Bibliography - N. Cassar

1. Hamme, Roberta C., N. Cassar, Veronica P. Lance, Robert D. Vaillancourt, Michael Bender, Peter G. Strutton, Tommy S. Moore, Michael D. DeGrandpre, C. L. Sabine, D. T. Ho, and Bruce R. Hargreaves, 2012: Dissolved O₂/Ar and other methods reveal rapid changes in productivity during a Lagrangian experiment in the Southern Ocean. Journal of Geophysical Research, American Geophysical Union, 117(C00F12), doi:10.1029/2011JC007046
   [ Abstract ]

   We use continuous and discrete measurements of the dissolved O₂/Ar ratio in the mixed layer to investigate the dynamics of biological productivity during the Southern Ocean Gas Exchange Experiment in March and April 2008. Injections of SF₆ defined two water masses (patches) that were followed for up to 2 weeks. In the first patch, dissolved O₂/Ar was supersaturated, indicating net biological production of organic carbon. In the second patch, rapidly decreasing O₂/Ar could only be reasonably explained if the mixed layer was experiencing a period of net heterotrophy. The observations rule out dominant contributions from vertical mixing, lateral dilution, or respiration in the ship's underway seawater supply lines. We also compare nine different estimates of net community, new, primary, or gross production made during the experiment. Net community and new production estimates agreed well in the first patch but disagreed in the second patch, both during an initial net heterotrophic period but also during the apparently autotrophic period at the end of the observations. Rapidly changing productivity during the second patch complicated the comparison of methods that integrate over daily and several week timescales. Primary productivity values from on-deck 24 h ¹⁴C incubations and gross carbon production values from photosynthesis-irradiance experiments were nearly identical even during highly dynamic periods of net heterotrophy, while gross oxygen production measurements were 3.5—4.2 times higher but with uncertainties in that ratio near ±2. These comparisons show that the photosynthesis-irradiance experiments based on 1—2 h ¹⁴C incubations underestimated gross carbon production.

2. Huang, Kuan, Hugh Ducklow, Maria Vernet, N. Cassar, and Michael Bender, 2012: Export production and its regulating factors in the West Antarctica Peninsula region of the Southern Ocean. Global Biogeochemical Cycles, American Geophysical Union, 26(GB2005), doi:10.1029/2010GB004028
   [ Abstract ]

   In connection with the Palmer LTER program, mixed layer water samples were collected during the cruise of the L.M. Gould in Jan., 2008 at 49 stations on a 20 x 100 km grid in the West Antarctica Peninsula (WAP) region of the Southern Ocean. In this study, [O₂]/[Ar] ratios and the triple isotope composition of dissolved O₂ were measured, and were used to estimate net community O₂ production (NCP) and gross primary O₂ production (GPP), respectively. These estimates are further converted to carbon export production, primary production and the f-ratio. Our measurements give NCP ranging from -3 to 76 mmol O₂ m^-2 day ^-1 (-25 to 650 mg C m ^-2 day ^-1), and GPP from 40 to 220 mmol O₂ m ^-2 day ^-1 (180 to 1010 mg C m ^-2 day ^-1). The O₂ NCP/GPP ratios range from -0.04 to 0.43, corresponding to f-ratios of -0.08 to 0.83. NCP and the NCP/GPP ratio are highest in the northern coastal areas, and decrease to lower values toward the southern coastal area and the open ocean. The inshore-offshore gradient appears to be regulated primarily by iron availability, as supported by the positive correlation between NCP and F_v/F_m ratios (r² = 0.22, p < 0.05). Mixed layer depth (MLD) is inversely correlated with NCP (r² = 0.21, p < 0.002) and NCP/GPP (r² = 0.21, p < 0.02), and highest NCP occurred in the fresh water lenses probably formed from melted coastal glaciers. These results suggest that export production and the f-ratio increase where water stratification is intensified by input of fresh meltwater, and that mixed layer stratification is the major factor regulating NCP in the inner-shelf and coastal regions. Along-shelf variability of phytoplankton community composition is highly correlated with NCP, i.e., NCP increases when the diatom-dominated community in the south transitions to the cryptophyte-dominated one in the north. A high correlation is also observed between NCP and the logarithm of the surface chlorophyll concentration (r² = 0.72, p < 0.0001), which makes it possible to estimate carbon export as a function of Chl a concentration in this region.

3. Bender, Michael, Saul Kinter, N. Cassar, and R. Wanninkhof, 2011: Evaluating gas transfer velocity parameterizations using upper ocean radon distributions. Journal of Geophysical Research, American Geophysical Union, 116(C02010), doi:10.1029/2009JC005805
   [ Abstract ]

   Sea-air fluxes of gases are commonly calculated from the product of the gas transfer velocity (k) and the departure of the seawater concentration from atmospheric equilibrium. Gas transfer velocities, generally parameterized in terms of wind speed, continue to have considerable uncertainties, partly because of limited field data. Here we evaluate commonly used gas transfer parameterizations using a historical data set of ²²²Rn measurements at 105 stations occupied on Eltanin cruises and the Geosecs program. We make this evaluation with wind speed estimates from meteorological reanalysis products (from National Centers for Environmental Prediction and European Centre for Medium-Range Weather Forecasting) that were not available when the ²²Rn data were originally published. We calculate gas transfer velocities from the parameterizations by taking into account winds in the period prior to the date that ²²²Rn profiles were sampled. Invoking prior wind speed histories leads to much better agreement than simply calculating parameterized gas transfer velocities from wind speeds on the day of sample collection. For individual samples from the Atlantic Ocean, where reanalyzed winds agree best with observations, three similar recent parameterizations give k values for individual stations with an rms difference of ~40% from values calculated using ²²²Rn data. Agreement of basin averages is much better. For the global data set, the average difference between k constrained by ²²²Rn and calculated from the various parameterizations ranges from -0.2 to +0.9 m/d (average, 2.9 m/d). Averaging over large domains, and working with gas data collected in recent years when reanalyzed winds are more accurate, will further decrease the uncertainties in sea-air fluxes.

4. Cassar, N., P. J. DiFiore, B. Barnett, Michael Bender, A. R. Bowie, Bronte Tilbrook, K. Petrou, K. J. Westwood, S. W. Wright, and D. Lefevre, 2011: The influence of iron and light on net community production in the Subantarctic and Polar Frontal Zones. Biogeosciences, Copernicus Publications, 8, doi:10.5194/bg-8-227-2011 227-237
   [ Abstract ]

   The roles of iron and light in controlling biomass and primary productivity are clearly established in the Southern Ocean. However, their influence on net community production (NCP) and carbon export remains to be quantified. To improve our understanding of NCP and carbon export production in the Subantarctic Zone (SAZ) and the northern reaches of the Polar Frontal Zone (PFZ), we conducted continuous onboard determinations of NCP as part of the Sub-Antarctic Sensitivity to Environmental Change (SAZ Sense) study, which occurred in January-February 2007. Biological O₂ supersaturation was derived from measuring O₂/Ar ratios by equilibrator inlet mass spectrometry. Based on these continuous measurements, NCP during the austral summer 2007 in the Australian SAZ was approximately 43 mmol O₂ m^-2 d^-1. NCP showed significant spatial variability, with larger values near the Subtropical front, and a general southward decrease. For shallower mixed layers (<50 m), dissolved Fe concentrations and Fe sufficiency, estimated from variable fluorescence, correlated strongly with NCP. The strong correlation between NCP and dissolved Fe may be difficult to interpret because of the correlation of dissolved Fe to MLD and because the concentration of iron may not be a good indicator of its availability. At stations with deeper mixed layers, NCP was consistently low, regardless of iron sufficiency, consistent with light availability also being an important control of NCP. Our new observations provide independent evidence for the critical roles of iron and light in mediating carbon export from the Southern Ocean mixed layer.

5. Cassar, N., B. Barnett, Michael Bender, J. Kaiser, Roberta C. Hamme, and Bronte Tilbrook, 2009: Continuous High-Frequency Dissolved O₂/Ar Measurements by Equilibrator Inlet Mass Spectrometry. Analytical Chemistry, 81, doi:10.1021/ac802300u 1855-1864
   [ Abstract ]

   The oxygen (O₂) 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 O₂ supersaturation and determine the biological oxygen supersaturation. Biological O₂ 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 O₂/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 ship’s 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 O₂/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 O₂/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 N₂/Ar and pCO₂ 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.

6. Cassar, N., G. McKinley, Michael Bender, R. Mika, and M. Battle, 2008: An improved comparison of atmospheric Ar/N₂ time series and paired ocean-atmosphere model predictions. Journal of Geophysical Research – Atmosphere, 113(D21122), doi:10.1029/2008JD009817
   [ Abstract ]

   Ar/N₂ variations in the atmosphere reflect ocean heat fluxes, air-sea gas exchange, and atmospheric dynamics. Here atmospheric Ar/N₂ time series are compared to paired ocean-atmosphere model predictions. Agreement between Ar/N₂ observations and simulations has improved in comparison to a previous study because of longer time series and the introduction of automated samplers at several of the atmospheric stations, as well as the refinement of the paired ocean-atmosphere models by inclusion of Ar and N₂ as active tracers in the ocean component. Although analytical uncertainties and collection artifacts are likely to be mainly responsible for observed Ar/N₂ outliers, air parcel back-trajectory analysis suggests that some of the variability in Ar/N₂ measurements could be due to the low-altitude history of the air mass collected and, by extension, the local oceanic Ar/N₂ signal. Although the simulated climatological seasonal cycle can currently be evaluated with Ar/N₂ observations, longer time series and additional improvements in the signal-to-noise ratio will be required to test other model predictions such as interannual variability, latitudinal gradients, and the secular increase in atmospheric Ar/N₂ expected to result from ocean warming.

7. Bender, Michael, D. T. Ho, M. B. Hendricks, R. Mika, M. Battle, P. P. Tans, T. J. Conway, B. Sturtevant, and N. Cassar, 2005: Atmospheric O₂/N₂ changes, 1993–2002: Implications for the partitioning of fossil fuel CO₂ sequestration. Global Biogeochemical Cycles, 19(GB4017), doi:10.1029/2004GB002410
   [ Abstract ]

   Improvements made to an established mass spectrometric method for measuring changes in atmospheric O₂/N₂ 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 O₂ in air. Records of changes in O₂/N₂ from six global sampling stations (Barrow, American Samoa, Cape Grim, Amsterdam Island, Macquarie Island, and Syowa Station) are presented. Combined with measurements of CO₂ 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 CO₂ 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 O₂. Interannual variability calculated from these data shows a strong land carbon source associated with the 1997–1998 El Nin˜o event, supporting many previous studies indicating that high atmospheric growth rates observed during most El Ninõ 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 O₂. The origin of these fluxes is not yet understood.

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