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| Titel |
Non-conservative behavior of Cd in estuarine systems - new constraints from combined Cd concentration and isotope analyses |
| VerfasserIn |
Myriam Lambelet, Mark Rehkamper, Zichen Xue, Tina van de Flierdt, Don Porcelli, Per Andersson |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250057622
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| Zusammenfassung |
| Cadmium (Cd) is a micronutrient with an important role in marine biological productivity. In
the open ocean, the distribution of dissolved Cd is correlated with that of phosphate, with low
concentrations in surface waters due to biological utilization and higher abundances at depth
from remineralization of organic material [1]. Previous studies have furthermore shown that
biological uptake of isotopically light Cd generates significant Cd isotope variability in
surface seawater (typically É114-110Cd - +5 to +40), whilst the deep ocean is characterized
by a relatively constant Cd isotope composition of É114-110Cd - +3 [2,3] (É114-110Cd is the
deviation of the 114Cd/110Cd ratio of a sample from the NIST 3108 Cd isotope standard in
parts per 10,000).
Riverine fluxes are an important source of marine Cd but the behavior of Cd in estuaries
is only poorly constrained. To further investigate the cycling of Cd in estuarine
systems, we determined the Cd concentrations and isotope compositions of 19
water samples that were collected during the ISSS-08 (International Siberian Shelf
Study) cruise along the Arctic Siberian Shelf, with emphasis on the Laptev and East
Siberian Seas of the Arctic Ocean. This is the first study to investigate the cycling of
Cd in such environments with the aid of combined Cd isotope and concentrations
measurements.
The majority of the samples define a linear correlation in a diagram of [Cd] vs. salinity
(with S - 3 to 32) and a curvilinear trend for the É114-110Cd vs. [Cd] data. These
observations are in accord with conservative behavior of Cd during mixing of riverine water
and seawater. In addition, they suggest that the seawater endmember is characterized by [Cd]
- 0.2 nM, in accord with previous work [4,5,6], and É114-110Cd - +5. These characteristics
resemble Cd-rich surface waters from other locations, where biological uptake
of Cd is incomplete. For the riverine endmember, the data suggest [Cd] -¤ 0.04
nM, in general agreement with previous analyses [5,6]. The isotope composition of
É114-110Cd -¤ +2 is similar to or slightly more positive than the upper continental crust,
which is inferred to be characterized by É114-110Cd - 0 [7]. This indicates that Cd
isotope fractionation during weathering and release of Cd from crustal materials is
limited.
Six of the 19 samples, however, show clear deviations from the conservative mixing trend
defined by the majority. One sample from the Ob Estuary (S - 8) features a highly negative
É114-110Cd value of about -7±1. This signature might reflect Cd pollution, as anthropogenic
emissions of Cd to the atmosphere are commonly characterized by light Cd isotope
compositions with É114-110Cd < 0 [8,9]. The remaining five samples, and in particular three
from the vicinity of the Indigirka Estuary, display positive deviations from the conservative
[Cd] vs. S mixing trend. The Cd isotope data support the interpretation that this deviation
reflects release of Cd from particles and suggest that the desorbed Cd is characterized by an
isotope composition similar to that of dissolved riverine Cd, with É114-110Cd -¤
+2.
Taken together, these results underline the important role that trace metal stable isotope
analyses can play in investigations of marine biogeochemical element cycles. For Cd, the
isotopic data (i) confirm the reactive behavior of Cd in estuarine systems, where Cd is
released from suspended particles and (ii) may be of value for fingerprinting anthropogenic
Cd additions.
References:
[1] Boyle (1988), Paleoceanography 3: 471-489. [2] Lacan et al. (2006), GCA 70:
5104-5118. [3] Ripperger et al. (2007), EPSL 261: 670-684. [4] Martin et al. (1993), Mar.
Chem. 43: 185-199. [5] Dai & Martin (1995), EPSL 131: 127-141. [6] Guieu et al. (1996),
Mar. Chem. 53: 255-267. [7] Schmitt et al. (2009), EPSL 277: 262-272. [8] Cloquet et al.
(2006), ES&T 40: 2525-2530. [9] Shiel et al. (2010), Sci. Total Environ. 408: 2357-2368. |
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