Sulfur stable isotopes indicate the source of sinking materials in a coastal bay: Otsuchi Bay, Sanriku, Japan

Through 2004 and 2005, δ ³⁴S of sinking material from Otsuchi Bay was measured at the center and rocky shore of the bay. At the center of the bay δ ³⁴S was high (18∼21‰) in the material collected from April to November. However, δ ³⁴S was low (9∼14‰) in the material collected from December to March. The increase in δ ³⁴S in April was attributed to an increase in phytoplankton biomass because marine phytoplanktonic δ ³⁴S is high. When δ ³⁴S of sinking material was low, input of riverine material or bottom sediment resuspension were considered as the probable causes, because their δ ³⁴S is low. Marine sulfur was always high (more than 70%) at both stations. The difference between the δ ³⁴S of sinking material collected from the different sampling stations indicates that marine macroalgae contribute to sinking material near the shore when phytoplankton is scarce. In conclusion, the relative influence of different material sources to sinking materials could be successfully estimated using δ ³⁴S.