Thomas, Helmuth

Permanent URI for this collectionhttps://hdl.handle.net/10222/22179

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Now showing 1 - 12 of 12

Open Access
Sediment-water column fluxes of carbon, oxygen and nutrients in Bedford Basin, Nova Scotia, inferred from Ra-224 measurements
(2013) Burt, W. J.; Thomas, H.; Fennel, K.; Horne, E.
Exchanges between sediment pore waters and the overlying water column play a significant role in the chemical budgets of many important chemical constituents. Direct quantification of such benthic fluxes requires explicit knowledge of the sediment properties and biogeochemistry. Alternatively, changes in water-column properties near the sediment-water 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 Ra-224 activity in order to yield vertical eddy diffusivities (K-Z), based upon which we assess the diffusive exchange of dissolved inorganic carbon (DIC), nutrients and oxygen (O-2), across the sediment-water interface in a coastal inlet, Bedford Basin, Nova Scotia, Canada. Numerical model results are consistent with the assumptions regarding a constant, single benthic source of Ra-224, the lack of mixing by advective processes, and a predominantly benthic source and sink of DIC and O-2, respectively, with minimal water-column respiration in the deep waters of Bedford Basin. Near-bottom observations of DIC, O-2 and nutrients provide flux ratios similar to Redfield values, suggesting that benthic respiration of primarily marine organic matter is the dominant driver. Furthermore, a relative deficit of nitrate in the observed flux ratios indicates that denitrification also plays a role in the oxidation of organic matter, although its occurrence was not strong enough to allow us to detect the corresponding AT fluxes out of the sediment. Finally, comparison with other carbon sources reveal the observed benthic DIC release as a significant contributor to the Bedford Basin carbon system.
Open Access
Enhanced ocean carbon storage from anaerobic alkalinity generation in coastal sediments
(European Geosciences Union, 2008-09) Thomas, H.; Schiettecatte, L-S; Suykens, K.; M Kone, YJ; Shadwick, E. H.; F Prowe, AE; Bozec, Y.; W de Baar, HJ; Borges, A. V.
The coastal ocean constitutes the crucial link between land, the open ocean and the atmosphere. Furthermore, its shallow water column permits close interactions between the sedimentary and atmospheric compartments, which otherwise are decoupled at short time scales (<1000 yr) in the open oceans. Despite the prominent role of the coastal oceans in absorbing atmospheric CO sub(2) and transferring it into the deep oceans via the continental shelf pump, the underlying mechanisms remain only partly understood. Evaluating observations from the North Sea, a NW European shelf sea, we provide evidence that anaerobic degradation of organic matter, fuelled from land and ocean, generates alkalinity (A sub(T)) and increases the CO sub(2) buffer capacity of seawater. At both the basin wide and annual scales anaerobic A sub(T) generation in the North Sea's tidal mud flat area irreversibly facilitates 7-10%, or taking into consideration benthic denitrification in the North Sea, 20-25% of the North Sea's overall CO sub(2) uptake. At the global scale, anaerobic A sub(T) generation could be accountable for as much as 60% of the uptake of CO sub(2) in shelf and marginal seas, making this process, the anaerobic pump, a key player in the biological carbon pump. Under future high CO sub(2) conditions oceanic CO sub(2) storage via the anaerobic pump may even gain further relevance because of stimulated ocean productivity.
Open Access
Preferential recycling of nutrients - the ocean's way to increase new production and to pass nutrient limitation?
(1999-12) Thomas, H.; Ittekkot, V.; Osterroht, C.; Schneider, B.
Uptake of atmospheric CO2 by the oceans and the export of carbon into deeper waters via the biological CO2 pump is driven by the production of particulate organic matter (POM). The elemental ratios of carbon, nitrogen, and phosphorus within POM follow Redfield ratios, suggesting that the concentrations of dissolved inorganic carbon (DIC) and nutrients decrease during new production corresponding to these ratios. Subsequently, new production and the efficiency of the biological CO2 pump are usually estimated using Redfield ratios. However, our observations in the Baltic Sea and observations elsewhere show significantly greater decreases in DIC during the productive season than that predicted from the decline in nutrients with reference to Redfield ratios. Using new DIC, nutrient, and oxygen data from the Baltic Sea, we discuss this discrepancy and provide evidence that preferential recycling of nutrients fuels the productive community with nutrients. Limiting nutrients are preferentially recycled and become available for new production. These findings derived from hydrochemical bulk data confirm the picture of the microbial loop but question the common description of new production and nutrient limitation. Finally, we argue for a carbon-based efficiency estimate of the biological CO2 pump probably exceeding significantly nutrient-based estimates.
Open Access
Carbon cycling in the Arctic Archipelago: the export of Pacific carbon to the North Atlantic
(European Geosciences Union, 2009-01) Shadwick, E. H.; Papakyriakou, T.; Prowe, AEF; Leong, D.; Moore, SA; Thomas, H.
The Arctic Ocean is expected to be disproportionately sensitive to climatic changes, and is thought to be an area where such changes might be detected. The Arctic hydrological cycle is influenced by: runoff and precipitation, sea ice formation/melting, and the inflow of saline waters from Bering and Fram Straits and the Barents Sea Shelf. Pacific water is recognizable as intermediate salinity water, with high concentrations of dissolved inorganic carbon (DIC), flowing from the Arctic Ocean to the North Atlantic via the Canadian Arctic Archipelago. We present DIC data from an east-west section through the Archipelago, as part of the Canadian International Polar Year initiatives. The fractions of Pacific and Arctic Ocean waters leaving the Archipelago and entering Baffin Bay, and subsequently the North Atlantic, are computed. The eastward transport of carbon from the Pacific, via the Arctic, to the North Atlantic is estimated. Altered mixing ratios of Pacific and freshwater in the Arctic Ocean have been recorded in recent decades. Any climatically driven alterations in the composition of waters leaving the Arctic Archipelago may have implications for anthropogenic CO sub(2) uptake, and hence ocean acidification, in the subpolar and temperate North Atlantic.
Open Access
Controls of the surface water partial pressure of CO2 in the North Sea
(2005) Thomas, H.; Bozec, Y.; Elkalay, K.; de Baar, HJW; Borges, AV; Schiettecatte, LS
The seasonal variability of the partial pressure of CO2 (pCO(2)) has been investigated in the North Sea, a northwest European shelf sea. Based on a seasonal and high spatial resolution data set the main controlling factors - biological processes and temperature - have been identified and quantified. In the central and northern parts being a CO2-sink all year round, the biological control dominates the temperature control. In the southern part, the temperature control dominates the biological control at an annual scale, since the shallow water column prevents stronger net-CO2 removal from the surface layer due to the absence of seasonal stratification. The consequence is a reversal of the CO2 seatoair flux during the spring bloom period, the only time, when CO2 is taken up from the atmosphere in the southern region. Net community production in the mixed layer has been estimated to 4 mol Cm-2 yr(-1) with higher values (4.3 mol Cm-2 yr(-1)) in the northern part and lower values in the southern part (2.6 mol Cm-2 yr(-1)).
Open Access
Assessment of the processes controlling seasonal variations of dissolved inorganic carbon in the North Sea
(2006-11) Bozec, Yann; Thomas, Helmuth; Schiettecatte, Laure-Sophie; Borges, Alberto V.; Elkalay, Khalid; de Baar, Hein J. W.
We used a seasonal North Sea data set comprising dissolved inorganic carbon (DIC), partial pressure of CO2 (pCO(2)), and inorganic nutrients to assess the abiotic and biological processes governing the monthly variations of DIC. During winter, advection and air-sea exchange of CO2 control and increase the DIC content in the surface and deeper layers of the northern and central North Sea, with the atmosphere supplying CO2 on the order of 0.2 mol C m(-2) month(-1) to these areas. From February to July, net community production (NCP) controls the seasonal variations of DIC in the surface waters of the entire North Sea, with a net uptake ranging from 0.5 to 1.4 mol C m(-2) month(-1). During the August-December period, NCP controls the seasonal variations of DIC in the southern North Sea, with a net release ranging from 0.5 to 0.8 mol C m(-2) month(-1). Similarly, during the April-August period in the deeper layer of the northern North Sea, the NCP was the main factor controlling DIC concentrations, with a net release ranging from 0.5 to 5.5 mol C m(-2) month(-1). In the surface layer of the North Sea, NCP on the basis of DIC was 4.3 +/- 0.4 mol C m(-2) yr(-1), whereas, NCP on the basis of nitrate was 1.6 +/- 0.2 mol C m(-2) yr(-1). Under nutrient-depleted conditions, preferential recycling (extracellular) of nutrients and intracellular mechanisms occurred and were responsible for the non-Redfield uptake of DIC versus nitrate and phosphate.
Open Access
Direct observations of diel biological CO2 fixation on the Scotian Shelf, northwestern Atlantic Ocean
(2012) Thomas, H.; Craig, S. E.; Greenan, B. J. W.; Burt, W.; Herndl, G. J.; Higginson, S.; Salt, L.; Shadwick, E. H.; Urrego-Blanco, J.
Much of the variability in the surface ocean's carbon cycle can be attributed to the availability of sunlight, triggering 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 cycle regulation, occurring at time scales of a day or less, however, have undergone few investigations, most of which have been limited to time spans of several days to months. Optical methods have helped to infer short-term biological variability, but complementing investigations of the oceanic CO2 system are lacking. We employ high-frequency CO2 and 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 in the surface CO2 system 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. 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.
Open Access
Seasonal variability of the inorganic carbon system in the Amundsen Gulf region of the southeastern Beaufort Sea
(2011-01) Shadwick, E. H.; Thomas, H.; Chierici, M.; Else, B.; Fransson, A.; Michel, C.; Miller, L. A.; Mucci, A.; Niemi, A.; Papakyriakou, T. N.; Tremblay, J. -E
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 CO(2) (pCO(2)) 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 pCO(2). Surface waters were undersaturated with respect to atmospheric CO(2) 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 CO(2) uptake during winter. CO(2) 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 m(-2) 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.
Open Access
Air-Sea CO2 fluxes on the Scotian Shelf: seasonal to multi-annual variability
(2010) Shadwick, E. H.; Thomas, H.; Comeau, A.; Craig, S. E.; Hunt, C. W.; Salisbury, J. E.
We develop an algorithm to compute pCO(2) in the Scotian Shelf region (NW Atlantic) from satellite-based estimates of chlorophyll-a concentration, sea-surface temperature, and observed wind speed. This algorithm is based on a high-resolution time-series of pCO(2) observations from an autonomous mooring. At the mooring location (44.3 degrees N and 63.3 degrees W), the surface waters act as a source of CO2 to the atmosphere over the annual scale, with an outgassing of -1.1 mol C m(-2) yr(-1) in 2007/2008. A hindcast of air-sea CO2 fluxes from 1999 to 2008 reveals significant variability both spatially and from year to year. Over the decade, the shelf-wide annual air-sea fluxes range from an outgassing of -1.70 mol C m(-2) yr(-1) in 2002, to -0.02 mol C m(-2) yr(-1) in 2006. There is a gradient in the air-sea CO2 flux between the northeastern Cabot Strait region which acts as a net sink of CO2 with an annual uptake of 0.50 to 1.00 mol C m(-2) yr(-1), and the southwestern Gulf of Maine region which acts as a source ranging from -0.80 to -2.50 mol C m(-2) yr(-1). There is a decline, or a negative trend, in the air-sea pCO(2) gradient of 23 mu atm over the decade, which can be explained by a cooling of 1.3 degrees C over the same period. Regional conditions govern spatial, seasonal, and interannual variability on the Scotian Shelf, while multi-annual trends appear to be influenced by larger scale processes.
Open Access
Satellite observations reveal high variability and a decreasing trend in CO sub(2) fluxes on the Scotian Shelf
(European Geosciences Union, 2010-07) Shadwick, E. H.; Thomas, H.; Comeau, A.; Craig, SE; Hunt, C. W.; Salisbury, JE
We develop an algorithm to compute pCO sub(2) in the Scotian Shelf region (NW Atlantic) from satellite-based estimates of chlorophyll-a concentration, sea-surface temperature, and observed wind speed. This algorithm is based on a high-resolution time-series of pCO sub(2) observations from an autonomous mooring. At the mooring location (44.3 degree N and 63.3 degree W), the surface waters act as a source of CO sub(2) to the atmosphere over the annual scale, with an outgassing of -1.1 mol C m super(-2) yr super(-1) in 2007/2008. A hindcast of air-sea CO sub(2) fluxes from 1999 to 2008 reveals significant variability both spatially and from year to year. Over the decade, the shelf-wide annual air-sea fluxes range from an outgassing of -1.7 mol C m super(-2) yr super(-1) in 2002, to -0.02 mol C m super(-2) yr super(-1) in 2006. There is a gradient in the air-sea CO sub(2) flux between the northeastern Cabot Strait region which acts as a net sink of CO sub(2) with an annual uptake of 0.5 to 1.0 mol C m super(-2) yr super(-1), and the southwestern Gulf of Maine region which acts as a source ranging from -0.8 to -2.5 mol C m super(-2) yr super(-1). There is a decline, or a negative trend, in the air-sea pCO sub(2) gradient of 23 mu atm over the decade, which can be explained by a cooling of 1.3 degree C over the same period. Regional conditions govern spatial, seasonal, and interannual variability on the Scotian Shelf, while multi-annual trends appear linked to the North Atlantic Oscillation.
Open Access
The carbon budget of the North Sea
(2005) Thomas, H.; Bozec, Y.; de Baar, HJW; Elkalay, K.; Frankignoulle, M.; Schiettecatte, LS; Kattner, G.; Borges, AV
A carbon budget has been established for the North Sea, a shelf sea on the NW European continental shelf. The carbon exchange fluxes with the North Atlantic Ocean dominate the gross carbon budget. The net carbon budget more relevant to the issue of the contribution of the coastal ocean to the marine carbon cycle - is dominated by the carbon inputs from rivers, the Baltic Sea and the atmosphere. The North Sea acts as a sink for organic carbon and thus can be characterised as a heterotrophic system. The dominant carbon sink is the final export to the North Atlantic Ocean. More than 90% of the CO2 taken up from the atmosphere is exported to the North Atlantic Ocean making the North Sea a highly efficient continental shelf pump for carbon.
Open Access
Spatiotemporal variations of fCO(2) in the North Sea
(2010) Omar, A. M.; Olsen, A.; Johannessen, T.; Hoppema, M.; Thomas, H.; Borges, A. V.
Data from two Voluntary Observing Ship (VOS) (2005-2007) augmented with data subsets from ten cruises (1987-2005) were used to investigate the spatiotemporal variations of the CO2 fugacity in seawater (fCO(2)(sw)) in the North Sea at seasonal and inter-annual time scales. The observed seasonal fCO(2)(sw) variations were related to variations in sea surface temperature (SST), biology plus mixing, and air-sea CO2 exchange. Over the study period, the seasonal amplitude in fCO(2)(sw) induced by SST changes was 0.4-0.75 times those resulting from variations in biology plus mixing. Along a meridional transect, fCO(2)(sw) normally decreased northwards (-12 mu atm per degree latitude), but the gradient disappeared/reversed during spring as a consequence of an enhanced seasonal amplitude of fCO(2)(sw) in southern parts of the North Sea. Along a zonal transect, a weak gradient (-0.8 mu atm per degree longitude) was observed in the annual mean fCO(2)(sw). Annually and averaged over the study area, surface waters of the North Sea were CO2 undersaturated and, thus, a sink of atmospheric CO2. However, during summer, surface waters in the region 55.5-54.5 degrees N were CO2 supersaturated and, hence, a source for atmospheric CO2. Comparison of fCO(2)(sw) data acquired within two 1 degrees x1 degrees regions in the northern and southern North Sea during different years (1987, 2001, 2002, and 2005-2007) revealed large interannual variations, especially during spring and summer when year-to-year fCO(2)(sw) differences (approximate to 160-200 mu atm) approached seasonal changes (approximate to 200-250 mu atm). The springtime variations resulted from changes in magnitude and timing of the phytoplankton bloom, whereas changes in SST, wind speed and total alkalinity may have contributed to the summertime interannual fCO(2)(sw) differences. The lowest interannual variation (10-50 mu atm) was observed during fall and early winter. Comparison with data reported in October 1967 suggests that the fCO(2)(sw) growth rate in the central North Sea was similar to that in the atmosphere.

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