Source of the Magnetic Susceptibility Variations in Southern Ocean Sediments Over the Last Glacial Cycle

Abstract:

Changes in the sources, mineralogy and rates of iron supply to the Southern Ocean may have global impact and significance, by influencing plankton growth rates and nutrient take-up in this, the largest of the high nutrient low chlorophyll (HNLC) regions of the world ocean. Iron ‘fertilization’ in the Southern Ocean may increase rates of carbon export production and thus the ocean uptake flux of atmospheric CO₂, and also diminish the northward flow of residual nutrients to the extra-polar ocean, especially the HNLC regions of the Pacific. Changes in Southern Ocean export production may contribute to global climate change over glacial-interglacial timescales.

The key sources of iron for the Southern Ocean are reported to be windblown dust and sedimentary supply; their relative significance an issue of much long-standing debate. Links between aeolian dust fluxes to the Southern Ocean and to the Antarctic ice cores have been proposed for the Scotia Sea region of the Southern Ocean, downwind from the South American land mass. Regional downcore variations in the magnetic susceptibility of sediments from the Scotia Sea show remarkable similarity to variations in dust concentration and flux in East Antarctic ice cores (with glacial stages characterised by increases in ice dust and sediment magnetic susceptibility). Indeed, the strength of the ice dust/sediment magnetism correlations (r ~ 0.7) provides a pragmatic basis for use of the sedimentary magnetic susceptibility records as a chronostratigraphic proxy, a boon in the carbonate-free deep-sea sediments of the Southern Ocean.

However, the source and causal basis of the sediment magnetism/ice dust co-variations remain controversial; aeolian dust, bacterial magnetite and wind-driven current transport of marine sediment have all been invoked as possible key sources. Here, we use magnetic and isotopic methods to resolve this debate, and identify and quantify the sources of magnetic material to the Scotia Sea for the last glacial cycle.