Subsurface Seawater Research Articles

Photo-induced degradation of tracer phenols added to marine surface microlayers

To determine photochemical reactivity of surface microlayer components and to estimate residence times of phenolic materials at the ocean surface, we evaluated photo-induced degradation of three tracer phenols (phloroglucinol [1,3,5-trihydroxybenzene), TMP (2,4,6-trimethylphenol), and cresol (3-methylphenol)) added to samples of surface microlayers and subsurface waters. When all samples were exposed to the same natural sunlight conditions, first-order degradation rates of phloroglucinol in microlayer samples from surface slicks were always faster than degradation rates in microlayer samples not from slicks or in samples from subsurface seawaters. TMP was less photo-reactive than phloroglucinol, probably because of its lower dissociation to phenoxide ion at seawater pH. Degradation rates of phloroglucinol and TMP in microlayer samples from surface slicks were depressed by the presence of 50 μM NaN 3, a common singlet oxygen quencher, suggesting the involvement of photo-induced singlet oxygen. Generally, degradation of cresol was slower than either phloroglucinol or TMP, although it should have had a higher degree of dissociation than TMP at seawater pH; degradation of cresol in seawater may be not via singlet oxygen. Dark control experiments exhibited < 10% decreases in tracer phenol concentrations, indicating that microbial activities had little effect on degradation of these phenols. Because photoreactivity in samples from surface slicks was generally much greater than in other samples, microlayer photochemistry cannot be estimated by simply extrapolating near-surface reaction rates to surface light intensities.

Nitrification rates and 15N abundances of N2O and NO3− in the western North Pacific

NITROUS oxide plays an important part in atmospheric chemistry; it is a 'greenhouse' gas and has the potential to destroy ozone. Concentrations of N2O in surface waters of the oceans are generally close to equilibrium with the atmosphere, but sub-surface sea water is supersaturated with N2O1–5. The oceans are now regarded as a source4 for atmospheric N2O by production through the oxidation of ammonium ('nitrification')1,3, except in extremely oxygen-deficient waters6. Nevertheless, the possibility that N2O might be produced through the reduction of nitrate ('denitrification') cannot be ruled out5,7. The distribution of 15N in atmospheric and oceanic N2O can be used to investigate the behaviour of N2O in the oceans8,9. Here we report simultaneous measurements of the nitrification rates and nitrogen isotope data for N2O and NO3– produced in the western North Pacific Ocean. The amount of N2O produced by nitrification is much lower than that expected from the production of NO3–. The N2O in the oxygen-deficient layer is more enriched in 15N than NO3–. These results imply that the contribution of nitrification to the production of N2O is lower than previously thought and that denitrification is primarily responsible for the production of N2O.

Current subsurface seawater intrusion to base levels below sea level

The imperial Valley, Jordan-Dead Sea Rift, and the Afar and Oattara depressions, all regions below sea level and the only regions in the world so situated, are characterized by saline and hypersaline groundwaters and lakes, phenomena that to date have been solely attributed to ancient lagoons, salt dissolution, and evaporation Current subsurface seawater intrusion is herewith suggested as an additional mechanism responsible for the salination of these regions This type of dynamic seawater flow is feasible where there is (a) a base level below sea level with a hydrological continuity between the two levels, and (b) a low groundwater divide between the base levels with a shallow seawater/freshwater interface situated above the base of the aquifer

A Large-Volume Sampling Assembly for the Determination of Synthetic Organic and Petroleum Compounds in the Dissolved and Particulate Phases of Seawater

A system for the in situ sampling of subsurface seawater for the determination of synthetic organic and petroleum compounds in concentrations of picograms per litre is described. Seawater from in front of the bow of the vessel was pumped through a stainless steel tube into a cleanroom, through a glass fiber filter, and through parallel or serial Teflon® columns packed with a porous polyurethane foam (PPF) solid adsorbent. The majority of hydrocarbons associated with seawater particles were quantitatively recovered. The efficiency of the foam columns in extracting the dissolved phase was determined by intercaiibration with liquid–liquid extraction using cyclohexane and by separate analyses of two columns mounted in series. Recovery of polychlorinated biphenyls, several chlorinated hydrocarbon biocides, and aromatic hydrocarbons was quantitative with a single PPF column. Measured concentrations were in the low picogram per litre range for chlorinated organics and the nanogram per litre range for total petroleum. Reduction of contamination during sampling remains of paramount importance in the determination of hydrocarbon concentrations in seawater.

Study on the extraction of organic compounds from seawater with XAD-2 resin

Approximately twenty filtered sub-surface seawater samples were pumped through XAD-2 resin columns which were subsequently extracted and the extracts analyzed by GC and GC/MS. Conditions of adsorption and of extraction, including filters and pumps used, water flow rates and the solvent for extraction, were varied during the study and the resulting extracts were compared. The most prevalent compounds encountered in all samples were the straight chain fatty acids typical of phytoplankton (predominantly even carbon numbers ranging from C 12 to C 22 with significant proportion of unsaturated compounds). The proportions of individual compounds varied somewhat over the study, but the major components such as C 16:0 were always present as a high fraction of the total acids. Variations from sample to sample appeared to be more related to day-to-day variation in the water than to minor changes in method. Other compounds observed in the samples included two and three ring aromatic compounds, straight chain aliphatic compounds and alcohols. In all these cases the levels were considerably lower than the acids and were variable from sample to sample. Based on the individual members of the classes present, however, it was possible to determine the probable source of these compounds. In addition to blanks and spikes to check on the effectiveness of the method, a direct comparison of the XAD method to a continuous flow liquid—liquid extractor was made by splitting the flow of a filtered surface water sample (highly polluted) between the two extractors. The compounds observed in the twó samples were very similar both qualitatively and quantitatively. It would appear the XAD-2 method is a viable method for analyzing organic compounds in seawater.

Thermohaline steps induced by melting of the Erebus Glacier Tongue

A vertically stable, step‐like thermohaline structure is observed throughout a continuous, 400 m conductivity‐temperature‐depth (CTD) profile taken near the Erebus Glacier Tongue, McMurdo Sound, Antarctica. The pattern is best developed between the sea surface and 250 m depth, the interval corresponding to that of the irregular underwater profile of the Glacier Tongue. The steps average 17 m in thickness and typically display discontinuities of 0.1°C in temperature, 0.04‰ in salinity and 3.5 × 10−4 g cm−3 in density. The observations are compared with theory and laboratory experiments of cell development and lateral flow near ice melting into vertically stratified salt water. At this location, subsurface seawater is inferred to remain above the in situ freezing point year‐round, and contains sufficient heat to account for much of the Glacier Tongue thinning by basal melting. An adequate volume of meltwater would result to produce the measured salinity steps. We discuss related observations and some implications of this process for ocean circulation and biological productivity in the Antarctic.

The sea surface microlayer: Measurements of dissolved iodine species and nutrients in coastal waters1

Sea surface microlayer concentrations of the dissolved iodine species iodate, iodide, and total iodine as well as the nutrients nitrite, nitrate, ammonium, and phosphate have been compared with their amounts in subsurface seawater. Little or no microlayer enhancement was found for any of these species in any of the coastal samples studied. The reason for this may be that for the species investigated there is no mechanism by which elevated microlayer concentrations can be maintained in the face of processes leading to vertical mixing into the subsurface water.

Processes affecting particulate trace metals in the sea surface microlayer

Abstract Concentrations of particulate Fe, Mn, Ni, Cu, Zn, Cd and Pb have been measured in surface microlayer and subsurface seawater samples collected in the North Sea adjacent to the East Anglian coast, in an area subject to a considerable fluvial input of clay minerals. The results are interpreted by estimating the magnitudes of different processes affecting particulate matter in the microlayer: atmospheric deposition, Brownian diffusion, gravitational settling, bubble flotation and mixing. Both Fe and Mn are strongly depleted in the microlayer, evidently as a result of gravitational settling of Fe- and Mn-bearing mineral particles out of the microlayer. These particles are mixed into the surface region from the water column beneath. Microlayer enrichment of Cu, Zn and Pb was also observed and probably results from flotation of particles attached to rising bubbles. In one set of samples, however, the marked enrichment of these elements, as well as Ni, may result instead from deposition of particles from the atmosphere directly onto the water surface.

Current subsurface intrusion of Mediterranean seawater — a possible source of groundwater salinity in the Rift Valley system, Israel

Various theories have been put forward regarding the salination mechanism in the Jordan-Dead Sea Rift Valley and its branching valleys. None of these have suggested current subsurface seawater penetration to the internally drained Judea Group aquifer, as a part of the system. The hydrogeological configuration is a combination of a low water table and a rather low groundwater divide, resulting in a shallow sea-fresh water interface. In most of the area the interface is expected to be situated above the base of the aquifer. As a result current seawater intrusion is possible across the ground water divide through the Yizre'el and Beersheva valleys which dissect Israel and connect the Mediterranean and the Jordan-Dead Sea Rift Valley. The model suggested herein is in accordance with, or cannot be rejected on the basis of the chemistry and isotope composition of the saline waters. In the highly flushed aquifers of the internal valleys, brackish water with marine affinity suggests current infiltration of seawater. This mechanism is additional to other sources, such as deep-seated highly-concentrated brines and entrapped fossil seawater, all of which are diluted by the flow of cyclic freshwaters.

Concentration of magnesium in carbonate nodules of soils: An indication of fresh groundwater contamination by intruding seawater

Carbonate nodules from soils in Israel, only exposed to meteoric water, have a m Mg 2+ m Ca 2+ ratio of < 0.04. Higher ratios, mostly between 0.035 and 0.089, were found in similar nodules from regions of emergence of brackish waters formed as a result of subsurface seawater intrusion from the interface underneath. These nodules consist of Mg-bearing calcite or dolomite. The m Mg 2+ m Ca 2+ ratio in the nodules agrees with the same ratio measured in the groundwater solutions precipitating them.

Artificial upwelling induced by ocean currents —Theory and experiment

A submerged apparatus, which consists of a buoy, several horizontal contraction and expansion tubes (Venturi-type tubes) and a long pipe, is expected to be used to pump the subsurface sea-water (200–300 meter depth) containing abundant nutrients to surface layer (50–100 m) by the dynamic of ocean currents. i.e. an artificial upwelling without energy cost. A preliminary experiment and analysis are undertaken and shows that the capacity of pumping the nutrient-rich sea-water is worth to build a pilot prototype model.

Comparison determinations of lead by investigators analyzing individual samples of seawater in both their home laboratory and in an isotope dilution standardization laboratory

To determine photochemical reactivity of surface microlayer components and to estimate residence times of phenolic materials at the ocean surface, we evaluated photo-induced degradation of three tracer phenols (phloroglucinol [1,3,5-trihydroxybenzene), TMP (2,4,6-trimethylphenol), and cresol (3-methylphenol)) added to samples of surface microlayers and subsurface waters. When all samples were exposed to the same natural sunlight conditions, first-order degradation rates of phloroglucinol in microlayer samples from surface slicks were always faster than degradation rates in microlayer samples not from slicks or in samples from subsurface seawaters. TMP was less photo-reactive than phloroglucinol, probably because of its lower dissociation to phenoxide ion at seawater pH. Degradation rates of phloroglucinol and TMP in microlayer samples from surface slicks were depressed by the presence of 50 μM NaN3, a common singlet oxygen quencher, suggesting the involvement of photo-induced singlet oxygen. Generally, degradation of cresol was slower than either phloroglucinol or TMP, although it should have had a higher degree of dissociation than TMP at seawater pH; degradation of cresol in seawater may be not via singlet oxygen. Dark control experiments exhibited < 10% decreases in tracer phenol concentrations, indicating that microbial activities had little effect on degradation of these phenols. Because photoreactivity in samples from surface slicks was generally much greater than in other samples, microlayer photochemistry cannot be estimated by simply extrapolating near-surface reaction rates to surface light intensities.