Showing papers by "Wade R. McGillis published in 2009"

Advances in Quantifying Air-Sea Gas Exchange and Environmental Forcing*

Abstract: The past decade has seen a substantial amount of research on air-sea gas exchange and its environmental controls. These studies have significantly advanced the understanding of processes that control gas transfer, led to higher quality field measurements, and improved estimates of the flux of climate-relevant gases between the ocean and atmosphere. This review discusses the fundamental principles of air-sea gas transfer and recent developments in gas transfer theory, parameterizations, and measurement techniques in the context of the exchange of carbon dioxide. However, much of this discussion is applicable to any sparingly soluble, non-reactive gas. We show how the use of global variables of environmental forcing that have recently become available and gas exchange relationships that incorporate the main forcing factors will lead to improved estimates of global and regional air-sea gas fluxes based on better fundamental physical, chemical, and biological foundations.

Effects of freezing, growth, and ice cover on gas transport processes in laboratory seawater experiments

Abstract: [1] Gas exchange through sea ice is a determining factor in the polar ocean budget of climatically-active gases. We use SF6 and O2 as conservative gas tracers to observe transport between the water, ice and air during conditions of freezing and partial ice cover in artificial seawater. During ice growth, O2 and SF6, as non-polar solutes, were rejected from the ice into the underlying water at a faster rate than that observed for salt. Measurements of the gas exchange rate, k, through partial ice cover exceeded that expected from linear scaling between 100% open water (k100%) and complete ice cover: at 15% open water, k was 25% of k100%. These results indicate that the net flux of gas through the ice pack may not scale linearly with open water area, as circulation processes under the ice affect the gas exchange rate.

Episodic riverine influence on surface DIC in the coastal Gulf of Maine

Abstract: Anomalously high precipitation and river discharge during the spring of 2005 caused considerable freshening and depletion of dissolved inorganic carbon (DIC) in surface waters along the coastal Gulf of Maine Surface pCO2 and total alkalinity (TA) were monitored by repeated underway sampling of a cross-shelf transect in the western Gulf of Maine (GOM) during 2004–05 to examine how riverine fluxes, mixing, and subsequent biological activity exert control on surface DIC in this region Most of the variability in surface DIC concentration was attributable to mixing of low DIC river water with higher DIC, saline GOM waters, but net biological uptake of DIC was significant especially during the spring and summer seasons The extent and persistence of the coastal freshwater intrusion exerted considerable influence on net carbon dynamics Integrated over the 10-m surface layer of our study region (∼5 × 104 km2), net biological DIC uptake was 048 × 108 mol C during April–July of 2004 compared to 133 × 108 mol C during April–July of 2005 We found the temporal signature and magnitude of DIC cycling to be different in adjacent plume-influenced and non-plume regions Extreme events such as the freshwater anomaly observed in 2005 will affect mean estimates of coastal carbon fluxes, thus budgets based on short time series of observations may be skewed and should be viewed with caution

Rain‐induced turbulence and air‐sea gas transfer

Abstract: =4 for a range of rain rates with broad drop size distributions. The hydrodynamic measurements elucidate the mechanisms responsible for the rain-enhanced k results using SF6 tracer evasion and active controlled flux technique. High-resolution k and turbulence results highlight the causal relationship between rainfall, turbulence, stratification, and air-sea gas exchange. Profiles of e beneath the air-sea interface during rainfall, measured for the first time during a gas exchange experiment, yielded discrete values as high as 10 �2 Wk g �1 . Stratification modifies and traps the turbulence near the surface, affecting the enhancement of the transfer velocity and also diminishing the vertical mixing of mass transported to the air-water interface. Although the kinetic energy flux is an integral measure of the turbulent input to the system during rain events, e is the most robust response to all the modifications and transformations to the turbulent state that follows. The Craig-Banner turbulence model, modified for rain instead of breaking wave turbulence, successfully predicts the near-surface dissipation profile at the onset of the rain event before stratification plays a dominant role. This result is important for predictive modeling of k as it allows inferring the surface value of e fundamental to gas transfer.

Sea surface pCO2 and carbon export during the Labrador Sea spring-summer bloom: An in situ mass balance approach

Abstract: [1] We report depth-resolved in situ time series of the partial pressure of CO2 (pCO2) and other carbon-related parameters spanning the development and decline of a high-latitude phytoplankton bloom. A suite of sensors was deployed on a mooring in the Labrador Sea from June to August 2004. The study became quasi-Lagrangian when the mooring broke free in late June. Measured parameters included pCO2, chlorophyll a fluorescence, beam c, optical backscatter, and photosynthetically active radiation. During the bloom, the pCO2 was drawn down from 330 to 260 μatm, corresponding to a 70 μmol kg−1 decrease of dissolved inorganic carbon (DIC). One-dimensional model results suggest that the observed drawdown was primarily driven by local processes and contributions from horizontal advection were minimal. A mass balance using the DIC and particulate organic carbon found that 47 mmol C m−2 d−1 of DIC was assimilated into biomass. The bloom biomass was not remineralized in the mixed layer but was rapidly exported below 35 m within 15 days of the bloom. As a consequence, the large air-sea pCO2 gradient persisted, and approximately 30% of the DIC was regained through air-sea exchange by the end of the study. It is likely that all of the exported organic matter, corresponding to 5.4 ± 1.9 Tg of carbon, was replaced by atmospheric CO2 prior to the onset of deep convective mixing.

O 2 -MAVS: An instrument for measuring oxygen flux

Abstract: With the awareness that the surface ocean is becoming more acidic due to the uptake of anthropogenic C0 2 and that the resulting decrease in carbonate ion concentration is contributing to a decline in the calcification rate of many organisms, there is a pressing need to develop methods that can easily and reliably measure the calcification rates of these organisms under natural conditions so that any changes that may be occurring can be detected. Recent advances in control volume [1] and benthic boundary layer eddy correlation methods [2, 3] promise to make continuous non-invasive measurements of net oxygen community production and respiration a reality. These methods, however, cannot be extended to the measurement of calcification rate because no suitable sensor for the measurement of total alkalinity (TA) exists at the present time. We have a solution that is based on measuring the vertical gradients in current velocity and the chemical constituent of interest in the boundary layer. The vertical gradient in velocity gives us K z and K z times oC/δζ gives us the flux of constituent C into or out of the seafloor. The method can be applied to the measurement of calcification but could also be applied to the measurement of the flux of any chemical or biological constituent that can be determined from a water sample, in this case 0 2 . Water samples could be collected with an automated water sampler preferably slowly over the period of an hour so that high frequency variability that cannot be sampled would be averaged out. Here we present preliminary data using an Acoustic Doppler Velocimeter (ADV), Modular Acoustic Velocity Sensor (MAVS) and two Aanderaa oxygen optodes. Three test deployments were made on shallow, warm-water coral reefs in La Parguera, Puerto Rico. Time series of net production obtained using the boundary layer gradient flux method (GF) were compared with measurements based on the Eulerian upstream-downstream flow respirometry method and a 1-D water column mass balance method with air-sea gas exchange. The agreement between methods was very encouraging, thus validating the 0 2 -MAVS as an instrument capable of measuring oxygen flux.