Vema Channel Research Articles

Paleoceanographic conditions on the São Paulo Ridge, SW Atlantic Ocean, for the past 30 million years inferred from Os and Pb isotopes of a hydrogenous ferromanganese crust

Hydrogenous ferromanganese (Fe-Mn) crusts can provide records of long-term environmental changes during the Cenozoic. To understand the paleoceanographic conditions in the southwestern Atlantic Ocean, we investigated depth profiles of major- and trace-element concentrations as well as Os and Pb isotopic compositions in a Fe-Mn crust collected from the southern flank of the São Paulo Ridge in the southwestern Atlantic. Major and trace element data plotted on ternary Mn–Fe–10×(Ni+Co+Cu) and rare-earth element plus yttrium (REY) discrimination diagrams indicate that the analyzed sample is a typical hydrogenous Fe-Mn crust. The obtained 187Os/188Os data were matched to the Cenozoic seawater Os isotope evolution curve reconstructed from pelagic sediments. The result suggests that the Fe-Mn crust has accreted over ~30Myr with growth rates of 0.5–3mm/Myr, although the sample likely grew in two directions during the early stage of its growth. We found no evidence of growth hiatus in the sample, which may contrast with the growth histories of many Pacific Fe-Mn crusts. Hence, the conditions favorable for the accretion of hydrogenous Fe-Mn crusts were likely to have developed on the São Paulo Ridge over the past ~30Myr. The Pb isotopic compositions show very limited ranges (e.g., 206Pb/204Pb=18.80–18.85), and are similar to those of pre-anthropogenic seawater in the Southern Ocean. As the São Paulo Ridge is located near the Vema Channel, which is presently a major path of Antarctic Bottom Water, we suggest that a strong northward bottom current has continuously swept detrital and biogenic sediments from the ridge, and played a vital role in the Fe-Mn crust formation since ~30Ma.

Late Pliocene–Pleistocene stratigraphy and history of formation of the Ioffe calcareous contourite drift, Western South Atlantic

The high-resolution SES-2000 deep seismic profiles performed by R/V Akademik Ioffe (2010) recorded a calcareous contourite drift over the Florianopolis fracture zone ridge, South Atlantic, named the Ioffe Drift. Core AI-2436 retrieved from the drift summit recovered calcareous nannofossil–foraminiferal ooze contourites with small amounts of fine-grained terrigenous siliciclastic material. The calcareous ooze is intercalated with foraminiferal sand and more clayey ooze layers in the Upper Pleistocene section. According to planktonic foraminiferal and nannofossil biostratigraphy, the age of the uppermost section of the drift ranges from the Late Pliocene to the Late Pleistocene. It includes several stratigraphic hiatuses due to erosion, especially at the end of Pliocene–Early Pleistocene and in the intervals 1.51–0.81 and 0.44–0.27Ma. The Ioffe Drift accumulated under the influence of the eastern branch of the Antarctic Bottom Water (AABW) outflow from the Vema Channel. Temporary intensification of the AABW flow, likely due to its increased production in the Weddell Sea, led to deep erosion ultimately resulting in long-term hiatuses and hence contributing to the enormously compressed Upper Pliocene–Middle Pleistocene section of the drift.

Bottom sediments and near-bottom currents in the Southwestern Atlantic

On the basis of the author’s data on the composition of sediments and seismic cross sections, together with literature data, the bottom topography was described and the main structural features of the top 10–100 m thick sedimentary sequence in the Southwestern Atlantic (Brazil Basin) were identified. The presence of a heavy northward flow of Antarctic bottom water (AABW) and its active erosive activity were confirmed. The AABW caused the erosion or redeposition of red pelagic clays and hemipelagic clays, which accumulated in the Brazil Basin in the Holocene and Pleistocene; the clays contain abundant redeposited Pleistocene diatoms and Neogene and Paleogene discoasters. In most of the sediment cores of the Brazil Basin, the red pelagic clays are of Pleistocene age. Contourites and sandy microlayers have been found in the sediments at the foot of the continental slope of South America; this is the effect of the Deep Western Boundary Current on the ocean floor. The AABW transfers Antarctic diatom species along the continental slope of South America to 10°-5° S. The presence of the Equatorial Midocean Channel with a relative depth of 149 m in the western pelagic equatorial part of the Atlantic was confirmed, and new channels, such as Vavilov and Akademik Ioffe, have been found. The AABW flows northward along the Equatorial Mid-Ocean Channel. Apparently, the Akademik Ioffe Channel is not a proper midocean channel. At 20° S (at a depth of 5000 m), Pleistocene diatomic (Ethmodiscus rex) ooze containing up to 74% amorphous SiO2 was detected. On the Amazon-Mid-Atlantic Ridge profile, the AABW flows into the Guyana Basin through only one valley of the Nara Plain, with a depth of 4620 m. Near the Ceara Rise and on the Amazon Fan, no geologic traces of the AABW flow into the Guyana Basin were found. Near the Rio Grande Rise, the AABW might have appeared in the Eocene. The formation of the Vema Channel, which separates the Rio Grande Rise from South America, also began at that time. The AABW flows were the heaviest before the largest glaciations (particularly at isotopic stages 7/6 and 3/2).

Flow of Antarctic Bottom Water at the output of the Vema Channel

The pathways of the coldest part of the Antarctic Bottom Water (AABW) with potential temperature θ < 0.0°C in the Vema Channel and the pathways of this water flowing out of the channel to the Brazil Basin are studied on the basis of the data collected during the Russian expeditions in 2003, 2009–2012, and historical CTD data. It is shown that the AABW flows to the north in the Vema Channel as two streams, one of which is located in the deep channel and the other approximately 300 m higher over the western slope of the channel. It was found that the northern end of the deep channel is not located near 26°40′ S, 34°00′ W as a widening in the northern direction to the Brazil Basin as follows from the dataset of digital topography (Smith and Sandwell, 1997) but continues in the eastern direction. A weaker northerly flow of the cold AABW was also found from the results of the measurements in 2012, which is confined to a branch of the Vema Channel continuation.

Flow of Antarctic Bottom Water at the Output of the Vema Channel

Flow of Antarctic Bottom Water at the Output of the Vema Channel

Antarctic Bottom Water temperature changes in the western South Atlantic from 1989 to 2014

AbstractWarming of abyssal waters in recent decades contributes to global heat uptake and sea level rise. Repeat oceanographic section data in the western South Atlantic taken mostly in 1989 (1995 across the Scotia Sea), 2005, and 2014 are used to quantify warming in abyssal waters that spread northward through the region from their Antarctic origins in the Weddell Sea. While much of the Scotia Sea warmed between 1995 and 2005, only the southernmost portion, on the north side of the Weddell Gyre, continued to warm between 2005 and 2014. The abyssal Argentine Basin also warmed between 1989 and 2005, but again only the southernmost portion continued to warm between 2005 and 2014, suggesting a slowdown in the inflow of the coldest, densest Antarctic Bottom Waters into the western South Atlantic between 1989 and 2014. In contrast, the abyssal waters of the Brazil Basin warmed both between 1989 and 2005 and between 2005 and 2014, at a rate of about 2 m°C yr−1. This warming is also assessed in terms of the rates of change of heights above the bottom for deep isotherms in each deep basin studied. These results, together with findings from previous studies, suggest the deep warming signal observed in the Weddell Sea after the mid‐1970s Weddell Polynya was followed by abyssal warming in the Argentine Basin from the late 1970s through about 2005, then warming in the deep Vema Channel from about 1992 through at least 2010, and warming in the Brazil Basin from 1989 to 2014.

The flow of antarctic bottom water from the Vema channel to the Brazil Basin

The flow of antarctic bottom water from the Vema channel to the Brazil Basin

Measurements of currents in abyssal channels during cruise 32 of the research vessel Akademik Ioffe and cruise 34 of the research vessel Akademik Sergey Vavilov

The investigations of the flows in the abyssal chan� nels of the Atlantic were continued during cruise 32 of the R/V Akademik Ioffe (2010) and cruise 34 of the R/V Akademik Sergey Vavilov (2011). The measure� ments were made in the Discovery Gap (37°20′ N, 15°45′ W), the Kane Gap (9°20′ N, 19°50′ W), in two study regions in the Romanche Fracture Zone (0°47′ N, 13°15′ W; 1°00′ S, 22°00′ W), and in a study region in the Northern part of the Vema Channel (26°43′ S, 34°12′ W). The measurements were carried out using an SBE�19 profiler (CTD profiles). The CTD profiles were combined with measurements of the currents using a Workhorse Sentinel lowered acoustic Doppler current profiler (LADCP). The routes of the vessels and the studied regions are shown in the figure. The cruise of the R/V Akademik Ioffe in 2010 started in Kaliningrad and ended in Ushuaia, and the cruise of the R/V Akademik Sergey Vavilov in 2011 started in Rotterdam and ended in Ushuaia. We studied the propagation of the Antarctic Bottom Water (the AABW, the potential temperature θ < 2°C) from the Southern Ocean to the North Atlantic [1, 2], into which this water spreads mainly through the basins of the West Atlantic beginning from the Argen� tine Basin. The Antarctic Bottom Water propagates from the Argentine Basin to the Brazil Basin through the Vema Channel and then to the basins of the East Atlantic through the Romanche and Chain fracture zones as well as through the Vema Fracture Zone. The Antarctic Bottom Water propagates through the Vema Fracture Zone to the Cabo Verde Basin and Canary Basin, while, through the Romanche and Chain frac� ture zones, this water propagates to the Sierra Leone and Guinea basins. The Kane Gap is the deepest pas� sage connecting the Cabo Verde and Sierra Leone basins. It is located between the Sierra Leone Rise and the Guinea Plateau near the African continent. The Discovery Gap is the terminal point of the AABW’s propagation in the sense of its definition as water with

Structure and evolution of the abyssal jet in the Vema Channel of the South Atlantic

The Vema Channel represents the only major conduit through which the deepest and coldest (<0.2°C potential temperature) Antarctic Bottom Water (AABW) flows from the Argentine into the Brazil Basin. From 2003 to 2007 two current meter moorings were present on each side of the Vema Sill, close to the narrowest spot of the Vema Channel. The data from the moorings are compared with earlier current and temperature observations. On average the maximum current core lies ∼100m above the bottom of the sill with a mean northward speed of 0.3ms−1. Farther up in the water column where Lower Circumpolar Deep Water and North Atlantic Deep Water prevail, one finds a level of sluggish currents with a southward tendency in the sub-centimeter-per-second range. The lower boundary of a layer of ‘no’ motion was observed at ∼3700m depth where the mean potential temperature amounts to 1.5°C.The evolution of the abyssal warming phenomenon over the last decades with notable fluctuations at the choke point between the Argentine and the Brazil Basin differs from the more stable attitude of deep horizontal currents. Starting with CTD observations in 1972 we find a steady increase of temperatures of the coldest AABW in the Vema Channel. This general trend of rising abyssal potential temperatures of almost 2mKelvin per year is based on mostly annual CTD observations. The overall warming trend is fully compatible with our three-year moored temperature series in agreement with earlier records with high temporal resolution.Distinct frequently fluctuating horizontal current shear between the western and eastern sides of the Vema Sill may be explained by two different catchment areas for AABW at the mouth of the Vema Channel. One pathway originates at the American continental rise and advects bottom water in form of the deep western boundary current. A second pathway is supplied by an eastern boundary current along the Mid Atlantic Ridge in the Argentine Basin. Both source waters merge at the channel entrance, mix, and their respective strengths can alternate within the sill area.

SPREADING OF ANTARCTIC BOTTOM WATER IN THE ATLANTIC OCEAN

This paper describes the transport of bottom water from its source region in the Weddell Sea through the abyssal channels of the Atlantic Ocean. The research brings together the recent observations and historical data. A strong flow of Antarctic Bottom Water through the Vema Channel is analyzed. The mean speed of the flow is 30 cm/s. A temperature increase was found in the deep Vema Channel, which has been observed for 30 years already. The flow of bottom water in the northern part of the Brazil Basin splits. Part of the water flows through the Romanche and Chain fracture zones. The other part flows to the North American Basin. Part of the latter flow propagates through the Vema Fracture Zone into the Northeast Atlantic. The properties of bottom water in the Kane Gap and Discovery Gap are also analyzed.

Potencial de transporte sedimentar pelas correntes de fundo na região do Canal de Vema (Atlântico Sul)

Vema Channel is an important passageway to the flow of bottom water between the Argentine and the Brazil basins. The channel morphology causes confinement and intensification of bottom currents that affect the ocean floor through sediment resuspension, transport and deposition, consequently playing a fundamental role in the formation of deep ocean deposits. The main goal of this paper is to evaluate the potential for sediment suspension and transport by the local currents through estimates of nearbed shear stresses as well as to assess whether the present dynamic conditions are conducive to the buildup of contouritic deposits in the Vema channel area. The available datasets include (i) current velocity and direction from the World Ocean Circulation Experiment (WOCE) and from moorings deployed by the Woods Hole Oceanographic Institute (WHOI); (ii) hydrographic parameters from the World Ocean Database 2005; and (iii) the description of bottom sediment characteristics and nephelometric profiles available in publications such as research reports and scientific articles. Calculated shear stresses indicate that local currents would be able to erode and transport medium silt with density similar to that of quartz during up to 87% of the deployment duration, a finding that is reinforced by the nephelometric profiles and by suspended particulate matter concentrations documented by filtered water samples. Channel enlargement downstream leads to loss of transport capacity along the channel. These results suggest that a modern drift deposit may still be under construction downstream the channel mouth wherea contourite fan has intermittently been built since the late Oligocene.

Transport of antarctic waters in the deep channels of the Atlantic Ocean

Starting from 2002, the Shirshov Institute of Oceanology has been conducting investigations of Antarctic water transport in deep channels of the Atlantic Ocean, namely, the Vema Channel (31 ° S), Romanche Fracture Zone (0 ° N), and Vema Fracture Zone (11 ° N). The flow of bottom water in the Vema Channel is estimated as 4 Sv (1 Sv = 10 6 m 3 /s). Velocities in the channel reach 60 cm/s. A strong flow with velocities up to 30 cm/s was recorded in the Vema Fracture Zone. This flow includes the upper part of bottom waters of Antarctic origin and the lower part of North Atlantic Deep waters. The easterly transport of Antarctic waters is 0.1–0.7 Sv. In the Romanche Fracture Zone, maximum velocities reach 10 cm/s, while the entire easterly water transport is estimated as 0.1–0.8 Sv. The dominating propagation of Antarctic waters into the deep basins of the Northeast Atlantic occurs through the Vema Fracture Zone but not through the Romanche Fracture Zone due to strong mixing of deep waters in the latter channel caused by internal tidal waves. Bottom waters are formed at polar latitudes of the World Ocean. In the classic work by Wust [15], all waters of Antarctic origin are called Antarctic Bottom Waters (AABW). They propagate near the bottom in the Atlantic, being formed mainly in the Weddell Sea near the Antarctic slope as a result of mixing of cold and heavy Antarctic Shelf Water with lighter and warmer more saline Circumpolar Deep Waters. The pathways of Antarctic water propagation between the basins of the Atlantic are confined to depressions in the bottom topography. Antarctic Bottom Water from the Weddell Sea propagates through four passages in the South Scotia Ridge and through the South Sandwich Trench. The further propagation of AABW to the north into the Argentine Basin occurs through the Falkland Gap in the Falkland Ridge [14]. Part of this flow propagates along the southern and western margins of the Argentine Basin. The other part is trapped by the Subantarctic Front and flows to the east in the field of the Antarctic Circumpolar Current [14]. The waters of Antarctic origin are later transported to the Brazil Basin along three pathways: in the Vema Channel, in the Hunter Channel, and over the Santos Transport of Antarctic Waters in the Deep Channels of the Atlantic Ocean

Temperature fluctuations and current shear in Antarctic Bottom Water at the Vema Sill

The Vema Channel acts as a major conduit for the equatorward spreading of Antarctic Bottom Water between the Argentine and Brazil Basins. For almost two years the thermal stratification above its saddle depth (4660 m) – called Vema Sill – was recorded by moored thermistors and current meters. The lowest 490 m of the water column was instrumented to monitor the well-developed benthic boundary layer of Antarctic Bottom Water. The latter can be subdivided into Weddell Sea Deep Water on the sea bed and lower Circumpolar Deep Water above it. The data show fluctuations on various scales including periods, each about 1–2 weeks long, when the abyssal stratification virtually disappeared. Assuming a stable ratio between density and temperature, time series of bulk Richardson numbers are estimated from temperature and current shear data. The results suggest a potential for intermittent episodes of locally generated vertical mixing.

Decadal warming of the coldest Antarctic Bottom Water flow through the Vema Channel

A decadal warming trend of Antarctic Bottom Water flowing through the Vema Channel is reanalyzed. Our data base consists of 94 high precision, full depth stations from 19 visits to the Vema Sill plus twelve stopovers at two additional key locations. Originally a long‐term temperature increase in the near‐bottom jet was noted from 1992 onward, after a period of rather constant abyssal temperatures since 1972. From today's perspective the apparent stagnant temperature level until 1991 can be interpreted as a period of feeble rising in comparison with a perspicuous warming trend of 2.8 mK yr−1 in the following 15 years. However, the clearly manifested temperature rise is superimposed with fluctuations. For the first time the available time series appears long enough to indicate an associated slight freshening of the bottom water. An attempt is made to trace the observed variability back to its source region in the Weddell Sea.

Effects of Strait Mixing on Ocean Stratification

The density distribution in the abyssal Atlantic Ocean suggests that mixing associated with overflows across deep sills may account for substantial amounts of deep mixing. Estimates of the strait mixing are made from published estimates of the overflows and the difference between Antarctic Bottom Water densities across the Vema Channel and the Romanche Fracture Zone to demonstrate that the strait mixing is an order of magnitude larger than the abyssal mixing estimated for a standard diffusivity of 1 {times} 10{ndash}4 m2 s{ndash}1

Water masses and baroclinic transports in the South Atlantic and Southern oceans

We describe the World Ocean Circulation Experiment (WOCE) A23 hydrographic section from Antarctica to Brazil, nominally along 35W. The section crossed the center of the Weddell Gyre, the Antarctic Circumpolar Current (ACC) and the Subtropical Gyre in the Argentine Basin. We precisely define the locations of fronts, the changes in water mass properties across them, and their transports. The Antarctic Slope Front was crossed above the continental slope of Antarctica, with a baroclinic transport of 4 Sv, part of the cyclonic Weddell Gyre circulation of 19 Sv. We repeated a section in the Weddell Sea occupied in 1973, and saw a marked warming of the inflowing Warm Deep Water layer by some 0.2°C, but no discernible change in the outflowing northern limb of the gyre. An inflow of recently ventilated water with the same characteristics as Weddell Sea Deep Water (WSDW) was observed flowing into the Weddell Sea from the east. The Weddell Front was crossed at 61°7'S and the Southern Boundary (SB) of the ACC (often referred to as the Scotia Front) at 58°38'S. Between these lay the Weddell-Scotia Confluence, contributing 16 Sv of eastward transport. The first crossing of the Southern ACC Front (SACCF) lay south of South Georgia at ~55°30'S. It then wrapped anticyclonically around South Georgia and was encountered at 53°40'S before retroflecting and returning eastward at ~53°30'S. The baroclinic transport was ~15 Sv at each crossing. In this region the SACCF is most clearly identified by a decrease in the salinity of the temperature minimum layer. The core of the Polar Front (PF) lay at ~49S where the isotherms plunged down sharply to the north, and transported 67 Sv. The PF and Subantarctic Front (SAF) were barely distinguishable with only one station clearly in the Polar Frontal Zone. The SAF, transporting 57 Sv, was encountered at ~48°45'S where the subsurface salinity minimum of Antarctic Intermediate Water (AAIW) began to descend. The Subtropical Front (STF) marks the boundary between the waters of the subtropical gyre and the colder, fresher subantarctic waters to the south; its southernmost crossing was at 44-45S transporting ~25 Sv. This is several hundred kilometers farther south than historical locations of the STF at this longitude. The Brazil Current Front (BCF) was encountered at ~38S transporting 43 Sv. Whereas previous observations found the STF to be the primary means for eastward flow of the waters of the Brazil Current after it has separated from the coast, during A23 we find that the BCF carries the majority of this transport. A further Deep Front, possibly marking the center of the subtropical gyre, was crossed at 34°22'S associated with a transport of 2.5 Sv. In the western Vema Channel we encountered a recirculation of the Brazil Current, flowing to the northeast with a transport of 23 Sv. The section ended prematurely without crossing the Brazil Current. To close the subtropical gyre transport would require a Brazil Current significantly in excess of the historical estimates based on shallow reference levels. Both the SACCF and STF exhibited meanders probably caused by bathymetry. The STF meander may be caused by the eastern end of the circulation around the Zapiola Rise. Both the SB and PF had associated eddies. The SB had shed a cyclonic eddy to the north, which had subsequently been capped by local water. It transported 8 Sv azimuthally. The PF had shed an anticyclonic eddy to the south, which was also capped and circulated at least 3 Sv. A small anticyclonic subsurface lens of AAIW was observed in the Vema Channel with a transport of the order of I Sv. In the Vema Channel, a level of no motion between the North Atlantic Deep Water and the Lower Circumpolar Deep Water (LCDW) gives a net northward flow of 1.2 Sv WSDW and 4 Sv LCDW from the Argentine to the Brazil Basin.

On the circulation of bottom water in the region of the Vema Channel

The circulation and transport of Antarctic Bottom Water ( σ 4<45.87) in the region of the Vema Channel are studied along three WOCE hydrographic lines, the geostrophic velocities referenced to previously published direct current measurements. The primary supply of water to the deep Vema Channel is from the Argentine Basin's deep western boundary current, with no indication of an inflow from the southeast. In the northern Argentine Basin, detachment of lower North Atlantic Deep Water from the continental slope is associated with a deep thermohaline front near 34°S. To the north of this front, the upper part of the AABW bound for the Vema Channel ( σ 4<46.01) exhibits a significant NADW influence. Further modification of the throughflow water occurs near 30°30′S, where the channel orientation changes by ∼50°. Southward flow of bottom water on the eastern flank of the Vema Channel, amounting to ∼1.5 Sv, represents a significant countercurrent to the deep channel transport. Inclusion of this countercurrent reduces the net flow of AABW through the Vema Channel from 3.2±0.7 to 1.7±1.1 Sv. Water properties imply that the near-zero net flow over the Santos Plateau results from a near-closed cyclonic circulation fed by the deep Vema Channel throughflow. A disruption of the northward boundary current in the upper AABW (lower circumpolar water) is required by this flow pattern. The extension of the cyclonic circulation on the Santos Plateau enters the Brazil Basin as a ∼1 Sv flow distinct from the outflow in the Vema Channel Extension (6.2 Sv). The high magnitude of the latter suggests a southward recirculation of bottom water near the western boundary to the north of the region of study.

Deep lenses of circumpolar water in the Argentine Basin

Three deep anticyclonic eddies of a species only reported once before [Gordon and Greengrove, 1986] were intersected by hydrographic lines of the World Ocean Circulation Experiment (WOCE) and South Atlantic Ventilation Experiment (SAVE) programs in the Argentine Basin. The vortices are centered near 3500 m depth at the interface between North Atlantic Deep Water and Bottom Water. They have ∼1500‐m‐thick cores containing Lower Circumpolar Deep Water and a dynamic influence that may span up to two thirds of the water column. As one eddy was observed just downstream of the western termination of the Falkland Escarpment, a destabilization of the deep boundary current by the sudden slope relaxation is suggested as a potential cause of eddy formation. Besides isopycnal interleaving at the eddy perimeters, strongly eroded core properties in the upper parts of the lenses, associated with low density ratios, hint at double diffusion at the top of the structures as another major decay mechanism. The presence of an eddy in the northern Argentine Basin shows the possibility for a northward drift of the vortices, in this basin at least. Deep events in recent current measurements from the Vema Channel are presented that raise the question of further equatorward motion to the Brazil Basin.

About this title

Countourites are w widespread but poorly known group of sediments linked to the action of powerful bottom currents in deep water. Although we know they are especially common along continental margins and through oceanic gateays, they have been surrounded by contoversy since they were first recognized in the early 1960s. Where correctly recognuzed and decoded they can provide one of the keys to our better understanding of bottom water circulation and of the ocean–climate link. They are part of the spectrum of deposits that confronts the oil industry as exploration moves into progressively greater water depths. This memoir is an important outcome of the International Geological Correlation Project 432 on BottomCurrents, Contourites and Palaeocirculation . It includes 30 papers involving over 75 key scientists from around the world. Following and introductory state–of–the–art paper by the editors, there are 25 separate case studies on modern drifts and four on ancient contourite series. Each contribution highlights the specific geological and oceanographic setting, bathymetry, physiographic and stratigraphic context, seismic attributes and sedimentary characteristics of that drift. Case studies range from some of the well-documented North Atlantic drifts to those much less known from the Mediterrenean, from important syntheses of the Gulf of Caidz and Vema Channel Gateway, to completely new data on South Atlantic, Pacific and Antartic margin systems. The four papers on ancient series from Japan, China and Cyprus serve to emphasise the complex nature and subtle characteristics of contourites, which make their identification a scientific challenge. This volume is dedicated to the memory of Charlie Hollister (1936–1999), one of the founding fathers and pioneers of countourite research.

Chlorofluorocarbon‐derived formation rates of the deep and bottom waters of the Weddell Sea

The spreading and mixing of deep and bottom waters from the Weddell Sea (originating with potential temperature &lt;0°C) are investigated through analysis of new and historical CFC measurements. We investigate the component that enters the southern Indian Ocean with a simple one‐dimensional advection‐diffusion model of the deep boundary current. This model does not require a condition of mixing solely with tracer‐free water to be imposed. As boundary conditions, we derive time histories for the CFC concentrations of newly formed Weddell Sea deep and bottom waters. A range of best fit velocities is obtained by determining which model velocities produce model tracer concentrations closest to measured concentrations. From this, we derive a formation rate of 3.2±1.5 Sv for the waters originating with temperatures lower than 0°C that subsequently enter the southern Indian Ocean. This compares to an upper limit for these waters of 5.7 Sv derived from the CFC‐11/CFC‐12 ratio. Whilst the former value is dependent on the highly idealized concepts of the model used, the latter is a more robust upper limit. Using the CFC‐11/CFC‐12 ratio measured at Vema Channel during the World Ocean Circulation Experiment cruise A23, an upper limit of 0.9 Sv is derived for the formation rate of waters colder than 0°C that enter the South Atlantic. Overall, an upper limit of 6.6 Sv is derived for the rate of formation of the deep and bottom waters of the Weddell Sea. A better representation is obtained by combining the upper limit to the South Atlantic component with the model‐derived southern Indian Ocean estimate; we thus estimate the formation rate of the deep and bottom waters in the Weddell Sea to be 3.7±1.6 Sv. Whilst the range obtained is large and the assumptions implicit in the calculations are potentially significant, we believe this more realistically represents the true formation rate than estimates derived using an imposed condition of tracer‐free mixing. The range can be narrowed in future as more data become available.