Alexander N. Demidov

Abyssal Channels in the Atlantic Ocean: Water Structure and Flows

Foreword. Preface. Acknowledgements. 1. Geological and geophysical characteristics of the transform fault zones. 1.1 General description. 1.2 Charlie Gibbs Fracture Zone. 1.3 Vema Fracture Zone. 1.4 Romanche Fracture Zone. 1.5 Chain Fracture Zone. 1.6 Vema Channel. 2. Deep water masses of the South and North Atlantic. 2.1 General description. 2.2 Antarctic Intermediate Water. 2.3 Upper Circumpolar Water and Upper Circumpolar Deep Water. 2.4 North Atlantic Deep Water. 2.5 Lower Circumpolar Water and Lower Circumpolar Deep Water, Circumpolar Bottom Water, Southeast Pacific Deep Water, and Warm Deep Water. 2.6 Antarctic Bottom Water. 3. Source Regions, Abyssal Pathways, and Bottom Flow Channels (for waters of the Antarctic origin). 3.1 General description. 3.2 Weddell Sea and Weddell Gyre. 3.3 Agulhas and Cape basins. 3.4 Drake Passage, Scotia Sea, and Georgia Basin. 3.5 Antarctic Bottom Water in the Argentine Basin. 4. Exchange between the Argentine and Brazil basins Abyssal pathways and bottom flow channels (for waters of the Antarctic origin). 4.1 General description. 4.2 Vema Channel. 4.3 Santos Plateau. 4.4 Hunter Channel. 5. Further propagation of Antarctic Bottom Water from the Brazil Basin. 5.1 Brazil Basin. 5.2 Flow in the Guiana Basin and westward equatorial channels. 5.3 North American Basin. 5.4 Eastward Equatorial Channels. The Romanche and Chain Fracture Zones. 5.5 Vema Fracture Zone. 5.6 Eastern Basin Pathways and further propagation of Antarctic Bottom Water in the East Atlantic. 5.7 Kane Gap. 5.8 Angola Basin. 6. Flows through the Mid-Atlantic Ridge in the northern channels. Charlie Gibbs Fracture Zone and other fracture zones. Integrated conclusions. References. Index.

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

Deep Water Masses of the South and North Atlantic

The Southern Ocean and Antarctic Circumpolar Current isolate the Antarctic continent from other regions of the Earth. Thus, conditions in the study region provide the formation of a special water structure around Antarctica, whereas water structure in the northern regions is determined by interactions between waters of the North Atlantic and those of Arctic origin.

On the warming of intermediate and deep waters in the equatorial North Atlantic

In July 2000, a transatlantic hydrographic section was made on board the Russian R/V Akademik Ioffe in the northern equatorial region at ∼6.5° N on the WOCE (World Ocean Circulation Experiment) A06 line. A significant warming in the layers of intermediate and deep waters in the interior eastern basin is determined from comparison of the section temperature data and those obtained on the WOCE A06 line in 1993. This result, together with the results of the previous studies, indicates a substantial warming of intermediate and upper deep waters above 2800–3000 m in the eastern equatorial North Atlantic during the second half of the 20th century. In the 1000–2000 m layer, temperature has increased by 0.13–0.14°C since 1957.

Structure and Transport of Bottom Waters through the Chain Fracture Zone of the Mid-Atlantic Ridge

During the 27th cruise of the research vessel Akademik Ioffe in 2009, the measurements in the area of the main saddle of the Chain fracture zone were carried out. They included the CTD-sounding and determination of current speeds by lowered acoustic Doppler current profiler (LADCP). As a result, the structure of waters was determined and the transport of bottom waters was estimated as 0.11–0.17 Sv that is significantly less than the previous estimates. The variability of bottom potential temperature values as compared with the measurements carried out in 1991–1994 is registered. At the station situated on the main saddle, the potential temperature increased by 0.1°C and at the station located behind the saddle, it decreased by 0.034°C.

Exchange Between the Argentine and Brazil Basins; Abyssal Pathways and Bottom Flow Channels (for Waters of the Antarctic Origin)

The zonally aligned Rio Grande Rise separates the Argentine Basin in the south from the Brazil Basin in the north. It is a high topographic obstacle for bottom water propagation to the north. Two meridional gaps intersect the Rise at ~39° W and ~28° W (Vema and Hunter channels, respectively). The depth in the Vema Channel exceeds 4,600 m as compared to the background depths of 4,200 m. The Hunter Channel is much shallower and the greatest depth does not exceed 4,000 m.

Further Propagation of Antarctic Bottom Water from the Brazil Basin

In this chapter we assume Wust’s (1936) sense of Antarctic Bottom Water propagation. In other words, this is the bottom water of the Antarctic rather than North Atlantic origin. However there is no generally accepted isotherm or value of any characteristics in literature for the upper boundary of Antarctic Bottom Water in the Equatorial and North Atlantic. Therefore, it is essential that quantitative estimates of Antarctic Bottom Water properties and transport depend on the choice of this boundary.

Flows through the Mid-Atlantic Ridge in the Northern Channels. Charlie Gibbs Fracture Zone and Other Fracture Zones

In the winter period, cold dense water is formed in the Norwegian and Greenland seas due to severe cooling and intense heat release to the atmosphere. The water mass from the Greenland Sea overflows the shallow threshold between Greenland and Iceland and flows into the Irminger Basin. The water mass from the Norwegian Sea overflows the threshold between Iceland, Faeroe, and Shetland Islands and flows into the Iceland Basin. The water mass, which is formed during the overflow over the latter thresholds, is called Iceland Scotland Overflow Water (ISOW). In the Iceland Basin this water is mixed with the significantly warmer and more saline waters in the Northeastern Atlantic, which are subject to the influence of Mediterranean waters. As a result, Iceland Scotland Overflow Water appears more saline and warmer than the deep waters propagating into the Atlantic through the Denmark Strait between Greenland and Iceland (Denmark Strait Overflow Water, DSOW). Iceland Scotland Overflow Water flows first to the west with the Deep Northern Boundary Current and then to the south along the eastern slope of the Reykjanes Ridge. The circulation scheme of different layers of the North Atlantic is shown in Fig. 2.6.

Source Regions, Abyssal Pathways, and Bottom Flow Channels (for Waters of the Antarctic Origin)

Generally, propagation of Antarctic waters in the bottom layer of the Atlantic Ocean is confined to depressions in the bottom topography. The general flow of these waters can be presented as follows (Fig. 3.1).

Geological and Geophysical Characteristics of the Transform Fault Zones

From the point of view of the new concept of global tectonics, oceanic fracture zones consist of one or more transform faults. Their geomorphological descriptions were given for the first time in (Wilson 1965; Menard 1966; Menard and Chase 1970). The authors defined the transform faults as long narrow zones of strongly rugged bottom topography characterized by the existence of linear forms that usually separate topographic provinces with different regional depths. In the transform zones, fault-line ridges are extended parallel to the troughs. It is noteworthy that remains of shallow-water sediments are found on some of the fault-line ridges and bottom terraces. A fracture zone located between contiguous spreading axes is called an active zone. The fracture walls between neighboring spreading axes are characterized by opposite directions of motion. Passive parts of transform faults are located beyond the active zone, but the direction of their motion is the same. Transform faults are distinguished well not only in the ocean bottom topography, but also in anomalous geophysical fields. High fault-line ridges near the walls of faults (mainly between the neighboring parts of spreading axes), deep troughs, faults, and fissures are characteristic of the fracture zones, which represent an assemblage of deep and bottom structures. Anomalies of the magnetic and gravity fields, undulations of the heat flux, and other geophysical data testify to a complex dynamic regime of lithosphere in the fracture zones. Active parts of transform faults are also characterized by the most intense seismicity with clear manifestation of the shear component in earthquake sources.