N. I. Makarenko

Flows of Antarctic bottom water through fractures in the southern part of the North Mid-Atlantic Ridge

We study the flows of bottom waters of the Antarctic origin in deep fracture zones of the southern part of the North Mid-Atlantic Ridge. In the autumn of 2014, an expedition onboard the RV Akademik Sergey Vavilov carried out measurements of current velocities and thermohaline properties of bottom water in several quasi-zonal fractures in the southern part of the Northern Mid-Atlantic Ridge, which connect the deep basins of the West and East Atlantic, the Vema Fracture Zone (FZ) (10°50′ N) and a group of sub-equatorial fractures: Doldrums (8°15′ N), Vernadsky (7°40′ N), and a nameless fracture at 7°30′ N. The estimates of bottom water (θ < 2.0°C) transport through this group based on measurements from 2014 are approximately 0.28 Sv (1 Sv = 106 m3/s), which is close to 25% of the transport estimate through the Vema FZ (1.20 Sv) obtained in the same expedition. The coldest bottom water temperatures among the investigated fractures were recorded in the Vema FZ.

Antarctic bottom water flow in the western part of the Romanche Fracture Zone based on the measurements in October of 2011

The properties of the Antarctic Bottom Water flow in the region of its inflow to the channel of the Romanche Fracture Zone at 22°10′–22°30′ W are studied on the basis of CTD and LADCP profiling in the western part of the equatorial fracture zone. A deep water cataract was found at the sill over the southern wall of the fracture with a depth of approximately 4600 m, which is associated with the abyssal flow, whose potential temperature is lower than 1°C. The inflow of water into the channel of the fracture in this temperature range is fully localized over this sill. The minimum potential temperature θ recorded in 2011 near the bottom was equal to 0.51°C, which is lower approximately by 0.12°C than the minimum temperatures ever measured in the western part of the fracture. The water transport in the cataract was estimated at 0.2 Sv (1 Sv = 106 m3/s), which is approximately 30% of the known estimates of the total transport of Antarctic Bottom Water (θ < 1.9°C) through the fracture. The extremely high intensity of the cross isothermal mixing in the cataract region was found. The analysis of the bottom topography data, including the historical WOD09 dataset, shows that the inflow of water with 1.00° < θ < 1.70°C into the channel of the fracture is most likely fully localized in a few passages in the region of the survey in 2011, while the water exchange with the abyssal waters with θ > 1.70°C through the Romanche Fracture Zone between the West and East Atlantic can also occur through the depressions in the southern and northern walls of the fracture in the region of the Vema Deep.

Abyssal cataracts in the Romanche and Chain fracture zones

1211 The Romanche Fracture Zone is a deep (up to 7850 m in the Vema Deep at 18°30′ W), long passage in the equatorial zone of the Mid Atlantic Ridge in the Atlantic. This fracture along with the Chain Fracture Zone located a few degrees to the south (Fig. 1) pro vides the inflow of cold Antarctic waters from the western part of the Atlantic to the eastern basins of the equatorial zone. Then this water spreads to the south into the Angola Basin and, possibly, to the north through the Kane Gap. In the latter case, this water merges with the water transported from the West Atlantic through the Vema Fracture Zone at 11° N and fills the abyssal of the northeastern Atlantic basins. Thus, the Romanche and Chain fracture zones are very important elements in the water exchange of bot tom waters between the West and East Atlantic.

Measurements of bottom currents in underwater channels of the atlantic during cruise 36 of the R/V Akademik Sergey Vavilov

762 The investigation of the pathways of the Antarctic Bottom Water’s propagation in the abyssal channels of the Atlantic were continued during cruise 36 of the R/V Akademik Sergey Vavilov (October–November, 2012). We carried out measurements of the tempera ture, salinity, and current velocity in the Kane Gap, in the western part of the Romanche Fracture Zone, and in the northern part of the Vema Channel. The Con ductivity–Temperature–Depth profiling (CTD pro filing) was performed using an SBE 19 profiler. The current’s velocity was measured by a lowered acoustic doppler current profiler (LADCP RDI WHS 300 kHz). The cruise started in Rotterdam and ended in Ushuaia. The route of the cruise and the studied regions are shown in the figure. The Vema Channel, Romanche Fracture Zone, and Kane Gap are related to the key abyssal channels through which the Antarctic Bottom Water (AABW) with a potential temperature of θ < 2.0°C is trans ported from the South Atlantic to the northern basins of the Atlantic. The Vema Channel is a long (more than 700 km) and the deepest channel connecting the abyssal depths of the Argentine and Brazil basins. It is a pathway for the coldest part of the AABW (θ < 0.0°C) [2]. The equatorial Romanche Fracture Zone in the Mid Atlantic Ridge (figure) along with the Chain Fracture Zone located two degrees to the south are the main sources of cold bottom waters in the equatorial basins of the East Atlantic and in the Angola Basin [2]. The Vema Fracture Zone, through which the AABW also spreads to the East Atlantic and then flows to the north reaching the West European Basin, is located at 11° N. The flows of the AABW that propagated from the West Atlantic through the Vema and Romanche fracture zones merge in the region of the Kane Gap, which is the deepest bottom depression (approxi mately 4560 m) connecting the Sierra Leone Basin in the south and the Cabo Verde Basin in the north [2]. Unlike the measurements in the southern part of the Vema Channel, the measurements in its northern wide part were not numerous. In the 1990s, German scientists started the works in this region. Russian sci entists from the Shirshov Institute of Oceanology con tinued these investigations in 2003, 2004, 2009, and 2010 [1]. The main goals of the works in 2012 were the measurements of the thermohaline parameters and the assessment of the flow intensity of the coldest part of the AABW, which, as was supposed based on the digital data on the bottom topography [3], should flow out of the deep channel in this region (26°40′ S, 34°00′ W; figure). An echo sounder survey was an important component of the research in 2012. The results of the survey determined the selection of the locations of five CTD/LADCP profiling stations. The measurements revealed significant differences from database [3], which were manifested, first of all, in the principally different orographic forms of the bottom topography. In particular, it was found that the deep channel is directed here to the east northeast. It is not finished with a widening to the Brazil Basin but continues in the quasi zonal direction. The joint analysis of the data measured in 2010 and 2012 showed that the AABW flowing in the deep channel splits in the studied region into two branches. The most intense flow of cold water continues its motion along the channel to the east northeast while the second less intense jet of slightly warmer water flows to the north along a branch of the channel approximately 70 m deep. Both flows displace to the right to submarine elevations. We performed an echo sounder survey in the mid dle part of the Vema Channel near to a threshold (sill) revealed from database [3] with a depth of 4420 m (28°55′ S, 38°20′ W) (figure). These measurements revealed a sill, but its depth was estimated at 4620– 4670 m. The Romanche Fracture Zone (figure) is a deep passage in the Mid Atlantic Ridge in the equatorial Atlantic. Its length is approximately 800 km and its width ranges from 10 to 40 km. Its depth in the western part (0°50′ S, 22°25′ W; 1°10′ S, 22°35′ W) where the Measurements of Bottom Currents in Underwater Channels of the Atlantic during Cruise 36 of the R/V Akademik Sergey Vavilov

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

Bottom Water Flows in the Vema Channel and over the Santos Plateau Based on the Field and Numerical Experiments

The properties of Antarctic Bottom Water flows in the Southwest Atlantic were studied on the basis of hydrographic measurements and numerical modeling of the oceanic circulation. The CTD and LADCP profiles in the region of the Vema Channel and Santos Plateau were measured onboard the R/V “Akademik Sergey Vavilov”. Hydrographic observations at several locations over the Santos Plateau were carried out for the first time. The numerical simulation was performed using the Institute of Numerical Mathematics Ocean Model (INMOM). The observations of velocities were used for verification of the numerical model. The simulated three-dimensional velocity fields with high spatial resolution in the lower layer allow us to study the bottom currents over the entire length of the Vema Channel.

Measurements of bottom currents in fractures of the southern part of the Mid-Atlantic Ridge on cruises 39, 40, and 41 of the R/V Akademik Sergey Vavilov

The bottom waters of Antarctic origin can be traced far to the north up to the Grand Banks of Newfoundland in the West Atlantic and to the abyssal depths of the North European Basin in the East Atlantic [4]. The main pathway for these waters from their origin in the Weddell Sea is in the West Atlantic through the Argentine and Brazil basins. Here, the f low of Antarctic waters splits into two branches. One is directed east and crosses the Mid-Atlantic Ridge through the equatorial Romanche and Chain fracture zones, which form deep channels in the ridge. This branch fills the abyssal depths of the equatorial basins in the East Atlantic. The other branch continues its motion to the north in the West Atlantic. The other branch separates from it at 11° N and crosses the northern Mid-Atlantic Ridge (part of the Mid-Atlantic Ridge in the North Atlantic) through the Vema Fracture Zone. This fracture is known as the main source of cold abyssal waters in the northern basins of the East Atlantic. In the Southern Hemisphere, the cold and dense core of Antarctic water propagates to the north in the abyssal depths and it moves to the left of the general direction of the f low owing to geostrophic balance. Thus, it moves to the continental slope of South America. After crossing the equator, this core should move to the right and to the western slope of the MidAtlantic Ridge also due to geostrophic balance. Such a reconstruction of the f low should facilitate the overflow of Antarctic Bottom Water to the basins of the East Atlantic through fractures due to the deep location of fractures crossing the ridge and the existence of the zonal baroclinic pressure gradient that forms between the West and East Atlantic at depths exceeding 3000 m. The easterly f low in the Vema Fracture Zone is definitely related to these overflows. On cruises 39 (October 2014), 40 (September 2015), and 41 (April 2016) of the R/V Akademik Sergey Vavilov, we continued the program of the Shirshov Institute of Oceanology, Russian Academy of Sciences to study the flows in deep channels of the Atlantic, conducted since 2003. The hydrophysical characteristics were measured in the abyssal currents through fractures in the southern part of the north Mid-Atlantic Ridge (Fig. 1). For the majority of these fractures, such targeted studies were the first in the world practice. A section of five hydrographic stations crossing the rift valley of the ridge was established along 16°39′ N within a program of investigating sulfide ore deposits on the Mid-Atlantic Ridge. A total of 46 stations were occupied in the region (Fig. 1).

Flows of bottom water in fractures of the North Mid-Atlantic Ridge

It has been shown that the total transport of Antarctic Bottom Water (AABW) in the northern fractures (Kane, Cabo Verde, Marathon) are one order of magnitude smaller than in the southern fractures (Vema, Doldrums, Vernadsky). The estimates of AABW transport through this group of fractures based on measurements in 2014 were approximately 0.28 Sv, which is about 25% of the transport through the Vema Fracture Zone. However, the coldest water flows through the Vema Fracture Zone.

Transport of Antarctic Bottom Water through Passages in the East Azores Ridge (37° N) in the East Atlantic

The structure of northerly overflow of Antarctic Bottom Water (AABW) through passages in the East Azores Ridge (37° N) in the East Atlantic from the Madeira Basin to the Iberian Basin is studied on the basis of hydrographic measurements carried out by the Institute of Oceanology, Russian Academy of Sciences (RAS) in October 2011, historical World Ocean Data Base 2009, and recent data on the bottom topography. The overflow of the coldest layers of this water occurs through two passages with close depths at 16° W (Discovery Gap) and at 19°30′ W (nameless Western Gap). It is shown that it is likely that the role of the latter passage in water transport was underestimated in earlier publications because the water (2.01°C) found in the region north of the Western Gap was cooler than in the region north of the Discovery Gap (2.03°C). In 2011, we found a decrease of 0.01°C in the AABW temperature near the bottom compared to previous measurements in 1982 (from 2.011°C to 2.002°C). Analysis of the historical database shows that this decrease is most likely caused by the cooling trend in the abyssal waters in the East Atlantic basins.