Louis I. Gordon

Submarine Thermal Springs on the Galápagos Rift

The submarine hydrothermal activity on and near the Gal�pagos Rift has been explored with the aid of the deep submersible Alvin. Analyses of water samples from hydrothermal vents reveal that hydrothermal activity provides significant or dominant sources and sinks for several components of seawater; studies of conductive and convective heat transfer suggest that two-thirds of the heat lost from new oceanic lithosphere at the Gal�pagos Rift in the first million years may be vented from thermal springs, predominantly along the axial ridge within the rift valley. The vent areas are populated by animal communities. They appear to utilize chemosynthesis by sulfur-oxidizing bacteria to derive their entire energy supply from reactions between the seawater and the rocks at high temperatures, rather than photosynthesis.

Ridge crest hydrothermal activity and the balances of the major and minor elements in the ocean: The Galapagos data

Samples collected by the deep submersible “Alvin” from four hot spring fields (T = 3–13°C) on the crest of the Galapagos spreading ridge show pronounced and varied compositional anomalies. If it is assumed that these have a general significance, that they are associated with hydrothermal reactions between seawater and basalt wherever new oceanic crust is being produced then global fluxes can be computed. These are large. For Mg and SO4 they balance the river input. For Li and Rb they exceed it by factors of between five and ten. Calcium is supplied at a rate equivalent to that of non-carbonate Ca from the continents. The additions of K, Ba and Si are between one third and two thirds of the river load. There are large positive and negative anomalies for Cl and Na indicating that substantial amounts of Cl may be taken up by the newly formed crust and transported deep into the subduction zones. Where there are data in common, the field measurements agree with the experimental findings at low (<5) water/rock ratios.

On the formation of metal-rich deposits at ridge crests

Data from the hot springs at the Galapagos spreading center (T = 3–13°C) show depletions of the exiting waters in Cu, Ni, Cd, Se, Cr and U relative to ambient seawater. Manganese is strongly enriched. Iron shows highly variable behavior between vent fields but is in general low. The data confirm the occurrence of extensive subsurface mixing between the primary high-temperature, acid, reducing hydrothermal fluids and “groundwater”. The composition of the latter is indistinguishable from that of the free water column adjacent to the ridge axis. The final solutions are on the boundary between those forming MnO2 crusts and those producing iron-manganese rich sediments. The suite of metal rich deposits observed at ridge crests — Mn-O, Fe-Mn-O, Fe-S — can be explained as the manifestation of the degree of subsurface mixing, decreasing from 100 : <1 to <1 : 1 across the series (assuming an end-member temperature of 350°C).

Hyperproductivity of the Ross Sea (Antarctica) polynya during austral spring

Although satellite data of surface layer pigments have suggested that the daily productivity in the Ross Sea is among the largest found in any marine system, no modern oceanographic cruise has entered the Ross Sea polynya to quantitatively assess the austral spring productivity over time scales of days to weeks. We conducted a cruise to the Ross Sea polynya in November–December, 1994 to measure the contribution of phytoplankton during the austral spring to the annual productivity of the region and found markedly enhanced levels of phytoplankton biomass. Chlorophyll concentrations were greater than 3 µg l−1 in mid-November, a time when the polynya was covered by a thin (ca. 20–30 cm) layer of ice. Particulate matter concentrations increased through time, and by early December chlorophyll and particulate carbon concentrations exceeded 10 µg l−1 and 53 µmol l−1, respectively. Primary productivity also increased through time: the mean productivity in early December equaled 3.53 g C m−2 d−1, and maximum measured rates exceeded 6 g C m−2 d−1. Productivity based on nitrate disappearance averaged 1.52 g C m−2 d−1 (with a maximum rate of 2.49 g C m−2 d−1), suggesting that the blooms new production was also substantial. The Ross Sea polynya is the most southerly location in the Antarctic where phytoplankton growth is initiated this early and which supports such high standing stocks by early December. Inclusion of this production in a carbon budget for the region suggests that this area supports an annual production of 200 g C m−2, the largest of any region in the Southern Ocean, and confirms its hyperproductive nature.

Nitrous oxide in the oxygen minimum of the eastern tropical North Pacific: evidence for its consumption during denitrification and possible mechanisms for its production

Investigation of the nitrous oxide (N2O), oxygen, and nutrient distributions in the eastern tropical North Pacific Ocean (ETNP) indicated biochemical N2O consumption around the core of the oxygen minimum where denitrification takes place. Subsurface N2O minima were found at stations where intermediate waters contained near-zero oxygen concentrations and well-developed secondary nitrite maxima, characteristic of denitrification. The N2O minima were absent at stations where there was little or no evidence for denitrification. Surface waters of the ETNP were on the average 110% saturated with N2O relative to a marine air content of 287 ppbv N2O. The area appears to be a source for atmospheric N2O with an average magnitude of 0.14 pg N2O cm−2s−1. Except for the oxygen-deficient layer where the N2O minima were found, N2O is highly supersaturated at all stations from just below the mixed layer to about 1000 m. Supersaturations varied from 224 to 600% in shallow waters above the base of the pycnocline and from 120 to 250% in deep waters below 1000 m. Possible mechanisms for biochemical N2O production in the ocean were examined and it is concluded that in the ETNP, nitrification is the most important.

Production and pelagic dissolution of biogenic silica in the Southern Ocean

Vertical distributions of particulate silica, and of production and dissolution rates of biogenic silica, were determined on two N-S transects across the Pacific sector of the Antarctic Circumpolar Current during the austral spring of 1978. Particulate silica profiles showed elevated levels in surface water and near the bottom, with low (35–110 nmol Si · 1−1) and vertically uniform values through the intervening water column. Both the particulate silica content of the upper 200 m and the production rate of biogenic silica in the photic zone increased from north to south, reaching their highest values near the edge of the receding pack ice. A significant, but variable, fraction (18–58%) of the biogenic silica produced in the surface layer was redissolving in the upper 90–98 m. Net production of biogenic silica in the surface layer (production minus dissolution) was proceeding at a mean rate of ca. 2 mmol Si · m−2 · day−1. This is ca. 4 times greater than the most recent estimate of the mean accumulation rate of siliceous sediments beneath the ACC. We estimate, based on mass balance, that the mean dissolution rate of biogenic silica in subsurface water column in the Southern Ocean is 1.2–2.9 mmol Si · m−2 · day−1.

Distribution of CO2 species, estimates of net community production, and air-sea CO2 exchange in the Ross Sea polynya

Measurements of surface total carbon dioxide (TCO2), alkalinity, and calculated pCO2, along with water column nutrients and hydrography, were made on two cruises to the Ross Sea polynya (NBP 94-6, November-December 1994 and NBP 95-8, December 1995 to January 1996). The polynya experiences an intense phytoplankton bloom during a short period of open water conditions from mid-December to mid-February each year. Our biogeochemical observations were used to determine the temporal variability of CO2, fluxes of carbon within the ocean, and rates of air-sea exchange of CO2. Depletions of TCO2, pCO2, and nitrate+nitrite were considerable (?70–150 ?mol kg?1, 80–150 ?atm, and 10–20 ?mol kg?1, respectively) and associated primarily with biological uptake during Phaeocystis and diatom blooms. Alkalinity was a conservative tracer of salinity and nitrate+nitrite. Surface ?CO2 was undersaturated by ?50–150 ?atm, and air-sea gas exchange of CO2 during open water conditions was directed from atmosphere to ocean. Observed surface stoichiometric C:N ratios were 6.66: 1 and 6.77:1 for the 2 years, consistent with global “Redfield” ratios, while C:P and N:P ratios were variable (75–141:1, 12–18:1). Estimates of net community production (NCP) rates were made using in situ changes in TCO2 and nitrate+nitrite across repeated transects along 76°30?S. Mean NCP rates across the polynya ranged from 0.86 to 0.98 g C m?2 d?1. These values may be underestimated by 5–25% because of the contribution of atmospheric CO2 to the surface layer through gas exchange. Export of carbon from the surface to depth was at least 55–60% of NCP rates.

Particulate matter and nutrient distributions in the ice-edge zone of the Weddell Sea: relationship to hydrography during late summer

Previous estimates of the marginal ice zones quantitative contribution to biogeochemical cycles and annual productivity in the Southern Ocean may be conservative because of assumptions that phytoplankton blooms are associated only with actively retreating ice edges. Observations during March 1986, near an almost stationary ice edge in the northwestern Weddell Sea, revealed very low geostrophic currents, no appreciable horizontal gradients in temperature or salinity and no significant net melting or freezing in the ice-edge region. Vertical stratification within the upper 50 m was evident throughout the study area, and resulted primarily from prior melting of pack ice. In contrast with previous observations in marginal ice zones, the distribution of phytoplankton biomass showed little correlation with the meltwater field; here, significant horizontal biomass gradients occurred in an area where vertical stability was almost uniform laterally and both elevated biomass and diminished nutrient levels extended well below the pycnocline. Absolute levels of chlorophyll were modest (generally 10, 1.5 and 3.5 μmol l−1, respectively) were similar to those found previously during a spring phytoplankton bloom at the Weddell-Scotia Sea ice edge. The mean mole ratio of biogenic silica to organic carbon within the particle assemblage was 0.44, which is very high for surface seawater and about three times higher than that typically found in pure diatom cultures. Therefore, despite relatively low chlorophyll levels, the ice edge remained a localized maximum in biogenic particulate matter at least through March and this material was unusually rich in silica. Combined, these physical and biological data show that elevated phytoplankton biomass in the ice-edge zone can persist well past the time when net melting stops and the ice edge becomes stationary.

Nutrient dynamics and new production in a warm-core eddy from the Eastern Mediterranean Sea

The nutrient dynamics of a warm-core eddy from the Eastern Mediterranean was studied through a complete annual cycle. Winter mixing of the upper 450 m of the eddy core resulted in phytoplankton biomass build-up (70 mg Chl a m−2) of stock well above that present in the boundary (44 mg m−2). During this biomass build-up phosphate was reduced to undetectable levels while excess nitrate (0.6 μM) remained which, taken with other evidence, showed that this area of the Eastern Mediterranean is phosphorus limited. After stratification in late March, nitrate and phosphate concentrations were at undetectable levels in the upper 120 m, a deep chlorophyll maximum had developed (90–120 m), and a gradual systematic accumulation of nutrients occurred in the zone from 120 to 450 m. The permanent nutricline lay between 450 and 650 m. The net supply of nutrients to the euphotic zone (0–120 m), which is the annual new production, was 300 mmoles N m−2 y−1 and 14.9 mmoles P m−2 y−1. Most of this new production (∼90%) was supplied by deep winter mixing and occurred over a limited period of time (days to a few weeks). Previous estimates for new production in the Eastern Mediterranean by Dugdale and Wilkerson (1988, Oceanologica Acta, 9, 170–184), based on bottle incubations, were low because the major input of nutrients by deep winter mixing was not included in the calculation. Most of the nutrients supplied to the euphotic zone in the eddy (60–70%) were derived from the decomposition of the previous years productivity.

Early spring phytoplankton blooms in ice platelet layers of the southern Weddell Sea, Antarctica

Abstract A dense diatom bloom growing in a shallow stratified layer maintained in position by loose ice platelets was found underlying pack-ice bordering the coastal polynyas of the Weddell Sea ice shelf south of 74°S in early spring well before the onset of seasonal melt. This rich bloom, which covered ca 20,000 km 2 , contrasted with the barrenness of the entire area between 74°S and the northern edge of the pack-ice at 58°S; its presence is explained by favourable conditions for accumulation of several decimetre-thick ice platelet layers under pack-ice of the southern shelf. Nutrient exhaustion and mass sinking of diatom chains were observed in this layer. Centric diatoms suspended in interstitial water dominated this bloom, which contrasted strongly with the flora of attached pennates typical of ice platelet layers underlying fast ice. Superblooms have been described previously from the southern Weddell Sea, although their developmental dynamics were not known at the time. We provide explanations for several perplexing features of this superbloom and show that they are significant in enhancing productivity of the Weddell Sea.