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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. 

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. 

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. 

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. 

Tropospheric reactive bromine (Br_y) influences the oxidation capacity of the atmosphere by acting as a sink for ozone and nitrogen oxides. Aerosol acidity plays a crucial role in Br_yabundances 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 Br_ychanges, consistent with the observed relationship between acidity and bromine. The influence of atmospheric acidity on Br_yshould be considered in interpretation of ice‐core bromine trends.

 

Abstract. The injection of sulfur into the stratosphere by volcanic eruptions is thedominant driver of natural climate variability oninterannual to multidecadal timescales. Based on a set of continuous sulfateand sulfur records from a suite of ice cores from Greenland and Antarctica,the HolVol v.1.0 database includes estimates of the magnitudes andapproximate source latitudes of major volcanic stratospheric sulfurinjection (VSSI) events for the Holocene (from 9500 BCE or 11 500 years BP to1900 CE), constituting an extension of the previous record by 7000 years.The database incorporates new-generation ice-core aerosol records with asub-annual temporal resolution and a demonstrated sub-decadal dating accuracyand precision. By tightly aligning and stacking the ice-core records on theWD2014 chronology from Antarctica, we resolve long-standing inconsistenciesin the dating of ancient volcanic eruptions that arise from biased (i.e.,dated too old) ice-core chronologies over the Holocene for Greenland. Wereconstruct a total of 850 volcanic eruptions with injections in excess of 1 teragram of sulfur (Tg S); of these eruptions, 329 (39 %) are located in the low latitudes with bipolarsulfate deposition, 426 (50 %) are located in the Northern Hemisphere extratropics (NHET) and 88 (10 %) are located in the Southern Hemisphere extratropics (SHET). The spatial distribution of the reconstructed eruption locationsis in agreement with prior reconstructions for the past 2500 years. Intotal, these eruptions injected 7410 Tg S into thestratosphere: 70 % from tropical eruptions and 25 % from NHextratropical eruptions. A long-term latitudinally and monthly resolvedstratospheric aerosol optical depth (SAOD) time series is reconstructed fromthe HolVol VSSI estimates, representing the first Holocene-scalereconstruction constrained by Greenland and Antarctica ice cores. These newlong-term reconstructions of past VSSI and SAOD variability confirm evidencefrom regional volcanic eruption chronologies (e.g., from Iceland) in showingthat the Early Holocene (9500–7000 BCE) experienced a higher number ofvolcanic eruptions (+16 %) and cumulative VSSI (+86 %) compared withthe past 2500 years. This increase coincides with the rapid retreat of icesheets during deglaciation, providing context for potential future increasesin volcanic activity in regions under projected glacier melting in the 21stcentury. The reconstructed VSSI and SAOD data are available at https://doi.org/10.1594/PANGAEA.928646 (Sigl et al., 2021).