Trace Element Profiles in Sediments as Proxies of Dead Zone History; Rhenium Compared to Molybdenum
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- 作者：George R. Helz ; Jordan M. Adelson
- 刊名：Environmental Science & Technology (ES&T)
- 出版年：2013
- 出版时间：February 5, 2013
- 年：2013
- 卷：47
- 期：3
- 页码：1257-1264
- 全文大小：347K
- 年卷期：v.47,no.3(February 5, 2013)
- ISSN：1520-5851
文摘
Warm-season dead zones鈥攙olumes of coastal water containing too little O2 to support macrofauna鈥攁re a growing global menace. Trace elements that are deposited in sediments in response to reducing or sulfidic conditions can provide proxy records for reconstructing dead zone evolution. Based on relative enrichment in reduced vs oxidized marine sediments, Re seems promising as a dead zone proxy. Here, Re is determined by isotope dilution mass spectrometry in sediments underlying the summertime dead zone in Chesapeake Bay. Contrary to expectation, Re becomes only modestly (2-fold) elevated during the 20th century and fails to track the historic record of summertime O2 depletion. Rhenium enrichments are watershed-specific and apparently controlled by anthropogenic sources, not by redox-linked authigenic processes. In contrast, Mo enrichments do track historic O2 depletion. Three factors cause redox control to override anthropogenic control in the case of Mo: relative to weathering fluxes, anthropogenic Mo fluxes are weaker than Re fluxes; during anoxic periods, Mn refluxing amplifies Mo but not Re concentrations near the sediment surface; and high pore water sulfide-polysulfide promotes Mo fixation in pyrite while promoting formation of organo-Re adducts; the latter are too mobile and reactive to preserve a reliable historic record under seasonally fluctuating redox conditions.