The Southern Ocean is a centerpiece in global oceanography, linked to all major ocean basins via its wind-driven zonal and meridional circulation. As such it can not only reflect changes in global climate, but also moderate them by varying the amounts of deep and intermediate water supply to lower latitudes, as well as potentially being a large sink for atmospheric CO2. One of the important questions about the Southern Ocean is how the Antarctic Polar Front (APF) responds to the advancing and retreating ice shelf on glacial/interglacial timescales. The interplay between the two governs ocean-atmosphere gases exchange and the efficiency of the biological pump at high southern latitudes. Research to date, however, on the last glacial/interglacial transition does not provide a definitive answer to the question whether the APF has migrated at these times.

Ocean Drilling Program (ODP) Leg 177 recovered laminated diatom mats (LMD) deposited across the modern APF throughout the Plio-Pliestocene. A critical component of LDM is the presence of the long, pennate diatom Thalassiothrix antarctica at the 5% relative abundance level or greater. Their occurrence throughout the Plio-Pliestocene tracks the long term position of the APF and accumulation rates of LDM deposits can reach 45cm kyr-1. Scanning electron microscope analysis of the fine lamination reveals a sequence of contrasting deposits which agree well with the seasonally varying diatom assemblages across the Antarctic Circumpolar Current (ACC) as sampled by the USJGOFS-AESOPS sediment trap array. As a result sedimentation rates based on this annual sedimentation model of LDM can be locally as high as 55-80cm kyr-1. There is no evidence of along bottom sediment focusing, but focusing in the water column could still be a contributing factor to such high pelagic sedimentation rates.

Pooling all the available data on T. antarctica water column observations, concentration in sediment trap samples from the USJGOFS-AESOPS program, and abundance in sediment cores from ODP Leg 177 and others, it becomes evident that LDM can potentially track both the position of the APF as well as the stabilizing effect on the surface waters of meltwater runoff near the ice edge. T. antarctica is an eurythermal diatom and as such offers a temperature-independent proxy for the past position of the APF and potentially the seasonal ice zone. Provided sediment cores are monitored for LDM and T. antarctica, this new proxy can help resolve the question whether APF and surface isotherms decouple on glacial/interglacial timescales. If interpretations of the limited data are correct, the spatial and temporal distribution of LDM and T. antarctica can be used to determine the areal extent of deep circumpolar water exposed to the atmosphere.

Although there is no known modern analogue to the formation and deposition of LDM, even at the present low abundance <3% in cores and sediment trap samples, T.antarctica can export opal from the base of the mixed layer, and bury it into the sediments in comparable quantities to the very prolific Fragilariopsis kerguelensis.

A collection of electron microscopy images of diatom frustules, both extinct and living, can be seen on http://www.soton.ac.uk/~ibg. For contact email ivo_grigorov@hotmail.com