Sio Guyot: A complex volcanic edifice in the western Mid-Pacific Mountains

Sio Guyot, in the westernmost edge of the Mid-Pacific Mountains, is a large, complex volcanic edifice rising to more than 1200 m below sea level. The summit is divided into two flat-topped areas by a WNW-trending sediment-filled trough. Seismic reflection profiles reveal three acoustic units: an upper transparent layer (pelagic cap), a lower opaque layer (reef- and lagoon-derived sediments), and an acoustic (volcanic) basement. Free-air gravity anomalies indicate three eruptive centers or conduits within the main edifice, which apparently was constructed during late Cretaceous time on a broad basement swell or plateau that today is more than 3400 m below sea level (1500 m above regional abyssal depths).     

Using ocean ambient noise for array self-localization and self-synchronization

Estimates of the travel times between the elements of a bottom hydrophone array can be extracted from the time-averaged ambient noise cross-correlation function (NCF). This is confirmed using 11-min-long data blocks of ambient noise recordings that were collected in May 1995 near the southern California coast at an average depth of 21 m in the 150-700 Hz frequency range. Coherent horizontal wavefronts emerging from the time derivative of the NCF are obtained across the array's aperture and are related to the direct arrival time of the time-domain Green's function (TDGF). These coherent wavefronts are used for array element self-localization (AESL) and array element self-synchronization (AESS). The estimated array element locations are used to beamform on a towed source.     

Null broadening with snapshot-deficient covariance matrices in passive sonar

Adaptive-array beamforming achieves high resolution and sidelobe suppression by producing sharp s in the adaptive beampattern. Large-aperture sonar arrays with many elements have small resolution cells; interferers may move through many resolution cells in the time required for accumulating a full-rank sample covariance matrix. This leads to "snapshot-deficient" processing. In this paper, the -broadening technique originally developed for an ideal stationary problem is extended to the snapshot-deficient problem combined with white-noise constraint (WNC) adaptive processing. Null broadening allows the strong interferers to move through resolution cells and increases the number of degrees of freedom, thereby improving the detection of weak stationary signals.     

An ecosystem model with iron limitation of primary production in the equatorial Pacific at 140°W

We construct a one-dimensional ecosystem model (nitrate, ammonium, phytoplankton, zooplnakton and detritus) with simple physics and biology in order to focus on the structural relations and intrinsic properties of the food web that characterizes the biological regime in the central equatorial Pacific at 140°W. When possible, data collected during the EgPac and other cruises were used to calibrate model parameters for two simulations that differ in the limiting nutrient, i.e. nitrogen or iron. Both simulations show annual results in good agreement with the data, but phytoplankton biomass and primary production show a more pronounced annual variability when iron is used as the limiting nutrient. This more realistically reproduces the variability of biological production and illustrates the greater coupling between vertical physical processes and biological production when the limiting nutrient is iron rather than nitrogen. The iron simulation also illustrates how iron supply controls primary production variability, how grazingbalances primary production and controls phytoplankton biomass, and how both iron supply and grazingcontrol primary production. These results suggest that it is not possible to capture primary production variability in the central equatorial Pacific with biological models using nitrogen as the limiting nutrient. Other indirect results of this modeling study were: (1) partitioning of export production between dissolved and particulate matter is almost equal, suggesting that the importance of DOC export may have been previously overestimated; (2) lateral export of live biomass has to be taken into account in order to balance the nitrogen budget on the equator at 140°W; and (3) preferential uptake of ammonium (i.e. nitrate uptake inhibition by ammonium) associated with high regeneration of nitrogen (low f ratio as a consequence of the food web structure imposed by iron limitation) largely accounts for the surface build-up of upwelled nitrate.     

Chemical oceanography, vol. 8: Edited by J.P. Riley and R. Chester. Academic Press, London, 1983, xvi + 398 pp., £45.00/US $75.00 (hardcover)

Representative profiles of inorganic nitrogenous species dissolved in interstitial waters of coral reef sands are presented. Ammonium is the dominant nitrogenous species in these pore waters with concentrations of up to 40 μm. Nitrate is present but in lower concentrations. Nitrite is found only occasionally in trace amounts. Computations of diffusive fluxes and inferences concerning microbial activity are derived from the profile structures. Computed flux rates of nitrogenous species from the sediment to the water column range between 0.75 and 1.37 μM m^?1 h^?1. These inputs may represent a significant source of recycled nitrogen to the primary producers of the coral reef ecosystem.     

Reconstruction of ancient sea conditions with an example from the Swiss Molasse

Ancient sea conditions can be estimated from the grain size, spacing and steepness of preserved ripple-marks. The element of greatest uncertainty in such reconstructions is the relationship between near-bed orbital diameter of water particles and the ripple spacing. This relationship is simple for vortex ripples of high steepness but is problematical for the low-steepness forms known as post-vortex, rolling-grain or anorbital ripples.

The existence field for wave ripples is between the threshold velocity for sediment movement and the onset of sheet flow, most low-steepness forms occurring close to the bed planation threshold. A range of maximum period of formative waves can be obtained using combinations of orbital diameter and orbital velocity, assuming linear wave theory to be a reasonable approximation.

Probable wave heights, wave lengths and water depths can be investigated using the transformation of wave parameters in shallowing waters and the constraints on wave dimensions provided by the wave-breaking condition. Given reasonable estimates of wave height, crude estimates of wave power allow a comparison of ancient wave-influenced sequences with modern counterparts.

Wave ripple-marks preserved in the Upper Marine Molasse of western Switzerland have been investigated. Results, which are in agreement with regional geology, suggest deposition in a seaway of approximately 100 km width, where moderate period waves (T = 3–6 s) were generated. The depositional facies belts were adjusted to the prevailing waves, tides and fluvial outflows.     

A community-wide intercomparison exercise for the determination of dissolved iron in seawater

The first large-scale international intercomparison of analytical methods for the determination of dissolved iron in seawater was carried out between October 2000 and December 2002. The exercise was conducted as a rigorously “blind” comparison of 7 analytical techniques by 24 international laboratories. The comparison was based on a large volume (700 L), filtered surface seawater sample collected from the South Atlantic Ocean (the “IRONAGES” sample), which was acidified, mixed and bottled at sea. Two 1-L sample bottles were sent to each participant. Integrity and blindness were achieved by having the experiment designed and carried out by a small team, and overseen by an independent data manager. Storage, homogeneity and time-series stability experiments conducted over 2.5 years showed that inter-bottle variability of the IRONAGES sample was good (< 7%), although there was a decrease in iron concentration in the bottles over time (0.8–0.5 nM) before a stable value was observed. This raises questions over the suitability of sample acidification and storage.     

Distribution of As, Cr and V species in the Po-Adriatic mixing area, (Italy)

The distribution of dissolved and participate concentrations of As, Cr and V was investigated in the mixing area of the Po river in five surveys over the period March 1992-March 1995. The data obtained indicate noticeable contamination of coastal waters of the northern Adriatic with Cr but not As or V. Total dissolved concentrations for As and V were comparable with oceanic levels but were higher by a factor of 4 for Cr. These results suggest either low anthropogenic inputs of As and V, from terrigenous and atmospheric sources, or efficient removal mechanisms. The investigated metals had low reactivity during freshwater-seawater mixing and the distribution of concentrations in the dissolved and solid phases suggests that the dilution process is the main factor controlling the transport of these metals to the sea in low-to-medium solid transport conditions. Further investigations are required to elucidate the behavior of these metals during high solid transport conditions. Significant changes in the speciation of As occurred during the transition between the two end-member matrices. In the coastal waters of the northern Adriatic, the arsenite fraction in winter, and the arsenite and dimethylarsinate fractions in summer increased with respect to riverine waters. Changes in the speciation of chromium were minor, but also showed a small increase in reduced species in marine waters.