Recent chronostratigraphic evidence suggests that the central Australian Great Barrier Reef formed within the past 780 k.y. Periplatform sediments of the same age recovered from the western Coral Sea record a progressive decrease in the delta (super 18) O of planktonic foraminifera to the present. Several investigators have proposed that this trend represents an appreciable late Pleistocene warming ( approximately 4 degrees C) of ocean surface temperatures, which they posit catalyzed the growth of the Great Barrier Reef. Contrary to this hypothesis, we demonstrate using alkenone paleothermometry (U (sub 37) (super k') ) on sediments from Ocean Drilling Program (ODP) Site 820 that sea-surface temperatures (SSTs) in the western Coral Sea changed by approximately 1.5 degrees C or less during the past approximately 800 k.y. If the central Great Barrier Reef rose in the late Quaternary, it was therefore not due to a warming of SSTs. We explore whether a major moisture balance change and/or diagenetic alteration of calcareous microfossils can explain the higher delta (super 18) O values observed at depth in the planktonic delta (super 18) O record at ODP Site 820. Our results suggest that diagenesis provides a large isotopic overprint.