Strontium, dissolved and particulate loads in fresh and brackish waters: the Baltic Sea and Mississippi Delta
A study was conducted of the isotopic composition and concentration of Sr and of major elements in dissolved and suspended loads of fresh and brackish waters. The purpose was to establish the contributions of different parent rocks and minerals to Sr during weathering and transport and to identify the role of Fe—Mn oxyhydroxides in the redistribution of Sr in the water column during the sedimentary cycle. Studies were conducted on a profile across an oxic-anoxic boundary in the Baltic and on rivers covering behavior over an annual cycle. In general, the 87Sr/86Sr ratios differ between particulate and dissolved loads, with more radiogenic Sr in the particulate loads. These differences are attributed to differential weathering of minerals, where high Rb/Sr minerals dominate the particulate load and low Rb/Sr the dissolved load. There is broad correlation of ^(87)Sr/^(86)Sr with K/Al in the suspended load. The differences in ^(87)Sr/^(86)Sr between suspended and dissolved load are highly variable and are related to the Fe or Mn concentration on the particulates. In samples with high Fe/Al, the difference becomes small. A good correlation was found between Sr/Al and Fe/Al or Mn/Al in the particulates both in brackish and fresh waters. Sr is removed from solution both in rivers and in the Baltic Sea whenever there is formation of Fe—Mn oxyhydroxide particulates. This precipitation greatly diminishes the difference in isotopic composition of the dissolved and suspended loads. As the particles containing Fe—Mn oxyhydroxides settle, they dissolve in anoxic zones and release Sr. This provides a mechanism for Sr redistribution in the water column. Sr is thus only quasi-conservative in environments where Fe—Mn oxyhydroxides form or dissolve. From consideration of the isotopic differences in Sr between dissolved and suspended loads, it follows that the net Sr input depends upon weathering characteristics of the contributing mineral phases. Changes in weathering mechanisms due to climate change may cause Sr isotopic shifts in the marine environment.
© 1994 Elsevier Science B.V. Received November 8, 1993; revision accepted March 24, 1994. We acknowledge the advice and aid of D.A. Papanastassiou. We thank the SMHI (Swedish Meteorological and Hydrological Institute) for allowing us to participate in the R.V. Argos cruises. Thanks also to Dr. M.R. Scott for allowing M. Stordal to participate in the R.V. Gyre cruise of 1983. This work was supported by a grant from the Department of Energy (DOE-DE-FG03-88ER13851) and the National Science Foundation (NSF OCE 90-18534). Per S. Andersson was supported by a post-doctoral fellowship from the Swedish Natural Science Research Council (NFR G-PD 6331-300 and G-GU 6331-300). This is Division Contribution 5309(818).