Early diagenetic imprint on temperature proxies in holocene corals : a case study from French Polynesia
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Coral-based reconstructions of sea surface temperatures (SSTs) using Sr/Ca, U/Ca and δ18O are important tools for quantitative analysis of past climate variabilities. However, post-depositional alteration of coral aragonite, particularly early diagenesis, restrict the accuracy of calibrated proxies even on young corals. Considering the diagenetic effects, we present new Mid to Late Holocene SST reconstructions on well-dated (U/Th: ∼70 yr to 5.4 ka) fossil Porites sp. collected from the Society Islands, French Polynesia. For few corals, quality pre-screening routines revealed the presence of secondary aragonite needles inside primary pore space, resulting in a mean increase in Sr/Ca ratios between 5-30%, in contrast to the massive skeletal parts. Characterized by a Sr/Ca above 10 mmol/mol, we interpret this value as the threshold between diagenetically altered and unaltered coral material. At a high-resolution, observed intra-skeletal variability of 5.4 to 9.9 mmol/mol probably reflects the physiological control of corals over their trace metal uptake, and individual variations controlled by CaCO3– precipitation rates. Overall, the Sr/Ca, U/Ca and δ18O trends are well correlated, but we observed a significant offset up to ± 7°C among the proxies on derived palaeo-SST estimates. It appears that the related alteration process tends to amplify temperature extremes, resulting in increased SST-U/Ca and SST-Sr/Ca gradients, and consequently their apparent temperature sensitivities. A relative SST reconstruction is still feasible by normalizing our records to their individual mean value defined as ΔSST. This approach shows that ΔSST records derived from different proxies agree with an amplitudinal variability of up to ± 2°C with respect to their Holocene mean value. Higher ΔSST values than the mean SSTs (Holocene warm periods) were recorded from ∼1.8 to ∼2.8 ka (Interval I), ∼3.7 to 4.0 ka (Interval III) and before ∼5 ka, while lower ΔSST values (Holocene cold periods, Interval II and IV) were recorded in between. The ensuing SST periodicity of ∼1.5 ka in the Society Islands record is in line with the solar activity reconstructed from 10Be and 14C production (Vonmoos et al., 2006), emphasizing the role of solar activity on climate variability during the Late Holocene.
Rashid , R , Eisenhauer , A , Liebetrau , V , Fietzke , J , Böhm , F , Wall , M , Krause , S , Rüggeberg , A , Dullo , W-C , Jurikova , H , Samankassou , E & Lazar , B 2020 , ' Early diagenetic imprint on temperature proxies in holocene corals : a case study from French Polynesia ' , Frontiers in Earth Science , vol. 8 , 301 . https://doi.org/10.3389/feart.2020.00301
Frontiers in Earth Science
Copyright © 2020 Rashid, Eisenhauer, Liebetrau, Fietzke, Böhm, Wall, Krause, Rüggeberg, Dullo, Jurikova, Samankassou and Lazar. This is an open-access article distributed under the terms of the Creative Commons Attribution License (CC BY). The use, distribution or reproduction in other forums is permitted, provided the original author(s) and the copyright owner(s) are credited and that the original publication in this journal is cited, in accordance with accepted academic practice. No use, distribution or reproduction is permitted which does not comply with these terms.
DescriptionThis study was supported through funds from ESF/CHECKREEF project (EI272/22-1 to AE; DU129/141-1 to W-CD; 20MA21-115944 and 200020-140618 to ES), by the collaborative research initiative CHARON (DFG Forschergruppe 1644 – Phase II) funded by the German Research Foundation, and by the GEOMAR Helmholtz Centre for Ocean Research Kiel. The Ph.D. program of RR was financially supported by the Tanzania Ministry of Education and Vocational Training (MoEVT) in collaboration with DAAD (Deutscher Akademischer Austauschdienst) fellowship. AR acknowledges support from Swiss National Science Foundation Project Number SNF 200021_149247.
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