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Hydrographic properties and ventilation of the Black Sea

Using hydrographic data collected by CTD during five cruises of the 1988 Black Sea Oceanographic Expedition, from 16 April to 29 July 1988, we describe the distribution of potential temperature (θ), salinity (S), and potential density (σθ) throughout the water column. The salinity and density increase rapidly with depth, while temperature decreases to a minimum at 50 m in the cold intermediate layer (CIL). All three variables increase slowly with depth in the deep water. The hydrographic properties of the upper 200 m varied little over the duration of the 1988 expedition. Significant differences are observed when the 1988 data are compared with the 1969 Atlantis II data set. All of the 1969 data are warmer at a given salinity than the 1988 data to a depth with a salinity of about 21.0%o. Possible causes for these changes are increased heat loss to the atmosphere and decreased freshwater input. The most distinctive feature in the deep water is a homogeneous benthic bottom layer that extends from about 1700 m to the bottom. There is a single pronounced step in all hydrographic properties at the top of this layer. Vertical transport across the upper boundary may be controlled by double diffusion driven by geothermal heat flow. The predicted double-diffusive heat flux agrees with geothermal heat flow to within a factor of 5. A simple box model with surface, entrainment and deep-water reservoirs is used to model the entrainment process and the residence time of deep water in the Black Sea. The results suggest that the Bosporus inflow entrains water with properties of the CIL. The ratio of entrainment to Bosporus inflow is 3.3. Assuming a Bosporus inflow of 312 km3 y−1, the resulting residence time of the deep water is 387 years. A total CO2 balance is used to calculate the flux of carbon into the deep water and Δ14C balance is used to calculate the pre-nuclear value of Δ14C = −200%o in the entrainment water. This highly depleted value would have resulted in an apparent age of 1400 years for the CIL and, probably, the surface water as well. If the carbon flux of biological origin was depleted to the same extent this may account for some of the differences in sedimentary chronology based on 14C dates and varve counts.

The evolution of Mediterranean water in the Black Sea: interior mixing and material transport by double diffusive intrusions

Abstract After its entry into the Black Sea from the Bosphorus Strait, Mediterranean Water first evolves by mixing with the Cold Intermediate Water on the shelf region, and later sinks along the continental slope, reaching the halocline in the form of cold anomalies. The intrusion of the modified waters drives a series of intermediate depth nepheloid layers spreading from the southwestern margin into the interior of the Black Sea basin. In many cases, the temperature, salinity, suspended matter and other properties of the intruding layers in the interior can be traced back to the southwest shelf region. The opposing effects of the existing temperature and salinity gradients on the stability of the interior density stratification, and the anomalous temperature and salinity of the intrusions themselves, result in a unique mechanism of double diffusive convection at intermediate depth. The mixture of Mediterranean and Cold Intermediate Water is injected at a wide range of intermediate depths below the anoxic interface, leading to time-dependent, filamented and layered structures in the interior. The net entrainment and convective mixing produced by the intrusions are significant in determining the overall boundary transport and mixing across the permanent halocline. The time-dependent penetration of the sinking water implies a statistical control of the Black Sea interior stratification, and may explain the peculiar features of the permanent halocline. Local shelf dynamics and coherent structures of the Black Sea circulation modify the transport by the intrusions and their mixing with the interior waters. Boundary currents transport the anomalous properties east along the shelf, and interactions of the currents with shelf topography near Sakarya Canyon lead to suspension and spreading of shelf sediments.

Use of tritium and helium to define groundwater flow conditions in Everglades National Park

The concentrations of tritium ( 3 H) and helium isotopes ( 3 He and 4 He) were used as tracers of groundwater flow in the surficial aquifer system (SAS) beneath Everglades National Park (ENP), south Florida. From ages determined by 3 H/ 3 He dating techniques, groundwater within the upper 28 m originated within the last 30 years. Below 28 m, waters originated prior to 30 years before present with evidence of mixing at the interface. Interannual variation of the 3 H/ 3 He ages within the upper 28 m was significant throughout the 3 year investigation, corresponding with varying hydrologic conditions. In the region of Taylor Slough Bridge, younger groundwater was consistently detected below older groundwater in the Biscayne Aquifer, suggesting preferential flow to the lower part of the aquifer. An increase in He with depth in the SAS indicated that radiogenic 4 He produced in the underlying Hawthorn Group migrates into the SAS by diffusion. Higher Δ 4 He values in brackish groundwaters compared to fresh waters from similar depths suggested a possible enhanced vertical transport of 4 He in the seawater mixing zone. Groundwater salinity measurements indicated the presence of a wide (6-28 km) seawater mixing zone. Comparison of groundwater levels with surface water levels in this zone indicated the potential for brackish groundwater discharge to the overlying Everglades surface water.

Isotopic, geophysical and biogeochemical investigation of submarine groundwater discharge: IAEA-UNESCO intercomparison exercise at Mauritius Island

Submarine groundwater discharge (SGD) into a shallow lagoon on the west coast of Mauritius Island (Flic-en-Flac) was investigated using radioactive ((3)H, (222)Rn, (223)Ra, (224)Ra, (226)Ra, (228)Ra) and stable ((2)H, (18)O) isotopes and nutrients. SGD intercomparison exercises were carried out to validate the various approaches used to measure SGD including radium and radon measurements, seepage rate measurements using manual and automated meters, sediment bulk conductivity and salinity surveys. SGD measurements using benthic chambers placed on the floor of the Flic-en-Flac Lagoon showed discharge rates up to 500 cm/day. Large variability in SGD was observed over distances of a few meters, which were attributed to different geomorphological features. Deployments of automated seepage meters captured the spatial and temporal variability of SGD with a mean seepage rate of 10 cm/day. The stable isotopic composition of submarine waters was characterized by significant variability and heavy isotope enrichment and was used to predict the contribution of fresh terrestrially derived groundwater to SGD (range from a few % to almost 100%). The integrated SGD flux, estimated from seepage meters placed parallel to the shoreline, was 35 m(3)/m day, which was in reasonable agreement with results obtained from a hydrologic water balance calculation (26 m(3)/m day). SGD calculated from the radon inventory method using in situ radon measurements were between 5 and 56 m(3)/m per day. Low concentrations of radium isotopes observed in the lagoon water reflected the low abundance of U and Th in the basalt that makes up the island. High SGD rates contribute to high nutrients loading to the lagoon, potentially leading to eutrophication. Each of the applied methods yielded unique information about the character and magnitude of SGD. The results of the intercomparison studies have resulted a better understanding of groundwater-seawater interactions in coastal regions. Such information is an important pre-requisite for the protection and management of coastal freshwater resources.

Double Diffusive Intrusions, Mixing and Deep Sea Convection Processes in the Black Sea

Hydrographic measurements obtained during Leg 4 oceanographic expedition of the RV Knorr in the Black Sea point to important processes deserving scientific attention. The destabilizing effects of the cooling near the surface and the influx of geothermal heat at the bottom combined with the stable salinity gradients provide an environment suitable for double-diffusion in the water column. The hydrographic data do not provide any evidence for the replenishment of the bottom water by the inflowing Mediterranean waters. On the other hand, there is clear evidence that the renewal of intermediate waters takes place at or below the pycnocline level by the transformed intrusion waters. Temperature-salinity anomalies at the shelf break demonstrate that the Mediterranean water mixes with the ambient cold intermediate waters and then sinks in the form of a plume with high aspect ratio. Later, horizontal transport of water in the form of multiple layers with anomalous properties is often evident and is also traced by the transport of suspended matter from the continental slope towards the interior region of the Black Sea. The geothermal heating gives rise to the convective turnover of the deep waters in a bottom mixing layer. The extent and features of this convective layer are quite unique. On the other hand, models derived from laboratory experiments and the available theory appear to be insufficient to describe its evolutionary aspects. The identification of such unique physical processes in the Black Sea make it an ideal environment for scientific studies of these processes, as well as providing new insights into some of its chemical, and sedimentological puzzles.

Evidence for the removal of CFC-11, CFC-12, and CFC-113 at the groundwater–surface water interface in the Everglades

Abstract Poor agreement between 3 H/ 3 He ages and CFC-11 and CFC-12 ages suggests that CFCs may not be conservative tracers in the Everglades National Park. 3 H/ 3 He ages were used to calculate the expected concentration of CFC-11 and CFC-12 in groundwater from wells 2 to 73 m deep. The expected concentrations of CFCs were compared to the measured concentrations and plots of the % CFC-12 and CFC-11 remaining offered no evidence that significant CFC removal was occurring in the groundwater at depths ≥2 m, suggesting that CFC removal occurs at shallower depths. Except where CFC contamination was suspected, CFC-11, CFC-12 and CFC-113 concentrations in fresh surface water were nearly always below solubility equilibrium with the atmosphere. Measurements of CFC-11, CFC-12 and CFC-113 in pore water indicate a 50–90% decrease in concentration 5 cm below the groundwater–surface water (GW–SW) interface. In the same 5 cm interval CH 4 concentrations increased by 300–1000%. This suggested that CFCs were removed at the GW–SW interface, possibly by methane-producing bacteria. CFC derived recharge ages should therefore be viewed with caution when recharging water percolates through anoxic methanogenic sediments.

Mantle 3He distribution and deep circulation in the Indian Ocean

[1] The World Ocean Circulation Experiment Indian Ocean helium isotope data are mapped and features of intermediate and deep circulation are inferred and discussed. The 3He added to the deep Indian Ocean originates from (1) a strong source on the mid-ocean ridge at about 19°S/65°E, (2) a source located in the Gulf of Aden in the northwestern Indian Ocean, (3) sources located in the convergent margins in the northeastern Indian Ocean, and (4) water imported from the Indonesian Seas. The main circulation features inferred from the 3 He distribution include (1) deep (2000-3000 m) eastward flow in the central Indian Ocean, which overflows into the West Australian Basin through saddles in the Ninetyeast Ridge, (2) a deep (2000-3000 m) southwestward flow in the western Indian Ocean, and (3) influx of Banda Sea Intermediate Waters associated with the deep core (1000-1500 m) of the through flow from the Pacific Ocean. The large-scale 3 He distribution is consonant with the known pathways of deep and bottom water circulation in the Indian Ocean.

Helium isotopes, neon and tritium in the Black Sea: A comparison with the 1975 observations

Abstract Measurements of dissolved helium, neon and tritium in the Black Sea are described. A comparison of the present measurements with those made in 1975 reveals some permanent and transitory characteristics of the basin. Helium, with a large source from the sediments, is found to have essentially the same profile as in 1975, whereas neon shows small deviations from the previous profile, probably as a result of variations in the influence of the Mediterranean input. The present temperature-salinity structure also appears to have changed from that in 1975. In the preliminary set of analyses no measurable tritium was found below about 500 m, in contrast to the 1975 measurements which showed a small concentration below 1700 m. The maximum in 3 He, the radioactive decay product of tritium, was found to have migrated from a depth of 70 to 130 m since 1975. 3 He concentrations in some bottom samples from stations near the southern coast are slightly elevated. This may be related to the tectonic structure of the region.

Modification of the intermediate waters in the northeastern subpolar Pacific

The modification of the intermediate layers in and around the Alaskan Gyre are investigated in relation to general circulation features of the region, using tracer data from the World Ocean Circulation Experiment P17N line. The Alaskan Gyre (AG) core lies between the North Pacific Current and the Alaskan Stream (AS) in the 52–54.5°N range and to the west of 144°W. The North Pacific Current feeds the Transition Zone (TZ) and the Western Subpolar Waters (WSW), which consequently constitute the inflow to the region from the northwestern Pacific. The Alaska Current (AC) is observed to the west of 144°W, between the North American coast and the AG. The AC halocline (σθ < 26.7) has distinct property distributions (low chlorofluorocarbon and oxygen saturations and high nutrient concentrations), which are correlated with eddies that are characterized by higher salt content and warmer temperatures (most significantly at 26.0 σθ). The AG is found to act as a reservoir where water mass modification takes place and the AS transports a mixture of waters from the AG and the AC west-ward out of the region. The residence time estimate for the intermediate and upper layer water in the AG (σθ ≤ 27.2) is about 2 years, which corresponds to a volume exchange between the gyre and the surrounding circulation of 4.7 Sv in the 0–1000 m range. The layers above 26.9 σθ (∼0–200 m) are modified by vertical mixing mostly of WSW in the AG; the flux of modified waters out of the AG is approximately 0.9 Sv. Comparison of the present data with the 1984–1989 period suggests that mixing of water from different regimes has not changed dramatically.

Evidence of Tectonism from 3He and residence time of helium in the Black Sea

The presence of 3He in excess of its saturation in the deep ocean is an unmistakable sign for exchange between water and material of magmatic origin. For example, in locations such as the Galapagos Spreading Center or the Red Sea, substantial excesses of this isotope bear witness to present and past hydrothermal activity at the seafloor [Jenkins 1978; Lupton et al., 1977]. Our measurements of 3He in the Black Sea in 1988 show that there is about 13% excess of this isotope in the water column of the southern half of the basin with indications that its injection may be occurring along the southern coast. This must be related to tectonic activity, also observed to be happening along the south coast.