Mary I. Scranton

Seasonal and interannual variation in the hydrography of the Cariaco Basin: implications for basin ventilation

The hydrography of the Cariaco Basin (temperature, salinity, density, dissolved oxygen concentration) was studied using monthly observations collected between November 1995 and August 1998 at the CArbon Retention In A Colored Ocean (CARIACO) time-series station (10.51N, 64.661W). Satellite scatterometer wind estimates showed that changes in the wind preceded changes in hydrography by 1–2 weeks. Upward migration of isopleths within the upper 150 m was observed between November and May each year, when the Trade Wind was more intense. A seasonal deepening of the isopleths was observed when winds relaxed. A secondary upwelling event was observed every year between July and August, in response to an intensification of the southward component of the Trade Wind. Interannual variation in the upwelling cycle was driven in part by variations in wind intensity and in part by other events at time scales of 1–3 months. The latter were associated with 90–140 m deep intrusions of Caribbean Sea water that forced waters above them to the surface. Satellite-derived sea surface height anomaly maps demonstrated that these events were related to cyclonic and anticyclonic eddies moving along the continental shelf. Waters deeper than 1200 m showed small temperature and salinity increases of 0.00751 Cy r � 1 and 0.0016 yr � 1 , consistent with previous estimates.

Controls on temporal variability of the geochemistry of the deep Cariaco Basin

Studies of the Cariaco Basin on the continental shelf of Venezuela, as a part of the Carbon Retention In A Colored Ocean (CARIACO) program, have revealed that the chemistry of the deeper waters of the system is more variable than previously believed. Small oxygen maxima have been observed on a number of occasions at depths where oxygen was previously absent, suggesting the occurrence of intrusions of oxygenated water into the region of the oxic/anoxic interface (250}300 m). Apparently because of these events, the oxic/anoxic interface deepened by about 100 m during the period of our observations. We also observed a dramatic decrease in H S concentrations at all depths below the oxic/anoxic interface during this same period. Bottom waters, for example, had an H S concentration of about 75Min November 1995, but since November 1997, concentrations in bottom water have not exceeded 55M. Water of su

Temporal changes in the hydrography and chemistry of the Cariaco Trench

cient density to sink to the bottom of the Basin has been observed on one occasion at sill depth just north of the eastern sill. However, based on a simple box model, the decrease in deep-water sulde does not appear to be due to intrusion of oxygenated water alone, as concentrations of other measured species, and of hydrographic parameters, have remained constant with time. Instead, we postulate that an earthquake that took place in July 1997 resulted in a turbidity current that transported large quantities of coastal sediment containing oxidized iron into the deep waters of the basin. If the nal products of reaction were elemental sulfur and iron sulde, the sediment associated with the oxidized iron would have produced a turbidite layer about 10 cm thick. Previous earthquakes have produced turbidites of similar thickness. 2001 Elsevier Science Ltd. All rights reserved.

Si cycle in the Cariaco Basin, Venezuela: Seasonal variability in silicate availability and the Si:C:N composition of sinking particles

The Cariaco Trench has been considered a classic example of a marine anoxic basin for nearly 30 years. Although most workers assume that the chemistry of the basin is at steady-state, detailed data sets collected for hydrographic and nutrient parameters in 1973 and 1982 indicate strong temporal variations on a decadal time scale. Over this 8 year period deep-water temperature increased by 0.06°C, salinity by 0.01%, and hydrogen sulfide by 14 μM. Silica concentrations may also have increased by up to 14 μM. We can explain most of the temporal variability of these parameters in the Cariaco Trench deep waters using a time-dependent box model that includes the effects of vertical eddy diffusion and flux of the chemical species from the sediments. Deep-water temperature and salinity increases are primarily the result of downward diffusion of heat and salt, while geothermal heat flux seems to be responsible for only a small fraction of the observed temperature increase. Hydrogen sulfide profiles can be predicted fairly well by the model if a sediment-water flux of 2.7 × 10−6 μmol cm−2 s−1 (measured using pore waters collected with the Woods Hole Insitu Marine Probe) is used. For hydrogen sulfide, in situ production seems to be much less important than diffusion on sulfide from the pore waters. On the other hand, silica profiles cannot be adequately modeled unless an in situ production term is included.

Temporal variations in the methane content of the Cariaco Trench

result of rapid utilization. In most years, the upper water column during winter and spring is marked by Si(OH)4 :NO3 and Si* values of less than 1. This indicates that silicate limitation in Cariaco Basin is most severe during upwelling and may restrict diatom production. Conversely, during the summer and fall when upwelling is reduced, Si(OH)4 :NO3 ratios in the upper 50 m of the water column exceed 10, implying that nitrate rather than silicate is acting to limit production during this time of year. On average, sinking particles collected at 150-m depth in the Cariaco Basin have Si:C and Si:N values of 0.17 ± 0.01 and 1.14 ± 0.10, respectively. These ratios increase with depth to 400 m and then remain relatively constant, suggesting minimal selective removal of elements with remineralization in the anoxic portion of the water column. Similar depth-dependent changes in these ratios are seen in surface sediments from the basin. Seasonally, particulate Si:C and Si:N are highest during the early part of the year when upwelling is most intense, while both ratios decrease to their lowest values during summer and fall. The observed seasonal variability in these ratios is due to changes in both nutrient utilization by diatoms and the contribution of diatoms to the total phytoplankton. The high ratios during upwelling suggest enhanced export of Si relative to C and N during this time of year.

TEMPORAL VARIABILITY IN THE NUTRIENT CHEMISTRY OF THE CARIACO BASIN

Abstract The deep waters of the Cariaco Trench are not in steady state with respect to distributions of either hydrographical or chemical properties. Over the 15 years, methane concentrations at the bottom of the anoxic zone have increased steadily from about 7–9 to 12.5 μM. A simple model for the anoxic waters of the Cariaco Trench suggests that methane distributions may be explained as the result of the flux of methane from the sediments (about 12.5–17.5 μmol cm−2 y−1), vertical eddy diffusion within the water column, and methane oxidation in the previously measured, although available data are inadequate to determine whether the methane oxidation near the interface occurs under anaerobic conditions or whether it occurs when oxygenated overlying water is mixed down into the top of the anoxic zone. It also is possible that methane fluxes from the shallower sediments of the Cariaco Trench are very low compared to those from the deeper sediments.

The importance of episodic events in controlling the flux of methane from an anoxic basin

Nutrient data have been collected monthly at the CARIACO time series site in the Cariaco Basin since 1995, providing a unique picture of the cycling of NO3 − , NO2 − ,N H 4 + ,P O 4 3− and SiO2 in this permanently anoxic system underlying a major coastal upwelling zone. Our data indicate that nutrients for phytoplankton growth are primarily supplied by upwelling of subsurface water on a seasonal basis. In addition, coastal runoff seems to supply important amounts of silica and ammonium to surface waters. We saw no indication of local nitrogen fixation in the Cariaco surface waters. In the suboxic zone, our data to date are not of sufficiently high enough resolution to resolve all important features. However, at least partial phosphate removal appears to occur in a zone above the first appearance of sulfide, and associated with intermittent intrusions of oxygenated water. In the suboxic zone, there appear to be thin layers where ammonium and nitrite coexist, potentially permitting anaerobic ammonium oxidation (anammox) to take place. In the deep waters, concentrations of ammonium, phosphate and silica continue to increase at a rate consistent with prior studies. However, in the upper part of the anoxic zone, there is evidence for sulfide removal, probably associated with oxygen intrusions.

Hydrogen cycling in the waters near Bermuda: the role of the nitrogen fixer, Oscillatoria thiebautii

Methane distributions and fluxes were determined in the more southerly of two permanently anoxic basins at the head of the otherwise shallow and tidal Pettaquamscutt River. Fluxes out of the sediment were approximately equal to fluxes across the air-sea interface as determined by flux chamber and were of the order of 3 - 6 mg CH4 m−2 d−1. Ebullition, observed during one set of flux chamber experiments, increased air-sea fluxes by several orders of magnitude. Methane oxidation rates within the waters of the basin were measured by using 14CH4, and a maximum in oxidation rate was observed at the oxic-anoxic interface. Oxidation rates were about a factor of 10 lower than either measured sediment-water or air-sea fluxes. Fluxes across the pycnocline also were lower than sediment-water fluxes and fell at the low end of the range of calculated air-sea fluxes (which were lower than measured air-sea fluxes). Therefore, on the basis of a short term survey, the system appeared to be roughly at steady state. However, in spite of the appearance of steady state, methane profiles measured over a period of several years varied dramatically suggesting that episodic events (ebullition, tidal mixing, upwelling, and intrusion events) must be major factors in the methane budget not obvious from short-term experiments. Episodic events are probably also important for other systems such as productive coastal environments, suboxic estuaries or any other systems in which reduced gases might accumulate within or below a pycnocline. When the pycnocline is tilted or breaks down, substantial amounts of gases may be released to the atmosphere.

Fatty acid oxidation in anoxic marine sediments: the importance of hydrogen sensitive reactions

Oceanic surface waters are frequently supersaturated with hydrogen gas relative to atmospheric equilibrium. One possible source for the excess hydrogen is production by nitrogen-fixing organisms, many of which are known to reduce hydrogen ion, H+, to H2. Results from short-term, shore-based incubations in Bermuda have demonstrated that Oscillatoria thiebautii, a marine cyanobacterium, produces hydrogen in significant quantities, confirming previous work from St. Croix. Measurement of an acetylene reduction:15N2 fixation ratio of 2.85 ± 0.36:1 for Oscillatoria thiebautii near Bermuda was in agreement with our relatively low rates of hydrogen production (0.002 to 0.05 nmol per colony per hour) compared with nitrogen fixation rates (0.3 to 1.5 nmol per colony per hour). Although hydrogen production was slow, rates would be sufficient to produce surface supersaturations if no removal mechanisms were important. However, data on water column distributions indicated that surface waters near Bermuda at the time of our experiment were undersaturated with hydrogen. Diffusion of hydrogen into the thermocline and in situ biological consumption appear to be the most likely hydrogen sinks.

Hydrogen (H2) distributions in the Carmans River estuary

In anoxic marine sediments fatty acids may be oxidized directly by sulfate reducing bacteria, or may be oxidized by pathways which result in hydrogen production. Some of these latter reactions are quite sensitive to hydrogen concentrations ... in other words if hydrogen concentrations become elevated, fatty acid oxidation will cease. Thus sulfate reducers may actually play two important roles in the metabolism of fatty acids in marine sediments. The sulfate reducers both can utilize fatty acids directly, and also can oxidize hydrogen and thus control hydrogen partial pressures in the sediments. Therefore sulfate reducers may act indirectly to facilitate fatty acid oxidation by hydrogen-producing pathways. We carried out a series of incubations of slurried salt marsh sediment under high and low hydrogen partial pressures and in the presence and absence of molybdate to investigate the relative importance of sulfate reducers and other bacteria mediating hydrogen-sensitive reactions. Our results suggest that both classes of bacteria contribute significantly to fatty acid turnover in marine sediments. Studies of low molecular weight fatty acid turnover in sediment must explicitly recognize that manipulation of sediment (including addition of molydbate to inhibit sulfate reducers) may have a large impact on hydrogen partial pressures in sediment, and may thus significantly alter the pathways and/or rates of fatty acid turnover.