Abstract
Coral-based reconstructions of sea surface temperatures (SSTs) using Sr/Ca, U/Ca and δ 18 O 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 CaCO 3 − precipitation rates. Overall, the Sr/Ca, U/Ca and δ 18 O 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 SST. This approach shows that SST records derived from different proxies agree with an amplitudinal variability of up to ± 2 • C with respect to their Holocene mean value. Higher SST 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 SST 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 10 Be and 14 C production (Vonmoos et al., 2006), emphasizing the role of solar activity on climate variability during the Late Holocene.
Highlights
Rashid et al (2020) presented mid- to late-Holocene SST reconstructions obtained on fossil Porites corals from French Polynesia
During the mid- to late-Holocene, low-amplitude (∼1 m) relative sea-level (RSL) changes were mostly governed by Glacial Isostatic Adjustment (GIA) processes (e.g., Camoin and Webster, 2015 and references therein)
The reconstruction of RSL changes at high resolution requires the use of reliable sea-level indicators, including salt marsh microfossils, archaeological remains, vermetid constructions, and fossil corals
Summary
Rashid et al (2020) presented mid- to late-Holocene SST reconstructions obtained on fossil Porites corals from French Polynesia. They especially report centennial to millennial SST trends on the order of ±2◦C and relate them to sea-level changes that were previously published by Rashid et al (2014). The authors conclude that cyclic SST variations may superimpose Glacial Isostatic Adjustment (GIA)-controlled sea-level height
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