James F. Gendron

Hydrothermal plumes along the East Pacific Rise, 8°40′ to 11°50′N: Particle distribution and composition

The discovery of a volcanic eruption at the 9°45.52′N site on the East Pacific Rise (EPR) led us to investigate how the composition of suspended matter in hydrothermal plumes may be used to provide a better understanding of the relative age and vent fluid characteristics of a ridge crest system during surface ship surveys. During the TROUGHS (Tropical Ridge Observations of Underwater Geochemical Hydrothermal Signals) expedition we collected hydrothermal plume particles along a 350 km section, extending from approximately 8°40′N to 11°50′N, of the EPR north and south of the Clipperton Transform Fault. Over the segment south of the Clipperton Transform Fault, the SFe ratio in the particles ranged from about 0.9 to 10, with the highest values immediately over the new vent field at the 9°45.52′N site. These enrichments exactly coincide with the elevated 3He/heat and CH4Mn ratios in the plumes over the same vent field. The S-rich particles are primarily organic and may be related to the ‘bacteria blizzard’ associated with the eruption. In addition, very high CuFe and ZnFe ratios were observed in the hydrothermal plumes south of the Clipperton Transform Fault. These high ratios are indicative of the predominance of high-temperature, H2S-rich vents between 9°35′N and 9°50′N. In contrast, the samples from the segment north of Clipperton Transform Fault had SFe ratios which ranged from 0.04 to 0.82, typical of mature, Fe-rich hydrothermal plumes. These results suggest that the gas-rich vent fluids at the 9°45.52′N site produced the sulfur-enriched particles in the water column. Gas-rich plumes and sulfur-enriched plume particles were also observed underneath the 1986 Megaplume on the Juan de Fuca Ridge (JdFR), suggesting a similar mode of formation. North of the Clipperton Transform Fault, the hydrothermal particles are dominated by Fe-rich oxyhydroxide particles, characteristic of mature high-temperature vent fields. The plume maximum is centered at 11°08′N. The Fe oxyhydroxide particles scavenge Ca, Si, P, V, Cr and As from seawater. For P and V the mean element/Fe ratios in the EPR hydrothermal particles are intermediate between JdFR and TAG hydrothermal systems. For As the mean element/Fe ratios of all three systems are about the same. New evidence for Si scavenging by hydrothermal Fe oxyhydroxide particles indicates that this sink for dissolved Si in the oceans is small.

Winter-summer variations of calcite and aragonite saturation in the Northeast Pacific

Abstract New carbonate data obtained on February-March and June-July cruises in the northeast Pacific during 1985 were utilized to describe processes affecting seasonal variations of calcite and aragonite saturation. Large gradients in saturation state occur in the region between the Subtropical and the Subarctic Fronts in the north-south direction. These gradients are a function of largescale mixing and biological processes in the North Pacific. The saturation values in the upper kilometer of the water column were observed to be significantly lower in winter than in summer. These decreases were due to a number of processes including: (i) the seasonal decrease in the temperature of the water column; (ii) the seasonal increase in vertical mixing causing CO 2 -enriched deep waters to be upwelled; and (iii) the seasonal enhancement of respiration over photosynthesis. Seasonal changes in total CO 2 (TCO 2 ) concentrations appeared to have the greatest overall effect on the saturation state. Aragonite dissolution rate experiments were conducted during the June-July cruise to provide an independent verification of the saturation calculations. In all cases, significant increases in aragonite dissolution were observed below the 100% saturation depth. In the northeast Pacific, shallow undersaturation horizons provide for significant dissolution rates at depths below 400 m.

Possible evidence for enrichment of trace elements in the hydrous manganese oxide phases of suspended matter from an urbanized embayment

Abstract Total and weak-acid-soluble trace elements in suspended matter from the Duwamish River and Elliott Bay were determined from samples collected in February and September 1980. The results indicate that Mn scavenging in the water column is coincident with enrichments of Cr, Ni, Cu, Zn, and Pb in the suspended matter, suggesting a possible enrichment of these trace elements in a hydrous Mn oxide phase. This process occurs primarily in the sub-surface waters of Elliott Bay. Since mass balance calculations show a net export of Mn out of the bay, this mechanism may be an important means of transporting toxic trace metals from polluted estuaries and embayments to cleaner coastal environments.

Seasonal and vertical variations in the elemental composition of suspended and settling particulate matter in Puget Sound, Washington

Abstract The distributions of C, Al, Si, P, Cr, Mn, Fe, Ni, Cu, Zn and Pb in suspended and settling particulate matter from Puget Sound were studied to investigate the processes affecting the uptake of trace metals by particulate phases. Particulate material was collected by water filtration procedures and sequentially-sampling sediment traps during approximately bimonthly surveys from December 1980 through December 1981 at a station located in the center of the main basin of Puget Sound. The samples were selectively extracted for trace metals associated with oxide, organic, and residual phases and analysed by graphite furnace atomic absorption spectrophotometry and X-ray energy spectrometry. The results show that for Mn, Fe, Zn and Pb, residual and oxide phases were the most important phases controlling the vertical flux; whereas residual and organic phases controlled the vertical flux of Cu. The average annual flux data were used along with dissolved metal profiles to compute scavenging residence times for several trace metals in Puget Sound. Relatively short scavenging residence times (i.e. 100 days) were calculated for metals enriched in organic phases, such as Cu.

Excess 222Rn above the Cleft segment of the Juan de Fuca Ridge

The distributions of excess 222Rn above the North Cleft segment of the Juan de Fuca Ridge were determined from 1988 to 1991 in order to obtain a better understanding of the temporal variability of heat flux from this ridge segment. Based on the inventories of excess 222Rn, the hydrothermal heat flux was estimated to be 2.7±1.3 GW in 1990. The large uncertainty results from errors in calculating the standing crop of 222Rn, the difficulty in determining the 222Rn to heat ratio, and the problem of entrainment of low-temperature waters into the rising plume. The high-resolution sampling program carried out in 1990 permitted the mapping of apparent plume ages for different sections of the neutrally buoyant plume. About 3 km to the east of the ridge axis there is a sharp gradient in the apparent age of the plume, while to the west the plume appears to be younger. This plume age distribution could indicate a current flow to the northwest during the time this transect was sampled. The mean 222Rn/heat ratios for the high- and low-temperature vent fluids were 0.0017 disintegrations per minute (dpm) g−1 °C−1 and 0.0044 dpm g−1 °C−1, respectively. The extrapolated end-member concentrations of excess 222Rn in low-temperature vent fluids are a factor of 6 greater than the high-temperature fluids, based on the regressions of excess 222Rn activity and Mg concentration in North Cleft segment vent fluid samples.

Partitioning of Cu in estuarine waters, I. Partitioning in a poisoned system

Abstract The importance of dissolved hydrophobic organic matter in controlling the partitioning of Cu between the dissolved and particulate phases in natural waters was determined. Estuarine particulate matter was suspended in two solutions: (1) filtered seawater, and (2) filtered seawater in which the hydrophobic organic matter had been removed. Radioactive Cu was then added to each suspension. The removal of hydrophobic dissolved organic matter resulted in a two-fold increase in the distribution coefficient ( K D ) of added radioactive Cu (from 10 4.50 to 10 4.81 1/kg). The partitioning of 64 Cu in the suspensions was controlled by chemical processes dominated by complexation with particulate and dissolved organic matter.

Behavior of Fe, Mn, Cu and Cd in the duwamish river estuary downstream of a sewage treatment plant

Abstract The behavior of dissolved Fe, Mn, Cu and Cd in the Duwamish River downstream of a sewage treatment plant located near Seattle, Washington, was investigated in three distinct zones: (1) immediately downstream of the effluent outfall, (2) further downstream in the freshwater portion of the river and (3) in the estuarine mixing zone. A three-end-member tracer model utilizing salinity and dissolved inorganic nitrogen was developed to distinguish physical mixing processes from biogeochemical reactions. The results of this investigation indicate that Fe was removed from the dissolved phase onto particulates near the plant. Although no additional reactions controlled the distribution of dissolved Fe, Mn and Cu in the freshwater portion of the river, Cd was removed from the dissolved phase. In the estuarine mixing zone, a significant fraction of the dissolved Fe and Cu was removed from solution, while Mn and Cd desorbed from particulates. The chemical forms of the trace metals rather than their sources appear to determine their participation in these reactions.

Partitioning of Cu in estuarine waters, II. Control of partitioning by the biota

Abstract In two sets of suspensions containing biotic particles, organisms indirectly controlled the partitioning of added Cu between the dissolved and particulate phases by releasing dissolved organic ligands. Organisms present at total suspended matter concentrations as low as 10 mg/l released enough organic compounds during 1 day to negate the initial removal of the hydrophobic dissolved organic matter by Sep-Pak columns. In these experiments, the coating of particulate inorganic surfaces with organic matter may have also prevented Cu from establishing a chemical equilibrium between these particulate surfaces and the solution. The behavior of added radioactive Cu is consistent with an equilibrium model if the transfer of organic matter between dissolved and particulate phases by organisms is considered. In contrast, the behavior of natural Cu in these biotic suspension indicate that the partitioning of Cu was controlled as much by active biological processes as by chemical equilibria.

Age estimate for the 1987 megaplume on the southern Juan de Fuca Ridge using excess radon and manganese partitioning

Abstract In September 1987 we collected samples for the determination of 222 Rn, 226 Ra, and dissolved and particulate manganese in “Megaplume 2” above the Juan de Fuca Ridge spreading center. The results of these analyses permit us to estimate the apparent age of the megaplume by two independent methods. The age estimate based on excess 222 Rn is 20±9 days. Based on the partitioning of manganese between the dissolved and particulate phases the age is estimated to be between 6 and 60 days. Both techniques confirm that Megaplume 2 was a separate hydrothermal event and that it was not Megaplume 1 sampled 1 year after it was first discovered.