Bottom Water Flow Research Articles

Antarctica is less isolated with increasing depth - evidence from pycnogonids

No other group of animals typifies the uniqueness of Antarctic life more than Pycnogonida (sea spiders), with 20% of all known species found in the Southern Ocean, and 64% of these endemic to the Antarctic. Despite nearly 200 years of research into pycnogonids and other benthic phyla in Antarctica, the parameters which drive the distribution and diversity of benthic fauna are still poorly understood. This study aimed to investigate the diversity and connectivity of pycnogonid communities on either side of the Antarctic Polar Front, with an emphasis on the role of water depth, using an occurrence dataset containing 254 pycnogonid species from 2187 sampling locations. At depths shallower than 1000 m, communities to the north and south of the Antarctic Polar Front were distinct, while below this depth this geographic structure disintegrated. The Polar Front, or the expanse of deep ocean it bisects, seemingly acts as a semipermeable barrier to species exchange between well-sampled shallow communities. The less sampled and less understood deep sea appears to be better connected, with high levels of shared species following the northward flow of Antarctic Bottom Water. The exceptionally high diversity and endemism of Antarctic pycnogonids may reflect an apparent competitive advantage in cold waters which leaves them vulnerable to ongoing ocean warming, with increased competition and predation pressures.

Multiproxy approach to characterize the sedimentary process of Cape Darnley Bottom Water flow through the Wild Canyon, East Antarctica

To reconstruct the interaction between Antarctic Bottom Water (AABW) and global climate change, extracting information about past AABW formation from the sedimentary record is crucial. The Prydz Bay region, East Antarctica, has high glacier outflow and is a region of vigorous formation of AABW associated with sea ice formation. The Cape Darnley Bottom Water (CDBW), a primary AABW precursor, flows into Wild Canyon west of Prydz Bay. Thus, the sedimentary record of Wild Canyon can serve as an archive for CDBW flow. In this study, to identify the sedimentary processes and mineral compositions characteristic of CDBW flow, we evaluate deep-sea camera imagery, X-ray computed tomography images, grain-size data, and optical and chemical mineralogical analyses. Heavy minerals in the surface sediments are characterized by pyrope-rich almandine garnets (up to 80% of the heavy-mineral assemblage, 0.2%–3.7% of the dry bulk sediment by weight) typical of nearshore sediment derived from Mac. Robertson Land. In addition, ripples on the channel floor indicate sediment transport occurred as tractional bedload, whereas settling of suspended sediment prevailed on the canyon levee. We propose that modern CDBW flow is the dominant process of sediment reworking through sustained high flow velocities, distinctly different from turbidity currents triggered by ice sheet expansion and rapid retreat. Our results further suggest that fine-grained sediments are transported to the canyon levee as CDBW flows through the submarine canyon. These findings will be a key for reconstructing past AABW formation, which is crucial for understanding the response of AABW to future climate change.

Laboratory Evaluation on the Water Flooding Characteristics in Bottom Water Reservoir Containing Interbeds.

Sandstone reservoirs with bottom water drive are widely distributed all over the world, which are characterized by the complex process of oil and water storage and transmission. At present, the research on the water flooding process and oil-water evolution characteristics in bottom water reservoirs containing interbeds needs to be strengthened. In this study, water flooding experiments with different placements of the interbeds were conducted using a two-dimensional (2D) vertical model. The results demonstrated that the interbeds make the bottom water flow upward more evenly, resulting in decreased incursion speed, increased displacement area, and better displacement effect. Moreover, compared with the tilted interbed model, the horizontal model has a 6% higher oil recovery rate, exhibiting a better oil displacement effect. The results presented herein will provide important guidance on water control in bottom-aquifer oil reservoirs containing interbeds and will promote unconventional petroleum resources recovery.

Properties of Antarctic Bottom Water in the Western Gap (Azores-Gibraltar Fracture Zone, Northeast Atlantic) in 2021

We present the results of measurements of thermohaline and dynamical structures, as well as hydrochemical parameters of near-bottom water in the Western Gap (Azores-Gibraltar Fracture Zone, Northeast Atlantic). The data were collected during cruise 59 of the research vessel Akademik Ioffe in October 2021. The results of the analysis showed that water with potential temperatures lower than 2.00 °C had crossed the Entrance Sill, but was not detected inside the interior basin of the Western Gap. The velocities of currents at the Entrance Sill were high (up to 30 cm/s) leading to mixing of the water overflowing the sill. While Antarctic Bottom Water (AABW) flows from the Entrance Sill to the Exit Sill it is subjected to modification. Bottom water becomes less dense after the sill due to mixing during overflow and in addition not all cold water overflows the sill. The dissolved oxygen concentration in the bottom layer increases from the Entrance Sill (239–242 μmol/kg) to the Exit Sill (241–248 μmol/kg) indicating the transformation of AABW as it circulates in the Western Gap and mixes with North Atlantic Deep Water propagating from the north. All velocity measurements indicate that north velocity component is a characteristic property of the bottom flow. Thus, water with specific chemical composition of AABW is transported to the Iberian Basin despite that its potential temperature becomes higher than 2 °C.

Decoupling of the Surface and Bottom‐Intensified Antarctic Slope Current in Regions of Dense Shelf Water Export

AbstractThe Antarctic Slope Current is guided by the topographic gradient of the Antarctic continental slope and creates a dynamical barrier between the continental shelf and the open ocean. The current's vertical structure varies around the continent affecting cross‐slope water mass exchange with consequences for Antarctic mass loss, ventilation of the deep ocean, and carbon uptake. The Antarctic Slope Current is surface‐intensified in many regions but bottom‐intensified in regions of dense overflows. This study investigates the role of dense overflows in modifying the dynamics of the bottom‐intensified flow using a 0.1° global ocean‐sea ice model. The occurrence of bottom‐intensification is tightly linked with dense overflows and bottom speeds correlate with dense overflows on interannual time scales. A lack of vertical connectivity between the bottom and surface flow, however, suggests that the along‐slope bottom water flows are coincidentally co‐located with the Antarctic Slope Current, rather than dynamically a part of the current.

Antarctic Bottom Water in the Vema Fracture Zone

AbstractA section of 46 CTD/Lowered Acoustic Doppler Current Profiler stations along the Vema Fracture Zone (VFZ) was made for the first time with a repeat of the transverse section on the main sill (at 41° W) for the sixth time. Potential temperature of Antarctic Bottom Water when it flows into the fracture increases from 1.3°С to 1.6°C. This temperature increase is much smaller than in the Romanche Fracture Zone because in the VFZ there are not many transverse sills that force the flow to become turbulent. A zone with minimal velocities of the bottom flows exists in the western part of the VFZ up to the main sill. A current with velocities of 0.25 m/s flows above the low velocity zone at a depth of about 4,000 m. Underwater spillways with hydraulically controlled flow of bottom water down the slope from 4,500 m to 5,000 m at speeds up to 0.40 m/s were found east of the sill (in the central and southern channels). The bottom current accelerates as it flows down, but then its kinetic energy decreases. The bottom current slows down and mixes with surrounding waters. A thin (∼30 m) bottom flow descends further to the deep depression. In the southern channel of the VFZ, bottom water flows into a deep (5,400 m) basin. The coldest and densest water reaches this depression during rare inflows.

A North Pacific Meridional Section (U.S. GEOTRACES GP15) of Helium Isotopes and Noble Gases I: Deep Water Distributions

AbstractThe noble gas signature of incoming Pacific Bottom Water (PBW), when compared to North Atlantic Deep Water, indicates the addition of 450 ± 70 GT a−1glacial melt water to form AABW and subsequently PBW. The downstream evolution of this signature between the southern (20°S to equator) and northern (25°–45°N) bottom waters indicates a decrease in sea level pressure around Antarctica over the past two millennia. Vertical profiles of noble gases in the deep Pacific show exponential relationships with depth with scale heights identical to temperature and salinity. Unlike the other noble gases, helium isotopes show evidence of mid‐depth injection of non‐atmospheric helium. Using observed deviations from exponential behavior, we quantify its magnitude and isotope ratio. There is a clear latitude trend in the isotope ratio of this added helium that decreases from a high exceeding 9 RA(atmospheric3He/4He ratio) in the south to around 8 RAnear the equator. North of 30–40°N, it systematically decreases northward to a low of ∼2 RAnorth of 50°N. This decline results from a combination of northward decline in seafloor spreading, release of radiogenic helium from increased sediment thickness, and the possible emission of radiogenic helium through cold seeps along the Alaskan and North American margins. Finally, we derive an improved method of computing the excess helium isotope concentrations and that the distributions of bottom water3HeXS/4HeXSare consistent with what is known about bottom water flow patterns and the input of low3He/4He sedimentary helium.

Tides regulate the flow and density of Antarctic Bottom Water from the western Ross Sea

Antarctic Bottom Water (AABW) stores heat and gases over decades to centuries after contact with the atmosphere during formation on the Antarctic shelf and subsequent flow into the global deep ocean. Dense water from the western Ross Sea, a primary source of AABW, shows changes in water properties and volume over the last few decades. Here we show, using multiple years of moored observations, that the density and speed of the outflow are consistent with a release from the Drygalski Trough controlled by the density in Terra Nova Bay (the “accelerator”) and the tidal mixing (the “brake”). We suggest tides create two peaks in density and flow each year at the equinoxes and could cause changes of ~ 30% in the flow and density over the 18.6-year lunar nodal tide. Based on our dynamic model, we find tides can explain much of the decadal variability in the outflow with longer-term changes likely driven by the density in Terra Nova Bay.

Oceanographic Investigations in the Tropical Atlantic (Cruise 52 of the Research Vessel “Akademik Boris Petrov”)

Information is presented on the work performed to study the flow of Antarctic Bottom Water along the entire length of the Vema Fracture Zone in the tropical part of the North Atlantic Ridge and Russian-Brazilian field studies in the largest river plume of the World Ocean, on the shelf of the northern part of Brazil near the mouth of the Amazon River. A total of 46 stations were completed in the Vema Fracture Zone and 28 stations offshore Brazil. The new expedition that took place was an important step in the development of international scientific cooperation between the BRICS countries: Russia and Brazil.

Transport of Antarctic Bottom Water Entering the Brazil Basin in a Planetary Geostrophic Inverse Model

AbstractThe abyssal circulation in the interior portions of the Brazil Basin and northern Argentine Basin were estimated using an overdetermined inverse method conserving neutral surface planetary potential vorticity and salinity. Antarctic Bottom Water (AABW) flows with a northward component through 30°S west of the Rio Grande Rise (RGR), as expected. However, in the area east of the RGR, the model shows southward transport in the upper part of the AABW across 30°S. This finding suggests that previous calculations of northward transport through Hunter Channel reaching the broader Brazil Basin may be overestimates. Reduced northward transport through the Hunter Channel has significant implications for the heat budget, implying less diapycnal mixing across the warmer AABW classes, compared to earlier studies.

Tritium and radiocarbon in the water column of the Red Sea

Despite being the busiest transient sea in the world due to the Suez Canal, radionuclide distribution studies in seawater and sediment of the Red Sea remain rare. A sampling expedition in the Red Sea was conducted from June 9 to July 6, 2021, visiting a transect of several deep sampling stations located along the central axis of the basin from the Gulf of Aqaba to the southern Red Sea (near Farasan Island, Saudi Arabia). The collected seawater profile samples were analyzed for tritium, radiocarbon and oxygen-18. The observed tritium levels in surface waters of the Red Sea peaked at 0.3–0.4 TU, similar to the values observed in the western Arabian Sea (decay corrected). The values observed at waters below 150 m were around 0.2 TU, however, at depths of 450 and 750 m, tritium minima (<0.2 TU) were observed, which could be associated with a partial return flow of bottom waters from the southern to the northern Red Sea. At two stations at the depth of about 550 m, deep Δ14C minima were observed as well (−4‰ and −10‰), documenting ongoing transport of carbon in the water column, important for sink of anthropogenic carbon.

Deep Flow of Antarctic Bottom Water in the Atlantic and Internal Waves

Deep Flow of Antarctic Bottom Water in the Atlantic and Internal Waves

Hydraulically Controlled Bottom Flow in the Orkney Passage

Supercritical hydraulically controlled overflow of Antarctic Bottom Water from the Weddell Sea has been observed in the Orkney Passage during field measurements in February 2022. The Orkney Passage is the main pathway for the densest layer of Antarctic Bottom Water flow from the Weddell Sea to the Scotia Sea. The bottom current overflows the sill across the passage and flows down from the crest of the sill at 3600 m deeper than 4000 m. The descending flow accelerates because of the difference in the height of the sill and its foot. An estimate of the Froude number of this flow was greater than unity. Near the foot of the slope the kinetic energy of the flow becomes insufficient to continue moving in this regime. The flow slows down, and strong mixing and warming of the bottom water occurs due to the exchange with the surrounding waters. This hydrodynamic phenomenon is called supercritical hydraulically controlled flow. However, the flow of bottom water continues further and eventually fills the abyssal depths of the Atlantic.

Northeast Greenland: ice‐free shelf edge at 79.4°N around the Last Glacial Maximum 25.5–17.5 ka

The size of the last glacial ice sheet on the Northeast (NE) Greenland shelf and its interaction with ocean circulation have been the subject of debate. Here we provide insights into the extent of the ice sheet around the Last Glacial Maximum (LGM) and investigate timing and strength of changes in the flow of Atlantic Water. The study is based on a multiproxy investigation of a marine sediment core, gravity core DA17‐NG‐ST01‐019G, from 323‐m water depth at the NE Greenland shelf edge at 79.4°N. We present benthic and planktic foraminiferal distribution data, AMS‐14C dates, sedimentological (ice‐rafted debris (IRD) and grain sizes), and geochemical (XRF) data in combination with geophysical (sub‐bottom profiler) data. The oldest sediments at the study site are dated to 25.5–17.5 ka, encompassing the time frame from the beginning of the LGM to the early deglaciation. This part is overlain by sediments from the late deglaciation and Holocene. The deposits dating from the LGM are very rich in both planktic and benthic foraminifera and macrofossils of excellent preservation. The faunas show that the site generally was affected by a strong flow of relatively warm subsurface Atlantic Water during the LGM and Early Holocene. Conditions turned more polar with cold bottom water flow in the Middle–Late Holocene (c. 7.5 ka to Recent) with presence of mainly agglutinated benthic foraminiferal species. Our data from the LGM also indicate that the deposits were mixed by iceberg scouring, confirmed by the geophysical data showing extensive ploughing of the sediments on the outer shelf area. The results further indicate that the Greenland Ice Sheet did not reach to the edge of the NE Greenland shelf at 79.4°N during the LGM 24–18 ka.

EXPERIENCE OF USING SIMULTANEOUS-SEPARATE DOWNLOADS AT THE AKSHABULAK TSENTRALNY DEPOSIT

The traditional joint injection of water into several reservoirs with different permeability does not control bottom hole pressures and water flow for each of the reservoirs (by their current geological parameters), which leads to uneven watering of oil deposits. As a result, advanced watering of highly permeable oil reservoirs occurs, the degree of coverage decreases, uneven impact, and the development of each of the reservoirs with different reservoir properties, namely, permeability occurs. In order to reduce capital costs for drilling a new injection well in multi-layer fields, one injection well accounts for more than two perforated reservoirs of heterogeneous permeability to maintain reservoir pressure. The article describes the successful experience of implementing simultaneous-separate injection and analyzes the effectiveness of implementing the technology of simultaneous-separate injection using the example of the M-II-1 horizon of the Akshabulak Central field. Among the successfully conducted experiments, there is an increase in oil recovery, cost savings due to the effectiveness of the technology, and the absence of any complications during repair work. However, also, there are disadvantages: a decrease in injectivity for individual choke due to contamination of the internal section and an increase in injectivity due to wear of the choke, difficulties in assessing the depletion of separate layers due to the insufficient number of measurements of the inflow profile at production wells. For better results in the future, it is planned to redistribute the history of production across reservoirs at production wells on a capacitive-resistive model (CRM) using neural network technologies, using the account of PLT studies at production wells, and distributed injection across reservoirs at injection wells.

Hydraulic control of flow in a multi-passage system connecting two basins

When a fluid stream in a conduit splits in order to pass around an obstruction, it is possible that one branch will be critically controlled while the other remains not so. This is apparently the situation in Pacific Ocean abyssal circulation, where most of the northward flow of Antarctic bottom water passes through the Samoan Passage, where it is hydraulically controlled, while the remainder is diverted around the Manihiki Plateau and is not controlled. These observations raise a number of questions concerning the dynamics necessary to support such a regime in the steady state, the nature of upstream influence and the usefulness of rotating hydraulic theory to predict the partitioning of volume transport between the two paths, which assumes the controlled branch is inviscid. Through the use of a theory for constant potential vorticity flow and accompanying numerical model, we show that a steady-state regime similar to what is observed is dynamically possible provided that sufficient bottom friction is present in the uncontrolled branch. In this case, the upstream influence that typically exists for rotating channel flow is transformed into influence into how the flow is partitioned. As a result, the partitioning of volume flux can still be reasonably well predicted with an inviscid theory that exploits the lack of upstream influence.

Probing the Mysteries of Deep, Dense Antarctic Seawater

Twelve freely drifting Deep Argo floats reveal year-round dynamics of bottom water flow in the Australian-Antarctic Basin.

High‐Frequency Fluctuations in Antarctic Bottom Water Transport Driven by Southern Ocean Winds

AbstractNorthward flow of Antarctic Bottom Water (AABW) across the Southern Ocean comprises a key component of the global overturning circulation. Yet AABW transport remains poorly constrained by observations and state estimates, and there is presently no means of directly monitoring any component of the Southern Ocean overturning. However, AABW flow is dynamically linked to Southern Ocean surface circulation via the zonal momentum balance, offering potential routes to indirect monitoring of the transport. Exploiting this dynamical link, this study shows that wind stress (WS) fluctuations drive large AABW transport fluctuations on time scales shorter than 2 years, which comprise almost all of the transport variance. This connection occurs due to differing time scales on which topographic and interfacial form stresses respond to wind variability, likely associated with differences in barotropic versus baroclinic Rossby wave propagation. These findings imply that AABW transport variability can largely be reconstructed from the surface WS alone.

Ontogenetic Changes in Behavioral and Energetic Responses of Juvenile Atlantic Croaker to an Estuarine Salinity Gradient

AbstractLarval Atlantic Croakers Micropogonias undulatus ingress into estuaries from offshore spawning sites during fall and winter. Larvae and early juveniles migrate up‐estuary to oligohaline nurseries where they reside for several months before emigrating seaward. We examined the ontogenetic changes in behavioral salinity preference and avoidance as potential contributors to these movement patterns along the estuarine salinity gradient. Four size‐classes (26–40, 41–55, 56–70, and 71–85 mm SL) were acclimated to 10‰ and 18°C and exposed to horizontal salinity gradients providing five discrete salinity choices: 2, 6, 10, 14, and 18‰. Behavioral salinity preference varied ontogenetically: 26–40‐mm juveniles avoided salinities ≤10‰ and preferred higher salinities, 41–55‐mm juveniles showed a shift toward lower salinities, and 56–70‐mm fish continued this trend by preferring 2‰ and avoiding 18‰. The largest fish tested (71–85 mm) showed no significant preference or avoidance behavior, although they tended to avoid the low and prefer the high salinities that were tested. The preference for higher salinities among early (26–40 mm) juveniles may facilitate orientation in higher‐salinity bottom waters by assisting up‐estuary immigration to oligohaline nurseries via the residual bottom‐layer inflow. The preference for low salinity of 56–70‐mm juveniles is consistent with their residence in oligohaline areas. We also conducted feeding and growth experiments, at 18°C, on 30–40‐mm and 75–85‐mm fish, corresponding to the smallest and largest fish in the salinity preference trials. Smaller juveniles showed no significant energetic advantage in oligohaline versus mesohaline conditions. However, larger juveniles grew significantly faster at 18‰ than at 2‰, indicating that down‐estuary movement during summer and fall improves growth capacity later in the nursery season. The increased precipitation and river discharge that is predicted from climate change and its associated stronger net up‐estuary flow of saline bottom waters may facilitate the up‐estuary immigration of early juvenile Atlantic Croakers through their attraction to higher salinity, as is demonstrated by the present work.

Structure of the bottom water flow in the Vema Channel based on the measurements from the R/V Akademik Sergey Vavilov

Structure of the bottom water flow in the Vema Channel based on the measurements from the R/V Akademik Sergey Vavilov