Impact of skeletal dissolution and secondary aragonite on trace element and isotopic climate proxies in Porites corals

Abstract

[1] Restricted zones of recent dissolution and secondary aragonite infilling were identified in a coral core collected in 1986 from a living massive Porites colony from the central Great Barrier Reef, Australia. Secondary aragonite needles, !20 mm long, cover skeletal surfaces deposited from 1972 to late 1974 and increase bulk density by 10%. Dissolution is observed above this zone, whereas older skeleton is pristine. We investigate the impact of both types of early marine diagenesis on skeletal geochemistry and coral paleoclimate reconstructions by comparison with similar records from eight contemporary Porites colonies collected at nearby reefs. Secondary aragonite overgrowth causes anomalies in skeletal density, Mg/Ca, Sr/Ca, U/Ca, d Dissolution, through incongruent leaching, also causes cool SST artifacts but only for trace element ratios (Mg/Ca-SST À1.2°C; Sr/Ca-SST À1.2°C; U/Ca-SST À2.1°C). The sequence of preference with respect to dissolution of coral skeleton in seawater is Mg > Ca > Sr > U. Rigorous screening of coral material for paleoclimate reconstructions is therefore necessary to detect both dissolution and the presence of secondary minerals. The excellent agreement between apparent SST anomalies generated by different modes of diagenesis means that replication of tracers within a single coral cannot be used to validate climate-proxy interpretations. Poor replication of records between different coral colonies, however, provides a strong indication of nonclimatic artifacts such as dissolution and secondary aragonite.

Intra-skeletal calcite in a live-collected Porites sp.: Impact on environmental proxies and potential formation process

“…This is the case for intra-skeletal calcite, such as described by Rabier et al (2008) and McGregor and Gagan (2003) for example. Moreover, although the impacts of diagenesis have been studied for almost each proxy independently, only very few studies report on multi-proxy approach allowing for a more global picture of a specific archive (Allison et al, 2007;Griffiths et al, 2013;Hendy et al, 2007). Lastly, the impacts of diagenesis on the recently validated Li/Mg and δ 11 B proxies are poorly documented.…”

Diagenetic Screening in Porites Fossil Corals from South Pagai, Kendari, and Banten Bay, Indonesia

“…XRD is the most convenient and widespread method for mineralogy assessment [11,13]. However, screening for diagenesis cannot rely on XRD alone since it is still problematic to detect the occurrence of secondary aragonite [6,14,18,19,20]. Another diagenetic texture that is essential to be screened for is dissolution since it may cause cool SST artifacts [14].…”

“…The dark color of COC dissolution visible in the thin sections is due to the internal reflection of the light in minute voids between the dissolved primary aragonite [27]. Dissolution is a diagenetic texture that can cause cool SST artifacts [14,16]. One percent secondary calcite is enough to cause 1°C warming artifacts in absolute Sr/Ca SST reconstruction, although seasonal variability in Sr/Ca SST may still be preserved [7,8,10,16,28,29].…”

“…Primary coral aragonite cannot be distinguished from secondary aragonite using XRD, therefore it is still problematic to detect occurrences of secondary aragonite [6,14]. Dissolution, another essential diagenetic texture that may cause cool SST artifacts [14][15][16], also cannot be detected by XRD.…”

“…Dissolution, another essential diagenetic texture that may cause cool SST artifacts [14][15][16], also cannot be detected by XRD. In contrast, petrography and SEM analysis allow to screen for secondary aragonite and dissolution better.…”