The “salt wedge pump”: Convection‐driven pore‐water exchange as a source of dissolved organic and inorganic carbon and nitrogen to an estuary

Abstract

Hypoxia and anoxia in coastal waters have typically been explained by the respiration of sinking organic matter associated with nutrient over-enrichment and phytoplankton blooms. Here, we assess whether submarine groundwater discharge and seawater recirculation in sediments can explain widespread chemical anomalies, including low dissolved oxygen, in salt wedge estuaries. We rely on high-resolution radon (a natural groundwater and pore-water tracer), and dissolved carbon concentrations and stable isotope observations in the Yarra River estuary in Melbourne, Australia. Radon was highly enriched within the salt wedge, demonstrating enhanced porewater exchange at this area. We use the term ''salt wedge pump'' to describe convection-driven advective porewater exchange at the sediment-water interface during the upstream propagation of the salt wedge. Radonderived convection-driven pore-water exchange rates within the salt wedge were estimated at 2.8 cm d 21 , a value equivalent to 2.4% of the total river freshwater runoff to the estuary. Pore-water exchange led to pulsed dissolved inorganic carbon (DIC) and ammonium fluxes , 10-fold higher than measured diffusive fluxes. In contrast, diffusive sediment oxygen uptake was 5-fold higher than oxygen uptake related to advective pore-water exchange. Estimated fluxes, associated with the nonconservative DIC, d 13 C-DIC, and ammonium behavior within the estuary support convective pore-water exchange as a major source of DIC and ammonium to the estuary, but not of dissolved organic carbon, nitrate, dissolved organic nitrogen, and anoxia. Accounting for seawater recirculation in sediments may help reconcile unbalanced carbon and nitrogen budgets in several coastal systems.

Groundwater dynamics in subterranean estuaries of coastal unconfined aquifers: Controls on submarine groundwater discharge and chemical inputs to the ocean

“…This neglects transformations in the STE, consequently overestimating fluxes from contaminated CUAs (as in Figure 4A) and underestimating fluxes from CUAs where nitrate is produced (as in Figure 4B). An approach often used to bypass the assessment of biogeochemical transformations in the STE consists of sampling pore water in the seepage zone just before it discharges across the CUA-ocean interface [Santos et al, 2012a]. For this approach, the pore water concentrations in the seepage zone may be lower due to dilution of the terrestrial groundwater with large quantities of circulating seawater, and therefore consideration needs to be made for the high volume flux through the seepage zone.…”

Groundwater‐derived dissolved inorganic and organic carbon exports from a mangrove tidal creek: The missing mangrove carbon sink?

“…This unreactive pipe is defined as the flow of terrestrial-derived organic matter via surface waters (i.e., rivers) to the ocean. However, studies suggest that significant transformations of the organic matter pool occur during transport (Peterson et al 1994;Maher and Eyre 2010), and that there is a substantial, yet poorly constrained, flow of terrestrial carbon through subsurface pathways (Cai et al 2003;Santos et al 2009). In addition, our understanding of the transport and transformation of carbon as it transits from terrestrial to oceanic biomes suffers from a lack of data from a variety of ecosystems and latitudes, including mangroves (Bouillon et al 2008;Kristensen et al 2008a;Alongi 2009).…”

Submarine groundwater discharge revealed by radium isotopes (Ra-223 and Ra-224) near a paleochannel on the Southern Brazilian continental shelf

“…The intrusion of cold (and hence, denser) SASW into the paleochannel may lead to density inversion at the sediment-water interface and may potentially release the radium-and nutrient-enriched interstitial water. This process, which may occur in diverse environments, acts on temporal/spatial scales that may vary from hours to years and from centimeters to kilometers (SANTOS et al, 2012a). For instance, in Florida, groundwater reaches the shelf through fractures in the seabed and creates convection cells on a kilometer scale (KOHOUT, 1967).…”

“…For instance, in Florida, groundwater reaches the shelf through fractures in the seabed and creates convection cells on a kilometer scale (KOHOUT, 1967). The balance of the interstitial and overlying waters involved in this mechanism may reach 0.14 m d -1 in estuaries (WEBSTER et al, 1996) and determine the chemical composition of estuarine waters (SANTOS et al, 2012a).…”