Melissa B. Hendricks

Space and time variation of δ18O andδD in precipitation: Can paleotemperature be estimated from ice cores?

A one-dimensional model of meridional water vapor transport is used to evaluate the factors that control the spatial and temporal variations of oxygen (δ18O) and hydrogen (δD) isotopic ratios in global precipitation. The model extends Rayleigh descriptions of isotopes in precipitation by including (1) effects of recharge to air masses by evaporation and (2) horizontal transport by both eddy fluxes and advection. Globally, spatial variations in precipitation δ18O and δD depend on the ratio of evaporation to the product of horizontal moisture flux and horizontal temperature gradient. At low latitudes, where this ratio is large, precipitation δ18O and δD are closely tied to the isotopic ratios of oceanic evaporation. At high latitudes the ratio is small, and δ18O and δD are controlled by the ratio of advective transport to eddy transport. Transport by eddy fluxes induces less fractionation than transport by advection, resulting in a smaller gradient of isotopic ratios with temperature. The model-predicted temporal relationships between δ18O (or δD) of Antarctic precipitation and temperature do not necessarily coincide with the modern spatial relationship and depend strongly on the proximity of the precipitation site to the ocean evaporation source. Sensitivity of δ18O to temporal changes in local surface temperature is low at coastal sites and increases with distance inland. These results suggest a possible explanation of the apparent discrepancy between borehole temperature inversion estimates of glacial temperatures and temperatures inferred from the modern spatial δ18O—surface temperature relationship.

Biological oxygen productivity during the last 60,000 years from triple oxygen isotope measurements

during isotope exchange between O2 and CO2 in the stratosphere. The relative rates of biologic O2 production and stratospheric processing determine the relationship between d 17 O and d 18 Oo f O2 in the atmosphere. Variations of this relationship thus allow us to estimate changes in the rate of mass-dependent O2 production by photosynthesis versus the rate of O2-CO2 exchange in the stratosphere with about equal fractionations of d 17 O and d 18 O. In this study we reconstruct total oxygen productivity for the last glacial, the last glacial termination, and the early Holocene from the triple isotope composition of atmospheric oxygen trapped in ice cores. With a box model we estimate that total biogenic productivity was only � 76–83% of today for the glacial and was probably lower than today during the glacial-interglacial transition and the early Holocene. Depending on how reduced the oxygen flux from the land biosphere was during the glacial, the oxygen flux from the glacial ocean biosphere was 88–140% of its present value. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 4870 Oceanography: Biological and Chemical: Stable isotopes; 1615 Global Change: Biogeochemical processes (4805); 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; KEYWORDS: GISP2, SIPLE ice cores, oxygen isotopes, past oxygen productivity, stratospheric isotope exchange, respiration

Atmospheric O2/N2 changes, 1993–2002: Implications for the partitioning of fossil fuel CO2 sequestration

Improvements made to an established mass spectrometric method for measuring changes in atmospheric O 2 /N 2 are described. With the improvements in sample handling and analysis, sample throughput and analytical precision have both increased. Aliquots from duplicate flasks are repeatedly measured over a period of 2 weeks, with an overall standard error in each flask of 3-4 per meg, corresponding to 0.6-0.8 ppm O 2 in air. Records of changes in O 2 /N 2 from six global sampling stations (Barrow, American Samoa, Cape Grim, Amsterdam Island, Macquarie Island, and Syowa Station) are presented. Combined with measurements of CO 2 from the same sample flasks, land and ocean carbon uptake were calculated from the three sampling stations with the longest records (Barrow, Samoa, and Cape Grim). From 1994-2002, We find the average CO 2 uptake by the ocean and the land biosphere was 1.7 ± 0.5 and 1.0 ± 0.6 GtC yr -1 respectively; these numbers include a correction of 0.3 Gt C yr -1 due to secular outgassing of ocean O 2 . Interannual variability calculated from these data shows a strong land carbon source associated with the 1997-1998 El Nifio event, supporting many previous studies indicating that high atmospheric growth rates observed during most El Nino events reflect diminished land uptake. Calculations of interannual variability in land and ocean uptake are probably confounded by non-zero annual air sea fluxes of O 2 . The origin of these fluxes is not yet understood.

Measurements and models of the atmospheric Ar/N2 ratio

The Ar/N 2 ratio of air measured at 6 globally distributed sites shows annual cycles with amplitudes of 12 to 37 parts in 10 6 . Summertime maxima reflect the atmospheric Ar enrichment driven by seasonal warming and degassing of the oceans. Paired models of air-sea heat fluxes and atmospheric tracer transport predict seasonal cycles in the Ar/N 2 ratio that agree with observations, within uncertainties.