Douglas S. Introne

Atmospheric composition 1 million years ago from blue ice in the Allan Hills, Antarctica

Significance Bubbles of ancient air trapped in ice cores permit the direct reconstruction of atmospheric composition and allow us to link greenhouse gases and global climate over the last 800 ky. Here, we present new ice core records of atmospheric composition roughly 1 Ma from a shallow ice core drilled in the Allan Hills blue ice area, Antarctica. These records confirm that interglacial CO2 concentrations decreased by 800 ka. They also show that the link between CO2 and Antarctic temperature extended into the warmer world of the mid-Pleistocene. Here, we present direct measurements of atmospheric composition and Antarctic climate from the mid-Pleistocene (∼1 Ma) from ice cores drilled in the Allan Hills blue ice area, Antarctica. The 1-Ma ice is dated from the deficit in 40Ar relative to the modern atmosphere and is present as a stratigraphically disturbed 12-m section at the base of a 126-m ice core. The 1-Ma ice appears to represent most of the amplitude of contemporaneous climate cycles and CO2 and CH4 concentrations in the ice range from 221 to 277 ppm and 411 to 569 parts per billion (ppb), respectively. These concentrations, together with measured δD of the ice, are at the warm end of the field for glacial–interglacial cycles of the last 800 ky and span only about one-half of the range. The highest CO2 values in the 1-Ma ice fall within the range of interglacial values of the last 400 ka but are up to 7 ppm higher than any interglacial values between 450 and 800 ka. The lowest CO2 values are 30 ppm higher than during any glacial period between 450 and 800 ka. This study shows that the coupling of Antarctic temperature and atmospheric CO2 extended into the mid-Pleistocene and demonstrates the feasibility of discontinuously extending the current ice core record beyond 800 ka by shallow coring in Antarctic blue ice areas.

Discovery of a nanodiamond-rich layer in the Greenland ice sheet

We report the discovery in the Greenland ice sheet of a discrete layer of free nanodiamonds (NDs) in very high abundances, implying most likely either an unprecedented influx of extraterrestrial (ET) material or a cosmic impact event that occurred after the last glacial episode. From that layer, we extracted n-diamonds and hexagonal diamonds (lonsdaleite), an accepted ET impact indicator, at abundances of up to about 5×10 6 times background levels in adjacent younger and older ice. The NDs in the concentrated layer are rounded, suggesting they most likely formed during a cosmic impact through some process similar to carbon-vapor deposition or high-explosive detonation. This morphology has not been reported previously in cosmic material, but has been observed in terrestrial impact material. This is the first highly enriched, discrete layer of NDs observed in glacial ice anywhere, and its presence indicates that ice caps are important archives of ET events of varying magnitudes. Using a preliminary ice chronology based on oxygen isotopes and dust stratigraphy, the ND-rich layer appears to be coeval with ND abundance peaks reported at numerous North American sites in a sedimentary layer, the Younger Dryas boundary layer (YDB), dating to 12.9 ± 0.1 ka. However, more investigation is needed to confirm this association.

A coastal transect of McMurdo Dry Valleys (Antarctica) snow and firn: marine and terrestrial influences on glaciochemistry

Samples of snow and firn from accumulation zones on Clark, Commonwealth, Blue and Victoria Upper Glaciers in the McMurdo Dry Valleys (� 77-788 S, 161-1648 E), Antarctica, are evaluated chemically and isotopically to determine the relative importance of local (site-specific) factors vs regional-scale influences in defining glaciochemistry. Spatial variation in snow and firn chemistry confirms documented trends within individual valleys regarding major-ion deposition relative to elevation and to distance from the coast. Sodium and methylsulfonate (MS - ), for example, follow a decreasing gradient with distance from the coast along the axis of Victoria Valley (350-119 gL -1 for Na + ; 33-14 gL -1 for MS - ); a similar pattern exists between Commonwealth and Newall Glaciers in the Asgaard Range. When comparing major-ion concentrations (e.g. Na + ,M S - ,C a 2+ ) or trace metals (e.g. Al, Fe) among different valleys, however, site-specific exposures to marine and local terrestrial chemical sources play a dominant role. Because chemical signals at all sites respond to particulates with varying mixtures of marine and terrestrial sources, each of these influences on site glaciochemistry must be considered when drawing temporal climate inferences on regional scales.

Industrial-age doubling of snow accumulation in the Alaska Range linked to tropical ocean warming

Future precipitation changes in a warming climate depend regionally upon the response of natural climate modes to anthropogenic forcing. North Pacific hydroclimate is dominated by the Aleutian Low, a semi-permanent wintertime feature characterized by frequent low-pressure conditions that is influenced by tropical Pacific Ocean temperatures through the Pacific-North American (PNA) teleconnection pattern. Instrumental records show a recent increase in coastal Alaskan precipitation and Aleutian Low intensification, but are of insufficient length to accurately assess low frequency trends and forcing mechanisms. Here we present a 1200-year seasonally- to annually-resolved ice core record of snow accumulation from Mt. Hunter in the Alaska Range developed using annual layer counting and four ice-flow thinning models. Under a wide range of glacier flow conditions and layer counting uncertainty, our record shows a doubling of precipitation since ~1840 CE, with recent values exceeding the variability observed over the past millennium. The precipitation increase is nearly synchronous with the warming of western tropical Pacific and Indian Ocean sea surface temperatures. While regional 20th Century warming may account for a portion of the observed precipitation increase on Mt. Hunter, the magnitude and seasonality of the precipitation change indicate a long-term strengthening of the Aleutian Low.

A 400‐Year Ice Core Melt Layer Record of Summertime Warming in the Alaska Range

Warming in high-elevation regions has societally important impacts on glacier mass balance, water resources, and sensitive alpine ecosystems, yet very few high-elevation temperature records exist from the middle or high latitudes. While a variety of paleoproxy records provide critical temperature records from low elevations over recent centuries, melt layers preserved in alpine glaciers present an opportunity to develop calibrated, annually resolved temperature records from high elevations. Here we present a 400-year temperature proxy record based on the melt layer stratigraphy of two ice cores collected from Mt. Hunter in Denali National Park in the central Alaska Range. The ice core record shows a sixtyfold increase in water equivalent total annual melt between the preindustrial period (before 1850 Common Era) and present day.We calibrate themelt record to summer temperatures based onweather station data from the ice core drill site and find that the increase inmelt production represents a summer warming rate of at least 1.92 ± 0.31°C per century during the last 100 years, exceeding rates of temperature increase at most low-elevation sites in Alaska. The Mt. Hunter melt layer record is significantly (p< 0.05) correlated with surface temperatures in the central tropical Pacific through a Rossby wave-like pattern that enhances high temperatures over Alaska. Our results show that rapid alpine warming has taken place in the Alaska Range for at least a century and that conditions in the tropical oceans contribute to this warming. Plain Language Summary Warming in mountainous areas affects glacier melt, water resources, and fragile ecosystems, yet we know relatively little about climate change in alpine areas, especially at high latitudes. We use ice cores drilled on Mt. Hunter, in Denali National Park, to develop a record of summer temperatures in Alaska that extends 400 years into the past, farther than any other mountain record in the North Pacific region. The ice core record shows that 60 times more snowmelt occurs today than 150 years ago. This corresponds to roughly a 2°C increase in summer temperature, which is faster than summertime warming in Alaska near sea level. We suggest that warming of the tropical Pacific Ocean has contributed to the rapid warming on Mt. Hunter by enhancing high-pressure systems over Alaska. Our ice core record indicates that alpine regions surrounding the North Pacific may continue to experience accelerated warming with climate change, threatening the already imperiled glaciers in this area.

Denali Ice Core Methanesulfonic Acid Records North Pacific Marine Primary Production

The high-nutrient, low-chlorophyll region of the northeastern (NE) subarctic Pacific is one of the most biologically productive marine ecosystems in the world, supporting fisheries worth over

Coupled North Atlantic slope water forcing on Gulf of Maine temperatures over the past millennium

5 billion annually. Phytoplankton are the primary producers in this ecosystem and are also a major source of biogenic sulfur emissions, important in Earth’s climate system. However, variability in marine primary production through time is not well constrained. Here we establish methanesulfonic acid (MSA) concentrations in the Denali ice core as a proxy for marine primary production in the NE Pacific. Using Hybrid Single-Particle Lagrangian Integrated Trajectory (HYSPLIT; Stein et al., 2015, https://doi.org/10.1178/BAMS-D-14-00110.1) modeling, we identify moisture source regions for the core site and correlate Sea-Viewing Wide Field-of-View Sensor-derived chlorophyll a concentrations with ice core MSA. From 1998 to 2007 we find that areas of significant positive correlation overlap with the HYSPLIT-inferred moisture source region in the western Gulf of Alaska on an annual basis (r = 0.85, p < 0.001). We identify an MSA response to a localized bloom related to ash deposition from a 2009 Mt. Redoubt eruption. An anomalous upwelling-driven bloom in spring 2008 did not impact the ice core MSA record due to unfavorable transport conditions. Despite this, we observe that bloom events are rarely missed in the MSA record, which we attribute to the consistent and high snow accumulation rate at the ice core drill site. Our findings suggest that Denali ice core MSA is a reliable recorder of changes in marine primary production through time in the NE subarctic Pacific. Plain Language Summary The base of the marine food web is composed of single-celled photosynthetic organisms that are collectively termed primary producers. Because these microscopic organisms support all marine life, changes in their biomass can impact the entire food web. Over the past three decades, satellite data have shown that primary producers are declining around the world with some of the greatest declines occurring in the North Pacific Ocean. The reasons for these declines may include changes in ocean temperatures, nutrient availability, and wind-driven ocean mixing, all of which are related to climate. To place these changes within a longer-term context, we seek to validate regionally a proxy tool by measuring a chemical produced by phytoplankton called methanesulfonic acid (MSA). MSA is transported through the atmosphere by storms and deposited on mountain glaciers in the North Pacific region. WemeasuredMSA in a new ice core fromDenali National Park, Alaska. We describe strong, statistically significant correlations between ice core MSA concentrations and chlorophyll concentrations in the western Gulf of Alaska. We suggest that the ice core MSA proxy record can help us understand how primary production in this region has changed through time.