Surface Seawater Concentrations Research Articles

Sulfur gas fluxes and horizontal inhomogeneities in the marine boundary layer

Real‐time total gaseous sulfur concentrations were measured (sampling frequency 1 Hz) from the R/V Oceanus as part of the Atlantic Stratocumulus Transition Experiment/Marine Aerosol and Gas Exchange (ASTEX/MAGE) field campaign in the Azores during June of 1992. The measurements were used to estimate sulfur gas sea‐to‐air flux, determine the size scale of sulfur gas inhomogeneities in the marine boundary layer, and the timescale of sampling necessary to characterize a meaningful air mass. Using the time scale of sampling and wind speed measurements, the size scale of sampling can also be determined. Sea‐to‐air sulfur gas flux estimates were obtained using a variance method and the inertial‐dissipation method. Values from five 1‐hour measurement periods on two different days ranged from 21 to 28 μmol/m2 d and 11 to 17 μmol/m2 d, respectively. These values are above the range of 1 to 13 μmol/m2 d reported by Blomquist et al. (this issue) during ASTEX/MAGE, based on the dimethyl sulfide (DMS) concentration in surface seawater, using the stagnant boundary layer model of air‐sea exchange. A power spectrum of the sulfur gas time series shows the contribution of each turbulent eddy size to the total signal variance. In a representation of the power spectrum, the area under any portion of the curve is proportional to variance. From these power spectra we have obtained the fraction of the total variance in ground level sulfur concentration fluctuations as a function of time. The data indicate that air samples should be integrated for 1000 s for ground‐based measurements.

Laboratory production of bromoform, methylene bromide, and methyl iodide by macroalgae and distribution in nearshore southern California waters

Production rates of bromoform (CHBr3), methylene bromide (CH2Br2), and methyl iodide (CH3I) were measured in the laboratory for 11 species of marine macroalgae. Production rates of the volatile bromomethanes extrapolated to a global scale suggest that marine macroalgae produce 2 × 1011 g Br yr−1(1 × 109 mol Br yr−1), 98% of which is bromoform. Laminarians (kelps) produce 61% of this organic Br. These calculations suggest that marine macroalgae are important in the biogeochemical cycling of Br. Seawater concentrations of CHBr3, CH2Br2, and CH3I were determined from various southern California coastal locales. High concentrations were measured in seawater from the canopy and the bottom of a dense bed of Macrocystis as compared to other sites. Surface seawater concentrations of these halomethanes showed a strong cross‐shore gradient with the highest concentration in the kelp canopy and the lowest at 5 km offshore. Seawater adjacent to decaying macroalgae on the bottom of a submarine canyon was not enriched in halomethanes relative to surface water. Water exiting a productive estuary was enriched only with CH2Br2, although two algal species that are abundant there (Ulva and Enteromorpha) showed high laboratory production rates of both CHBr3 and CH2Br2.

Limitation of productivity by trace metals in the sea

Some trace metals such as Fe, Ni, Cu, and Zn are essential for the growth OF phytoplankton. The concentrations of these essential trace elements in seawater are so low as to limit their availability to aquatic microbiota. Trace element uptake is ultimately limited by kinetics of reaction with transport ligands or by diffusion to the cell. From what we know of the characteristics of the uptake systems of phytoplankton and their trace metal requirements we can estimate that Fe and Zn may at some times in some places limit phytoplankton productivity, which is in accord with available field data on trace metal enrichmeuts: Although the founding fathers of the field discussed many elements, including Fe, Mn, Zn, and Cu, as potentially limiting algal growth in seawater (Harvey 1945), for the past several decades biological oceanographers have focused almost exclusively on N and, to a lesser extent, on P and Si. Over that period we have learned that surface seawater concentrations of biologically interesting trace elements are much lower than previously thought (Bruland 1983). The pervasive contamination that distorted early measurements of trace elements would have also obfuscated their possible biological role. The application of so-called clean techniques to biological experiments has rekindled oceanographers’ interest in the biological and ecological function of trace elements in the sea, particularly the possible role of Fe in limiting primary production in oceanic regions where surface seawater is relatively rich in N and P. Several years ago we proposed that many bioactive elements may be colimiting phytoplankton growth in oceanic waters, that the stoichiometric concepts of Redfield

The investigation of air quality and acid rain over the Gulf of Mexico

A research cruise was conducted in the summer of 1986 by a group of scientist from the U.S.A. and Mexico to investigate air chemistry over the Gulf of Mexico. Chemical, physical, meteorological and oceanographic measurements were carried out to survey temporal and spatial variations of diverse parameters throughout the Gulf. Emphases were placed on air-sea-land exchange of gases and aerosols, natural air quality, transport of anthropogenic air pollution, and acid rain deposition to the Gulf. Although the prevailing winds were easterly from the sea during the cruise, the air was highly polluted with continental aerosols, probably caused by local shifting winds and the oscillation between sea breeze and land breeze. Aerosol number concentrations were measured from 10 5 cm −3 at ports to 10 3 cm −3 in the open Gulf. The average aerosol mass concentration was ∼25 μg M −3, consisting of 60% insoluble crustal particles that contained Si, Al, Fe; 30% seasalt particles that contained Na + and Cl −; and 10% anthropogenic sulfate and nitrate particles. Samples of rain water collected near the coast were acidic (pH ∼4). The concentrations of dimethyl sulfide correlated with bio-particle concentrations in surface seawater and could be a significant precursor of atmospheric SO 4 2− particles. The life cycles of the aerosols in the Gulf, including sources, transport, transformation, and wet and dry deposition are discussed.

Sulfide complexation in surface seawater

The speciation of inorganic sulfides in surface seawater has been calculated using available stability constants for metal sulfides and constants estimated from the dithizone extraction constants, together with mean trace concentrations of inorganic sulfide and some metal ions that form strong sulfide complexes. The main species is CuS. Practically all mercury (II) is found to be in the form of HgS while chloride complexation is more important for copper (I) and silver (I). Cobalt, nickel, zinc, cadmium and lead are not affected by sulfide concentrations in surface seawater of ∼ 0.5 nM. The influence of the dissolved FeS complex on the solubility curve for iron (II) sulfides is shown.

Biogenic sulfur emissions from the Subantarctic and Antarctic Oceans

Measurements of the atmospheric distributions of dimethylsulfide (DMS), sulfur dioxide (SO2), methanesulfonate (MSA), non‐seasalt sulfate (nss‐SO42−), and methylmercaptan (MeSH) were made over the Drake Passage and the coastal and inshore waters west of the Antarctic Peninsula during the 1986 austral fall season. DMS concentrations were also measured in the surface ocean, and vertical profiles were obtained at several locations in the inshore water areas. Surface seawater concentrations of DMS were on the order of 2 nmol L−1, similar to values observed in temperate latitudes. The vertical profiles showed maximum values at depths between 5–40 m. DMS concentrations in the atmosphere were near 4 nmol m−3 (96 parts per trillion by volume, (pptv)), close to the global average for remote oceanic regions. The sea‐to‐air flux of DMS ranged between 1–5 μmol m−2 d−1, with the highest values found in the open ocean areas. The total sea‐to‐air flux of DMS from the Southern Ocean is estimated to be 0.2 Tmol yr −1. MeSH was detected occasionally at concentrations up to 0.15 nmol m−3 (3.6 pptv). The average aerosol MSA concentration (0.27 nmol m−3) was surprisingly high relative to the level of nss‐SO42− (0.31 nmol m−3). Both compounds were found predominantly in fine particles. The nss‐SO42− values were extremely low, comparable to winter values reported for the South Pole. SO2 concentrations were also very low (0.45 nmol m−3 (11 pptv)), consistent with the low levels of nss‐SO42−. The residence time of DMS in the Antarctic marine atmosphere is estimated to be about 2–3 days during clear weather and about 6–7 days during overcast conditions. Photooxidation of DMS in the Southern Ocean atmosphere may lead to a higher yield of MSA and a lower yield of SO2 compared to other world ocean areas.

Mercury in surface waters of the open ocean

Mercury was determined in samples of surface seawater and rainfall from coastal and open ocean areas of the northwest Atlantic and Pacific Oceans. Mercury concentrations in surface seawater and open ocean rainfall ranged from 0.5 to 11 pM and 6 to 130 pM, respectively. The pluvial flux of Hg to the open ocean was estimated at 1.7 × 109 g/yr, using mean concentrations of Hg in rainfall and annual rainfall volume estimates for the major ocean basins. Fluvial input, while difficult to reliably assess, was a less important (about 0.18 × 109 g/yr) source of Hg to the ocean. The mean residence time of Hg in the surface mixed layer (to 75 m) was calculated at 4–7 years, which is similar to other very reactive elements such as Al, Pb and Bi. Surface seawater Hg distributions are markedly influenced by atmospheric sources. Due to an elevated supply of Hg to the northwest Atlantic by rain, higher Hg values were observed in surface seawater in the northwest Atlantic Ocean (4.0 ± 0.7 pM) than at low latitudes in the central North Pacific Ocean (2.1 ± 0.4 pM). Surface seawater Hg concentrations between coastal New England and the Sargasso Sea do not show the large offshore concentration gradient typical of elements which have relatively large fluvial or coastal sources. Rather, the Hg distribution in this region was similar to that obtained for the atmospherically derived constituents Pb and 210Pb. Surface seawater Hg measurements in the central Pacific Ocean along 160°W between 20°N and 20°S showed a depression (>60%) in the equatorial upwelling area which coincided with the transect region exhibiting low 234Th/238U activity ratios. This relationship implies that Hg has enhanced scavenging and removal from surface seawater in biologically productive oceanic zones. In contrast to the surface distribution in the northwest Atlantic, a pronounced on‐shore gradient in sea surface Hg concentration was observed in the Tasman Sea. This may have resulted from the upwelling of water high in Hg content into the near coastal zone.

Methyl halides in and over the eastern Pacific (40°N–32°S)

Methyl chloride, methyl bromide, and methyl iodide measurements in and over the eastern Pacific (40°N and 32°S latitude) show mean air concentrations of 633 parts per trillion (ppt), 23 ppt, and 2 ppt, and mean surface seawater concentrations of 11.5 ng/l, 1.2 ng/l, and 1.6 ng/l respectively. Long‐term ambient measurements at a marine Pacific site (39°N) show an essentially unchanging background, with some indication of high concentrations of methyl bromide and methyl iodide during summertime. The oceanic data indicate a mean surface supersaturation of 275%, 250%, and 340%, respectively, for these three methyl halides. Depth profiles show that methyl halides are most abundant in the top mixed layer of the ocean. Oceanic concentrations of methyl chloride and methyl bromide are significantly correlated (linear regression coefficient of 0.85), suggesting a common source. No relationship between oceanic methyl iodide concentrations and the other two halides could be found. Coexistence of high concentrations of methyl iodide with relatively low concentrations of methyl chloride and vice versa provided no direct support for the hypothesis that chloride ion reactions with methyl iodide may be the dominant oceanic source of methyl chloride. For the eastern Pacific, mean ocean to air fluxes (in units of 10−7 g cm−2 yr−1) of 13, 1, and 1 are determined for methyl chloride, methyl bromine, and methyl iodide, respectively. When extrapolated to global waters, they provide an adequate source to explain the atmospheric reservoir of these organic halides. Total organic bromine and iodine measurements would suggest that methyl bromide and methyl iodide contribute predominately to the tropospheric bromine and iodine organic reservoir.

Latitudinal distribution and origin of particulate silica in the surface water of the North Pacific

The concentrations of particulate silica and inorganic suspended matter in the surface water of the North Pacific were determined. The concentration of particulate silica depends on latitude, and increases with increasing degree of the north latitude. The concentration of inorganic suspended matter shows a similar distribution pattern, but it is of much smaller magnitude. There exists a high positive correlation between the concentrations of particulate silica and nutrients. On the other hand, a weak negative correlation is found between the concentrations in surface seawater of particulate silica and radioactive 210Pb which originate chiefly from precipitation and dry fallout. This evidence suggests that the particulate silica in the sea is mainly of biogenic origin and the contribution of nonbiogenic silica, i.e., of terrestrial origin, is small.

The Th 232 concentration of surface ocean water

Previous workers have reported Th 232 concentrations in surface sea water ranging from 0.3 to 0.6 μg/1000 1. Whereas these estimates were based on the analysis of single samples, the present work is based on a composite of 24 analyses of water collected from widely spaced parts of the ocean. A careful interpretation of the alpha spectrometer data indicates that the true concentration is less than 0.07 μg Th 232 1000 1 .