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Chromium‐isotope signatures in scleractinian corals from the R ocas A toll, T ropical S outh A tlantic
Author(s) -
Pereira N. S.,
Voegelin A. R.,
Paulukat C.,
Sial A. N.,
Ferreira V. P.,
Frei R.
Publication year - 2016
Publication title -
geobiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.859
H-Index - 72
eISSN - 1472-4669
pISSN - 1472-4677
DOI - 10.1111/gbi.12155
Subject(s) - seawater , isotope , environmental chemistry , coral , geochemistry , geology , chemistry , biology , oceanography , physics , quantum mechanics
Chromium‐isotope compositions (expressed as δ 53 C r) of recent and ancient skeletal and non‐skeletal carbonates are currently explored as a (paleo‐) redox‐proxy for shallow seawater. The idea behind this approach is that biogenic and non‐biogenic carbonates could potentially be used as archives recording the C r‐isotope composition of seawater in which they formed, and with this contribute to the reconstruction of past paleo‐environmental changes in the marine realm, and potentially to climate changes on land. However, investigations addressing the behavior and uptake mechanism of C r, and the potential isotope fractionations between seawater and biogenic carbonates are scarce. Here, we present a study of C r‐isotope variations in three species of corals and contemporary seawater from the R ocas A toll, NE , B razil. C r‐isotope values of the studied coral species ( S iderastrea stellata, P orites sp. , and M ontastrea cavernosa ) vary from −0.5 to +0.33‰ and point to significant isotopic disequilibrium with coexisting seawater characterized by a C r‐isotope value of +0.92 ± 0.2‰. This isotopic offset requires reduction of hexavalent C r( VI ) in the sequestration process of all the studied coral species. C r‐isotope values in a profile across an S . stellata colony returned homogeneous, slightly positively fractioned δ 53 C r values of +0.07 ± 0.08‰ ( n = 8, 2σ), which we interpret to reflect a constant reductive uptake during the 20‐year growth period recorded in this coral. In contrast, samples across a 12‐year growth profile from Porites sp. display rather heterogeneous C r‐isotope values with δ 53 C r varying from −0.50 to +0.10‰, indicating C r incorporation under changing redox processes during its growth intervals. We propose a mechanism whereby initial photoreduction of isotopically heavy C r( VI ) to isotopically lighter C r( III ) in the endodermal layer of corals must be followed by efficient and effective re‐oxidation of reduced C r species to favor subsequent chromate ( CrO 4 2 − ) substitution during the calcifying processes ultimately leading to the formation of the coral skeleton.