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

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 mu m 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, delta O-18, and delta C-13. The secondary aragonite is consistently associated with a cool temperature anomaly for each of the sea surface temperature (SST) proxies (delta O-18-SST-1.6 degrees C; Sr/Ca-SST -1.7 degrees C; Mg/Ca-SST -1.9 degrees C; U/Ca-SST -2.8 degrees C). Dissolution, through incongruent leaching, also causes cool SST artifacts but only for trace element ratios (Mg/Ca-SST -1.2 degrees C; Sr/Ca-SST -1.2 degrees C; U/Ca-SST -2.1 degrees 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.