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USGS2007

Coupling alongshore variations in wave energy to beach morphologic change using the SWAN wave model at Ocean Beach, San Francisco, CA

Coastal managers have faced increasing pressure to manage their resources wisely over the last century as a result of heightened development and changing environmental forcing. It is crucial to understand seasonal changes in beach volume and shape in order to identify areas vulnerable to accelerated erosion. Shepard (1950) was among the first to quantify seasonal beach cycles. Sonu and Van Beek (1971) and Wright et al. (1985) described commonly occurring beach states. Most studies utilize widest spaced 2-D cross shore profiles or shorelines extracted from aerial photographs (e.g. Winant et al. 1975; Aubrey, 1979, Aubrey and Ross, 1985; Larson and Kraus, 1994; Jimenez et al., 1977; Lacey and Peck, 1998; Guillen et al., 1999; Norcorss et al., 2002) to analyzed systematic changes in beach evolution. But with the exception of established field stations, such as Duck, NC (Birkemeier and Mason, 1984), ans Hazaki Oceanographical Research Station (HORS) in Japan (Katoh, 1997), there are very few beach change data sets with high temporal and spatial resolutions (e.g. Dail et al., 2000; Ruggiero et al., 2005; Yates et al., in press). Comprehensive sets of nearshore morphological data and local in situ measurements outside of these field stations are very rare and virtually non-existent high-energy coasts. Studied that have attempted to relate wave statistics to beach morphology change require some knowledge of the nearshore wave climate, and have had limited success using offshore measurement (Sonu and Van Beek, 1971; Dail et al., 2000). The primary objective of this study is to qualitatively compare spatially variable nearshore wave predictions to beach change measurements in order to understand the processes responsible for a persistent erosion 'hotspot' at Ocean Beach, San Francisco, CA. Local wave measurements are used to calibrate and validate a wave model that provides nearshore wave prediction along the beach. The model is run for thousands of binned offshore wave conditions to help isolate the effects of offshore wave direction and period on nearshore wave predictions. Alongshore varying average beach change statistics are computed at specific profile locations from topographic beach surveys and lidar data. The study area is located in the San Francisco Bight in central California. Ocean Beach is a seven kilometer long north-south trending sandy coastline located just south of the entrance to the San Francisco Bay Estuary (Figure 1). It contains an erosion hotspot in the southern part of the beach which has resulted in damage to local infrastructure and is the cause of continued concern. A wide range of field data collection and numerical modeling efforts have been focused here as part of the United States Geological Survey's (USGS) San Francisco Bight Coastal Processes Study, which began in October 2003 and represents the first comprehensive study of coastal processes at the mouth of San Francisco Bay. Ocean Beach is exposed to very strong tidal flows, with measured currents often in excess of 1 m/s at the north end of the beach. Current profiler measurements indicate that current magnitudes are greater in the northern portion of the beach, while wave energy is greater in the southern portion where erosion problems are greatest (Barnard et al., 2007). The sub-aerial beach volume fluctuates seasonally over a maximum envelope of 400,000 m 3 for the seven kilometer stretch (Barnard et al, 2007). The wave climate in the region is dominated by an abundance of low frequency energy (greater than 20 s period) and prevailing northwest incident wave angles. The application of a wave model to the region is further complicated by the presence of the Farallon Islands 40 kilometers west, and a massive ebb tidal delta at the mouth of San Francisco Bay (~150 km 2 ), which creates complicated refraction patterns as wave energy moves from offshore Ocean Beach; however the cost and threat of the energetic nearshore environment have limited the temporal and spatial resolution of these measurements. Applying numerical models to predict wave and current patterns along the beach can help supplement the filed data that exists and provide opportunities to make prediction about the impacts of changing environmental forcing.

California↗

USGS2016

Ocean acidification buffering effects of seagrass in Tampa Bay

The Intergovernmental Panel on Climate Change has identified ocean acidification as a critical threat to marine and estuarine species in ocean and coastal ecosystems around the world. However, seagrasses are projected to benefit from elevated atmospheric pCO2, are capable of increasing seawater pH and carbonate mineral saturation states through photosynthesis, and may help buffer against the chemical impacts of ocean acidification. Additionally, dissolution of carbonate sediments may also provide a mechanism for buffering seawater pH. Long-term water quality monitoring data from the Environmental Protection Commission of Hillsborough County indicates that seawater pH has risen since the 1980‘s as seagrass beds have continued to recover since that time. We examined the role of seagrass beds in maintaining and elevating pH and carbonate mineral saturation state in northern and southern Tampa Bay where the percent of carbonate sediments is low (<3%) and high (>40%), respectively. Basic water quality and carbonate system parameters (including pH, total alkalinity, dissolved inorganic carbon, partial pressure of CO2, and carbonate mineral saturation state) were measured over diurnal time periods along transects (50-100 m) including dense and sparse Thalassia testudinum. seagrass beds, deep edge seagrass, and adjacent bare sand bottom. Seagrass density and productivity, sediment composition and hydrodynamic parameters were also measured, concurrently. Results indicate that seagrass beds locally elevate pH by up to 0.5 pH unit and double carbonate mineral saturation states relative to bare sand habitats. Thus, seagrass beds in Tampa Bay may provide refuge for marine organisms from the impacts of ocean acidification.

Florida↗

USGS2014

Breaking the oceanic lithosphere of a subducting slab: the 2013 Khash, Iran earthquake

[1] Large intermediate depth, intraslab normal faulting earthquakes are a common, dangerous, but poorly understood phenomenon in subduction zones owing to a paucity of near field geophysical observations. Seismological and high quality geodetic observations of the 2013 M w 7.7 Khash, Iran earthquake reveal that at least half of the oceanic lithosphere, including the mantle and entire crust, ruptured in a single earthquake, confirming with unprecedented resolution that large earthquakes can nucleate in and rupture through the oceanic mantle. A rupture width of at least 55 km is required to explain both InSAR observations and teleseismic waveforms, with the majority of slip occurring in the oceanic mantle. Combining our well-constrained earthquake slip distributions with the causative fault orientation and geometry of the local subduction zone, we hypothesize that the Khash earthquake likely occurred as the combined result of slab bending forces and dehydration of hydrous minerals along a preexisting fault formed prior to subduction.

Geophysical Research Letters↗

USGS2014

Elders recall an earlier tsunami on Indian Ocean shores

Ten years on, the Indian Ocean tsunami of 26 December 2004 still looms large in efforts to reduce coastal risk. The disaster has spurred worldwide advances in tsunami detection and warning, tsunami-risk assessment, and tsunami awareness [Satake, 2014]. Nearly a lifetime has passed since the northwestern Indian Ocean last produced a devastating tsunami. Documentation of this tsunami, in November 1945, was hindered by international instability in the wake of the Second World War and, in British India, by the approach of independence and partition. The parent earthquake, of magnitude 8.1, was widely recorded, and the tsunami registered on tide gauges, but intelligence reports and newspaper articles say little about inundation limits while permitting a broad range of catalogued death tolls. What has been established about the 1945 tsunami falls short of what's needed today for ground-truthing inundation models, estimating risk to enlarged populations, and anchoring awareness campaigns in local facts. Recent efforts to reduce coastal risk around the Arabian Sea include a project in which eyewitnesses to the 1945 tsunami were found and interviewed (Fig. 1), and related archives were gathered. Results are being made available through UNESCO's Indian Ocean Tsunami Information Center in hopes of increasing scientific understanding and public awareness of the region's tsunami hazards.

Eos, Earth and Space Science News↗

USGS2020

Genetic diversity, population structure, and historical demography of a highly vagile and human‐impacted seabird in the Pacific Ocean: The red‐tailed tropicbird, Phaethon rubricauda

Many seabird breeding colonies have recovered from heavy anthropogenic disturbance after conservation actions. The widely distributed red‐tailed tropicbird, Phaethon rubricauda , was used as a model species to assess potential anthropogenic impacts on the genetic diversity of breeding colonies in the Pacific Ocean. Cytochrome c oxidase subunit I and control region sequences analyses were conducted across the range of the species in the Pacific Ocean. The study sites were at islands without human‐related disturbance (non‐impacted islands) and with human‐related disturbance (impacted islands). We hypothesized that (i) breeding colonies of the red‐tailed tropicbird on impacted islands have lower genetic diversity compared with colonies on non‐impacted islands, and (ii) breeding colonies of the red‐tailed tropicbird show significant fine and broad‐scale genetic structure across the Pacific Ocean. Bayesian skyline analyses were conducted to infer past changes in population sizes. Genetic diversity was similar between impacted and non‐impacted islands. There was significant broad‐scale genetic structure among colonies separated by over 6,000 km, but a lack of significant fine‐scale genetic structure within Australasia and Hawai'i, although a significant level of differentiation was found within Chile with Φ ST analyses. Skyline analyses showed that effective population sizes remained relatively constant through time, but experienced either a slight decrease or the end of an expansion event through the last 1,000 years. These changes may be related to the arrival of humans on Pacific islands. Impacted islands may have received immigrants from other relatively close islands, buffering the loss of genetic diversity. However, it is also possible that colonies have retained ancestral variation or that a large effective population size coupled with a long generation time (13 years) has prevented the loss of genetic diversity in human‐impacted islands. Future research using higher‐resolution markers is needed to resolve the population genetic structure of the red‐tailed tropicbird in an ecological time‐scale.

Hawaii↗

USGS2019

Avian influenza virus prevalence in marine birds is dependent on ocean temperatures

Waterfowl and shorebirds are the primary hosts of influenza A virus (IAV), however, in most surveillance efforts, large populations of birds are not routinely examined; specifically marine ducks and other birds that reside predominately on or near the ocean. We conducted a long-term study sampling sea ducks and gulls in coastal Maine for IAV and found a virus prevalence (1.7%) much lower than is typically found in freshwater duck populations. We found wide year-to-year variation in virus detection in sea ducks and that the ocean water temperature was an important factor affecting IAV prevalence. In particular, the ocean temperature that occurred 11 d prior to collecting virus positive samples was important while water temperature measured concurrently with host sampling had no explanatory power for viral detection. We also experimentally showed that IAV is relatively unstable in sea water at temperatures typically found during our sampling. This represents the first report of virus prevalence and actual environmental data that help explain the variation in marine IAV transmission dynamics.

Maine↗

USGS2016

Dissolved methane in the Beaufort Sea and the Arctic Ocean, 1992-2009; sources and atmospheric flux

Methane concentration and isotopic composition was measured in ice-covered and ice-free waters of the Arctic Ocean during eleven surveys spanning the years of 1992-1995 and 2009. During ice-free periods, methane flux from the Beaufort shelf varies from 0.14 to 0.43 mg CH4 m-2 day-1. Maximum fluxes from localized areas of high methane concentration are up to 1.52 mg CH4 m-2 day-1. Seasonal buildup of methane under ice can produce short-term fluxes of methane from the Beaufort shelf that varies from 0.28 to 1.01 to mg CH4 m-2 day-1. Scaled-up estimates of minimum methane flux from the Beaufort Sea and pan-Arctic shelf for both ice-free and ice-covered periods range from 0.02 Tg CH4 yr-1 and 0.30 Tg CH4 yr-1 respectively to maximum fluxes of 0.18 Tg CH4 yr-1 and 2.2 Tg CH4 yr-1 respectively. A methane flux of 0.36 Tg CH4 yr-1from the deep Arctic Ocean was estimated using data from 1993-94. The flux can be as much as 2.35 Tg CH4 yr-1 estimated from maximum methane concentrations and wind speeds of 12 m/s, representing only 0.42% of the annual atmospheric methane budget of ~560 Tg CH4 yr-1. There were no significant changes in methane fluxes during the time period of this study. Microbial methane sources predominate with minor influxes from thermogenic methane offshore Prudhoe Bay and the Mackenzie River delta and may include methane from gas hydrate. Methane oxidation is locally important on the shelf and is a methane sink in the deep Arctic Ocean.

Limnology and Oceanography↗

USGS2011

Book review: Nonlinear ocean waves and the inverse scattering transform

Nonlinear Ocean Waves and the Inverse Scattering Transform is a comprehensive examination of ocean waves built upon the theory of nonlinear Fourier analysis. The renowned author, Alfred R. Osborne, is perhaps best known for the discovery of internal solitons in the Andaman Sea during the 1970s. In this book, he provides an extensive treatment of nonlinear water waves based on a nonlinear spectral theory known as the inverse scattering transform. The writing is exceptional throughout the book, which is particularly useful in explaining some of the more difficult mathematical concepts. Review info: N onlinear Ocean Waves and the Inverse Scattering Transform. B y Alfred R. Osborne, 2010. ISBN: 978-125286299, 917 pp.

Pure and Applied Geophysics↗

USGS2017

Challenges to oil spill assessment for seabirds in the deep ocean

We synthesize impediments for evaluating effects to seabirds from open ocean hydrocarbon releases. Effects on seabirds from ship discharges, spills, and well blowouts often are poorly detected and monitored far from land. Regulatory regimes for ocean spills can result in monitoring efforts that are not entirely transparent. We illustrate how interdisciplinary technologies address deficits that hamper individual or population level assessments for seabirds, and we demonstrate where emerging technologies might be engaged to bridge gaps in oil spill monitoring. Although acute mortality from direct oil exposure poses the greatest risk to seabirds, other hazards from light-attraction, flaring, collisions, chronic pollution, and hydrocarbon inhalation around oil infrastructure also may induce bird mortality in the deep ocean.

Archives of Environmental Contamination and Toxico↗

USGS2011

Recent summer precipitation trends in the Greater Horn of Africa and the emerging role of Indian Ocean sea surface temperature

We utilize a variety of climate datasets to examine impacts of two mechanisms on precipitation in the Greater Horn of Africa (GHA) during northern-hemisphere summer. First, surface-pressure gradients draw moist air toward the GHA from the tropical Atlantic Ocean and Congo Basin. Variability of the strength of these gradients strongly influences GHA precipitation totals and accounts for important phenomena such as the 1960s–1980s rainfall decline and devastating 1984 drought. Following the 1980s, precipitation variability became increasingly influenced by the southern tropical Indian Ocean (STIO) region. Within this region, increases in sea-surface temperature, evaporation, and precipitation are linked with increased exports of dry mid-tropospheric air from the STIO region toward the GHA. Convergence of dry air above the GHA reduces local convection and precipitation. It also produces a clockwise circulation response near the ground that reduces moisture transports from the Congo Basin. Because precipitation originating in the Congo Basin has a unique isotopic signature, records of moisture transports from the Congo Basin may be preserved in the isotopic composition of annual tree rings in the Ethiopian Highlands. A negative trend in tree-ring oxygen-18 during the past half century suggests a decline in the proportion of precipitation originating from the Congo Basin. This trend may not be part of a natural cycle that will soon rebound because climate models characterize Indian Ocean warming as a principal signature of greenhouse-gas induced climate change. We therefore expect surface warming in the STIO region to continue to negatively impact GHA precipitation during northern-hemisphere summer.

Greater Horn of Africa↗

USGS2014

La Niña diversity and Northwest Indian Ocean Rim teleconnections

The differences in tropical Pacific sea surface temperature (SST) expressions of El Niño-Southern Oscillation (ENSO) events of the same phase have been linked with different global atmospheric circulation patterns. This study examines the dynamical forcing of precipitation during October–December (OND) and March–May (MAM) over East Africa and during December–March (DJFM) over Central-Southwest Asia for 1950–2010 associated with four tropical Pacific SST patterns characteristic of La Niña events, the cold phase of ENSO. The self-organizing map method along with a statistical distinguishability test was used to isolate La Niña events, and seasonal precipitation forcing was investigated in terms of the tropical overturning circulation and thermodynamic and moisture budgets. Recent La Niña events with strong opposing SST anomalies between the central and western Pacific Ocean (phases 3 and 4), force the strongest global circulation modifications and drought over the Northwest Indian Ocean Rim. Over East Africa during MAM and OND, subsidence is forced by an enhanced tropical overturning circulation and precipitation reductions are exacerbated by increases in moisture flux divergence. Over Central-Southwest Asia during DJFM, the thermodynamic forcing of subsidence is primarily responsible for precipitation reductions, with moisture flux divergence acting as a secondary mechanism to reduce precipitation. Eastern Pacific La Niña events in the absence of west Pacific SST anomalies (phases 1 and 2), are associated with weaker global teleconnections, particularly over the Indian Ocean Rim. The weak regional teleconnections result in statistically insignificant precipitation modifications over East Africa and Central-Southwest Asia.

Climate Dynamics↗

USGS2009

On the occurrence, trace element geochemistry, and crystallization history of zircon from in situ ocean lithosphere

We characterize the textural and geochemical features of ocean crustal zircon recovered from plagiogranite, evolved gabbro, and metamorphosed ultramafic host-rocks collected along present-day slow and ultraslow spreading mid-ocean ridges (MORs). The geochemistry of 267 zircon grains was measured by sensitive high-resolution ion microprobe-reverse geometry at the USGS-Stanford Ion Microprobe facility. Three types of zircon are recognized based on texture and geochemistry. Most ocean crustal zircons resemble young magmatic zircon from other crustal settings, occurring as pristine, colorless euhedral (Type 1) or subhedral to anhedral (Type 2) grains. In these grains, Hf and most trace elements vary systematically with Ti, typically becoming enriched with falling Ti-in-zircon temperature. Ti-in-zircon temperatures range from 1,040 to 660°C (corrected for a TiO2 ≈ 0.7, a SiO2 ≈ 1.0, pressure ≈ 2 kbar); intra-sample variation is typically ~60–150°C. Decreasing Ti correlates with enrichment in Hf to ~2 wt%, while additional Hf-enrichment occurs at relatively constant temperature. Trends between Ti and U, Y, REE, and Eu/Eu* exhibit a similar inflection, which may denote the onset of eutectic crystallization; the inflection is well-defined by zircons from plagiogranite and implies solidus temperatures of ~680–740°C. A third type of zircon is defined as being porous and colored with chaotic CL zoning, and occurs in ~25% of rock samples studied. These features, along with high measured La, Cl, S, Ca, and Fe, and low (Sm/La) N ratios are suggestive of interaction with aqueous fluids. Non-porous, luminescent CL overgrowth rims on porous grains record uniform temperatures averaging 615 ± 26°C (2SD, n = 7), implying zircon formation below the wet-granite solidus and under water-saturated conditions. Zircon geochemistry reflects, in part, source region; elevated HREE coupled with low U concentrations allow effective discrimination of ~80% of zircon formed at modern MORs from zircon in continental crust. The geochemistry and textural observations reported here serve as an important database for comparison with detrital, xenocrystic, and metamorphosed mafic rock-hosted zircon populations to evaluate provenance.

Contributions to Mineralogy and Petrology↗

USGS2009

Declining global per capita agricultural production and warming oceans threaten food security

Despite accelerating globalization, most people still eat food that is grown locally. Developing countries with weak purchasing power tend to import as little food as possible from global markets, suffering consumption deficits during times of high prices or production declines. Local agricultural production, therefore, is critical to both food security and economic development among the rural poor. The level of local agricultural production, in turn, will be determined by the amount and quality of arable land, the amount and quality of agricultural inputs (fertilizer, seeds, pesticides, etc.), as well as farm-related technology, practices and policies. This paper discusses several emerging threats to global and regional food security, including declining yield gains that are failing to keep up with population increases, and warming in the tropical Indian Ocean and its impact on rainfall. If yields continue to grow more slowly than per capita harvested area, parts of Africa, Asia and Central and Southern America will experience substantial declines in per capita cereal production. Global per capita cereal production will potentially decline by 14% between 2008 and 2030. Climate change is likely to further affect food production, particularly in regions that have very low yields due to lack of technology. Drought, caused by anthropogenic warming in the Indian and Pacific Oceans, may also reduce 21st century food availability in some countries by disrupting moisture transports and bringing down dry air over crop growing areas. The impacts of these circulation changes over Asia remain uncertain. For Africa, however, Indian Ocean warming appears to have already reduced rainfall during the main growing season along the eastern edge of tropical Africa, from southern Somalia to northern parts of the Republic of South Africa. Through a combination of quantitative modeling of food balances and an examination of climate change, this study presents an analysis of emerging threats to global food security.

Food Security↗

USGS2015

Biological response to climate change in the Arctic Ocean: The view from the past

The Arctic Ocean is undergoing rapid climatic changes including higher ocean temperatures, reduced sea ice, glacier and Greenland Ice Sheet melting, greater marine productivity, and altered carbon cycling. Until recently, the relationship between climate and Arctic biological systems was poorly known, but this has changed substantially as advances in paleoclimatology, micropaleontology, vertebrate paleontology, and molecular genetics show that Arctic ecosystem history reflects global and regional climatic changes over all timescales and climate states (10 3 –10 7 years). Arctic climatic extremes include 25°C hyperthermal periods during the Paleocene-Eocene (56–46 million years ago, Ma), Quaternary glacial periods when thick ice shelves and sea ice cover rendered the Arctic Ocean nearly uninhabitable, seasonally sea-ice-free interglacials and abrupt climate reversals. Climate-driven biological impacts included large changes in species diversity, primary productivity, species’ geographic range shifts into and out of the Arctic, community restructuring, and possible hybridization, but evidence is not sufficient to determine whether or when major episodes of extinction occurred.

arktos↗

USGS1973

K, Rb, Sr and 87Sr 86Sr in rocks from the Mid-Indian Oceanic Ridge

The 87 Sr/ 86 Sr ratios of Mid-Indian Oceanic Ridge (MIOR) basalts are nearly identical (0.7032 to 0.7035), with the exception of one more highly radiogenic sample (0.7043). These values are consistently higher than the strontium isotopic ratios of the ridge basalts from Atlantic and Pacific Oceans, suggesting that the source of the MIOR basalts was depleted in alkalies more recently and/or to a lesser degree than the basalts from other oceans.

Earth and Planetary Science Letters↗

USGS1973

87Sr/86Sr ratios in basalts from islands in the Indian Ocean

87 Sr/ 86 Sr ratios of basalts from islands in the Indian Ocean (0.7040) are higher than those of basalts dredged from the Mid-Indian Ocean Ridge (0.7034). The sources of the island basalts have apparently not been in equilibrium with the source of the ridge basalts for roughly 10 9 years. Both ridge and island basalts in the Indian Ocean are higher in 87 Sr/ 86 Sr than are rocks from similar settings in the eastern Pacific.

Earth and Planetary Science Letters↗

USGS1980

Some factors controlling the concentration of uranium in the world ocean

Low-temperature alteration of the oceanic crust is a major sink for the U supplied to the oceans and may account for about 50% of the estimated present-day input of this element. Uranium uptake by organic-rich sediments and coexisting phosphorites on continental margins is also important and may remove in excess of 10% of the total supply. High-temperature alteration of oceanic basalts, metalliferous sediments, carbonate sediments, and sediments in anoxic basins deeper than 200 m play a relatively minor role in the removal of U. Each of these sinks is responsible for the uptake of less than 5% of the overall input.

Geochimica et Cosmochimica Acta↗

USGS1977

Suspended particulate loads and transports in the nepheloid layer of the abyssal Atlantic Ocean

Vertical profiles of light scattering from over 1000 L-DGO nephelometer stations in the Atlantic Ocean have been used to calculate mass concentrations of suspended particles based on a calibration from the western North American Basin. From these data are plotted the distributions of particulate concentrations at clear water and in the more turbid near-bottom water. Clear water is the broad minimum in concentration and light scattering that occurs at varying mid-depths in the water column. Concentrations at clear water are as much as one-to-two orders of magnitude lower than those in surface water but still reflect a similar geographic distribution: relatively higher concentrations at ocean margins, especially underneath upwelling areas, and the lowest concentrations underneath central gyre areas. These distributions within the clear water reflect surface-water biogenic productivity, lateral injection of particles from shelf areas and surface circulation patterns and require that the combination of downward vertical and horizontal transport processes of particles retain this pattern throughout the upper water column. Below clear water, the distribution of standing crops of suspended particulate concentrations in the lower water column are presented. The integration of mass of all particles per unit area (gross particulate standing crop) reflects a relative distribution similar to that at the surface and at clear water levels, superimposed on which is the strong imprint of boundary currents along the western margins of the Atlantic. Reducing the gross particulate standing crop by the integral of the concentration of clear water yields a net particulate standing crop. The distribution of this reflects primarily the interaction of circulating abyssal waters with the ocean bottom, i.e. a strong nepheloid layer which is coincident with western boundary currents and which diminishes in intensity equatorward. The resuspended particulate loads in the nepheloid layer of the basins west of the Mid-Atlantic Ridge, resulting from interaction of abyssal currents with the bottom, range from ∼ 2 · 10 6 tons in the equatorial Guyana Basin to ∼ 50 · 10 6 tons in the North American Basin. The total resuspended particulate load in the western basins (111 · 10 6 tons) is almost an order of magnitude greater than that in the basins east of the Mid-Atlantic Ridge (13 · 10 6 tons). The net northward flux of resuspended particles carried in the AABW drops from ∼ 8 · 10 6 tons/year between the southern and northern ends of the Brazil Basin and remains ∼ 1 · 10 6 tons/year across the Guyana Basin.

Marine Geology↗