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The geomorphometry of the of the Amazon Continental Margin was analyzed using the Benthic Terrain Model, a spatial analysis technique that defines physical habitat classes based on seafloor relief heterogeneities. A compilation of available bathymetric and novel multibeam data was used to define the megahabitats, with emphasis on shelf-slope transitions and shelf-edge reefs. The analyses revealed a complex mosaic of benthic megahabitats associated to short and long-term geologic and sedimentary evolution of the margin. The continuous terrigenous sediment input is associated to a smooth muddy deposit along the inner and mid shelf (Regular Continental Shelf megahabitat). The portions of the shelf that are less influenced by riverine sediment accumulation are rougher and characterized by either sand (Irregular Sand Continental Shelf megahabitat) or carbonate-dominated bottom (Irregular Reef Continental Shelf megahabitat). The most notable difference in terms of morphometric analysis and megahabitats can be observed along the outer shelf and shelf-break. The Shelf-Slope Transition megahabitat is marked by ridges in the shelf break and by a more acute depth gradient that forms a distinct outer shelf edge. Three different alongshore sectors were explored in order describe the heterogeneous megahabitat mosaic in terms of slope and bottom morphology. The western-most sector (S3) is remarkable due to an indistinct separation between ridges and the outer shelf edge, as well as for presenting reefs with up to 20 m high, between 110 and 210 m water depths. The central sector (S2) presents no shelf-break and lacks ridges, a feature that is associated with the long-term sediment accumulation (Amazon Fan). The southern-most sector (S1) does not present an outer shelf edge, only ridges, and a number of shelf-incised channels, comprising a sediment bypass across the shelf, and carbonate sedimentation. The continental slope is divided into a Featured Slope megahabitat with numerous canyons and ravines, and areas that lack such features, including a Shallow Gentle Slope megahabitat, a Steep Slope megahabitat and a Deep Gentle Slope megahabitat. Our results confirm the usefulness of geomorphometric analyses to define benthic megahabitats and can be used as a starting point in a much-needed marine spatial planning process for the area.

Marine Protected Areas (MPAs) are a widely‐used tool for conserving biodiversity. Features that support marine mammal foraging have been suggested as important components to include in MPAs, but research is needed to understand the relationship between these features and diversity. For example, the Northeast Canyons and Seamounts Marine National Monument represents an area known to support marine mammal foraging and was designated to protect an area of high marine mammal diversity. However, no comparisons have been made between marine mammal diversity in the Monument and other areas. We used 3,174,167 km of survey effort and 189,175 sightings to assess alpha and beta diversity in the Monument and 500 randomly selected sites along the east coast of the United States. We used linear models to relate diversity to variables that represent marine mammal foraging areas. Our analyses showed a gradient of higher to lower diversity from north to south and that the shelf‐edge, canyons, and areas of likely upwelling support high diversity. We also found that the Monument protects a diverse and unique marine mammal community. Our analyses contribute to efforts to designate MPAs to conserve habitat that is important for protecting species by identifying drivers of biodiversity and potential sites for protecting 30% of the planet by 2030.

Mesophotic coral ecosystems are extensive light-dependent habitats that typically form between 30 – 150 m depth in the tropical oceans. The forces that structure the benthic communities in these ecosystems are poorly understood but this is rapidly changing with technological advances in technical diving and remote observation that allow large-scale scientific investigation. Recent observations of southeastern Puerto Rican Shelf of the US Virgin Islands have shown that this Caribbean mesophotic coral ecosystem has distinct habitats within the same depth ranges and across small horizontal distances (25%. High-resolution bathymetric mapping of the shelf edge revealed a topographically distinct semi-continuous 71 km-long relict barrier reef bank system. The purpose of this study was to characterize the pattern of mesophotic habitat development of the shelf edge and use this data to narrow the potential long-term and large-scale structuring forces of this mesophotic coral ecosystem. We hypothesized from limited preliminary observations that the shelf edge coral cover was limited in shallower portions of the bank and on the seaward orientation. Through stratified random surveys we found that increasing depth and decreasing wave driven benthic orbital velocities were positively related to coral abundance on the shelf edge. In addition, low coral cover habitats of the shelf edge contrasted strongly with adjacent on shelf banks surveyed previously in the same depth range, which had relatively high coral cover (>30%). Predictions of benthic orbital velocities during major storms suggested that mechanical disturbance combined with low rates of coral recovery as a possible mechanism structuring the patterns of coral cover, and these factors could be targets of future research.

本文试从沉积动力学视角重新剖析河流三角洲沉积体系特征。根据“河流三角洲是同一河流入海物质所形成的集中堆积体”的定义，传统上根据径流、潮汐和波浪而构建的三端元分类图似乎不能概括三角洲的所有类型，河口湾形态、陆架环流和海面变化也有同等的重要性，可形成海湾充填三角洲、远端泥、陆架边缘三角洲这样的端元形态。沉积物重力流也是不可忽视的因素。融合以上各个因素所形成的河流三角洲形态谱系，有助于过程-产物关系的建立。需进一步开展的相关研究包括：① 地层层序中三角洲沉积类型的识别标志，以区分水下三角洲、远端泥，确定陆架边缘三角洲的归属；② 三角洲体系的时空分布及其与沉积记录完整性之间的关系；③ 三角洲演化的终极形态、规模与沉积物收支过程的关系。

A long‐standing hypothesis postulates that the elevated productivity at continental shelf break regions is stimulated by enhanced nutrient supply to the surface sunlit depths, however, direct, quantitative evidence has been lacking. We tested this hypothesis with a set of high‐resolution physical and biogeochemical observations from repeated summertime surveys across the Northeast U.S. continental shelf break front. We found direct evidence during summer at this shelf break frontal region that: (a) near‐bottom flow convergence, a necessary process leading to frontal upwelling, was a common occurrence, happening over 75% of the time; (b) nitrate concentrations were elevated, characterized by a dome‐shaped cross‐shelf distribution approximately 30 m tall, extending from the foot of the shelf break front up to the base of the euphotic layer; (c) subsurface chlorophyll enhancement was a persistent feature. Together, these findings link bottom flow convergence, nutrient upwelling, and phytoplankton enhancement, elucidating the shelf break frontal upwelling dynamics.

The properties and heat budget of marine heat waves (MHWs) on the northern South China Sea (SCS) continental shelf are investigated. MHWs with warming amplitudes above 1.5°C occur mainly along the coast, and their temperature anomaly decreases toward the open sea. MHWs with 1°–1.5°C warming and duration < 20 days dominate the northern SCS continental shelf. A heat budget analysis indicates that the main heat source is the sea surface net heat flux. Oceanic processes are dominated by the advection of mean temperature by the anomalous horizontal velocity (advha). The net contribution of advha always cools the upper layer of the ocean, resulting in the decay of MHWs. Active cross-slope water exchanges exist at the east and west sides of the northern SCS continental shelf edge, which makes the dominant contributions to the advha. In the MHW developing phase, the west (east) side makes a positive (negative) contribution to the advha. In the decay phase, both sides make a negative contribution to the advha, resulting in the rapid decay of MHWs. Although the contribution of advha to the heat budget varies along the northern SCS continental shelf edge, its net effect always cools the MHWs over the shelf. These results provide new insight into the characteristics and formation mechanism of MHWs on the northern SCS continental shelf; in particular, they clarify the respective contributions of air–sea flux and oceanic processes to MHWs.

The response of a wide shelf to subinertial and barotropic offshore pressure signals from the shelf edge was investigated. By relaxing the semigeostrophic approximation, an elliptical wave structure equation was formulated and solved with the integral transform method. It was found that when the imposed offshore signal has an along-shelf length scale similar to the shelf width, it can efficiently break the potential vorticity barrier and propagate toward the coast, producing a significant coastal sea level setup. Thereafter, the pressure signal reflects from the coast or the sloping topography, producing a transient eddy and propagates to the downshelf. The intensities of the coastal setup and the eddy increase as the along-shelf scale of the subinertial signal decreases or when its time scale is close to the inertial period. For a signal with longer time scale, the eddy is insignificant. The nature of the shelf response is controlled by the shelf conductivity κ ≡ r /( fsB ), in which r is the Rayleigh friction coefficient, f is the Coriolis parameter, s is the shelf slope, and B is the shelf width, respectively. For a given offshore signal, coastal setup increases with κ . For large κ , the eddy energy is concentrated at low modes, producing a large eddy, whereas a small κ produces a small eddy. The proposed theory can explain coastal sea level fluctuations under eddy impingement in the Mid-Atlantic Bight or other similar areas.

The Weddell Sea supplies 40%–50% of the Antarctic Bottom Water that fills the global ocean abyss, and therefore exerts significant influence over global circulation and climate. Previous studies have identified a range of different processes that may contribute to dense shelf water (DSW) formation and export on the southern Weddell Sea continental shelf. However, the relative importance of these processes has not been quantified, which hampers prioritization of observational deployments and development of model parameterizations in this region. In this study a high-resolution (1/12°) regional model of the southern Weddell Sea is used to quantify the overturning circulation and decompose it into contributions due to multiannual mean flows, seasonal/interannual variability, tides, and other submonthly variability. It is shown that tides primarily influence the overturning by changing the melt rate of the Filchner–Ronne Ice Shelf (FRIS). The resulting ∼0.2 Sv (1 Sv ≡ 10 6 m 3 s −1 ) decrease in DSW transport is comparable to the magnitude of the overturning in the FRIS cavity, but small compared to DSW export across the continental shelf break. Seasonal/interannual fluctuations exert a modest influence on the overturning circulation due to the relatively short (8-yr) analysis period. Analysis of the transient energy budget indicates that the nontidal, submonthly variability is primarily baroclinically generated eddies associated with dense overflows. These eddies play a comparable role to the mean flow in exporting dense shelf waters across the continental shelf break, and account for 100% of the transfer of heat onto the continental shelf. The eddy component of the overturning is sensitive to model resolution, decreasing by a factor of ∼2 as the horizontal grid spacing is refined from 1/3° to 1/12°.

All exchanges between the open ocean and the Antarctic continental shelf must cross the Antarctic Slope Current (ASC). Previous studies indicate that these exchanges are strongly influenced by mesoscale and tidal variability, yet the mechanisms responsible for setting the ASC’s transport and structure have received relatively little attention. In this study the roles of winds, eddies, and tides in accelerating the ASC are investigated using a global ocean–sea ice simulation with very high resolution (1/48° grid spacing). It is found that the circulation along the continental slope is accelerated both by surface stresses, ultimately sourced from the easterly winds, and by mesoscale eddy vorticity fluxes. At the continental shelf break, the ASC exhibits a narrow (~30–50 km), swift (>0.2 m s −1 ) jet, consistent with in situ observations. In this jet the surface stress is substantially reduced, and may even vanish or be directed eastward, because the ocean surface speed matches or exceeds that of the sea ice. The shelfbreak jet is shown to be accelerated by tidal momentum advection, consistent with the phenomenon of tidal rectification. Consequently, the shoreward Ekman transport vanishes and thus the mean overturning circulation that steepens the Antarctic Slope Front (ASF) is primarily due to tidal acceleration. These findings imply that the circulation and mean overturning of the ASC are not only determined by near-Antarctic winds, but also depend crucially on sea ice cover, regionally-dependent mesoscale eddy activity over the continental slope, and the amplitude of tidal flows across the continental shelf break.

The seasonal properties and heat budget of marine heatwaves (MHWs) on the northern South China Sea (SCS) continental shelf are investigated. The winter MHWs are generally the warmest, gradually weakening in the following seasons. In winter and spring, all events have a temperature elevated by more than 1 °C while approximately half of the MHWs in summer and fall have temperatures elevated by less than 1 °C. The MHW duration is longest in winter, second longest in fall, and shortest in spring. Three types of MHWs have been defined based on the main heat source; i.e., air–sea heat flux dominant, ocean advection dominant, and mixed types. Air–sea heat flux is the dominant source for more than 80% of MHWs in winter and fall, ~ 70% of MHWs in spring, and ~ 50% in summer. There are slightly more ocean advection dominant events than Mixed-MHWs. Ocean advection always weakens an air–sea heat flux dominant MHW and enhances other types of events, so that the net heat supply is smallest for air–sea heat flux dominant MHW types. Ocean advection is mainly modulated by cross-slope water exchanges along the continental shelf edge. The offshore (onshore) cross-slope flow induces a negative (positive) contribution to MHW development. These results provide new insight into the seasonal cycle of MHWs on the northern SCS continental shelf.