H. Thomas

Dalhousie University, Halifax, Nova Scotia, Canada

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Publications (43)37.86 Total impact

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    ABSTRACT: Sediment-water column exchange plays an important role in coastal biogeochemistry. We utilize short-lived radium isotopes (224Ra and 223Ra) to understand and quantify the dominant processes governing sediment-water column exchange throughout the North Sea. Our comprehensive survey, conducted in September 2011, represents the first of its kind conducted in the North Sea. We find that two main sources regulate surface Ra distributions: minor coastal input from rivers and shallow mudflats, and North Sea sediments as the dominant source. Porewaters show 100-fold larger activities than the water column. North Sea sediment characteristics such as porosity and mean grain size, as well as turbulence at the sediment-water interface, are the dominant factors contributing to variability of Ra efflux. Ra inventory and mass-balance approaches consistently yield high benthic Ra effluxes in the southern North Sea, driven by strong tidal and wind mixing, which in turn cause high sediment irrigation rates. These results exceed incubation-based Ra flux estimates, and the majority of previously reported Ra flux estimates for other regions. Ra-based estimates of benthic alkalinity fluxes compare well to observed values and the high rates of Ra efflux imply a potentially significant exchange of other products of sedimentary reactions, including carbon and nutrient species. Passive tracer simulations lend strong support to the Ra source attribution and imply seasonal variation in the surface water Ra distribution depending on stratification conditions.
    Global Biogeochemical Cycles. 07/2014;
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    ABSTRACT: Polar oceans are chemically sensitive to anthropogenic acidification due to their relatively low alkalinity and correspondingly weak carbonate buffering capacity. Here, we compare unique CO2 system observations covering complete annual cycles at an Arctic (Amundsen Gulf) and Antarctic site (Prydz Bay). The Arctic site experiences greater seasonal warming (10 vs 3°C), and freshening (3 vs 2), has lower alkalinity (2220 vs 2320 μmol/kg), and lower summer pH (8.15 vs 8.5), than the Antarctic site. Despite a larger uptake of inorganic carbon by summer photosynthesis, the Arctic carbon system exhibits smaller seasonal changes than the more alkaline Antarctic system. In addition, the excess surface nutrients in the Antarctic may allow mitigation of acidification, via CO2 removal by enhanced summer production driven by iron inputs from glacial and sea-ice melting. These differences suggest that the Arctic system is more vulnerable to anthropogenic change due to lower alkalinity, enhanced warming, and nutrient limitation.
    Scientific Reports 08/2013; 3:2339. · 5.08 Impact Factor
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    ABSTRACT: As a response to public demand for a welldocumented, quality controlled, publically available, global surface ocean carbon dioxide (CO2) data set, the international marine carbon science community developed the Surface Ocean CO2 Atlas (SOCAT). The first SOCAT product is a collection of 6.3 million quality controlled surface CO2 data from the global oceans and coastal seas, spanning four decades (1968–2007). The SOCAT gridded data presented here is the second data product to come from the SOCAT project. Recognizing that some groups may have trouble working with millions of measurements, the SOCAT gridded product was generated to provide a robust, regularly spaced CO2 fugacity (fCO2) product with minimal spatial and temporal interpolation, which should be easier to work with for many applications. Gridded SOCAT is rich with information that has not been fully explored yet (e.g., regional differences in the seasonal cycles), but also contains biases and limitations that the user needs to recognize and address (e.g., local influences on values in some coastal regions).
    Earth System Science Data. 07/2013; 5:145-153.
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    ABSTRACT: A well-documented, publicly available, global data set of surface ocean carbon dioxide (CO2) parameters has been called for by international groups for nearly two decades. The Surface Ocean CO2 Atlas (SOCAT) project was initiated by the international marine carbon science community in 2007 with the aim of providing a comprehensive, publicly available, regularly updated, global data set of marine surface CO2, which had been subject to quality control (QC). Many additional CO2 data, not yet made public via the Carbon Dioxide Information Analysis Center (CDIAC), were retrieved from data originators, public websites and other data centres. All data were put in a uniform format following a strict protocol. Quality control was carried out according to clearly defined criteria. Regional specialists performed the quality control, using state-of-the-art web-based tools, specially developed for accomplishing this global team effort. SOCAT version 1.5 was made public in September 2011 and holds 6.3 million quality controlled surface CO2 data points from the global oceans and coastal seas, spanning four decades (1968–2007). Three types of data products are available: individual cruise files, a merged complete data set and gridded products. With the rapid expansion of marine CO2 data collection and the importance of quantifying net global oceanic CO2 uptake and its changes, sustained data synthesis and data access are priorities
    Earth System Science Data. 07/2013; 5:125-143.
  • W.J. Burt, H. Thomas, J.-P. Auclair
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    ABSTRACT: Radium (Ra) isotopes have become an established tool for investigating mixing rates on continental shelves, and more recently have been used to quantify the release of dissolved compounds enriched in pore-waters into the water column. We present results from Ra sampling of the Scotian Shelf region of the Canadian northwestern Atlantic Ocean, which reveal cross-shelf Ra distributions that are unique compared to other coastal regions. We explain the observations of lower 224Ra activities near the coast, relatively high activities at large distances offshore (> 100 km), and gradients in both offshore and onshore directions by referring to regional geomorphology, as well as shelf bathymetry and circulation patterns. Vertical mixing above shallow offshore banks allows for Ra enrichments in offshore surface waters, while horizontal dispersion of this bottom-generated signal can transport Ra off the shelf break in surface waters, and towards the shore beneath the surface mixed layer. On the Scotian Shelf, estimating horizontal diffusivity (KX) using a 1-D Ra diffusion model is limited by the presence of two distinct Ra sources (land and sediment), by three dimensional shelf topography, as well as by complex hydrodynamic conditions. Here, a numerical 2-D diffusion model reproduces Ra distributions on the shelf using both coastal and benthic sources. Horizontal and vertical mixing coefficients are then calculated by minimizing deviations between model output and observations. Onshore gradients in CO2 and nutrient species combined with model-derived KX values can yield onshore carbon and nutrient fluxes in subsurface waters, which in turn supply the CO2 outgassing from the Scotian Shelf. Our results thus provide constraints for cross-shelf transports of carbon and nutrients on the Scotian Shelf in order to guide mass balance or model based budget approaches in future studies.
    Marine Chemistry 01/2013; 156:120–129. · 3.00 Impact Factor
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    ABSTRACT: Underway and in situ observations of surface ocean pCO2, combined with satellite data, were used to develop pCO2 regional algorithms to analyze the seasonal and interannual variability of surface ocean pCO2 and sea-air CO2 flux for five physically and biologically distinct regions of the eastern North American continental shelf: the South Atlantic Bight (SAB), the Mid-Atlantic Bight (MAB), the Gulf of Maine (GoM), Nantucket Shoals and Georges Bank (NS+GB), and the Scotian Shelf (SS). Temperature and dissolved inorganic carbon variability are the most influential factors driving the seasonality of pCO2. Estimates of the sea-air CO2 flux were derived from the available pCO2 data, as well as from the pCO2 reconstructed by the algorithm. Two different gas exchange parameterizations were used. The SS, GB+NS, MAB, and SAB regions are net sinks of atmospheric CO2 while the GoM is a weak source. The estimates vary depending on the use of surface ocean pCO2 from the data or algorithm, as well as with the use of the two different gas exchange parameterizations. Most of the regional estimates are in general agreement with previous studies when the range of uncertainty and interannual variability are taken into account. According to the algorithm, the average annual uptake of atmospheric CO2 by eastern North American continental shelf waters is found to be between -3.4 and -5.4 Tg C yr-1 (areal average of -0.7 to -1.0 mol CO2 m-2 yr-1) over the period 2003-2010.
    Journal of Geophysical Research 01/2013; 118:5439–5460. · 3.17 Impact Factor
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    W J Burt, H Thomas, K Fennel, E Horne
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    ABSTRACT: Exchanges between sediment pore waters and the overlying water column play a significant role in the chemical budgets of many important chemical constituents. Quantification of such benthic fluxes requires explicit knowledge of the sediment properties and biogeochemistry. Alternatively, changes in water column properties near the sediment-water column interface can be exploited to gain insight into the sediment biogeochemistry and benthic fluxes. Here, we apply a 1-D diffusive mixing model to near-bottom water column profiles of 224Ra activity in order to yield vertical eddy diffusivities (KZ), based upon which we assess the diffusive exchange of inorganic carbon (DIC), nutrients and oxygen (O2), across the sediment-water interface in a coastal inlet, Bedford Basin, Nova Scotia, Canada. Near-bottom observations of DIC, O2 and nutrients provide flux ratios similar to Redfield values, suggesting that benthic respiration of primarily marine organic matter is the dominant driver. Furthermore, we did not observe any significant release of alkalinity (AT) from the sediments to the overlying water column, providing further insight into the dominant reactions taking place within sediments: the respiration of organic matter occurs largely under aerobic conditions or products of anaerobic processes are reoxidized quickly in oxygenated layers of the sediments. Finally, comparison with other carbon sources reveal the observed benthic DIC release as a significant contributor to the Bedford Basin carbon system.
    Biogeosciences 01/2013; 10(1):53-66. · 3.75 Impact Factor
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    ABSTRACT: we made continuous measurements of sea surface partial pressure of CO 2 (pCO 2sw) in three regions of the southeastern Beaufort Sea (Canada): the Amundsen Gulf, the Banks Island Shelf, and the Mackenzie Shelf. All three regions are seasonally ice covered, with mobile winter ice and an early spring opening that defines them as polynya regions. Amundsen Gulf was characterized by undersaturated pCO 2sw (with respect to the atmosphere) in the late fall, followed by an under-ice increase to near saturation in winter, a return to undersaturation during the spring, and an increase to near saturation in summer. The Banks Island Shelf acted similarly, while the Mackenzie Shelf experienced high supersaturation in the fall, followed by a spring undersaturation and a complex, spatially heterogeneous summer season. None of these patterns are similar to the annual cycle described or proposed for other Arctic polynya regions. We hypothesize that the discrepancy reflects the influence of several previously unconsidered processes including fall phytoplankton blooms, upwelling, winter air-sea gas exchange, the continental shelf pump, spring nutrient limitation, summer surface warming, horizontal advection, and riverine input. In order to properly predict current and future rates of air-sea CO 2 exchange in such regions, these processes must be considered on a location-by-location basis. (2012), Annual cycles of pCO 2sw in the southeastern Beaufort Sea: New understandings of air-sea CO 2 exchange in arctic polynya regions, J. Geophys. Res., 117, C00G13, doi:10.1029/2011JC007346.
    Journal of Geophysical Research 11/2012; 117(9). · 3.17 Impact Factor
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    ABSTRACT: Rising atmospheric and in turn oceanic CO2 concentrations cause an ongoing acidification of the marine environment. The pH variations in coastal- and shelf regions can be up to an order of magnitude higher than in the open ocean. The near-shore effects of acidification are difficult to determine, because of the shallow water column and the tight coupling to the benthic environment. Significant variations in fluxes of total alkalinity (TA) exist in association with inflow of nutrients from rivers and pore-water exchange in sediments. TA is an essential part of the carbonate system and hence vital to understand and reliably attribute to the observed decreasing pH values. Dependencies of carbon fluxes and TA in the North Sea, a shelf-sea of the eastern North Atlantic, considering especially the TA produced in the Wadden Sea, is under debate (Thomas et al.,2009). Based on observations in the southern North Sea in 2001 and 2002 they estimated in a first approach a flux of 72.5 Gmol TA yr-1 from the Wadden Sea into the southern North Sea, generated by anaerobic degradation of organic matter. Especially in summer the data reveal a larger amplitude than could be caused by riverine TA alone. The aim of our study is to verify the estimates of Thomas et al. (2009) and to investigate the sensitivity of TA on riverine input. We use the ecosystem model ECOHAM4 (Lorkowski et al., 2012) with a prognostic treatment of TA that is based on the "explicit conservative form of TA" (Wolf-Gladrow, 2007) in order to simulate pelagic TA changes due to biogeochemical and physical processes. Our focus is on different approaches of simulating riverine input of TA and DIC for the years 2001 to 2009: Fixed concentrations (endmember values) for riverine TA and DIC Monthly concentrations for riverine TA and DIC based on observations in the Elbe estuary and different observations and calculations in other rivers We compare our results with observations from 2001, 2002 (Thomas et al., 2009) and 2005 (Bozec et al., 2005) and discuss them against the background of sensitivity on these different treatments. Finally, we recalculate the amount of missing TA that is possibly contributed by the Wadden Sea.
    04/2012;
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    ABSTRACT: Much of the surface ocean carbon cycle variability can be attributed to the availability of sunlight, through processes such as heat fluxes or photosynthesis, which regulate the ocean carbon cycle over a wide range of time scales. The critical processes occurring on timescales of a day or less, however, have undergone few investigations, and most of those have been limited to a time span of several days to months, or exceptionally, for longer periods. Optical methods have helped to infer short-term biological variability, however lacking corresponding investigations of oceanic CO2 system. Here, we employ high-frequency CO2 system and optical observations covering the full seasonal cycle on the Scotian Shelf, Northwestern Atlantic Ocean, in order to unravel daily periodicity of the surface ocean carbon cycle and its effects on annual budgets. We show that significant daily periodicity occurs only if the water column is sufficiently stable as observed during seasonal warming. During that time biological CO2 drawdown, or net community production (NCP), is delayed for several hours relative to the daylight cycle due the daily build-up of essential Chlorophyll a, to cell physiology and to grazing effects, all restricting or hindering photosynthesis in the early morning hours. NCP collapses in summer by more than 90%, when the mixed layer depth reaches the seasonal minimum, which eventually makes the observed daily periodicity of the CO2 system vanish.
    04/2012;
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    ABSTRACT: High biological activity causes a distinct seasonality of surface water pH in the North Sea, which has been identified as a strong sink for atmospheric CO2 via a particularly effective shelf pump. The intimate connection between the North Sea and the North Atlantic suggests that the variability of the CO2 system of the North Atlantic Ocean may in part be responsible for the observed, but hitherto poorly understood variability of pH and CO2 in the North Sea. Here we investigate the role of the North Atlantic Oscillation (NAO), the dominant climate mode for the North Atlantic hemisphere in governing this variability. Based on three extensive observational records covering the relevant levels of the NAO index, we provide evidence that the North Sea pH and CO2 system strongly responds to external and internal expressions of the NAO. We argue that under NAO+ conditions higher rates of inflow of water from the North Atlantic Ocean limits seasonal shoaling of the summer mixed layer in the northern North Sea, diminishing the biological potential to lower pCO2 and raise pH. In addition the faster circulation of the North Sea enhances the shelf pump efficiency. These clear patterns are obscured by changing properties of the North Sea waters, masking or enforcing these effects on various time scales. Such controls indicate that inter-annual trends in the North Sea CO2 system must be carefully examined with consideration to the North Atlantic Oscillation.
    04/2012;
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    ABSTRACT: Much of the variability in the surface ocean's carbon cycle can be attributed to the availability of sunlight, through processes such as heat fluxes and photosynthesis, which regulate over a wide range of time scales. The critical processes occurring on timescales of a day or less, however, have undergone few investigations, and most of these have been limited to a time span of several days to months, or exceptionally, for longer periods. Optical methods have helped to infer short-term biological variability, however corresponding investigations of the oceanic CO2 system are lacking. We employ high-frequency CO2and optical observations covering the full seasonal cycle on the Scotian Shelf, Northwestern Atlantic Ocean, in order to unravel diel periodicity of the surface ocean carbon cycle and its effects on annual budgets. Significant diel periodicity occurs only if the water column is sufficiently stable as observed during seasonal warming. During that time biological CO2 drawdown, or net community production (NCP), is delayed for several hours relative to the onset of photosynthetically available radiation (PAR), due to diel cycles in chlorophyll-a concentration and to grazing, both of which, we suggest, inhibit NCP in the early morning hours. In summer, NCP decreases by more than 90 %, coinciding with the seasonal minimum of the mixed layer depth and resulting in the disappearance of the diel CO2 periodicity in the surface waters.
    Biogeosciences Discussions 02/2012; 9(2):2153-2168.
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    ABSTRACT: We present an Arctic seasonal survey of carbon dioxide partial pressure (pCO2) dynamics within sea ice brine and related air-ice CO2 fluxes. The survey was carried out from early spring to the beginning of summer in the Arctic coastal waters of the Amundsen Gulf. High concentrations of pCO2 (up to 1834 μatm) were observed in the sea ice in early April as a consequence of concentration of solutes in brines, CaCO3 precipitation and microbial respiration. CaCO3 precipitation was detected through anomalies in total alkalinity (TA) and dissolved inorganic carbon (DIC). This precipitation seems to have occurred in highly saline brine in the upper part of the ice cover and in bulk ice. As summer draws near, the ice temperature increases and brine pCO2 shifts from a large supersaturation (1834 μatm) to a marked undersaturation (down to almost 0 μatm). This decrease was ascribed to brine dilution by ice meltwater, dissolution of CaCO3 and photosynthesis during the sympagic algal bloom. The magnitude of the CO2 fluxes was controlled by ice temperature (through its control on brine volume and brine channels connectivity) and the concentration gradient between brine and the atmosphere. However, the state of the ice-interface clearly affects air-ice CO2 fluxes.
    Journal of Geophysical Research (Oceans). 02/2012; 117(C2).
  • 01/2012: pages 109-122;
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    ABSTRACT: Much of the variability in the surface ocean's car-bon cycle can be attributed to the availability of sunlight, trig-gering surface heat flux and photosynthesis, which in turn regulate the biogeochemical cycling of carbon over a wide range of time scales. The critical processes of this carbon cy-cle regulation, occurring at time scales of a day or less, how-ever, have undergone few investigations, most of which have been limited to time spans of several days to months. Opti-cal methods have helped to infer short-term biological vari-ability, but complementing investigations of the oceanic CO 2 system are lacking. We employ high-frequency CO 2 and op-tical observations covering the full seasonal cycle on the Sco-tian Shelf, northwestern Atlantic Ocean, in order to unravel diel periodicity of the surface ocean carbon cycle and its ef-fects on annual budgets. Significant diel periodicity in the surface CO 2 system occurs only if the water column is suffi-ciently stable as observed during seasonal warming. During that time biological CO 2 drawdown, or net community pro-duction (NCP), is delayed for several hours relative to the onset of photosynthetically available radiation (PAR), due to diel cycles in chlorophyll a concentration and to grazing. In summer, NCP decreases by more than 90 %, coinciding with the seasonal minimum of the mixed layer depth and result-ing in the disappearance of the diel CO 2 periodicity in the surface waters.
    Biogeosciences 01/2012; 9:2301-2309. · 3.75 Impact Factor
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    ABSTRACT: Relative to their surface areas, coastal oceans and continental shelves host a disproportionately large fraction of ocean productivity. The Scotian Shelf is a biologically productive coastal region of the Northwestern Atlantic Ocean. This subpolar region is influenced by the outflow of the St. Lawrence Estuary system and acts as an annual source for atmospheric CO2. As part of the Atlantic Zone Monitoring Program, dissolved inorganic carbon (DIC), total alkalinity, and surface CO2 partial pressure measurements were made throughout the Scotian Shelf in 2007. A shelf-wide assessment of the spatio-temporal variability of the inorganic carbon system was made relying on observations in April and September. Between these periods, biological production results in a significant drawdown of inorganic nutrients and DIC in the surface mixed-layer, while hydrographic controls also influence seasonal changes in DIC. Net community production (NCP) over the spring and summer seasons was estimated on the basis of inorganic carbon data. We find significant spatial variability in NCP with the largest values in the Southwestern Browns Bank region and a general trend of increasing NCP with distance offshore. A bulk seasonal carbon budget suggests that along-shore and cross-shelf transport may result in the export of subsurface DIC from this region.
    Biogeosciences Discussions 12/2011; 8(6):12013-12050.
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    ABSTRACT: During an east-to-west transect through the Canadian Arctic Archipelago, dissolved inorganic carbon (DIC) and total alkalinity (TA) were measured. The watermass composition throughout the Archipelago is determined using TA and the seawater oxygen isotope fractionation (δ18O) data, and the carbon characteristics of these waters are examined. The influence of the Mackenzie River is primarily limited to the upper water column in the western Archipelago while the fraction of sea-ice melt water in the surface waters increases eastward with maximum values at the outflows of Jones and Lancaster Sounds. The depth of Pacific-origin upper halocline waters increases eastward through the Archipelago. In the western Archipelago, non-conservative variations in deep water DIC are used to compute a subsurface carbon surplus, which appears to be fueled by organic matter produced in the surface layer and by benthic respiration. The eastward transport of carbon from the Pacific, via the Arctic Archipelago, to the North Atlantic is estimated, and the impact of increased export of sea-ice melt water to the North Atlantic is discussed.
    Continental Shelf Research 01/2011; 31(7):806-816. · 1.89 Impact Factor
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    ABSTRACT: During a year-round occupation of Amundsen Gulf in the Canadian Arctic Archipelago dissolved inorganic and organic carbon (DIC, DOC), total alkalinity (TA), partial pressure of CO2 (pCO2) and related parameters were measured over a full annual cycle. A two-box model was used to identify and assess physical, biological, and chemical processes responsible for the seasonal variability of DIC, DOC, TA, and pCO2. Surface waters were undersaturated with respect to atmospheric CO2 throughout the year and constituted a net sink of 1.2 mol C m-2 yr-1, with ice coverage and ice formation limiting the CO2 uptake during winter. CO2 uptake was largely driven by under ice and open-water biological activity, with high subsequent export of organic matter to the deeper water column. Annual net community production (NCP) was 2.1 mol C m-2 yr-1. Approximately one-half of the overall NCP during the productive season (4.1 mol C m22 from Apr through Aug) was generated by under-ice algae and amounted to 1.9 mol C m-2 over this period. The surface layer was autotrophic, while the overall heterotrophy of the system was fueled by either sedimentary or lateral inputs of organic matter.
    Limnology and Oceanography. 01/2011; 56(1):303-322.
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    ABSTRACT: High rates of CO2 exchange were observed over flaw leads using eddy covarianceEnhanced gas exchange is hypothesized to be due to ice formation in leadsEnhanced gas exchange is important to annual air-sea CO2 budgets of this region
    Journal of Geophysical Research 01/2011; 116. · 3.17 Impact Factor
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    ABSTRACT: The coastal oceans have relatively large fluxes of CO2, but the temporal variability of these systems is high. Compared to open ocean systems, the variability of the CO2 system in coastal regions remains poorly understood. The Scotian Shelf is a highly productive area of the Canadian northwestern Atlantic Ocean. The region is host to active fisheries, is heavily influenced by river water input from the Gulf of St. Lawrence, and receives a downstream flow of Arctic water from the Labrador Sea. The seasonal variability of inorganic carbon in the surface waters of the Scotian Shelf is assessed using hourly measurements of the partial pressure of CO2, (pCO2), from an autonomous moored instrument, and covering a full annual cycle. These measurements are complemented by frequent shipboard sampling of dissolved inorganic carbon (DIC), total alkalinity (TA), and pCO2 over the larger spatial scale. Biology dominates changes in mixed-layer DIC, while competing effects of temperature and biology influence surface pCO2 in roughly equal magnitude. The mixed-layer in the Scotian Shelf is overall autotrophic; the region acts as a net source of CO2 to the atmosphere on the annual scale. An algorithm to compute pCO2 from satellite-based estimates of chlorophyll-a concentration, sea-surface temperature, and wind speed is developed. A hind-cast of air-sea CO2 fluxes from 1999 to 2008 reveals significant variability both spatially and from year to year. Regional conditions govern spatial and interannual variability on the Scotian Shelf, while multi-annual trends appear to be correlated with the North Atlantic Oscillation.
    AGU Fall Meeting Abstracts. 12/2010;

Publication Stats

197 Citations
37.86 Total Impact Points

Institutions

  • 2004–2014
    • Dalhousie University
      • Department of Oceanography
      Halifax, Nova Scotia, Canada
  • 1999–2002
    • Koninklijk Nederlands Instituut voor Onderzoek der Zee - NIOZ
      • Department of Biological Oceanography (BIO)
      Burg, North Holland, Netherlands