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

Abstract

A B S T R A C TSubmarine Groundwater Discharge (SGD) has been recognized as an important component of the ocean-continent interface. The few previous studies in Brazil have focused on nearshore areas. This paper explores SGD on the Southern Brazilian Continental Shelf using multiple lines of evidence that include radium isotopes, dissolved nutrients, and water mass observations. The results indicated that SGD may be occurring on the Continental Shelf in the Albardão region, near a paleochannel located 50 km offshore. This paleochannel may thus be a preferential pathway for the delivery of nutrientand metal-enriched groundwater and porewater into continental shelf waters. R E S U M OA descarga de água subterrânea ("Submarine Groundwater Discharge"; SGD) é um importante elo entre continente-oceano. No Brasil, embora haja um crescente interesse em estudos sobre este tema, eles ainda são raros e se restringem às zonas costeiras. A presente investigação explora as evidências SGD na Plataforma Continental do Sul do Brasil, as quais incluem isótopos de rádio, nutrientes dissolvidos e distribuição das massas d'água. Os resultados indicam que a SGD pode ocorrer na Plataforma Continental na região do Albardão, próximo a um paleocanal localizado a 50 km da costa. Esse paleocanal pode, assim, ser o caminho preferencial de entrada de nutrientes e de águas subterrâneas ricas em metais na plataforma continental.

Environmental and biological factors controlling the spring phytoplankton bloom at the Patagonian shelf-break front – Degraded fucoxanthin pigments and the importance of microzooplankton grazing

“…Recently, Rijkenberg et al (2014) reported that the higher DFe concentrations (up to 6.1 nM) of the southwest Atlantic were found at the BMC(35-40°S). Due to the fact that the high DFe concentrations were not associated with a minimum in salinity, Rijkenberg et al (2014) suggest that these high DFe concentrations were not part of the Rio de la Plata plume; instead, the enrichment was due to the offshore export of Brazilian shelf waters (Attisano et al, 2013) near the BMC (see Matano et al, 2014). Complementary to the in situ growth hypothesis, the high diatom biomass observed in the northernmost station (St8) may be also due to their transport from the adjacent shelf-break bloom station, as occurring near the BMC (Matano et al, 2010;Guerrero et al, 2014;Strub et al, 2015).…”

Distribution of 222RN on the Continental Shelf in Southern Brazil and its Relation with Groundwater Discharge

“…Studies have shown that the main source of recharge of the surface aquifers of this barrier system is precipitation [26]. Previous studies have shown that SGDprocesses are important sources of iron and other nutrients (nitrogen, phosphate and silicate) to the platform adjacent to the 240 km barrier that separates the Atlantic Ocean from the Patos Lagoon [16,17].In the south, adjacent to the Mangueira Lagoon, paleochannels facilitates this process [14,15]. In general, groundwater flow towards the ocean, but, depending on precipitation conditions, the subterranean estuary, formed when fresh water meets sea water, may cause water to flow to a region farther away from the beach, into the mainland.…”

“…However, it is known that SGD can also occur farther offshore. For example, south of the Patos Lagoon, in a region about 100 km from its mouth, [15] analyzed nutrients, radio isotopes and bodies of waterand showed that SGD may occur near a paleocanal 50 km from the coast. Distribution of 222 Rn in the four transects perpendicular to the beach line are shown in Figure 4.…”

“…Widespread use of this isotope is mainly due to its high concentration in groundwater (on average, 1,000 times higher than in surface water) and to its completely inert character, that is, its activity is not altered by biochemical reactions, thus making it an ideal tracer [13] Currentlyradon and radium isotopes remain as the most popular geochemical tracers to identify and quantify SGD processes [9]. The study was carried out in the southern region in Brazil, mainly on the coast of Rio Grande do Sul (RS) state, where a group of researchers have conducted studiesof SGD processes [14][15][16][17]. It is due to the existence of many coastal lagoons in the region and because it is separated from the ocean by permeable sediments, which influence the flow of groundwater to the ocean directly.…”