Bottom Water Values Research Articles

Predicted Episode of Submarine Groundwater Discharge Onto the South Carolina, USA, Continental Shelf and Its Effect on Dissolved Oxygen

AbstractSubmarine groundwater discharge (SGD) may directly influence the dissolved oxygen (DO) content of coastal bottom waters. Here, we report a predicted episode of enhanced SGD that caused low DO concentrations on the South Carolina continental shelf. The prediction model linked episodes of SGD to upwelling‐favorable winds. The data revealed these waters were a factor of 2–6 higher in 226Ra and 228Ra compared to typical bottom water values and were significantly depleted in DO (<130 μM). The tight 228Ra:226Ra correlation of these data was similar to values during a strong hypoxic event off SC in 2012. Water ages from 224Ra and 223Ra indicated the event occurred 2–9 days before sampling. The success of the prediction lends added credence to the correlation of upwelling‐favorable winds—but not necessarily accompanied by upwelling—to episodic SGD events. This prediction from wind data represents a major advance for quantifying SGD in the region.

Time-series CH4 measurements from Saanich Inlet, BC, a seasonally anoxic fjord

We present 7 years of water column methane (CH4) measurements from Saanich Inlet, a seasonally anoxic fjord in British Columbia, Canada. Our time-series observations show that CH4 concentrations and sea-air fluxes vary significantly over seasonal cycles, driven primarily by changes in O2-availability throughout the water column. Methane concentrations were always supersaturated in near-surface waters (mean 1160 ± 590% saturation at 10 m depth) and remained high between 10 and 85 m depth. We observed a persistent mid-depth CH4 minimum between 85 and 110 m associated with a turbidity maximum near the base of the oxycline. CH4 accumulated steadily below 150 m depth from the onset of anoxia until the subsequent annual renewal, with maximum bottom water values indicative of sedimentary sources. Beyond the strong seasonal cycle in CH4 concentrations, we also observed significant inter-annual variability. Most notably, the moderate 2009–2010 El Niño resulted in a weak deep basin renewal, and anomalously high CH4 concentrations in both the deep basin and upper 85 m. Over the time-series, we observed statistically significant shoaling of the hypoxic boundary, associated with declining CH4 concentrations in the upper water column (10–85 m depth). Our results provide insight into the potential effects of expanding hypoxia on CH4 cycling in coastal waters.

The Mechanism of Methane Gas Migration Through the Gas Hydrate Stability Zone: Insights From Numerical Simulations

AbstractFree gas migration through the gas hydrate stability zone (GHSZ) and subsequent gas seepage at the seabed are characteristic features in marine gas hydrate provinces worldwide. The biogenic or thermogenic gas is typically transported along faults from deeper sediment strata to the GHSZ. Several mechanisms have been proposed to explain free gas transport through the GHSZ. While inhibition of hydrate formation by elevated salinities and temperatures have been addressed previously in studies simulating unfocused, area‐wide upward advection of gas, which is not adequately supported by field observations, the role of focused gas flow through chimney‐like structures has been underappreciated in this context. Our simulations suggest that gas migration through the GHSZ is, fundamentally, a result of methane gas supply in excess of its consumption by hydrate formation. The required high gas flux is driven by local overpressure, built up from gas accumulating below the base of the GHSZ that fractures the overburden when exceeding a critical pressure, thereby creating the chimney‐like migration pathway. Initially rapid hydrate formation raises the temperature in the chimney structure, thereby facilitating further gas transport through the GHSZ. As a consequence, high hydrate saturations form preferentially close to the seafloor, where temperatures drop to bottom water values, producing a prominent subsurface salinity peak. Over time, hydrates form at a lower rate throughout the chimney structure, while initial temperature elevation and salinity peak dissipate. Thus, our simulations suggest that the near‐surface salinity peak and elevated temperatures are a result of transient high‐flux gas migration through the GHSZ.

In Search of a Field-Based Relationship Between Benthic Macrofauna and Biogeochemistry in a Modern Brackish Coastal Sea

During several cruises in the southern Baltic Sea conducted in different seasons from 2014 to 2016, sediment cores were collected for the investigation of pore-water biogeochemistry and associated nutrient fluxes across the sediment-water interface. Six stations were positioned along a salinity gradient (ranging from 22 to 8) and covered various sedimentary habitats ranging from mud to sand. Integrated fluxes of nutrients in the supernatant water and sediment oxygen consumption were additionally derived from incubations of intact sediment cores. Subsequently, sediment from the pore-water and incubation cores was sieved for taxonomic identification and estimation of benthic macrofauna density. This combined dataset was used to determine the dominant factors influencing the vertical distribution of geochemical parameters in the pore-waters of the studied habitats and to find similarities and patterns explaining significant variations of solute fluxes across the sediment-water interface. A statistical relationship between the thickness of sulfide-free surface sediments, solute fluxes of sulfide, ammonium and phosphate as well as oxygen consumption and taxonomic and functional characteristics of macrobenthic communities were tested. Our data and modeling results indicate that bioturbation and bioirrigation alter near-surface pore-water nutrient concentrations towards bottom water values. Besides sediment properties and microbial activity, the biogeochemical fluxes can further be explained by the functional structure of benthic macrofauna. Community bioturbation potential, species richness and biomass of biodiffusers were the best proxies among the tested set of biotic and abiotic parameters and could explain 63% of multivariate total benthic flux variations. The effects of macrobenthos on ecosystem functioning differ between sediment types, specific locations and seasons. Both, species distribution and nutrient fluxes are temporally dynamic. Those natural patterns, as well as potential anthropogenic and natural disturbances (e.g. fishery, storm events), may cause impacts on field data in a way beyond our present capability of quantitative prediction, and require more detailed seasonal studies. The data presented here adds to our understanding of the complexity of natural ecosystem functioning under anthropogenic pressure.

Modeling benthic–pelagic nutrient exchange processes and porewater distributions in a seasonally hypoxic sediment: evidence for massive phosphate release by <i>Beggiatoa</i>?

Abstract. This study presents benthic data from 12 samplings from February to December 2010 in a 28 m deep channel in the southwest Baltic Sea. In winter, the distribution of solutes in the porewater was strongly modulated by bioirrigation which efficiently flushed the upper 10 cm of sediment, leading to concentrations which varied little from bottom water values. Solute pumping by bioirrigation fell sharply in the summer as the bottom waters became severely hypoxic (< 2 μM O2). At this point the giant sulfide-oxidizing bacteria Beggiatoa was visible on surface sediments. Despite an increase in O2 following mixing of the water column in November, macrofauna remained absent until the end of the sampling. Contrary to expectations, metabolites such as dissolved inorganic carbon, ammonium and hydrogen sulfide did not accumulate in the upper 10 cm during the hypoxic period when bioirrigation was absent, but instead tended toward bottom water values. This was taken as evidence for episodic bubbling of methane gas out of the sediment acting as an abiogenic irrigation process. Porewater–seawater mixing by escaping bubbles provides a pathway for enhanced nutrient release to the bottom water and may exacerbate the feedback with hypoxia. Subsurface dissolved phosphate (TPO4) peaks in excess of 400 μM developed in autumn, resulting in a very large diffusive TPO4 flux to the water column of 0.7 ± 0.2 mmol m−2 d−1. The model was not able to simulate this TPO4 source as release of iron-bound P (Fe–P) or organic P. As an alternative hypothesis, the TPO4 peak was reproduced using new kinetic expressions that allow Beggiatoa to take up porewater TPO4 and accumulate an intracellular P pool during periods with oxic bottom waters. TPO4 is then released during hypoxia, as previous published results with sulfide-oxidizing bacteria indicate. The TPO4 added to the porewater over the year by organic P and Fe–P is recycled through Beggiatoa, meaning that no additional source of TPO4 is needed to explain the TPO4 peak. Further experimental studies are needed to strengthen this conclusion and rule out Fe–P and organic P as candidate sources of ephemeral TPO4 release. A measured C/P ratio of < 20 for the diffusive flux to the water column during hypoxia directly demonstrates preferential release of P relative to C under oxygen-deficient bottom waters. This coincides with a strong decrease in dissolved inorganic N/P ratios in the water column to ~ 1. Our results suggest that sulfide oxidizing bacteria could act as phosphorus capacitors in systems with oscillating redox conditions, releasing massive amounts of TPO4 in a short space of time and dramatically increasing the internal loading of TPO4 to the overlying water.

Water mass circulation on Demerara Rise during the Late Cretaceous based on Nd isotopes

Circulation in the North Atlantic during the Late Cretaceous has implications for poleward heat transport and nutrient distribution during an extreme greenhouse interval with episodes of ocean anoxia. Nd isotopes of fossil fish teeth and debris represent one of the few water mass tracers that can be used to reconstruct deep ocean circulation. We present Nd isotopic data interpreted as bottom water values for 290 samples from three Ocean Drilling Program sites on Demerara Rise (Sites 1258, 1260, and 1261) along with 102 analyses from four other North Atlantic sites (Cape Verde, Goban Spur, Bermuda Rise and Blake Nose) that provide additional geographic and bathymetric control. Our results confirm the presence of a water mass with low εNd values (−14 to −17) that are believed to be influenced by continental material during local water mass formation at low latitudes. This Demerara Bottom Water (DBW) is the primary water mass in the region from the Cenomanian to Coniacian and from the late Campanian through early Maastrichtian following a hiatus of ~10my. A positive 8 εNd unit excursion occurs during Ocean Anoxic Event 2 (OAE2) that cannot be explained by changes in weathering inputs, diagenesis or magmatic sources of Nd; instead, it appears to represent an influx of bottom waters sourced from the Tethys or North Atlantic. This replacement of DBW during OAE2 argues for decreased production of DBW or enhanced production of Tethys/North Atlantic waters during peak greenhouse conditions. From the late Campanian through early Maastrichtian, DBW becomes the only water mass recorded at Demerara Rise and it appears to expand to abyssal depths at Cape Verde in the Campanian. This water mass is ultimately replaced by waters that appear to be sourced from the North Atlantic starting in the late Maastrichtian. Observed variations in circulation in the tropical North Atlantic during the Late Cretaceous can account for extensive deposition of black shales in the region prior to the hiatus, as well as local warming during a global cooling event in the Maastrichtian.

Denitrification in anoxic sediments supported by biological nitrate transport

Biologically available nitrogen (fixed N) is removed from the oceans by metabolic conversion of inorganic N forms (nitrate and ammonium) to N 2 gas. Much of this removal occurs in marine sediments, where reaction rates are thought to be limited by diffusion. We measured the concentration and isotopic composition of major dissolved nitrogen species in anoxic sediments off the coast of California. At depths below the diffusive penetration of nitrate, we found evidence of a large nitrate pool transported into the sediments by motile microorganisms. A ∼20‰ enrichment in 15N and 18O of this biologically transported nitrate over bottom water values and elevated [N 2] and δ 15N–N 2 at depth indicate that this nitrate is consumed by enzymatic redox reactions with the production of N 2 as the end product. Elevated N 2O concentrations in pore waters below the nitrate diffusion depth confirm that these reactions include the denitrification pathway. A data-constrained model shows that at least 31% of the total N 2 production in anoxic sediments is linked to nitrate bio-transport. Under suboxic/anoxic regimes, this nitrate bio-transport augments diffusive transport, thus increasing benthic fixed nitrogen losses and the reducing burial efficiency of sedimentary organic matter.

Seawater Pb isotopes extracted from Cenozoic marine sediments

Nd and Pb isotopic compositions extracted from bulk deep sea sediments have been shown to be robust proxies for deep water circulation as well as weathering provenance and intensity over geologically young time scales. In this study we evaluated ten deep sea samples from Ocean Drilling Program (ODP) site 1090 ranging in age from mid Eocene to early-Miocene to test whether Pb isotopic compositions extracted from geologically older sediments record reliable seawater isotopic ratios and to evaluate the source of the extracted Pb. The sequential extraction protocol used in this study is similar to protocols reported for previous studies and produces acetic acid, hydroxylamine hydrochloride (HH) and residue fractions. Each extracted fraction was analyzed for Pb isotopes, rare earth elements (REEs), and a suite of major elements. Similar 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb ratios are recorded from the acetic acid and HH fractions for ~ 70–80% of the samples, suggesting that either the acetic acid dissolves Fe–Mn oxides or multiple phases are recording the same seawater isotopic value. Several indirect tests, such as Al mass balance, comparison of Sr isotopes in HH extracts to contemporaneous seawater Sr isotopes, and comparison of Nd isotopic compositions in HH extracts to published fossil fish teeth values, provide evidence that Pb isotopic compositions measured in our bulk HH extracts record bottom water values. The relationship between Pb, Mn and Ca concentrations in HH fractions indicates that Fe–Mn oxides and a Mn-bearing carbonate are the dominant phases contributing seawater Pb. Comparison of REE patterns derived from the HH fraction and total digestions of Fe–Mn nodule standards reveals that the trivalent REEs exhibit patterns consistent with the parent archive, but Ce can be fractionated during extraction. Ratios of REEs also produce unique fields for each fraction and can be used to test the purity of the seawater signal of the extraction protocol. Finally, an initial evaluation of Pb isotopic compositions in fossil fish indicates that this archive is not suitable for bottom water Pb isotope studies.

Seep mounds on the Southern Vøring Plateau (offshore Norway)

Multidisciplinary study of seep-related structures on Southern Vøring Plateau has been performed during several UNESCO/IOC TTR cruises on R/V Professor Logachev. High-resolution sidescan sonar and subbottom profiler data suggest that most of the studied fluid discharge structures have a positive relief at their central part surrounded by depression. Our data shows that the present day fluid activity is concentrated on the top of these “ seep mounds”. Number of high hydrocarbon (HC) gas saturated sediment cores and 5 cores with gas hydrate presence have been recovered from these structures. δ 13C of methane (between −68 and −94.6‰ VPDB) and dry composition of the gas points to its biogenic origin. The sulfate depletion generally occurs within the upper 30–200 cm bsf and usually coincides with an increase of methane concentration. Pore water δ 18O ranges from 0.29 to 1.14‰ showing an overall gradual increase from bottom water values (δ 18O ∼ 0.35‰). Although no obvious evidence of fluid seepage was observed during the TV surveys, coring data revealed a broad distribution of living Pogonophora and bacterial colonies on sea bottom inside seep structures. These evidences point to ongoing fluid activity (continuous seepage of methane) through these structures. From other side, considerable number and variety of chemosynthetic macro fauna with complete absence of living species suggest that present day level of fluid activity is significantly lower than it was in past. Dead and subfossil fauna recovered from various seep sites consist of solemyid ( Acharax sp.), thyasirid and vesicomyid (cf. Calyptogena sp.) bivalves belonging to chemosymbiotic families. Significant variations in δ 13C (−31.6‰ to −59.2‰) and δ 18O (0.42‰ and 6.4‰) of methane-derived carbonates collected from these structures most probably related to changes in gas composition and bottom water temperature between periods of their precipitation. This led us to ideas that: (1) seep activity on the Southern Vøring Plateau was started with large input of the deep thermogenic gas and gradually decries in time with increasing of biogenic constituent; (2) authigenic carbonate precipitation started at the near normal deep sea environments with bottom water temperature around +5 °C and continues with gradual cooling up to negative temperatures recording at present time.

In-situ sediment temperature measurements and geochemical porewater data suggest highly dynamic fluid flow at Isis mud volcano, eastern Mediterranean Sea

Isis mud volcano is located on the upper slope of the Nile deep-sea fan and has been described as the surface expression of a deep-seated gas chimney [Loncke, L., Gaullier, V., Mascle, J., Vendeville, B., 2002. Shallow structure of the Nile deep-sea fan: interactions between structural heritage and salt tectonics; consequences on sedimentary dispersal. In: CIESM (Ed.), Turbidite systems and deep-sea fans of the Mediterranean and the Black seas. CIESM Workshop Series. vol. 17. Monaco]. Detailed geothermal and geochemical investigations of Isis MV have been carried out during the NAUTINIL (2003) and MIMES (2004) cruises within the framework of the Euro-margins/Mediflux project. Sediment temperatures of more than 40 °C at 10 m below the seafloor at the center of the mud volcano indicate an exceptionally high level of activity. Rapidly decreasing temperature gradients away from the center support the hypothesis of a dominantly axisymmetric functioning of Isis MV. The microbathymetry along dive tracks of the submersible Nautile reveals the presence of terraces and shows that the temperature gradients at shallow sediment depth follow the morphology. At the center, porewater profiles are characterized by a rapid decrease of chlorinity within the uppermost meter of the sediments, whereas the chlorinity of cores taken at short distances away is equal to bottom water values. Applying simple analytical models to the data provides evidence of recent mud volcano activity but no straightforward explanation of the observed anomalies, which points to a transient regime of variable fluid flow, possibly accompanied by episodic mud eruptions. Numerical modeling suggests that rapid cooling of the sediment column and downward progression of the sharp decrease in porewater chlorinity observed between the two cruises is related to episodic infiltration of bottom seawater into the mud.

Biogeochemistry of sulfur and iron in Thioploca-colonized surface sediments in the upwelling area off central chile

The biogeochemistry of sedimentary sulfur was investigated on the continental shelf off central Chile at water depths between 24 and 88 m under partial influence of an oxygen minimum zone. Dissolved and solid iron and sulfur species, including the sulfur intermediates sulfite, thiosulfate, and elemental sulfur, were analyzed at high resolution in the top 20 cm. All stations were characterized by high rates of sulfate reduction, but only the sediments within the Bay of Concepción contained dissolved sulfide. Due to advection and/or in-situ reoxidation of sulfide, dissolved sulfate was close to bottom water values. Whereas the concentrations of sulfite and thiosulfate were mostly in the submicromolar range, elemental sulfur was by far the dominant sulfur intermediate. Although the large nitrate- and sulfur-storing bacteria Thioploca were abundant, the major part of S 0 was located extracellularly. The distribution of sulfur species and dissolved iron suggests the reaction of sulfide with FeOOH as an important pathway for sulfide oxidation and sulfur intermediate formation. This is in agreement with the sulfur isotope composition of co-existing elemental sulfur and iron monosulfides. In the Bay of Concepción, sulfur isotope data suggest that pyrite formation proceeds via the reaction of FeS with polysulfides or H 2S. At the shelf stations, on the other hand, pyrite was significantly depleted in 34S relative to its potential precursors FeS and S 0. Isotope mass balance considerations suggest further that pyritization at depth includes light sulfide, potentially originating from bacterial sulfur disproportionation. The δ 34S-values of pyrite down to −38‰ vs. V-CDT are among the lightest found in organic-rich marine sediments. Seasonal variations in the sulfur isotope composition of dissolved sulfate indicated a dynamic non-steady-state sulfur cycle in the surface sediments. The 18O content of porewater sulfate increased with depth at all sites compared to the bottom water composition due to intracellular isotope exchange reactions during microbial sulfur transformations.

Fluorescent dissolved organic matter in marine sediment pore waters

Fluorescent dissolved organic matter (FDOM) in sediment pore waters from contrasting sites in the Chesapeake Bay and along the mid-Atlantic shelf/slope break was studied using three-dimensional fluorescence spectroscopy. Benthic fluxes of FDOM were also examined at the Chesapeake Bay sites. The major fluorescence peaks observed in these pore waters corresponded to those observed in the water column. These included peaks ascribed to the fluorescence of humic-like material (peaks A, C and M), as well as protein-like peaks that appear to result from the fluorescence of the aromatic amino acids tryptophan and tyrosine. In these pore waters we also observed a fourth humic-like fluorescence peak (A′). These four humic-like peaks appeared to occur in pairs (peaks A and M in one pair and peaks A′ and C in another pair) with near identical emission maxima but different excitation maxima. Peaks A′ and C were red shifted relative to peaks A and M. Humic-like fluorescence increased with sediment depth at almost all stations, and was closely correlated with total DOC. This fluorescence appeared to be a tracer for the refractory, relatively low molecular weight pore water DOM that accumulates with depth during sediment diagenesis. Fluorescence–DOC relationships indicated that larger relative amounts of humic-like FDOM were seen in anoxic sediments versus sub-oxic or mixed redox sediments. By extension, these observations suggest that refractory humic-like compounds (in general) are preferentially preserved in sediment pore waters under anoxic conditions. A simple conceptual model is presented here which proposes that different types of organic matter (e.g., marine vs. terrestrial) as well as internal transformations of DOM or FDOM may lead to the occurrence of these humic-like fluorophores. This model is consistent with a wide range of data on FDOM in marine as well as freshwater systems. Protein-like fluorescence showed no coherent depth trends in sediment pore waters, other than the fact that pore water fluorescence intensities were greater than bottom water values. Protein-like fluorescence in pore waters may be associated with refractory DOM, although this observation is somewhat equivocal. In contrast, the results of benthic flux studies suggested that here protein-like fluorescence was associated with reactive DOM intermediates of organic matter diagenesis (e.g., dissolved peptides and proteins) produced near the sediment–water interface. Furthermore, the interplay between transport processes and the depth zonation of DOM cycling in bioirrigated sediments leads to molecular diffusion (rather than bioirrigation) playing a much more important role in transporting protein-like fluorescence out of the sediments. In contrast, bioirrigation dominates sediment–water exchange of humic-like fluorescence (and therefore most DOC in general). Finally, benthic flux studies indicated that sediments represent a source of chromophoric DOM to coastal waters, although further work will be needed to quantify their significance in terms of other known sources of this material (e.g., riverine input, phytoplankton degradation products).

Sulfur and oxygen isotopes of coeval sulfate–sulfide in pore fluids of cold seep sediments with sharp redox gradients

Leakage of crude oil and gas through fault conduits intersecting the seafloor gives rise to scores of point-source anoxic enclaves on the oxic northern Gulf of Mexico slope. A study of 13 short cores recovered with a manned submersible from these seepage-affected sediments reveals that microbial processes fueled by hydrocarbons cause extensive sulfur diagenesis. Sulfate reduction and sulfide release occurring in the pore fluids reach completion 10–25 cm below the sediment–water interface. Bacterial sulfate reduction (BSR) rates are highly variable between sites but maximum values (97 and 917 μmol SO 4 cm −3 year −1) in a bacterial mat and mussel bed, respectively, are unusually high for cold, deepwater habitats. δ 34S and δ 18O values of the residual sulfate range from 20.7‰ to 70.8‰ (CDT) ( n=45) and from 11.1‰ to 23.6‰ (SMOW) ( n=33) compared to the overlying Gulf of Mexico bottom water values of 20.3‰ and 9.7‰, respectively. δ 34S values of H 2S yield a mean of 12.4±5.4‰ (CDT) ( n=15). δ 34S (SO 4) data yield an integrated fractionation factor of α S=1.015 under a closed system assumption but a substantial higher fractionation of α S=1.023 under an open system assumption. Paired SO 4–H 2S inventory indicates that up to 28% of sulfide is removed from the system and supports the contention that seep sediments constitute an open system. The sulfur isotope fractionations reported here compare well with experimental data for cold-adapted sulfate-reducing bacteria but are substantially smaller than the “geological” fractionation of α S=1.055 derived from coeval sulfate-sulfide in Phanerozoic sediments. Isotope enrichments in the SO 4 are 2.4 times greater in δ 34S than in δ 18O and the relations documented with f(SO 4) are indicative of mixing between two end-member sulfate sources; seawater sulfate cycled through microbial dissimilatory sulfate reduction at depth and secondary sulfate produced by oxidation of H 2S at near-surface through bacterial disproportionation (BDS) processes. The evidence for superimposed metabolic reactions in the reducing and oxidative parts of the anaerobic sulfur cycle in seep sediments has important implications regarding the proposed use of pore-water sulfate profiles as proxies of upward methane fluxes resulting from dissociation of marine-based gas hydrates.

Hydrography and local sources of dissolved trace metals Mn, Ni, Cu, and Cd in the northeast Atlantic Ocean

Abstract Vertical profiles of dissolved Ni and Cd in the northeast Atlantic Ocean are dominated by conservative mixing of North Atlantic Deep Water and Antarctic Bottom Water. Local regenerative input of Ni and Cd is restricted to thermocline waters. In deep waters Cu is affected by mixing of water masses, but input from the sediment is also important. The North Atlantic Deep Water (NADW) contains 3.5–4.0 nM Ni, 1.2–2.0 nM Cu, 200–240 pM Cd and has a Cd/PO4 ratio of 180–220 pmol/μmol. The bottom waters (Lower Deep Water, LDW) are a mixture of 69% NADW and 31% Antarctic Bottom Water (AABW). The LDW contains 4.9 ± 0.6 nM Ni, 2.47 ± 0.56 nM Cu, 315 ± 30 pM Cd and has a Cd/PO4 ratio of 216 ± 0.14 pmol/μmol. For Cd and Ni the bottom water (LDW) values are consistent with conservative mixing of NADW and AABW, for Cu an excess is ascribed to additional benthic input. Local sources and sinks dominate the dissolved Mn distribution. The deep water concentration of Mn varies between 0.2 and 0.5 nM and is not related to the presence of watermasses.

GEOSECS Atlantic 32Si profiles

Measurements of five cosmogenic 32Si vertical profiles in Atlantic waters (27°N to 60°S) are presented. The amounts of dissolved SiO 2 extracted range from 2 to 54 g; the amounts of water from which SiO 2 was extracted range between 540 kg and 270, 000 kg. In additon, SiO 2 recovered from four surface particulate composites (64°N to 61°S) were also analyzed for 32Si. 32Si measurements were made by milking and counting the daughter activity, 32P. The net 32P activities range from 0.7 to 6.8 cph; typical errors in measurements of the 32P activities are 20–30%. The 32Si concentrations vary from 0.6 dpm/10 6 kg of water in the North Atlantic surface waters to 235 dpm/10 6 kg at 400 m depth in the circumpolar waters. The vertical profiles of 32Si at the five Atlantic stations approximately follow the Si profiles but the depth gradients are different. This would be expected also considering the in-situ release mechanisms due to dissolution and advection/diffusion from the bottom waters. Except for the circumpolar station 89, where the Si and 32Si profiles show the effect of marked vertical mixing (nearly depth independent profiles), the profiles show the following features: (1) specific activities of 32Si ( 32Si/SiO 2 ratios) are lowest at intermediate depths, and (2) on an average the surface specific activities are higher, by 2–4 times, than the bottom water values. These data are consistent with generation of the highest specific activity 32Si waters at the surface, where Si concentrations are lowest and precipitation adds cosmogenic 32Si scavenged from the troposphere. Rapid removal of biogenic silica to the water-sediment interface, without much dissolution during transit, leads to the second regime of high 32Si specific activities. The 32Si inventories in the water column in the latitude belt 27°N-27°S are in the range (1–1.4) × 10 −2 dpm 32Si/cm 2, which is consistent with the expected fallout of cosmogenic 32Si. However, the 32Si column inventories south of 40°S are higher by a factor of ∼ 5–7, whereas the corresponding Si inventories increase by only a factor of 3. This excess 32Si in the Southern Ocean cannot be explained by direct fallout from the stratosphere or by melting of Antarctic snow and ice. Instead, this excess is maintained primarily by the southward deep-water transport of 32Si dissolved from sinking particulates.

Seasonal Variation of Temperature, Alkalinity and Dissolved Oxygen in the Sagar Lake

AbstractThe present study deals with the seasonal variation of temperature, alkalinity and dissolved oxygen in the Sagar lake. Different abiotic and biotic factors were investigated and a diurnal study was made, too. The investigation shows that the surface water values were always higher than the bottom water values. The water temperature ranged from 16.0 to 31.0 °C, carbonate alkalinity ranged from 0.0 to 46.0 mg/1, bicarbonate alkalinity ranged from 62.0 to 138.0 mg/1, dissolved oxygen ranged between 1.6 and 13.2 ppm.The water temperature was found to be positively correlated with carbonate alkalinity while an inverse correlation existed with bicarbonate alkalinity. Dissolved oxygen had a significant positive correlation with carbonate alkalinity, while a negative correlation was observed with bicarbonate alkalinity during all the seasons.

Early diagenesis in sediments from the eastern equatorial Pacific, II. Pore water metal results

Concentrations of Mn, Fe, Ni and Cu were measured in pore waters extracted from pelagic sediments at two sites in the eastern equatorial Pacific and at two locations in the eastern equatorial Atlantic. The results from these four sites suggest that the pore water profiles of these metals are controlled by processes involving the major oxidants (O 2, NO 3 −, Mn and Fe oxides). Dissolved copper concentrations in the top 2 cm of sediment are 10 times the bottom water value. This shallow regeneration is an important factor in the oceanic budget of copper. Manganese, nickel and iron concentrations near the interface are indistinguishable from bottom water values. At depth in the sediment, Mn oxides are reduced and manganese diffuses upwards to be reoxidized and trapped within the sediment column. The highest nickel concentrations in the Mn reduction zone are 40 times that of bottom water. This nickel gradient drives a flux into the Mn oxidation zone which is large enough to influence the nickel concentration of the bulk sediment. Dissolved Mn concentrations range from 10 nmol/kg in the oxygenated zone to 100 μmol/kg in the reduction zone.

Interstitial Nitrate Profiles and Oxidation of Sedimentary Organic Matter in the Eastern Equatorial Atlantic

Pore-water nitrate concentrations in six pelagic eastern equatorial Atlantic cores increase from bottom water values (22 micromolar) to 40 micromolar at a depth of about 5 centimeters, then decrease to undetectable levels at depths as shallow as 40 centimeters. These nitrate concentrations and concentration gradients reflect zones of oxygen reduction, nitrate reduction, and sulfate reduction in the sediments. The estimated benthic flux of nitrate to the ocean from our data is much less than the total globalflux of nitrate to deep waters, even though these equatorial sediments underlie a productive upwelling zone. The estimated denitrification rate in our study area suggests that pelagic sediments may be important sites of marine denitrification.

N2 : Ar, nitrification and denitrification in southern California borderland basin sediments1

The first quantitative observations of dissolved molecular nitrogen (N2) in deep‐sea sedimentary pore waters show increases of up to 17% above adjacent bottom water values. The N2 : Ar concentration ratio similarly increases. This increase is greater than expected from reduction of seawater nitrate in the sediment. A mechanism is suggested in which ammonia, derived from nitrogen‐containing organic matter, is converted to N2 through a nitrite intermediate under conditions of low oxygen tension. Ammonia is not converted to N2 in anoxic sediments that are undergoing sulfate reduction even though N2 is the thermodynamically stable species.