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  1. Growing season temperatures play a crucial role in controlling treeline elevation at regional to global scales. However, understanding of treeline dynamics in response to long-term changes in temperature is limited. In this study, we analyze pollen, plant macrofossils, and charcoal preserved in organic layers within a 10,400-year-old ice patch and in sediment from a 6000-year-old wetland located above present-day treeline in the Beartooth Mountains, Wyoming, to explore the relationship between Holocene climate variability and shifts in treeline elevation. Pollen data indicate a lower-than-present treeline between 9000 and 6200 cal yr BP during the warm, dry summer and cold winter conditions of the early Holocene. Increases in arboreal pollen at 6200 cal yr BP suggest an upslope treeline expansion when summers became cooler and wetter. A possible hiatus in the wetland record at ca. 4200–3000 cal yr BP suggests increased snow and ice cover at high elevations and a lowering of treeline. Treeline position continued to fluctuate with growing season warming and cooling during the late-Holocene. Periods of high fire activity correspond with times of increased woody cover at high elevations. The two records indicate that climate was an important driver of vegetation and treeline change during the Holocene. Early Holocene treeline was governed by moisture limitations, whereas late-Holocene treeline was sensitive to increases in growing season temperatures. Climate projections for the region suggest warmer temperatures could decrease effective growing season moisture at high elevations resulting in a reduction of treeline elevation. 
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    Free, publicly-accessible full text available March 1, 2025
  2. Abstract

    Estimating fire emissions prior to the satellite era is challenging because observations are limited, leading to large uncertainties in the calculated aerosol climate forcing following the preindustrial era. This challenge further limits the ability of climate models to accurately project future climate change. Here, we reconstruct a gridded dataset of global biomass burning emissions from 1750 to 2010 using inverse analysis that leveraged a global array of 31 ice core records of black carbon deposition fluxes, two different historical emission inventories as a priori estimates, and emission-deposition sensitivities simulated by the atmospheric chemical transport model GEOS-Chem. The reconstructed emissions exhibit greater temporal variabilities which are more consistent with paleoclimate proxies. Our ice core constrained emissions reduced the uncertainties in simulated cloud condensation nuclei and aerosol radiative forcing associated with the discrepancy in preindustrial biomass burning emissions. The derived emissions can also be used in studies of ocean and terrestrial biogeochemistry.

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  3. Abstract Black carbon emitted by incomplete combustion of fossil fuels and biomass has a net warming effect in the atmosphere and reduces the albedo when deposited on ice and snow; accurate knowledge of past emissions is essential to quantify and model associated global climate forcing. Although bottom-up inventories provide historical Black Carbon emission estimates that are widely used in Earth System Models, they are poorly constrained by observations prior to the late 20th century. Here we use an objective inversion technique based on detailed atmospheric transport and deposition modeling to reconstruct 1850 to 2000 emissions from thirteen Northern Hemisphere ice-core records. We find substantial discrepancies between reconstructed Black Carbon emissions and existing bottom-up inventories which do not fully capture the complex spatial-temporal emission patterns. Our findings imply changes to existing historical Black Carbon radiative forcing estimates are necessary, with potential implications for observation-constrained climate sensitivity. 
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    Free, publicly-accessible full text available December 1, 2024
  4. Ice cores contain stratigraphic records of microbial cells, buried through thousands of years of snow accumulation and spanning significant climatic periods. It is well established that microorganisms are transported to and preserved within the West Antarctic Ice Sheet. From the total assemblage of microorganisms that land on the ice sheet, we do not know how or if microorganisms survive burial and persist long-term in glacial ice equally. We cannot accurately interpret microbial cell stratigraphic records or utilize these cellular records as proxies until we understand post-depositional processes and the genomic adaptations of microbial cells in glacial ice. Here, we quantify cell concentrations in meltwater from four flow paths of a continuous flow analysis melter system in order to evaluate the efficacy of these flow paths for the successful collection of intact cells archived in ice cores. Using this information, we melted eight sections from the WAIS Divide ice core and quantified the cell concentrations, assayed the viability of the microbial cells, and sorted individual cells for genome sequencing. We will present preliminary data from the flow path cell recovery experiment, and genomic and viability results from the WAIS Divide ice core, with the hope to stimulate further discussion around single cell genomes and how they can be leveraged to complement paleoclimate information from ice cores. 
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  5. Warming temperatures and prolonged drought periods cause rapid changes of fire frequencies and intensities in high-latitude ecosystems. Associated smoke plumes deposit dark particles from incomplete combustion on the Greenland ice sheet that reduce albedo but also provide a detailed record of paleofire history. Here, we apply an emerging microscopic charcoal technique in combination with established black carbon and lead pollution measurements to an array of 10 ice cores from southern to central Greenland that span recent decades. We found that microscopic charcoal deposition is highly variable among sites, with a few records suggesting recently increasing biomass burning possibly in response to growing fire activity in boreal forest ecosystems. This stands in contrast to decreasing trends in black carbon measured in the same ice cores, consistent with contributions from industrial fossil fuel emissions. Decreasing trends of lead pollution and occurrence of microscopic spheroidal carbonaceous particles (SCP), a microfossil tracer of fossil fuel emissions, further support our interpretation that black carbon in this region is influenced by industrial emissions during recent decades. We conclude that microscopic charcoal analyses in ice may help disentangle biomass burning from fossil-fuel emissions during the industrial period and have potential to contribute to better understanding of regional high-latitude fire regimes. 
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  6. Abstract

    Tropospheric reactive bromine (Bry) influences the oxidation capacity of the atmosphere by acting as a sink for ozone and nitrogen oxides. Aerosol acidity plays a crucial role in Bryabundances through acid‐catalyzed debromination from sea‐salt‐aerosol, the largest global source. Bromine concentrations in a Russian Arctic ice‐core, Akademii Nauk, show a 3.5‐fold increase from pre‐industrial (PI) to the 1970s (peak acidity, PA), and decreased by half to 1999 (present day, PD). Ice‐core acidity mirrors this trend, showing robust correlation with bromine, especially after 1940 (r = 0.9). Model simulations considering anthropogenic emission changes alone show that atmospheric acidity is the main driver of Brychanges, consistent with the observed relationship between acidity and bromine. The influence of atmospheric acidity on Bryshould be considered in interpretation of ice‐core bromine trends.

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