Comparison of Equatorial Pacific Sea Surface Variability and Trends with Sr/Ca Records from Multiple Corals

Abstract:

The short duration of instrumental SST records makes it difficult to reliably separate anthropogenic-related trends from natural variability, especially in the tropical Pacific. The Sr/Ca ratio of massive corals has been used to lengthen the record and fill data gaps but its reliability has been questioned because different coral colonies on the same reef can have different Sr/Ca-temperature relationships and yield opposing derived SST trends. We exploited a unique multi-year dataset of in situ temperature data to assess whether inter-colony differences are attributable to small-scale oceanographic variability or “vital effects” and to quantify the effect of these differences on temperature estimates and uncertainties. Sr/Ca records from four massive Porites colonies growing on the east and west sides of Jarvis Island, central equatorial Pacific, were compared with temperature data from in situ loggers also placed on the east and west sides. In general, Sr/Ca captured – albeit inconsistently – the occurrence of inter-annual SST events but their amplitude was not consistently recorded by any of the corals. However, the Sr/Ca-temperature regressions derived from the four different colonies were statistically distinct. Assuming these relationships represent the full range of Sr/Ca-temperature calibrations in Jarvis Porites, we assessed how well Sr/Ca of a fossil coral with an unknown Sr/Ca-temperature relationship can constrain past temperatures. Our results indicate that the standard error of prediction methods underestimate the actual error as we could not reliably reconstruct the amplitude or frequency of ENSO events as large as ±2°C. Further, Sr/Ca of one coral implied a statistically significant cooling trend while that of its neighbor implied a warming trend whereas no long-term trend was identified in the instrumental data. This observation of a derived trend where none exists is consistent with observations made independently at other coral reef sites in the Pacific and Indian Oceans. Our results underscore the importance of characterizing the full range of temperature-Sr/Ca relationships at each study site to estimate the true uncertainty associated with this proxy.