Marine electromagnetic imaging and volumetric estimation of large-scale freshwater plumes offshore Hawai'i

<p>Submarine groundwater discharge (SGD) is a flow of cold and buoyant freshwater from the seafloor the ocean surface. Because SGD contains carbon, nutrients, metals, and green-house gases, it changes the oceanographical and biochemical properties of coastal waters. Therefore, SGD is an important phenomenon that governs hydrological cycles at the land-to-ocean transition zone. Due to the high spatial distribution and variability of SGD at the ocean surface, it is nontrivial to map SGD seep location and fluxes using traditional oceanographic methods. Here, we present electromagnetic imaging of large freshwater plumes in high-resolution, offshore west of Hawai‘i island. Our electrical resistivity models detect multiple vertical freshwater plumes (SGD point-sources) as well as spatially distributed surface freshwater, extending to a distance of ~3 km offshore Hawai‘i. Plume-scale salinity distribution indicates that these plumes contain up to 87% of freshwater. Thus, a substantial volume of freshwater occupies Hawaiian water column plumes. Our findings provide valuable information to elucidate hydrogeologic and oceanographic processes affecting biogeochemical cycles in coastal waters worldwide. This is the first study to demonstrate the marine electromagnetic method’s capability to image and delineate freshwater plumes from the seafloor to the ocean surface.</p><p><strong>Keywords</strong>: Freshwater Plumes, SGD, Hawai'i, Surface-towed CSEM, high-resolution 2D electrical imaging.   </p>

Impact of Shelf Valleys on the Spread of Surface Trapped River Plumes

This study focuses on mechanisms of shelf valley bathymetry affecting the spread of riverine freshwater in the nearshore region. In the context of Changjiang River, a numerical model is used with different no-tide idealized configurations to simulate development of unforced river plumes over a sloping bottom, with and without a shelf valley off the estuary mouth. All simulated freshwater plumes are surface-trapped with continuously growing bulges near the estuary mouth and narrow coastal currents downstream. The simulations indicate that a shelf valley tends to compress the bulge along the direction of the valley long axis and modify the incident angle of the bulge flow impinging toward the coast, which then affects the strength of the coastal current. The bulge compression results from geostrophic adjustment and isobath-following tendency of the depth-averaged flow in the bulge region. Generally, the resulting change in the direction of the bulge impinging flow enhances down-shelf momentum advection and freshwater delivery into the coastal current. Sensitivity simulations with altered river discharges (Q), Coriolis parameter, shelf bottom slope, valley geometry, and ambient stratification show that enhancement of down-shelf freshwater transport in the coastal current, ΔQc, increases with increasing valley depth within the bulge region and decreasing slope Burger number of the ambient shelf. Assuming potential vorticity conservation, a scaling formula of ΔQc?Q is developed, and it agrees well with results of the sensitivity simulations. Mechanisms of valley influences on unforced river plumes revealed here will help future studies of topographic influence on river plumes under more realistic conditions.

Buoyancy-driven effects on turbulent diffusivity induced by a river plume in the southern Brazilian shelf

Freshwater plumes are important flow structures that influence the dynamics and water properties of coastal regions and continental shelves. Turbulence in plume regions is mainly driven by shear instabilities at the interface between plume and oceanic waters, which, in turn, depend on the geometry and outflow of a specific plume region. The Southern Brazilian Shelf presents a highly variable hydrographic distribution modulated by the seasonal wind variation and the freshwater discharge from the La Plata River estuary, which has a significant impact on the continental shelf circulation. This buoyant plume creates strong density gradients and interacts with local water masses resulting in a complex hydrographic pattern. In this study, high resolution hydrography and microstructure measurements were obtained in order to verify the effect of freshwater stratification on vertical mixing in this highly dynamic continental shelf. Results show that the plume is highly stable at southern portions of the shelf, as density displacements, or Thorpe displacements, δT, heat diffusivity, KT, buoyancy flux, Bf, and density gradient ratio, Rp are reduced when compared to the northern areas. Moreover, hydrographic data suggests that the large-scale La Plata River plume has a dynamic mid-field region due to instabilities generated when reaching the shelf break.