Diatom Silica Oxygen Isotope Records from the Late Quaternary Antarctic Margin

Lead Research Organisation: NERC British Geological Survey
Department Name: NERC Isotope Geosciences Laboratory

Abstract

Antarctica and its ice sheets have played, and continue to play, a major role in the global ocean-atmosphere system, hence, it is critical that we have a sound understanding of the past behaviour of Antarctica and it's ice sheets with a view to understanding their potential future variability under a warming climate. The Southern Ocean is a key component of the thermohaline circulation of the world's oceans and the re-distribution of heat and salt around the oceans is integral to processes that regulate rapid climate transitions. Computer modelling results have shown that sufficient melt water input to the Antarctic continental shelf area is capable of shutting down the formation of cold, salty deep water in Antarctica hence upsetting the balance of the thermohaline circulation and the ocean-climate system of the Northern Hemisphere. In order to further investigate these processes that originate in Antarctica, it is necessary to understand the transfer mechanisms of ocean-climate signals from the Antarctic ice sheets, across the continental margin seas, into the Southern Ocean. Exceptionally well-preserved Antarctic margin sediment cores, recovered during the last decade, contain an excellent archive of these ice-ocean-climate interactions, often on seasonal timescales, from the end of the last ice age and throughout the recent warm interglacial (the Holocene). The cores are seasonally layered through the deglaication, intermittently layered through the Holocene, and the layers are dominated by fossil planktonic diatoms (algae); individual species of which are sensitive to sea surface conditions including sea ice concentration, fresh water influx, and open ocean influence upon the margin. Following the last ice age, these Holocene Antarctic sediments record climate fluctuations of tens to thousands of years long and whatever environmental forcing mechanism is responsible for these fluctuations, the changes are likely to be felt in the Antarctic coastal regions first, and the cores proposed for this research are located in prime positions to record these changes. Diatom oxygen isotope measurements represent an under-utilised technique that provides a means of obtaining oxygen isotope records in high latitude environments. The measurement of oxygen isotopes in diatoms is a widely used proxy in the study of the history of lakes, however, to date there have been many fewer attempts to use records of diatom oxygen isotopes in the oceans. Studies that have taken place have demonstrated the sensitivity of diatom oxyegn isotope measurements in polar and sub-polar waters to changes in surface ocean environmental parameters such as salinity, freshwater input and sea surface temperature. The research proposed here will be the first attempt to produce diatom oxygen isotope records from the Antarctic margin, a region sensitive to the waxing and waning of the Antarctic ice sheets in terms of melt water through-put to the Southern Ocean. We propose to investigate the evolution of seasonality along the Antarctic margin since the last ice age, and also the processes involved in producing the sediment record, by relating diatom oxygen isotope measurements on season-specific diatom taxa (i.e. diatom species that thrived particularly in spring or autumn) to relative freshwater influx to the coast, from either melted terrestrial ice or sea ice. We also hope to show that the diatom oxygen isotope measurements will be low at the end of the last ice age, as a large quantity of old ice sheets were melting, and will be higher during warmer time periods of the Holocene when ice sheets were at a minimum.

Publications

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Description The Antarctic Peninsula Ice Sheet is the most northerly of the Antarctic ice sheets and the most vulnerable to climate warming. The western Antarctic Peninsula (WAP) has experienced one of the largest and fastest atmospheric temperature rises since observations began with more than 3°C of atmospheric warming per century. Much of the recent increase in surface air temperature has been attributed to an atmospheric circulation pattern characterised by a positive Southern Annular Mode (SAM) associated with a deepening circumpolar trough, increasing cyclonicity, strengthened westerly winds and the delivery of warm, moist maritime air to the WAP. In light of recent instrumental records of warming along the WAP1, there has been much debate about what has influenced rising temperatures along the WAP during the Holocene, with terrestrial and oceanic proxies often suggesting different, sometimes opposing, mean conditions. However, the warming trend began prior to the recent positive phase of the SAM, hence, the SAM is unlikely to be the only contributor to Antarctic Peninsula warming. Concomitant with this atmospheric warming, although not forced by it, mean surface ocean temperatures have increased by >1°C. These factors have contributed to 87% of marine glacier fronts along the Antarctic Peninsula retreating over the past six decades6 and an increase in the mean annual melting period7. Given the likely increased melting rates to come in the future under warming atmospheric conditions, we place these contemporary observations into their historical context by analysing the oxygen isotope composition of marine diatom silica (d18Odiatom) over the past c. 13 ka from Palmer Deep, west Antarctic Peninsula (ODP Site 1098), including an ultra-high resolution investigation of during the rapid climate change of the last deglaciation. We interpret d18Odiatom in terms of Holocene glacial (terrestrial ice) melt water and show that during the past 5.0 ka, the El Niño - Southern Oscillation circulation pattern, in particular La Niña, and peak summer insolation at 60ºS exert a stronger influence on glacial melt water inflow to the WAP margin that oceanic processes driven by the southern westerly winds.
Exploitation Route We need more cores from geographically different places around Antarctica.
Sectors Environment