Ikaite of sediment cores from the Zaire deep-sea fan

Most concentration profiles of sulfate in continental margin sediments show constant or continuously increasing gradients from the benthic boundary layer down to the deep sulfate reduction zone. However, a very marked change in this gradient has been observed several meters below the surface at many locations, which has been attributed to anoxic sulfide oxidation or to non-local transport mechanisms of pore waters. The subject of this study is to investigate whether this feature could be better explained by non-steady state conditions in the pore-water system. To this end, data are presented from two gravity cores recovered from the Zaire deep-sea fan. The sediments at this location can be subdivided into two sections. The upper layer, about 10 m thick, consists of stratified pelagic deposits representing a period of continuous sedimentation over the last 190 kyr. It is underlain by a turbidite sequence measuring several meters in thickness, which contains large crystals of authigenic calcium carbonate (ikaite: CaCO3·6H2O). Ikaite delta13C values are indicative of a methane carbon contribution to the CO2 pool. Radiocarbon ages of these minerals, as well as of the adjacent bulk sediments, provide strong evidence that the pelagic sediments have overthrust the lower section as a coherent block. Therefore, the emplacement of a relatively undisturbed sediment package is postulated. Pore-water profiles show the depth of the sulfate-methane transition zone within the turbiditic sediments. By the adaptation of a simple transport-reaction model, it is shown that the change in the geochemical environmental conditions, resulting from this slide emplacement, and the development towards a new steady state are fully sufficient to explain all features related to the pore-water profiles, particularly, [SO4]2- and dissolved inorganic carbon (DIC). The model shows that the downslope transport took place about 300 yr ago.

Geochemistry of pore fluids from ODP Leg 134 sites

Depending on the temperature and the extent of diagenetic alteration of fluid chemistry, fluid flow at convergent margins may transfer important quantities of heat and mass between the crust and seawater, thereby influencing global mass, isotopic and heat budgets. In the North Aoba Basin, an intra-arc basin located at the New Hebrides Island Arc, alteration of volcanic ash to clay minerals and zeolites forms a CaCl2 brine, perhaps in less than 1 to 3 m.y. The brine results from an exchange of Ca for Na, K, and Mg, and an increase in Cl concentrations to a maximum of 1241 mM. The Cl increase is partly due to the transfer of H2O from the pore fluid into authigenic minerals, but water mass balances, d18O-Cl correlations, and Br/Cl ratios suggest that there is a source of Cl in the sediments. Concentration profiles indicate that Li is transferred from the fluid to solid phase at depths <300 meters below seafloor (mbsf), but at greater depths it is transferred from the solid to fluid phase, at temperatures possibly as low as 25°C. In the accretionary wedge extensive fluid flow appears to be confined to highly faulted regions. Although Cl concentrations less than seawater value are common at convergent margins, the New Hebrides margin contains little low-Cl fluid. Br/Cl ratios suggest the low-Cl fluid is from dilution, and d18O values indicate the water may be derived from mineral dehydration and mixing with meteoric water. The New Hebrides margin exhibits few surface manifestations of venting (e.g., sulfide-oxidizing benthic biological communities, carbonate crusts, mud volcanoes) and thus fluid fluxes may be smaller than at many other margins.

Concentrations of rare earth elements in pore waters of IODP Holes 323-U1345A and 323-U1345B

We studied the diagenetic behavior of rare earth elements (REEs) in a highly productive passive margin setting of the Bering Sea Slope. Site U1345 was drilled during the Integrated Ocean Drilling Program Expedition 323 at a water depth of 1008 m currently in the center of an oxygen minimum zone. Pore water concentrations of fourteen REEs were determined down to ~ 140 meters below the seafloor (mbsf). The REE concentrations were higher in the pore water than the deep seawater, indicating that there was significant liberation from the sediments during diagenesis. There was a major peak at ~ 10 mbsf that was more pronounced for the heavy REE (HREE); this peak occurred below the sulfate-methane transition zone (6.3 mbsf) and coincided with high concentrations of dissolved iron and manganese. At ~ 2 mbsf, there was a minor peak in REE and Mn contents. Below ~ 40 mbsf, the REE concentration profiles remained constant. The Ce anomaly was insignificant and relatively constant (PAAS-normalized Ce/Ce = 1.1 ± 0.2) throughout the depth profile, showing that the Ce depleted in seawater was restored in the pore water. HREE-enrichment was observed over the entire 140 m except for the upper ~ 1 m, where a middle REE (MREE)-bulge was apparent. REE release in shallow depths (2-4 mbsf) is attributed to the release of light REEs (LREEs) and MREEs during the organoclastic reduction of Mn oxides in anoxic sediments. The high HREE concentrations observed at ~ 10 mbsf can be attributed to the reduction of Fe and Mn minerals tied to anaerobic oxidation of methane or, less significantly, to ferromagnesian silicate mineral weathering. The upward diffusion flux across the sediment-water interface was between 3 (for Tm) and 290 (for Ce) pmol/m**2/y.

Biogeochemical parameters (oxygen, nitrate, phosphate, TOC and strontium) at three sites from the Clarion-Clipperton Fracture Zone

The Clarion-Clipperton Fracture Zone (CCFZ) in the Pacific Ocean is characterized by organic carbon-starved sediments and meter-scale oxygen penetration into the sediment. Furthermore, numerous seamounts occur throughout its deep-sea plain, which may serve as conduits for low-temperature hydrothermal circulation of seawater through the oceanic crust. Recent studies in deep-sea environments of the Pacific and Atlantic Oceans have suggested and presented evidence of an exchange of dissolved constituents between the seawater flowing in the basaltic crust and the pore water of the overlying sediments. Through high-resolution pore-water oxygen and nutrient measurements, we examined fluxes and geochemical interactions between the seamount basaltic basement and pore waters of the overlying sediments at three sites located on a radial transect from the foot of Teddy Bare, a small seamount in the CCFZ. At three sites, located 1000, 700 and 400 m away from the foot of the seamount, we found that oxygen concentrations initially decrease with sediment depth but start to increase at depths of 3 and 7 m towards the basaltic basement. NO32- concentrations mirror the oxygen concentration profiles, as they increase with sediment depth but decrease towards the basement. We performed transport reaction modeling and determined at one site the 87Sr/86Sr ratio of the pore water and the bottom water overlying the sediments, which indicated that the 87Sr/86Sr ratio of the pore water at the bottom of the sediment column is similar to the seawater Transport-reaction modeling revealed that (1) the diffusive flux of oxygen from the basaltic basement outpaces the oxygen consumption through organic matter oxidation and nitrification in the basal sediments and (2) the nutrient exchange between the sediment and the underlying basaltic crust occurs at orders-of-magnitude lower rates than between the upper sediment and the overlying bottom water. Our results suggest an upward diffusion of oxygen from seawater circulating within the seamount crust into the overlying basal sediments. The oxygen profiles presented here represent the first of their kind ever measured in the Pacific Ocean, as they indicate an upward flux of molecular oxygen from a basaltic aquifer, something that has so far only been documented - at one other location worldwide - the North Pond site in the Atlantic Ocean. We show that the diffusion of oxygen from the seamount basaltic basement into the overlying pore waters affects the preservation of organic compounds and helps to maintain a completely oxygenated sedimentary column at all 3 sites near the seamount.

(Table 1) Pore water halogen concentrations in ODP Holes 169-1033B and 169-1034B

The entire suite of halogens was measured in the pore fluids of Hole 1033B and 1034B from Saanich Inlet: ODP Leg 169S. The fast sedimentation rates and large amount of organic carbon burial coupled with anoxia of the overlying waters promotes an advanced stage of diagenesis within the sediment column. Chloride interstitial water profiles suggest salinity variations within the waters of Saanich Inlet. Concentration profiles for iodide and bromide support the argument that they are produced through the degradation of organic matter. Although the concentration increases in I- and Br- indicate that these halides are not regenerated in similar proportions to marine organic matter, it appears that iodide and bromide are regenerated to similar degrees within the sediment column and in similar proportions to the sediment halide concentrations. Fluoride porewater values show a complicated pattern, most likely caused by secondary reactions involving complexation with Mg2+, carbonate fluorapatite precipitation, carbonate mineral diagenesis, and/or uptake into alumino-silicate minerals.

Shell preservation of Limacina inflata in surface sediments of the Central and South Atlantic

Over 300 surface sediment samples from the Central and South Atlantic Ocean and the Caribbean Sea were investigated for the preservation state of the aragonitic test of Limacina inflata. Results are displayed in spatial distribution maps and are plotted against cross-sections of vertical water mass configurations, illustrating the relationship between preservation state, saturation state of the overlying waters, and overall water mass distribution. The microscopic investigation of L. inflata (adults) yielded the Limacina dissolution index (LDX), and revealed three regional dissolution patterns. In the western Atlantic Ocean, sedimentary preservation states correspond to saturation states in the overlying waters. Poor preservation is found within intermediate water masses of southern origin (i.e. Antarctic intermediate water (AAIW), upper circumpolar water (UCDW)), which are distinctly aragonite-corrosive, whereas good preservation is observed within the surface waters above and within the upper North Atlantic deep water (UNADW) beneath the AAIW. In the eastern Atlantic Ocean, in particular along the African continental margin, the LDX fails in most cases (i.e. less than 10 tests of L. inflata per sample were found). This is most probably due to extensive "metabolic" aragonite dissolution at the sediment-water interface combined with a reduced abundance of L. inflata in the surface waters. In the Caribbean Sea, a more complex preservation pattern is observed because of the interaction between different water masses, which invade the Caribbean basins through several channels, and varying input of bank-derived fine aragonite and magnesian calcite material. The solubility of aragonite increases with increasing pressure, but aragonite dissolution in the sediments does not simply increase with water depth. Worse preservation is found in intermediate water depths following an S-shaped curve. As a result, two aragonite lysoclines are observed, one above the other. In four depth transects, we show that the western Atlantic and Caribbean LDX records resemble surficial calcium carbonate data and delta13C and carbonate ion concentration profiles in the water column. Moreover, preservation of L. inflata within AAIW and UCDW improves significantly to the north, whereas carbonate corrosiveness diminishes due to increased mixing of AAIW and UNADW. The close relationship between LDX values and aragonite contents in the sediments shows much promise for the quantification of the aragonite loss under the influence of different water masses. LDX failure and uncertainties may be attributed to (1) aragonite dissolution due to bottom water corrosiveness, (2) aragonite dissolution due to additional CO2 release into the bottom water by the degradation of organic matter based on an enhanced supply of organic matter into the sediment, (3) variations in the distribution of L. inflata and hence a lack of supply into the sediment, (4) dilution of the sediments and hence a lack of tests of L. inflata, or (5) redeposition of sediment particles.

Bulk geochemical and lipid biomarker data for sediment core W8402A-14

Eleven sediment samples taken downcore and representing the past 26 kyr of deposition at MANOP site C (0°57.2°N, 138°57.3°W) were analyzed for lipid biomarker composition. Biomarkers of both terrestrial and marine sources of organic carbon were identified. In general, concentration profiles for these biomarkers and for total organic carbon (TOC) displayed three common stratigraphic features in the time series: (1) a maximum within the surface sediment mixed layer (<=4 ka); (2) a broad minimum extending throughout the interglacial deposit; and (3) a deep, pronounced maximum within the glacial deposit. Using the biomarker records, a simple binary mixing model is described that assesses the proportion of terrestrial to marine TOC in these sediments. Best estimates from this model suggest that ~20% of the TOC is land-derived, introduced by long-range eolian transport, and the remainder is derived from marine productivity. The direct correlation between the records for terrestrial and marine TOC with depth in this core fits an interpretation that primary productivity at site C has been controlled by wind-driven upwelling at least over the last glacial/interglacial cycle. The biomarker records place the greatest wind strength and highest primary productivity within the time frame of 18 to 22 kyr B.P. Diagenetic effects limit our ability to ascertain directly from the biomarker records the absolute magnitude that different types of primary productivity have changed at this ocean location over the past 26 kyr.

Chemistry of interstitial waters of ODP Leg 117 samples (Table 1)

Interstitial water analyses made at 12 sites during Leg 117 are used to define the nature of diagenetic reactions in organic-rich sediments on the Owen Ridge and Oman Margin. Minor variations in chloride concentration profiles are ascribed to past changes in bottom water salinity at two mid-depth margin sites and to upward migration of low salinity water at another. There is no evidence for subsurface brine movement, unlike the case on the Peru Margin. Dolomitization is widespread and accounts for the depletions of magnesium observed in pore waters at variable depths at nearly all sites. The mineral occurs both as disseminated euhedral limpid crystals and, in at least one location, in massive stringers. Formation of the latter is suggested to reflect precipitation during sea level transgressions when the sedimentation rate was low, but when productivity was high. Authigenic carbonate fluorapatite is also widespread, the phosphorus being derived from the breakdown of organic matter. Sulfate is quantitatively depleted at depth at most locations but the rate of depletion is markedly less than that observed on the Peru Margin where sedimentation is also similarly influenced by high rates of upwelling. The reason for this contrast is not clear and merits further investigation.

(Table 1) Stable nitrogen isotopes of ammonium in interstitial waters from ODP Site 164-997

Ammonium (NH4+) concentration profiles in piston-core sediments of the Carolina Rise and Blake Ridge generally have linear concentration profiles within the sulfate reduction zone (Borowski, 1998). Deep Sea Drilling Project (DSDP) Site 533, located on the Blake Ridge, also displayed a linear ammonium concentration profile through the sulfate reduction zone and the profile linearity continues into the upper methanogenic zone to a depth of ~200 meters below seafloor (mbsf), where the first methane gas hydrates probably occur (Jenden and Gieskes, 1983, doi:10.2973/dsdp.proc.76.114.1983; Kvenvolden and Barnard, 1983, doi:10.2973/dsdp.proc.76.106.1983). Sediments from the Ocean Drilling Program (ODP) Leg 164 deep holes (Sites 994, 995, and 997) also exhibit linear ammonium profiles above the top of the gas hydrate zone (~200 mbsf) (Paull, Matsumoto, Wallace, et al., 1996, doi:10.2973/odp.proc.ir.164.1996). We hypothesized that a possible cause of linear ammonium profiles was diffusion of ammonium from a concentrated ammonium source at depth. We further reasoned that if this ammonium were produced by microbial fermentation reactions at depth, that a comparison of the nitrogen isotopic composition of sedimentary organic nitrogen and the nitrogen with pore-water ammonium would test this hypothesis. Convergence with depth of d15N values of the nitrogen source (sedimentary organic matter) and the nitrogen product (dissolved NH4+) would strongly suggest that ammonium was produced within a particular depth zone by microbial fermentation reactions. Here, we report d15N values of pore-water ammonium from selected interstitial water (IW) samples from Site 997, sampled during ODP Leg 164.

Stable oxygen isotope composition of planktonic foraminifera in the Canary Islands region

Seasonal depth stratified plankton tows, sediment traps and core tops taken from the same stations along a transect at 29°N off NW Africa are used to describe the seasonal succession, the depth habitats and the oxygen isotope ratios (delta18O(shell)) of five planktic foraminiferal species. Both the delta18O(shell) and shell concentration profiles show variations in seasonal depth habitats of individual species. None of the species maintain a specific habitat depth exclusively within the surface mixed layer (SML), within the thermocline, or beneath the thermocline. Globigerinoides ruber (white) and (pink) occur with moderate abundance throughout the year along the transect, with highest abundances in the winter and summer/fall season, respectively. The average delta18O(shell) of G. ruber (w) from surface sediments is similar to the delta18O(shell) values measured from the sediment-trap samples during winter. However, the delta18O(shell) of G. ruber (w) underestimates sea surface temperature (SST) by 2 °C in winter and by 4 °C during summer/fall indicating an extension of the calcification/depth habitat into colder thermocline waters. Globigerinoides ruber (p) continues to calcify below the SML as well, particularly in summer/fall when the chlorophyll maximum is found within the thermocline. Its vertical distribution results in delta18O(shell) values that underestimate SST by 2 °C. Shell fluxes of Globigerina bulloides are highest in summer/fall, where it lives and calcifies in association with the deep chlorophyll maximum found within the thermocline. Pulleniatina obliquiloculata and Globorotalia truncatulinoides, dwelling and calcifying a part of their lives in the winter SML, record winter thermocline (~180 m) and deep surface water (~350 m) temperatures, respectively. Our observations define the seasonal and vertical distribution of multiple species of foraminifera and the acquisition of their delta18O(shell).

(Table 1) Pore-water composition of ODP Leg 126 sites

The Ocean Drilling Program Leg 126 sites may be classified into two categories depending on the presence (Group I: Sites 787, 792, and 793) or absence (Group II: Sites 788, 790, and 791) of steep concentration gradients. Shipboard X-ray diffraction analyses of bulk sediments from Group I sites revealed the presence of a number of diagenetic minerals (some of which are incompatible), but no systematic diagenetic zonation. The results of the chemical analyses of the pore waters from Group I have been used to estimate the activities of dissolved species. Thermodynamic analyses of the composition of the pore waters and the stability of authigenic minerals (gypsum, zeolites, feldspars, smectites, chlorites, and micas) show that the pore waters are close to equilibrium with most of the observed phases. Thus, only a small perturbation of the system (substitution in minerals and fluctuations in pore-water composition, in particular, in pH and SiO2 activity) will cause any of these phases to precipitate. Therefore, one would not expect mineralogical observations to show systematic vertical zonations at these sites. It is suggested that chlorites and high-temperature zeolites are not diagenetic sensu stricto, but were eroded from volcaniclastic highs. The absence of concentration gradients at the Group II sites has been analyzed in terms of reaction kinetics, hydrothermal advection, and temperature distribution. The absence of diagenetic imprints on the pore-water concentration profiles at these sites is probably caused by the slow nucleation of silica phases.

10Be and barium concentrations of sediment core GeoB1008-3

Particle reactive elements are scavenged to a higher degree at ocean margins than in the open ocean due to higher fluxes of biogenic and terrigenous particles. In order to determine the influence of these processes on the depositional fluxes of 10Be and barium we have performed high-resolution measurements on sediment core GeoB1008-3 from the Congo Fan. Because the core is dominated by terrigenous matter supplied by the Congo River, it has a high average mass accumulation rate of 6.5 cm/kyr. Biogenic 10Be and Ba concentrations were calculated from total concentrations by subtracting the terrigenous components of10Be and Ba, which are assumed to be proportional to the flux of Al2O3. The mean Ba/Al weight ratio of the terrigenous component was determined to be 0.0045. The unusualy high terrigenous 10Be concentrations of 9.1 * 10**9 atoms/g Al2O3 are either due to input of particles with high10Be content by the Congo River or due to scavenging of oceanic 10Be by riverine particles. The maxima of biogenic 10Be and Ba concentrations coincide with maxima of the paleoproductivity rates. Time series analysis of the 10Be and of Ba concentration profiles reveals a strong dominance of the precessional period of 24 kyr, which also controls the rates of paleoproductivity in this core. During the maxima of productivity the flux of biogenic Ba is enhanced to a larger extent than that of biogenic 10Be. Applying a model for coastal scavenging, we ascribe the observed higher sensitivity of Ba to biogenic particle fluxes to the fact that the ocean residence time of Ba is approximately 10 times longer than that of 10Be.

Gas hydrate investigation at the Amsterdam mud volcano

We investigated gas hydrate in situ inventories as well as the composition and principal transport mechanisms of fluids expelled at the Amsterdam mud volcano (AMV; 2,025 m water depth) in the Eastern Mediterranean Sea. Pressure coring (the only technique preventing hydrates from decomposition during recovery) was used for the quantification of light hydrocarbons in near-surface deposits. The cores (up to 2.5 m in length) were retrieved with an autoclave piston corer, and served for analyses of gas quantities and compositions, and pore-water chemistry. For comparison, gravity cores from sites at the summit and beyond the AMV were analyzed. A prevalence of thermogenic light hydrocarbons was inferred from average C1/C2+ ratios <35 and d13C-CH4 values of -50.6 per mil. Gas venting from the seafloor indicated methane oversaturation, and volumetric gas-sediment ratios of up to 17.0 in pressure cores taken from the center demonstrated hydrate presence at the time of sampling. Relative enrichments in ethane, propane, and iso-butane in gas released from pressure cores, and from an intact hydrate piece compared to venting gas suggest incipient crystallization of hydrate structure II (sII). Nonetheless, the co-existence of sI hydrate can not be excluded from our dataset. Hydrates fill up to 16.7% of pore volume within the sediment interval between the base of the sulfate zone and the maximum sampling depth at the summit. The concave-down shapes of pore-water concentration profiles recorded in the center indicate the influence of upward-directed advection of low-salinity fluids/fluidized mud. Furthermore, the SO42- and Ba2+ pore-water profiles in the central part of the AMV demonstrate that sulfate reduction driven by the anaerobic oxidation of methane is complete at depths between 30 cm and 70 cm below seafloor. Our results indicate that methane oversaturation, high hydrostatic pressure, and elevated pore-water activity caused by low salinity promote fixing of considerable proportions of light hydrocarbons in shallow hydrates even at the summit of the AMV, and possibly also of other MVs in the region. Depending on their crystallographic structure, however, hydrates will already decompose and release hydrocarbon masses if sediment temperatures exceed ca. 19.3°C and 21.0°C, respectively. Based on observations from other mud volcanoes, the common occurrence of such temperatures induced by heat flux from below into the immediate subsurface appears likely for the AMV.

Ex situ microsensor profiles of sulfide concentration and pH in/outside mat 1 (MSM15/1_492-PUC1) measured during the MSM15/1 cruise in 2010 (Black Sea)

Biogeochemical measurements in sediment cores collected with the submersible JAGO (pusch cores) and a TV-MUC in the Black Sea during MSM15/1, Northwest Crimea (HYPOX Project), at water depths between 152-156 m. A series of microbial mats were sampled on the hypoxic region of the Crimean Shelf. Concentrations of organic carbon (Corg) and nitrogen (N) were measured on finely powdered, freeze-dried subsamples of sediment using a using a Fisons NA-1500 elemental analyzer. For organic carbon determination samples were pre-treated with 12.5% HCl to remove carbonates. Chlorophyll a (chl a), phaeopigments (PHAEO) and chloroplastic pigment equivalents (CPE) was measured according to Schubert et al., (2005; doi:10.1029/2004GC000837) and total hydrolyzable amino acids (THAA) and single amino acid: ASP, GLU, SER, HIS, GLY, THR, ARG, ALA, TYR, MET, VAL, PHE, ILE, LEU, LYS following Dauwe et al., 1998. High-resolution ex situ sulfide and pH microprofiles, were assessed only for station MSM15/1_492_PUC1. "in mat 1, 2 and 3" refers to 3 different profiles in 3 different spots of the microbial mat, whereas "outside mat", a profile outside the microbial mat.

Profiles of iron concentration from GoFlow bottles during the CARUSO-EISENEX experiment

An in situ iron enrichment experiment was carried out in the Southern Ocean Polar Frontal Zone and fertilized a patch of water within an eddy of the Antarctic Circumpolar Current (EisenEx, Nov. 2000). During the experiment, a physical speciation technique was used for iron analysis in order to understand the changes in iron distribution and size-fractionations, including soluble Fe (<200 kDa), colloidal Fe (200 kDa-0.2 µm) and labile particle Fe (>0.2 µm), throughout the development of the phytoplankton bloom. Prior to the first infusion of iron, dissolved (<0.2 µm) iron concentrations in the ambient surface seawater were extremely low (0.06±0.015 nM) with colloidal iron being a minor fraction. For the iron addition, an acidified FeSO4 solution was released three times over a 23-day period to the eddy. High levels of dissolved iron concentrations (2.0±1.1 nM) were measured in the surface water until 4 days after the first iron infusion. After every iron infusion, when high iron concentrations were observed before storm events, there was a significant correlation between colloidal and dissolved iron concentrations ([Colloidal Fe]=0.7627[Dissolved Fe]+0.0519, R2=0.9346). These results indicate that a roughly constant proportion of colloidal vs. dissolved iron was observed after iron infusion (~76%). Storm events caused a significant decrease in iron concentrations (<0.61 nM in dissolved iron) and changed the proportions of the three iron size-fractions (soluble, colloidal and labile particle). The changes in each iron size-fraction indicate that colloidal iron was eliminated from surface mixed layer more easily than particulate and soluble fractions. Therefore, particle and soluble iron efficiently remain in the mixed layer, probably due to the presence of suspended particles and naturally dissolved organic ligands. Our data suggest that iron removal through colloidal aggregation during phytoplankton bloom should be considered in the oceanic iron cycle.