Chemistry of sediment profile GeoB4906

Mineralization of organic matter and the subsequent dissolution of calcite were simulated for surface sediments of the upper continental slope off Gabon by using microsensors to measure O2, pH, pCO2 and Ca2+ (in situ), pore-water concentration profiles of NO3-, NH4+, Fe2+, and Mn2+ and SO42- (ex situ), as well as sulfate reduction rates derived from incubation experiments. The transport and reaction model CoTReM was used to simulate the degradation of organic matter by O2, [NO3]-, Fe(OH)3 and [SO4]2-, reoxidation reactions involving Fe2+ and Mn2+, and precipitation of FeS. Model application revealed an overall rate of organic matter mineralization amounting to 50 µmol C cm**-2 yr**-1, of which 77% were due to O2, 17% to [NO3]- and 3% to Fe(OH)3 and 3% to [SO4]2-. The best fit for the pH profile was achieved by adapting three different dissolution rate constants of calcite ranging between 0.01 and 0.5% d-1 and accounting for different calcite phases in the sediment. A reaction order of 4.5 was assumed in the kinetic rate law. A CaCO3 flux to the sediment was estimated to occur at a rate of 42 g m**-2 yr**-1 in the area of equatorial upwelling. The model predicts a redissolution flux of calcite amounting to 36 g m**-2 yr**-1, thus indicating that ~90% of the calcite flux to the sediment is redissolved.

Chemical composition of bulk sediments from ODP Hole 199-1215A

Elemental concentrations were determined on 21 samples from Hole 1215A to evaluate the influence of hydrothermal sources on bulk sediment composition. Rare earth element (REE) concentrations were also determined on 10 of these samples. Concentration profiles and REE patterns are consistent with a strong hydrothermal influence on sediment composition at the base of Hole 1215A.

(Table 3) Stable oxagen isotope record of planktonic foraminifera from Central Walvish Ridge

Above the Walvis Ridge, in the SE Atlantic Ocean, we collected living plantkic foraminifera from the upper water column using depth stratified plantkon tows. The oxygen isotope composition (d18Oc) in shells of foraminifera and shell concentration profiles show seasonal and depth habitats of individual species. The tow results are compared with the average annual deposition d18Oc from sediment traps and the interannual average d18Oc of fossil specimens in top sediments at the same site. The species Globigerinita glutinata best reflects the austral winter/spring sea surface temperature (SST). Its d18Oc signal in top sediments remains pristine. In contrast, tow results also show that Globigerinoides ruber continues to calcify below the surface mixed layer (SML), i.e., down to the deep chlorophyll maximum (DCM); hence its d18Oc signature of exported specimens reflects the SST only when SML incorporates the DCM. Deep tow and sediment trap results show that both Globorotalia truncatulinoides and Globorotalia inflata record the temperature between 150 and 350 m, depending on the season and the shell size. However, for all fossil taxa in sediments apart from Globigerinita glutinata, we observe a positive d18Oc shift with respect to the sediment trap and plankton tow values, likely related to the interannual flux changes and deep encrustation.

Methane and hydrogen sulfide in porewater of sediment cores from the upwelling area off Namibia

Porewater concentrations of sulfate, methane, and other relevant constituents were determined on four sediment cores from the high productivity upwelling area off Namibia which were recovered from the continental slope at water depths of 1300 and 2000 m. At all four stations a distinct sulfate-methane transition zone was observed several meters below the seafloor in which both sulfate and methane are consumed. Nutrient porewater concentration profiles do not show gradient slope changes at the depths of the transition zones. Flux calculations carried out on the basis of the determined porewater profiles revealed that anaerobic methane oxidation accounts for 100% of deep sulfate reduction within the sulfate-methane transition zone and consumes the total net diffusive sulfate flux. A significant contribution of organic carbon oxidation to the reduction of sulfate at these depths could, therefore, be excluded. We state that porewater profiles of sulfate with constant gradients above the transition zones are indicative for anaerobic methane oxidation controlling sulfate reduction.

Sedimentary lipids and palynomorphs of sediment core ABC26 from the Eastern Mediterranean

Selective degradation of organic matter in sediments is important for reconstructing past environments and understanding the carbon cycle. Here, we report on compositional changes between and within lipid classes and kerogen types (represented by palynomorph groups) in relation to the organic matter flux to the sea floor and oxidation state of the sediments since the early Holocene for central Eastern Mediterranean site ABC26. This includes the initially oxic but nowadays anoxic presapropelic interval, the still unoxidised lower part of the organic rich S1 sapropel, its postdepositionally oxidised and nowadays organic-poor upper part as well as the overlying postsapropelic sediments which have always been oxic. A general ~ 2.3 times increase in terrestrial and marine input during sapropel formation is estimated on the basis of the total organic carbon (TOC), pollen, spore, dinoflagellate cyst, n-alkane, n-alkanol and n-alkanoic acid concentration changes in the unoxidised part of the sapropel. The long-chain alkenones, 1,15 diols and keto-ols, loliolides and sterols indicate that some plankton groups, notably dinoflagellates, may have increased much more. Apart from the terrestrial and surface water contributions to the sedimentary organic matter, anomalous distributions and preservation of some C23-C27 alkanes, alkanols and alkanoic acids have been observed, which are interpreted as a contribution by organisms living in situ. Comparison of the unoxidised S1 sapropel with the overlying oxidised sapropel and the organic matter concentration profiles in the oxidised postsapropelic sediments demonstrates strong and highly selective aerobic degradation of lipids and palynomorphs. There seems to be a fundamental difference in degradation kinetics between lipids and pollen which may be possibly related with the absence of sorptive preservation as a protective mechanism for palynomorph degradation. The n-alkanes, Impagidinium, and Nematosphaeropsis are clearly more resistant than TOC. The n-alkanols and n-carboxylic acids are about equally resistant whereas the pollen, all other dinoflagellate cysts and other lipids appear to degrade considerably faster, which questions the practice of normalising to TOC without taking diagenesis into account. Selective degradation also modifies the relative distributions within lipid classes, whereby the longer-chain alkanes, alcohols and fatty acids disappear faster than their shorter-chain equivalents. Accordingly, interpretation of lipid and palynomorph assemblages in terms of pre- or syndepositional environmental change should be done carefully when proper knowledge of the postdepositional preservation history is absent. Two lipid-based preservation proxies are tested the diol-keto-ol oxidation index based on the 1,15C30 diol and keto-ols (DOXI) and the alcohol preservation index (API) whereby the former seems to be the most promising.

Chemical composition of bulk sediments from ODP Hole 199-1221C

A record of inorganic geochemical variability was produced from a contiguous sequence of 35 samples, with 1 cm spacing, recovered from Hole 1221C. This record covers from 153.91 to 154.27 meters below seafloor and spans the Carbon Isotope Excursion (CIE) associated with the Paleocene/Eocene boundary interval. Elemental concentrations were determined for Al, As, Ba, Ca, Fe, K, Mg, Mn, P, Si, Sr, Ti, Cd, Co, Cr, Cu, Hf, Mo, Nb, Ni, Pb, Pt, Re, Sc, V, Y, Zn, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Most concentration profiles exhibit a marked peak coincident with or just prior to the CIE. In addition, the rare earth element pattern exhibits a significant flattening of the typical, prominent negative Ce anomaly across the same interval.

Chemical and Sr isotopic compositions of pore fluids and marine carbonates from ODP Hole 130-807A

The 87Sr/86Sr ratios and Sr concentrations in sediment and pore fluids are used to evaluate the rates of calcite recrystallization at ODP Site 807A on the Ontong Java Plateau, an 800-meter thick section of carbonate ooze and chalk. A numerical model is used to evaluate the pore fluid chemistry and Sr isotopes in an accumulating section. The deduced calcite recrystallization rate is 2% per million years (%/Myr) near the top of the section and decreases systematically in older parts of the section such that the rate is close to 0.1/age (in years). The deduced recrystallization rates have important implications for the interpretation of Ca and Mg concentration profiles in the pore fluids. The effect of calcite recrystallization on pore fluid chemistry is described by the reaction length, L, which varies by element, and depends on the concentration in pore fluid and solid. When L is small compared to the thickness of the sedimentary section, the pore fluid concentration is controlled by equilibrium or steady-state exchange with the solid phase, except within a distance L of the sediment-water interface. When L is large relative to the thickness of sediment, the pore fluid concentration is mostly controlled by the boundary conditions and diffusion. The values of L for Ca, Sr, and Mg are of order 15, 150, and 1500 meters, respectively. L_Sr is derived from isotopic data and modeling, and allows us to infer the values of L_Ca and L_Mg. The small value for L_Ca indicates that pore fluid Ca concentrations, which gradually increase down section, must be equilibrium values that are maintained by solution-precipitation exchange with calcite and do not reflect Ca sources within or below the sediment column. The pore fluid Ca measurements and measured alkalinity allow us to calculate the in situ pH in the pore fluids, which decreases from 7.6 near the sediment-water interface to 7.1+/-0.1 at 400-800 mbsf. While the calculated pH values are in agreement with some of the values measured during ODP Leg 130, most of the measurements are artifacts. The large value for L_Mg indicates that the pore fluid Mg concentrations at 807A are not controlled by calcite-fluid equilibrium but instead are determined by the changing Mg concentration of seawater during deposition, modified by aqueous diffusion in the pore fluids. We use the pore fluid Mg concentration profile at Site 807A to retrieve a global record for seawater Mg over the past 35 Myr, which shows that seawater Mg has increased rapidly over the past 10 Myr, rather than gradually over the past 60 Myr. This observation suggests that the Cenozoic rise in seawater Mg is controlled by continental weathering inputs rather than by exchange with oceanic crust. The relationship determined between reaction rate and age in silicates and carbonates is strikingly similar, which suggests that reaction affinity is not the primary determinant of silicate dissolution rates in nature.

Pore water chemistry of ODP Sites 201-1225, 201-1226 and 201-1231

High-resolution analyses of the oxygen isotope ratio (18O/16O) of dissolved sulfate in pore waters have been made to depths of >400 meters below seafloor (mbsf) at open-ocean and upwelling sites in the eastern equatorial Pacific Ocean. d18O values of dissolved sulfate (d18O-SO4) at the organic-poor open-ocean Site 1231 gave compositions close to modern seawater (+9.5 per mil vs. Vienna-standard mean ocean water, providing no chemical or isotopic evidence for microbial sulfate reduction (MSR). In contrast, the maximum d18O values at Sites 1225 and 1226, which contain higher organic matter contents, are +20 per mil and +28 per mil, respectively. Depth-correlative trends of increasing d18O-SO4, alkalinity, and ammonium and the presence of sulfide indicate significant oxidation of sedimentary organic matter by sulfate-reducing microbial populations at these sites. Although sulfate concentration profiles at Sites 1225 and 1231 both show similarly flat trends without significant net MSR, d18O-SO4 values at Site 1225 reveal the presence of significant microbial sulfur-cycling activity, which contrasts to Site 1231. This activity may include contributions from several processes, including enzyme-catalyzed equilibration between oxygen in sulfate and water superimposed upon bacterial sulfate reduction, which would tend to shift d18O-SO4 toward higher values than MSR alone, and sulfide oxidation, possibly coupled to reduction of Fe and Mn oxides and/or bacterial disproportionation of sulfur intermediates. Large isotope enrichment factors observed at Sites 1225 and 1226 (epsilon values between 42 per mil and 79 per mil) likely reflect concurrent processes of kinetic isotope fractionation, equilibrium fractionation between sulfate and water, and sulfide oxidation at low rates of sulfate reduction. The oxygen isotope ratio of dissolved pore water sulfate is a powerful tool for tracing microbial activity and sulfur cycling by the deep biosphere of deep-sea sediments.

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.

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.

Interstitial water chemistry at DSDP Leg 79 Holes

Interstitial water analyses of samples collected at Sites 544-547 of DSDP Leg 79 are presented. In Site 547 chloride concentrations increase to almost 80% of the halite saturation values. Gypsum occurrences in the sediments immediately overlying the halite deposit can be explained in terms of migration of Ca**2+ and SO2**2- from the underlying evaporites. At shallower depths sulfate concentrations decrease rapidly as a result of sulfate reduction processes. The same processes lead to the removal of calcium in the form of calcium carbonate. At Site 547, the chloride concentration depth profile suggests a maximum of dissolved chloride which may be the result of advective flow from nearby (abput 6 km) evaporite salt diapirs.