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 an d brackish waters. The purpose was to establish the contributions of d ifferent parent rocks and minerals to Sr during weathering and transpo rt and to identify the role of Fe-Mn oxyhydroxides in the redistributi on of Sr in the water column during the sedimentary cycle. Studies wer e conducted on a profile across an oxic-anoxic boundary in the Baltic and on rivers covering behavior over an annual cycle. In general, the Sr-87/Sr-86 ratios differ between particulate and dissolved loads, wit h more radiogenic Sr in the particulate loads. These differences are a ttributed to differential weathering of minerals, where high Rb Sr min erals dominate the particulate load and low Rb/Sr the dissolved load. There is broad correlation of Sr-87/Sr-86 with K/Al in the suspended l oad. The differences in Sr-87/Sr-86 between suspended and dissolved lo ad are highly variable and are related to the Fe or Mn concentration o n 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 f rom solution both in rivers and in the Baltic Sea whenever there is fo rmation of Fe-Mn oxyhydroxide particulates. This precipitation greatly diminishes the difference in isotopic composition of the dissolved an d suspended loads. As the particles containing Fe-Mn oxyhydroxides set tle, they dissolve in anoxic zones and release Sr. This provides a mec hanism for Sr redistribution in the water column. Sr is thus only quas i-conservative in environments where Fe-Mn oxyhydroxides form or disso lve. From consideration of the isotopic differences in Sr between diss olved and suspended loads, it follows that the net Sr input depends up on weathering characteristics of the contributing mineral phases. Chan ges in weathering mechanisms due to climate change may cause Sr isotop ic shifts in the marine environment.