Assessing the Paleoceanographic Potential of the coral Montipora venosa at Fanning Atoll, Central Equatorial Pacific

As interest in global climate change increases, so does the need for better and more extensive climate proxies. The central equatorial Pacific has been established as the region with the largest ENSO-related sea surface temperature (SST) and precipitation (PPT) anomalies, which are known to impact global interannual climate variability. To date, the coral genus Porites has been most commonly utilized for coral-based paleo-reconstructions of ENSO and lower frequency phenomena. However, due to questions of biological artifacts in coral, to more fully understand coral-based reconstruction, different coral genera need to be analyzed. In this study, oxygen (δ18O) and carbon (δ13C) isotopic time series as well as Sr/Ca time series, were generated from a colony of the massive hermatypic coral Montipora venosa, (a genus not previously studied) from an ENSO sensitive site. This coral core was recovered from Fanning Island (3º 52’N, 159º 20’W) in the central equatorial Pacific, located in the heart of the important Niño 3.4 region for monitoring ENSO. Oxygen isotopic data from Montipora venosa core (FI4) is compared to previously unpublished δ18O data from a Porites sp. core (FI5) taken at the same location. Even though core FI4 M. venosa is the shorter core (662mm), it spans 111yr (1997-1887), while the longer FI5 Porites core (1260mm) spans only 75 yr (1997-1922). Core FI5 Porites contains a deviation of undetermined cause towards increased δ18O values between 1950 and 1951, which is not recorded in core FI4 Montipora venosa. This deviation is not found among other central equatorial Pacific Porites records and thus supports the need for a multi-core replication strategy. There has been a suggestion by Cane et al. (1997) that the pattern of 20th century SST change across the Pacific may be due to an increase in the west to east temperature gradient. However, comparison of the M. venosacore FI4 δ18O time series with δ18O time series from Maiana (1ºN, 173ºE) and Urvina Bay (0º 24.5’S, 91º 14’W) indicates there has been no change in SST gradient across the Pacific during the studied time period (1997-1887), assuming that δ18O is primarily affected by temperature at all these sites.Coral data from several locations throughout the Pacific Ocean show a shift in 1976 towards lighter δ18O values, which is often interpreted as a trend towards warmer SSTs. However, it is not clear how much of this trend is real and how much might be attributable to biological effects. The results of this study indicate that the magnitude of the shift varies not only by location, but also by coral genus. Again, this substantiates the need for a replication strategy