The continental shelf benthic iron flux and its isotope composition

Benthic iron fluxes from sites along the Oregon-California continental shelf determined using in situ benthic chambers, range from less than 10 μmol m⁻² d⁻¹ to values in excess of ∼300 μmol m⁻² d⁻¹. These fluxes are generally greater than previously published iron fluxes for continental shelves contiguous with the open ocean (as opposed to marginal seas, bays, or estuaries) with the highest fluxes measured in the regions around the high-sediment discharge Eel River and the Umpqua River. These benthic iron fluxes do not covary with organic carbon oxidation rates in any systematic fashion, but rather seem to respond to variations in bottom water oxygen and benthic oxygen demand. We hypothesize that the highest rates of benthic iron efflux are driven, in part, by the greater availability of reactive iron deposited along these river systems as compared to other more typical continental margin settings. Bioirrigation likely plays an important role in the benthic Fe flux in these systems as well. However, the influence of bottom water oxygen concentrations on the iron flux is significant, and there appears to be a threshold in dissolved oxygen (∼60-80 μM), below which sediment-ocean iron exchange is enhanced. The isotope composition of this shelf-derived benthic iron is enriched in the lighter isotopes, and appears to change by ∼3‰ (δ⁵⁶Fe) during the course of a benthic chamber experiment with a mean isotope composition of −2.7 ± 1.1‰ (2 SD, n = 9) by the end of the experiment. This average value is slightly heavier than those from two high benthic Fe flux restricted basins from the California Borderland region where δ⁵⁶Fe is −3.4 ± 0.4‰ (2 SD, n = 3). These light iron isotope compositions support previous ideas, based on sediment porewater analyses, suggesting that sedimentary iron reduction fractionates iron isotopes and produces an isotopically light iron pool that is transferred to the ocean water column. In sum, our data suggest that continental shelves may export a higher efflux of iron than previously hypothesized, with the likelihood that along river-dominated margins, the benthic iron flux could well be orders of magnitude larger than non-river dominated shelves. The close proximity of the continental shelf benthos to the productive surface ocean means that this flux is likely to be essential for maintaining ecosystem micronutrient supply.     

Seasonal and tidal monthly patterns of particulate matter dynamics in the Columbia River estuary

We investigated seasonal and tidal-monthly, suspended particulate matter (SPM) dynamics in the Columbia River estuary from May to December 1997 using acoustic backscatter (ABS) and velocity data from four long-term Acoustic Doppler Profiler (ADP) moorings in or near the estuarine turbidity maximum (ETM). ABS profiles were calibrated and converted to total SPM profiles using pumped SPM samples and optical backscatter (OBS) data obtained during three seasonal cruises. Four characteristic settling velocity (W _s) classes were defined from Owen Tube samples collected during the cruises. An inverse analysis, in the form of a non-negative least squares minimization, was used to determine the contribution of the four,W _s-classes to each, total SPM profile. The outputs from the inverse analyses were 6–8 mo time-series ofW _s-specific SPM concentration and transport profiles at each mooring. The profiles extended from the free surface to 1.8–2.7 m from the bed, with 0.25–0.50 m resolution. These time series, along with Owen Tube results and disaggregated size data, were used to investigate SPM dynamics. Three non-dimensional parameters were defined to investigate how river flow and tidal forcing affect particle trapping: Rouse numberP (balance between vertical mixing and settling) trapping efficiencyE (ratio of maximum SPM concentration in the estuary to fluvial source concentration), and advection numberA (ratio of height of maximum SPM concentration to friction velocity). The most effective particle trapping (maximum values ofE) occurs on low-flow neap tides. The location of the ETM and the maximal trapping migrated seasonally in a manner consistent with the increase in salinity intrusion length after the spring freshet. Maximum advection (high values ofA) occurred during highly stratified neap tides.     

An experimental study of alteration of oceanic crust and terrigenous sediments at moderate temperatures (51 to 350°C): insights as to chemical processes in near-shore ridge-flank hydrothermal systems

Studies of hydrothermal circulation within partly buried basement on the eastern flank of the Juan de Fuca Ridge (JFR) have shown that ridge-flank geochemical fluxes are potentially important for the global budgets of some elements. There are major uncertainties in these flux calculations, however, because the composition of these basement fluids is strongly dependent on temperature and because they may be modified by interaction with the overlying terrigenous sediments, either by diffusive exchange with basement or during upwelling to the seafloor. To better understand the nature and temperature control of basalt-fluid and sediment-fluid reactions at the JFR flank, we have conducted laboratory experiments between 51 and 350°C and at 400 bars pressure. K, Rb, and Si are leached from basalt between 150 and 351°C, and Sr and U are taken up. The direction of exchange of Li and Ca with basalt varies with temperature. Li and Sr are removed from fluid at 150°C, but isotope studies show that there is simultaneous release of both elements from basalt, indicating that uptake is controlled by the formation of secondary minerals. Moreover, our experiments confirm that Sr isotope exchange with oceanic crust occurs at moderate temperature and is not confined to high-temperature axial hydrothermal systems. Our data and field data from the JDR flank indicate that uptake of U into basalt at moderate temperature could remove between 9.9 and 15 × 10⁶ mol U yr⁻¹ from the oceans. This is higher than a recent estimate based on measurements of U in altered ocean crust (5.7 ± 3.3 × 10⁶ mol yr⁻¹), which concords with arguments that the Δ_element/heat ratios of JDR flank fluids are too large to be representative of average global flank fluids. K, Ca, Sr, Ba, Li, Si, and B are leached from terrigenous sediments between 51 and 350°C, and U is taken up. Cs and Rb are removed from the fluid below 100°C and leached from the sediment at higher temperature. Sr isotope data show that Sr is preferentially mobilised from volcanic components within terrigenous sediments, which may lead to an overestimation of the ridge-flank Sr isotope flux at the JDR if there is exchange of sediment pore fluids with basement.     

Paleomagnetic evidence of northward movement of the Pacific plate in deep-sea cores from the Central Pacific Basin

Seven deep-sea sediment cores recovered in the central equatorial Pacific collectively span a magneto- and biostratigraphically determined age interval ranging from about 0.1 to 21 m.y. B.P. Measured values of paleomagnetic inclination and their systematic variation with depth in these cores denote relative motion between the central Pacific lithosphere and the magnetic field of the earth. Assuming that the position of the earth's dipole field remained essentially parallel to the present spin axis during the interval, the data provide evidence of a marked decrease in the northward rate of plate motion from about 11 cm/yr to about 6 cm/yr at approximately 12 m.y. B.P. This date of change of motion as well as the northward direction and overall average rate of about 8 cm/yr throughout the last 21 m.y., agree reasonably well with results of other studies of the tectonic history of the Pacific plate and ridge system. More significantly, however, these preliminary results demonstrate the usefulness of the paleomagnetic record in deep-sea sediment cores spanning sufficiently long intervals of time as an aid in reconstructing plate motions.     

Spinelloid phases in the nickel gallosilicate system

The phase diagram of the NiO-Ga₂O₃- SiO₂ system has been investigated at high temperatures (up to 1550° C) and 1 atm. pressure. The only ternary phases observed in this system are the spinelloid phases I, II, and V, forming on the NiGa₂O₄-Ni₂SiO₄ pseudo-binary join at temperatures above 1400° C. Our results show that phase V is stable at higher temperature than phase I, while phase II may be metastable. The stability of phases I and V has been confirmed by the successful growth of single crystals from a silica-rich flux. Phases I, II, and V have been characterized by powder X-ray diffraction and transmission electron microscopy. The dimensions of their orthorhombic unit-cells are: (I) a= 5.7741(5), b=11.712(1), c=8.2387(9) Å; (II) a= 5.765(1), b= 17.619(3), c= 8.238(2) Å; (V) a= 5.7914(4), b=8.7809(7), c=8.2346(6) Å. High resolution electron microscopy has also revealed the formation of microscopic intergrowths similar to those previously observed for the spinelloid phases of the NiO-Al₂O₃-SiO₂ system.