Reconstructing sea ice conditions and migration of the polar frontal zone in the warm late Pliocene, Wilkes Land margin, East Antarctica

Taylor-Silva, Briar Isabella, 1993 (Briar)

The warm late Pliocene (~3.3 -3.0 Ma) is the most recent period with a warm climate similar to that predicted by climate models for the next century. Late-Pliocene temperatures were ~1.84 – 3.6°C warmer than the preindustrial era, while reconstructed sea level was ~25 – 35 m higher than today due to reduced global ice volume (Haywood et al., 2015, Dowsett et al., 2009). The East Antarctic Ice Sheet (EAIS) is largely grounded above sea level, therefore is inferred to be relatively stable due to reduced ice- ocean interaction. Along the Wilkes Land margin, however, a large sector of the EAIS is grounded below sea level, which makes it vulnerable to retreat. A collapse of the EAIS within the Wilkes Land sector would raise sea level by 3-4 m, and understanding the response of this region to Pliocene warmth may therefore help us evaluate future vulnerability.

To investigate late-Pliocene EAIS behaviour, focus has been on a sediment core collected from the Wilkes Land margin by IODP Expedition 318. Site U1361A is located at 64°25’S, 143°53’E, at a water depth of 3454 m. Biogenic silica concentrations range from 56.5 - 85.4 mbsf (~3.8 to 2.8 Ma). Sediments with high concentrations of biogenic silica record periods when surface conditions were favourable to diatom productivity, while silica-poor sediments record periods of low productivity due to perennial sea ice cover or increased sediment delivery. Diatom assemblages within each silica rich interval provide further insight into sea ice distribution and sea surface conditions during late-Pliocene interglacials. By observing the distribution and relationship between warm, open ocean and cool, seasonal sea ice taxa inferences could be made about the state of the climate during this time.

Diatom assemblages were found to be predominantly composed of warm, open ocean Fragilariopsis spp and Thalssiothrix spp. Between ~3.15 Ma corresponding the KM3 interglacial isotope excursion, unexpected high abundances of F. weaveri were observed. Using the model of (Barron, 1996) allowed us to assess the location of the polar frontal zone using a relationship between F. barronii, F. weaveri and Rouxia spp. Results from this indicated a shift in the polar frontal zone towards the south by ≥ 4 °S relative to modern day, which occurred during interglacials centered at ~3.15 Ma. During the late Pliocene, global warmth was great enough that it has migrated the entire southern ocean.
system southwards. The late Pliocene warmth was also associated with a large increase in sedimentation rate from 30.1 m/m.y to 53.8 m/m.y which may reflect ice retreat and active erosion documented within the Wilkes subglacial basin during this (Cook et al., 2013).

Species assemblages infer that climatic warmth began in the Wilkes Land margin ~3.2 Ma which is in agreement with records from Prydz Bay (Whitehead et al., 2005). The Northern Hemisphere glaciation began at ~2.7 Ma, however, diatom assemblages suggest that during the late Pliocene the Wilkes Land margin was largely ice free indicating that cooling may not have occurred until the early Pleistocene.

81 pages

2015Taylor-Silva

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