Nicolas Cassar

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.

Bicarbonate uptake by Southern Ocean phytoplankton

uptake observed was attributable to direct HCO3 uptake, the other half being direct CO2 uptake mediated either by passive diffusion or active uptake mechanisms. The increase in growth rates and decrease in CO2 concentration associated with the iron fertilization did not trigger any noticeable changes in the mode of DIC acquisition, indicating that under most environmental conditions the carbon concentrating mechanism (CCM) is constitutive. A low-CO2 treatment induced an increase in uptake of CO2, which we attributed to increased extracellular carbonic anhydrase activity, at the expense of direct HCO3 transport across the plasmalemma. Isotopic disequilibrium experimental results are consistent with Southern Ocean carbon stable isotope fractionation data from this and other studies. Although iron fertilization has been shown to significantly enhance phytoplankton growth and may potentially increase carbon flux to the deep ocean, an important source of the inorganic carbon taken up by phytoplankton in this study was HCO3 � , whose concentration is negligibly affected by the anthropogenic rise in CO2. We conclude that biological productivity in this region of the world’s ocean is unlikely to be directly regulated by natural or anthropogenic variations in atmospheric CO2 concentrations because of the presence of a constitutive CCM. INDEX TERMS: 4207 Oceanography: General: Arctic and Antarctic oceanography; 4806 Oceanography: Biological and Chemical: Carbon cycling; 4853 Oceanography: Biological and Chemical: Photosynthesis; 4870 Oceanography: Biological and Chemical: Stable isotopes; KEYWORDS: bicarbonate, phytoplankton, Southern Ocean

13C discrimination patterns in oceanic phytoplankton: likely influence of CO2 concentrating mechanisms, and implications for palaeoreconstructions

The isotopic composition of organic carbon buried in marine sediments is an appealing proxy for palaeo CO2 concentrations due to the well-documented effect of CO2 concentrations on carbon fractionation by phytoplankton. However, a number of factors, in addition to CO2 concentrations, influence this fractionation. Included among these factors are cell geometry, in particular surface/volume ratios, growth rate, and the presence of CO2 concentrating mechanisms. Other potentially confounding factors are calcification, diagenesis, and the nature of the growth-rate-limiting factor, e.g. light vs nutrients. Because of these confounding factors, palaeoreconstructions based on the isotopic composition of organic carbon (δ13C) will almost certainly have to be based on the isotopic signatures of organic compounds that can be associated with a single species, or group of physiologically similar species. Long-chain alkenones produced by certain species of coccolithophores may provide a suitable diagnostic marker. By combining the δ13C of the alkenone carbon with the δ13C of coccolith carbon and the Sr/Ca ratio of the coccoliths, it is possible to calculate the extent of carbon fractionation (εp) and estimate growth rates. However, active transport of inorganic carbon tends to make εp insensitive to CO2 concentrations when the ratio of growth rate to CO2 concentration exceeds 0.285/rkg mol-1d-1, where r is the effective spherical radius of the cell in microns. Palaeo CO2 concentrations calculated from alkenone and coccolith δ13C data capture the gross features of CO2 concentrations in the Vostok ice core, but explain only 30-35% of the variance in the latter. The absence of a higher correlation may in part reflect the impact of active transport, particularly during glacial times. The impact of active transport may have been less severe prior to the Pleistocene, since CO2 concentrations are believed to have been higher than present-day values during most of Phanerozoic time.

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.

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.

The Southern Kalahari: a potential new dust source in the Southern Hemisphere?

Most sources of atmospheric dust on Earth are located in the Northern Hemisphere. The lower dust emissions in the Southern Hemisphere in part limit the supply of micronutrients (primarily soluble iron) to the Southern Ocean, thereby constraining its productivity. Climate and land use change can alter the current distribution of dust source regions on Earth. Can new dust sources be activated in the Southern Hemisphere? Here we show that vegetation loss and dune remobilization in the Southern Kalahari can promote dust emissions comparable to those observed from major contemporary dust sources in the Southern African region. Dust generation experiments support the hypothesis that, in the Southern Kalahari, aeolian deposits that are currently mostly stabilized by savanna vegetation are capable of emitting substantial amounts of dust from interdune areas. We show that dust from these areas is relatively rich in soluble iron, an important micronutrient for ocean productivity. Trajectory analyses show that dust from the Kalahari commonly reaches the Southern Ocean and could therefore enhance its productivity.

The influence of sea-ice cover on air-sea gas exchange estimated with radon-222 profiles

Air-sea gas exchange plays a key role in the cycling of greenhouse and other biogeochemically important gases. Although air-sea gas transfer is expected to change as a consequence of the rapid decline in summer Arctic sea ice cover, little is known about the effect of sea ice cover on gas exchange fluxes, especially in the marginal ice zone. During the Polarstern expedition ARK-XXVI/3 (TransArc, August/September 2011) to the central Arctic Ocean, we compared 222Rn/226Ra ratios in the upper 50 m of 14 ice-covered and 4 ice-free stations. At three of the ice-free stations, we find 222Rn-based gas transfer coefficients in good agreement with expectation based on published relationships between gas transfer and wind speed over open water when accounting for wind history from wind reanalysis data. We hypothesize that the low gas transfer rate at the fourth station results from reduced fetch due to the proximity of the ice edge, or lateral exchange across the front at the ice edge by restratification. No significant radon deficit could be observed at the ice-covered stations. At these stations, the average gas transfer velocity was less than 0.1 m/d (97.5% confidence), compared to 0.5–2.2 m/d expected for open water. Our results show that air-sea gas exchange in an ice-covered ocean is reduced by at least an order of magnitude compared to open water. In contrast to previous studies, we show that in partially ice-covered regions, gas exchange is lower than expected based on a linear scaling to percent ice cover.

Estimating net community production in the Southern Ocean based on atmospheric potential oxygen and satellite ocean color data

(1) The seasonal cycle of atmospheric potential oxygen (APOO2 + 1.1 CO2) reflects three seasonally varying ocean processes: 1) thermal in- and outgassing, 2) mixed layer net community production (NCP) and 3) deep water ventilation. Previous studies have isolated the net biological seasonal signal (i.e., the sum of NCP and ventilation), after using air-sea heat flux data to estimate the thermal signal. In this study, we resolve all three components of the APO seasonal cycle using a methodology in which the ventilation signal is estimated based on atmospheric N2O data, the thermal signal is estimated based on heat flux or atmospheric Ar/N2 data, and the production signal is inferred as a residual. The isolation of the NCP signal in APO allows for direct comparison to estimates of NCP based on satellite ocean color data, after translating the latter into an atmospheric signal using an atmospheric transport model. When applied to ocean color data using algorithms specially adapted to the Southern Ocean and APO data at three southern monitoring sites, these two independent methods converge on a similar phase and amplitude of the seasonal NCP signal in APO and yield an estimate of annual mean NCP south of 50°S of 0.8-1.2 Pg C/yr, with corresponding annual mean NPP of � 3 Pg C/yr and a mean growing season f ratio of � 0.33. These results are supported by ocean biogeochemistry model simulations, in which air-sea O2 and N2O fluxes are resolved into component thermal, ventilation and (for O2) NCP contributions. Citation: Nevison, C. D., R. F. Keeling, M. Kahru, M. Manizza, B. G. Mitchell, and N. Cassar (2012), Estimating net community production in the Southern Ocean based on atmospheric potential oxygen and satellite ocean color data, Global Biogeochem. Cycles, 26, GB1020, doi:10.1029/2011GB004040.

Potential contribution of β-carboxylases to photosynthetic carbon isotope fractionation in a marine diatom

N. Cassar and E.A. Laws. 2007. Potential contribution of β-carboxylases to photosynthetic carbon isotope fractionation in a marine diatom. Phycologia 46: 307–314. DOI: 10.2216/06-50.1 In vitro activities of phosphoenolpyruvate carboxylase (PEPC) and phosphoenolpyruvate carboxykinase (PEPCK) were measured in chemostat cultures of the marine diatom Phaeodactylum tricornutum grown under either phosphate-or nitrate-limited conditions and at low (1 μmol kg−1) and high (70–100 μmol kg−1) CO2 concentrations. A comparison of in vitro methods for measuring PEPC activity underscored the importance of including glycerol in the reaction mixture because of its ability to stabilize the quaternary structure of PEPC. Assays for total β-carboxylase activity (PEPC + PEPCK) identified the importance of substituting Mn2+ for Mg2+ as an activator for PEPC since Mg2+ is inhibitory to PEPCK activity. Results indicated that PEPC was constitutive, that is, present in a constant ratio relative to the limiting nutrient over a wide range of growth conditions. PEPCK acclimation was greater, and its intracellular concentration was positively correlated with CO2 concentrations. In vitro β-carboxylase activity averaged 13% of in vivo carbon fixation rates. The low percentage of β- carboxylase activity and the positive correlation between PEPCK and CO2 concentration argue against a C4-type pathway in P. tricornutum; that is, β-carboxylase activity appears related to strictly anaplerotic processes. Calculations indicate that β -carboxylase activity reduces the fractionation associated with carbon fixation but by no more than 2‰.

Correcting oceanic O2/Ar-net community production estimates for vertical mixing using N2O observations

The O2/Ar approach has become a key method to estimate oceanic net community production (NCP). However, in some seasons and regions of the ocean, strong vertical mixing of O2-depleted deepwater introduces a large error into O2/Ar-derived NCP estimates. In these cases, undersaturated-O2/Ar observations have for all intents and purposes been ignored. We propose to combine underway O2/Ar and N2O observations into a composite tracer that is conservative with respect to the influence of vertical mixing on the surface biological O2 inventory. We test the proposed method with an ocean observing system simulation experiment (OSSE) in which we compare N2O-O2/Ar and O2/Ar-only gas flux estimates of NCP to the model-simulated true NCP in the Southern Ocean. Our proof-of-concept simulations show that the N2O-O2/Ar tracer significantly improves NCP estimates when/where vertical mixing is important.