Gary P. Klinkhammer

Evidence of a dissolution effect on benthic foraminiferal shell chemistry: δ13C, Cd/Ca, Ba/Ca, and Sr/Ca results from the Ontong Java Plateau

Core-top benthic foraminifera (Cibicidoides wuellerstorfi) from a depth transect of Soutar box cores from the Ontong-Java Plateau (1.6–4.4 km) were analyzed for cadmium, barium, and strontium (Cd/Ca, Ba/Ca, and Sr/Ca) and for their stable isotopic composition (δ13C and δ18O). We also measured bottom water δ13C, Cd, and Ba at these sites. Foraminiferal δ13C values remain roughly constant over the entire depth range while bottom water δ13C values increase slightly, such that the δ13C difference between C. wuellerstorfi and bottom water ranges from about +0.2 ‰ in cores above 2.5 km to about −0.2 ‰ in cores below 4 km. This apparent depth dependence has not been previously reported, but this range in Δδ13C values is comparable to the uncertainty in published δ13C calibration studies. We observe strong decreases in foraminiferal Cd/Ca, Ba/Ca, and Sr/Ca ratios (50, 25, and 15 percent, respectively) at water depths greater than about 2.5 km. These decreases are substantially larger than the corresponding changes in bottom water trace element concentrations, and they are not correlated with variations in pore water Cd and Ba concentrations at these sites. Together, the foraminiferal and bottom water Cd/Ca, Ba/Ca, and Sr/Ca data yield decreases in the apparent distribution coefficients for these metals into calcite with increasing water depth, again a pattern which has not been previously reported. These results when combined with the data from published core-top calibration studies suggest that a preferential loss of Cd, Ba, and Sr occurs during the dissolution of benthic foraminiferal calcite on the sea floor and raise the possibility of a dissolution-driven decrease in benthic foraminiferal δ13C values.

DETERMINATION OF THE RARE EARTH ELEMENTS IN NATURAL WATERS BY ISOTOPE-DILUTION MASS SPECTROMETRY

A method is described for the determination of ten rare earth elements (La, Ce, Nd, Sm, Eu, Gd, Dy, Er, Yb, Lu) in natural waters by isotope-dilution mass spectrometry. A 1-l sample is used for sea water, and proportionately less for other natural waters. The rare earth elements are extracted by co-precipitation with hydrated iron (III) oxide and purified on a single cation-exchange column, with hydrochloric and nitric acids as eluents. Final measurements are from a triple Re/ Ta filament in the mass spectrometer, run automatically under computer control. Relative standard deviations are better than 4% for the analysis of standard solutions, with accuracy in the same range. The analytical blank is low ( <0.03 pmol, 4 pg, for Nd) producing a sample/blank concentration ratio greater than 100 for all ten rare earth elements when determined in a 1-l seawater sample. Concentration depth profiles are given for an ocean water and normalised abundance patterns for three natural waters.

Dispersal patterns for hydrothermal plumes in the South Pacific using manganese as a tracer

Abstract Manganese profiles over the spreading axis of the East Pacific Rise (EPR) between 22°15′S and 19°25′S disclose a series of plumes which are formed by buoyant emanations of hot, hydrothermal vent fluids. The plume ceilings, or anomaly maxima, can occur several hundred meters above the sea floor. These plumes are dispersed laterally and are detectable as mid-depth manganese anomalies 2000 km from the spreading axis. Along this section, the plumes are dispersed mainly westward but at 30°S the plumes are advected eastward. The asymmetrical dispersal pattern inferred from these data is coincident with the asymmetrical distribution of metalliferous sediments about the EPR which suggests that plumes, generated by high-temperature venting, play a dominant role in the formation of these sediments.

THE BEHAVIOR OF BARIUM IN ANOXIC MARINE WATERS

Abstract The present day distributions of Ba in the water columns at three anoxic marine sites, namely the Cariaco Trench, Framvaren Fjord, and Black Sea, are presented. Dissolved Ba levels generally increase with depth, ranging from 45–85, 64–280, and 180–460 nM in surface and bottom waters for the three basins, respectively. Small maxima are observed in the vicinity of the in redox interface in both the Framvaren Fjord and Black Sea. Comparison of the dissolved and particulate Ba, Fe, and Mn distributions show that the maxima do not result from adsorption onto freshly precipitated Fe and/or Mn oxyhydroxides. As for the open ocean, Ba cycling in all three basins is dominated by its uptake, primarily in the form of barite, into particulate matter associated with productivity in surface waters, followed by its regeneration at depth or in the sediments. Microbiological activity near the redox interface promotes the breakdown of settling particulate matter and the release of barite just above the O 2 H 2 S interface in the Black Sea, and most likely in the Framvaren Fjord, thus providing in part for the observed maxima. Dissolution of such barite in the marginal sediments of these basins probably also contributes to the maxima. Thermodynamic calculations show deep Black Sea Ba concentrations exceed saturation with respect to pure barite by at least a factor of 2. However, the uniformity of the deep water concentrations suggests thermodynamic control by some phase; it is likely that impurities, incorporated into barite during its rapid formation near the surface in microenvironments provided by decaying organisms, are responsible for the levels observed. Additional factors controlling the distributions of Ba in each basin are also discussed.

Manganese geochemistry near high-temperature vents in the Mid-Atlantic Ridge rift valley

In July 1985 the first high-temperature, hydrothermal vents ever discovered on the MAR were found at 26°N (TAG). A transponder-navigated survey of a 4 × 8 km area was begun when on-board measurements of manganese revealed concentrated hydrothermal plumes in the water column comparable in size and intensity to those found over geothermal fields on the EPR. These plumes also possess anomalously high light scattering properties and anomalously warm potential temperatures. Using relay-transponder navigation with a rosette-CTD system, it was possible to sample bottom waters with temperature anomalies of several decidegrees. Samples collected near vent orifices possessed silica anomalies of up to 17 μM and dissolved manganese anomalies as large as 360 nmol/kg. These manganese and silica anomalies are linearly related and predict aMn/Si ratio of 20 mmol/mol for these MAR vent fluids, a value essentially identical to that measured in 350°C fluid from the EPR at 21°N. The distribution of temperature anomalies in the water column over this area requires the presence of more than one active vent site. A rough estimate of the convective heat loss from this section of the MAR based onθ-z profiles is 1.2 × 108 cal/s which is similar to the heat loss estimated for the vent field on the EPR at 21°N.

Determination of barium in marine waters by isotope dilution inductively coupled plasma mass spectrometry

Abstract A simple and accurate method is presented for determining barium in marine waters. Equal amounts of an enriched isotope spike and sample are diluted 100-fold with 1% nitric acid. The diluted sample is introduced into an inductively coupled plasma mass spectrometer with a peristaltic pump and the mass region from 135 to 138 is scanned 480 times. The entire procedure takes less than 10 min per sample and is accurate to better than 1% as determined by intercalibration with isotope dilution mass spectrometry.

Rare earth elements in hydrothermal fluids and plume particulates by inductively coupled plasma mass spectrometry

Abstract This paper describes the measurement of the rare earth elements (REE) in fluids from deep-sea hydrothermal vents and in the particles that form as these fluids mix with sea water. The REE are extracted from fluids by coprecipitation with hydrated iron (III). The REE are isolated from other elements on miniature cation exchange columns before being measured by inductively coupled plasma mass spectrometry (ICP-MS). Column separations overcome the problem of sensitivity depression caused by high dissolved solids at the orifice of the mass spectrometer and minimize isobaric interferences from barium. Inter-laboratory calibrations with isotope-dilution thermal ionization mass spectrometry (IDTIMS) demonstrate that the ICP-MS results are accurate at the 6% (2σ) level.

Porewater cadmium geochemistry and the porewater cadmium:δ13C relationship

Results from two continental margin cores collected off Pt. Sur, California, make possible a direct determination of the relationship between dissolved cadmium and the carbon isotopic composition of dissolved inorganic carbon in the interstitial water of suboxic marine sediments. The δ13C values of dissolved inorganic carbon decrease nearly 1‰ in the top 0.5 cm of the sediments and display a more gradual decrease through the top 20 cm of the sediments. Dissolved cadmium concentrations increase by 0.6 to 1.5 nmol/kg in the top 0.5 cm of the sediments, and then rapidly decrease to values of approximately 0.12 nmol/kg between 5 and 10 cm into the sediments, apparently as a result of scavenging onto iron oxide surfaces. This pattern contrasts with porewater cadmium profiles previously reported for pelagic sediments, which show no evidence of cadmium scavenging by the sediments. However, porewater cadmium concentrations are poorly correlated with porewater δ13C values in both oxic and suboxic sediments. These results suggest that the cadmiumcalcium ratios and δ13C values of infaunal benthic foraminifera need not be tightly linked, and that the relationship between these two aspects of foraminiferal shell chemistry may vary in response to changes in sediment geochemistry.

Real-time observation of dispersed hydrothermal plumes using nephelometry: examples from the Mid-Atlantic Ridge

Abstract As part of the 1984–1985 NOAA VENTS program on the Mid-Atlantic Ridge, nephelometry was used to provide real-time detection and tracking of dispersed hydrothermal plumes. At all nine 1984 study sites, hydrothermal activity was detected by in-situ, real-time nephelometer measurements and later confirmed by dissolved Mn and particulate Fe measurements. These same techniques were employed in a site-specific survey of the Trans-Atlantic Geotraverse (TAG) area in 1985 where large water-column anomalies in turbidity and in dissolved Mn helped lead to the discovery of high-temperature black smokers. The optical response of the nephelometer was to hydrothermally-derived particulate matter. Thus strong correlations existed between the nephelometer readings and total suspended matter (r = 0.98, n = 34), and particulate Fe (r = 0.88, n = 32). In addition, digital nephelometer data correlated well with dissolved Mn (r = 0.88; n = 78) throughout a large concentration range (0.2–31.0 nmol/kg). These data provide good evidence for the utility of in-situ nephelometer measurements for locating and surveying plumes from hydrothermal vents. It also appears possible, within limits, to predict concentrations of in-situ total suspended matter, of particulate Fe and of dissolved Mn.