Snow and ice chemistry study of the Greenland ice sheet

The Greenland ice sheet preserves high resolution records of environmental and climate change spanning seasons to hundreds and thousands of years. In this study, we utilize major chemical species measurements of surface snowpit samples and ice cores collected at GISP2, 20D, Mount Logan, and Sentik Glacier to investigate major chemical species spatial distribution, temporal variations, and possible sources for the major chemical species in these ice cores. A two-sided t-distribution test (a = 0.05) was applied for the study of major chemical species spatial distribution over a portion of central Greenland. The results suggest that concentrations of major chemical species in snow do not vary significantly over the central Greenland. The relationship between chemical concentration and snow accumulation rate was investigated using the snowpits and ice core samples. Chemical concentrations do not vary with snow accumulation rate over the Greenland ice sheet We further suggest that it is improper to study the relationship between chemical concentration and snow accumulation rate by using data collected from different geographic sites. Of all the chemical series studied, only NO\sb3\sp- concentration data are normally distributed. NO\sb3\sp- concentration in snow is affected by post-depositional exchange with the atmosphere. The persistent summer maxima in NO\sb3\sp- observed in Greenland snow is believed mainly due to NO\rm\sb{x} released from peroxyacetyl nitrate (PAN) by thermal decomposition in the presence of higher OH concentrations in summer. A nearly complete depletion of NO\sb3\sp- (94%) and depletion of Cl\sp- (63%) correlates with the largest volcanic eruption horizon in the GISP2 core. It is suggested that such depletion is due to the large amounts of SO\sb4\sp{2-} disturbing atmospheric photochemistry and extremely acidic condition in ice layer. The 110,000 year long major chemical species fluctuations in the GISP2 ice core demonstrate that chemical concentrations are inversely correlated with \delta\sp{18}O during last glacial period, suggesting that chemical concentration increases while temperature decreases, and vice versa. During the Holocene the atmosphere was acidic, during interstadials the atmosphere was a neutral or alkalescent, and during stadials the atmosphere was alkaline. Changes in major chemical composition and ratios also indicate that source regions differed during these periods

Chemical species spatial distribution and relationship to elevation and snow accumulation rate over the Greenland Ice Sheet

Major chemical species (Cl−, NO−3, SO2−4, Na+, K+, Mg2+, Ca2+) from 24 snowpits (sampled at a resolution of 3 cm, total 2995 samples) collected from northern, central, and southern Greenland were used for this investigation. The annual and seasonal (winter and summer) concentration of each chemical species was calculated and used to study the spatial distribution of chemical species over the central portion of the Greenland Ice Sheet. A two‐sided t‐distribution test (α = 0.05) suggests that concentrations of major chemical species in snow do not vary significantly over this portion of central Greenland. The relationship between chemical concentration and snow accumulation rate was investigated using annual data from two groups of snowpits: those from coastal sites (northern and southern Greenland); and those from high‐altitude inland sites (central Greenland). The snowpit data from a single group, when examined independently of the other group, show that chemical concentrations do not vary with snow accumulation rate. However, when data from the two groups are integrated into a single data set, pseudorelationships appear, with NO−3 concentration decreasing and Na+, K+, Mg2+, and Cl− increasing as snow accumulation rate increases. Therefore we suggest that it is improper to study the relationship between chemical concentration and snow accumulation rate by using data collected from different geographic sites. The relationship between elevation and chemical concentration was investigated using the same suite of annual data sets. We find that Cl−, Na+, and Mg2+ concentrations decrease, while NO−3 concentration increases, with increasing elevation on the Greenland Ice Sheet

Anthropogenic Sulfate and Asian Dust Signals in Snow from Tien Shan, Northwest China

Snow samples were collected from a 0.5 m snowpack at Glacier No. I and near Bogda Feng, eastern Tien Shan, northwest China. Samples that were melted in the field were analyzed for chloride, nitrate, sulfate, sodium, potassium, magnesium, calcium, and microparticles. Eight samples were returned frozen and were analyzed for the above ions plus ammonium, acetate, formate, methylsulfonate, and hydrogen peroxide. There was no significant difference in measured major ion concentrations between the melted and frozen samples. Measured cations in both sets of samples were two to three times greater than measured anions. Calcium and sodium are the dominant cations while sulfate is the dominant measured anion. High ion burdens are associated with dusty layers in the snowpack, indicating that dust from the vast arid regions of central Asia is the dominant source for major ions in Tien Shan snow. The significant increase in sulfate and decrease in the cation: anion ratio in Bogda Feng snow relative to Glacier No. I snow suggest that anthropogenic emissions from Urtimqi are an important source of sulfate to precipitation downwind from the city

Depletion of atmospheric nitrate and chloride as a consequence of the Toba Volcanic Eruption

Continuous measurements of SO42− and electrical conductivity (ECM) along the GISP2 ice core record the Toba mega‐eruption at a depth 2590.95 to 2091.25 m (71,000±5000 years ago). Major chemical species were analyzed at a resolution of 1 cm per sample for this section. An ∼6‐year long period with extremely high volcanic SO42− coincident with a 94% depletion of nitrate and 63% depletion of chloride is observed at the depth of the Toba horizon. Such a reduction of chloride in a volcanic layer preserved in an ice core has not been observed in any previous studies. The nearly complete depletion of nitrate (to 5 ppb) encountered at the Toba level is the lowest value in the entire ∼250,000 years of the GISP2 ice core record. We propose possible mechanisms to explain the depletion of nitrate and chloride resulting from this mega‐eruption

Soluble species in aerosol and snow and their relationship at Glacier 1, Tien Shan, China

Simultaneous sampling of aerosol (n = 20) and snow (n = 114) was made at Glacier 1, Tien Shan, between May 19 and June 29, 1996. Similar temporal patterns of some major ion (calcium, magnesium, potassium, sodium, chloride, and sulfate) concentrations between snow and aerosol show that snow chemistry basically reflects changes in the chemistry of the atmosphere. This gives us confidence in the reconstruction of past atmospheric change using some snow data. There are no significant correlations between aerosol and snow samples for ammonium and nitrate. This suggests that post-depositional and/or post-collection processes may alter ammonium and nitrate concentrations in snow. The fact that the measured cations in aerosol and snow always exceed the measured anions suggests that the atmosphere is alkaline over Glacier 1, Tien Shan. In aerosol and snow samples, calcium is the dominant cationic species, with sulfate and presumed carbonate being the dominant anions. There is a very good inverse relationship (r = 0.96) between the equivalence ratio of calcium to sulfate and the ratio of ammonium to sulfate in aerosols, but this relationship does not hold for snow. This further suggests that post depositional and/or post collection processes exert important controls on ammonium concentrations in snow. Although melt-freeze cycles might increase the concentration of all crustal species through progressive dissolution of dust, these cycles seem most important for magnesium and carbonate

Global perspective of nitrate flux in ice cores

The relationships between the concentration and the flux of chemical species (Cl-, NO3 - , SO42-, Na +, K + , NH4 + , Mg 2+ , Ca 2+) versus snow accumulation rate were examined at GISP2 and 20D in Greenland, Mount Logan from the St. Elias Range, Yukon Territory, Canada, and Sentik Glacier from the northwest end of the Zanskar Range in the Indian Himalayas. At all sites, only nitrate flux is significantly (a = 0.05) related to snow accumulation rate. Of all the chemical series, only nitrate concentration data are normally distributed. Therefore we suggest that nitrate concentration in snow is affected by postdepositionaJ exchange with the atmosphere over a broad range of environmental conditions. The persistent summer maxima in nitrate observed in Greenland snow over the entire range of record studied (the last 800 years) may be mainly due to NO• released from peroxyacetyl nitrate by thermal decomposition in the presence of higher OH concentrations in summer. The late winter/early spring nitrate peak observed in modern Greenland snow may be related to the buildup of anthropogenically derived N Oy in the Arctic troposphere during the long polar winter

Major features and forcing of high‐latitude northern hemisphere atmospheric circulation using a 110,000‐year‐long glaciochemical series

The Greenland Ice Sheet Project 2 glaciochemical series (sodium, potassium, ammonium, calcium, magnesium, sulfate, nitrate, and chloride) provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid‐low latitudes of the northern hemisphere. Interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants, volcanic and biomass burning events, storminess over marine surfaces, continental aridity and biogenic source strength plus information related to the controls on both high‐ and low‐frequency climate events of the last 110,000 years. Climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long‐term behavior of atmospheric circulation patterns through changes in ice volume (sea level), events such as the Heinrich events (massive discharges of icebergs first identified in the marine record) that are found to operate on a 6100‐year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics, and rapid climate change events (massive reorganizations of atmospheric circulation) that are demonstrated to operate on 1450‐year cycles. Changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events. Explanation for the exact timing and global synchroneity of these events is, however, more complicated. Preliminary evidence points to possible solar variability‐climate associations for these events and perhaps others that are embedded in our ice‐core‐derived atmospheric circulation records

Climatological Significance of δ\u3csup\u3e18\u3c/sup\u3eO in Precipitation and Ice Cores: a Case Study at the Head of the Ürütnqi River, Tien Shan, China

Stable-oxygen-isotope ratios (δ18O) collected from the headwaters of the Ürümqi river, Tien Shan, China, were used to test the relationship between δ18O temporal relationship is found between δ18O monthly averages which remove synoptic-scale influences such as changes in condensation level, condensation temperature and moisture sources (Yao and others, 1996). Linear fits as high as 0.95‰°C-1 for precipitation events and 1.23‰°C-1 for monthly averages are found. Although the δ18O (∼2 km from the precipitation sampling site) decreased dramatically compared to the precipitation samples , the ice-core records of annually averaged δ18O with contemporaneous air temperature, especially summer air temperature, at the nearby Daxigou meteorological station. Nevertheless, the relationship between the ice core δ18O records and contemporaneous air temperature is less significant than that [or the precipitation samples due to depositional and post-depositional modification processes, which are highlighted by the successive snow-pit δ18O No. 1. Our results might extend the application of high-altitude and subtropical ice-core δ18

Major Features and Forcing of High-atitude Northern Hemisphere Atmospheric Circulation using a 110,000-year-long Glaciochemical Series

The Greenland Ice Sheet Project 2 glaciochemical series (sodium, potassium, ammonium, calcium, magnesium, sulfate, nitrate, and chloride) provides a unique view of the chemistry of the atmosphere and the history of atmospheric circulation over both the high latitudes and mid-low latitudes of the northern hemisphere. Interpretation of this record reveals a diverse array of environmental signatures that include the documentation of anthropogenically derived pollutants, volcanic and biomass burning events, storminess over marine surfaces, continental aridity and biogenic source strength plus information related to the controls on both high- and low-frequency climate events of the last 110,000 years. Climate forcings investigated include changes in insolation of the order of the major orbital cycles that control the long-term behavior of atmospheric circulation patterns through changes in ice volume (sea level), events such as the Heinrich events (massive discharges of icebergs first identified in the marine record) that are found to operate on a 6100-year cycle due largely to the lagged response of ice sheets to changes in insolation and consequent glacier dynamics, and rapid climate change events (massive reorganizations of atmospheric circulation) that are demonstrated to operate on 1450-year cycles. Changes in insolation and associated positive feedbacks related to ice sheets may assist in explaining favorable time periods and controls on the amplitude of massive rapid climate change events. Explanation for the exact timing and global synchroneity of these events is, however, more complicated. Preliminary evidence points to possible solar variability-climate associations for these events and perhaps others that are embedded in our ice-core-derived atmospheric circulation records

Soluble Species in Aerosol and Snow and Their Relationship at Glacier 1, Tien Shan, China

Simultaneous sampling of aerosol (n = 20) and snow (n = 114) was made at Glacier 1, Tien Shan, between May 19 and June 29, 1996. Similar temporal patterns of some major ion (calcium, magnesium, potassium, sodium, chloride, and sulfate) concentrations between snow and aerosol show that snow chemistry basically reflects changes in the chemistry of the atmosphere. This gives us confidence in the reconstruction of past atmospheric change using some snow data. There are no significant correlations between aerosol and snow samples for ammonium and nitrate. This suggests that postdepositional and/or postcollection processes may alter ammonium and nitrate concentrations in snow. The fact that the measured cations in aerosol and snow always exceed the measured anions suggests that the atmosphere is alkaline over Glacier 1, Tien Shan. In aerosol and snow samples, calcium is the dominant cationic species, with sulfate and presumed carbonate being the dominant anions. There is a very good inverse relationship (r = 0.96) between the equivalence ratio of calcium to sulfate and the ratio of ammonium to sulfate in aerosols, but this relationship does not hold for snow. This further suggests that postdepositional and/or postcollection processes exert important controls on ammonium concentrations in snow. Although melt-freeze cycles might increase the concentration of all crustal species through progressive dissolution of dust, these cycles seem most important for magnesium and carbonate