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 Δ18O and Sr/Ca data for two long coral cores spanning 1839--1994 AD at Ningaloo Reef, Western Australia, one of which includes significant secondary precipitation of marine inorganic aragonite. Long-term trends in reconstructed sea surface temperatures (SSTs) for the well preserved coral correlate strongly with instrumental SST records spanning the 20th century. In contrast, the Δ18O and Sr/Ca for the diagenetically altered coral give identical cool SST anomalies of 4--5¿C, as a consequence of the addition of secondary aragonite enriched in 18O and Sr. Our results indicate that cross-checking of paleoclimate reconstructions with two supposedly independent paleothermometers may not be valid, and that coral records showing cooler SSTs in the past need to be interpreted with caution. Furthermore, modern coral records with long-term trends in Δ18O indicating recent warming and freshening of the ocean can be potentially explained by early marine diagenesis. ¿ 2001 American Geophysical Union