228Ra/ 226Ra and 226Ra/Ba ratios to track barite formation and transport in the water column

Abstract

We measured 228Ra ex/ 226Ra ex and 226Ra ex/Ba ex ratios in suspended and sinking particles collected at the Oceanic Flux Program (OFP) time-series site in the western Sargasso Sea and compared them to seawater ratios to provide information on the origin and transport of barite (BaSO 4) in the water column. The 228Ra ex/ 226Ra ex ratios of the suspended particles down to 2000 m are nearly identical to those of seawater at the same water depth. These ratios are much lower than expected if suspended barite was produced in surface waters and indicate that barite is produced throughout the mesopelagic layer. The 228Ra ex/ 226Ra ex activity ratios of sinking particles collected at 1500 and 3200 m varied mostly between 0.1 and 0.2, which is intermediate between the seawater ratio at these depths (<0.03) and the seawater ratios found in the upper 250 m (0.31–0.42). This suggests that excess Ba (i.e., Ba ex = Ba total − Ba lithogenic), considered to be mainly barite, present in the sinking flux is a mixture of crystals formed recently in the upper water column, formed several years earlier in the upper water column, or formed recently in deeper waters. We observe a sizeable temporal variability in the 228Ra ex/ 226Ra ex ratios of sinking particles, which indicates temporal variability in the relative proportion of barite crystals originating from surface (with a high 228Ra ex/ 226Ra ex ratio) and mesopelagic (with a low 228Ra ex/ 226Ra ex ratio) sources. However, we could not discern a clear pattern that would elucidate the factors that control this variability. The 226Ra/Ba ratios measured in seawater are consistent with the value reported from the GEOSECS expeditions (2.3 dpm μmol −1) below 500 m depth, but are significantly lower in the upper 500 m. High 226Ra ex/Ba ex ratios and elevated Sr concentrations in suspended particles from the upper water column suggest preferential uptake of 226Ra over Ba during formation of SrSO 4 skeletons by acantharians, which must contribute to barite formation in shallow waters. Deeper in the water column the 226Ra ex/Ba ex ratios of suspended particles are lower than those of seawater. Since 228Ra ex/ 226Ra ex ratios demonstrate that suspended barite at these depths has been produced recently and in situ, their low 226Ra ex/Ba ex ratios indicate preferential uptake of Ba over Ra in barite formed in mesopelagic water.