Movement Of Deep Water Research Articles

Southern Ocean pinnipeds provide bathymetric insights on the East Antarctic continental shelf

Poor coverage of the Antarctic continental shelf bathymetry impedes understanding the oceanographic processes affecting Antarctica’s role in global climate. Continental shelf bathymetry influences warm modified Circumpolar Deep Water movement onto the shelf, making it an important factor promoting ice shelf melting and influencing the flow of ice shelves into the ocean. Building on previous work using seal dives to redefine bathymetry, our longitudinal study of ocean physics and animal behaviour provided new depth information from over 500,000 individual seal dives on the East Antarctic continental shelf. About 25% of these seal dives were 220 m (sometimes over 1000 m) deeper than the interpolated seafloor from IBCSO V2. Focusing on four well-sampled regions, we show that the bathymetry of 22% to 60% of the sampled area was improved by incorporating seal dive data. This revealed new bathymetric features, including troughs off the Shackleton Ice Shelf and Underwood Glacier and a deep canyon near the Vanderford Glacier. This deep canyon, the Mirounga-Nuyina Canyon, was confirmed by a recent multi-beam echo sounder survey. Further acquisitions of seal data will improve our understanding and modelling of Antarctic coastal ocean processes and ice-sheet dynamics.

Oceanography: Mixed up at the sea floor.

Circulation of the ocean's densest waters modulates millennial-scale shifts in climate. Contrary to conventional wisdom, a study finds that the shape of the sea floor constrains where these waters rise towards the surface. See Article p.181 Deep ocean circulation, below about 2.5 kilometres (km), is a key driver of oceanic uptake or release of heat and carbon. A few regions, such as the North Atlantic, are known sources of deep water formation, but the processes that control the movement of deep waters remain as opaque as the abyss itself. Now, Casimir de Lavergne and colleagues show that the characteristic flow patterns—northward below 4 km depth and southward between 2.5 km and 4 km—are tied to the distribution of the ocean floor at certain depths. Essentially, the bathymetry of the ocean, rather than extrinsic properties such as climate gradients, drives key features of deep ocean circulation. The authors incorporate an analysis of radiocarbon into their model that reveals a shadow layer between the surface and deep ocean, with stagnant properties and high potential for long-term carbon storage.

New Re-Os isotopic constrains on the formation of the metalliferous deposits of the Lower Cambrian Niutitang formation

The Terreneuvian Epoch (541–521 Ma)is also an important period for metallogenesis in South China, as was represented by the widespread occurrences of Ni-Mo polymetallic layers on the antecedent shallow platform margin and the V-enriched black shales in deeper slope-basin settings. In this study, we have measured Re-Os isochron ages of Ni-Mo polymetallic layers (Songlin, Niuchang, Sancha, Chuanpengwan), V-rich black shales (Bahuang), and non-metalliferous black shales (Shuidong) in the basal Niutitang Formation in Guizhou and Hunan province, South China. The Ni-Mo polymetallic layers and V-enriched black shales have similar Re-Os isochron ages, suggesting concurrent deposition of these two types of metalliferous ores. This suggestion is consistent with the traditional stratigraphic correlation by using the nodular phosphorite bed directly underlying these metalliferous layers as a stratigraphic marker. Furthermore, the metalliferous ores and non-metalliferous black shales have similar initial 187Os/188Os ratios of 0.8–0.9, arguing for a dominant seawater origin with minor contribution of hydrothermal activity. Furthermore, Re-Os isotopic data also imply that Ni-Mo and V ore might have derived from the same source. We suggest that the spatial distribution of metalliferous ores can be explained by the development of non-sulfidic anoxic-suboxic wedge (NSASW) in the slope-basin and sulfidic wedge in the previous platform margin. Upwelling of deep water first transects the mildly reduced, organic rich NSASW, in which V (V) is reduced to V (IV), and is preferentially removed from seawater by organometallic complex formation. As a result, V-rich black shale deposits in the slope-basin of Yangtze Platform. Further movement of deep water into the sulfidic platform margin results in Ni-Mo polymetallic layer formation.

An Open Ocean Trial of Controlled Upwelling Using Wave Pump Technology

AbstractIn 1976, John D. Isaacs proposed to use wave energy to invert the density structure of the ocean and pump deep, nutrient-rich water into the sunlit surface layers. The basic principle is simple: a length of tubing attached to a surface buoy at the top, and a one-way valve at the bottom can be extended below the euphotic zone to act as a conduit for deep water. The vertical motion of the ocean forces the attached valve to open on the downslope of a wave and close on the upslope, thus generating upward movement of deep water to the surface ocean. Although Isaacs’s wave-powered pump has taken many forms, from energy production to aquaculture to the more recent suggestion that artificial upwelling could be used to stimulate primary productivity and carbon sequestration, the simple engineering concept remains the same. In June 2008, the authors tested a commercially available wave pump (Atmocean) north of Oahu, Hawaii, to assess the logistics of at-sea deployment and the durability of the equipment under open ocean conditions. This test was done as part of an experiment designed to evaluate a recently published hypothesis that upwelling of water containing excess phosphate (P) relative to nitrogen (N) compared to the canonical “Redfield” molar ratio of 16N:1P would generate a two-phased phytoplankton bloom. The end result of this field experiment was rapid delivery (<2 h for a 300-m transit) of deep water to the surface ocean followed by catastrophic failure of pump materials under the dynamic stresses of the oceanic environment. Wave-driven upwelling of cold water was documented for a period of ∼17 h, with a volumetric upwelling rate of ∼45 m3 h−1 and an estimated total input of 765 m3 of nutrient-enriched deep water. The authors discuss the deployment of a 300-m wave pump, the strategy to sample a biogeochemical response, the engineering challenges faced, and the implications of these results for future experiments aimed at stimulating the growth of phytoplankton.

Distribution of deep-sea black shales of Cretaceous age in the eastern Equatorial Atlantic from seismic profiling

The presence of thick (∼ 200 m) carbonaceous shales of Cretaceous age in the deep waters of the eastern Equatorial Atlantic was first revealed by drilling at DSDP sites 367 and 368 near the Cape Verde Islands. More recent seismic investigations show that black shales are widely distributed in this region. In the Gambia Basin they are associated with a group of seismic reflectors lying at depths of 400–600 m below the sea floor. These can be traced southwards from the drill sites into the Sierra Leone Basin and eastwards beneath the African continental rise to depths at which ambient temperatures fall within the oil generation window. In the Gambia Basin the reflectors continue westwards from the Cape Verde Archipelago before terminating on Turonian oceanic basement (∼ 92 Ma). South of the Guinea Fracture Zone near 10° N extension of the same seismic sequence on to younger oceanic crust indicates that deep-sea carbonaceous deposits formed close to the present equator as late as the early Campanian (∼ 80 Ma), long after a series of oceanic anoxic events in the Aptian–Albian (OAE1) and over the Cenomanian–Turonian boundary (OAE2) led to widespread accumulation of black shales. This late period of deposition may correspond to the more locally developed OAE3. High surface productivity in equatorial upwelling zones during the late Cretaceous probably played an important role in delivering large amounts of organic carbon to the sea floor when meridional movement of deep water between the South and North Atlantic was restricted by closely-spaced, east–west fracture zones. In some areas seismic data show that the black shales level out rather than drape pre-existing topography, implying significant downslope transport of organic-rich material from upwelling cells near the African margin.

Abyssal circulation change in the equatorial Atlantic: Evidence from Cenozoic sedimentary drifts off West Africa

Seismic profiling in the equatorial Atlantic reveals deep-water (> 4500 m) sediment bodies formed by current-controlled deposition near the intersection of large-offset fracture zones with the African margin. A 600 km-long drift accumulation, the Ivory Coast Rise, lies north of the St Paul Transform near 3° N. A smaller drift deposit has been identified along the northern side of the Guinea Transform at 10° N. Antarctic Bottom Water (AABW), which presently enters the eastern Atlantic basins through the Romanche and Vema Fracture Zones, probably played an important role in the development of these features. Proto-AABW may have reached the equatorial region as early as mid-Eocene time, before the establishment of permanent ice sheets in Antarctica. The Ivory Coast Rise existed as a distinct sedimentary drift by the mid-Eocene as a result of deposition from bottom water moving southwards along the African margin and westwards parallel to the St Paul Fracture Zone. This early flow pattern is in the opposite sense to the present movement of deep water in the Sierra Leone Basin. A reversal in abyssal circulation may have been caused by the northward passage of the region across the paleoequator during the Cenozoic.

Measuring the salinity of the Clyde Sea from remotely sensed ocean colour

This paper describes a novel method of deriving surface salinity from remotely sensed ocean colour. The method is based on two important observations of optical properties in regions of freshwater influence (ROFI). The first is the strong effect that a form of dissolved organic matter (yellow substance) has on ocean colour when present in relatively high concentrations. The second is the close relationship between salinity and yellow substance originating from fresh water runoff. In this paper these relationships are demonstrated for the Clyde Sea, Scotland, and applied to SeaWiFS imagery for the derivation of yellow substance and surface salinity. The empirical relationships demonstrated in this study allow the satisfactory prediction of yellow substance and salinity in the Clyde Sea from remotely sensed ocean colour. The r.m.s. difference between the observed and predicted parameters are 0.19 m −1 and 1.1 for yellow substance and salinity, respectively, over a range of salinity from 16 to 34. Salinity maps created from the satellite-retrieved ocean colour identify features in the surface salinity distribution that to date have only been observed by in situ instrumentation. For example, the salinity distribution derived in this study is suggestive of an anticlockwise circulation in the Clyde Basin, driven both by the input of fresh water from the River Clyde and the intrusion of more saline water from the North Channel. The movement of deep water in the Clyde Sea has been well documented (Proc. R. Soc. Edinb. 90B (1986) 67), however, the residual circulation of surface water remains uncertain. The ability to obtain synoptic views of salinity such as those presented in this paper therefore provides great potential in furthering the understanding of Shelf Sea and coastal dynamics.

Constraints from paleotracer data on the North Atlantic circulation during the Last Glacial Maximum

We investigate the extent to which the paleotracers, δ13C and δ18O, constrain the North Atlantic Ocean general circulation during the last glacial maximum (LGM). The distinction is made in this work between the depiction of water mass distributions and the inference of the oceanic circulation. The δ13C data indicate that water masses formed in the North Atlantic and in the Southern Ocean were distributed differently during the LGM, but the extent of the differences cannot be quantified because of uncertainties in the end‐member composition. Reasonable choices of values for the end‐members produce volumes of Lower North Atlantic Deep Water (NADW) significantly smaller than in the modern Atlantic, in agreement with previous studies. To explore the range of circulations consistent with this water mass distribution, a coarse resolution inverse model of the North Atlantic is constrained with δ13C and δ18O paleodata. The model shows that the glacial period tracer distribution is fully consistent with the circulation rates estimated for the modern ocean. However, the paleodata are also consistent with a circulation rate of one‐half the modern flux of NADW, and indeed there are an infinite number of rates of deep water movement that are possible. To remove this ambiguity, the central need is for a data type capable of setting the overall rates of water movement, i.e., a “clock”, such as that provided by geostrophy or a well‐sampled radioactive tracer.

A conservative tracer for glacial ocean circulation from carbon isotope and palaeo-nutrient measurements in benthic foraminifera

THE ratio of cadmium to calcium (Cd/Ca) and the carbon isotope ratio (δ13C) in the calcite tests of benthic foraminifera both record nutrient distributions in the ocean1,2. Strict interpretation of both δ13C and Cd as nutrient tracers has led to conflicting views of glacial ocean circulation3–5. Here we show that, when one takes into account the fact that δ13C reflects air–sea exchange as well as nutrient distributions, these two proxies can provide complemen-tary information about the movement of deep water in the glacial ocean. We use the Cd concentration (assumed to be controlled primarily by biological cycling) to infer the age history of glacial deep water, and deduce the sources of deep water from the carbon isotope air–sea exchange signature, a conservative tracer that we construct using both Cd and δ13C measurements. Our analysis suggests that there were at least two sources of glacial deep water: a less dense component originating in the North Atlantic Ocean, and a more dense component which may have originated in the Pacific Ocean. As well as demonstrating the potential of this approach, our findings provide further support for a Pacific glacial deep water source, evidence for which has until now been both scarce and conflicting3,5–12.

The mineralogy of Lake Malawi ferromanganese nodules

In Lake Malawi, ferromanganese nodules and oolites occur in oxic areas at water depths of 80 to 160 m at the facies boundary between sands and muds. In most cases, the nodules have been found at the water/sediment interface. The nodules and micronodules range in colour from dark brown to rusty or yellowish. Some tend to be of metallic lustre on the outside. The size of the nodules ranges from about a millimeter to about a centimeter in diameter while the average is 3 mm. Three main mineral species have been identified in Lake Malawi nodules: 1) manganite (λ-MnOOH); 2) geothite [∝-FeO (OH)] and 3) vivianite [Fe 3 (PO 4) 2. 8H 2O]. A likely origin of Fe and Mn that form the nodules in Lake Malawi is from tthe sediments themselves. Another possible source can be through the movement of deep waters from the geothermally active areas of the lake to the shallow oxic areas where nodules are presently forming. The vivianite in the vivianite-rich nodules originates from dissolution of apatite (from fish debris) within the sediment and redeposition as Fe-phosphate under reducing conditions. Studies of the vivianite-rich micronodules using a scanning electron microscope (S.E.M.) and an electron microprobe have shown that in some samples, vivianite is associated with nontronite [(Ca Mg).5Fe 2(Si Al) 4O 10]. The nontronite occurs as a coating around the vivianite.

Some Aspects of the Thermal Regime, Hydrodynamics, and Petroleum Occurrences in Central Tunisia.

Recent literature has emphasized the possible relation between positive thermal anomalies, deep water movement, and hydrocarbon occurrences. In Tunisia, parallels have been shown between some hydrocarbon accumulations and the geothermal gradient in the Central part of the country. Other studies showed the relation between some deep aquifers hydrodynamics (Aptian, Ypresian, etc.) and the corresponding oil fields. For Central Tunisia, 8 of the 10 main existing hydrocarbon occurrences are located within a region of positive geothermal gradient values ( 50C/km). The study of the deep reservoirs in central Tunisia revealed that oil accumulations of the Aptian (Douleb field), Ypresian (Lytaiem field), etc. are located within zones of hydrodynamic convergence and discharge areas. Hydrogeologic studies of other reservoirs (Abiodh and Serj formations) revealed new interesting areas of deep waters convergence, favorable to oil accumulations and not yet assessed. Moving water within the deep reservoirs seems to be the main vehicle of temperature and oil. Even if the primary reason of this parallel between oil, temperature, and hydrodynamics, is not yet clearly established, empirical evidence and numerous examples throughout the world make it worthwhile to consider geothermal and hydrologic explorationmore » as integral part of any petroleum exploration program.« less

Terrestrial heat flow in Canada

Heat flow has been measured at 214 separate sites in Canada and on the surrounding continental shelves. Most geological provinces show average heat flow that conforms approximately to world averages for areas of similar tectonic age, but the northern Prairies show an anomalously high heat flow that is probably controlled by deep water movement, and the coastal zone of southern British Columbia shows a low heat flow that is believed to be associated with recent subduction of small ocean plates.

Calcium: chlorinity ratio and carbonate dissolution in the northwestern Indian Ocean

Analysis of 128 water samples from the northwestern Indian Ocean gave an average calcium: chlorinity ratio of 0.02164 ± 0.000009. An identical ratio (0.02165) was obtained using the data from selected Meteor stations in the region. The ratio in the northern Indian Ocean is significantly higher than the oceanic averages. The northern Indian Ocean, occupying only 3% of the global oceanic are, receives about 9% of the total global river runoff annually. It is suggested that the high calcium flux from the rivers in a relatively small area increases the Ca:Cl ratio in the upper layers of the region. The ratio in the deep waters in the region is also higher than in other oceanic areas, presumably due to a high rate of calcium removal at the surface by biological activity and its subsequent input to the bottom waters. A more rapid upward movement of deep waters in the Indian Ocean than in the Pacific and Atlantic oceans appears to result in a faster upward flux of calcium in the region. The ratio increases below the lysocline. The calcite saturation depth lies between 2500 and 3000 m. Carbonate content of the sediments, however, shows no appreciable decrease above 4000 m, which supports kinetic models for CaCO 3 dissolution. The data support the Broecker-Takahashi model ( Deep-Sea Research, 25, 65–91, 1978), indicating that variations in the lysocline depth can be accounted for largely by variations in carbonate ion concentration. The “critical carbonate ion concentration” for the region is 90 ± 5 μM kg −1. It is concluded that calcite dissolution does not occur above 4000 m.

Lacunes de sedimentation, series condensees et nodules de manganese dans les depots neogenes et quaternaires des bassins de l'ocean Indien austral

Abstract This is a detailed stratigraphic study of core samples from basins in the southern Indian Ocean used to characterize early milieux which controlled neogenic and quaternary sedimentation. Distribution of the principal pelagic facies identified--coccolithic calcareous oozes, red clays, grey and brown diatoms--depended more on depth than on latitudinal hydrologic zonation. Stratigraphic succession of neogenic and quaternary sediment layers--in general, red clays at the bottom, calcareous oozes above--is usually discontinuous. Condensation sequences and manganese nodules are comparable to those from the Jurassic, Cretaceous and Paleogene in alpine areas. Their stratigraphic character indicates that lithological condensation was due to carbonate dissolution and sedimentation hiatuses. Their formation seems to depend on 3 principal phenomena: depth variation of the accelerated calcium zone in the course of time; crustal subsidence produced by dorsal ridges; and winnowing of sediments by deepwater movement. These results suggest a solution to the paradoxical problem of slow nodule growth--that it takes place during breaks in the sedimentation process.

Offshore Characteristics in the Deep Waters of the Strait of Georgia as Indicated by Bathypelagic Fish

An attempt is made from oceanographical observations to explain the occurrence of certain bathypelagic species of fish which have been captured in the bottom waters of the southern Strait of Georgia. It is noted that there is a considerable seaward surface Sow of water from the Fraser River. The water from intermediate depths over the continental shelf forms the inflowing deep water of Juan de Fuca Strait mixing with the Fraser River water in the turbulent channels of the San Juan Archipelago. This mixture forms the deep inflowing water of southern Strait of Georgia and the outflowing surface water of the Juan de Fuca Strait as shown by salinity distribution and current measurements. The net inward movement of deep water is suggested as an agent of transport or a directive factor for the occurrence of these fish in this region. Physical and chemical conditions of the deep water in the Strait of Georgia are shown to be only slightly different from those found in the intermediate offshore water. It is probable that a combination of factors provides conditions suitable for survival of these species in the deep water of the southern Strait of Georgia.