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Dataset that includes 40 Ar atm and δ18O data from Allan Hills ice cores ALHIC2301, ALHIC2302, and ALHIC2201. 40Ar atm dating method is based on the deficit of radiogenic 40 Ar in ancient ice sample compared to the modern atmosphere as a standard (Bender et al., 2008). The argon isotope ratios (δ40Ar/36Ar, δ40Ar/38Ar and δ38Ar/36Ar ) were analysed using the MAT253plus with Dual Inlet systems. The δ40Ar/38Ar was corrected from gravitational settling or fractionation occuring in the firn using δ38Ar/36Ar to get the 40Ar atm. The amount of 40Ar atm divided by the degassing rate of radiogenic 40Ar into atmosphere gives the Argon age of the sample. The δ40Ar/38Ar was corrected from gravitational settling /fractionation occuring in the firn using δ38Ar/36Ar to get the 40Ar atm. The amount of 40Ar atm divided by the degassing rate of radiogenic 40Ar into atmosphere gives the Argon age of the sample. All δ18Ο data were analyzed using either a Picarro L2130-i or a L2140-i cavity-ring-down laser spectrometer, calibrated to the VSMOW-SLAP scale using references waters previously calibration to the VSMOW and SLAP international standards. Precision and accuracy (one standard deviation), based on independent analysis of a quality-control reference water, is <0.1‰. 

Previous analyses of dust concentration and size distribution in ice cores are limited to the past 800,000 years; however, the ALHIC1901 ice core drilled at the Allan Hills Blue Ice Area (BIA) in East Antarctica provides a unique opportunity to examine older discontinuous records of ice ranging in age from 4000-–500 ka. Here we present a discrete record of insoluble particles within ALHIC1901 from the bottom 25 m of the core. We investigated the particle mass concentration, size distribution, and mineralogy within the core to assess the preservation of dust records in BIAs with complex flow histories. We find that the insoluble particle concentrations are likely altered by entrainment of basal sediment for depths 5 m above bedrock. For shallower depths less affected by subglacial input, the record lacks expected peaks in dust concentration during glacial periods, which have been termed “long snapshots,” implying that low net accumulation rates during glacial periods at the Allan Hills BIA results in the preferential loss or attenuation of glacial ice and a corresponding bias toward the preservation of interglacial ice. The dust concentrations may also be further smoothed due to ice thinning. A subset of particles from both the upper and lower ranges of depths analyzed shows evidence of mineral weathering and/or in situ production of secondary minerals, and insoluble particle concentration correlates well with non-atmospherically derived carbon dioxide concentrations. These results highlight the importance of identifying signs of basal ice-rock interactions and/or complicated accumulation and ablation histories as these affect our interpretation of paleoclimate records preserved in ice cores from BIAs. 

An extreme negative (<-6‰) carbon isotope (δ¹³C) excursion is recognized globally in strata recording the Ediacaran-Cambrian boundary. This excursion has been termed the BAsal Cambrian carbon isotope Excursion (BACE). It, like other carbon isotope excursions throughout the geologic record, has been interpreted to record a perturbation to the global carbon cycle based on the assumption that shallow water carbonate sediments and rock preserve an accurate time-series of the δ¹³C composition of the global ocean. However, this assumption has been demonstrated to be inaccurate in some shallow water settings; pervasive early diagenetic alteration of shallow water carbonate δ¹³C records and local carbon cycling within the platform environment can result in the decoupling of global ocean and shallow water carbonate δ¹³C values. Here, we test the extent and isotopic effects of early diagenetic alteration of shallow water carbonate sediments that record the BACE in southwestern Laurentia using carbonate stable isotope (δ¹³C, δ¹⁸O, δ^44/40Ca, δ²⁶Mg) and major and minor element (Mg/Ca, Sr/Ca) geochemistry. The δ^44/40Ca values of the three studied sections are consistent with different modes of diagenetic alteration, ranging from more sediment-buffered to more fluid-buffered early marine diagenesis. Dolostone δ²⁶Mg and δ¹⁸O values suggest that diagenetic alteration and dolomitization occurred prior to significant burial. Because the δ¹³C chemostratigraphic records are reproducible despite the variable diagenetic regimes among the three sites, we argue that the prominent δ¹³C stratigraphic trends were insensitive to diagenetic overprinting and instead reflect changes in the primary δ¹³C composition of dissolved inorganic carbon of platform seawater. Finally, we explore a potential link between the BACE, an increase in early marine fluid-buffered marine diagenesis of platform carbonates, and magmatism associated with the rifting of the southwest margin of Rodinia. 

Between about 2.8-0.9 Ma, Earth’s climate was characterized by 40 kyr cycles, driven or paced by changes in the tilt of Earth’s spin axis. Much is known about the 40k world from studies of deep-sea sediments, but our understanding of climate change during this period and the transition between the 40kyr glacial cycles from 2.8-0.9 Ma and the 100kyr glacial cycles of the last 0.9 Myr is incomplete because we lack records of Antarctic climate and direct records of atmospheric greenhouse gas concentrations. We propose to address these issues by building on our recent studies of >1 Ma ice discovered in shallow ice cores in the Allan Hills Blue Ice Area (BIA), Antarctica. During the 2015-2016 field season we recovered ice from two nearby drill cores that dates to > 2 million years in age using the 40Ar/38Ar ratio of the trapped gases. Our discovery of ice of this antiquity in two cores demonstrates that there is gas-record quality ice from the 40k world in the Allan Hills BIA. To further characterize the composition of Earth’s atmosphere and Antarctic climate during the 40k world we request support for two field seasons to drill new large-volume (4” or 9” diameter) ice cores at sites where we have previously identified >1 Ma ice and nearby sites where ground penetrating radar has identified bedrock features conducive to the preservation of old ice. 

Between about 2.8-0.9 Ma, Earth’s climate was characterized by 40 kyr cycles, driven or paced by changes in the tilt of Earth’s spin axis. Much is known about the 40k world from studies of deep-sea sediments, but our understanding of climate change during this period and the transition between the 40kyr glacial cycles from 2.8-0.9 Ma and the 100kyr glacial cycles of the last 0.9 Myr is incomplete because we lack records of Antarctic climate and direct records of atmospheric greenhouse gas concentrations. We propose to address these issues by building on our recent studies of >1 Ma ice discovered in shallow ice cores in the Allan Hills Blue Ice Area (BIA), Antarctica. During the 2015-2016 field season we recovered ice from two nearby drill cores that dates to > 2 million years in age using the 40Ar/38Ar ratio of the trapped gases. Our discovery of ice of this antiquity in two cores demonstrates that there is gas-record quality ice from the 40k world in the Allan Hills BIA. To further characterize the composition of Earth’s atmosphere and Antarctic climate during the 40k world we request support for two field seasons to drill new large-volume (4” or 9” diameter) ice cores at sites where we have previously identified >1 Ma ice and nearby sites where ground penetrating radar has identified bedrock features conducive to the preservation of old ice. 

Between about 2.8-0.9 Ma, Earth’s climate was characterized by 40 kyr cycles, driven or paced by changes in the tilt of Earth’s spin axis. Much is known about the 40k world from studies of deep-sea sediments, but our understanding of climate change during this period and the transition between the 40kyr glacial cycles from 2.8-0.9 Ma and the 100kyr glacial cycles of the last 0.9 Myr is incomplete because we lack records of Antarctic climate and direct records of atmospheric greenhouse gas concentrations. We propose to address these issues by building on our recent studies of >1 Ma ice discovered in shallow ice cores in the Allan Hills Blue Ice Area (BIA), Antarctica. During the 2015-2016 field season we recovered ice from two nearby drill cores that dates to > 2 million years in age using the 40Ar/38Ar ratio of the trapped gases. Our discovery of ice of this antiquity in two cores demonstrates that there is gas-record quality ice from the 40k world in the Allan Hills BIA. To further characterize the composition of Earth’s atmosphere and Antarctic climate during the 40k world we request support for two field seasons to drill new large-volume (4” or 9” diameter) ice cores at sites where we have previously identified >1 Ma ice and nearby sites where ground penetrating radar has identified bedrock features conducive to the preservation of old ice. 

Field report for 2019/2020 Allan Hill shallow drilling season