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Silicon isotopic chemistry in the C hangjiang E stuary and coastal regions: Impacts of physical and biogeochemical processes on the transport of riverine dissolved silica
Author(s) -
Zhang A. Y.,
Zhang J.,
Hu J.,
Zhang R. F.,
Zhang G. S.
Publication year - 2015
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1002/2015jc011050
Subject(s) - estuary , salinity , fractionation , biogeochemical cycle , seawater , water column , oceanography , environmental science , environmental chemistry , biogenic silica , geology , hydrology (agriculture) , chemistry , diatom , organic chemistry , geotechnical engineering
The dissolved silica (DSi) concentration and silicon isotopic composition (δ 30 Si) of surface water samples from the Changjiang Estuary was measured in summer and winter to study the behavior of DSi fluvial inputs into the estuary. The DSi concentration decreased away from the estuary and had a linear relationship with salinity, suggesting that mixing between river water and seawater is the dominant effect on DSi levels in the study area. Measured δ 30 Si in the Changjiang Estuary ranged from +1.48‰ to +2.35‰ in summer, and from +1.54‰ to +1.95‰ in winter. As a result of low light levels and abundant DSi riverine inputs, DSi remains relatively unaffected by biological utilization and fractionation in the near‐shore region, and the isotopic imprint of water from the Changjiang can still be detected up to a salinity level of 20 in summer. An obvious increase in δ 30 Si was observed beyond this salinity level, indicating a significant increase in biological utilization and fractionation of DSi in high salinity waters. Lower water temperatures and light levels that prevail over the winter lead to the reduced fractionation of DSi compared with that in summer. The fractionation factor ( 30 ɛ) was estimated using a steady state model to the high salinity waters, yielding a value of −0.95‰, which is in agreement with previous results obtained for Skeletonema costatum in cultivation experiments. The results of this study suggest that silicon isotopes can be used to identify the impact of biological utilization on the behavior of DSi in highly dynamic estuarine environments.