Subthermocline Water Research Articles

Black shales on the basin margin: a model based on examples from the Upper Jurassic of the Boulonnais, northern France

Abstract Organic-rich shales were deposited during two phases of relative sea-level rise in the Kimmeridgian (Late Jurassic) basin margin sections of the Boulonnais (N. France). Pyrite framboid analysis indicates that bottom waters were euxinic during substantial periods of organic-rich shale formation, and Th/U ratios and degree of pyritisation of Fe suggest severe oxygen-restriction, whilst palaeoecological and sedimentological evidence suggests that seafloor oxygenation events were frequent. Interbedded, hummocky cross-stratified sandstones indicate deposition above storm wave base. Correlation of lateral facies changes reveals that the black shales record deposition beneath a nearshore zone of oxygen-poor waters that contracted in lateral extent during parasequence formation. The Boulonnais black shales are similar to other varieties of transgressive black shale, in that they are developed upon surfaces of marine starvation, but they differ in that they pass down-dip into better oxygenated facies. They are therefore classified as transgressive, nearshore (TN) black shales that do not record the ‘feather edge’ of basinal conditions. Deposition is interpreted to have occurred beneath a small volume of sub-thermocline bottom waters that were readily de-oxygenated, whilst anoxia was less readily developed in more offshore settings due to the greater volume of sub-thermocline water. Why such conditions should pertain only during initial stages of base-level rise remains obscure.

Dynamics of the North Brazil Current retroflection region from the Western Tropical Atlantic Experiment observations

Hydrographic and velocity observations of the North Brazil Current (NBC) retroflection region during the 1990–1991 Western Tropical Atlantic Experiment (WESTRAX) are examined with the intent of extracting dynamical information about the NBC eddy shedding. A comparison is performed between the depth structure of empirical orthogonal functions and the dynamical normal modes of the NBC retroflection region on a beta plane centered on 5°N. The barotropic and first two baroclinic modes account for about 75% of the vertical structure of the NBC flow. Thus, a three‐layer model of the NBC region is suitable for dynamical studies of the NBC retroflection. In terms of flow structure the upper layer represents the retroflection of the surface layers of the NBC to feed the North Equatorial Countercurrent. The middle layer represents the separating subthermocline waters of the NBC which feed the North Equatorial Undercurrent. The third layer represents a weak meandering flow that may be thought of as the Deep Western Boundary Current signature in the three‐layer ocean. In terms of PV, a well‐defined front separates the NBC waters from the North Equatorial Current in the upper layer. Both upper and middle layers present closed PV contours associated with the eddy in the process of pinching off from the retroflecting NBC. The NBC separation region, although equatorial, complies reasonably well with the basic quasigeostrophic (QG) assumptions. Therefore QG methods are applied to investigate the NBC meander growth. By isolating the effect of PV anomalies in each of the layers on each of the layers, baroclinic growth is verified to occur during the NBC eddy shedding.

Nutrient distributions in an anticyclonic eddy in the northeast Atlantic Ocean, with reference to nanomolar ammonium concentrations

The micromolar concentrations of nitrate, nitrite, silicate, phosphate and ammonium; and the nanomolar concentrations of nitrate, nitrite and ammonia were studied during a Lagrangian study of an anti-cyclonic eddy situated in the Iceland Basin of the North Atlantic Ocean. Underway surface transects, carried out across the eddy at the beginning and end of the survey period of 2 weeks, showed a decrease in the surface-water concentrations of nitrate and phosphate, with a concomitant increase in the phytoplankton biomass and surface-water temperature. The trajectory of the eddy was tracked by a release of SF 6 tracer, and regular water column sampling was carried out at the centre of the tracer patch over a period of 11 days. A storm after the first 5 days of the cruise brought about a transport into the surface mixed layer of colder sub-thermocline waters, which resulted in significant nutrient enrichment of nitrate, phosphate and silicate in the euphotic zone. This transport of nutrients, in particular nitrate, into the surface-mixed layer demonstrates the importance of eddies in the open ocean for sustaining ‘new’ production within the euphotic zone. We also report observations of ammonium at concentrations from 45 to 935 nmol l −1 for the northeast Atlantic using an automated analytical system. The presence of an ammonium maximum in the water column related to the thermocline, chlorophyll maximum, and the primary nitrite maximum.

Tidal mixing near the Kimberley coast of NW Australia

A complex of reefs and islands atop raised topography that is 140 km long and up to 35 km wide extends roughly westward from the Kimberley coast of NW Australia. The surrounding seafloor has depths of 60–80 m. Satellite images of sea surface temperature in spring, summer and autumn suggest that tides commonly mix sub-thermocline waters to the surface along this ridge, particularly along its western half, which includes Adele Island and Churchill, Albert, Beagle and Mavis reefs. The cool surface waters (~3˚C cooler than surroundings) are seen where the depths are less than ~50 m, such that they closely reflect the bottom topography. The semi-diurnal tides have a maximum daily range of ~10 m and it is when the range exceeds ~3 m that mixing to the surface occurs. Thus, the mixing can cease during some neap periods. Historical data from the shelf show the sub-thermocline water to be nutrient-rich (nitrate concentrations of 6.0 µM), so the mixing is likely to be important for the marine ecosystem. As winter approaches, the distinctive cool mixed region around the Adele–Mavis banks is overwhelmed by surface cooling, and historical ship data reveal that the stratification on the surrounding shelf disappears.

Direct measurement of the deep circulation within the Brazil Basin

In support of the Deep Basin Experiment, part of the World Ocean Circulation Experiment, a large number of neutrally buoyant floats were released within the Brazil Basin during the 1990s in an attempt to measure directly the circulation in the deep ocean interior. Three levels corresponding to the three major subthermocline water masses were selected, and results from the deeper two (North Atlantic Deep Water, NADW, and Antarctic Bottom Water, AABW) are described. At this writing processing of acquired tracking data is incomplete. Hence, this paper reports on the progress of the observational program and gives our initial conclusions. It appears that the flow in the deep Brazil Basin is unlike previous conjectures in which the circulation patterns can be characterized as being primarily meridional, both along the western boundary and in the interior. The existence of a deep western boundary current (DWBC) is quite clear in the float data at the NADW level, but less prominent in the AABW, and the interior flow is dominantly zonal with unexpectedly small meridional space scales. Integral time scales are long, of order 20–30 days, and eddy kinetic energy levels are low, of order 1 cm 2/s 2. In spite of the low energy levels a surprising number of our floats became caught up in vortices. A line of seamounts extending offshore near 20°S, known as the Vitória–Trindade Seamounts, interrupts the DWBCs and is the location for eddy formation and apparent flow away from the boundary into the interior. Although it has been speculated that this could feed a narrow zonal current of NADW (the “Namib Col Current”) our float trajectories suggest a return to the western boundary, rather than a continuation to the east.

A Regional Model of Shelf Circulation near Bass Strait: A New Upwelling Mechanism

Abstract A study is made of the low-frequency, three-dimensional circulation and upwelling for the western region of Bass Strait, using oceanographic data and the Princeton Ocean Model. A novel radiation condition, which simulates the effect of an eastern shelf on Kelvin wave scattering, is successfully tested against analytic solutions for coastal-trapped wave (CTW) scattering and generation. Using a realistic bathymetry for the Bass Strait region, the model was forced with 8-day period zonal winds within the strait and a mode 1 CTW at the northwestern boundary. Results demonstrate the existence of a new mechanism for upwelling whereby at the cessation of westward winds within the strait, the residual poleward flow over the steeply sloping Tasmanian shelf separates from a gyre that develops over the more gently sloping topography near the mouth of the strait. The resultant divergence of the velocity field leads to large vertical velocities (34 m day−1), and a plume of subthermocline water that is upwelle...

Mercury in the North Atlantic

Reactive mercury (Hg) concentrations (0.80±0.44 pM), measured during a cruise in the sub-polar North Atlantic (50–70°N) in August 1993 were lower those of more southerly regions of this ocean. A large fraction of the Hg in the surface waters was elemental Hg (Hg°; 88% of the reactive Hg, on average; 0.65±0.50 pM). Little dimethylmercury (DMHg) or monomethylmercury (MMHg) was found in surface waters but higher concentrations were found at depth. The high surface water Hg° concentrations likely reflect production of Hg° in concert with the short season of primary production in these waters. Overall, gas exchange plays a dominant role in the cycle of Hg in the upper waters of this region. For the sub-thermocline waters, a comparison of the Hg speciation in different water masses suggests that surface waters accumulate reactive Hg and methylated Hg—due to input of reactive Hg via remineralization and conversion of reactive Hg to methylated Hg—after sinking during winter to form North Atlantic deep water. Reactive Hg concentrations varied between water masses with the lowest concentrations being found in the waters north of Iceland (the source waters for deep water formation). DMHg was found throughout the sub-thermocline water column and the limited data set for MMHg suggests that this species was also prevalent. These observations suggest that methylated Hg production in the ocean is not confined to low oxygen or anoxic regions, as has been found in lakes. Concentrations of DMHg (0.08±0.07 pM, on average) were lower than those previously measured in the equatorial Pacific. Total Hg concentrations averaged 2.4±1.6 pM but little of this was particulate Hg (<10 pg l−1, typically). There was evidence of colloidally-bound Hg in surface waters. Overall, the data strengthen our hypotheses concerning Hg biogeochemical cycling in the ocean and confirm the importance of gas exchange and Hg methylation in the fate and transport of Hg in the ocean.

Pulsed delivery of subthermocline water to Conch Reef (Florida Keys) by internal tidal bores

Internal tidal bores generated by breaking internal waves cause dramatic, high‐frequency variation in temperature, salinity, water velocities, and concentration of chlorophyll a on Conch Reef, Florida Keys. The arrival of bores on the reef slope is linked to a semidiurnal internal tide and is marked by temperature drops of up to 5.4°C and salinity increases of up to 0.6‰ in 1–20 min. These changes are accompanied by the sudden onset of upslope flow 1–15 m above the bottom with speeds of 10–30 cm s−1. Cool, high‐salinity water is transported from below the thermocline seaward of the reef and is resident on the reef slope for up to 4 h before it mixes with surface waters and recedes downslope. Compared with ambient surface water, this deep water can contain significantly elevated concentrations of dissolved nitrate. Physical variability produced by this mechanism increases significantly with depth on the reef slope. Analysis of 3‐yr temperature records indicates the arrival of internal bores is a consistent feature at this site from May through November, with peak activity in July–September. Pulsed delivery of subthermocline water appears to significantly affect the temperature, nutrient, and particle flux regimes on this coral reef.

The water masses and circulation at 10°N in the Pacific

The circulation and distribution of water masses near the southern boundary of the North Pacific Basin are described, based on a recent hydrographic survey made at 10°N. A circulation scheme is found, using both the tracer data and a box inverse model. To ensure the best possible realization of the mean state, repeat survey data are used in the boundary current. Historical data are used to check the representativeness of the transport across the onetime section outside of the boundary current. The upper 400 m are dominated by the North Pacific tropical Sverdrup cell, where the net interior flow is to the north, compensated by an equatorward low-latitude boundary flow: the Mindanao Current. The tropical cell is highly baroclinic in the sense that 18 Sv (1 Sv = 1 × 10 6 m 3 s −1) of tropical surface water flows northwards and is returned southward entirely within the shallow tropical thermocline: half in a broad interior flow and half in the southward boundary current. The strongly baroclinic structure of the tropical cell allows for the efficient transport of North Pacific subtropical water masses across the gyre boundary. The tropical cell is responsible for nearly all of the 0.7 ± 0.5 PW of heat transported northwards across the section. There is essentially zero freshwater divergence over the North Pacific north of 10°N. Critical to the salt balance of the North Pacific basin is a poleward flux of salty subthermocline water which ultimately derives from the South Pacific subtropical gyre. The South Pacific water found along 10°N has temperatures and salinities characteristic of the 13°C Thermostad found at the equator, suggestive of a link via the eastern Pacific. At depth, the analysis reveals a deep meridional overturning in the North Pacific consisting of the import across 10°N of 8 Sv of bottom waters of Antarctic origin and their subsequent conversion and export as North Pacific Deep Water. Dominating the mid-depth property distribution and circulation is a deep anticyclonic cell which horizontally cycles 8 Sv of deep water through the North Pacific basin. We find that there is little interaction between the North Pacific wind-driven circulation and the abyssal volume, which contrasts with the Atlantic Ocean, in which there is strong communication between the cold and warm water volumes.

Nutrients and trace metals in the northwestern Mediterranean under coastal upwelling conditions

Salinity, temperature, turbidity, nutrients and iron, manganese and copper were measured in 50 surface water samples collected from the Rhoˆne river and river plume, the Gulf of Lions and along a vertical profile in the open northwestern Mediterranean. The data obtained bring new information on the distribution of nutrients and trace metals in the northwestern Mediterranean under wind induced coastal upwelling. Persistent northwesterly winds lead to complex physical processes such as upwelling along the northwestern coast of the Gulf, local eddies and oscillating currents. These processes result in the redistribution of dissolved and particulate components, enriched in the upwelling water, throughout the water column. The effect of nutrient-enriched upwelling water on the primary production of the Gulf is expected to be of secondary importance relative to the river input. In addition to the enhanced wind transport of metal-rich continental dust, the effect of upwelling of the sub-thermocline water and particularly the nepheloid layer has been demonstrated. The behaviour of Fe, Mn and Cu in the mixed water appears to be governed by the common estuarine processes particularly the liquid-solid exchange. Great estuarine reactivity was exhibited by Cu and Mn.

Satellite observations of sea surface temperature variability on southern Georges Bank

Sea surface temperature (SST) residual time series, derived from declouded Advanced Very High Resolution Radiometer (AVHRR) satellite imagery for southern Georges Bank, were examined for the period April–October 1987. Significant negative correlations were computed between the low-pass filtered SST residuals and daily-averaged tidal current magnitude, suggesting that for periods of ∼15 and ∼28 days, SST variability was related to the spring-neap tidal cycle in both the stratified and unstratified portions of southern Georges Bank during the stratified season. Maximum negative correlations occurred at a lag of ∼3 days, indicating that negative (positive) SST residuals lagged the maximum spring (minimum neap) tidal current by this amount. This lag and the measured 3σ SST residual of ∼3.0°C are in agreement with the summer hydrography on southern Georges Bank and a one-dimensional model in which the depth-independent vertical eddy diffusivity varies with time. Because meteorological forcing at the sea surface occurs on time scales of ∼1–7days, the data suggest that the observed, negative SST residuals are caused by periodic, enhanced vertical mixing of colder, sub-thermocline water into surface waters as a result of spring tidal currents during the stratified season. Given the inverse temperature-nitrate correlation for the region during the stratified season, the data suggest the occurrence of increased nitrate flux into the stratified and well-mixed regions of southern Georges Bank during spring tides. Periods of spring tides may result in the transport of a large fraction of the new nitrogen needed to sustain high primary production observed in the chlorophyll maximum within the stratified and well-mixed regions of southern Georges Bank during the stratified season.

Response of the Thermal Structure of Lake Ontario to Deep Cooling Water Withdrawals and to Global Warming

It has been proposed to cool buildings in downtown Toronto using cold, deep water withdrawn from Lake Ontario. 50 m 3 s −1 of sub-thermocline water would be withdrawn from a depth of 80 m at a temperature close to 4°C, distributed to chillers throughout the city, and discharged on the surface in the nearshore zone at a temperature of 12°C. Compared with electrically-powered chillers, the Deep Lake Water Cooling (DLWC) scheme is thermodynamically elegant and environmentally beneficial in many ways. Using a one-dimensional thermodynamic model of Lake Ontario, this paper assesses the physical impact of the DLWC scheme on the whole lake under present and future conditions. We conclude that the lake could presently absorb the heat from the proposed Toronto installation and 20 others like it without major lake-wide physical changes. At a conservative estimate of 1000 m 3 s −1 with an 8°C temperature differential, the DLWC scheme would reject approximately 33.5 GW of “waste heat” to Lake Ontario. Under our best estimate of a 2 x CO 2 climate scenario, the deep waters could be 2–3°C warmer than they are now. The DLWC system would be less efficient, but the lake would retain a similar overall cooling capacity. Neither biological consequences nor the local physical impact of the discharge of heated subsurface water from the DLWC system are addressed in this study.

Mercury speciation in open ocean waters

Vital to our understanding of the biogeochemical cycling of Hg and the origin of the enhanced monomethylmercury (MMHg) concentrations in biota is knowledge of the sources, behavior and fate of methylated Hg species in natural waters. Methylated Hg species, dissolved gaseous and reactive mercury were measured in the equatorial Pacific Ocean in early 1990. Both dimethylmercury (DMHg) and MMHg were found in the subthermocline waters of the equatorial Pacific Ocean. Maxima in alkylmercury species in the O2 minimum region coincided with a decrease in reactive mercury. A significant inverse correlation between DMHg, and MMHg, concentration and O2 content was observed. A maximum in reactive mercury was observed in the region of the thermocline, with similar concentrations in the surface and deeper waters. Atmospheric deposition is not a significant source of MMHg to this region. The data suggest formation of alkylmercury species in the low oxygen zone, with Hg(II) being the most likely substrate. A model for the cycling of Hg species in the equatorial Pacific Ocean is presented. These results are the first direct evidence of a significant open ocean source of methylated Hg species and suggest a pathway for mercury accumulation in pelagic fish.

Inputs of subthermocline waters and nitrate onto the Campeche Bank

Geostrophic tilting of isopycnal surfaces in the Yucatan Current raises the thermocline and nitrate nutricline (approximately the 22°C surface) above the level of the shelf break along the eastern margin of the Campeche Bank. Along the northern and western margins of the bank, these features lie 20–30 m below the shelf break. Cool nitrate-rich water can move laterally from the Yucatan Current along isopycnal surfaces beneath the thermocline onto the Campeche Bank. Drogues launched on the bank moved westward, corroborating transport away from the Yucatan Current. Near-bottom water on the bank is denser than water at similar depths in the Gulf of Mexico and Campeche Gulf. This dense near-bottom water is trapped within the euphotic zone on the bank for periods between days and weeks. Anomalies of temperature and chlorophylla at 30 m depth on the bank were directly proportional to density anomalies, indicating the mixing of subthermocline water with surface water. The development of surface patches of phytoplankton or elevated nitrate levels on the bank results from the vertical mixing of the stratified water column. During the summer, lateral transport of subthermocline water from the Yucatan Current appears to be an important source of nitrate nitrogen for the Campeche Bank.

Sources and effects of oxygen deficiencies in the south-eastern North Sea

Abstract Oxygen concentrations, oxygen comsumption and its possible sources were investigated in the southeastern North Sea (east of 6°N20′E, south of 56°50′N) in 1983–1985. 1983 was a year with extremely low, 1984 with high oxygen concentrations in the bottom water, but even in 1984 only 40–70 % oxygen saturation was found in some areas in the subthermocline waters. BOD values suggest that the oxygen content in the deep water could be used up within 20–30 days, given a calm weather period. This oxygen consumption, extrapolated from the BOD measurements, was fastest well north of areas with highest organic seston stocks. This supports the assumption of a northward transport of seston clouds with the residual currents. The theoretical oxygen demand for the degradation of measured organic seston stocks of the water column was as high as about half of the saturation oxygen content of the subthermocline water. Thus, even in years with average plankton and other organic seston stocks the oxygen balance of the bottom water would be endangered quickly, should a prolonged vertical density stratification develop. Mean oxygen consumption for macrobenthos inhabited sediment was 18 mg · m−2 · h−1. Average benthic community oxygen consumption amounted to 14 % of total biological oxygen demand of subthermocline water. Possible impact of oxygen depletion on benthic communities was investigated. Cluster analysis, diversity index and evenness index showed that benthic community structures were different in years with and without oxygen deficiency in the bottom water, suggesting effects of low bottom water oxygen levels on benthos.

Discovery of Pogonophora in warm shallow waters of the Florida Shelf

A new small species of Siboglinum has been found in sediments of sub-tropical waters off southwest Florida, at 24 to 90 m depth, where bottom temperatures range from 17 to 26°C This is unexpectedly warm for a pogonophoran. The phylum in general is mostly deep sea except for a few species that reach shelf depths in cold boreal or arctic regions. The new find suggests Pogonophora may be more widely distributed than previously thought. Pogonophora are predominantly deep-sea animals. In the tropics they have not previously been reported from less than 300 m, seeming to be limited to subthermocline waters cooler than 10°C (Southward 1971b, 1972, Gureeva 1981). A few shallow water species have been found in Arctic shelf seas and in boreal stratified seas where bottom temperatures are low for most of the year (Southward 19?1a, 1971b, 1979). The recently discovered giant vestimentiferan worms (Riftia pachyptila Jones 1980) from the Pacific at 2500 m are related to pogonophores, and their occurrence close to hydrothermal vents at first sight seemed to contradict the idea that pogonophores are restricted to cold water. However, it appears that these vestimentiferan worms live in places where the hot vent waters mix with cold bottom waters, and although a recent measurement suggests an upper limit of 35 C, most of the vestimentiferans experience temperatures from 2 to 14 C (Desbruyeres & Laubier 1983, Hessler & Smithey 1983, Canadian American Seamount Expedition 1985). It is notable that related vestimentiferans occur in a brine seep at the foot of the Florida Escarpment (4.4C) and in an oil-rich seep area on the Louisiana Slope, also in the Gulf of Mexico, at only 600 m depth with no indication of elevated temperatures (Paul1 et al. 1984, Brooks et al. 1985). A clear exception to the 'cold-water rule' has now been discovered on the south-west Florida Shelf. A new small species of Siboglinum has been found quite O Inter-Research/Printed in F. R. Germany widely distributed in a region where the bottom temperatures range from 17 to 26°C. The Florida shelf pogonophore was found during a survey of the softbottom benthos by the Mote Marine Laboratory, sponsored by the United States Department of the Interior, Minerals Management Service. Thirty stations were sampled over the course of 6 cruises: Nov 1980, AprMay 1981, Jul-Aug 1981, Jan-Feb 1982, Dec 1982, May-Jun 1983 (Fig. 1). At each of 15 soft-bottom stations 5 samples were taken with a modified Reineck box corer 19 X 30 cm, 30 cm deep. The top 15 cm of each core was washed through a 0.5 mm sieve and the residue preserved in formalin for later sorting in the laboratory (Woodward-Clyde Consultants 1983a, b). Pogonophores were found at 8 stations sampled during Cruises I, I1 and IV. They were most common at Station 8, where there were 45 m-2 in 1980. These pogonophores have a single tentacle, which shows that they probably belong to the genus Siboglinurn. They live in colourless, transparent tubes not wider than 0.2 mm and about 40 mm long. The presence of mature females, incubating embryos, shows that the population is adult and not merely a sporadic settlement of juveniles from some deep-water population. These pogonophores belong to a new species not closely related to other species known from nearby deepwater habitats (Ivanov 1971, Southward 1971b, 1972, Gureeva 1981). The stations at which pogonophores were found were fairly widely distributed in the mid-shelf region, with 6 stations at 38 to 58 m, 1 at 24 m and 1 at 90 m. At most of these the sediment was fine or medium sand, but at Station 26 it was a carbonate ooze of silt/clay sized particles. The CaCO, content exceeded 90 % at all except Station 2, where it was 42.5 %. The occurrence of macroalgae indicated that the range of depths occupied by the pogonophores was within the euphotic Mar Ecol. Prog. Ser 28: 287-289, 1986

The Vema Channel: Physiography, structure, and sediment—Current interactions

The east flank of the Vema Channel in the southwestern Atlantic is an opportune location for interpreting the Pleistocene/Holocene record of deep thermohaline circulation. The channel and the adjacent sediment-covered Rio Grande Rise intersect each of the principal sub-thermocline water masses of the western Atlantic. We analyzed a suite of 25 giant gravity cores in transects extending between 1400 and 4150 m depth; this interval includes the upper and lower branches of Circumpolar Water, North Atlantic Deep Water, and Antarctic Bottom Water. The late Pleistocene/Holocene sediments along these transects are pelagic carbonates with excellent stratigraphic continuity; shallower and deeper regions are relatively unsuitable due to poor core recovery or widespread erosional unconformities. Extensive hydrographic and direct current observations near the location of our prime coring transect at 30°S indicate a notable cross-channel asymmetry in the abyssal hydrography and velocity structure which is reflected in the underlying lithofacies distribution. The near-bottom flow field shows marked temporal and spatial variability on a local scale, even in the channel axis where one would anticipate relatively steady, uniform flow. Much of this variability appears to reflect the effects of transient mesoscale eddies rather than regional variations in the integrated north ward transport of AABW. In spite of temporal variations in the flow field, the NADW/AABW transition zone appears to have remained remarkably stable over time scales of 10 5–10 6 yrs, on geological evidence discussed elsewhere in this volume (Jones, 1984; Jones and Johnson, 1984). This observation is somewhat surprising, because in the modern oceans the density contrast between NADW and AABW is relatively small. If the transition zone between the two water masses has indeed remained relatively stationary, then the density-determining water properties ( T and S) may also have remained relatively constant, with only the rates of production changing. If this interpretation proves correct upon further studies, it implies that pale-oceanographic interpretations of deep thermohaline flow should in some instances utilize geological indices of non-conservative water properties (e.g. CO 2, δ 13 C for monitoring paleocirculation changes.

Dissolution of carbonate minerals in a subtropical shallow marine environment

Carbonate saturation state with respect to calcite and three different biogenic magnesian calcites has been determined by a modified saturometry technique. Measurements were made in the water column of Devil's Hole, a 25 m-deep basin in the Harrington Sound (Bermuda), which exhibits oxygen depletion in subthermocline waters during summer. Supersaturation in the entire water column was attained with respect to marble calcite. The biogenic carbonates analyzed became undersaturated below the thermocline within a narrow depth range, soon after P CO 2 exceeded 600 ppm. It is concluded that high magnesian calcites can be attacked during a significant portion of the year in these waters. Possible implications for the global CO 2 cycle are discussed.