Early marine diagenesis in corals and geochemical consequences for paleoceanographic reconstructions

Detecting the potential geochemical consequences of early marine diagenesis is essential for establishing the validity of past climate reconstructions from coral. We present coral skeletal delta O-18 and Sr/Ca data for two lo ng coral cores spanning 1839-1994 AD at Ningaloo Reef, Western Australia, o ne of which includes significant secondary precipitation of marine inorgani c aragonite. Longterm trends in reconstructed sea surface temperatures (SST s) for the well preserved coral correlate strongly with instrumental SST re cords spanning the 20th century. In contrast, the delta O-18 and Sr/Ca for the diagenetically altered coral give identical cool SST anomalies of 4-5 d egreesC, as a consequence of the addition of secondary aragonite enriched i n O-18 and Sr. Our results indicate that cross-checking of paleoclimate rec onstructions with two supposedly independent paleothermometers may not be v alid, and that coral records showing cooler SSTs in the past need to be int erpreted with caution. Furthermore, modern coral records with long-term tre nds in delta O-18 indicating recent warming and freshening of the ocean can be potentially explained by early marine diagenesis.