John P. Christensen

Benthic fluxes and nitrogen cycling in sediments of the continental margin of the eastern North Pacific

The exchange of Oz, Nz, NO;, NH:, Si(OH)d, and POi3 between the sediments and the overlying water (benthic flux) was determined at 18 locations on the Washington State continental margin using an in situ benthic tripod. Oxygen consumption by the sediments ranged from 21.2pmole cm-* s-l on the shelf to 2.85pmole cm-2 s-l on the slope. Nitrogen gas fluxes were from the sediments to the overlying water. They varied 5.5 to 1.2pmole-N cm-* SK* and were always greater than the corresponding NO; flux into the sediments. A nitrogen mass balance indicated that the difference between the N2 flux out and the NO; flux in could be accounted for by oxidation of NH: produced during aerobic and anaerobic carbon remineralization to NO; and subsequent denitrification to Nz, Comparison of the benthic fluxes of 02, NO; and Si(OH)4 with the fluxes predicted from molecular diffusion across the sediment water interface showed that for all three solutes the benthic fluxes were up to three times greater than the molecular fluxes and indicated the importance of macrobenthic irrigation in these sediments. However, several existing empirical irrigation models were not able to describe all three solutes. The overall carbon oxidation rate, as estimated from the sum of the O2 flux, the N2 flux and the measured SOT reduction rate, could be fit with a normalized power function; i.e., carbon oxidation rate (gC m-* y-l) = 110 . (z/~OO)-~,~*. The exponent describing the rate of attenuation with depth (-0.91) was similar to the carbon rain rate attenuation coefficient determined from sediment traps in the pelagic, eastern North Pacific.

Summer and winter denitrification rates in western Arctic shelf sediments

Abstract We report estimates of sedimentary denitrification rates from the Bering, Chukchi and Beaufort Seas made during August–September 1992, and rates obtained in the Beaufort Sea off Pt. Barrow, Alaska, under late wintertime conditions (March 1993). The estimates made during August–September were based on both deployments of an automated in situ benthic flux chamber tripod and on pore water chemical profiles from cores analyzed aboard the R/V Alpha Helix. Wintertime observations were obtained using scaled-down versions of the in situ flux chamber that were deployed through land-fast ice. The results of these determinations suggested that there was no drastic decrease in shelf denitrification rates at the end of winter off Pt. Barrow despite the fact that a supply of “new” organic material to the sediments via primary production had not occurred for several months. In addition, the ensemble of our data suggest appreciable year-round average rates (∼ 1 mg-atom N2 m−2 day−1) that fall within the range of modern estimates for moderately productive mid-latitude shelf regions, and are about 40% as high as values reported for the Washington (NE Pacific) shelf. The results have some significance for understanding the oceanic combined nitrogen budget because denitrification in shallow and hemipelagic sediments is a major loss term and because the Arctic Ocean and its marginal and adjacent seas (such as the Bering Sea) contain 25% of the oceans shelf sediments and 14% of the ocean sediments with depths between 0.2–1 km, despite the fact that this region accounts for only 4% of the total oceanic surface area.

Carbon export from continental shelves, denitrification and atmospheric carbon dioxide

Abstract To investigate the long-term role of continental shelves, a global carbon ☐ model which included continental shelf waters and sediments, ocean surface waters, the deep-sea and the atmosphere was constructed. With nitrogen limiting oceanic primary production, the model included balanced nitrogen inputs (from the continents and atmosphere) and losses (primarily via denitrification). Carbon export to the deep-sea (without deposition or burial in sediments) affected the average conditions found in the shelves but did not influence atmospheric carbon dioxide content. Similarly, redistribution of nitrogen inputs to the surface oceans had no major effect on atmospheric pCO2. In contrast, atmospheric pCO2 changes were caused by redistribution of denitrification rates between the continental shelf sediments and the deep-sea. When denitrification in continental shelf sediments increased from 10% of the total oceanic rate to 95%, shelf denitrification removed more nitrogen from the surface ocean, supporting less of a flux of sinking particulate carbon (SPC) into the deep-sea. This increased atmospheric pCO2 by 11 ppm. When overall rates of nitrogen cycling were decreased by half to todays level, the atmospheric pCO2 content was increased by an additional 5 ppm. These effects may have influenced the ice-ages. As the glaciers expanded, shelf denitrification was lessened by the reduction in continental shelf area. Moving the site of denitrification from the shelves to the deep-sea would have increased both oceanic new production and the SPC flux into the deep-sea, thereby lowering atmospheric pCO2 levels during the initial periods of glaciation. Increased new production may have enhanced water column denitrification which in turn lowered the oceanic inorganic nitrogen content and restricted oceanic productivity. With less oceanic nitrogen, water column denitrification would have decreased resulting in a more equal proportioning of the total removal rate between the deep-sea and the continental shelves. Also, the total cycling rate of nitrogen would have lessened. In addition, inundation of the continental shelves during glacial retreat would have increased shelf denitrification. From the model, these trends would have released CO2 from the ocean, accentuating global warming and hastening the return to the interglacial climate.

Sulfate reduction and carbon oxidation rates in continental shelf sediments, an examination of offshelf carbon transport

Sediments from 12 sites in the Gulf of Maine were sampled for sulfate reduction rates and interstitial water solute profiles. The profiles of redox sensitive solutes confirmed that sulfate reduction was probably the major mechanism of organic matter oxidation below the upper 2 cm of sediments. Sulfate reduction rates were assessed using35S-sulfate tracer conversion to reduced sulfur in both the acid volatile sulfide fraction and the chromate reducible sulfur fraction. Rates, integrated over the upper 30 cm, decreased exponentially with water column depth from 20,000 to 1800 pmol sulfate cm−2 h−1 between 50 and 300 m. For the entire Gulf, the areal mean rate of carbon oxidation by sulfate reduction was 20.4 g C m−2 y−1. Based on the relationship from other regions of sediment trap carbon flux to water depth and primary productivity (Betzer et al., 1984, Deep-Sea Research, 31, 1–11), the carbon flux to the sediments would be 47.0 g C m−2 y−1. Because the Gulf of Maine is a semi-enclosed sea, little export of organic carbon to the adjacent continental slope is expected. With estimated rates of oxygen consumption, denitrification, and carbon burial of 22.7, 4, and 5 g C m−2 y−1, the sediment respiration and burial rates, totaling 52.1 g C m−2 y−1, closely balanced the carbon input indicating no significant export of organic material. In contrast, a similar analysis of published data from the narrow exposed Washington state continental shelf showed that the carbon input (264 g C m−2 y−1) greatly exceeded the sediment respiration and burial rate (99.5 g C m−2 y−1), indicating between 22 and 50% of the primary production may have been exported. These and previous results from the New York Bight, the Bering Sea, and the Peru shelf suggest that shelf ecosystems may differ considerably in the proportion of shelf production which escapes seaward. Although only a fraction of the exported carbon may accumulate in slope sediments, shelf export may be important in redistributing organic carbon in particular geographic regions.

Biological enhancement of solute exchange between sediments and bottom water on the Washington continental shelf

Solute exchange between the interstitial waters and overlying waters on the Washington continental shelf was investigated based on measurements of the pore-water sulfate distribution and sulfate reduction rates as well as through models describing the distribution of sulfate in anaerobic pore waters. The depth-integrated sulfate reduction rate greatly exceeded the influx of sulfate attributable to molecular diffusion and sediment accumulation acting on the measured vertical sulfate gradients, and indicated that additional transport mechanisms must have been operating. Sediment mixing was probably not the primary mechanism since high eddy diffusivities would be required to depths of 30 cm to maintain the observed sulfate distribution, whereas previously measured210Pb distributions indicated sediment mixing is primarily restricted to depths <7 cm. Irrigation of bottom water through animal burrows was the most likely mechanism. To describe this process, a general diffusive irrigation coefficient, B, was formulated. Vertical profiles of B showed the main irrigation zone occurred between 2 and 10 cm with reduced irrigation rates occurring below this. These coefficients calculated from the sulfate distribution were similar to ones calculated from previously published radon measurements at the same stations, indicating that this formulation of irrigation exchange may be useful in modelling the exchange of dissolved solutes between the pore water and the bottom water.

Benthic nutrient regeneration and denitrification on the Washington continental shelf

Abstract Benthic nutrient regeneration on the Washington continental shelf was investigated using vertical profiles of pore-water nutrient concentrations and whole sediment sulfate reduction rates. In August, carbon oxidation rates by sulfate reduction frequently exceeded those calculated from previously reported oxygen consumption rates. Total carbon oxidation rates averaged 18.9 pmol CO 2 cm −2 s −1 , and using the 106 C: 16 N atomic ratio, 2.86 pmol N cm −2 s −1 would have been regenerated. Nutrient fluxes were calculated from pore-water profiles using diagenetic equations containing terms for vertical molecular diffusion, sedimentation, adsorption and macrobenthic irrigation which was estimated by modeling published radon distributions. In August, the calculated nitrate influx was 0.81 pmol N cm −2 s −1 . The outward ammonium flux of 0.53 pmol N cm −2 s −1 , calculated from pore-water ammonium profiles, was only 19% of the N regeneration rate. The missing ammonium could be accounted for by coupled nitrification and denitrification (1.16 pmol N cm −2 s −1 ) and by consumption of organic matter with high C:N ratios (9.8 by atoms). A comparison of annual regeneration and burial rates indicated that 84% of the deposited carbon is regenerated. For nitrogen, the annual outward ammonium flux was about half of the sum of the organic N burial rate and nitrate influx suggesting that these shelf sediments were a net sink for nitrogen. If the annual nitrate influx (0.57 pmol NO 3 − cm −2 s −1 ) to this shelf approximated that in other shelves, the global denitrification rate in continental shelf sediments wound be 69 Tg N y −1 , a value similar to the oceanic water-column denitrifications rate.

Formation of the Alboran oxygen minimum zone

Abstract The enhanced oxygen minimum in the western Alboran Sea is the result of a chain of processes starting with nutrient injection into the inflowing Atlantic water at the Strait of Gibraltar. These nutrients originate in the outflowing Levantine Intermediate Water, outflowing Mediterranean deep water, and inflowing North Atlantic Central Water (from 200 m). They are injected into the inflowing Atlantic surface water by strong mixing at the eastern end of the Strait. They move with Atlantic surface waters along the Spanish coast, mix with nutrients upwelling in the northwestern Alboran Sea and stimulate phytoplankton productivity. The organic matter produced by this mechanism is transported both with the anticyclonically flowing waters of the Alboran gyre and with the waters that converge at the center of the gyre. Sedimentation in this convergence zone helps to deliver this organic matter to the Levantine Intermediate. Water where bacteria metabolize it to CO 2 at the expense of the existing oxygen. This mechanism develops the most intense oxygen minimum zone in the Mediterranean Sea.

Respiration and vertical carbon flux in the Gulf of Maine water column

The transport of carbon from ocean surface waters to the deep sea is a critical factor in calculations of planetary carbon cycling and climate change. This vertical carbon e ux can be calculated by integrating the vertical proe le of the seawater respiration rate but is rarely done because measuring seawater respiration is so dife cult. However, seawater respiratory oxygen consumption is the product of the combined activity of all the respiratory electron transfer systems in a seawater community of bacterioplankton, phytoplankton, and zooplankton. This respiratory electron transfer system (ETS) is the membrane bound enzymatic system that controls oxygen consumption and ATP production in all eukaryots and in almost all bacteria and archaea. As such, it represents potential respiratory oxygen consumption. Exploiting this, we measured plankton-community ETS activity in water column proe les in the Gulf of Maine to give the potential-respiration of the water column. To interpret these potentials in terms of actual seawater respiration we made use of previous measurements of respiratory oxygen consumption and ETS activity in the Gulf of Maine to calculate a ratio of respiratory potential to actual respiration. Armed with this ratio we calculated seawater respiration depth proe les from the ETS activity measurements. These proe les were characterized by: (1) high oxygen consumption rates in the euphotic zone; (2) subsurface maxima near the subsurface chlorophyll maxima (SCM); (3) rapid declines associated with thermoclines; (4) low declining rates below 50 m; (5) and elevated values occasionally near the bottom. Sea surface values ranged from 229 to 489 pmol O 2 min 2 1

Water column nutrients and sedimentary denitrification in the Gulf of Maine

Sedimentary denitrification acts to remove nitrogen from both the sediments and water column in continental shelf ecosystems, so that in enclosed shelf areas where water residence times are long (about a year in the Gulf of Maine), significant rates of sedimentary denitrification might lower inorganic nitrogen concentrations. We examined this using a basin-wide suite of hydrographic and nutrient data collected in mid-summer. Total inorganic nitrogen (TIN) concentrations (nitrate + nitrite + ammonium) were highest in the aged North Atlantic continental slope waters found at depth in the Jordan Basin on the eastern side of the Gulf. Phosphate and silicate concentrations were moderately high in these waters. On the western side of the Gulf, Wilkinson Basin receives much less of the nutrient-rich slope waters. In these deep waters, TIN concentrations were lower and phosphate and silicate concentrations higher than in the Jordan Basin. In the intermediate and deep waters, TIN/PO4 ratios averaged 19 on the eastern side but only 15–16 on the western side of the Gulf. Partially isolated regions within the Gulf had even lower TIN/PO4 ratios. The differences in this ratio suggested the occurrence of either a non-stoichiometric reduction in nitrogen or an enrichment in phosphate between the eastern and western sides of the Gulf. The relationship between phosphate and silicate was identical in open waters on both sides of the Gulf, indicating that the difference in the TIN/PO4 ratio was due to a loss of nitrogen. A parameter, delta-N, quantified the non-stoichiometric nitrogen loss and was defined as, ΔN = α[PO4] − [TIN], where [PO4] and [TIN] were the measured concentrations in an individual water sample and α was the average TIN/PO4 ratio in the Jordan Basin. Within and below the Maine Intermediate Waters (50–120 m), the distribution of ΔN showed the removal of 2–4 μgat N l−1 in most of the waters west and south-west of the Jordan Basin. ΔN was greater in waters close to the sediments suggesting that sediments were the site of nitrogen removal. The overall rate of denitrification was estimated to be 31.2–46.8 × 109 gat N y−1 based on the average ΔN, the water volume within the Gulf and the water residence time. When normalized to sediment area, this rate was 0.80–1.21 pgat N cm−2 s−1. The classical inorganic nitrogen budget of the Gulf of Maine, based on inflow rates and nutrient concentrations of the source waters, was reassessed and found to be unbalanced by 43.1 × 109 gat N y−1 with TIN inputs (138.6 × 109 gat N y−1) exceeding advective TIN losses (−95.5 × 109 gat N y−1). Organic matter burial and net organic nitrogen export at the rate of 1.5 % of the primary productivity could account for 23 % of the imbalance (−10.0 × 109 gat N y−1). Denitrification, at the aerial rate of 0.85 pgat N cm−2 s−1 accounted for the remainder (−33.1 × 109 gat N y−1).

Electron transport system activity and oxygen consumption in marine sediments

Abstract Electron transport system (ETS) activity was investigated as an indicator of the respiratory potential of shallow-water and deep-sea marine sediments. ETS activities per volume of whole sediment (meq m −2 cm −1 h −1 ) were high in shallow water and decreased with water depth ( X in meters) according to the equation ETS = 1230 X −1.03 Associated with the decrease the thickness of the oxygenated sediments increased approximately exponentially from about 1 cm in shallow water to over 60 cm at 5000 m in the western North Atlantic, whereas published oxygen consumption rates decrease exponentially. The ratios of the published oxygen consumption rate to the ETS activity integrated over the oxygenated strata from the same sites averaged 0.17 ± 0.030 in shallow-water sediments and decreased with increasing water depth to 0.00036 at 5000 m. The three orders of magnitude decrease may be due to depression of the oxygen consumption rate relative to ETS activity within the deeply buried strata of long, aerobic cores from the deep sea.