Iron isotope fractionation in a sulfide-bearing subterranean estuary and its potential influence on oceanic Fe isotope flux

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• 作者：Moutusi Roy^a ; ^{mroy@coas.oregonstate.edu} ; Olivier Rouxel^b ; ^c ; Jonathan B. Martin^a ; Jaye E. Cable^d
• 关键词：Submarine groundwater discharge (SGD) ; Iron isotopes ; Subterranean estuary ; Indian River Lagoon ; Iron diagenesis ; Coastal sediments
• 刊名：Chemical Geology
• 出版年：2012
• 期刊代码：12_00092541
• 类别：ear
• 出版时间：18 March, 2012
• 卷：300-301
• 期：Complete
• 页码：133-142
• 文件大小：888 K

摘要

We trace pathways of Fe reactions in the Indian River Lagoon (Florida, USA) subterranean estuary using Fe isotopes to provide new constraints on Fe-isotopic fractionation in a sulfide-bearing subterranean estuary. Porewater 未⁵⁶Fe values increase from 鈭?#xA0;1.16鈥?at 115 cm depth to + 0.2鈥?at 7 cm depth due to isotope fractionation in three distinct lithostratigraphic zones. The deepest zone contains orange sands with elevated Fe-oxide contents (0.2 wt.%) that dissolve through diagenetic Fe-oxide reduction and elevate Fe concentrations in porewaters (100 to 300 渭M/l). This reaction causes porewater 未⁵⁶Fe values to be ~ 1鈥?lighter than the sediment 未⁵⁶Fe values. An intermediate zone contains white Fe-poor sands, with Fe-oxide contents < 0.1 wt.%and dissolved Fe concentrations < 20 渭M/l. This zone is a sink for dissolved Fe through adsorption of isotopically heavy dissolved Fe(II) onto mineral surfaces. This adsorption results in porewater 未⁵⁶Fe values that are as much as 1.8鈥?lighter than sediment 未⁵⁶Fe values. The uppermost zone contains organic carbon and Fe-sulfide rich black sediments with low dissolved Fe (< 1 渭M/l) and elevated porewater sulfide (up to 600 渭M/l) concentrations. Precipitation of isotopically light Fe-sulfides increases the porewater 未⁵⁶Fe values as much as 0.68鈥?more than corresponding sediment 未⁵⁶Fe values. The near-surface Fe-sulfide precipitation delivers to the lagoon dissolved Fe with slightly positive 未⁵⁶Fe values, averaging about + 0.24鈥? via submarine groundwater discharge (SGD). Iron-sulfide precipitation in sulfide-containing subterranean estuaries thus may result in a previously unidentified source of isotopically heavy Fe to the coastal oceans.