Black Sea Surface Waters Research Articles

On the possibility of iodide oxidation in the near-surface of the Black Sea and its implications to iodine in the general ocean

Iodide oxidation to iodate in near-surface waters of the open oceans is an elusive process, and an unequivocal demonstration of it would simplify modelling of the marine iodine system. In the open ocean, the upward advection of iodate complicates any mathematical treatment of the problem. In this context, the high concentration (0.1 μM) of iodate in the Black Sea surface waters suggested that this Sea might be a place where oxidation might be demonstrated. Hydrologically, the surface waters of the Black Sea appear to be downstream of the deeper waters and, given the latter's anoxicity, the surface waters seemed likely to gain most of their iodine as iodide by upward advection. To test this further, prior to experimentation, an iodine budget for the near-surface waters, based upon the latest hydrological model of the Sea was prepared; this predicts a minimum oxidation flux of 3.89×10 −4 mol I m −2 a −1. The chemistry of this oxidation is discussed in the light of existing knowledge of the sulfide system. It is argued that as the redox potential of the IO 3 −/I − and I 2/I − couples at pHs typical of the Black Sea (7.75) are much higher than that of the sulfate–sulfide couple, iodide is probably oxidized in the near-surface domain. This contrasts with sulfide oxidation in the suboxic zone. The possible role of nitrifying bacteria in the oxidation is discussed.

Cycling of radium and barium in the Black Sea

We report the first data for the concentrations of Ba and 226Ra in rivers draining into the Black Sea. For Ba, concentrations are in the range 215–405 nM L -1; for 226Ra, concentrations are in the range 0.4–4 Bq m -3. These are dissolved concentrations and do not represent additional inputs due to desorption from fluvial particles. We also report new data for the concentrations of Ba and 226Ra in Black Sea surface waters from 1991 to 1995. Earlier studies documented significant reductions in the concentrations of these elements in Black Sea surface waters for the periods 1957–1988 (Ra) and 1975–1988 (Ba). During the period 1988–89 to 1995, Ba concentrations followed the earlier trend and decreased by an additional 35%. On the other hand, 226Ra concentrations reversed the trend and increased by about 60% from 1988 to 1995. The reason for the recent divergence in behavior of these two elements is unknown.

Effect of Danube River dam on Black Sea biogeochemistry and ecosystem structure

Rivers contribute significantly to the pollution and eutrophication that have caused drastic changes to the ecosystem of the Black Sea1–3. Although damming is known to affect riverborne nutrient loads, and thus riverine ecosystems, evidence for significant effects in open coastal waters is sparse4–6. Here we present long-term data sets of water and nutrient discharge from the River Danube to the Black Sea. These data reveal a reduction in the dissolved silicate load of the river by about two-thirds since dam constructions in the early 1970s. A concomitant decrease in wintertime dissolved silicate concentrations by more than 60% was observed in central Black Sea surface waters. The consequent changes in silicon to nitrogen ratio of the Black Sea nutrient load appear to be larger than those caused by eutrophication alone, and seem to be responsible for dramatic shifts in phytoplankton species composition from diatoms (siliceous) to coccolithophores and flagellates (non-siliceous). Our results strongly suggest that the damming of the Danube has been instrumental in causing the observed changes in Black Sea surface waters3,7–9, and that the large number of dams in operation around the world today could similarly affect the food web structure and biogeochemical cycling in coastal seas.

Rhenium in the Black Sea: comparison with molybdenum and uranium

Rhenium concentrations were measured in the Black Sea to determine the behavior of this redox-sensitive element in an anoxic basin. Under reducing conditions, Re is removed from solution, leading to its depletion from Black Sea deep waters. Compared with uranium, water column Re and Mo profiles show evidence of removal from mid-depths, suggesting that these elements are scavenged either in the water column or slope sediments to a greater degree than U. Black Sea surface waters are enriched in Re, reflecting Re in rivers draining into the northern Black Sea that are enhanced up to 80-fold over other large world rivers. This enrichment is most likely anthropogenic and may be due to coal burning in the region. A simple two-box model is used to demonstrate that Re concentrations in the Black Sea are not in steady state with respect to current riverine inputs and authigenic fluxes. Riverine inputs of U and Mo are not as significant, perhaps reflecting the fractionation of Re from the other elements during a high-temperature combustion process (Re 2O 7 is volatile above 270°C).

226Ra in the Black Sea and Sea of Marmara

Water column distributions of 226Ra were determined at stations in the Sea of Marmara and the Black Sea as part of the 1988 Joint U.S.—Turkish Black Sea Expedition. Black Sea surface water 226Ra concentrations were a factor of three to four lower than measurements made 20 years earlier. The most likely cause is increased removal of 226Ra and Ba [35] due to increased surface biological activity; a secondary effect is decreased fluvial discharge and related dimunition of inputs by desorption from fluvial suspended sediments. The amount of 226Ra missing from the surface waters of the Black Sea over this period is accounted for in the high-porosity surficial “fluff” sediment layer. Throughout the Black Sea, depth profiles of 226Ra exhibited pronounced maxima of approximately 25 dpm/100 L at about σ θ = 16.2–16.3 , in the vicinity of a bacterial maximum, but slightly shallower than the total dissolved Mn and Fe maxima ( σ θ = 16.4–16.5 ) reported by Lewis and Landing [38]. While the 226Ra maximum may, in part, be linked to the cycling of Mn and Fe oxyhydroxides near the O 2H 2S interface, its distribution appears to be more plausibly explained as a result of the microbial breakdown of particulate organic matter and the subsequent release and partial dissolution of associated barite in this region. A simple steady-state two-☐ model has been used to obtain a semiquantitative understanding of the behavior of 226Ra in the Black Sea. By incorporating reasonable estimates for the input and removal of 226Ra in the Black Sea, an excellent agreement between predicted and observed (1988) 226Ra concentrations was achieved. The model suggests that the dominant variables controlling the distribution of 226Ra in the Black Sea are riverine input and cycling with Ba.

Depletion of barium and radium-226 in Black Sea surface waters over the past thirty years

THE nearly landlocked waters of the Black Sea support a valuable fishery, but are also particularly vulnerable to anthropogenic disturbance. Here we use dissolved barium and radium-226 as tracers, to investigate the biogeochemical health of the sea. Both elements are brought to surface waters by vertical mixing of deeper, enriched waters1,2, and by rivers3–8; these inputs should ordinarily be balanced by outflow of surface waters at the Bosporus, and by biologically mediated removal of 226Ra-bearing barite. We show, however, that surface-water inventories have been substantially depleted over the past few decades: recent (1988–89) barium concentrations were 1.6 times lower than in 19759; and recent 226Ra concentrations were 3.5 times lower than in 195810 and 196711. These observations suggest that steady-state cycling of these elements has been perturbed by increased primary productivity, presumably fuelled by nutrients from industry and agricultural runoff, and to a lesser extent by decreased fluvial sediment loads owing to extensive impoundment of rivers in the region.

Mixing between oxic and anoxic waters of the Black Sea as traced by Chernobyl cesium isotopes

The Chernobyl nuclear power station accident in 1986 released readily measureable quantities of fallout 134Cs and 137Cs to Black Sea surface waters. This pulse-like input of tracers can be used to follow the physical mixing of the surface oxic waters, now labeled with the Chernobyl tracers, and the deeper anoxic waters, which were initially Chernobyl free. By 1988, there is clear evidence of Chernobyl Cs penetration below the oxic/anoxic interface at deep water stations in the western and eastern basins of the Black Sea. This rapid penetration of surface waters across the pycnocline cannot be explained by vertical mixing processes alone. Data from profiles at the mouth of the Bosporus suggest that significant ventilation of intermediate depths can occur as the outflowing Black Sea waters are entrained with the inflowing Mediterranean waters, forming a sub-surface water mass which is recognized by its surface water characteristics, i.e. initially a relatively high oxygen content and Chernobyl Cs signal. The lateral propagation of this signal along isopycnals into the basin interior would provide a rapid and effective mechanism for ventilating intermediate depths of the Black Sea. This process could also account for the lateral injection of resuspended margin sediments into the basin interior. The temperature and salinity data suggest that the entrainment process occurs at depths of 50–80 m, mixing waters from the Cold Intermediate Layer with the incoming, denser Mediterranean waters.