Publications by authors named "Joseph R McConnell"

31 Publications

Hemispheric black carbon increase after the 13th-century Māori arrival in New Zealand.

Nature 2021 10 6;598(7879):82-85. Epub 2021 Oct 6.

Instituto Antártico Argentino, Centro Regional de Investigaciones Cientifícas y Teconológicas, Mendoza, Argentina.

New Zealand was among the last habitable places on earth to be colonized by humans. Charcoal records indicate that wildfires were rare prior to colonization and widespread following the 13th- to 14th-century Māori settlement, but the precise timing and magnitude of associated biomass-burning emissions are unknown, as are effects on light-absorbing black carbon aerosol concentrations over the pristine Southern Ocean and Antarctica. Here we used an array of well-dated Antarctic ice-core records to show that while black carbon deposition rates were stable over continental Antarctica during the past two millennia, they were approximately threefold higher over the northern Antarctic Peninsula during the past 700 years. Aerosol modelling demonstrates that the observed deposition could result only from increased emissions poleward of 40° S-implicating fires in Tasmania, New Zealand and Patagonia-but only New Zealand palaeofire records indicate coincident increases. Rapid deposition increases started in 1297 (±30 s.d.) in the northern Antarctic Peninsula, consistent with the late 13th-century Māori settlement and New Zealand black carbon emissions of 36 (±21 2 s.d.) Gg y during peak deposition in the 16th century. While charcoal and pollen records suggest earlier, climate-modulated burning in Tasmania and southern Patagonia, deposition in Antarctica shows that black carbon emissions from burning in New Zealand dwarfed other preindustrial emissions in these regions during the past 2,000 years, providing clear evidence of large-scale environmental effects associated with early human activities across the remote Southern Hemisphere.
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http://dx.doi.org/10.1038/s41586-021-03858-9DOI Listing
October 2021

North Atlantic jet stream projections in the context of the past 1,250 years.

Proc Natl Acad Sci U S A 2021 09;118(38)

Division of Hydrologic Sciences, Desert Research Institute, Reno, NV 89512.

Reconstruction of the North Atlantic jet stream (NAJ) presents a critical, albeit largely unconstrained, paleoclimatic target. Models suggest northward migration and changing variance of the NAJ under 21st-century warming scenarios, but assessing the significance of such projections is hindered by a lack of long-term observations. Here, we incorporate insights from an ensemble of last-millennium water isotope-enabled climate model simulations and a wide array of mean annual water isotope ([Formula: see text]O) and annually accumulated snowfall records from Greenland ice cores to reconstruct North Atlantic zonal-mean zonal winds back to the 8th century CE. Using this reconstruction we provide preobservational constraints on both annual mean NAJ position and intensity to show that late 20th- and early 21st-century NAJ variations were likely not unique relative to natural variability. Rather, insights from our 1,250 year reconstruction highlight the overwhelming role of natural variability in thus far masking the response of midlatitude atmospheric dynamics to anthropogenic forcing, consistent with recent large-ensemble transient modeling experiments. This masking is not projected to persist under high greenhouse gas emissions scenarios, however, with model projected annual mean NAJ position emerging as distinct from the range of reconstructed natural variability by as early as 2060 CE.
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http://dx.doi.org/10.1073/pnas.2104105118DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8463874PMC
September 2021

Improved estimates of preindustrial biomass burning reduce the magnitude of aerosol climate forcing in the Southern Hemisphere.

Sci Adv 2021 May 28;7(22). Epub 2021 May 28.

Division of Hydrologic Sciences, Desert Research Institute, Reno, NV 89512, USA.

Fire plays a pivotal role in shaping terrestrial ecosystems and the chemical composition of the atmosphere and thus influences Earth's climate. The trend and magnitude of fire activity over the past few centuries are controversial, which hinders understanding of preindustrial to present-day aerosol radiative forcing. Here, we present evidence from records of 14 Antarctic ice cores and 1 central Andean ice core, suggesting that historical fire activity in the Southern Hemisphere (SH) exceeded present-day levels. To understand this observation, we use a global fire model to show that overall SH fire emissions could have declined by 30% over the 20th century, possibly because of the rapid expansion of land use for agriculture and animal production in middle to high latitudes. Radiative forcing calculations suggest that the decreasing trend in SH fire emissions over the past century largely compensates for the cooling effect of increasing aerosols from fossil fuel and biofuel sources.
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http://dx.doi.org/10.1126/sciadv.abc1379DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC8163089PMC
May 2021

The magnitude and impact of the 431 CE Tierra Blanca Joven eruption of Ilopango, El Salvador.

Proc Natl Acad Sci U S A 2020 10 28;117(42):26061-26068. Epub 2020 Sep 28.

Gerencia de Geología del Observatorio Ambiental, Ministerio de Medio Ambiente y Recursos Naturales, San Salvador 76230, El Salvador.

The Tierra Blanca Joven (TBJ) eruption from Ilopango volcano deposited thick ash over much of El Salvador when it was inhabited by the Maya, and rendered all areas within at least 80 km of the volcano uninhabitable for years to decades after the eruption. Nonetheless, the more widespread environmental and climatic impacts of this large eruption are not well known because the eruption magnitude and date are not well constrained. In this multifaceted study we have resolved the date of the eruption to 431 ± 2 CE by identifying the ash layer in a well-dated, high-resolution Greenland ice-core record that is >7,000 km from Ilopango; and calculated that between 37 and 82 km of magma was dispersed from an eruption coignimbrite column that rose to ∼45 km by modeling the deposit thickness using state-of-the-art tephra dispersal methods. Sulfate records from an array of ice cores suggest stratospheric injection of 14 ± 2 Tg S associated with the TBJ eruption, exceeding those of the historic eruption of Pinatubo in 1991. Based on these estimates it is likely that the TBJ eruption produced a cooling of around 0.5 °C for a few years after the eruption. The modeled dispersal and higher sulfate concentrations recorded in Antarctic ice cores imply that the cooling would have been more pronounced in the Southern Hemisphere. The new date confirms the eruption occurred within the Early Classic phase when Maya expanded across Central America.
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http://dx.doi.org/10.1073/pnas.2003008117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7584997PMC
October 2020

Comparison of co-located ice-core and tree-ring mercury records indicates potential radial translocation of mercury in whitebark pine.

Sci Total Environ 2020 Nov 4;743:140695. Epub 2020 Jul 4.

Desert Research Institute, Division of Hydrologic Sciences, Reno, NV 89512, United States.

Tree-ring records are a potential archive for reconstructing long-term historical trends in atmospheric mercury (Hg) concentrations. Although Hg preserved in tree rings has been shown to be derived largely from the atmosphere, quantitative relationships linking atmospheric concentrations to those in tree rings are limited. In addition, few tree-ring-based Hg records have been evaluated against co-located proxies of atmospheric Hg deposition or direct atmospheric measurements. Here we develop long-term Hg records extending from 1800 to 2018 CE using cores collected from two stands of whitebark pine located near the Upper Fremont Glacier in the Wind River Range, Wyoming, where a long-term record of atmospheric Hg deposition previously was developed from an ice core. The tree ring record showed that Hg concentrations increased beginning in 1800 CE to a broad peak centered at ~1960 CE, before decreasing to present, generally paralleling the ice-core record of Hg deposition. The exact timing and magnitude of the Hg increases in the trees, however, is offset earlier relative to the ice-core record. These discrepancies potentially arise from biotic processes that impact Hg uptake and preservation in whitebark pine, and results from an advection-diffusion model indicate that the temporal differences are consistent with radial movement of Hg within the trees. The forms of atmospheric Hg and seasonality may also impact the Hg record preserved by each archive, but are less likely to affect long-term trends. Further work is needed to assess radial Hg translocation in more controlled studies with larger sample sizes.
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http://dx.doi.org/10.1016/j.scitotenv.2020.140695DOI Listing
November 2020

Extreme climate after massive eruption of Alaska's Okmok volcano in 43 BCE and effects on the late Roman Republic and Ptolemaic Kingdom.

Proc Natl Acad Sci U S A 2020 07 22;117(27):15443-15449. Epub 2020 Jun 22.

Physics of Ice, Climate, and Earth, University of Copenhagen, 1017 Copenhagen, Denmark.

The assassination of Julius Caesar in 44 BCE triggered a power struggle that ultimately ended the Roman Republic and, eventually, the Ptolemaic Kingdom, leading to the rise of the Roman Empire. Climate proxies and written documents indicate that this struggle occurred during a period of unusually inclement weather, famine, and disease in the Mediterranean region; historians have previously speculated that a large volcanic eruption of unknown origin was the most likely cause. Here we show using well-dated volcanic fallout records in six Arctic ice cores that one of the largest volcanic eruptions of the past 2,500 y occurred in early 43 BCE, with distinct geochemistry of tephra deposited during the event identifying the Okmok volcano in Alaska as the source. Climate proxy records show that 43 and 42 BCE were among the coldest years of recent millennia in the Northern Hemisphere at the start of one of the coldest decades. Earth system modeling suggests that radiative forcing from this massive, high-latitude eruption led to pronounced changes in hydroclimate, including seasonal temperatures in specific Mediterranean regions as much as 7 °C below normal during the 2 y period following the eruption and unusually wet conditions. While it is difficult to establish direct causal linkages to thinly documented historical events, the wet and very cold conditions from this massive eruption on the opposite side of Earth probably resulted in crop failures, famine, and disease, exacerbating social unrest and contributing to political realignments throughout the Mediterranean region at this critical juncture of Western civilization.
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http://dx.doi.org/10.1073/pnas.2002722117DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7354934PMC
July 2020

Pervasive Arctic lead pollution suggests substantial growth in medieval silver production modulated by plague, climate, and conflict.

Proc Natl Acad Sci U S A 2019 07 8;116(30):14910-14915. Epub 2019 Jul 8.

Centre for Ice and Climate, University of Copenhagen, DK-1017 Copenhagen, Denmark.

Lead pollution in Arctic ice reflects large-scale historical changes in midlatitude industrial activities such as ancient lead/silver production and recent fossil fuel burning. Here we used measurements in a broad array of 13 accurately dated ice cores from Greenland and Severnaya Zemlya to document spatial and temporal changes in Arctic lead pollution from 200 BCE to 2010 CE, with interpretation focused on 500 to 2010 CE. Atmospheric transport modeling indicates that Arctic lead pollution was primarily from European emissions before the 19th-century Industrial Revolution. Temporal variability was surprisingly similar across the large swath of the Arctic represented by the array, with 250- to 300-fold increases in lead pollution observed from the Early Middle Ages to the 1970s industrial peak. Superimposed on these exponential changes were pronounced, multiannual to multidecadal variations, marked by increases coincident with exploitation of new mining regions, improved technologies, and periods of economic prosperity; and decreases coincident with climate disruptions, famines, major wars, and plagues. Results suggest substantial overall growth in lead/silver mining and smelting emissions-and so silver production-from the Early through High Middle Ages, particularly in northern Europe, with lower growth during the Late Middle Ages into the Early Modern Period. Near the end of the second plague pandemic (1348 to ∼1700 CE), lead pollution increased sharply through the Industrial Revolution. North American and European pollution abatement policies have reduced Arctic lead pollution by >80% since the 1970s, but recent levels remain ∼60-fold higher than at the start of the Middle Ages.
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http://dx.doi.org/10.1073/pnas.1904515116DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6660774PMC
July 2019

Method for Correcting Continuous Ice-Core Elemental Measurements for Under-Recovery.

Environ Sci Technol 2019 05 9;53(10):5887-5894. Epub 2019 May 9.

Alfred Wegener Institute , Am Alten Hafen 26 , 27568 Bremerhaven , Germany.

Measurement of elemental concentrations in ice cores are critical for determining atmospheric aerosol variations. For such measurements, acidified ice-core meltwater typically is analyzed continuously (<5 min after acidification) or discretely (∼3 months after acidification). The reduced acidification time during continuous analysis may result in a measured elemental concentration that is lower than the concentration of discrete analysis if particulates are not fully dissolved. To evaluate this, sections of three ice cores from Greenland and Antarctica were measured both continuously (4.5 min after acidification) and discretely (repeatedly from 1 to 151 days after continuous measurements), with discrete samples collected from the meltwater sample stream prior to continuous measurement. We show that elements such as Na, Sr, and S dissolved readily and therefore were fully recovered during continuous measurements. Average recovery for other elements was between 70 to 100% for Cd, Gd, Mg, Mn, U, and Yb, 50 to 90% for Ca, Ce, Sm, and V, and less than 50% for Al, Fe, and La. Given the advantages of continuous measurements, we conclude that the preferred method for ice-core measurements is continuous analysis with simultaneous discrete sample collection, followed by adjustment of the continuous measurements based on discrete sample analysis at least 3 months after acidification.
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http://dx.doi.org/10.1021/acs.est.9b00199DOI Listing
May 2019

Industrial-era decline in subarctic Atlantic productivity.

Nature 2019 05 6;569(7757):551-555. Epub 2019 May 6.

Department of Earth System Science, University of California, Irvine, Irvine, CA, USA.

Marine phytoplankton have a crucial role in the modulation of marine-based food webs, fishery yields and the global drawdown of atmospheric carbon dioxide. However, owing to sparse measurements before satellite monitoring in the twenty-first century, the long-term response of planktonic stocks to climate forcing is unknown. Here, using a continuous, multi-century record of subarctic Atlantic marine productivity, we show that a marked 10 ± 7% decline in net primary productivity has occurred across this highly productive ocean basin over the past two centuries. We support this conclusion by the application of a marine-productivity proxy, established using the signal of the planktonic-derived aerosol methanesulfonic acid, which is commonly identified across an array of Greenlandic ice cores. Using contemporaneous satellite-era observations, we demonstrate the use of this signal as a robust and high-resolution proxy for past variations in spatially integrated marine productivity. We show that the initiation of declining subarctic Atlantic productivity broadly coincides with the onset of Arctic surface warming, and that productivity strongly covaries with regional sea-surface temperatures and basin-wide gyre circulation strength over recent decades. Taken together, our results suggest that the decline in industrial-era productivity may be evidence of the predicted collapse of northern Atlantic planktonic stocks in response to a weakened Atlantic Meridional Overturning Circulation. Continued weakening of this Atlantic Meridional Overturning Circulation, as projected for the twenty-first century, may therefore result in further productivity declines across this globally relevant region.
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http://dx.doi.org/10.1038/s41586-019-1181-8DOI Listing
May 2019

Four-fold increase in solar forcing on snow in western U.S. burned forests since 1999.

Nat Commun 2019 05 2;10(1):2026. Epub 2019 May 2.

Geological Sciences and Engineering, University of Nevada, Reno, 1664 N. Virginia Street, Reno, NV, 89557, USA.

Forest fires are increasing across the American West due to climate warming and fire suppression. Accelerated snow melt occurs in burned forests due to increased light transmission through the canopy and decreased snow albedo from deposition of light-absorbing impurities. Using satellite observations, we document up to an annual 9% growth in western forests burned since 1984, and 5 day earlier snow disappearance persisting for >10 years following fire. Here, we show that black carbon and burned woody debris darkens the snowpack and lowers snow albedo for 15 winters following fire, using measurements of snow collected from seven forested sites that burned between 2002 and 2016. We estimate a 372 to 443% increase in solar energy absorbed by snowpacks occurred beneath charred forests over the past two decades, with enhanced post-fire radiative forcing in 2018 causing earlier melt and snow disappearance in > 11% of forests in the western seasonal snow zone.
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http://dx.doi.org/10.1038/s41467-019-09935-yDOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6497640PMC
May 2019

Nonlinear rise in Greenland runoff in response to post-industrial Arctic warming.

Nature 2018 12 5;564(7734):104-108. Epub 2018 Dec 5.

Institute for Marine and Atmospheric Research, Utrecht University, Utrecht, Netherlands.

The Greenland ice sheet (GrIS) is a growing contributor to global sea-level rise, with recent ice mass loss dominated by surface meltwater runoff. Satellite observations reveal positive trends in GrIS surface melt extent, but melt variability, intensity and runoff remain uncertain before the satellite era. Here we present the first continuous, multi-century and observationally constrained record of GrIS surface melt intensity and runoff, revealing that the magnitude of recent GrIS melting is exceptional over at least the last 350 years. We develop this record through stratigraphic analysis of central west Greenland ice cores, and demonstrate that measurements of refrozen melt layers in percolation zone ice cores can be used to quantifiably, and reproducibly, reconstruct past melt rates. We show significant (P < 0.01) and spatially extensive correlations between these ice-core-derived melt records and modelled melt rates and satellite-derived melt duration across Greenland more broadly, enabling the reconstruction of past ice-sheet-scale surface melt intensity and runoff. We find that the initiation of increases in GrIS melting closely follow the onset of industrial-era Arctic warming in the mid-1800s, but that the magnitude of GrIS melting has only recently emerged beyond the range of natural variability. Owing to a nonlinear response of surface melting to increasing summer air temperatures, continued atmospheric warming will lead to rapid increases in GrIS runoff and sea-level contributions.
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http://dx.doi.org/10.1038/s41586-018-0752-4DOI Listing
December 2018

Abrupt ice-age shifts in southern westerly winds and Antarctic climate forced from the north.

Nature 2018 11 28;563(7733):681-685. Epub 2018 Nov 28.

Department of Earth and Space Science, University of Washington, Seattle, WA, USA.

The mid-latitude westerly winds of the Southern Hemisphere play a central role in the global climate system via Southern Ocean upwelling, carbon exchange with the deep ocean, Agulhas leakage (transport of Indian Ocean waters into the Atlantic) and possibly Antarctic ice-sheet stability. Meridional shifts of the Southern Hemisphere westerly winds have been hypothesized to occur in parallel with the well-documented shifts of the intertropical convergence zone in response to Dansgaard-Oeschger (DO) events- abrupt North Atlantic climate change events of the last ice age. Shifting moisture pathways to West Antarctica are consistent with this view but may represent a Pacific teleconnection pattern forced from the tropics. The full response of the Southern Hemisphere atmospheric circulation to the DO cycle and its impact on Antarctic temperature remain unclear. Here we use five ice cores synchronized via volcanic markers to show that the Antarctic temperature response to the DO cycle can be understood as the superposition of two modes: a spatially homogeneous oceanic 'bipolar seesaw' mode that lags behind Northern Hemisphere climate by about 200 years, and a spatially heterogeneous atmospheric mode that is synchronous with abrupt events in the Northern Hemisphere. Temperature anomalies of the atmospheric mode are similar to those associated with present-day Southern Annular Mode variability, rather than the Pacific-South American pattern. Moreover, deuterium-excess records suggest a zonally coherent migration of the Southern Hemisphere westerly winds over all ocean basins in phase with Northern Hemisphere climate. Our work provides a simple conceptual framework for understanding circum-Antarctic temperature variations forced by abrupt Northern Hemisphere climate change. We provide observational evidence of abrupt shifts in the Southern Hemisphere westerly winds, which have previously documented ramifications for global ocean circulation and atmospheric carbon dioxide. These coupled changes highlight the necessity of a global, rather than a purely North Atlantic, perspective on the DO cycle.
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http://dx.doi.org/10.1038/s41586-018-0727-5DOI Listing
November 2018

Alpine ice evidence of a three-fold increase in atmospheric iodine deposition since 1950 in Europe due to increasing oceanic emissions.

Proc Natl Acad Sci U S A 2018 11 12;115(48):12136-12141. Epub 2018 Nov 12.

Wolfson Atmospheric Chemistry Laboratories, Department of Chemistry, University of York, York, YO10 5DD, United Kingdom.

Iodine is an important nutrient and a significant sink of tropospheric ozone, a climate-forcing gas and air pollutant. Ozone interacts with seawater iodide, leading to volatile inorganic iodine release that likely represents the largest source of atmospheric iodine. Increasing ozone concentrations since the preindustrial period imply that iodine chemistry and its associated ozone destruction is now substantially more active. However, the lack of historical observations of ozone and iodine means that such estimates rely primarily on model calculations. Here we use seasonally resolved records from an Alpine ice core to investigate 20th century changes in atmospheric iodine. After carefully considering possible postdepositional changes in the ice core record, we conclude that iodine deposition over the Alps increased by at least a factor of 3 from 1950 to the 1990s in the summer months, with smaller increases during the winter months. We reproduce these general trends using a chemical transport model and show that they are due to increased oceanic iodine emissions, coupled to a change in iodine speciation over Europe from enhanced nitrogen oxide emissions. The model underestimates the increase in iodine deposition by a factor of 2, however, which may be due to an underestimate in the 20th century ozone increase. Our results suggest that iodine's impact on the Northern Hemisphere atmosphere accelerated over the 20th century and show a coupling between anthropogenic pollution and the availability of iodine as an essential nutrient to the terrestrial biosphere.
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http://dx.doi.org/10.1073/pnas.1809867115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6275475PMC
November 2018

Lead pollution recorded in Greenland ice indicates European emissions tracked plagues, wars, and imperial expansion during antiquity.

Proc Natl Acad Sci U S A 2018 05 14;115(22):5726-5731. Epub 2018 May 14.

Centre for Ice and Climate, University of Copenhagen, DK-1017 Copenhagen, Denmark.

Lead pollution in Arctic ice reflects midlatitude emissions from ancient lead-silver mining and smelting. The few reported measurements have been extrapolated to infer the performance of ancient economies, including comparisons of economic productivity and growth during the Roman Republican and Imperial periods. These studies were based on sparse sampling and inaccurate dating, limiting understanding of trends and specific linkages. Here we show, using a precisely dated record of estimated lead emissions between 1100 BCE and 800 CE derived from subannually resolved measurements in Greenland ice and detailed atmospheric transport modeling, that annual European lead emissions closely varied with historical events, including imperial expansion, wars, and major plagues. Emissions rose coeval with Phoenician expansion, accelerated during expanded Carthaginian and Roman mining primarily in the Iberian Peninsula, and reached a maximum under the Roman Empire. Emissions fluctuated synchronously with wars and political instability particularly during the Roman Republic, and plunged coincident with two major plagues in the second and third centuries, remaining low for >500 years. Bullion in silver coinage declined in parallel, reflecting the importance of lead-silver mining in ancient economies. Our results indicate sustained economic growth during the first two centuries of the Roman Empire, terminated by the second-century Antonine plague.
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http://dx.doi.org/10.1073/pnas.1721818115DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5984509PMC
May 2018

Prokaryotes in the WAIS Divide ice core reflect source and transport changes between Last Glacial Maximum and the early Holocene.

Glob Chang Biol 2018 05 16;24(5):2182-2197. Epub 2018 Feb 16.

Department of Land Resources and Environmental Sciences, Montana State University, Bozeman, MT, USA.

We present the first long-term, highly resolved prokaryotic cell concentration record obtained from a polar ice core. This record, obtained from the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core, spanned from the Last Glacial Maximum (LGM) to the early Holocene (EH) and showed distinct fluctuations in prokaryotic cell concentration coincident with major climatic states. The time series also revealed a ~1,500-year periodicity with greater amplitude during the Last Deglaciation (LDG). Higher prokaryotic cell concentration and lower variability occurred during the LGM and EH than during the LDG. A sevenfold decrease in prokaryotic cell concentration coincided with the LGM/LDG transition and the global 19 ka meltwater pulse. Statistical models revealed significant relationships between the prokaryotic cell record and tracers of both marine (sea-salt sodium [ssNa]) and burning emissions (black carbon [BC]). Collectively, these models, together with visual observations and methanosulfidic acid (MSA) measurements, indicated that the temporal variability in concentration of airborne prokaryotic cells reflected changes in marine/sea-ice regional environments of the WAIS. Our data revealed that variations in source and transport were the most likely processes producing the significant temporal variations in WD prokaryotic cell concentrations. This record provided strong evidence that airborne prokaryotic cell deposition differed during the LGM, LDG, and EH, and that these changes in cell densities could be explained by different environmental conditions during each of these climatic periods. Our observations provide the first ice-core time series evidence for a prokaryotic response to long-term climatic and environmental processes.
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http://dx.doi.org/10.1111/gcb.14042DOI Listing
May 2018

New Zealand supereruption provides time marker for the Last Glacial Maximum in Antarctica.

Sci Rep 2017 09 25;7(1):12238. Epub 2017 Sep 25.

SGEES, Victoria University, PO Box 600, Wellington, 6140, New Zealand.

Multiple, independent time markers are essential to correlate sediment and ice cores from the terrestrial, marine and glacial realms. These records constrain global paleoclimate reconstructions and inform future climate change scenarios. In the Northern Hemisphere, sub-visible layers of volcanic ash (cryptotephra) are valuable time markers due to their widespread dispersal and unique geochemical fingerprints. However, cryptotephra are not as widely identified in the Southern Hemisphere, leaving a gap in the climate record, particularly during the Last Glacial Maximum (LGM). Here we report the first identification of New Zealand volcanic ash in Antarctic ice. The Oruanui supereruption from Taupo volcano (25,580  ±  258 cal. a BP) provides a key time marker for the LGM in the New Zealand sector of the SW Pacific. This finding provides a high-precision chronological link to mid-latitude terrestrial and marine sites, and sheds light on the long-distance transport of tephra in the Southern Hemisphere. As occurred after identification of the Alaskan White River Ash in northern Europe, recognition of ash from the Oruanui eruption in Antarctica dramatically increases the reach and value of tephrochronology, providing links among climate records in widely different geographic areas and depositional environments.
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http://dx.doi.org/10.1038/s41598-017-11758-0DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5613013PMC
September 2017

Synchronous volcanic eruptions and abrupt climate change ∼17.7 ka plausibly linked by stratospheric ozone depletion.

Proc Natl Acad Sci U S A 2017 09 5;114(38):10035-10040. Epub 2017 Sep 5.

Lamont-Doherty Earth Observatory, Earth Institute at Columbia University, Palisades, NY 10964.

Glacial-state greenhouse gas concentrations and Southern Hemisphere climate conditions persisted until ∼17.7 ka, when a nearly synchronous acceleration in deglaciation was recorded in paleoclimate proxies in large parts of the Southern Hemisphere, with many changes ascribed to a sudden poleward shift in the Southern Hemisphere westerlies and subsequent climate impacts. We used high-resolution chemical measurements in the West Antarctic Ice Sheet Divide, Byrd, and other ice cores to document a unique, ∼192-y series of halogen-rich volcanic eruptions exactly at the start of accelerated deglaciation, with tephra identifying the nearby Mount Takahe volcano as the source. Extensive fallout from these massive eruptions has been found >2,800 km from Mount Takahe. Sulfur isotope anomalies and marked decreases in ice core bromine consistent with increased surface UV radiation indicate that the eruptions led to stratospheric ozone depletion. Rather than a highly improbable coincidence, circulation and climate changes extending from the Antarctic Peninsula to the subtropics-similar to those associated with modern stratospheric ozone depletion over Antarctica-plausibly link the Mount Takahe eruptions to the onset of accelerated Southern Hemisphere deglaciation ∼17.7 ka.
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http://dx.doi.org/10.1073/pnas.1705595114DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5617275PMC
September 2017

Reassessment of the Upper Fremont Glacier Ice-Core Chronologies by Synchronizing of Ice-Core-Water Isotopes to a Nearby Tree-Ring Chronology.

Environ Sci Technol 2017 04 4;51(8):4230-4238. Epub 2017 Apr 4.

Department of Earth System Science, University of California , Irvine, California 92697, United States.

The Upper Fremont Glacier (UFG), Wyoming, is one of the few continental glaciers in the contiguous United States known to preserve environmental and climate records spanning recent centuries. A pair of ice cores taken from UFG have been studied extensively to document changes in climate and industrial pollution (most notably, mid-19th century increases in mercury pollution). Fundamental to these studies is the chronology used to map ice-core depth to age. Here, we present a revised chronology for the UFG ice cores based on new measurements and using a novel dating approach of synchronizing continuous water isotope measurements to a nearby tree-ring chronology. While consistent with the few unambiguous age controls underpinning the previous UFG chronologies, the new interpretation suggests a very different time scale for the UFG cores with changes of up to 80 years. Mercury increases previously associated with the mid-19th century Gold Rush now coincide with early-20th century industrial emissions, aligning the UFG record with other North American mercury records from ice and lake sediment cores. Additionally, new UFG records of industrial pollutants parallel changes documented in ice cores from southern Greenland, further validating the new UFG chronologies while documenting the extent of late 19th and early 20th century pollution in remote North America.
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http://dx.doi.org/10.1021/acs.est.6b06574DOI Listing
April 2017

Multiradionuclide evidence for the solar origin of the cosmic-ray events of ᴀᴅ 774/5 and 993/4.

Nat Commun 2015 Oct 26;6:8611. Epub 2015 Oct 26.

PRIME Laboratory, Purdue University, West Lafayette, Indiana 47907, USA.

The origin of two large peaks in the atmospheric radiocarbon ((14)C) concentration at AD 774/5 and 993/4 is still debated. There is consensus, however, that these features can only be explained by an increase in the atmospheric (14)C production rate due to an extraterrestrial event. Here we provide evidence that these peaks were most likely produced by extreme solar events, based on several new annually resolved (10)Be measurements from both Arctic and Antarctic ice cores. Using ice core (36)Cl data in pair with (10)Be, we further show that these solar events were characterized by a very hard energy spectrum with high fluxes of solar protons with energy above 100 MeV. These results imply that the larger of the two events (AD 774/5) was at least five times stronger than any instrumentally recorded solar event. Our findings highlight the importance of studying the possibility of severe solar energetic particle events.
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http://dx.doi.org/10.1038/ncomms9611DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4639793PMC
October 2015

Paleoclimate. Enhanced tropical methane production in response to iceberg discharge in the North Atlantic.

Science 2015 May;348(6238):1016-9

Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA 92093-0244, USA.

The causal mechanisms responsible for the abrupt climate changes of the Last Glacial Period remain unclear. One major difficulty is dating ice-rafted debris deposits associated with Heinrich events: Extensive iceberg influxes into the North Atlantic Ocean linked to global impacts on climate and biogeochemistry. In a new ice core record of atmospheric methane with ultrahigh temporal resolution, we find abrupt methane increases within Heinrich stadials 1, 2, 4, and 5 that, uniquely, have no counterparts in Greenland temperature proxies. Using a heuristic model of tropical rainfall distribution, we propose that Hudson Strait Heinrich events caused rainfall intensification over Southern Hemisphere land areas, thereby producing excess methane in tropical wetlands. Our findings suggest that the climatic impacts of Heinrich events persisted for 740 to 1520 years.
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http://dx.doi.org/10.1126/science.1262005DOI Listing
May 2015

Centennial-scale changes in the global carbon cycle during the last deglaciation.

Nature 2014 Oct;514(7524):616-9

College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon 97331, USA.

Global climate and the concentration of atmospheric carbon dioxide (CO2) are correlated over recent glacial cycles. The combination of processes responsible for a rise in atmospheric CO2 at the last glacial termination (23,000 to 9,000 years ago), however, remains uncertain. Establishing the timing and rate of CO2 changes in the past provides critical insight into the mechanisms that influence the carbon cycle and helps put present and future anthropogenic emissions in context. Here we present CO2 and methane (CH4) records of the last deglaciation from a new high-accumulation West Antarctic ice core with unprecedented temporal resolution and precise chronology. We show that although low-frequency CO2 variations parallel changes in Antarctic temperature, abrupt CO2 changes occur that have a clear relationship with abrupt climate changes in the Northern Hemisphere. A significant proportion of the direct radiative forcing associated with the rise in atmospheric CO2 occurred in three sudden steps, each of 10 to 15 parts per million. Every step took place in less than two centuries and was followed by no notable change in atmospheric CO2 for about 1,000 to 1,500 years. Slow, millennial-scale ventilation of Southern Ocean CO2-rich, deep-ocean water masses is thought to have been fundamental to the rise in atmospheric CO2 associated with the glacial termination, given the strong covariance of CO2 levels and Antarctic temperatures. Our data establish a contribution from an abrupt, centennial-scale mode of CO2 variability that is not directly related to Antarctic temperature. We suggest that processes operating on centennial timescales, probably involving the Atlantic meridional overturning circulation, seem to be influencing global carbon-cycle dynamics and are at present not widely considered in Earth system models.
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http://dx.doi.org/10.1038/nature13799DOI Listing
October 2014

Climate change and forest fires synergistically drive widespread melt events of the Greenland Ice Sheet.

Proc Natl Acad Sci U S A 2014 Jun 19;111(22):7964-7. Epub 2014 May 19.

Thayer School of Engineering, Dartmouth College, Hanover, NH 03755; and.

In July 2012, over 97% of the Greenland Ice Sheet experienced surface melt, the first widespread melt during the era of satellite remote sensing. Analysis of six Greenland shallow firn cores from the dry snow region confirms that the most recent prior widespread melt occurred in 1889. A firn core from the center of the ice sheet demonstrated that exceptionally warm temperatures combined with black carbon sediments from Northern Hemisphere forest fires reduced albedo below a critical threshold in the dry snow region, and caused the melting events in both 1889 and 2012. We use these data to project the frequency of widespread melt into the year 2100. Since Arctic temperatures and the frequency of forest fires are both expected to rise with climate change, our results suggest that widespread melt events on the Greenland Ice Sheet may begin to occur almost annually by the end of century. These events are likely to alter the surface mass balance of the ice sheet, leaving the surface susceptible to further melting.
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http://dx.doi.org/10.1073/pnas.1405397111DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4050608PMC
June 2014

One hundred years of Arctic surface temperature variation due to anthropogenic influence.

Sci Rep 2013 ;3:2645

Canadian Centre for Climate Modelling and Analysis, Environment Canada, Victoria BC, V8W 3R4, Canada.

Observations show that Arctic-average surface temperature increased from 1900 to 1940, decreased from 1940 to 1970, and increased from 1970 to present. Here, using new observational data and improved climate models employing observed natural and anthropogenic forcings, we demonstrate that contributions from greenhouse gas and aerosol emissions, along with explosive volcanic eruptions, explain most of this observed variation in Arctic surface temperature since 1900. In addition, climate model simulations without natural and anthropogenic forcings indicate very low probabilities that the observed trends in each of these periods were due to internal climate variability alone. Arctic climate change has important environmental and economic impacts and these results improve our understanding of past Arctic climate change and our confidence in future projections.
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http://dx.doi.org/10.1038/srep02645DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3770965PMC
June 2014

Comparison of water isotope-ratio determinations using two cavity ring-down instruments and classical mass spectrometry in continuous ice-core analysis.

Isotopes Environ Health Stud 2013 29;49(3):387-98. Epub 2013 May 29.

a Desert Research Institute , Reno , NV , USA ;

We present a detailed comparison between subsequent versions of commercially available wavelength-scanned cavity ring-down water isotope analysers (L2120-i and L2130-i, Picarro Inc.). The analysers are used in parallel in a continuous mode by adaption of a low-volume flash evaporation module. Application of the analysers to ice-core analysis is assessed by comparison between continuous water isotope measurements of a glacial ice-core from Severnaya Zemlya with discrete isotope-ratio mass spectrometry measurements performed on parallel samples from the same ice-core. The great advances between instrument versions, particularly in the measurement of δ(2)H, allow the continuous technique to achieve the same high level of accuracy and precision obtained using traditional isotope spectrometry techniques in a fraction of the experiment time. However, when applied to continuous ice-core measurements, increased integration times result in a compromise of the achievable depth resolution of the ice-core records.
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http://dx.doi.org/10.1080/10256016.2013.781598DOI Listing
June 2014

High-resolution, continuous method for measurement of acidity in ice cores.

Environ Sci Technol 2012 Feb 10;46(3):1659-66. Epub 2012 Jan 10.

Division of Hydrologic Sciences, Desert Research Institute, Reno, Nevada 89512, United States.

The acid content of ice core samples provides information regarding the history of volcanism, biogenic activity, windblown dust, forest fires, and pollution-induced acid rain. A continuous ice core analysis allows for collection of high-resolution data in a very efficient manner, but this technique has not been readily applied to the measurement of pH and acidity in ice cores. The difficulty arises because the sample is highly undersaturated with respect to carbon dioxide (CO(2)) immediately after melting, making it difficult to maintain stable concentrations of dissolved carbon dioxide and carbonic acid (H(2)CO(3)). Here, we present a solution to this problem in the form of a small flow-through bubbling chamber that is supplied with a known concentration of CO(2). The bubbling action allows for quick equilibration while the small size of the chamber limits sample mixing in order to maintain high resolution. Thorough error analysis provides a measurement uncertainty of ±0.20 μM or ±5% of the acidity value, whichever is greater, and the T95 signal response time is determined to be 1.25 min. The performance of the technique is further evaluated with data from a 63-year ice core from northwest Greenland for which all major ion species were also measured. The measured acidity closely matches the acidity derived from a charge balance calculation, indicating that all of the analytes were measured accurately. The performance specifications that we provide are applicable to ice cores with low concentrations of alkaline dust (<500 ppb), which includes the vast majority of ice cores that are collected. To date, the method has not been evaluated with samples containing high alkaline dust concentrations, such as Greenland cores from the last glacial period, where measurement could be made difficult by memory effects as particles coat the internal surfaces of the sample stream.
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http://dx.doi.org/10.1021/es202668nDOI Listing
February 2012

Coal burning leaves toxic heavy metal legacy in the Arctic.

Proc Natl Acad Sci U S A 2008 Aug 18;105(34):12140-4. Epub 2008 Aug 18.

Desert Research Institute, Nevada System of Higher Education, Reno, NV 89512, USA.

Toxic heavy metals emitted by industrial activities in the midlatitudes are transported through the atmosphere and deposited in the polar regions; bioconcentration and biomagnification in the food chain mean that even low levels of atmospheric deposition may threaten human health and Arctic ecosystems. Little is known about sources and long-term trends of most heavy metals before approximately 1980, when modern measurements began, although heavy-metal pollution in the Arctic was widespread during recent decades. Lacking detailed, long-term measurements until now, ecologists, health researchers, and policy makers generally have assumed that contamination was highest during the 1960s and 1970s peak of industrial activity in North America and Europe. We present continuous 1772-2003 monthly and annually averaged deposition records for highly toxic thallium, cadmium, and lead from a Greenland ice core showing that atmospheric deposition was much higher than expected in the early 20th century, with tenfold increases from preindustrial levels by the early 1900s that were two to five times higher than during recent decades. Tracer measurements indicate that coal burning in North America and Europe was the likely source of these metals in the Arctic after 1860. Although these results show that heavy-metal pollution in the North Atlantic sector of the Arctic is substantially lower today than a century ago, contamination of other sectors may be increasing because of the rapid coal-driven growth of Asian economies.
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http://dx.doi.org/10.1073/pnas.0803564105DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2529165PMC
August 2008

20th-century industrial black carbon emissions altered Arctic climate forcing.

Science 2007 Sep 9;317(5843):1381-4. Epub 2007 Aug 9.

Desert Research Institute, Nevada System of Higher Education, Reno, NV 89512, USA.

Black carbon (BC) from biomass and fossil fuel combustion alters chemical and physical properties of the atmosphere and snow albedo, yet little is known about its emission or deposition histories. Measurements of BC, vanillic acid, and non-sea-salt sulfur in ice cores indicate that sources and concentrations of BC in Greenland precipitation varied greatly since 1788 as a result of boreal forest fires and industrial activities. Beginning about 1850, industrial emissions resulted in a sevenfold increase in ice-core BC concentrations, with most change occurring in winter. BC concentrations after about 1951 were lower but increasing. At its maximum from 1906 to 1910, estimated surface climate forcing in early summer from BC in Arctic snow was about 3 watts per square meter, which is eight times the typical preindustrial forcing value.
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http://dx.doi.org/10.1126/science.1144856DOI Listing
September 2007

20th-Century doubling in dust archived in an Antarctic Peninsula ice core parallels climate change and desertification in South America.

Proc Natl Acad Sci U S A 2007 Apr 26;104(14):5743-8. Epub 2007 Mar 26.

Desert Research Institute, Nevada System of Higher Education, Reno, NV 89512, USA.

Crustal dust in the atmosphere impacts Earth's radiative forcing directly by modifying the radiation budget and affecting cloud nucleation and optical properties, and indirectly through ocean fertilization, which alters carbon sequestration. Increased dust in the atmosphere has been linked to decreased global air temperature in past ice core studies of glacial to interglacial transitions. We present a continuous ice core record of aluminum deposition during recent centuries in the northern Antarctic Peninsula, the most rapidly warming region of the Southern Hemisphere; such a record has not been reported previously. This record shows that aluminosilicate dust deposition more than doubled during the 20th century, coincident with the approximately 1 degrees C Southern Hemisphere warming: a pattern in parallel with increasing air temperatures, decreasing relative humidity, and widespread desertification in Patagonia and northern Argentina. These results have far-reaching implications for understanding the forces driving dust generation and impacts of changing dust levels on climate both in the recent past and future.
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http://dx.doi.org/10.1073/pnas.0607657104DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1851562PMC
April 2007

Snowfall-driven growth in East Antarctic ice sheet mitigates recent sea-level rise.

Science 2005 Jun 19;308(5730):1898-901. Epub 2005 May 19.

Department of Electrical and Computer Engineering, University of Missouri-Columbia, Columbia, MO 65211, USA.

Satellite radar altimetry measurements indicate that the East Antarctic ice-sheet interior north of 81.6 degrees S increased in mass by 45 +/- 7 billion metric tons per year from 1992 to 2003. Comparisons with contemporaneous meteorological model snowfall estimates suggest that the gain in mass was associated with increased precipitation. A gain of this magnitude is enough to slow sea-level rise by 0.12 +/- 0.02 millimeters per year.
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http://dx.doi.org/10.1126/science.1110662DOI Listing
June 2005
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