Bruce Barnett

The Southern Ocean Biological Response to Aeolian Iron Deposition

Biogeochemical rate processes in the Southern Ocean have an important impact on the global environment. Here, we summarize an extensive set of published and new data that establishes the pattern of gross primary production and net community production over large areas of the Southern Ocean. We compare these rates with model estimates of dissolved iron that is added to surface waters by aerosols. This comparison shows that net community production, which is comparable to export production, is proportional to modeled input of soluble iron in aerosols. Our results strengthen the evidence that the addition of aerosol iron fertilizes export production in the Southern Ocean. The data also show that aerosol iron input particularly enhances gross primary production over the large area of the Southern Ocean downwind of dry continental areas.

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

Continuous High-Frequency Dissolved O2/Ar Measurements by Equilibrator Inlet Mass Spectrometry

The oxygen (O(2)) concentration in the surface ocean is influenced by biological and physical processes. With concurrent measurements of argon (Ar), which has similar solubility properties as oxygen, we can remove the physical contribution to O(2) supersaturation and determine the biological oxygen supersaturation. Biological O(2) supersaturation in the surface ocean reflects the net metabolic balance between photosynthesis and respiration, i.e., the net community productivity (NCP). We present a new method for continuous shipboard measurements of O(2)/Ar by equilibrator inlet mass spectrometry (EIMS). From these measurements and an appropriate gas exchange parametrization, NCP can be estimated at high spatial and temporal resolution. In the EIMS configuration, seawater from the ships continuous intake flows through a cartridge enclosing a gas-permeable microporous membrane contactor. Gases in the headspace of the cartridge equilibrate with dissolved gases in the flowing seawater. A fused-silica capillary continuously samples headspace gases, and the O(2)/Ar ratio is measured by mass spectrometry. The ion current measurements on the mass spectrometer reflect the partial pressures of dissolved gases in the water flowing through the equilibrator. Calibration of the O(2)/Ar ion current ratio (32/40) is performed automatically every 2 h by sampling ambient air through a second capillary. A conceptual model demonstrates that the ratio of gases reaching the mass spectrometer is dependent on several parameters, such as the differences in molecular diffusivities and solubilities of the gases. Laboratory experiments and field observations performed by EIMS are discussed. We also present preliminary evidence that other gas measurements, such as N(2)/Ar and pCO(2) measurements, may potentially be performed with EIMS. Finally, we compare the characteristics of the EIMS with the previously described membrane inlet mass spectrometry (MIMS) approach.

Contribution of soil respiration in tropical, temperate, and boreal forests to the 18O enrichment of atmospheric O2

[1] The 18 O content of atmospheric O2 is an important tracer for past changes in the biosphere. Its quantitative use depends on knowledge of the discrimination against 18 O associated with the various O2 consumption processes. Here we evaluated, for the first time, the in situ 18 O discrimination associated with soil respiration in natural ecosystems. The discrimination was estimated from the measured [O2] and d 18 Oo f O2 in the soilair. The discriminations that were found are 10.1 ± 1.5%, 17.8 ± 1.0%, and 22.5 ± 3.6%, for tropical, temperate, and boreal forests, respectively, 17.9 ± 2.5% for Mediterranean woodland, and 15.4 ± 1.6% for tropical shrub land. Current understanding of the isotopic composition of atmospheric O2 is based on the assumption that the magnitude of the fractionation in soil respiration is identical to that of dark respiration through the cytochrome pathway alone (� 18%). The discrimination we found in the tropical sites is significantly lower, and is explained by slow diffusion in soil aggregates and root tissues that limits the O2 concentration in the consumption sites. The high discrimination in the boreal sites may be the result of high engagement of the alternative oxidase pathway (AOX), which has high discrimination associated with it (� 27%). The intermediate discrimination (� 18%) in the temperate and Mediterranean sites can be explained by the opposing effects of AOX and diffusion limitation that cancel out. Since soil respiration is a major component of the global oxygen uptake, the contribution of large variations in the discrimination, observed here, to the global Dole Effect should be considered in global scale studies. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/ atmosphere interactions; 0365 Atmospheric Composition and Structure: Troposphere—composition and chemistry; 1040 Geochemistry: Isotopic composition/chemistry; 1615 Global Change: Biogeochemical processes (4805); KEYWORDS: Dole Effect, oxygen isotopes, soil respiration Citation: Angert, A., et al., Contribution of soil respiration in tropical, temperate, and boreal forests to the 18 O enrichment of

The contemporary degassing rate of 40Ar from the solid Earth

Knowledge of the outgassing history of radiogenic 40Ar, derived over geologic time from the radioactive decay of 40K, contributes to our understanding of the geodynamic history of the planet and the origin of volatiles on Earths surface. The 40Ar inventory of the atmosphere equals total 40Ar outgassing during Earth history. Here, we report the current rate of 40Ar outgassing, accessed by measuring the Ar isotope composition of trapped gases in samples of the Vostok and Dome C deep ice cores dating back to almost 800 ka. The modern outgassing rate (1.1 ± 0.1 × 108 mol/yr) is in the range of values expected by summing outgassing from the continental crust and the upper mantle, as estimated from simple calculations and models. The measured outgassing rate is also of interest because it allows dating of air trapped in ancient ice core samples of unknown age, although uncertainties are large (±180 kyr for a single sample or ±11% of the calculated age, whichever is greater).