Dissolved strontium in the subterranean estuary – Implications for the marine strontium isotope budget

Abstract

15Submarine groundwater discharge (SGD) to the ocean supplies Sr with less radiogenic 87 Sr/ 86 Sr than seawater, and thus constitutes an important term in the Sr isotope budget in the modern ocean. However, few data exist for Sr in coastal groundwater or in the geochemically dynamic subterranean estuary (STE). We examined Sr concentrations and isotope ratios from nine globally-distributed coastal sites and characterized the behavior of Sr in the STE. Dissolved Sr 20 generally mixed conservatively in the STE, although large differences were observed in the meteoric groundwater end-member Sr concentrations among sites (0.1 -24 µM Sr). Strontium isotope exchange was observed in the STE at five of the sites studied, and invariably favored the meteoric groundwater end-member signature. Most of the observed isotope exchange occurred in the salinity range 5-15, and reached up to 40% exchange at salinity 10.

C, O, and Sr isotopic variations in Neoproterozoic-Cambrian carbonate rocks from Sete Lagoas Formation (Bambuí Group), in the Southern São Francisco Basin, Brazil

“…This would explain the high fluctuation in 87 Sr/ 86 Sr ratios, without correlated detrital input. Several authors had already investigated the impact of SGD in marine Sr isotope budget (Basu et al, 2001;Dowling et al, 2003;Rahaman & Singh, 2012;Beck et al, 2013). The Sr flux to the ocean from SGD is a major component (13 to 31%) of the marine Sr isotope budget (Beck et al, 2013).…”

“…Several authors had already investigated the impact of SGD in marine Sr isotope budget (Basu et al, 2001;Dowling et al, 2003;Rahaman & Singh, 2012;Beck et al, 2013). The Sr flux to the ocean from SGD is a major component (13 to 31%) of the marine Sr isotope budget (Beck et al, 2013). However, there is some controversy regarding if groundwater sources are less or more radiogenic relative to 87 Sr/ 86 Sr seawater composition, which is influenced by both carbonate and silicate weathering, with a negative or positive effect on the SGD 87 Sr/ 86 Sr ratios.…”

“…However, there is some controversy regarding if groundwater sources are less or more radiogenic relative to 87 Sr/ 86 Sr seawater composition, which is influenced by both carbonate and silicate weathering, with a negative or positive effect on the SGD 87 Sr/ 86 Sr ratios. Even so, Beck et al (2013) proposed that global average SGD 87 Sr/ 86 Sr ratios vary from 0.7083 to 0.7094. In a similar way, the Eastern margin of the São Francisco basin might have also been subject to local isotopic exchange processes of seawater with SGD, incorporating external Sr.…”

“…Therefore, a Sr isotopic local trend dominated by SGD, independent from detrital input, is suggested. In the modern ocean Sr budget, Beck et al (2013) explain that the estimate of the less-radiogenic Sr input by SGD does not appear to be sufficient to balance the radiogenic river source, but it substantially reduces the imbalance. Groundwater input with less radiogenic 87 Sr/ 86 Sr than seawater buffers the rise in the 87 Sr/ 86 Sr of seawater caused by the fluvial input of radiogenic Sr, and is comparable in magnitude to the flux driven by submarine hydrothermal circulation through oceanic spreading centers and diagenetic processes.…”

“…Groundwater input with less radiogenic 87 Sr/ 86 Sr than seawater buffers the rise in the 87 Sr/ 86 Sr of seawater caused by the fluvial input of radiogenic Sr, and is comparable in magnitude to the flux driven by submarine hydrothermal circulation through oceanic spreading centers and diagenetic processes. The fact that the SGD-driven Sr flux is a major component of the budget implies that changes in the SGD volume inputs over geologic time may have contributed to temporal variations in marine 87 Sr/ 86 Sr ratios (Beck et al, 2013). The direct flow of SGD into the ocean and the chemical reactions of meteoric and seawater mixtures within coastal aquifers are processes that have been largely ignored when estimating material flows between land and sea (Moore, 2010).…”