Bibliography - M. G. Hastings

1. Hastings, M. G., Daniel Sigman, and E. J. Steig, 2005: Glacial/interglacial changes in the isotopes of nitrate from the Greenland Ice Sheet Project 2 (GISP2) ice core. Global Biogeochemical Cycles, 7, doi:10.1029/2005GB002502 611-625
   [ Abstract ]

   The ¹⁵N/¹⁴N and ¹⁸O/¹⁶O 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 ¹⁵N/¹⁴N and ¹⁸O/¹⁶O 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 ¹⁵N/¹⁴N 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 NO_x. The glacial/interglacial change in ¹⁸O/¹⁶O is 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 NO_x photochemistry and nitrate scavenging have not been fully characterized, the ¹⁵N/¹⁴N data may indicate glacial/interglacial changes in the relative contributions from different natural sources of NO_x on a hemispheric or global scale.

2. Hastings, M. G., E. J. Steig, and Daniel Sigman, 2004: Seasonal variations in N and O isotopes of nitrate in snow at Summit, Greenland: Implications for the study of nitrate in snow and ice cores. Journal of Geophysical Research – Atmosphere, 109(D20306), doi:10.1029/2004JD004991
   [ Abstract ]

   Nitrogen and oxygen isotopes of NO₃ have been measured in snow and firn from Summit, Greenland. The ¹⁵N/¹⁴N and ¹⁸O/¹⁶O ratios of NO₃ in recently fallen snow are similar to those of surface snow. Diurnal variation is observed in ¹⁵N/¹⁴N of NO₃, and possibly ¹⁸O/¹⁶O, suggesting fractionating loss of NO₃ from snow during the day, which is subsequently recovered at night. A larger seasonal variation is observed, with higher ¹⁵N/¹⁴N and lower ¹⁸O/¹⁶O of NO₃ in summer than winter, which cannot be explained by postdepositional fractionation. The generally high ¹⁸O/¹⁶O of NO₃ in Greenland snow (δ¹⁸O versus VSMOW = 65.2 to 79.6%) indicates that oxygen atoms from ozone have been incorporated into NO_x that was subsequently deposited as HNO₃. The lower mean δ¹⁸O of NO₃ in summer snow relative to winter (68.9% in summer 2000 and 70.5% in summer 2001 versus 77.5% in winter 2000–01) is a result of summertime HNO₃ production via NO₂ reaction with hydroxyl radical (OH), which dilutes the high δ¹⁸O imparted on NO₂ from ozone. The higher mean ¹⁵N/¹⁴N of NO₃ observed in snow from spring (δ¹⁸O versus air N₂ = +5.9% in 2000 and -1.4% in 2001) and summer (+0.1% in 2000 and -0.8% in 2001) than fall (-9.2% in 2000) and winter( -10.0% in 2000–01) is more difficult to explain with seasonal photochemistry, given current knowledge. The seasonal ¹⁵N/¹⁴N change may reflect NO_X sources, with a greater fall and wintertime contribution from fossil fuel emissions relative to other inputs of NO_X (i.e., biogenic soil emissions, biomass burning, and lightning).

3. Hastings, M. G., Daniel Sigman, and R. Lipshultz, 2003: Isotopic evidence for source changes of nitrate in rain at Bermuda. Journal of Geophysical Research – Atmosphere, 108(D24), doi:10.1029/2003JD003789
   [ Abstract ]

   Rainwater collected on the island of Bermuda between January 2000 and January 2001 shows pronounced seasonal variation in the nitrogen and oxygen isotopic composition of nitrate. Higher ¹⁵N/¹⁴N and lower ¹⁸O/¹⁶O ratios are observed in the warm season (April–September) in comparison to the cool season (October–March): The mean d15N of nitrate for the warm and cool seasons is -2.1% and -5.9% (versus air N2), respectively, while the mean δ:¹⁸O is 68.6% and 76.9% (versus Vienna Standard Mean Ocean Water). The few cool season rain events that had high ¹⁵N/¹⁴N and low ¹⁸O/¹⁶O exhibited trajectory paths originating from the south, similar to those of warm season samples. Accordingly, the region from which air is transported to the island determines the ¹⁵N/¹⁴N and ¹⁸O/¹⁶O of the nitrate. The source region provides precursor nitrogen oxides (NO_x), influencing the ¹⁵N/¹⁴N of nitrate, and contributes to the chemistry that produces nitrate from NO_x, which determines the ¹⁸O/¹⁶O of nitrate. While the range in nitrate ¹⁵N/¹⁴N observed during the cool season is consistent with anthropogenic emissions from North America, the higher warm season ¹⁵N/¹⁴N suggests that lightning is a significant source of nitrate to Bermuda. The isotopic evidence for a significant southern source of nitrate to Bermuda helps to explain the previous observation of unexpectedly high nitrate concentrations in warm season rain. The ¹⁸O/¹⁶O of nitrate in rain at Bermuda is high throughout the year (δ:¹⁸O = 60.3 to 86.5%) as a result of interactions of precursor NO_x with ozone, which has a high ¹⁸O/¹⁶O ratio. The lower nitrate ¹⁸O/¹⁶O in the warm season and in cool season air masses from the south is consistent with elevated concentrations of hydroxyl radical (OH), which dilutes the isotopic signal of ozone. Our limited data set suggests that the relative importance of the OH sink for NO_x during the cool season varies spatially over as large a range as is observed between the warm and cool seasons.

4. Carrillo, J. H., M. G. Hastings, Daniel Sigman, and B. J. Huebert, 2002: Atmospheric deposition of inorganic and organic nitrogen and base cations in Hawaii. Global Biogeochemical Cycles, 16(4), doi:10.1029/2002GB001892
   [ Abstract ]

   Atmospheric deposition of nitrogen (N) and base cations was measured for 5–7 years on the island of Hawaii and for 1.5 years on Kauai. On Hawaii, mean annual fluxes of K⁺, Mg²⁺, and Ca²⁺ were 15, 17, and 13 kg ha^-1 yr^-1, respectively. Fog interception was the largest deposition pathway. Sea salt contributed the majority of cations, although biomass burning and Asian dust were significant sources for some years. Total N deposition (inorganic and organic) averaged 17 kg N ha^-1 yr^-1. Fog interception was also the largest source of N, depositing 16 kg N ha^-1 yr^-1. Precipitation deposition was 1.0 and 0.2 kg N ha^-1 yr^-1, respectively on Hawaii and Kauai. Dry deposition on Hawaii was 0.1 kg N ha^-1 yr^-1. Organic N averaged 16 and 12% of total N in rain and fog, respectively. The δ¹⁵N values for NO₃-N are consistent with long-range transport of N from Asia in the spring/summer and from North America in the fall/winter as nonvolcanic sources. Atmospheric deposition on Hawaii may completely account for a previously identified soil N imbalance.

5. Casciotti, K. L., Daniel Sigman, M. G. Hastings, J. K. Böhlke, and A. Hilkert, 2002: Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Analytical Chemistry, 74(19), doi:10.1021/ac020113w 4905-4912
   [ Abstract ]

   We report a novel method for measurement of the oxygen isotopic composition (¹⁸O/¹⁶O) of nitrate (NO₃ -) from both seawater and freshwater. The denitrifier method, based on the isotope ratio analysis of nitrous oxide generated from sample nitrate by cultured denitrifying bacteria, has been described elsewhere for its use in nitrogen isotope ratio (¹⁵N/¹⁴N) analysis of nitrate.1 Here, we address the additional issues associated with ¹⁸O/¹⁶O analysis of nitrate by this approach, which include (1) the oxygen isotopic difference between the nitrate sample and the N₂O analyte due to isotopic fractionation associated with the loss of oxygen atoms from nitrate and (2) the exchange of oxygen atoms with water during the conversion of nitrate to N₂O. Experiments with ¹⁸O labeled water indicate that water exchange contributes less than 10%, and frequently less than 3%, of the oxygen atoms in the N₂O product for Pseudomonas aureofaciens. In addition, both oxygen isotope fractionation and oxygen atom exchange are consistent within a given batch of analyses. The analysis of appropriate isotopic reference materials can thus be used to correct the measured ¹⁸O/¹⁶O ratios of samples for both effects. This is the first method tested for ¹⁸O/¹⁶O analysis of nitrate in seawater. Benefits of this method, relative to published freshwater methods, include higher sensitivity (tested down to 10 nmol and 1 μM NO₃ -), lack of interference by other solutes, and ease of sample preparation.

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