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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 Ϸ1°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.aluminosilicate dust ͉ global warming ͉ human impacts ͉ Patagonia ͉ radiative transfer C rustal dust in the atmosphere has a direct impact on climate forcing in two significant ways: modifying the radiation balance and affecting cloud nucleation and optical properties (1, 2). Atmospheric crustal dust also supplies iron, an essential nutrient for phytoplankton, to ocean surface waters and may indirectly affect climate by modulating the biological export of carbon to the deep ocean (3). Impacts of atmospheric dust on regional radiation budgets are similar in magnitude to those from sulfate and biomass burning aerosols (4) but can be either negative or positive (1). Estimates of the optical properties of dust have been revised recently as a result of improved in situ and remote sensing measurements (5, 6), but warming has been predicted across areas of high albedo (7, 8) such as snow-and ice-covered regions of the Antarctic Peninsula where recent warming has been pronounced (9). Although atmospheric dustiness has been linked to large-amplitude, large-scale temperature changes in past ice core studies of glacial to interglacial transitions (10, 11), it is unclear whether projected climate warming in coming decades to centuries will result in more or less atmospheric dust (12). Decadal changes in dust flux have been reported for ice cores from the Antarctic Peninsula (13, 14), but reliable, high-time-resolution records of changes in dust levels during recent decades and centuries are sparse (15).Ice core records offer the possibility of reconstructing past changes in dust concentration (10,11,(13)(14)(15)(16)(17)(18)(19)(20). Most previous high-resolution ice core studies used as proxies of atmospheric dust the non-sea-salt component of soluble calcium (nssCa) or magnesium (nssMg) that are computed by using estimated elemental ratios in sea salt aerosols (11,19). At many ice core sites, particularly coastal locations, the nssCa ...

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[1] Accurate accumulation records are fundamental to understanding mass balance of the Greenland ice sheet. Ice cores can provide multicentury, high-resolution ground-based point estimates of water accumulation. In 2003 and 2004, four new ice cores were collected in the central western (D4 and D5) and Summit (Sandy and Katie) regions of Greenland. Annual layer counting based on multiple glaciochemical parameters (primarily hydrogen peroxide, sea salt, and dust proxies) was used to date the ice cores, resulting in annual dating. The bottom depth ages for D4, D5, Sandy, and Katie were estimated at 1738, 1673, 1844, and 1934 A.D., with mean annual accumulation rates of 41.4, 35.2, 22.4, and 22.4 cm weq yr À1 , respectively. These new records were in good agreement with previous accumulation maps and four previously published records from nearby multicentury ice cores. Spatial variability was quantified through an analysis of variance, thereby allowing for more meaningful comparisons to atmospheric processes. More regionally representative accumulation records were constructed for the central western and Summit regions spanning recent centuries using a principle component analysis of both the new and previously reported ice core measurements. These regional records exhibited 6-7% interannual variability (1 standard deviation of the mean) and had lower spatial variability uncertainties than individual accumulation records. Correlations indicate that $20% of the variance in the central western region was explained by the North Atlantic Oscillation (NAO), and suggests that an array of more northern ice cores may correlate more closely with NAO.

Many karst regions are undergoing rapid population growth and expansion of urban land accompanied by increases in wastewater generation and changing patterns of nitrate (NO3(-)) loading to surface and groundwater. We investigate variability and sources of NO3(-) in a regional karst aquifer system, the Edwards aquifer of central Texas. Samples from streams recharging the aquifer, groundwater wells, and springs were collected during 2008-12 from the Barton Springs and San Antonio segments of the Edwards aquifer and analyzed for nitrogen (N) species concentrations and NO3(-) stable isotopes (δ(15)N and δ(18)O). These data were augmented by historical data collected from 1937 to 2007. NO3(-) concentrations and discharge data indicate that short-term variability (days to months) in groundwater NO3(-) concentrations in the Barton Springs segment is controlled by occurrence of individual storms and multi-annual wet-dry cycles, whereas the lack of short-term variability in groundwater in the San Antonio segment indicates the dominance of transport along regional flow paths. In both segments, longer-term increases (years to decades) in NO3(-) concentrations cannot be attributed to hydrologic conditions; rather, isotopic ratios and land-use change indicate that septic systems and land application of treated wastewater might be the source of increased loading of NO3(-). These results highlight the vulnerability of karst aquifers to NO3(-) contamination from urban wastewater. An analysis of N-species loading in recharge and discharge for the Barton Springs segment during 2008-10 indicates an overall mass balance in total N, but recharge contains higher concentrations of organic N and lower concentrations of NO3(-) than does discharge, consistent with nitrification of organic N within the aquifer and consumption of dissolved oxygen. This study demonstrates that subaqueous nitrification of organic N in the aquifer, as opposed to in soils, might be a previously unrecognized source of NO3(-) to karst groundwater or other oxic groundwater systems.

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[1] High-resolution trace element records from polar ice cores are fundamental to identifying spatial and temporal variability in dust and sea salt aerosols. Here, we developed high temporal resolution glaciochemical records over recent centuries from an array of seven recently collected Greenland ice cores. Positive matrix factorization (PMF) was used to assess apportionment of trace elements between three unique sources, which were identified as proxies of carbonate dust, aluminous dust, and sea salt. The three PMF factors agree well with literature values and represent the first time carbonate and aluminous dust sources have been quantifiably differentiated using high-resolution glaciochemistry. The concentrations of all three sources varied with ice core site. However, the large array of ice cores provided a regional-scale understanding of the sources and magnitude of dust and sea salt impurities deposited on the Greenland ice sheet. At the regional level, all three sources varied on seasonal, annual, and multiyear timescales. The seasonal deposition of sea salt impurities reached a maximum in the winter, followed by spring peaks of carbonate and aluminous dust. Over the 331 year composite record, only aluminous dust exhibited a longterm increasing trend that peaked in the 1930s and was followed by a 20% decline in recent decades.Components: 9899 words, 7 figures, 9 tables.

In south-central Texas the lower Guadalupe River has incised into the outcrop of the Carrizo-Wilcox aquifer. The river and the aquifer are hydraulically connected across the outcrop, although the connectivity is obscured at the surface by alluvium and surface-water and groundwater exchange dynamics are currently poorly understood. To investigate surface-water and groundwater exchange dynamics between the lower Guadalupe River and the Carrizo-Wilcox aquifer, a geophysical study was completed along a 14.86 km reach of the river by using water-borne gradient self-potential (SP) profiling and two-dimensional direct-current electric resistivity tomography. This paper explores the applicability of these water-borne geoelectric methods in delineating gaining and losing channel reaches, and demonstrates that geoelectric signals in the form of total electric field strength can be logged with an electric dipole and decomposed into component SP signals depicting regional and local groundwater flow patterns attributable to regional and localized hydraulic gradients. Localized SP anomalies of several tens of millivolts, indicative of hyporheic exchange flows, are observed and superimposed upon a 124 mV regional SP anomaly indicative of ambient groundwater exchange flows between the river and the aquifer. The observed SP signals are interpreted through two-dimensional finite-element modeling of streaming potentials attributable to ambient groundwater exchange and hyporheic exchange flow patterns. Variables of the channel environment such as temperature and concentration gradients, depth, and velocity are considered and subsequently eliminated as alternative sources of the SP signals that are presented.