Can coral skeletal-bound nitrogen isotopes be used as a proxy for past bleaching?

Abstract

There is currently no reliable geochemical proxy of historical coral bleaching. Here we test the hypothesis that increases in the nitrogen isotopic signature of coral tissue, which is recorded in skeletal-bound organic material (CS-δ15N), can be used to detect coral bleaching events in the past. We measured CS-δ15N in coral colonies that showed high density thermal stress bands associated with the 1998 and 2002 mass bleaching events on the central Great Barrier Reef. Analysis of coral cores from 2 inshore and 1 mid-shelf reef found that increases in CS-δ15N occurred either side of the thermal stress events and were therefore unlikely to be a physiological response to bleaching. We then assessed whether CS-δ15N could be used to detect shelf-wide upwelling, which has been previously shown to coincide with mass bleaching events. For the inshore sites the response of CS-δ15N to upwelling was not uniform between reefs or between colonies from the same reef. However, on the mid-shelf reef, increased CS-δ15N during the known upwelling of 1998 was more consistent between cores. Analysis of a 23-year old coral core from the mid-shelf reef showed increased CS-δ15N between 1983–1985 and 1997–2000, periods where strong El Nino events were proceeded by intense wind-driven upwelling. We conclude that coral CS-δ15N is not a direct proxy for coral bleaching but can be used to detect prolonged and pervasive upwelling that coincides with mass bleaching events. Further validation with longer core records is required.