Sophia M. Wensman

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.

Studying coastal ecogeomorphic change relies on reliably and accurately dating recent sediment deposits. While a handful of short-lived radioisotopes are proven suitable geochronometers, all have limitations. One particularly useful anthropogenic radionuclide, ¹³⁷ Cs, is rapidly approaching extinction due to its half-life. We evaluate whether the bomb-produced radionuclide ²⁴¹ Am is a reliable alternative to ¹³⁷ Cs. In 75 cores from eight Oregon intertidal zones, ²⁴¹ Am was readily detectable in higher-elevation marsh sediments and, when present, was less mobile post-deposition than ¹³⁷ Cs. Accretion derived from the depth of the ²⁴¹ Am and ¹³⁷ Cs peaks associated with the height of nuclear proliferation were statistically similar to rates determined from excess ²¹⁰ Pb. Although ²⁴¹ Am activities are relatively low in these environments, their detectability is expected to increase as ²⁴¹ Pu continues to decay. ²⁴¹ Am may therefore be an accurate event-horizon chronometer in high marsh sediments, and will likely become an increasingly preferable dating technique compared to ¹³⁷ Cs geochronology.

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.