation of palaeo wind strength.