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Till J J Hanebuth - One of the best experts on this subject based on the ideXlab platform.
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Storm‐driven bottom sediment transport on a high‐energy narrow shelf (NW Iberia) and development of mud depocenters
Journal of Geophysical Research, 2016Co-Authors: Wenyan Zhang, Ana Santos, Till J J HanebuthAbstract:Bottom sediment transport on the NW Iberian shelf was monitored during a Downwelling storm in September 2014. Collected data was analyzed and fed into a 3D coastal ocean model to understand storm-driven sediment transport on the shelf and its impact on mid-shelf mud depocenters (MDCs). A significantly enhanced level of bottom sediment resuspension, nearly two orders of magnitude higher than that in the pre-storm period, was recorded at the mooring site. Field data analysis reveals that it was induced by a short-lasting strong bottom current in combination with enhanced wave-current interaction. Simulation results indicate that this strong current was part of a coastal jet resulted from Downwelling. An across-shelf horizontal density gradient as high as 0.32 g/m4 occurred at the interface between the Downwelling and the bottom waters, forming a remarkable front. Due to buoyancy effect, the Downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region, resulting in a northward-directed coastal jet. Simulation results suggest that during the storm, local near-bottom sediment suspensions with concentrations on the order of 10 kg/m3 would be triggered by wave-current interaction and flow convergence associated with the front. Direct impact on the development of MDCs by transport and deposition of concentrated sediment suspensions is indicated by model results. The seaward limit of the front coincided with the shoreward edge of the MDC nucleus, suggesting the front as a primary control on the deposition of fine-grained sediment. This article is protected by copyright. All rights reserved.
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storm driven bottom sediment transport on a high energy narrow shelf nw iberia and development of mud depocenters
Journal of Geophysical Research, 2016Co-Authors: Wenyan Zhang, Ana Santos, Till J J HanebuthAbstract:Bottom sediment transport on the NW Iberian shelf was monitored during a Downwelling storm in September 2014. Collected data was analyzed and fed into a 3D coastal ocean model to understand storm-driven sediment transport on the shelf and its impact on mid-shelf mud depocenters (MDCs). A significantly enhanced level of bottom sediment resuspension, nearly two orders of magnitude higher than that in the pre-storm period, was recorded at the mooring site. Field data analysis reveals that it was induced by a short-lasting strong bottom current in combination with enhanced wave-current interaction. Simulation results indicate that this strong current was part of a coastal jet resulted from Downwelling. An across-shelf horizontal density gradient as high as 0.32 g/m4 occurred at the interface between the Downwelling and the bottom waters, forming a remarkable front. Due to buoyancy effect, the Downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region, resulting in a northward-directed coastal jet. Simulation results suggest that during the storm, local near-bottom sediment suspensions with concentrations on the order of 10 kg/m3 would be triggered by wave-current interaction and flow convergence associated with the front. Direct impact on the development of MDCs by transport and deposition of concentrated sediment suspensions is indicated by model results. The seaward limit of the front coincided with the shoreward edge of the MDC nucleus, suggesting the front as a primary control on the deposition of fine-grained sediment. This article is protected by copyright. All rights reserved.
Alan L. Shanks - One of the best experts on this subject based on the ideXlab platform.
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Testing the intermittent upwelling hypothesis: upwelling, Downwelling, and subsidies to the intertidal zone
Ecological Monographs, 2017Co-Authors: Alan L. Shanks, Steven G MorganAbstract:© 2017 by the Ecological Society of America The Intermittent Upwelling Hypothesis (IUH) posits that subsidies of larvae and phytoplankton to intertidal communities should vary unimodally along a gradient of upwelling from persistent upwelling to persistent Downwelling with most subsidies occurring where upwelling is of intermediate strength and intermittent. Furthermore, the hypothesis states that larvae and phytoplankton are transported far offshore by strong, persistent upwelling and fail to subsidize nearshore communities, whereas weak upwelling or Downwelling reduces nutrients for phytoplankton production limiting food for larvae and nearshore communities. We review studies conducted at sea and onshore and reanalyze published data to test the IUH and evaluate alternative hypotheses. To test the hypothesis, we examine five predictions that must hold if the IUH is true. (1) Larvae should inhabit the surface Ekman layer where they are transported offshore during upwelling. Larvae of many intertidal taxa occur deeper in the water column where currents flow shoreward during upwelling. (2) Larvae of nearshore species should occur farther offshore during upwelling than during relaxation or Downwelling. Larvae of many nearshore species remain within several kilometers of shore during both conditions. (3) Larval settlement in intertidal communities should be lower during upwelling than relaxation or Downwelling. Daily larval settlement has not observed to be higher during relaxation or Downwelling events; settlement has most often been seen to vary with the fortnightly tidal cycle likely due to onshore larval transport by internal tides. (4) Larval settlement and recruitment in intertidal communities should be lower in areas of strong, persistent upwelling than where upwelling is weaker and less persistent. Recruitment of mussels and barnacles to artificial and natural substrates did not vary with the strength of upwelling, but did vary inversely with two measures of desiccation potential, and directly with indicators of surf zone hydrodynamics; larval recruitment was higher where surf zones were more dissipative with rip currents. (5) Phytoplankton subsidies to nearshore communities should be highest where upwelling is moderate and intermittent. Like larval subsidies, phytoplankton subsidies varied spatially with surf zone hydrodynamics rather than upwelling. This reconsideration of the evidence for the IUH finds the hypothesis unsupported.
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Paradigm lost? Cross-shelf distributions of intertidal invertebrate larvae are unaffected by upwelling or Downwelling
Marine Ecology Progress Series, 2009Co-Authors: Alan L. Shanks, R. Kipp ShearmanAbstract:We tested the hypothesis that larvae of intertidal invertebrates are swept offshore dur- ing upwelling and shoreward during Downwelling. During summer 2007, vertically stratified zoo- plankton samples and oceanographic data were collected at 7 stations located from 0.7 to 27 km from shore near Coos Bay, Oregon, USA. Half the sample dates (27 June and 14 August) were character- ized by upwelling conditions (lines of constant temperature and salinity tilted upward, and a band of cold surface water was against the coast) and the other half (3 and 18 July) were characterized by Downwelling or relaxation (lines of constant temperature and salinity were flat, and warm surface waters were in contact with the shore). We identified and staged larvae of Neotrypaea californiensis, Balanus glandula, B. nubilus, Chthamalus dalli, Pollicipes polymerus, and Semibalanus cariosus/B. crenatus and identified (to species or taxa level) Mytilus californianus, M. trossulus, Hiatella arctica, Dendraster excentricus, and pinnotherid and pagurid zoea. On all sample dates, all taxa and larval stages were rare in surface waters (0 to 10 m depth) and, with one exception (B. nubilus cyprids), were abundant at the 3 inshore stations (0.7 to 4.5 km offshore) and very rare or absent at seaward stations. The average distance offshore of all taxa and larval stages ranged from 0.9 to 4 km from shore and did not vary with upwelling and Downwelling. Upwelling and Downwelling had no effect on the cross-shelf distribution of the larvae of intertidal invertebrates; the hypothesis that upwelling carries larvae offshore and Downwelling carries them back onshore was not supported.
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Upwelling, Downwelling, and cross-shelf transport of bivalve larvae: Test of a hypothesis
Marine Ecology Progress Series, 2005Co-Authors: Alan L. Shanks, Laura BrinkAbstract:Larval supply affects the structure of marine communities. Cross-shelf transport of larvae by upwelling and Downwelling may cause variation in larval supply. We tested the hypothesis that slowswimming bivalve larvae are swept offshore during upwelling and shoreward during Downwelling. We sampled a transect at Duck, North Carolina during a period when currents shifted from upwelling to Downwelling and back to upwelling. During each shift, nearshore water was exchanged with offshore water and currents were 10 to 100 times faster than larval swimming speeds. Larval Spisula solidissima and Ensis directus were found below the thermocline and, contrary to prediction, were swept onshore during upwelling and offshore during Downwelling. When S. solidissima larvae were found above the thermocline, cross-shelf transport was as predicted. Larval Tellina spp. and Mulinia lateralis remained within 5 km of shore despite cross-shelf currents and the exchange of nearshore waters with offshore waters. They did not behave as passive particles; they were not swept offshore by upwelling or onshore by Downwelling. For these taxa the hypothesis was rejected. These larvae may have remained close to shore by using behaviors analogous to those displayed by animals concentrated at convergent fronts. Given the relatively slow swimming speed of bivalve larvae, there is no reason to expect that any larval type is swept offshore by upwelling. The effect of upwelling and Downwelling on larval distributions varies with larval behavior and vertical distribution. Without careful sampling, one cannot invoke offshore transport of larvae by upwelling as a cause of variations in larval settlement.
Colin R. Townsend - One of the best experts on this subject based on the ideXlab platform.
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hyporheic community composition in a gravel bed stream influence of vertical hydrological exchange sediment structure and physicochemistry
Freshwater Biology, 2003Co-Authors: Dean A. Olsen, Colin R. TownsendAbstract:Summary 1. We studied the relative contributions of the magnitude and direction of vertical hydrological exchange, subsurface sediment composition and interstitial physicochemistry in determining the distribution of hyporheic invertebrates in the Kye Burn, a fourth order gravel-bed stream in New Zealand. 2. In winter 2000 and summer 2001, we measured vertical hydrological gradient (VHG), dissolved oxygen, water temperature and water chemistry using mini-piezometers, each installed in a different upwelling or Downwelling zone. Next to every piezometer, a freeze core sample was taken to quantify the sediment, particulate organic matter and invertebrates. 3. Dissolved oxygen concentration at 25 cm was high on both occasions (>9 mg L−1) but was higher in winter than summer. Interstitial water temperature was higher in down than upwellings and was substantially higher in summer than winter. Other features of the subsurface sediments and interstitial nitrate–nitrite concentrations were similar on both occasions and in up and Downwellings. Interstitial ammonium and soluble reactive phosphorous concentrations were higher in winter than summer and ammonium was higher in up than Downwelling areas. 4. The proportion of fine sediment (63 μm–1 mm), sediment heterogeneity and VHG accounted for the greatest proportion of variance in invertebrate distributions in both summer and winter. 5. The hyporheos was numerically dominated by early instar leptophlebiid mayfly nymphs and asellotan isopods. Water mites were a taxonomically diverse group with 13 genera. Taxonomic diversity (Shannon–Weaver), but not taxon richness, was higher in upwelling areas, reflecting lower numerical dominance by a few taxa in these locations. 6. Sediment composition (particularly the amount of fine sediments) and vertical hydrological exchange determined the composition and distribution of the hyporheos. Patchiness in these factors is important in planning sampling regimes or field manipulations in the hyporheic zone.
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Hyporheic community composition in a gravel‐bed stream: influence of vertical hydrological exchange, sediment structure and physicochemistry
Freshwater Biology, 2003Co-Authors: Dean A. Olsen, Colin R. TownsendAbstract:Summary 1. We studied the relative contributions of the magnitude and direction of vertical hydrological exchange, subsurface sediment composition and interstitial physicochemistry in determining the distribution of hyporheic invertebrates in the Kye Burn, a fourth order gravel-bed stream in New Zealand. 2. In winter 2000 and summer 2001, we measured vertical hydrological gradient (VHG), dissolved oxygen, water temperature and water chemistry using mini-piezometers, each installed in a different upwelling or Downwelling zone. Next to every piezometer, a freeze core sample was taken to quantify the sediment, particulate organic matter and invertebrates. 3. Dissolved oxygen concentration at 25 cm was high on both occasions (>9 mg L−1) but was higher in winter than summer. Interstitial water temperature was higher in down than upwellings and was substantially higher in summer than winter. Other features of the subsurface sediments and interstitial nitrate–nitrite concentrations were similar on both occasions and in up and Downwellings. Interstitial ammonium and soluble reactive phosphorous concentrations were higher in winter than summer and ammonium was higher in up than Downwelling areas. 4. The proportion of fine sediment (63 μm–1 mm), sediment heterogeneity and VHG accounted for the greatest proportion of variance in invertebrate distributions in both summer and winter. 5. The hyporheos was numerically dominated by early instar leptophlebiid mayfly nymphs and asellotan isopods. Water mites were a taxonomically diverse group with 13 genera. Taxonomic diversity (Shannon–Weaver), but not taxon richness, was higher in upwelling areas, reflecting lower numerical dominance by a few taxa in these locations. 6. Sediment composition (particularly the amount of fine sediments) and vertical hydrological exchange determined the composition and distribution of the hyporheos. Patchiness in these factors is important in planning sampling regimes or field manipulations in the hyporheic zone.
Wenyan Zhang - One of the best experts on this subject based on the ideXlab platform.
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Storm‐driven bottom sediment transport on a high‐energy narrow shelf (NW Iberia) and development of mud depocenters
Journal of Geophysical Research, 2016Co-Authors: Wenyan Zhang, Ana Santos, Till J J HanebuthAbstract:Bottom sediment transport on the NW Iberian shelf was monitored during a Downwelling storm in September 2014. Collected data was analyzed and fed into a 3D coastal ocean model to understand storm-driven sediment transport on the shelf and its impact on mid-shelf mud depocenters (MDCs). A significantly enhanced level of bottom sediment resuspension, nearly two orders of magnitude higher than that in the pre-storm period, was recorded at the mooring site. Field data analysis reveals that it was induced by a short-lasting strong bottom current in combination with enhanced wave-current interaction. Simulation results indicate that this strong current was part of a coastal jet resulted from Downwelling. An across-shelf horizontal density gradient as high as 0.32 g/m4 occurred at the interface between the Downwelling and the bottom waters, forming a remarkable front. Due to buoyancy effect, the Downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region, resulting in a northward-directed coastal jet. Simulation results suggest that during the storm, local near-bottom sediment suspensions with concentrations on the order of 10 kg/m3 would be triggered by wave-current interaction and flow convergence associated with the front. Direct impact on the development of MDCs by transport and deposition of concentrated sediment suspensions is indicated by model results. The seaward limit of the front coincided with the shoreward edge of the MDC nucleus, suggesting the front as a primary control on the deposition of fine-grained sediment. This article is protected by copyright. All rights reserved.
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storm driven bottom sediment transport on a high energy narrow shelf nw iberia and development of mud depocenters
Journal of Geophysical Research, 2016Co-Authors: Wenyan Zhang, Ana Santos, Till J J HanebuthAbstract:Bottom sediment transport on the NW Iberian shelf was monitored during a Downwelling storm in September 2014. Collected data was analyzed and fed into a 3D coastal ocean model to understand storm-driven sediment transport on the shelf and its impact on mid-shelf mud depocenters (MDCs). A significantly enhanced level of bottom sediment resuspension, nearly two orders of magnitude higher than that in the pre-storm period, was recorded at the mooring site. Field data analysis reveals that it was induced by a short-lasting strong bottom current in combination with enhanced wave-current interaction. Simulation results indicate that this strong current was part of a coastal jet resulted from Downwelling. An across-shelf horizontal density gradient as high as 0.32 g/m4 occurred at the interface between the Downwelling and the bottom waters, forming a remarkable front. Due to buoyancy effect, the Downwelling water was mostly confined to the coast with a depth limit of 80 m in the south and 120 m in the north of the region, resulting in a northward-directed coastal jet. Simulation results suggest that during the storm, local near-bottom sediment suspensions with concentrations on the order of 10 kg/m3 would be triggered by wave-current interaction and flow convergence associated with the front. Direct impact on the development of MDCs by transport and deposition of concentrated sediment suspensions is indicated by model results. The seaward limit of the front coincided with the shoreward edge of the MDC nucleus, suggesting the front as a primary control on the deposition of fine-grained sediment. This article is protected by copyright. All rights reserved.
Claudio Faccenna - One of the best experts on this subject based on the ideXlab platform.
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plate motions andean orogeny and volcanism above the south atlantic convection cell
Earth and Planetary Science Letters, 2012Co-Authors: Laurent Husson, Clinton P Conrad, Claudio FaccennaAbstract:article i nfo The geometric and kinematic evolution of the Andes provides insight onto the nature of the force balance be- neath the South American plate. While the Andean load is opposed on its western edge by the force induced by subduction of the Nazca plate, its more elusive eastern counterpart, which we explore herein, requires some contribution from the mantle beneath the South Atlantic. Using a mantle flow model, we show that the Andes owe their existence to basal drag beneath South America caused by a cylindrical convection cell under the South Atlantic. We find that the observed Andean uplift requires both westward push from active upwelling beneath Africa and westward drag toward the downgoing Nazca slab. These mutually-reinforcing Downwellings and upwellings amount to 38% and 23% of the total driving force, respectively. Further decomposition reveals that the South Atlantic cell is most vigorous near its center, rendering the net drag force higher where the Andes also reach their highest elevation. Kinematic reconstructions suggest that the South Atlantic cell could have grown owing to the migration of the Nazca slab until ~50 Ma. We propose that from 50 Ma onwards, the cell may have ceased growing westward because (i) it had reached an optimal aspect ratio and (ii) the Nazca slab became anchored into the lower mantle. Continued westward motion of the plates, however, moved the surface expressions of spreading and convergence away from the upwelling and Downwelling arms of this cell. Evidence for this scenario comes from the coeval tectonic, morphologic, and magmatic events in Africa and South America during the Tertiary.
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Plate motions, Andean orogeny, and volcanism above the South Atlantic convection cell
Earth and Planetary Science Letters, 2012Co-Authors: Laurent Husson, Conrad Clinton P., Claudio FaccennaAbstract:The geometric and kinematic evolution of the Andes provides insight onto the nature of the force balance beneath the South American plate. While the Andean load is opposed on its western edge by the force induced by subduction of the Nazca plate, its more elusive eastern counterpart, which we explore herein, requires some contribution from the mantle beneath the South Atlantic. Using a mantle flow model, we show that the Andes owe their existence to basal drag beneath South America caused by a cylindrical convection cell under the South Atlantic. We find that the observed Andean uplift requires both westward push fromactive upwelling beneath Africa andwestward drag toward the downgoing Nazca slab. These mutually-reinforcing Downwellings and upwellings amount to 38% and 23% of the total driving force, respectively. Further decomposition reveals that the South Atlantic cell is most vigorous near its center, rendering the net drag force higher where the Andes also reach their highest elevation. Kinematic reconstructions suggest that the South Atlantic cell could have grown owing to the migration of the Nazca slab until ~50 Ma. We propose that from 50 Ma onwards, the cell may have ceased growing westward because (i) it had reached an optimal aspect ratio and (ii) the Nazca slab became anchored into the lower mantle. Continued westward motion of the plates, however, moved the surface expressions of spreading and convergence away from the upwelling and Downwelling arms of this cell. Evidence for this scenario comes from the coeval tectonic, morphologic, and magmatic events in Africa and South America during the Tertiary.