North Atlantic intermediate water silicic acid during deglaciation: where do you come from, where do you go?

Abstract:

Ice cores from Greenland and Antarctica document a dramatic, step-wise increase in atmospheric carbon dioxide (pCO2) as the Earth warmed during the last deglaciation. Marine sediment archives point towards concurrent millennial-scale shifts in ocean circulation, nutrient cycling, and biological productivity; all of these processes would have been involved in carbon cycle feedbacks. These abrupt events, Heinrich Stadial One (HS1) and the Younger Dryas (YD), are associated in time with large freshwater anomalies and ice-berg rafting in the North Atlantic - which are, in turn, thought to have caused a disruption in the Meridional Overturning Circulation (MOC) through buoyancy forcing, and thus a major perturbation in heat and nutrient transport in the oceans.

Intermediate waters, formed by a combination of wind-driven mixing and deep winter convection between the Polar and Subantarctic Fronts, represent the upper limb of the MOC that supplies nutrients (albeit relatively depleted in silicic acid compared to other nutrients) to lower latitude thermocline waters. There is widespread evidence throughout the North Atlantic for pulses of diatom productivity during HS1 and YD in regions where there is a low supply of silicic acid today. Were these diatom pulses a result of changes in nutrient supply as a result of a large-scale reorganisation of ocean circulation; changes in the supply rate, extent or relative silicic acid content of the northward flowing upper MOC limb; or do they represent more localised supply processes?

Here, we investigate changes in intermediate water silicic acid supply using mid- and high-latitude North Atlantic sediment cores and the silicon isotope composition of deep-sea sponge spicules. Together with other proxy records our results, including new high-resolution records from off the south east of Iceland, shed light on the processes that influence silicic acid supply and uptake during periods of abrupt climatic change.