Search for: All records

The Denali Ice Cores were collected from the summit of Begguya (Mt. Hunter), Denali National Park, Alaska in the summer of 2013. Sampling permits were granted by Denali National Park for the drilling and removal of the ice cores. Here, we use the Cameca SX100 at the University of Maine to examine tephra particles recovered from the ice cores. 

In 2013, two parallel ice cores (commonly referred to as the Denali Ice Cores) were drilled to bedrock on the summit plateau of Begguya, Alaska (62.93 N 151.083 W, 3912 m asl; also known as Mount Hunter). A robust chronology has been developed using a combination of techniques including annual layer counting, sulfate peaks (volcanics), radiocarbon dating and the 1963 atmospheric nuclear weapons testing horizon. Here, we employed tephrochronology practices to isolate and document the presence of the Lena Ash Layer and White River Ash east (WRAe) volcanic eruptions within the ice. We separated tephra from the meltwater and analyzed them using SEM-EDS and EPMA methodologies. The data are not immediately conclusive, and work is still ongoing to understand the findings. 

The Denali Ice Cores were collected from the summit of Begguya (Mt. Hunter), Denali National Park, Alaska in the summer of 2013. Sampling permits were granted by Denali National Park for the drilling and removal of the ice cores. Here, we use the Tescan II at the University of Maine to examine tephra particles recovered from the ice cores. 

A robust chronology has been developed for the Denali Ice Cores, Begguya, Alaska (62.93 N 151.083 W, 3912 m asl (meters above sea level); also known as Mount Hunter) using a combination of techniques including annual‑layer counting, volcanics, radiocarbon dating, and the 1963 atmospheric nuclear‑weapons‑testing horizon. Radiocarbon dating confirms that there is early Holocene ice preserved at the bottom of the Denali Ice Cores. To confirm this, researchers at the University of Maine have produced oxygen‑isotope records. Examining the data from the twin cores, we see replicate isotope profiles in the bottom 8 meters of ice, showing a sharp decrease of δ^18O (oxygen‑18 isotope ratio) of nearly 6 ‰ (permil) near the bottom. To investigate whether this decrease is a climate signal or an artifact of basal‑ice dynamics, we collected trace‑element data across the oxygen‑isotope decrease. Because the basal ice of the Denali Ice Cores contains too high a sediment load to be melted and analyzed with aqueous inductively coupled plasma mass spectrometry (ICP‑MS), we analyzed Na (sodium), Mg (magnesium), Cu (copper), Pb (lead), Al (aluminum), Ca (calcium), Fe (iron), and S (sulfur) in the basal ice (207.35 m to 208.76 m depth) using laser‑ablation inductively coupled plasma mass spectrometry (LA‑ICP‑MS). The data are still being analyzed and compared with data from other methods to determine the cause of the oxygen‑isotope‑signal decrease. Researchers seeking to use this dataset should proceed with caution, as there is some evidence of contamination in the Pb and Cu analyses. 

In the North Pacific, large swings in climate, such as the so-called Little Ice Age, Medieval Climate Anomaly, and the 4.2 ka (thousand years ago) event, have all occurred during the Middle-Late Holocene, providing an opportunity to investigate the regional climate and environmental response to hemisphere-scale changes. Two surface-to-bedrock ice cores (210 meters) recovered from the Begguya plateau (Alaska) have been used to document late Holocene climate variability in the North Pacific, underpinned by an annual layer counted timescale that extends to ~800 AD (190 meters depth). Here we describe new data and approaches being used to investigate Holocene and late Pleistocene conditions on Begguya through stable water isotope analysis performed in the bottom 20 meters of the cores. We have completed a full δ18O-H2O isotope profile for both cores, showing relatively uniform values through the core section thought to contain the 4.2ka event. In contrast, a pronounced but continuous 5‰ (permil) increase in δ18O-H2O occurs approximately 2 meters above the bed. Based on the location and structure of these changes, we tentatively infer that the isotope and chemistry excursions near the bed represent the late Pleistocene-Holocene transition, and the isotope profile in that area possibly shows evidence of a climate reversal akin to the Younger Dryas. 

Abstract. Warming in high alpine regions is leading to an increase in glacier surface melt production, firn temperature, and firn liquid water content, altering regional hydrology and climate records contained in the ice. Here we use field observations and firn modeling to show that although the snowpack at Eclipse Icefield at 3000 m a.s.l. in the St. Elias Range, Yukon, Canada, remains largely dry, meltwater percolation is likely to increase with an increase in intense melt events associated with continued atmospheric warming. In particular, the development of year-round deep temperate firn at Eclipse Icefield is promoted by an increase in the number of individual melt events and in average melt event magnitude combined with warmer wintertime temperatures, rather than an earlier or prolonged melt season. Borehole temperatures indicate that from 2016 to 2023 there was a 1.67 °C warming of the firn at 14 m depth (to -3.37±0.01 °C in 2023). Results from the Community Firn Model show that warming of the firn below 10 m depth may continue over the next decade, with a 2 % chance of becoming temperate year-round at 15 m depth by 2033, even without continued atmospheric warming. Model results also show that the chance of Eclipse Icefield developing year-round temperate firn at 15 m depth by 2033 increases from 2 % with 0.1 °C atmospheric warming over the period 2023–2033 to 12 % with 0.2 °C warming, 51 % with 0.5 °C warming, and 98 % with 1 °C warming. As the majority of the St. Elias Range's glacierized terrain lies below Eclipse Icefield, the development of temperate firn at this elevation would likely indicate widespread meltwater percolation in this region and a wholesale change in its hydrological system, reducing its capacity to buffer runoff and severely limiting potential ice core sites. It is therefore urgent that a deep ice core be retrieved while the record is still intact. 

Lead (Pb) has been used in human civilization for centuries, but the quantity and source of Pb pollution released into the environment varies spatially and temporally. Ice cores and snowpits are an excellent record of past Pb use. 

Abstract. Dimethyl sulfide (DMS) is primarily emitted by marine phytoplankton and oxidized in the atmosphere to form methanesulfonic acid (MSA) and sulfate aerosols. Ice cores in regions affected by anthropogenic pollution show an industrial-era decline in MSA, which has previously been interpreted as indicating a decline in phytoplankton abundance. However, a simultaneous increase in DMS-derived sulfate (bioSO4) in a Greenland ice core suggests that pollution-driven oxidant changes caused the decline in MSA by influencing the relative production of MSA versus bioSO4. Here we use GEOS-Chem, a global chemical transport model, and a zero-dimensional box model over three time periods (preindustrial era, peak North Atlantic NOx pollution, and 21st century) to investigate the chemical drivers of industrial-era changes in MSA and bioSO4, and we examine whether four DMS oxidation mechanisms reproduce trends and seasonality in observations. We find that box model and GEOS-Chem simulations can only partially reproduce ice core trends in MSA and bioSO4 and that wide variation in model results reflects sensitivity to DMS oxidation mechanism and oxidant concentrations. Our simulations support the hypothesized increase in DMS oxidation by the nitrate radical over the industrial era, which increases bioSO4 production, but competing factors such as oxidation by BrO result in increased MSA production in some simulations, which is inconsistent with observations. To improve understanding of DMS oxidation, future work should investigate aqueous-phase chemistry, which produces 82 %–99 % of MSA and bioSO4 in our simulations, and constrain atmospheric oxidant concentrations, including the nitrate radical, hydroxyl radical, and reactive halogens. 

This project intends to use the Mount Denali ice core archive to develop the most comprehensive suite of North Pacific fire and summer climate proxy records since about 2500 years before present. Wildfire is a key component of summer climate in the North Pacific where wildfires are projected to increase with continued summer warming. Studies that combine paleorecords of summer climate and wildfire are therefore critically needed, especially in the North Pacific region where fire recurrence rate and decadal-to-centennial scale climate fluctuations occur over longer time periods than are covered by direct observations. The goal of the proposed research is to improve our understanding of relationships between summertime climate and wildfire activity, focusing especially on the Medieval Climate Anomaly (MCA), when regional temperatures were perhaps as warm as the 20th century. Recent advances now permit the measurement of new fire-related (pyrogenic) compounds in ice cores, enabling the development of a robust fire record capable of rigorous comparison with regional paleoclimate reconstructions. 

An industrial-era drop in Greenland ice core methanesulfonic acid (MSA) is thought to herald a collapse in North Atlantic marine phytoplankton stocks related to a weakening of the Atlantic Meridional Overturning Circulation. In contrast, stable levels of marine biogenic sulfur production contradict this interpretation and point to changes in atmospheric oxidation as a potential cause of the MSA decline. However, the impact of oxidation on MSA production has not been quantified, nor has this hypothesis been rigorously tested. Here we present a multi-century MSA record from the Denali, Alaska, ice core, which shows an MSA decline similar in magnitude but delayed by 93 years relative to the Greenland record. Box model results using updated chemical pathways indicate that oxidation by industrial nitrate radicals has suppressed atmospheric MSA production, explaining most of Denali’s and Greenland’s MSA declines without requiring a change in phytoplankton production. The delayed timing of the North Pacific MSA decline, relative to the North Atlantic, reflects the distinct history of industrialization in upwind regions and is consistent with the Denali and Greenland ice core nitrate records. These results demonstrate that multi-decadal trends in industrial-era Arctic ice core MSA reflect rising anthropogenic pollution rather than declining marine primary production.