Terrestrial meteorite ages indicate that some ice at the Allan Hills blue ice area (AH BIA) may be as old as 2.2 Ma. As such, ice from the AH BIA could potentially be used to extend the ice core record of paleoclimate beyond 800 ka. We collected samples from 5 to 10 cm depth along a 5 km transect through the main icefield and drilled a 225 m ice core (S27) at the midpoint of the transect to develop the climate archive of the AH BIA. Stable water isotope measurements (δD) of the surface chips and of ice core S27 yield comparable signals, indicating that the climate record has not been significantly altered in the surface ice. Measurements of 40Aratm and δ18Oatm taken from ice core S27 and eight additional shallow ice cores constrain the age of the ice to approximately 90–250 ka. Our findings provide a framework around which future investigations of potentially older ice in the AH BIA could be based.
Future changes in North Pacific wintertime climate will be largely determined by the response of the Aleutian Low (ALow) pressure system to anthropogenic forcing. Although the ALow has intensified over the twentieth century, global climate model projections of future ALow variability are equivocal. In order to evaluate decadal to centennial ALow forcing mechanisms and provide context for the modern intensification, here we combine a new Denali ice core (Alaska) sea‐salt sodium record with the Mount Logan ice core (Yukon) sodium record to develop a composite 1200 year record of ALow variability. The composite record indicates that the recent secular ALow intensification began circa 1741 and is unprecedented in magnitude and duration over the past millennium. North Pacific ice core snow accumulation and stable isotope records are consistent with this interpretation. The ALow intensification is associated with warming tropical Pacific sea surface temperatures, consistent with dynamic theory and instrumental correlations.
Abstract. Widespread existing geological records from above the modern ice sheet surface and outboard of the current ice margin show that the Antarctic Ice
Sheet (AIS) was much more extensive at the Last Glacial Maximum (∼ 20 ka) than at present. However, whether it was ever smaller than
present during the last few millennia, and (if so) by how much, is known only for a few locations because direct evidence lies within or beneath the
ice sheet, which is challenging to access. Here, we describe how retreat and readvance (henceforth “readvance”) of AIS grounding lines during the
Holocene could be detected and quantified using subglacial bedrock, subglacial sediments, marine sediment cores, relative sea-level (RSL) records,
geodetic observations, radar data, and ice cores. Of these, only subglacial bedrock and subglacial sediments can provide direct evidence for
readvance. Marine archives are of limited utility because readvance commonly covers evidence of earlier retreat. Nevertheless, stratigraphic
transitions documenting change in environment may provide support for direct evidence from subglacial records, as can the presence of transgressions
in RSL records, and isostatic subsidence. With independent age control, ice structure revealed by radar can be used to infer past changes in ice
flow and geometry, and therefore potential readvance. Since ice cores capture changes in surface mass balance, elevation, and atmospheric
and oceanic circulation that are known to drive grounding line migration, they also have potential for identifying readvance. A multidisciplinary
approach is likely to provide the strongest evidence for or against a smaller-than-present AIS in the Holocene.
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