World9s Oceans Research Articles

Dynamic deviation of fluid pressure from hydrostatic pressure in turbidity currents

Turbidity currents are large sediment-carrying submarine avalanches that flow over the continental slope and through submarine channels to the deeps of the world9s oceans. For more than 50 years researchers have extended the hydrostatic pressure simplification, which is valid in most hydraulic analyses of flow in rivers and channels, to analyses of turbidity currents. In this paper we present the first measurements of fluid pressure beneath experimental turbidity currents. Basal fluid pressures are much lower (30%–45%) than those predicted from bulk flow density, in proportion to the kinematic energy density of the flow, a parameter usually referred to as the dynamic pressure. This invalidates the commonplace assumption of a hydrostatic pressure regime in turbidity currents, and necessitates the incorporation of dynamic fluid pressure as a physical parameter in the analysis of turbidity currents. The contrast between turbidity currents and free surface flows is caused by the difference in their density structure. We highlight current enigmas and gaps in our knowledge of submarine currents, for which our result opens new opportunities of investigation.

Milankovitch-scale correlations between deeply buried microbial populations and biogenic ooze lithology

The recent discoveries of large, active populations of microbes in the subseafloor of the world9s oceans supports the impact of the deep biosphere biota on global biogeochemical cycles and raises important questions concerning the functioning of these extreme environments for life. These investigations demonstrated that subseafloor microbes are unevenly distributed and that cell abundances and metabolic activities are often independent from sediment depths, with increased prokaryotic activity at geochemical and/or sedimentary interfaces. In this study we demonstrate that microbial populations vary at the scale of individual beds in the biogenic oozes of a drill site in the eastern equatorial Pacific (Ocean Drilling Program Leg 201, Site 1226). We relate bedding-scale changes in biogenic ooze sediment composition to organic carbon (OC) and microbial cell concentrations using high-resolution color reflectance data as proxy for lithology. Our analyses demonstrate that microbial concentrations are an order of magnitude higher in the more organic-rich diatom oozes than in the nannofossil oozes. The variations mimic small-scale variations in diatom abundance and OC, indicating that the modern distribution of microbial biomass is ultimately controlled by Milankovitch-frequency variations in past oceanographic conditions.

Warming the World's Oceans

Several recent studies have provided evidence for a warming of the world9s oceans. In their Perspective, Hegerl and Bindoff highlight the report by Barnett et al., whose study substantially strengthens the evidence that human activities are responsible for the observed ocean warming. The observed space-time pattern of ocean warming with depth can only be explained if greenhouse gas forcing is considered in simulations of ocean climate change. The study has important implications for validating model simulations and hence for projections of future warming. Further study of the world9s oceans is still needed to better understand ocean variability, to quantify the likelihood of sudden ocean change, and to predict future changes in ocean biogeochemistry and ecosystems.

Whole flow field dynamics and velocity pulsing within natural sediment-laden underflows

Sediment-laden density underflows are important agents of erosion and deposition and are especially significant in the management of human-made reservoirs, pollutant dispersal, and sediment deposition in the world9s oceans. Quantification of continuous, sediment-laden underflows in Lillooet Lake, British Columbia, shows that the underflows descend along a distinct plunge line but, although the input from the source is constant, adopt a distinct pulsing in their velocity structure. Such velocity pulsing will produce temporally and spatially varying bed shear stresses, sediment erosion and/or deposition, and fluid mixing, and represents a central property of underflows that must be incorporated into models of density current behavior.

Productivity cycles of 200–300 years in the Antarctic Peninsula region: Understanding linkages among the sun, atmosphere, oceans, sea ice, and biota

Compared to the rest of the world9s oceans, high-resolution late Holocene paleoclimatic data from the Southern Ocean are still rare. We present a multiproxy record from a sediment core retrieved from a deep basin on the western side of the Antarctic Peninsula that reveals a dramatic perspective on paleoclimatic changes over the past 3700 yr. Analyses completed include measurement of magnetic susceptibility and granulometry, bed thickness, particle size, percent organic carbon, bulk density, and microscopic evaluation of diatom and benthic foraminiferal assemblages and abundances. Downcore variability of these parameters demonstrates the significance of both short-term cycles, which recur approximately every 200 yr, and longer term events (≈2500 yr cycles) that are most likely related to global climatic fluctuations. In the upper 600 cm of the core, lower values of magnetic susceptibility (MS) are correlated with lower bulk density, the presence of thinly laminated units, specific diatom assemblages, and generally higher total organic carbon content. Below 600 cm, magnetic susceptibility is uniformly low, though variability in other parameters continues. The magnetic susceptibility signal is controlled primarily by dilution of ferromagnetic phases with biosiliceous material. This signal may be enhanced further by dissolution of magnetite in the magnetic susceptibility lows (high total organic carbon). The role of variable primary productivity and its relationship to paleoclimate is assessed through the diatom data. In particular, magnetic susceptibility lows are characterized by higher than normal abundances of Chaetoceros resting spores. Corethron criophilum and/or Rhizosolenia spp. also are found, as is a higher ratio of the most common species of Fragilariopsis versus species of Thalassiosira . These assemblages are indicative of periods of high primary productivity driven by the presence of a meltwater stabilized water column. The 200 yr cyclicity noted in other paleoclimatic records around the world suggests a global forcing mechanism, possibly solar variability. In addition to the cyclic changes in productivity, overall elevated productivity is noted below 600 cm, or prior to ca. 2500 yr B.P. This increased productivity may represent the tail end of a Holocene climatic optimum, which is widely recognized in other parts of the world, but as yet is poorly documented in Antarctica.

Construction of a polynomial model of glacio-eustatic fluctuation: Estimating paleo-sea levels continuously through time

Sea-level curves for the late Quaternary are typically reconstructed with data from reef tracts and terraces preserved along actively uplifting coastlines, recording highstands, and in some cases lowstands, of the world9s oceans through time. As a result, sea-level curves reconstructed from these data are fixed at the maxima (highstands) and minima (lowstands), but either make no predictions about the intervening time or use arbitrary or free-hand methods of interpolation. These curves fail to meet the need of many applications of sea-level data for continuous, quantitative information. A new method for calculating sea levels during intervening times produces a polynomial expression of sea level through time. LaGrange polynomial interpolation is used to fit a series of fourth-order equations to existing data of late Quaternary sea-level fluctuations, producing a systematic and realistic model of continuous sea-level elevations with time. The method is germane to any and all models of sea level based on discrete data points. Here the model advanced by Bloom et a1. is reexamined, and a new polynomial model which estimates eustatic sea levels continuously in the range 0-140 ka is developed from the Huon data. The method includes an error-analysis procedure that accounts for the effects of error in radiometric dates and theodolite measurements in the original data set, and extrapolates this error through the continuous model. Polynomial form and error estimation are indispensible to rigorous quantitative applications of sea-level data and supplant less-rigorous methods used to approximate sea level and rates of sea-level rise.

Late Quaternary deposition of ice-rafted sand in the subpolar North Atlantic (lat 40° to 65°N)

A major change in the North Atlantic pattern of ice-rafting deposition, during the last interglacial-glacial cycle, occurred approximately 75,000 B.P. Prior to this time, deposition for a period of almost 50,000 yr during isotopic stage 5 was greatest in the northwest near Greenland and Newfoundland. The main glacial pattern was very different; the main depositional axis shifted abruptly to a zonal axis along lat 46° to 50°N, reflecting the passage of ice farther from the pole before reaching water warm enough in which to melt. This pattern remained essentially unchanged for 65,000 yr during the main Wurm glaciation. The peak interglacial depositional pattern can best be explained by analogy with the modern oceanic flow, except for the addition of a concentrated eastward component along lat 50°N. The glacial pattern is also best explained by counterclockwise flow. Laurentide and Greenlandic ice entering the western North Atlantic from the Labrador Sea moved to the east and southeast directly into the glacial depositional maximum. Scandinavian ice dropped part of its bed load near Norway, looped to the southwest into the North Atlantic in a counterclockwise passage south of Iceland, and finally melted along the primary depositional maximum. Total input rates of ice-rafted sediment to the Atlantic and Norwegian Sea increased slightly at 115,000 B.P. (the glacial inception), rose markedly at 75,000 B.P. (the major glacial transition), and continued to rise late in the Wurm toward the late-glacial maximum. North Atlantic ice-rafting deposition is thus positively correlated with ice-sheet size. During Quaternary time, roughly 70% of ice-rafted deposition of continental detritus in the world9s oceans has occurred in the subpolar North Atlantic south of Iceland. During the past 3 m.y., a mass of wet unconsolidated drift estimated at 200,000 km 3 has been moved from the continents to the deep Atlantic by ice-rafting alone. This is equivalent to a layer of drift 16 m thick over all parts of the continents thought to have supplied ice-rafted detritus to the North Atlantic.

Transport phases of transition metals in the Amazon and Yukon Rivers

Samples representing yearly averages of material transported by the Amazon and Yukon Rivers were analyzed to separate the transition metals (Cr, Mn, Fe, Co, Ni, and Cu) into the following transport phases: (1) crystalline particles, (2) metal hydroxide coatings, (3) solid organic material, (4) sorbed material, and (5) those in solution. The major transport phases are crystalline particles and metal hydroxide coatings, which, combined, carry 65% to 92% of the transition metals transported. Solid organic material, the next most important phase, transports between 5% and 19% of the total transported. Material carried in solution transports 0.6% to 17% of the total transported. Sorbed transition metals account for between 0.02% and 8% of the total transported. Metal hydroxide coatings represent the major transporting mechanism potentially available to organisms, since, for the Amazon and Yukon Rivers, respectively, 87% and 78% of the Fe, 69% and 73% of the Mn, and 71% and 69% of the Ni are transported in this form. Comparing the concentrations of transition metals carried to the oceans with the concentrations on the continents, a high Cu ratio (5 to 7) indicates continental depletion or river output enrichment; a moderate ratio (1.1 to 1.7) for Ni, Co, and Cr indicates intermediate depletion or enrichment, and a near-unity ratio for Fe and Mn indicates little depletion or enrichment. The sediments transport >97% of the total mass of transition metals to the world9s oceans.

Tectonic History of the Fiji Plateau

The Fiji Plateau is a high, hot area of young oceanic crust. It is bounded on the north by a Cretaceous Pacific archipelago, and to the east and west by the Tonga and New Hebrides island arcs which go back to the Eocene. The Fiji Islands are an Eocene and younger continental mass formed within the ocean basin. Plate tectonics provides the key for understanding the area. Marine geological and geophysical data from Scripps Institution of Oceanography expeditions, especially Nova, and published seismic, gravity, and island geologic information provide the basis for the interpretation. Fiji is now flanked by three active sea-floor spreading centers which are part of a very complicated transform linking the Tonga and New Hebrides crustal consumption zones. Extension in the Lau Basin is also taking place. Magnetic anomalies and seismicity permit six small blocks and the large Pacific and Australian plates to be distinguished, and some idea of their relative motions to be gained. From published magnetic anomaly and fracture zone data, a detailed history of the Tertiary motions of the Pacific and Australian plates with respect to Antarctica has been deduced. By a suitable choice of plate boundaries, horizontal movements of the larger tectonic units of the Fiji Plateau region can be worked out for the entire Tertiary. This reconstruction successfully accounts for many hitherto unexplained bathymetric and geologic features of the area. The history proposed for the Fiji area is probably unique among the world9s oceans.