West Antarctic ice sheet surface melting and Holocene climate variability

Surface melting is a rare event across Antarctica and melt features preserved in the ice sheet stratigraphy provide a unique paleoclimate record of changes in summer temperature. We have analyzed the visible stratigraphy of the Siple Dome deep ice core and use the record of melt layers to investigate changes in Holocene climate variability in West Antarctica. To understand the present-day temperature variability for the Ross Embayment region of West Antarctica, we first synthesized a new time-series of surface temperature for the last 20 years. We find high seasonal to inter-annual variability in both mean temperature and variance. In particular, fluctuations in seasonal to inter-annual temperature variance occur on an approximately five year cycle and correlate with variations in the Southern Oscillation Index. In a study of recent surface melting across this region, we compared the utility of three fundamentally different methods of detecting surface melting: automatic weather station air temperature records; changes in passive microwave brightness temperature; and snow and firn stratigraphy. We found melting occurs during brief (≤1 to 2 day) periods of positive air temperatures, and melt layers formed during the instrumental period can be detected reliably in cores as well as snow pits and that they form during warmer than average summers. In addition, we developed a new passive microwave threshold to use for melt onset in West Antarctica. In further work, we used field experiments at Siple Dome to investigate the formation of melt layers in the surface snow. Experimental results show that melt layers form when air temperatures exceeds a positive-degree-day threshold, and that melt-water migrates to and refreezes in surface- or near-surface crust layers, forming thin, laterally continuous, distinct ice features. Finally, we used our improved understanding of melt layer occurrence and formation to interpret the melt-layer record from the Siple Dome ice core. We find millennial-scale summer climate conditions in West Antarctica changed significantly during the Holocene. The most significant trend is an increase in melt frequency from the mid-Holocene to the present, which we have calibrated as a ≥2°C increase in summer temperature. This is in contrast to other East Antarctic climate records, which show an early Holocene climatic optimum followed by stable or decreasing temperatures through the mid to late Holocene. We interpret the mid to late Holocene rise in temperature as evidence of an increasing marine influence on the Ross Sea sector of West Antarctica. We believe this is due to the continued lateral retreat through the mid to late Holocene of the West Antarctic ice sheet from its furthest Last Glacial Maximum configuration, as well as a change in Pacific Ocean climatology bringing more variability to the Ross Sea sector of the South Pacific Ocean.