Meredith G. Hastings

Anthropogenic Impacts on Nitrogen Isotopes of Ice-Core Nitrate

The isotopic composition of nitrogen in nitrate deposited in Greenland has changed markedly over the past 150 years. A strong, unambiguous negative trend is found in the nitrogen isotopic composition (δ15N) of nitrate over the industrial period, on the basis of a 100-meter ice core from Summit, Greenland. This record indicates that ice-core nitrate reflects changes in nitrogen oxide (NOx) source emissions and that anthropogenic emissions of NOx have resulted in a 12 per mil decline in δ15N of atmospheric nitrate from preindustrial values to present. Variations in the isotopic composition of nitrate may affect the interpretation of other records of environmental change that are affected by atmospheric nitrate.

Glacial/interglacial changes in the isotopes of nitrate from the Greenland Ice Sheet Project 2 (GISP2) ice core

Received 3 March 2005; revised 23 September 2005; accepted 26 October 2005; published 24 December 2005. [1] The 15 N/ 14 N and 18 O/ 16 O ratios of nitrate in the Greenland Ice Sheet Project 2 (GISP2) (Summit, Greenland) ice core are much higher in ice from the last glacial period than in the pre-industrial Holocene, despite the lack of a significant glacial/interglacial change in nitrate concentration. While both the 15 N/ 14 N and 18 O/ 16 O records are anticorrelated with snow accumulation rate, neither is satisfactorily explained by accumulation changes or post-depositional processes. The similarity in the glacial/ interglacial change in 15 N/ 14 N from several different Greenland ice cores and the large amplitude of this change relative to observed seasonal variation raise the possibility that the isotopes of nitrate in ice cores indicate a large-scale glacial/interglacial change in the isotopic composition of atmospheric NOx. The glacial/interglacial change in 18 O/ 16 Oi s best explained by a greater contribution of HNO3 production from hydrolysis of N2O5, which has implications for reconstruction of past atmospheric oxidant levels. Although isotope effects associated with NOx photochemistry and nitrate scavenging have not been fully characterized, the 15 N/ 14 N data may indicate glacial/interglacial changes in the relative contributions from different natural sources of NOx on a hemispheric or global scale.

Isotopic evidence for a marine ammonium source in rainwater at Bermuda

Emissions of anthropogenic nitrogen (N) to the atmosphere have increased tenfold since preindustrial times, resulting in increased N deposition to terrestrial and coastal ecosystems. The sources of N deposition to the ocean, however, are poorly understood. Two years of event-based rainwater samples were collected on the island of Bermuda in the western North Atlantic, which experiences both continent- and ocean-influenced air masses. The rainwater ammonium concentration ranged from 0.36 to 24.6 μM, and the ammonium δ15N from −12.5 to 0.7‰; and neither has a strong relationship with air mass history (6.0 ± 4.2 μM, −4.1 ± 2.6‰ in marine air masses and 5.9 ± 3.2 μM, −5.8 ± 2.5‰ in continental air masses; numerical average ± standard deviation). A simple box model suggests that the ocean can account for the concentration and isotopic composition of ammonium in marine rainwater, consistent with the lack of correlation between ammonium δ15N and air mass history. If so, ammonium deposition reflects the cycling of N between the ocean and the atmosphere, rather than representing a net input to the ocean. The δ15N data appear to require that most of the ammonium/a flux to the ocean is by dissolution in surface waters rather than atmospheric deposition. This suggests that the atmosphere and surface ocean are near equilibrium with respect to air/sea gas exchange, implying that anthropogenic ammonia will equilibrate near the coast and not reach the open marine atmosphere. Whereas ~90% of the ammonium deposition to the global ocean has previously been attributed to anthropogenic sources, the evidence at Bermuda suggests that the anthropogenic contribution could be much smaller.

Influence of local photochemistry on isotopes of nitrate in Greenland snow

[1] To explore the seasonality in δ 15 N and δ 18 O of nitrate in Greenland snow, we describe a simple box model of local photochemistry. Isotope ratios of HNO 3 are controlled by the nitrogen isotope fractionation between NO and NO 2 , the ratio of NO 2 to NO, and seasonal variations in HNO 3 production. The observed seasonal range in δ 15 N requires either a large net fractionation (∼70‰) associated with NO x cycling or a seasonal change in the δ 15 N of NO x sources. The observed range in δ 18 O of nitrate is smaller than that calculated from HNO 3 production pathways, suggesting that seasonal transport may also be required to explain the seasonality in nitrate δ 18 O.

Marine biogenic source of atmospheric organic nitrogen in the subtropical North Atlantic

Significance Global models indicate that the human-derived nitrogen emissions that reach the ocean through atmospheric transport and deposition directly impact biology and the oceanic carbon dioxide (CO2) sink. Here, we find that the organic nitrogen in marine aerosols derives predominantly from biological production in the surface ocean rather than from pollution on land. Our previous work has shown significant anthropogenic influence on North Atlantic nitrate deposition, whereas ammonium cycles dynamically between the upper ocean and lower atmosphere. Collectively, these findings indicate that the ocean is not a passive recipient of anthropogenic nitrogen deposition, as it has previously been considered. This implies that the contribution of atmospheric nitrogen deposition to ocean fertility, oceanic CO2 removal, and nitrous oxide emissions has been overestimated. Global models estimate that the anthropogenic component of atmospheric nitrogen (N) deposition to the ocean accounts for up to a third of the ocean’s external N supply and 10% of anthropogenic CO2 uptake. However, there are few observational constraints from the marine atmospheric environment to validate these findings. Due to the paucity of atmospheric organic N data, the largest uncertainties related to atmospheric N deposition are the sources and cycling of organic N, which is 20–80% of total N deposition. We studied the concentration and chemical composition of rainwater and aerosol organic N collected on the island of Bermuda in the western North Atlantic Ocean over 18 mo. Here, we show that the water-soluble organic N concentration ([WSON]) in marine aerosol is strongly correlated with surface ocean primary productivity and wind speed, suggesting a marine biogenic source for aerosol WSON. The chemical composition of high-[WSON] aerosols also indicates a primary marine source. We find that the WSON in marine rain is compositionally different from that in concurrently collected aerosols, suggesting that in-cloud scavenging (as opposed to below-cloud “washout”) is the main contributor to rain WSON. We conclude that anthropogenic activity is not a significant source of organic N to the marine atmosphere over the North Atlantic, despite downwind transport from large pollution sources in North America. This, in conjunction with previous work on ammonium and nitrate, leads to the conclusion that only 27% of total N deposition to the global ocean is anthropogenic, in contrast to the 80% estimated previously.

Stable isotopic evidence of enhanced export of microbially derived \({\text{NO}}_{3}^{ - }\) following active layer slope disturbance in the Canadian High Arctic

Permafrost disturbance is expected to alter nitrogen (N) export in High Arctic watersheds by enhancing loads of dissolved inorganic N (DIN), particularly nitrate (NO3-\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}

Evaluating source, chemistry and climate change based upon the isotopic composition of nitrate in ice cores

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Investigating the preservation of nitrate isotopic composition in a tropical ice core from the Quelccaya Ice Cap, Peru

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