Antarctic Bottom Water Circulation Research Articles

Interplay of deep-marine sedimentary processes with seafloor morphology offshore Madeira Island (Central NE-Atlantic)

The deep-water sedimentary processes and morphological features offshore Madeira Island, located in the Central-NE Atlantic have been scantly studied. The analysis of new multibeam bathymetry, echo-sounder profiles and few multichannel seismic reflection profiles allowed us to identify the main geomorphologies, geomorphic processes and their interplay. Several types of features were identified below 3800 m water depth, shaped mainly by i) the interplay between northward-flowing Antarctic Bottom Water (AABW) and turbidity currents and ii) interaction of the AABW with oceanic reliefs and the Madeira lower slope. Subordinate and localized geomorphic processes consist of tectono-magmatic, slope instability, turbidity currents and fluid migration. The distribution of the morphological features defines three regional geomorphological sectors. Sector 1 represents a deep-seafloor with its abyssal hills, basement highs and seamounts inherited from Early Cretaceous seafloor spreading. Sector 2 is exclusively shaped by turbidity current flows that formed channels and associated levees. Sector 3 presents a more complex morphology dominated by widespread depositional and erosional features formed by AABW circulation, and localized mixed contourite system developed by the interplay between the AABW circulation and WNW-ESE-flowing turbidite currents. The interaction of the AABW with abyssal hills, seamounts and basement ridges leads to the formation of several types of contourites: patch drifts, double-crest mounded bodies, and elongated, mounded and separated drifts. The patch drifts formed downstream of abyssal hills defining an previously unknown field of relatively small contourites. We suggest they may be a result of localized vortexes that formed when the AABW's flow impinges these oceanic reliefs producingthe erosional scours that bound these features. The bottom currents in the area are known to be too weak (1–2 cm s−1) to produce the patch drifts and scours. Therefore, we suggest that these features could be relics at present, having developed when the AABW was stronger than today, as during glacial/end of glacial stages.

Southern Ocean Oxygenation Changes Inferred From Redox‐Sensitive Trace Metals Across Marine Isotope Stage 11

AbstractChanges in the circulation of the Southern Ocean are known to have impacted global nutrient, heat, and carbon cycles during the glacial and interglacial periods of the late Pleistocene. Proxy‐based records of these changes deserve continued scrutiny as the implications may be important for constraining future change. A record of authigenic uranium from the South Atlantic has been used to infer changes in deep‐sea oxygenation and organic matter export over the past 0.5 million years. Since sedimentary uranium has the possible complication of remobilization, it is prudent to investigate the behavior of other redox‐sensitive trace metals to confidently interpret temporal changes in oxygenation. Focusing here on the exceptionally long interglacial warm period, Marine Isotope Stage (MIS) 11, we found concurrent authigenic enrichments of uranium (U) and rhenium (Re) throughout MIS 12 to 10, overall supporting prior interpretations of low‐oxygen periods. However, there are differential responses of Re and U to oxygen changes and some evidence of small‐scale Re remobilization, which may involve differences in molecular‐level reduction mechanisms. Peaks in authigenic manganese (Mn) intervening with peaks in Re and U indicate increases in porewater oxygenation which likely relate to increased Antarctic Bottom Water circulation at the onset of MIS11c and during the peak warmth of the interglacial around 400 ka.

A Synoptic View of the Ventilation and Circulation of Antarctic Bottom Water from Chlorofluorocarbons and Natural Tracers.

Antarctic Bottom Water (AABW) is the coldest, densest, most prolific water mass in the global ocean. AABW forms at several distinct regions along the Antarctic coast and feeds into the bottom limb of the meridional overturning circulation, filling most of the global deep ocean. AABW has warmed, freshened, and declined in volume around the globe in recent decades, which has implications for the global heat and sea level rise budgets. Over the past three decades, the use of tracers, especially time-varying tracers such as chlorofluorocarbons, has been essential to our understanding of the formation, circulation, and variability of AABW. Here, we review three decades of temperature, salinity, and tracer data and analysis that have led to our current knowledge of AABW and how the southern component of deep-ocean ventilation is changing with time.

Magnetostratigraphic evidence for deep-sea erosion on the Pacific Plate, south of Mariana Trench, since the middle Pleistocene: potential constraints for Antarctic bottom water circulation

The Antarctic Bottom Water (AABW) current plays a crucial role in storing and transporting heat, water, and nutrients around the world. However, it is impossible to monitor AABW in the Plio-Pleistocene by direct measurement. Hence, abyssal erosion was usually chosen as an effective indicator of the presence of the AABW in the Indian and Eastern Pacific Oceans during that period. Here, we report a high-resolution magnetostratigraphy of a gravity core, the JL7KGC-01A from the south of the Mariana Trench, northwest Pacific Ocean. The main results are as follows: (1) polarity data suggest that the sequence recorded the late Gauss chron to the early Brunhes chron, including the Jaramillo, Cobb Mountain, and Olduvai normal subchrons; (2) the sedimentary processes in the study area since 2.9 Ma show three stages of sedimentation: 83 cm/Ma during 2.9–1.2 Ma, 183 cm/Ma during 1.2–0.7 Ma, and no sedimentation since ~0.7 Ma; (3) the area south of the Mariana Trench experienced a significant change in the deposition rate at 1.2 Ma, which could be correlated with the intensified desertification in inland Asia, and experienced a prominent depositional hiatus since the early middle Pleistocene, which likely resulted from the enhanced/expanded AABW. Based on these new polarity data and comparisons with previous studies around the Pacific Ocean, we therefore propose that the AABW experienced a notable change during the early–mid Pleistocene transition.

Spatial Distribution and Longitudinal Variation of Clay Minerals in the Central Indian Basin

Abstract:Grain size and clay mineral distribution up to 45 cm depth in the silty clay sediments from 26 box cores from 10° to 16° S along four longitudes (73.5°‐76.5°E) were studied for understanding spatial variability in the Central Indian Basin (CIB). It was observed that the average sand content in the basin is 3.8%, which decreases systematically and longitudinally to 0.3% towards south. The average illite and chlorite major clay mineral abundance also decrease southwards along the four longitudes from 10°S, and show the limit of influence of the Ganges‐ Brahmaputra river's supply up to 10° S. However, the average clay content increases from west to east in the basin, and southwards along 73.5°E and either side of the 76.5°E fracture zone (FZ), which strongly suggests the possibility of clay supply due to circulation of Antarctic Bottom Water (AABW) from the south through the FZ. The distribution of four clay minerals along 73° and 76.5° E FZ in the CIB shows dissimilar trends of increase and decrease, and indicate a mix environment in the basin. This study indicates that the FZ in the CIB has an important role in controlling the distribution of clay minerals.

Giant mounded drifts in the Argentine Continental Margin: Origins, and global implications for the history of thermohaline circulation

Partially buried giant drifts are located in the southern-most sector of the Argentine continental margin, generating a bathymetric jump at the base of the slope. They are characterised as giant, asymmetrical elongated, mounded contourite drifts, and are described in detail here for the first time. This description is mainly based on the bathymetric and multichannel seismic reflection profiles data. Their origin and evolution in the Argentine Basin are decoded and their implications in an area crucial for geologic and palaeoceanographic reconstruction between the Atlantic and Antarctica are discussed. The buried giant drifts are divided into two zones by a large seamount. The major giant-drift (50 km wide, 300 km long, and having a sedimentary thickness of nearly 1 km) is located to the south of this seamount and oriented to the north, where its summit outcrops at present seafloor. Here, its asymmetric external shape is characterised by a steep, western side and a gently dipping, smooth eastern side, with internal reflections prograding eastward. Based on its position, morphology and internal characteristics, it has been deduced that this giant-drift was generated in the Argentine Basin by the southward branch of a confined large loop of the Antarctic Bottom Deep Water, from the Eocene-Oligocene boundary (coevally with the Drake Passage opening) until the middle Miocene. However, after a major palaeoceanographic change in the mid-to-late Miocene a new oceanographic scenario was established that fossilised these giant drifts. These changes produced a new depositional style in the margin, resulting in initiation of the present contourite depositional system and morphologic features over the slope. Extension of North Atlantic Deep Water circulation into the Southern Hemisphere and deepening of Antarctic Bottom Water circulation in the Argentine Basin are herein postulated as the factors that controlled said changes. These giant drifts show strong seismic evidence of hydrates and free gas that are ripe for energy resources exploration, thus demonstrating the economic potential of contourite deposits in deep marine environments.

Contourite depositional system on the Argentine Slope: An exceptional record of the influence of Antarctic water masses

A significant contourite depositional system (CDS) on the continental slope of the southern Argentine margin is described here for the first time. This system contains both erosive and depositional features that have resulted from several factors, including topographic intensification of the Antarctic-sourced water masses, the systematic northward decrease in speed of these water masses, a northward increase of downslope sedimentary processes, and local tectonic influences. This system is an exceptional example of a CDS that started to develop at the time of the Eocene-Oligocene boundary, potentially coeval with the opening of the Drake Passage. However, a new margin morphology, characterized by a complex terraced slope lacking any continental rise, developed after a major paleoceanographic change in the middle to late Miocene. We infer that this change resulted from the extension of North Atlantic Deep Water circulation into the Southern Hemisphere and the deepening of Antarctic Bottom Water circulation in the Argentine Basin.

Nonuniform Upwelling in a Shallow-Water Model of the Antarctic Bottom Water in the Brazil Basin*

Abstract A numerical model based on the shallow-water equations is developed to represent the flow of Antarctic Bottom Water (AABW) in the Brazil Basin (southwest Atlantic Ocean). The aim is twofold. First, an attempt is made to identify in a model that includes both simplified dynamics and realistic bathymetry (at 1/6° resolution) the impacts of the elevated diapycnal mixing rates near the Mid-Atlantic Ridge (MAR) documented by dissipation data of the Deep Basin Experiment (DBE). To this end, different assumptions regarding the distribution of the velocity across the AABW layer interface (w) are considered. Second, the extent to which the shallow-water model can replicate observations relative to AABW circulation in the basin, in particular the trajectory and velocity of neutrally buoyant floats released in the AABW during the DBE, is examined. The model flows are characterized by small Rossby numbers, except in the northward-flowing western boundary current where kinetic energy is largely concentrated. To interpret the flows, model streamlines are compared with isopleths of linear potential vorticity f/h0 of the shallow-water theory (f is the planetary vorticity and h0 is the layer thickness in the absence of motion). The f/h0 contours are oriented northwest–southeast in the western part of the basin and southwest–northeast in the eastern part, reflecting the bowl-shaped topography of the Southern Hemisphere basin. With a spatially uniform (positive) w, the ubiquitous vortex stretching produces a flow to the southeast, consistent with the Stommel–Arons theory. This flow occurs in most of the basin interior, even in the east where f/h0 contours converge to the northeastern end of the basin. With strongly positive w near the ridge and zero or slightly negative w elsewhere, the flow follows more closely f/h0 contours in the western interior and intersects them near the ridge. The confinement of the diapycnal mass flux near the MAR drastically reduces the southward flow in the interior or even reverses its direction, leading to a circulation quite distinct from that of the Stommel– Arons theory. The model results compare favorably to some (but not all) hydrographic estimates of AABW circulation patterns and rates. On the other hand, the model streamlines and velocities show important differences with, respectively, the trajectory and the velocity of the floats launched in the AABW layer. The prescription of vanishing w in the interior does not systematically improve the fit of the model streamlines to the float trajectories, and the model velocities simulated with spatially uniform w or spatially variable w are on average smaller by one order of magnitude than the float velocities. A variety of mechanisms, which are not included in the numerical experiments, may explain the differences between the model results and the float data.

Parameterization of density-driven downsloping flow for a coarse-resolution ocean model in z-coordinate

In the World Ocean, densest waters found on the continental shelves induce density drivendownsloping currents that can influence the deep ocean water masses properties. This processis poorly represented in z-coordinate ocean models, especially in Ocean General CirculationModel (OGCM) with coarse resolution in both horizontal and vertical directions. Consequently,continental shelves appear to be too isolated from the open ocean, whereas the density remainstoo low in the deep ocean. This study presents a simple parameterization of downsloping flowdesigned for z-coordinate, coarse resolution ocean model. At the shelf break, when the densityon the shelf is higher than that in the neighbouring deep water column, a downsloping currentis set up. This current is linearly related to the horizontal density gradient between the twoadjacent boxes, using a prescribed coefficient. For simplicity, a uniform value of the coefficientis used here, although it should ideally vary in space. From the shelf, the downsloping flow isassumed to go downward along the slope until it reaches a level of equal density. An upwardreturn flow of equal magnitude maintains the conservation of mass. This parameterization hasbeen implemented in an OGCM and two experiments, with and without this scheme, have beenintegrated until equilibrium using restoring boundary conditions. The impact of the downslopingparameterization on the global ocean is dominated by the improvement of the Antarcticbottom water circulation and water mass properties. The parameterization increases the densityof the deep ocean and tends to reduce the intensity and depth of the North Atlantic deep watercirculation, which is in better agreement with observations. As a result of a higher exchangewith the open ocean, the properties of continental shelf waters are also improved, with a markedreduction of the Antarctic shelves salinities. Therefore, this simple parameterization leads to asignificant improvement of the model results, at little computational cost.

Manganese deposition through geological time: dominance of the post-eocene deep-sea environment

The total terrestrial manganese ore deposits amount to about 6.4×10 9 tonnes of which 5.3×10 9 tonnes (82%) are Precambrian and 1.1×10 9 tonnes (18%) Mesozoic and Cenozoic. Deep-sea manganese nodules formed since the lower Miocene unconformity (12 Ma B.P.) hold about 10 11 tonnes Mn (about 16 times the manganese in terrestrial deposits). The development of deep-sea manganese nodules is a reflection of global cooling and resulted from the transition of the sluggish Cretaceous ocean with warm saline bottom water to the post-Eocene ocean with cold, well-oxygenated bottom curents. Important climatic and oceanographic events occurred at the Eocene-Oligocene boundary and more particularly at the lower Miocene unconformity which marks the period of increased Antarctic Bottom Water circulation. By contrast, the giant shallow-water manganese deposits of the Lower Jurassic to Oligocene are associated with anoxia and high sea-level stands. The two types of deposit reflect different oceanographic conditions and tend not to be contemporaneous. Mass balance considerations suggest that the present-day input of manganese into the oceans is in balance with its deposition in manganese nodules and sediments with perhaps 90% of the input being from hydrothermal sources, although this is speculative. The scale of present-day deep-sea manganese nodule formation suggests that we live in the manganese era.

Change from “Atlantic-type” to “Pacific-type” carbonate stratigraphy in the middle Pliocene Equatorial Pacific Ocean

A paleomagnetically dated piston core from the Equatorial Pacific Ocean (V28-179) contains levels of highest carbonate percentage in association with lower δ 18 O values in early Pliocene sediments. This relationship is typical of the Atlantic during the Quaternary and is thus referred to as an “Atlantic-type” carbonate stratigraphy. The change from an “Atlantic-type” to a “Pacific-type” stratigraphy (high-carbonate glacial episodes) appears to have occurred during the earliest Gauss Magnetic Chron (3.18–3.41 Ma) of middle Pliocene age, and may be related to the development of Northern Hemisphere glaciation. Samples deposited prior to the development of Northern Hemisphere glaciation in V28-179 have a significant proportion of low-frequency ( > 400- ka period ) variance in both the benthic O 18 and carbonate spectra, and an “Atlantic-type” correlation. Both the oxygen-isotopic and the carbonate spectra of V28-179 have frequency-specific correlation (coherency) with the calculated Pliocene record of the eccentricity of Earth's orbit. Thus, orbital parameters may have affected Equatorial Pacific sedimentation in the Pliocene. The change from “Atlantic-type” to “Pacific-type” carbonate stratigraphy in the middle Pliocene may have been related to a change in the dominant frequency of variance of carbonate oscillations related to orbital parameters, and/or to intensification of Antarctic Bottom Water circulation during the middle Pliocene near the base of the Gauss Chron.

Response of deep-sea benthonic Foraminifera to development of the psychrosphere near the Eocene/Oligocene boundary

A dominant feature of present-day thermohaline circulation in oceans is the production of cold Antarctic Bottom Water (AABW) in the Weddell and Ross Seas and along the continental margin off Antarctica. The cold, dense AABW formed at or near the surface sinks to abyssal depths and travels throughout the ocean basins. AABW circulation was suggested to have begun near the Eocene/Oligocene boundary with the development of the psychrosphere (lower cold layer of a two-layer ocean with temperatures <10°C) inferred from ostracode faunal data1 and oxygen isotopic evidence from closely-spaced samples in DSDP site 277 (ref. 2). Previous studies suggested that the temperature change associated with the development of the psychrosphere caused a crisis for deep-sea ostrocodes1 and deep-sea benthonic Foraminifera3. I consider here the response of benthonic Foraminifera to the development of the psychrosphere.

Recent sedimentation in the southeast Indian Ocean with special reference to the effects of Antarctic Bottom Water circulation

In the southeast Indian Ocean the influence of the Anarctic Bottom Water (AABW) on sedimentation is evidenced by the intensity of the water turbidity near the bottom, and by the mineralogy (carbonate and quartz abundances) of the sediments and current markings on the sea floor. In the deep basins south of 50°S (South Indian and Tasman basins), besides high biogenic siliceous opal productivity, the greater influence of AABW is recorded in shallower carbonate compensation depths (<4000 m) than is the case in basins north of 50°S (5000–5200 m). Spreading of the AABW from the regions south of the Southeast Indian Ridge to the regions north of it is through fracture zones in the ridge at about 123–126°E and 145–155°E. These bottom-water passages are characterized by (1) low carbonate, high quartz, or high clay contents in the sediments, (2) distinct current markings suggesting the northward flow, and (3) high bottom-water turbidity. In large basins such as the South Indian and Tasman basins which have moderate-to-high near-bottom turbidity, central portions tend to be less turbid than the margins, suggesting strong marginal circulation. The Emerald Basin, situated east of the Tasman Basin and north of the Indian-Pacific Ridge, has high near-bottom turbidities. This suggests that the AABW in the Emerald Basin is derived directly from the source regions south of the ridge although some of the water may also be spreading from the Tasman Basin.

Clay mineralogy and sedimentation in the western Indian ocean

The clay mineralogy of 235 surface sediment samples from the western Indian Ocean reveals many sedimentary sources and processes: (1) some areas, mainly in the central Indian Ocean, where very little sediment transport is involved, have smectite-rich clays derived from the alteration of in situ submarine basalts. (2) In many other areas, the mineralogy is significantly influenced by several modes of long-distance sediment transport, and the continental climate or the southern ocean volcanism. These areas consist of: (a) smectite-rich sediments in the Crozet and Madagascar basins and in the eastern Arabian Sea; (b) illite-rich sediments in the Indus Cone, the Agulhas Basin, and the ocean adjacent to Africa west of the Agulhas Plateau; (c) illite-palygorskite-rich sediments in the Carlsberg Ridge, Owen Ridge and in the westernmost Arabian Sea; (d) illite-kaolinite-rich sediments in the Mozambique Basin and in the regions between Madagascar and Africa; (e) kaolinite-rich sediments adjacent to central Africa and Madagascar. (3) A third category of areas has mixed type sediments with no specific mineral in characteristically high amounts. Mixing of sediments derived from the alteration of in situ submarine basalts and/or from other areas rich in different minerals resulted in this group. The mineral distributions suggest that the Antarctic Bottom Water circulation is responsible for the sediment dispersal in the Crozet, Madagascar, Agulhas and Mozambique basins. Although the aeolian process could be effective throughout the Arabian Sea, it is dominant only in the western Arabian Sea.

Spreading of Antarctic Bottom Water and its effects on the floor of the Indian Ocean inferred from bottom-water potential temperature, turbidity, and sea-floor photography

Data on bottom-water potential temperature, turbidity and current indications show that in the Southern Ocean west of the Kerguelen Plateau, Antarctic Bottom Water (AABW) of Weddell Sea origin spreads northwards from the Atlantic—Indian Basin in two directions: (1) AABW enters the Agulhas Basin through relatively deep areas in the Mid-Indian Ridge at 20–25°E and possibly at 35°E, and flows northwards into the Mozambique Basin as far as its northern limits; (2) a more easterly spreading path extends from the Atlantic—Indian Basin through the Crozet into the Madagascar, Mascarene, Somali and Arabian Basins. The passage in the western branch of the Indian Ridge for the AABW spreading from the Crozet into the Madagascar Basin appears to be at 29-26°S and 60–64°E. East of the Kerguelen Plateau in the South Indian Basin, the bottom water formed mainly along the Adélie Coast and Ross Sea travels west towards the Kerguelen Plateau and then parallel to it. This water finally flows eastwards hugging the Southeast Indian Ridge. Significant deviations from this general circulation pattern occur due to local topographic effects. Some AABW in the South Indian Basin exits through a passage at 120–125°E in the region of the Australian—Antarctic discordance in the Southeast Indian Ridge and enters the South Australian Basin and subsequently the Wharton Basin. This passage is clearly indicated by the northward extension of a cold, bottom-water tongue as shown by the temperature distribution in the region; the bottom-water effects in the passage are reflected in the high turbidity and current lineations on the sea floor. In the Southern Ocean basins, bottom-water turbidity is generally high, reflecting in part the strong bottom-water activity. The effects of AABW circulation on the sea floor—in the form of well-developed small- or large-scale current ripples and erosional/depositional features, manganese-nodule formations, and unconformities and reworking of sediments observed in cores — are also marked in these basins. Even though the AABW in the Wharton Basin is cold, its spreading effects on the sea floor are minimal in this basin in contrast to the basins west of the Mid-Indian Ridge at comparable latitudes.