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The response of inner shelf circulation and bottom temperature variability to synoptic wind forcing and freshwater outflow is evaluated in an area with a wide continental shelf off central Chile. This forced circulation, with a strong seasonal evolution from upwelling- to downwelling-favorable conditions, is a key process modulating the exchange of water properties in a coastal zone characterized by multiple river outflows and high biological productivity. Ocean currents on the inner shelf (34 m depth) off the Itata River mouth were obtained and analyzed for a nine-month record (December 2008 to September 2009). The synoptic wind and current variability was defined between 2 and 16 days (0.02 to 0.0026 cph). The subinertial coastal circulation within the inner shelf off Itata River was dominated by the along-shelf flow, while cross-shelf flows driven by along-shelf winds were substantially reduced. The alongshore synoptic currents showed two distinct modal structures: (i) A mean two-layer flow field during upwelling-favorable wind stress with northward (southward) flow at the surface (bottom), and (ii) a mean southward flow through the entire water column during downwelling-favorable wind stress periods, which were intensified during peak river discharge events in winter. Calculations of the wind index clarified the relative importance of wind versus buoyancy forcing on the alongshore flow. The wind forcing dominated during summer when the river discharge was minimum, whereas the relative importance of the freshwater outflow from the Itata River became dominant in winter (May-August) when the buoyancy forcing, in conjunction with southward wind events, modulated strong barotropic southward flows. The change in the long-term regime of coastal winds and river discharges, with a dominance of upwelling winds and reduced river outflows in the last decade, is discussed in the context of observations of thinner river plumes under frequent upwelling conditions off central Chile. [ABSTRACT FROM AUTHOR]

A new seismic model for crust and upper mantle of the south Central Andes is derived from full waveform inversion, covering the Pampean flat subduction and adjacent Payenia steep subduction segments. Focused crustal low‐velocity anomalies indicate partial melts in the Payenia segment along the volcanic arc, whereas weaker low‐velocity anomalies covering a wide zone in the Pampean segment are interpreted as remnant partial melts. Thinning and tearing of the flat Nazca slab is inferred from gaps in the slab along the inland projection of the Juan Fernandez Ridge. A high‐velocity anomaly in the mantle below the flat slab is interpreted as relic Nazca slab segment, which indicates an earlier slab break‐off triggered by the buoyancy of the Juan Fernandez Ridge during the flattening process. In Payenia, large‐scale low‐velocity anomalies atop and below the re‐steepened Nazca slab are associated with the re‐opening of the mantle wedge and sub‐slab asthenospheric flow, respectively. Plain Language Summary: Taking advantage of the abundant information recorded in seismic waveforms, we imaged the seismic structure of the crust and upper mantle beneath central Chile and western Argentina, where the oceanic Nazca slab is subducting beneath the South American plate. The subducted Nazca slab is almost flat at a depth of 100–150 km in the north of the study area below the Pampean region, where the Juan Fernandez seamount ridge is subducting as part of the Nazca slab. The slab steepens again in the south in the Payenia region. Our model reveals pronounced low‐velocity anomalies within the Pampean flat slab along the inland projection of the Juan Fernandez Ridge, indicating that the Pampean flat slab is thinned or even torn apart. A high‐velocity anomaly is imaged beneath the flat slab, representing a former slab segment that was broken off during the slab flattening process and was overridden by the advancing young slab. Our model suggests a causal relationship between the oceanic ridge subduction and the flat slab formation. In the Payenia region, the slab re‐steepening resulted in the re‐establishment of the mantle wedge and induced hot mantle flow below the slab, which are characterized by low‐velocity anomalies in the model. Key Points: A new seismic model for the crust and upper mantle beneath central Chile and western Argentina is presentedThinning and tearing within the Pampean flat slab is detected along the inland projection of the Juan Fernandez RidgeA relic slab is imaged beneath the Pampean flat slab, reflecting slab break‐off during the flattening process [ABSTRACT FROM AUTHOR]

Long‐distance dispersal plays a key role in evolution, facilitating allopatric divergence, range expansions, and species movement in response to environmental change. Even species that seem poorly suited to dispersal can sometimes travel long distances, for example via hitchhiking with other, more intrinsically dispersive species. In marine macroalgae, buoyancy can enable adults—and diverse hitchhikers—to drift long distances, but the evolution and role of this trait are poorly understood. The southern bull‐kelp genus Durvillaea includes several non‐buoyant and buoyant species, including some that have only recently been recognized. In revising the genus, we not only provide updated identification tools and describe two new species (D. incurvata comb. nov. from Chile and D. fenestrata sp. nov. from the Antipodes Islands), but also carry out biogeographic analyses to determine the evolutionary history of buoyancy in the genus. Although the ancestral state was resolved as non‐buoyant, the distribution of species suggests that this trait has been both gained and lost, possibly more than once. We conclude that although buoyancy is a trait that can be useful for dispersal (creating evolutionary pressure for its gain), there is also evolutionary pressure for its loss as it restricts species to narrow environmental ranges (i.e., shallow depths). [ABSTRACT FROM AUTHOR]

Long‐distance dispersal plays a key role in evolution, facilitating allopatric divergence, range expansions, and species movement in response to environmental change. Even species that seem poorly suited to dispersal can sometimes travel long distances, for example via hitchhiking with other, more intrinsically dispersive species. In marine macroalgae, buoyancy can enable adults—and diverse hitchhikers—to drift long distances, but the evolution and role of this trait are poorly understood. The southern bull‐kelp genus Durvillaea includes several non‐buoyant and buoyant species, including some that have only recently been recognized. In revising the genus, we not only provide updated identification tools and describe two new species (D. incurvata comb. nov. from Chile and D. fenestrata sp. nov. from the Antipodes Islands), but also carry out biogeographic analyses to determine the evolutionary history of buoyancy in the genus. Although the ancestral state was resolved as non‐buoyant, the distribution of species suggests that this trait has been both gained and lost, possibly more than once. We conclude that although buoyancy is a trait that can be useful for dispersal (creating evolutionary pressure for its gain), there is also evolutionary pressure for its loss as it restricts species to narrow environmental ranges (i.e., shallow depths). [ABSTRACT FROM AUTHOR]

A wedge subduction can initiate after a ridge subduction and is characterized by the young and thin part of the plate subducting first. A series of 2D geodynamical models were built to investigate the factors in the development of a young flat-slab during the wedge subduction process after a ridge subduction. The factors include the convergence rate, the half spreading rate of the ridge, the initial subduction angle, and the thickness of the overriding plate. The results show that a flat-slab generally forms before the slab age reaches 40 Myr, except in special cases, for instance, when convergence rate ≤ 1 cm/yr, the half spreading rate is low, or the initial subduction angle ≥ 55°. The wedge subduction slab is prone to flatten because the young wedge subduction slab is naturally accompanied by buoyancy anomaly and has the potential to flatten at the relatively low convergence rate. There is no evidence as to whether a flat-slab exists south of Chile Triple Junction. According to the simulation results, the tomographic evidence, and the oceanic age distribution, it can be inferred that, from 46.5°S to ∼50°S, the wedge subduction is in the initiation stage and the formation of a flat-slap is impossible. Near 50–52°S, the slab possibly flattens when the initial subduction angle is 20–30°. From ∼52°S to ∼55°S, when the initial subduction angle is 20–40°, a flat subduction is likely to occur. • A wedge subduction is easier than a common subduction to form a flat-slab. • The wedge subduction zone exists in a post-ridge-subduction area south of Chile Triple Junction. • There is no evidence as to whether there is a flat-slab south of Chile Triple Junction. • A flat subduction is possible south of 50°S. [ABSTRACT FROM AUTHOR]

The emplacement of ultramafic blocks in accretionary complexes poses a geodynamic problem due to their negative buoyancy. In this study, we explore plausible emplacement scenarios for peridotite bodies in the Coastal Cordillera of south-central Chile by combining geophysical observations, numerical modeling and available petrological data for ultramafic blocks exhumed along the subduction channel. The La Cabaña massif is the largest serpentinized peridotite complex yet recognized in the Coastal Cordillera, however, its size and petrophysical characteristics remain unknown. The geophysical measurements were performed to determine the size of this body, involving magnetic airborne surveys and electrical resistivity tomography. Inversion data show that the largest ultramafic block in La Cabaña is 3 km long, 1.5 km wide and at least 1.2 km deep. This result constrains the characteristics of the block transported by the subduction channel. In the second step, we developed a numerical model for the subduction channel assuming a viscous rheology. In this modeling effort we search for tectonic scenarios that provide adequate conditions for the exhumation of the ultramafic body in La Cabaña. These scenarios included a combination of key parameters, subduction angle dip and velocity, subduction channel geometry, rheology and density contrast. Scenarios compatible with the exhumation of La Cabaña body type includes channel viscosity range of 1019−20 Pa s, fast exhumation rates at mantle depths (10–20 mm/yr; >30–40 km), steep subduction angles (30°–60°), subduction channel widths of 3–5 km, density contrast between −200 and −400 kg/m3 and a body diameter of 1.5 km which is consistent with our geophysical inversions. On the contrary, slow exhumation rates (∼1 mm/yr), low subduction angles (15°), high-density contrasts (−400 to −600 kg/m3) and bodies larger than 1.5 km wide, are not viable exhumation scenarios. The methodology developed provided insights to infer ancient subduction channel geometries and rheologies that include peridotite. • We test the concept that the subduction channel drives the exhumation of ultramafic blocks within accretionary prisms. • We perform a parametric study of the rheology, geometry and density for ultramafic blocks exhumed in accretionary prisms. • Critical conditions regarding the serpentinization degree and subduction channel rheology are also explored. [ABSTRACT FROM AUTHOR]

The radiation of notothenioid fishes (Perciformes) in Antarctic waters was likely the result of an absence of competition in the isolated Antarctic waters and key traits such as the production of antifreeze glycoprotein and buoyancy modifications. Although notothenioids lack a swim bladder, the buoyancy of Antarctic species, ranging from neutrally buoyant to relatively heavy, corresponds to diverse life styles. The buoyancy of South American notothenioids has not been studied. Static buoyancy was measured in adult notothenioids ( n = 263, from six species of the sub-order Notothenioidei, families Bovichtidae, Eleginopidae, Nototheniidae, and Harpagiferidae) from the Beagle Channel. Measurements were expressed as percentage buoyancy (%B). Buoyancy ranged from 3.88 to 6.96% (median, 4.0-6.7%), and therefore, all species could be considered benthic consistent with previous studies that found that neutral buoyancy in notothenioids is rare. Harpagifer bispinis, Patagonotothen cornucola, and Cottoperca gobio were significantly less buoyant than Paranotothenia magellanica. The buoyancy values of most species were concordant with known habitat preferences. These data, especially the data of C. gobio (sister lineage of all other nototehnioids) and E. maclovinus (sister lineage of the Antarctic clade of notothenioids), could be useful for understanding the diversification of this feature during the notothenioid radiation. [ABSTRACT FROM AUTHOR]

Long-distance dispersal plays a key role in evolution, facilitating allopatric divergence, range expansions, and species movement in response to environmental change. Even species that seem poorly suited to dispersal can sometimes travel long distances, for example via hitchhiking with other, more intrinsically dispersive species. In marine macroalgae, buoyancy can enable adults-and diverse hitchhikers-to drift long distances, but the evolution and role of this trait are poorly understood. The southern bull-kelp genus Durvillaea includes several non-buoyant and buoyant species, including some that have only recently been recognized. In revising the genus, we not only provide updated identification tools and describe two new species (D. incurvata comb. nov. from Chile and D. fenestrata sp. nov. from the Antipodes Islands), but also carry out biogeographic analyses to determine the evolutionary history of buoyancy in the genus. Although the ancestral state was resolved as non-buoyant, the distribution of species suggests that this trait has been both gained and lost, possibly more than once. We conclude that although buoyancy is a trait that can be useful for dispersal (creating evolutionary pressure for its gain), there is also evolutionary pressure for its loss as it restricts species to narrow environmental ranges (i.e., shallow depths)., (© 2019 Phycological Society of America.)