Joaquim Pereira Bento

The vertical variability in the oxygen minimum zone (OMZ) in the eastern South Pacific (ESP; 0–40°S) is characterized by the influence of anticyclonic intrathermocline eddies (ITEs), which are subsurface‐intensified mesoscale features that are frequently generated in the coastal upwelling zone off Peru and Chile. The unique lens‐shaped signatures that ITEs leave on temperature, salinity, and dissolved oxygen vertical distributions are used as proxies to assess their influences on vertical OMZ boundaries and thickness. Data from in situ profiles in the region (World Ocean Database and Argo databases) are used in correlation analyses between anomalous depths (and vertical displacements) of oxyclines and isopycnals/isotherms, together with an objective eddy detection method based on satellite altimetry to identify the location of such profiles (i.e., outside or inside cyclonic and anticyclonic eddies). The results indicate that most of the vertical fluctuations in the climatological OMZ have a lens‐shaped signature and that those at the mesoscale are largely due to ITEs. ITEs are a main driver of the upper oxycline variability in the coastal band, the coastal transition zone (CTZ, 3–10° from the coast) and beyond at mid‐latitudes (12–26°S). The influence of ITEs on the upper and lower oxycline variability is mostly observed within the horizontal (offshore) climatological boundary of the OMZ, suggesting that ITEs play a role in the OMZ offshore extension in the ESP. ITEs may produce mesoscale variations in the thickness of the surface‐oxygenated layer, resulting in potential changes in prey‐predator interactions and in food web functioning.

Large amounts of gas hydrate are present in marine sediments offshore Taitao Peninsula, near the Chile Triple Junction. Here, marine sediments on the forearc contain carbon that is converted to methane in a regime of very high heat flow and intense rock deformation above the downgoing oceanic spreading ridge separating the Nazca and Antarctic plates. This regime enables vigorous fluid migration. Here, we present an analysis of the spatial distribution, concentration, estimate of gas-phases (gas hydrate and free gas) and geothermal gradients in the accretionary prism, and forearc sediments offshore Taitao (45.5°–47° S). Velocity analysis of Seismic Profile RC2901-751 indicates gas hydrate concentration values −1). Gas hydrates are located in shallow sediments (90–280 m below the seafloor). The large amount of hydrate and free gas estimated (7.21 × 10¹¹ m³ and 4.1 × 10¹⁰ m³; respectively), the high seismicity, the mechanically unstable nature of the sediments, and the anomalous conditions of the geothermal gradient set the stage for potentially massive releases of methane to the ocean, mainly through hydrate dissociation and/or migration directly to the seabed through faults. We conclude that the Chile Triple Junction is an important methane seepage area and should be the focus of novel geological, oceanographic, and ecological research.

Recent studies have reported cold seeps offshore of Mocha Island. Gas hydrate occurrences along the Chilean margin could explain seeps presence. Gas-phase (gas hydrate and free gas) and geothermal gradients were estimated analysing two seismic sections. Close to Mocha Island (up to 20 km) were detected high (up to 1900 m/s) and low (1260 m/s) velocities associated with high gas hydrate (up to 20% of total volume) and free gas (up to 1.1% of total volume) concentrations, respectively. A variable and high geothermal gradient (65–110 °C/km) was obtained. These results are related to high supply of deep fluids canalised by faults and fractures. Faraway from Mocha Island (>60 km), free gas concentrations decrease to 0.3% of total volume and low geothermal gradient (from 35 to 60 °C/km) are associated with low fluids supply. Finally, we propose gas hydrate dissociation processes as the main supply source for seeps in the vicinity of Mocha Island. These processes can be caused by: (a) active faults and seismic activity; and (b) warm fluid expulsion from deeper zones altering hydrate stability conditions. In both cases, gas hydrate dissociation could generate slope instability and landslides, as occurred in the past in this region and reported in the literature.

Large amounts of gas hydrate are present in marine sediments offshore Taitao Peninsula, near the Chile Triple Junction. Here, marine sediments on the forearc contain carbon that is converted to methane in a zone of very high heat flow and intense rock deformation above the downgoing oceanic spreading ridge separating the Nazca and Antarctic plates. This regime enables vigorous fluid migration. Here we present an analysis of the spatial distribution, concentration, estimate of gas phases (gas hydrate and free gas) and geothermal gradients in the accretionary prism and forearc sediments offshore Taitao (45.5° - 47° S). Velocity analysis of Seismic Profile RC2901-751 indicates gas hydrate concentration values <10% of the total rock volume, and extremely high geothermal gradients (<190 °Ckm-1). Gas hydrates are located in shallow sediments (90-280 meters below the seafloor). The large amount of hydrate and free gas estimated (7.21x1011 m3 and 4.1x1010 m3, respectively), the high seismicity, the mechanically unstable nature of the sediments, and the anomalous geothermal conditions, set the stage for potential massive releases of methane to the ocean mainly through hydrate dissociation and/or migration directly to the seabed through faults. We conclude that the Chile Triple Junction is an important methane seepage area and should be the focus of novel geological and ecological research.

Recent studies have reported shallow and deep seep areas offshore Mocha island. Gas hydrate occurrences along the Chilean margin could explain seeps presence. Gas phases (gas hydrate and free gas) and geothermal gradients were estimated analysing two seismic sections. Close to Mocha island (up to 20 km) were detected high (up to 1900 m/s) and low (1260 m/s) velocities associated with high gas hydrate (up to 20 % of total volume) and free gas (up to 1.1% of total volume) concentrations respectively. These values are in agreement with a variable and high geothermal gradient (65 to 110 °C/km) related to high supply deep fluids canalised by faults and fractures. Faraway from Mocha island (more than 60 km), free gas concentrations decrease to 0.3 % of total volume and low geothermal gradient (from 35 to 60 °C/km) are associated with low fluids supply. Finally, we propose gas hydrate dissociation processes as the main supply source for seeps in the vicinity of Mocha island. These processes can be triggered by ancient sliding reported in literature.

In Eastern Boundary Upwelling Systems, cold coastal waters are separated from offshore by a strong cross-shore Sea Surface Temperature (SST) gradient zone. This upwelling front plays a major role for the coastal ecosystem. This paper proposes a method to automatically identify the front and define its main characteristics (position, width, and intensity) from high resolution data. The spatio-temporal variability of the front characteristics is then analyzed in a region off Central Chile (37°S), from 2003 to 2016. The front is defined on daily 1 km-resolution SST maps by isotherm T₀ with T₀ computed from mean SST with respect to the distance from the coast. The probability of detecting a front, as well as the front width and intensity are driven by coastal wind conditions and increased over the 2007–2016 period compared to the 2003–2006 period. The front position, highly variable, is related to the coastal jet configuration and does not depend on the atmospheric forcing. This study shows an increase by 14% in the probability of detecting a front and also an intensification by 17% of the cross-front SST difference over the last 14 years. No trend was found in the front position.

In eastern boundary current systems (EBCSs), submesoscale to mesocale variability contributes to cross‐shore exchanges of water properties, nutrients, and plankton. Data from a short‐term summer survey and satellite time series (January–February 2014) were used to characterize submesoscale variability in oceanographic conditions and phytoplankton distribution across the coastal upwelling and coastal transition zones north of Punta Lavapié, and to explore cross‐shelf exchanges of diatom taxa. A thermohaline front (FRN‐1) flanked by a mesoscale anticyclonic intrathermocline eddy (ITE‐1), or mode‐water eddy, persisted during the time series and the survey was undertaken during a wind relaxation event. At the survey time, ITE‐1 contributed to an onshore intrusion of warm oceanic waters (southern section) and an offshore advection of cold coastal waters (northern section), with the latter forming a cold, high chlorophyll‐a filament. In situ phytoplankton and diatom biomasses were highest at the surface in FRN‐1 and at the subsurface in ITE‐1, whereas values in the coastal zone were lower and dominated by smaller cells. Diatom species typical of the coastal zone and species dominant in oceanic waters were both found in the FRN‐1 and ITE‐1 interaction area, suggesting that this mixture was the result of both offshore and onshore advection. Overall, front‐eddy interactions in EBCSs could enhance cross‐shelf exchanges of coastal and oceanic plankton, as well as sustain phytoplankton growth in the slope area through localized upward injections of nutrients in the frontal zone, combined with ITE‐induced advection and vertical nutrient inputs to the surface layer.

Measuring small-scale physical processes and how they affect the spatial patterns of sea water's optical constituents plays a key role in understanding the functioning of complex coastal ecosystems such as estuaries. The spatial variations of physical and bio-optical water properties were investigated during one spring tidal cycle in austral summer, on two transects across the channel of the Medeiros and Itaqui sub-estuaries. These sub-estuaries are a biosphere reserve, and part of the Paranaguá Estuarine System, located on the Southern Brazilian coast. Both sub-estuaries were classified as Type 1a, wellmixed with low stratification. The salinity variations are in phase with the water level, and the tidal propagation is well represented by a standing wave. The vertical velocity profiles showed little vertical shear, and the intensity of the u-component of the velocity varied semi-diurnally. The upper estuary salt transport was dominated by tidal diffusion in an unstable water column. The optical environment presented a mixed dominance of optically active substances, as indicated by the absorption coefficients of dissolved and particulate matter. The colored dissolved organic matter (CDOM) showed overall conservative behavior and was dominant in light absorption below 550 nm in the Medeiros, while nonalgal particles play the most important role in light absorption in the Itaqui in the blue absorption band. The phytoplanktonic contribution is prominent in the red domain and increases as a function of saline intrusion. However, due to the influence of freshwater discharge and the re-suspension of bottom sediments induced by physical processes, the concentrations of the optical components in the water column do not generally have any simple relationships between them.