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

We measured Ra-228(ex)/Ra-226(ex) and Ra-226(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 (BaSO4) in the water column. The Ra-228(ex)/Ra-226(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 Ra-228(ex)/Ra-226(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 Ra-228(ex)/Ra-226(ex) ratios of sinking particles, which indicates temporal variability in the relative proportion of barite crystals originating from surface (with a high Ra-228(ex)/Ra-226(ex) ratio) and mesopelagic (with a low Ra-228(ex)/Ra-226(ex) ratio) sources. However, we could not discern a clear pattern that would elucidate the factors that control this variability. The Ra-226(ex)/Ba-ex ratios measured in seawater are consistent with the value reported from the GEOSECS expeditions (2.3 dpm mu mol(-1)) below 500 m depth, but are significantly lower in the upper 500 m. High Ra-226(ex)/Ba-ex ratios and elevated Sr concentrations in suspended particles from the upper water column suggest preferential uptake of Ra-226 over Ba during formation of SrSO4 skeletons by acantharians, which must contribute to barite formation in shallow waters. Deeper in the water column the Ra-226(ex)/Ba-ex ratios of suspended particles are lower than those of seawater. Since Ra-228(ex)/Ra-226(ex) ratios demonstrate that suspended barite at these depths has been produced recently and in situ, their low Ra-226(ex)/Ba-ex ratios indicate preferential uptake of Ba over Ra in barite formed in mesopelagic water. (c) 2006 Elsevier Inc. All rights reserved.