Robert W. Collier

Gas hydrate destabilization: enhanced dewatering, benthic material turnover and large methane plumes at the Cascadia convergent margin

Mixed methane–sulfide hydrates and carbonates are exposed as a pavement at the seafloor along the crest of one of the accretionary ridges of the Cascadia convergent margin. Vent fields from which methane-charged, low-salinity fluids containing sulfide, ammonia, 4He, and isotopically light CO2 escape are associated with these exposures. They characterize a newly recognized mechanism of dewatering at convergent margins, where freshening of pore waters from hydrate destabilization at depth and free gas drives fluids upward. This process augments the convergence-generated overpressure and leads to local dewatering rates that are much higher than at other margins in the absence of hydrate. Discharge of fluids stimulates benthic oxygen consumption which is orders of magnitude higher than is normally found at comparable ocean depths. The enhanced turnover results from the oxidation of methane, hydrogen sulfide, and ammonia by vent biota. The injection of hydrate methane from the ridge generates a plume hundreds of meters high and several kilometers wide. A large fraction of the methane is oxidized within the water column and generates δ13C anomalies of the dissolved inorganic carbon pool.

The Trace Element Geochemistry of Marine Biogenic Particulate Matter.

Abstract Plankton samples have been carefully collected from a variety of marine environments for major and trace-chemical analysis. The samples were collected and handled under the rigorous conditions necessary to prevent contamination of the trace elements. Immediately after collection, the samples were subjected to a series of physical and chemical leaching-decomposition experiments designed to identify the major and trace element composition of the biogenic particulate matter. Emphasis was placed on the determination of the trace element/major element ratios in the various biogenic phases important in biogeochemical cycling. The majority of the trace elements in the samples were directly associated with the non-skeletal organic phases of the plankton. These associations include a very labile fraction which was rapidly released into seawater immediately after collection and a more refractory component which involved specific metal-organic binding. Calcium carbonate and opal were not significant carriers for any of the trace elements studied. A refractory phase containing aluminum and iron in terrigenous ratios was present in all samples, even from remote pelagic environments. This non-biogenic carrier contributed insignificant amounts to the other trace elements studied. The plankton samples were collected from surface waters with a wide range in the dissolved trace element/nutrient ratios, however, the same elemental ratios in the bulk plankton samples were relatively constant in all these environments. The bulk compositions and the rapid release of the metals and nutrient elements (specifically phosphorus) from the plankton after collection were used to examine the systematics of depletions of the dissolved elements from surface waters. These elemental ratios were combined with known fluxes of the major biogenic materials to estimate the significance of the plankton in the vertical flux of the trace elements. In parallel with the major surface ocean cycles of carbon and nitrogen, significant fractions of the trace elements taken up by primary producers must be rapidly regenerated in order to explain the observed elemental compositions and fluxes.

Export production of particles to the interior of the equatorial Pacific Ocean during the 1992 EqPac experiment

Abstract Twenty-four time-series, moored sediment traps were deployed between 2/2/92 and 1/27/93 along 140°W at 9°N, 5°N, 2°N, 0°, 2°S, 5°S and 12°S at water depths of approximately 1200 m and 2200 m, and 700 m above the bottom. The opening/closing of the traps was synchronized at 17-day periods, for 21 events, covering a total of 357 days. The average annual particle flux in the oceans interior (2.2 to 4.4 km deep) from 5°N to 5°S was 28.5 g m−2 year−1, with 34.8 g−2 year−1 the maximum annual flux at the equator. Sixty-six per cent of settling particles were carbonate; 24% biogenic SiO2 and 5% organic carbon. The onset of tropical instability waves, marking the years El Nino/post-El Nino boundary, was associated with a succession of intervals with greater organic carbon and opal at 5°N, 2°S and 5°S that occurred synchronously with a meridional oscillation of instability waves, while net carbon flux during El Nino and post-El Nino periods did not change. Although organic carbon flux increased at 5°N, 2°S and 5°S during the post-El Nino period, it was counterbalanced by decreases at the upwelling stations (2°N and the equator), resulting in no net carbon flux increase across the 5°N to 5°S region. In February/March 1992, only 0.34% of the organic carbon fixed by primary production over the 5°N to 5°S zone arrived in the oceans interior. In August/September that year, zonal average of organic carbon flux increased slightly to 0.5% of primary production. Very little carbon reached the interior depths of the upwelling stations; however, the fraction of export was higher at the 5°N, 2°S and 5°S stations. The pattern of variability of particle flux at the shallow depths was observed also in deeper traps, without temporal offsets, suggesting a settling particle residence time shorter than the 17-day timeseries resolution during most of this experiment.

Particulate barium fluxes and their relationships to biological productivity

Abstract To understand better the processes that control the transport of particulate barium through the water column and its preservation in marine sediments, we measured particulate barium fluxes along an equatorial transect at 140°W using moored sediment traps. The fluxes of barium correlate strongly with the fluxes of organic carbon; however, this relationship is non-linear—higher carbon fluxes have proportionately less associated barium. As a result we observe spatial and temporal variations of roughly a factor of three in the barium-to-organic carbon ratio. Understanding this variability may help to define the processes that determine the geochemical behavior of Ba in the oceans. Several hypotheses that could influence the flux of Ba and its relationship to organic carbon flux have been proposed: barite formation in barium- and sulfate-enriched microenvironments formed during particle settling; lateral advection of carbon and barium from continental margins; the influence of seawater barium concentration; and Ba scavenging by aluminosilicates. Our study reveals temporal variability in the Ba/Corg values that occurs over timescales of less than one month. Also, depth profiles of carbon and Ba fluxes show that the variability originates at depths less than 1200 m and is conveyed throughout the water column. Both the rapid changes and the upper water column origin of the signals point to upper-ocean biological processes as the predominant cause of the variability in the barium-to-organic carbon ratios. We also observe, however, a 25% increase in Ba flux below 1200 m. The deep sources of Ba could result from barite formation linked to continued organic carbon degradation or from lateral sources of particulate barium. Because the spatial and temporal changes in Ba/Corg values correlated to changes in particulate opal and organic carbon fluxes, ocean ecology appears to have an important influence on barium fluxes. A better understanding of the processes that contribute to the particulate barium flux is needed before the accumulation of barium in marine sediments can be used as a quantitative proxy for ocean productivity.

Measurements of transience and downward fluid flow near episodic methane gas vents, Hydrate Ridge, Cascadia

Aqueous flux measurements within cold seep regions on northern Hydrate Ridge, Cascadia, indicate a high degree of variability, with extended periods of downflow and reversals of flow direction over periods of weeks to months. Local episodic venting of free methane gas was also observed. The instruments recorded similar changes in hydrologic flow patterns both on and off clam fields, the magnitude of the flow rates decreasing away from the clam field. The coincidence of episodic gas venting with nearby highly variable aqueous fluid flow suggests that they may be coupled. We propose that these observations are consistent with the action of a gas-driven pump that operates somewhat like a geyser. The hypothesis of gas-driven pumping of seawater through northern Hydrate Ridge has important ramifications for the mass fluxes through this region.

Deep-dwelling planktonic foraminifera of the northeastern Pacific Ocean reveal environmental control of oxygen and carbon isotopic disequilibria

Abstract We assess the utility of four species of living planktonic foraminifera as tracers of thermocline and intermediate water masses in the northern Pacific Ocean, based on their water-column distribution and shell isotopic composition. Assuming oxygen isotopic equilibria with the water column, we infer apparent calcification depths. This allows an estimate of apparent carbon isotopic disequilibria. We then relax the assumption of oxygen isotopic equilibrium to examine habitat effects on kinetic disequilibrium processes. In the California Current, left-coiling Neogloboquadrina pachyderma and Globigerina calida calcify in the upper thermocline. Globorotalia scitula resides within the Shallow Salinity Minumum (potential density σ θ = 25.1–26.7), while Globoquadrina hexagona is associated with the deeper, North Pacific Intermediate Water (σ θ = 26.7–26.9). Apparent carbon isotopic disequilibria corrected for oxygen isotopic disequilibria, range from 1.0–1.9‰ in these asymbiotic species. The carbon isotopic disequilibrium can be modeled as a single exponential function of temperature or a logarithmic function of potential food supply. We infer that carbon isotopic disequilibrium increases with metabolic rate, related to temperature and/or food supply. Kinetic processes of oxygen and carbon isotopic disequilibria yields reasonable depth habitats if the slope of the oxygen:carbon isotopic shift is about 0.35, consistent with culture data. Our finding of a link between environment, metabolism, and isotopic disequilibrium observed in oceanic settings suggests the potential to better reconstruct the structure and biological processes of the upper water column from geologic data.

Paleoproductivity and carbon burial across the California Current: The multitracers transect, 42°N

The Multitracers Experiment studied a transect of water column, sediment trap, and sediment data taken across the California Current to develop quantitative methods for hindcasting paleoproductivity. The experiment used three sediment trap moorings located 120 km, 270 km, and 630 km from shore at the Oregon/California border in North America. We report here about the sedimentation and burial of particulate organic carbon (Corg) and CaCO3. In order to observe how the integrated CaCO3 and Corg burial across the transect has changed since the last glacial maximum, we have correlated core from the three sites using time scales constrained by both radiocarbon and oxygen isotopes. By comparing surface sediments to a two-and-a-half year sediment trap record, we have also defined the modern preservation rates for many of the labile sedimentary materials. Our analysis of the Corg data indicates that significant amounts (20–40%) of the total Corg being buried today in surface sediments is terrestrial. At the last glacial maximum, the terrestrial Corg fraction within 300 km of the coast was about twice as large. Such large fluxes of terrestrial Corg obscure the marine Corg record, which can be interpreted as productivity. When we corrected for the terrestrial organic matter, we found that the mass accumulation rate of marine Corg roughly doubled from the glacial maximum to the present. Because preservation rates of organic carbon are high in the high sedimentation rate cores, corrections for degradation are straightforward and we can be confident that organic carbon rain rate (new productivity) also doubled. As confirmation, the highest burial fluxes of other biogenic components (opal and Ba) also occur in the Holocene. Productivity off Oregon has thus increased dramatically since the last glacial maximum. CaCO3 fluxes also changed radically through the deglaciation; however, they are linked not to CaCO3 production but rather to changes in deepwater carbonate chemistry between 18 Ka and now.

Distribution and height of methane bubble plumes on the Cascadia Margin characterized by acoustic imaging

Submersible investigations of the Cascadia accretionary complex have identified localized venting of methane gas bubbles in association with gas hydrate occurrence. Acoustic profiles of these bubble plumes in the water column in the vicinity of Hydrate Ridge offshore Oregon provide new constraints on the spatial distribution of these gas vents and the fate of the gas in the water column. The gas vent sites remained active over the span of two years, but varied dramatically on time scales of a few hours. All plumes emanated from local topographic highs near the summit of ridge structures. The acoustic images of the bubble plumes in the water column disappear at water depths between 500 to 460 m, independent of the seafloor depth. This coincides with the predicted depth of the gas hydrate stability boundary of 510 to 490 m, suggesting that the presence of a hydrate skin on the bubble surface prevents them from rapid dissolution. The upper limit of the acoustic bubble plumes at 460 m suggests that dissolution of the residual bubbles is relatively rapid above the hydrate stability zone.

Chemical reaction rates and entrainment within the Endeavour Ridge hydrothermal plume

The aging of the hydrothermal plume over the Endeavour segment of the Juan de Fuca Ridge was estimated by measuring the222Rn3He ratio in the plume as it dispersed. Despite uncertainties in the source function of hydrothermal input, it wa determined that the relative sequence of removal from the plume isH2 > Δc >222Rn>CH4 ≫Mn, whereΔc is a measure of particle concentration and the mean life of222Rn is 5.5 days. H2 is removed from the plume within hours of input while Mn is not removed within the two-week timescale of the radon-helium clock. Entrainment of bottom water within the buoyant plume may introduce additional chemical signatures into the spreading effluent layer over that which would be introduced by hydrothermal discharge alone. This is particularly significant for those chemical species which are not greatly enriched in the vent fluids relative to bottom water concentration and which display a nutrient-like profile in the deep ocean. Thus we found that significant fractions of the Si and226Ra anomalies in the plume were not of hydrothermal origin but were derived from entrained bottom water which has a higher concentration of these elements than ambient water at plume height.

The chemistry, biology, and vertical flux of particulate matter from the upper 1500 m of the Panama Basin☆

Abstract Particulate matter divided into 53- μ m size fractions was obtained using the Large Volume in situ Filtration System (LVFS) modified to collect four samples per deployment. Samples collected to 1500 m at 0°45′N, 86°10′W were analyzed for major and minor element, organism (species assemblage and population densities), and large particle size distributions. Vertical fluxes of fecal matter, fecal pellets, Foraminifera, and diatoms were calculated from size distribution data and two settling models. Estimates of the vertical mass, organic carbon, carbonate, and opal fluxes were compared with mean particle collection rates by sediment traps deployed at 2500 m for 234 days commencing at the time of LVFS sampling. The productivity of the surface layer at this station is controlled by the north-south position of the Equatorial Front, which separates low productivity Tropical Surface Water (TSW) from a mixture of warm Subtropical Surface Water (SSW) and cool, nutrient-rich water upwelled along the Peruvian coast. At the time of LVFS sampling the front was south of the station, with the lower productivity TSW dominating the surface water. Particle distributions and fluxes indicated lower than average productivity and highly efficient zooplankton grazing. Variability in productivity at the station was indicated by weekly sea surface temperature data and was manifested by an anomalous centrate diatom maximum centered at 400 m and by the indication that the particle flux through 1500 m was 1/40th of the rates particles were collected by the sediment traps at 2500 m. The depth of the nitrite maximum corresponded to that of strongest rate of decrease of the organic carbon flux. Fluxes of particulate opal and carbonate decreased markedly in the upper water column although it was supersaturated with respect to calcite. The occurrence of fecal pellets devoid of birefringent material in the water column indicates that the carbonate dissolution was biologically mediated. Calculations indicate that the strong carbonate dissolution in the upper waters at this station is not representative of “average” ocean conditions and is probably a local or rare phenomenon.