Nathan Chellman

Dansgaard-Oeschger (D-O) events refer to a series of rapid climate fluctuations in the Northern Hemisphere during the last glacial period. These fluctuations are recorded in Greenland ice cores by large, abrupt changes in both water isotopes and aerosol deposition, but the processes driving these changes remain unclear. Here we present new high-resolution, multi-proxy measurements from the Greenland GISP2 ice core spanning eight D-O events. In addition to multiple dust and sea-salt tracers, we provide the first high-resolution biomass burning proxy records from refractory black carbon (rBC) concentration and particle size during these transitions. Bayesian ramp fitting analysis shows that water-isotope and aerosol changes generally occurred within a narrow temporal window with no robust and reproducible sequence. Using a simplified aerosol transport and deposition framework, we find that changes in atmospheric transport and removal processes can explain much of the variability shared among aerosols from distinct source regions. Residual variability shows stronger relationships within than across source classes and, particularly for dust, differs between cold and warm climate states. These results suggest that rapid large-scale atmospheric and hydrological reorganization drove the common aerosol response, while changes in individual source systems contributed to the remaining variability. These new multi-proxy records provide valuable constraints for evaluating climate-model representations of abrupt climate transitions.

Paleoclimate reconstructions of wildfire using ice cores often rely on high‐resolution aerosol records of refractory black carbon (rBC) or ammonium (NH ₄ ⁺ ). The interpretation of such records, however, can be hindered by uncertainties related to long‐range atmospheric transport, spatial distribution and strength of emissions, as well as glaciochemical noise introduced to the record during and after deposition. Here, we use arrays of parallel ice cores collected from two sites in Greenland, each with differing characteristics including accumulation rate, elevation, and meteorology, to assess the spatial variability and site representativeness of their rBC and NH ₄ ⁺ records. The higher accumulation site, Summit, was found to preserve a more coherent signal of both aerosols across multiple ice core records and was more representative of the total site signal than the lower accumulation site, Tunu. Representativeness analyses indicate that a single core at Summit is as representative of the theoretical total site signal as a combined three core composite record from Tunu for rBC and NH ₄ ⁺ at annual resolution. This work suggests that high temporal resolution records of these aerosols using a single core from either site have considerable uncertainties, highlighting the need for careful consideration of the effects of site conditions on aerosol representativeness for wildfire reconstruction.

Transport of Saharan dust over Europe impacts climate and nutrient input to ecosystems, but its variability over the past is poorly documented and understood. Because Alpine ice potentially provides an archive of past dust deposition, here we used an ice core record of stable water isotopes and dust‐related chemical species from Dôme du Goûter (DDG, French Alps) to assess past climate and dust deposition in western Europe over the past 8,000 years. The DDG chemical records including calcium, a potential good proxy of Saharan dust, show a dust increase associated with Saharan aridification following the African Humid Period some 3,600 years ago. The more recent records spanning the last two millennia indicate warmer, drier, and dustier conditions during the Roman, Medieval, and Modern periods than during the Little Ice Ages, thereby elucidating a generally consistent relationship between Saharan dust deposition over western Europe and past climate.

Black carbon (BC) and iron oxide (FeO_x) aerosols represent important contributors to shortwave atmospheric heating and have been associated with adverse human health outcomes. Accurate measurement of BC and FeO_x is crucial for understanding their roles in climate forcing and air quality. However, observational data─particularly for anthropogenic FeO_x─remain limited. In this study, we use a modified Single Particle Soot Photometer to measure atmospheric concentrations of refractory BC (rBC) and FeO_x particles in urban (Atlanta, GA) and rural (Boone, NC) environments in the southeastern U.S. We identify likely emission sources and estimate light absorption properties to assess the relative contributions of rBC and FeO_x to atmospheric heating. Our findings indicate that rBC and FeO_x concentrations and absorption are substantially higher in the urban environment, reflecting significant anthropogenic contributions to pollutant emissions. Assuming external mixing, we estimate that FeO_x contributes 1.3% of total absorption by rBC at the urban site and approximately 0.6% at the rural site. These results highlight the spatial variability of light-absorbing aerosol components and the importance of localized assessments to better understand their impacts on air quality and climate.

Light absorbing impurities, such as black carbon (BC) and mineral dust (MD) particles, are important constituents of seasonal snowpacks, especially when they are present near the surface where they can influence snow albedo and energy balance. Such impurities can be directly deposited or accumulate on the snow surface during melt processes. While soluble chemical species are known to rapidly flush from snowpacks during melt, the behavior of BC and MD particles, which mostly consist of insoluble species, is not well constrained. Previous work has shown that BC particles can either be concentrated at the snow surface or flushed from the snowpack under different conditions. Here we develop a comprehensive data set of BC and MD particles within and eluting from a seasonal Sierra Nevada, CA, USA snowpack by combining timeseries of snow profile measurements with time‐resolved characterization of meltwater captured by snow lysimeters. Together, the snow and lysimeter measurements indicate that BC and MD particles are retained until the final few days of snowmelt, when they are rapidly flushed from the remaining, shallow snow column. During the final melt, BC and MD concentrations and mass fluxes closely parallel melt rates, indicating that snowpack properties and melt rates are the main controls on how these impurities behave in a given snowpack.

Ice cores from Mt. Logan, the second highest peak in North America located in the St. Elias mountains in southwest Yukon, Canada, have provided conflicting accumulation records, thus the hydroclimate response to changing atmospheric conditions in the highest elevation regions is not well constrained. Here, we present the accumulation record from the new 325 m Mt. Logan ice core drilled at 5,334 m asl on the summit plateau in May 2022. Multi‐parameter annual layer counting, confirmed with radionuclide and volcanic tephra measurements, extends to 1911 CE, associated with a depth of 257 m. The thinning‐corrected annual accumulation record reveals an average rate of 3.0 m water equivalent per year (m weq a ⁻¹ ) from 1912 to 2020 CE, greater than six times higher than the previous estimate of ∼0.41 m weq a ⁻¹ from the 2002 Mt. Logan Prospector Russell Col core. Correlation analysis between the annual accumulation record and regional climate data sets (e.g., Japanese 55‐year Reanalysis, weather stations) reveal a strong positive relationship with warm‐season total precipitable water and temperature. Thus, we suggest interdecadal precipitation variability on Mt. Logan is at least partially driven by warm‐season atmospheric water vapor loading, potentially related to atmospheric temperature responses associated with the warm‐season Alaska Blocking Index. Further, the record reveals a statistically significant increase in accumulation of 0.13 m weq per decade since 1970. These results reveal a drastically different Mt. Logan ice core record and provide a new warm‐season perspective on drivers of high‐elevation accumulation variability in the North Pacific.

Aerosol radiative forcing is an important but often poorly understood component of regional climate. While glacier ice contains the most detailed archives of past atmospheric aerosol composition and temperature, no well-preserved ice records extending into the last climatic transition have been reported for the historically important European region. Here, we use an Alpine ice core to document changes in European aerosols and climate from the end of the last glacial age (LGA) through the Holocene. The core was drilled on a glacier dome in the French Alps called the Dôme du Goûter (DDG), and it provides a stratigraphically intact record of aerosol and climate extending to at least 12 kyears (ky) before present. Although dating near the base of the glacier is not well constrained, the oldest DDG ice layers reflect glacial conditions in western Europe during the LGA. In addition to changes in atmospheric transport, increased sea-salt and dust deposition in western Europe recorded in the LGA ice suggest enhanced westerly winds and more active dust sources, possibly including North Africa. Deposition of terrestrial biogenic indicators during the cold LGA climate was lower, however, consistent with strongly reduced European vegetation. The DDG record of terrestrial biogenic emissions also suggests a decline of European forests throughout the Holocene, resulting from deterioration of climatic conditions and more recently from establishment of the first agricultural societies. The pronounced changes in atmospheric aerosol recorded in Alpine ice imply large variations in aerosol radiative forcing in western Europe during the last 12 ky.

Ancient texts and archaeological evidence indicate substantial lead exposure during antiquity that potentially impacted human health. Although lead exposure routes were many and included the use of glazed tablewares, paints, cosmetics, and even intentional ingestion, the most significant for the nonelite, rural majority of the population may have been through background air pollution from mining and smelting of silver and lead ores that underpinned the Roman economy. Here, we determined potential health effects of this air pollution using Arctic ice core measurements of Roman-era lead pollution, atmospheric modeling, and modern epidemiology-based relationships between air concentrations, blood lead levels (BLLs), and cognitive decline. Findings suggest air lead concentrations exceeded 150 ng/m ³ near metallurgical emission sources, with average enhancements of >1.0 ng/m ³ over Europe during the Pax Romana apogee of the Roman Empire. The result was blood lead enhancements in young children of about 2.4 µg/dl above an estimated Neolithic background of 1.0 µg/dl, leading to widespread cognitive decline including a 2.5-to-3 point reduction in intelligence quotient throughout the Roman Empire.

Polar ice cores and historical records evidence a large-magnitude volcanic eruption in 1831 CE. This event was estimated to have injected ~13 Tg of sulfur (S) into the stratosphere which produced various atmospheric optical phenomena and led to Northern Hemisphere climate cooling of ~1 °C. The source of this volcanic event remains enigmatic, though one hypothesis has linked it to a modest phreatomagmatic eruption of Ferdinandea in the Strait of Sicily, which may have emitted additional S through magma–crust interactions with evaporite rocks. Here, we undertake a high-resolution multiproxy geochemical analysis of ice-core archives spanning the 1831 CE volcanic event. S isotopes confirm a major Northern Hemisphere stratospheric eruption but, importantly, rule out significant contributions from external evaporite S. In multiple ice cores, we identify cryptotephra layers of low K andesite-dacite glass shards occurring in summer 1831 CE and immediately prior to the stratospheric S fallout. This tephra matches the chemistry of the youngest Plinian eruption of Zavaritskii, a remote nested caldera on Simushir Island (Kurils). Radiocarbon ages confirm a recent (−2 ) is comparable to the 1991 CE Pinatubo eruption and can readily account for the climate cooling in 1831–1833 CE. These data provide compelling evidence that Zavaritskii was the source of the 1831 CE mystery eruption and solve a confounding case of multiple closely spaced observed and unobserved volcanic eruptions.