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Chromium-isotope compositions (expressed as δ(53) Cr) 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 Cr-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 Cr, and the potential isotope fractionations between seawater and biogenic carbonates are scarce. Here, we present a study of Cr-isotope variations in three species of corals and contemporary seawater from the Rocas Atoll, NE, Brazil. Cr-isotope values of the studied coral species (Siderastrea stellata, Porites sp., and Montastrea cavernosa) vary from -0.5 to +0.33‰ and point to significant isotopic disequilibrium with coexisting seawater characterized by a Cr-isotope value of +0.92 ± 0.2‰. This isotopic offset requires reduction of hexavalent Cr(VI) in the sequestration process of all the studied coral species. Cr-isotope values in a profile across an S. stellata colony returned homogeneous, slightly positively fractioned δ(53) Cr 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 Cr-isotope values with δ(53) Cr varying from -0.50 to +0.10‰, indicating Cr incorporation under changing redox processes during its growth intervals. We propose a mechanism whereby initial photoreduction of isotopically heavy Cr(VI) to isotopically lighter Cr(III) in the endodermal layer of corals must be followed by efficient and effective re-oxidation of reduced Cr species to favor subsequent chromate (CrO42-) substitution during the calcifying processes ultimately leading to the formation of the coral skeleton.

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The Maastrichtian-Danian limestones of the Yacoraite Formation (northwestern Argentina) show carbon and oxygen isotopic values consistent with shallow marine conditions. The members of the formation respond to different sedimentary environments and are characterised by distinctive stable isotopes and geochemistry. The basal Amblayo Member is composed of high-energy dolomitic limestones and limestones with positive isotopic values (+2& d 13 C, +2& d 18 O). The top of the member reveals an isotopic shift of d 13 C (À5&) and d 18 O (À10&), probably related to a descent in the sea level. The sandy Gü emes Member has isotopically negative (À2& d 13 C, À1& d 18 O) limestones, principally controlled by water mixing, decreased organic productivity, and compositional changes in the carbonates. The isotopically lighter limestones are calcitic, with a greater terrigenous contribution and different geochemical composition (high Si-Mn-Fe-Na, low Ca-Mg-Sr). These isotopic and lithological changes relate to the Cretaceous-Palaeogene transition. The Alemanía Member, composed of dolomitic limestones and pelites, represents a return to marine conditions and shows a gradual increase in isotopic values, reaching values similar to those of the Amblayo Member. The Juramento Member, composed of stromatolite limestones, shows isotopic variations that can be correlated with the two well-defined, shallowing-upward sequences of the member. ResumenLas calizas maastrichtiano-danianas de la Formació n Yacoraite del noroeste argentino revelan valores de isó topos estables coherentes con el ambiente marino somero predominante del depó sito. Los miembros de la Formació n, que responden a variaciones ambientales, quedan caracterizados por valores isotó picos y geoquímicos. El Miembro Amblayo basal está compuesto por calizas dolomíticas y calizas de alta energía con valores isotó picos positivos (+2& d 13 C, +2& d 18 O). En el techo del miembro se observa un salto isotó pico del d 13 C (À5&) y del d 18 O (À10&) probablemente vinculado con un descenso del nivel del mar. El Miembro Gü emes, arenoso, posee calizas con valores isotó picos negativos (À2& d 13 C, À1& d 18 O) controlados principalmente por mezcla de aguas, descenso en la productividad orgánica y cambios composicionales del carbonato. Las calizas isotó picamente más livianas son calcíticas, poseen mayor contenido de elementos terrígenos y composició n geoquímica diferente (alto Si-Mn-Fe-Na, bajo Ca-Mg-Sr). Estos cambios estarían relacionados con la transició n Cretácico-Paleó geno. El Miembro Alemanía, compuesto por calizas dolomíticas y pelitas, representa un regreso a las condiciones marinas y muestra un paulatino aumento isotó pico hasta asemejarse a los valores del Miembro Amblayo. El Miembro 0895-9811/$ -see front matter Ó Juramento, compuesto por calizas estromatolíticas, muestra variaciones isotó picas que serían correlacionables con los dos ciclos de somerizació n bien definidos del miembro.

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