Chromium‐isotope signatures in scleractinian corals from the Rocas Atoll, Tropical South Atlantic

Abstract

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.

A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions

“…Methods used in this study for the purification and isotope analysis of Cr in seawater samples and biogenic carbonates follow those described in Paulukat et al (2016) and Pereira et al (2015), with small modifications. Briefly, filtered and spiked seawater samples were transferred into 1 L Savillex ™ teflon beakers, evaporated, redissolved in 50 mL of aqua regia, and evaporated again.…”

“…However, investigations addressing the behavior and uptake mechanism of Cr, and the potential isotope fractionations between seawater and biogenic carbonates are scarce. All studies so far conducted on marine biogenic carbonates have revealed the incorporation of isotopically lighter chromium into skeletal and non-skeletal carbonates compared to Cr isotope signatures of seawater at the respective sampling sites (e.g., Pereira et al, 2015;Holmden et al, 2016;Farkaš et al, 2018). Due to a lack of ambient seawater δ 53 Cr data to compare the 53 Cr data of various foraminifera analyzed by Wang et al (2016), conclusion with respect to using δ 53 Cr data of foraminiferal species as a reliable proxy of seawater δ 53 Cr could not be made in the respective study.…”

“…In contrast, results from biologically produced CaCO 3 minerals, such as foraminiferal calcite (Wang et al, 2016) and/or coral aragonite (Pereira et al, 2015) confirmed that these marine organisms produce CaCO 3 skeletons that are systematically negatively fractionated, up to ∼ 1 ‰, compared to ambient seawater. Similarly, data by Holmden et al (2016) from the modern Caribbean Sea show that the δ 53 Cr of bulk carbonate sediments is about 0.46 ± 0.14 ‰ lower relative to local seawater.…”

“…It is conceivable that the Cr isotope composition of seawater and marine chemical sediments reflect a complex signal of oxidation/reduction processes operating within the oceans (Scheiderich et al, 2015;Paulukat et al, 2016), and it is therefore clear that one must first understand the individual processes and mechanisms that govern the transfer of dissolved Cr in seawater into the respective potential archives. Recent studies (Rodler et al, 2015;Pereira et al, 2015;Wang et al, 2016) have paved the way, but further systematic investigations in both natural and laboratorycontrolled settings are required.…”

Population dynamics of Siderastrea stellata Verrill, 1868 from Rocas Atoll, RN: implications for predicted climate change impacts at the only South Atlantic atoll

“…Nevertheless, at Rocas Atoll, despite the existence of information on S. stellata abundance (Echeverría et al 1997, Fonseca et al 2012, percent coverage (Longo et al 2015), and its importance as a natural archive for paleoclimate studies (Mayal et al 2009, Oliveira 2012, Pereira et al 2016, there is no record of other aspects of its population dynamics. Part of the reason of the absence of population models for Brazilian coral species are due to this lack of information to produce them.…”