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Nineteen uranium-lead zircon ages of lower crustal gabbros from Atlantis Bank, Southwest Indian Ridge, constrain the growth and construction of oceanic crust at this slow-spreading midocean ridge. Approximately 75% of the gabbros accreted within error of the predicted seafloor magnetic age, whereas ~25% are significantly older. These anomalously old samples suggest either spatially varying stochastic intrusion at the ridge axis or, more likely, crystallization of older gabbros at depths of ~5 to 18 kilometers below the base of crust in the cold, axial lithosphere, which were uplifted and intruded by shallow-level magmas during the creation of Atlantis Bank., Slow- and ultraslow-spreading ridges with spreading rates of The SWIR separates the Antarctic and African plates (Fig. 1, inset). Atlantis Bank lie ~100 km south of the SWIR rift valley, [...]

The bounded derivative theory (BDT) for hyperbolic systems with multiple timescales was originally applied to the initialization problem for large-scale shallow-water flows in the midlatitudes and near the equator. Concepts from the theory also have been used to prove the existence of a simple reduced system that accurately describes the dominant component of a midlatitude mesoscale storm forced by cooling and heating. Recently, it has been shown how the latter results can be extended to tropospheric flows near the equator. In all of these cases, only a single type of flow was assumed to exist in the domain of interest in order to better examine the characteristics of that flow. Here it is shown how BDT concepts can be used to understand the dependence of developing mesoscale features on a balanced large-scale background flow. That understanding is then used to develop multiscale initialization constraints for the three-dimensional diabatic equations in any domain on the globe.

A comparison of model predictions of the flow of magma and solid mantle beneath mid-ocean ridges with the chemistry of well located and closely sampled basalts from across the ridge-crest of the rapidly spreading East Pacific Rise at 12 degrees N indicates that the off-axis melts at 12 degrees N seem to be more depleted in incompatible trace elements than the axial melts across a range of elements. This is consistent with convergent melt flow. These findings probably exclude substantial convergent solid flow within the partially molten region.

The chemical and isotopic compositions of oceanic biogenic and authigenic minerals contain invaluable information on the evolution of seawater, hence on the history of interaction between tectonics, climate, ocean circulation, and the evolution of life. Important advances and greater understanding of (a) key minor and trace element cycles with various residence times, (b) isotopic sources and sinks and fractionation behaviors, and (c) potential diagenetic problems, as well as developments in high-precision instrumentation, recently have been achieved. These advances provided new compelling evidence that neither gradualism nor uniformitarianism can explain many of the new important discoveries obtained from the chemistry and isotopic compositions of oceanic minerals. Presently, the best-developed geochemical proxies in biogenic carbonates are 18O/16O and Sr/Ca ratios (possibly Mg/Ca) for temperature; 87Sr/86Sr for input sources, Cd/Ca and Ba/Ca ratios for phosphate and alkalinity concentrations, respectively, thus also for ocean circulation; 13C/12C for ocean productivity; B isotopes for seawater pH;, U, Th isotopes, and 14C for dating; and Sr and Mn concentrations for diagenesis. The oceanic authigenic minerals most widely used for chemical paleoceanography are barite, evaporite sulfates, and hydrogenous ferromanganese nodules. Marine barite is an effective alternative monitor of seawater 87Sr/86Sr, especially where carbonates are diagenetically altered or absent. It also provides a high-resolution record of seawater sulfate S isotopes, (evaporite sulfates only carry an episodic record), with new insights on factors affecting the S and C cycles and atmospheric oxygen. High-resolution studies of Sr, Nd, and Pb isotopes of well-dated ferromanganese nodules contain invaluable records on climate driven changes in oceanic circulation.

Wave data on [H.sub.1/3], [H.sub.max], etc. observed in the sea area around Japan are analyzed to investigate the probability distribution of [H.sub.max]/[H.sub.1/3] and the occurrence properties of giant freak waves with wave heights exceeding 2[H.sub.1/3] and 10 m. The analyzed results show that the occurrence probability of the giant freak waves could be argued generally under the Rayleigh extreme distribution, although it varies from area to area. Four-ninths of the extreme waves having the largest maximum individual wave height [([H.sub.max]).sub.max] at each observation point are occupied by the giant freak waves and moreover, they occur in lower sea states rather than in the severest one.

Fluctuations of the Caspian Sea's mean monthly surface water level for the period of January 1986 to December 1993 were studied. The time series data showed an increasing trend and seasonal variations. Artificial neural network (ANN) and multiplicative autoregressive integrated moving average (ARIMA) modeling were used to predict the time series data. The ANN's input and output consisted of the last 12 months and the current month surface water levels, respectively. The selected ARIMA model required one-month regular differencing, 12-month seasonal differencing, and had a moving average component of lag 12. The ANN and ARIMA predictions for the period of January to December 1993 were very reasonable when compared with the recorded levels. On average, the ANN model underestimated the sea level by three cm, whereas the ARIMA model overestimated it by three cm. The monthly predictions for January to December 1994 presented a continuation of the Caspian Sea water surface level rise that would have various adverse effects or its neighboring countries.

The evolution of biogeochemical cycles are examined to determine the factors that influence total oceanic primary productivity. For the past two million years, the rise in the transportation of organic carbon from surface waters into marine sediments were linked to the decline in atmospheric carbon dioxide concentrations. This indicates that fixed nitrogen is a variable that controls primary productivity on geological timescales.

Laboratory experiments support observation-based theories that mantle plumes interact with mid-ocean ridges beneath the lithosphere. The fluid and essential physical aspects of off-axis mantle plumes were recreated in a simulation to explore plume-ridge channelling and the influence of surface cooling and plume source temperature. A rheological border layer is necessary for the communication between the plume and ridge with high temperatures undermining the interaction by eroding the lithosphere.

A decline in oceanic water-column denitrification occurred during glacial periods, as shown by 15N/14N marine sediment core records, could explain the decrease in atmospheric CO2 concentrations. Oceanic nitrate is a nutrient and so many have stimulated biological growth if in unusual abundance, resulting in a higher demand for atmospheric CO2.

Biogeochemical cycles were reorganized in the late Proterozoic according to an analysis of hydrocarbons from Proterozoic sediments around the globe, indicating an unusually efficient biodegradation mechanism fro algal products. The organic matter floated slowly to the ocean depths and oxygen was largely removed, probably by sulphate-reducing bacteria. The production of fecal matter by newly evolved Cambrian creatures caused rapid sinking of organic matter, allowing oxygen to reach the ocean depths and therefore allowing for the development of sea floor creatures.

In the oligotrophic North Atlantic and North Pacific, ultrafiltration studies show that concentrations of soluble iron and soluble iron-binding organic ligands are much lower than previously presumed 'dissolved' concentrations, which were operationally defined as that passing through a 0.4-micrometer pore filter. Our studies indicate that substantial portions of the previously presumed 'dissolved' iron (and probably also iron-binding ligands) are present in colloidal size range. The soluble iron and iron-binding organic ligands are depleted at the surface and enriched at depth, similar to distributions of major nutrients. By contrast, colloidal iron shows a maximum at the surface and a minimum in the upper nutricline. Our results suggest that 'dissolved' iron may be less bioavailable to phytoplankton than previously thought and that iron removal through colloid aggregation and settling should be considered in models of the oceanic iron cycle., Phytoplankton growth (1, 2) and nitrogen fixation (3) in the ocean are strongly influenced by Fe availability, which in turn is determined by the physicochemical speciation of dissolved Fe (4). [...]

The subseafloor at the mid-ocean ridge is predicted to be an excellent microbial habitat, because there is abundant space, fluid flow, and geochemical energy in the porous, hydrothermally influenced oceanic crust. These characteristics also make it a good analog for potential subsurface extraterrestrial habitats. Subseafloor environments created by the mixing of hot hydrothermal fluids and seawater are predicted to be particularly energy-rich, and hyperthermophilic microorganisms that broadly reflect such predictions are ejected from these systems in low-temperature ([approximately equals] 15 [degrees] C), basalt-hosted diffuse effluents. Seven hyperthermophilic heterotrophs isolated from low-temperature diffuse fluids exiting the basaltic crust in and near two hydrothermal vent fields on the Endeavour Segment, Juan de Fuca Ridge, were compared phylogenetically and physiologically to six similarly enriched hyperthermophiles from samples associated with seafloor metal sulfide structures. The 13 organisms fell into four distinct groups: one group of two organisms corresponding to the genus Pyrococcus and three groups corresponding to the genus Thermococcus. Of these three groups, one was composed solely of sulfide-derived organisms, and the other two related groups were composed of subseafloor organisms. There was no evidence of restricted exchange of organisms between sulfide and subseafloor habitats, and therefore this phylogenetic distinction indicates a selective force operating between the two habitats. Hypotheses regarding the habitat differences were generated through comparison of the physiology of the two groups of hyperthermophiles; some potential differences between these habitats include fluid flow stability, metal ion concentrations, and sources of complex organic matter.

Late Cambrian to early Ordovician trilobites, the family Olenidae, were tolerant of oxygen-poor, sulfur-rich sea floor conditions, and a case is made that they were chemoautotrophic symbionts. Olenids were uniquely adapted to this habitat in the Lower Paleozoic, which was widespread in the Late Cambrian over Scandinavia. This life habit explains distinctive aspects of olenid morphology: wide thoraces and large numbers of thoracic segments, thin cuticle and, in some species, degenerate hypostome, and the occasional development of brood pouches. Geochemical and field evidence is consistent with this interpretation. Olenids occupied their specialized habitat for 60 million years until their extinction at the end of the Ordovician.

Scientific researchers led by Falkowski have discovered that nitrogen is the ultimate controlling nutrient that limits carbon fixation in oceanic plants. To arrive at this conclusion, Falkowski examined the origin of enzyme systems involved in the fixation and denitrification of nitrogen. The low solubility of iron limited the fixation of nitrogen in oceanic waters during a certain period in the Earth's history.

It is possible to prove that large-scale bischofite formation took place during the Messinian salinity crisis by analyzing brines taken from the Discovery Basin in the eastern Mediterranean. It was discovered that the brine is basically a concentrated solution of MgCl2, and it was established that the brine is homogeneous throughout the basin. The chemistry of the brine indicates that there were extreme evaporite conditions in the eastern Mediterranean during the late Messinian.

Data from the Hawaiian Ocean Time Series (HOT) station ALOHA was used to indicate that the structure and productivity of the subtropical North Pacific's pelagic ecosystem had significantly changed. The validity of this assumption is questioned because the ALOHA station may not represent the anticyclonic central gyre. However, the ALOHA station was established as an oligotrophic ocean benchmark by the US Joint Global Ocean Flux Study and no equivalent, synoptic oceanographic data set has been proposed to replace ALOHA's data.

Three separate oceanic studies have established that the operations of the Atlantic Ocean's conveyor belt underwent rapid and recurring changes during the last glaciation. Results also suggest a correlation between changes in the operations of the Atlantic conveyor and climatic changes. The instability and erratic behavior of the Atlantic's conveyor as suggested by the results is consistent with the results achieved by simulations. Possible causes for the discrepancies in the results of the three studies are discussed.

An examination of the properties of updraft and downdraft cores using Electra data from TOGA COARE shows that they have diameters and vertical velocities similar to cores observed over other parts of the tropical and subtropical oceans. As in previous studies, a core is defined as having vertical velocity of the same sign and greater than an absolute value of 1 m [s.sup.-1] for at least 500 m. A requirement that the core contain either cloud or precipitation throughout is added, but this should not affect the results significantly. Since the Electra was equipped with the Ophir III radiometric temperature sensor, it was also possible to make estimates of core buoyancies. As in TAMEX and EMEX, where core temperatures were estimated using the modified side-looking Barnes radiometer on the NOAA P3s, a significant fraction of both updraft and downdraft cores had apparent virtual temperatures greater than their environments. In fact, the average virtual temperature deviation from the environment for downdraft cores was +0.4 K. Sixteen of the strongest downdraft cores were examined, all of which had positive virtual-temperature deviations, to find the source of this surprising result. It is concluded that the downdraft cores are artificially warm because 100% relative humidity was assumed in calculating virtual temperature. However, reducing core mixing ratios to more physically realistic values does not eliminate warm virtual potential temperature downdraft cores, nor does water loading make all cores negatively buoyant. Thus positively buoyant convective downdrafts do exist, though probably in smaller numbers than previously suggested.