| Summary: | A new paleoclimatic indicator based on ice core bubble number-density shows a ~1.7°C cooling from ~257 B.C.E. to ~1686 C.E. at the WAIS Divide site in West Antarctica. Independent estimates of paleoclimatic variables increase our confidence in reconstructions. It is known that density increase and grain growth in polar firn are both controlled by temperature and accumulation rate, and the integrated effects are recorded in the number-density of bubbles as the firn changes to ice (Spencer and others, 2006). Number-density is conserved in bubbly ice following pore close-off, allowing reconstruction of either paleotemperature, or paleo-accumulation rate if the other is known. In this study, a new quantitative late-Holocene paleoclimate reconstruction is presented for West Antarctica that made use of new data obtained from the WAIS Divide field site, and the new steady-state bubble number-density model developed by Spencer and others (2006). Using samples taken from the WDC06A ice core drilled during the 07-08 West Antarctic Ice Sheet (WAIS) Divide field season, bubble sections were prepared and digitally imaged at the National Ice Core Lab in Lakewood, CO. These images were then manipulated, error-checked and reduced into workable bubble number-density data. For dating purposes and annual layer thicknesses, a preliminary chemistry-based depth-age scale for the WDC06A core was used (McConnell, 2009). Accumulation rates were estimated from the layer thickness after correcting for ice-flow strain (a small correction made using a Nye model) and for densification. (The results are thus subject to revision pending finalization of the depth-age scale for WDC06A). The accumulation rates and bubble number-density data were then used to estimate paleotemperatures, finding a linear cooling of ~1.7°C ± 0.7 between ~257 B.C.E. and ~1686 C.E. This late-Holocene cooling is consistent with new δ18O isotope-derived temperature data (Steig, 2009), modeled borehole temperature data (personal communication from A. Orsi, 2009), and newly modeled elevation data (Pollard & DeConto, 2009; personal communication from D. Pollard, 2009) for WAIS Divide. These results could be indicative of a Little Ice Age (LIA) event in West Antarctica, although paleotemperature ¬data from additional sites would help to corroborate this. The bubble number-density model itself was further validated in this study by an observed ~9% increase in accumulation rate per degree of positive temperature change. This observation is consistent with saturation vapor pressure values (Denton and others, 2005).
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