Brine-seawater Interface Research Articles

Formation on the sea floor of the Hellyer volcanogenic massive sulfide deposit

New fluid inclusion data from the stockwork veins beneath the Hellyer orebody, a relatively undeformed, Late Cambrian polymetallic volcanogenic massive sulfide deposit, allow semiquantitative modeling of the behavior of the ore fluids on the sea floor and the manner of growth of the massive sulfide. The earliest cool fluids may have been denser than seawater and ponded in a basin identified by sedimentological and structural studies. Subsequent vent fluids, buoyant with respect to the brine pool and subsequently at higher temperature with respect to seawater, mixed with the cooler brine. During and after filling the basin to the level of its lowest outlet, density inhomogeneities within the brine pool were reduced by double-diffusive convection across the brine-seawater interface and internal boundaries, and mixing by currents within the brine pool. Estimates of likely temperatures and salinities in the brine pool for the first main mineralization stage (avg 200°C, 11 wt % salinity), assuming a steady or quasisteady state, had been established, and using volume fluxes similar to those observed in modern oceans, indicate temperatures ≪100°C. Sulfides (mostly pyrite, sphalerite, and galena) precipitated from the vent fluids after mixing with cooler basin fluid settled from the gravity current to the basin floor, a pattern that would have continued whether or not a true steady state was reached. If the early fluids were buoyant in seawater they ponded after mixing, forming a hybrid brine pool that would have required longer periods to homogenize. At the peak temperature (≥300°C) and volume flux the salinity of the vent fluids fell to <7.5 wt percent, but the flow paths would have been maintained unless the brine pool had been drained. At this time that part of the massive sulfide body over the vent (the Cu core) grew by dilatational veining together with leaching, cavity infilling, replacement, and recrystallization. Chalcopyrite was deposited both in the Cu core and probably at the same time in a layer over the flanking massive sulfide. In the final stage as temperature fell and salinity rose (avg 250°C, 11 wt % salinity), the fluid oxidation potential increased so that barite precipitation dominated, resulting in a formation of a discontinuous barite-rich cap over the massive sulfide. The brine pool model explains several features of the Hellyer ore that differ from those of the ores of the Hokuroko basin, namely, the large metal content, high Zn/Cu ratio, low aspect ratio, absence of chimney fragments, presence of mineral banding, and high barite sulfur isotope values, features seen in many other massive sulfide orebodies.

Yttrium and lanthanides in eastern Mediterranean seawater and their fractionation during redox-cycling

Concentrations of dissolved Y and rare-earth elements (REE) are reported for oxic eastern Mediterranean seawater from the western Levantine Basin, and for anoxic hypersaline brine and overlying oxic seawater from the Tyro sub-basin, the latter data allowing a comparative study of Y and REE behaviours during redox-cycling. All oxic waters show shale-normalized Rare-Earths and Yttrium (REYSN; Y inserted between Dy and Ho) patterns with HREE enrichment, negative CeSN anomalies, and positive anomalies of LaSN, EuSN, and, most pronounced, YSN. The anoxic brine in the Tyro sub-basin displays the highest REY concentrations and the least HREE enrichment of all samples, and positive anomalies of LaSN, EuSN, GdSN, YSN, and, in marked contrast to the oxic samples, CeSN. Compared to overlying oxic water, the anoxic brine shows enrichment factors that decrease from La (12.1) to Yb (2.7), with a pronounced positive deviation for Ce (51.2) and a strong negative deviation for Y (2.4, compared to 4.3 for Dy and 3.7 for Ho). The YHo molar ratio decreases from 102 above to only 67 below the seawater-brine interface, due to preferential sorption of Ho with respect to Y on Fe- and Mn-oxyhydroxide particles that eventually dissolve under anoxic conditions. The pronounced Y-Ho fractionation during redox-cycling of Mn and Fe is further evidence for the considerably lower marine particle reactivity of Y compared to Ho, resulting from lower stabilities of surface complexes of Y relative to those of its REE neighbours. Hence, the YHo ratio appears to be a sensitive indicator of the impact of particles on the distribution of dissolved trace elements in natural waters.

96/00124 Formation of pristene, pristane and phytane: Kinetic study by laboratory pyrolysis of Monterey source rock

The element compositions Si, Ca and Al of up to 21.1 ka old sediments in about 10 m long cores from the southern basin of the Shaban and Kebrit deeps in the northern Red Sea allowed a classification of major sediment types in carbonate sands and -muds and siliceous oozes. A FeOOH-enriched sediment horizon and a few samples with high Zn values in the Kebrit core indicate a hydrothermal origin probably near the brine–sea water interface with subsequent transport of hydrothermal compounds into the deep sediments.High organic carbon contents up to 8.4% are positively correlated with the Ba concentrations, which suggests that high bioproductivity and rapid deposition (14C dating suggests a sedimentation rate near 70 cm/ka) led to the formation of sapropelic sediments between 11.8 and 13.6 ka (Younger Dryas). Organic petrological observations showed that the sediment organic material largely consists of < 20 μm-sized roundish fecal pellets (intimate mixtures of organic matter and inorganic constituents) and bituminite. Terrestrial organic matter (pollens of land plants, fusinite etc.) is very rare in the sediment cores from both deeps.Organic-geochemical investigations of kerogens and organic extracts show that a significant (hydrothermal) hydrocarbon production did not occur in near-surface sediments of the Shaban and Kebrit deeps. Rock Eval pyrolysis of kerogens characterised the organic matter to be of type II quality. The δ13C values of the kerogens from the most prominent sapropel in the Shaban deep indicate an enrichment of (12C-rich) nutrients in the water column during postglacial sapropel formation in the Younger Dryas. The n-alkane spectra are dominated by short chain lengths between n-C15 and n-C25. Prevailing n-C15 to n-C25 alkanes in low mature sediments are indicative of algal and microbial source. Pristane/phytane ratios are generally low (< 1 to ∼ 1) which suggests that anoxic conditions prevailed within the anaerobic brine-filled deeps for the whole time covered by the sediments. This again indicates that sapropel formation was caused by high bioproductivity in the northern Red Sea rather than episodic stagnation with better preservation of the organic matter.Long-chain alkenones and sterols are the dominating compounds of the lipid fraction. Cholesterol contents in the sediment cores reflect phases of eukaryotes production in the water column, whereas the positive correlations of dinosterol with TOC and the amounts of total extract suggests that the major organic carbon source in the northern Red Sea during postglacial high-productivity stages were dinoflagellates. Another important carbon source, however, is indicated by the occurrence of 22,29,30-trisnorhopan-21-one (TNH). Although the formation of TNH from its precursors is not fully understood, this compound probably results from microbial degradation of intact bacteriohopanepolyols (BHP), which can be used as indicators for bacterial abundances and phyla. TNH is most likely produced at the brine–sea water interface where sedimenting organic matter accumulates and, if the redoxcline corresponds to the density gradient, the organic matter is subjected to efficient aerobic bacterial degradation processes. However, during high bioproductivity stage (Younger Dryas) the redoxcline was probably higher in the water column and thus, a significant TNH production at the brine–sea water interface did not occur at times of sapropel formation in the northern Red Sea deeps.

Thermal and Density Stratification in a Seafloor Brine Pool, Northern Gulf of Mexico: ABSTRACT

Brine Pool NR-1 is located on the continental slope of the Gulf of Mexico at a site where a salt diapir has penetrated the near subbottom. The pool sits atop a low cone that rises about 10 in above the surrounding seafloor. It has a distinct sea water-brine interface at a water depth of 650 m. It is about 10 m in diameter and is surrounded by a very dense colony of mussels that possess methanotrophic symbionts. The fluid in the upper layer has a salinity [approximately]120 psu and contains high concentrations of dissolved methane. A continuous stream of methane bubbles vents from a localized region near the center of the pool I used a small winch, mounted on submarine Johnson Sea-Link I to lower a CTD (modified for high salinity) into the pool at various points and also used a pole to probe the depth of the pool in the shallower parts. The pool was funnel-shaped, a couple of cm deep in the edges and less than 2 m deep across most of the pool area, but over 33 m deep at the point where the bubbles were venting. The CTD record shows that temperature increases abruptly at themore » brine-seawater interface and forms a second thermocline within the brine column. Conductivity also increases abruptly at the interface, consistent with the high salinity of the pool. Remarkably, conductivity decreases at the same brine depth as the thermocline, is erratic for several meters, and then becomes constant to the bottom of the pool. This unusual stratification is maintained by the mixing energy of the bubble stream, which is sufficient to maintain a large volume of suspended sediment. The suspended sediment interferes with the conductivity cell on the CTD to produce the anomalous reading. This stability of this system is determined by the gas venting rate and the shape of the pool and diatreme. Stratification of the fluid may serve to extend the life of the pool.« less

Vertical distributions and speciation of dissolved rare earth elements in the anoxic brines of Bannock Basin, eastern Mediterranean Sea

Vertical distributions of dissolved rare earth elements (REEs) are presented for the anoxic, highly sulfidic brines of Bannock Basin in the eastern Mediterranean Sea. REE concentrations at the seawater-brine interface are the highest ever recorded in the water column of an anoxic basin and comparable only to REE concentrations at depth in the porewaters of reducing sediments. Below the seawater-brine interface, throughout the upper of two chemically distinct brine layers, REE concentrations decrease with depth as a result of scavenging, while shale-normalized REE patterns are characterized by little or no fractionation between the light REEs (LREEs) and the heavy REEs (HREEs) and by a positive Ce anomaly. Throughout the lower of the two brine layers REE concentrations are constant, while REE patterns are characterized by the same lack of LREE or HREE enrichment, as well as by the absence of any Ce anomaly. A pronounced REE minimum at the interface between the two brine layers is apparently caused by uptake into dolomite that is formed through in situ recrystallization of Ca carbonate, a process that was previously reported to cause similar minima for Mn and Fe. Calculations of REE speciation, incorporated into a model of REE scavenging (developed by Byrne and Kim, 1990), which uses REE complexation by monocarboxylic acids to represent REE adsorption onto particulate matter, are surprisingly successful at reproducing the unusual shape of REE patterns in the brine. This suggests that this shape, although very different from that of REE patterns in the overlying seawater due to the very high ionic strength of the brine, is the result of much the same processes that constitute REE scavenging in the open ocean.

Trace-metal distributions in seawater and anoxic brines in the eastern Mediterranean Sea

The vertical distribution of dissolved and particulate trace metals (Mn, Fe, Ni, Cu, Cd) is reported for seawater and the anoxic brines of Tyro and Bannock Basins in the eastern Mediterranean. In normal Mediterranean seawater, the distribution of dissolved trace metals is governed by water mass circulation, as well as by local input sources. For Cd and possibly Cu, steady-state behaviour is not a viable assumption, probably as a result of anthropogenic sources. In agreement with previous reports, no correlation was observed between dissolved Ni, Cu, Cd and the major nutrients. At the seawater-brine interface, all trace metals show dramatic changes in concentration. In the brines, the concentrations of dissolved Fe and Mn are strongly elevated compared to overlying waters, yet probably limited by formation of solid sulfide and/or carbonate phases. The distributions of dissolved Fe and sulfide show a remarkable mirror image of those found previously in the Orca brine (Gulf of Mexico) and the ionic concentration products are approximately the same in all three basins. The difference in absolute Fe and sulfide concentrations is thought to be caused by a high supply of Fe to Orca Basin. Concentrations of dissolved Ni, Cu and Cd in the brines are at least one order of magnitude lower than in overlying waters, most likely due to formation of solid sulfide phases. We believe these data provide the first indication of substantial nickel-sulfide (co) precipitation within an anoxic basin. Particulate metal concentrations are elevated at the seawater-brine interface and are probably associated with the presence of solid sulfide phases. In addition, the extreme density gradient at the interface will slow the sinking rate of particles and lead to accumulation of particles at the interface due to accumulation.

Archaebacterial activity in the Orca Basin determined by the isolation of characteristic isopranyl ether-linked lipids

Phytanyl glycerol ether lipids characteristic of archaebacterial inputs have been quantified in 30 water samples taken in the Orca Basin, an anoxic hypersaline basin located in the northwestern Gulf of Mexico. Because of the Basin's anoxic hypersaline character, it seems likely that archaebacteria may play a significant role in the microbial ecology of the brine. Physical data, including temperature, salinity, per cent transmission, oxygen and nutrient concentrations, also were collected from six depths at five sampling sites in the Basin. Four of the five sites were characterized by a 200 m thick, anoxic brine (salinity ≈ 250 ppt) at an approximate water depth of 2240 m. A stepwise increase in salinity was associated with the brine-seawater interface, increasing from 38 to 150 ppt within the upper portion of the 10 m interface and to >250 ppt within the brine. Three distinct layers of particulate material were observed within the 10 m interface. Corresponding with the salinity gradient was a decrease in dissolved oxygen from 5.0 ml l −1 at 2040 m to 0 ml l −1 within the brine. Ammonia and phosphate concentrations increased from 0 and 2.5 μM above the brine to 519 and 63.5 μM within the brine. At the same time, nitrate concentrations decreased from 22 μM above the brine to negligible within the brine. Depletion of oxygen, with concomitant increases in ammonia and phosphate, decreased nitrate, and the production of methane suggest microbially mediated processes may be occurring at the brine-seawater interface. Highest concentrations of phytanyl ether lipids were observed within the interface, ranging from 29.7 to 84.1 ng l −1. Concentrations were negligible below the interface. Elevated phytanyl ether lipid concentrations in conjunction with microbial activity studies carried out by other investigators suggest that archaebacterial activity is occurring within the brine particulate layers. A decline in ether lipid concentration and microbial activity below this particulate interface indicates a reduction or slowing of microbial activity in the deeper anoxic brine. The bacteria responsible for methane production at the interface appear capable of growth under anoxic conditions at salinities up to 150 ppt. This unusual activity has not been commonly reported in other marine systems.

Hydrography of the brines in the Bannock and Tyro anoxic basins (eastern Mediterranean)

The anoxic brine-filled Tyro and Bannock Basins (eastern Mediterranean) show strong temperature and conductivity gradients at the interface between normal water and anoxic brine (at ∼ 3276 m in the Bannock Basin, at 3328 m in the Tyro Basin), with temperature and conductivity increasing toward the bottom. Temperature reaches values of 15.14°C in the Bannock area and of 14.13°C in the Tyro Basin, and conductivity at the bottom is respectively 177.44 and 191.26 mS cm −1. In the deepest basin of the Bannock area (named Libeccio) a sequence of nearly homogeneous layers, separated by sharp interfaces, is present below the seawater-brine interface. Near the bottom a homogeneous 244-m-thick layer has been observed. The hydrological structure of brines in Tyro Basin shows no evidence of thermal layering. The present geothermal flux seems to be sufficient to create and maintain the temperature structure observed in the brines. A minimum of 4500 years would be needed to raise the Bannock brine body to its current temperature. Only a minimum of 150 years is needed to bring the Tyro brine to its current condition.

Trace elements in suspended matter from the anoxic hypersaline Tyro and Bannock Basins (eastern Mediterranean)

In the anoxic hypersaline Tyro and Bannock Basins of the eastern Mediterranean, extremely high concentrations of Co (0.015%), Cu (1.35%) and Zn (0.28%) were found in suspended matter collected at the sharp interface between seawater and the anoxic brine. The high particulate Co, Cu and Zn concentrations can be explained by the sharp increase in dissolved sulphide at these interfaces, and the resultant precipitation of metal sulphides. The particulate As, Sb and Mo concentrations also showed a sharp maximum at or close to the interface. However, the contributions of As, Sb and Mo contents in suspended matter to the total concentrations in the water column are small. Scanning electron microscopy-energy-dispersive X-ray analysis (SEM-EDAX) of suspended particulate matter from the Tyro Basin revealed spherical particles strongly enriched in Fe, Cu and Zn at the seawater-brine interface.

Trace elements in suspended matter from the anoxic hypersaline Tyro and Bannock Basins (eastern Mediterranean)

In the anoxic hypersaline Tyro and Bannock Basins of the eastern Mediterranean, extremely high concentrations of Co (0.015%), Cu (1.35%) and Zn (0.28%) were found in suspended matter collected at the sharp interface between seawater and the anoxic brine. The high particulate Co, Cu and Zn concentrations can be explained by the sharp increase in dissolved sulphide at these interfaces, and the resultant precipitation of metal sulphides. The particulate As, Sb and Mo concentrations also showed a sharp maximum at or close to the interface. However, the contributions of As, Sb and Mo contents in suspended matter to the total concentrations in the water column are small. Scanning electron microscopy-energy-dispersive X-ray analysis (SEM-EDAX) of suspended particulate matter from the Tyro Basin revealed spherical particles strongly enriched in Fe, Cu and Zn at the seawater-brine interface.

Sulfur and oxygen isotopic compositions of dissolved sulfate in the Orca Basin: Implications for origin of the high-salinity brine and oxidation of sulfides at the brine-seawater interface

Analysis of sulfur and oxygen isotopic compositions of dissolved sulfate from a vertical water profile in the anoxic, hypersaline Orca Basin provides important constraints upon the provenance of the sulfate. Results support a dissolution origin of the brine from a Jurassic evaporitic salt. However, mass balance considerations and the observed minima in the vertical δ 18O-sulfate profile at the brine-seawater interface are not reconcilable with only a simple two box brine-seawater mixing model. Instead, the data imply contributions from the oxidation of sulfides.

Gelatinous pellicles in deep anoxic hypersaline basins from the Eastern Mediterranean

Gelatinous pellicles have been observed in three out of ten cores recovered from the Bannock Basin (or Bacino Bannock) and in two dredges from its flanks during a cruise of the R.V. Bannock in the Eastern Mediterranean. Similar pellicles have also been detected in two cores recovered from the Tyro Basin. The Bannock and Tyro basins are brine-filled depressions which are presently characterized by anoxic sedimentation; in the Bannock Basin also gypsum precipitation occurs. The gelatinous pellicles, some millimeters thick and dark grey to greenish in color, are always found in very dark colored sediments of Quaternary age. These pellicles have been investigated by optical microscopy, by scanning electron and transmission electron microscopy, and by flash pyrolysis-gas chromatography and flash pyrolysis-gas chromatography-mass spectrometry to decipher their nature. Abundant skeletal tests of siliceous and calcareous planktonic organisms (radiolaria, diatoms, silicoflagellates, formainifera and nannofossils) are observed, together with amorphous organic matter. The pellicles enclosing these remains consist of organic matter; they foliate into submillimetric undulating and anastomosing microlaminae. Cellular structures are never observed, neither in the pellicles nor in the microlaminae. Organic geochemical analyses point to a high contribution of terrestrial material. The organic matter of the pellicles investigated consists of a “coal-like” material. It is suggested that the pellicles are originally formed at the brine-seawater interface, but subsequently sink to the bottom due to increasing load of entrapped biogenic and non-biogenic detritus.

Organic geochemical characteristics of sediments from the anoxic brine-filled Tyro basin (eastern Mediterranean)

Soxhlet-extractable organic compounds of sediments from the anoxic brine-filled Tyro basin (eastern Mediterranean) were analyzed by gas chromatography and gas chromatography-mass spectrometry. Cholesterol and 22,29,30-trisnorhopan-21-one are the major compounds identified. The occurrence of this triterpenoid ketone, as well as 22,29,30-trisnorhop-6-en-21-one and diplopterol (both tentatively identified), is ascribed to an input of microbiota living at the brine-seawater interface.

The geochemistries of 210Po and 210Pb in waters overlying and within the Orca Basin, Gulf of Mexico

Vertical profiles of dissolved 210Po, 210Pb, and 226Ra were obtained from waters overlying and within the Orca Basin; an anoxic, hypersaline and virtually non-sulfide bearing basin in the Gulf of Mexico. The station had a depth of 2410 m. The seawater-brine interface was located at 2230 m. In oxic waters, near-surface (0–250 m) concentrations of dissolved 210Po and 210Pb ranged from 6 to 8 dpm/100 kg and decreased to about 3 dpm/100 kg at 740 m. Below this depth concentrations remained relatively constant. Near-surface 226Ra concentrations varied from about 7 to 9 dpm/100 kg, and then increased with depth to a relatively uniform deep-water value of approximately 13 dpm/100 kg, in agreement with previously reported data for the Gulf of Mexico. The slight deficiency of 210Pb in the surface waters may result from mixing with inner shelf waters that are enriched in 226Ra. At the seawater-brine interface, sharp maxima were observed in the profiles of 210Po, 210Pb, and 226Ra, where concentrations reached 56.0, 28.3, and 840 dpm/100 kg, respectively. These are among the highest values reported for the marine environment to date. Below the interface, concentrations decreased to about 9, 6, and 210 dpm/100 kg for 210Po, 210Pb, and 226Ra, respectively. These sharp maxima may be caused by a combination of the accumulation of particles in the strong pycnocline, the subsequent dissolution of particulate hydrous Mn and Fe oxides and hydroxides, and the decomposition of biogenic particulate material. The residence time of 210Pb in the brine is about one year, a value similar to that reported for the anoxic waters of the Cariaco Trench even though the concentration of 210Pb in that basin is much lower.

Variations of calcium carbonate, organic carbon and iron sulfides in anoxic sediment from the Orca Basin, Gulf of Mexico

Sediment from the anoxic Orca Basin is highly laminated and enriched in iron sulfides and organic carbon, unlike sediment from the surrounding northwest Gulf of Mexico continental slope. Black laminae are enriched in calcium carbonate (19 vs. 15%), organic carbon (1.8 vs. 0.7%) and iron sulfides (0.7 vs. 0.4%) compared to light gray laminae. However, more of the FeS is converted to FeS 2 in the light gray laminae. Dark gray laminae of intermediate composition are also common. The laminae probably reflect annual and longer-term variations in organic matter supply to the basin and variations in mixing at the brine—seawater interface.

Light hydrocarbons in Red Sea brines and sediments

Light hydrocarbon (C 1-C 3) concentrations in the water from four Red Sea brine basins (Atlantis II, Suakin, Nereus and Valdivia Deeps) and in sediment pore waters from two of these areas (Atlantis II and Suakin Deeps) are reported. The hydrocarbon gases in the Suakin Deep brine ( T = ~ 25° C, Cl − = ~ 85‰ , CH 4 = ~ 71 1 ) are apparently of biogenic origin as evidenced by C 1 (C 2 + C 3 ) ratios of ~ 1000. Methane concentrations (6–8 μl/l) in Suakin Deep sediments are nearly equal to those in the brine, suggesting sedimentary interstitial waters may be the source of the brine and associated methane. The Atlantis II Deep has two brine layers with significantly different light hydrocarbon concentrations indicating separate sources. The upper brine ( T = ~ 50° C, Cl − = ~ 73‰ , CH 4 = ~ 155 μl/ l) gas seems to be of biogenic origin [ C 1 (C 2 + C 3) = ~1100 ], whereas the lower brine ( T = ~ 61° C, Cl − = ~ 155‰ , CH 4 = ~ 120 μl/ l) gas is apparently of thermogenic origin [ C 1 (C 2 + C 3) = ~ 50 ]. The thermogenic gas resulting from thermal cracking of organic matter in the sedimentary column apparently migrates into the basin with the brine, whereas the biogenic gas is produced in situ or at the seawater-brine interface. Methane concentrations in Atlantis II interstitial waters underlying the lower brine are about one half brine concentrations; this difference possibly reflects the known temporal variations of hydrothermal activity in the basin.