ABSTRACT
The ENSO (El Nino and the Southern Oscillation) is a fundamental ocean-atmosphere phenomenon that dominates interannual global climate variability. Reconstructing past ENSO events is therefore important for documenting and understanding the past behavior of the global climate system and enabling us to predict future climate change. However, the present understanding of ENSO events has been mainly based on the instrumental record of Pacific climate which provides detailed data only for the past few decades. Recent studies have shown that the stable isotope record in coral skeletons can be used as a valuable indicator of paleoclimatic changes in tropical regions, and therefore can potentially be used as a tool for reconstructing past ENSO events. This study presents a complete 88-year oxygen and carbon isotopic record in corals from Clipperton Atoll in the eastern Pacific for the period 1906-1994. The data were analyzed using time-series statistical methods and discussed in terms of the varying extent to which they reflect the past ENSO events as well as other climatic changes. Although both sea surface temperature (SST) and precipitation have influenced coral d18O at Clipperton, multisample analysis suggests that the effect of SST on coral d18O appears to have played a more important role than that of precipitation/salinity. Growth rate does not appear to have much effect on coral d18O. In the case of d13C in these corals, there is a negative correlation between SST and d13C, although not as apparent as with skeletal d18O. Solar radiation intensity also shows weak negative correlation with d13C, which suggests a view contrary to what is generally held. To identify possible ENSO events and other possible climate changes in this region, the 88-year d18O record was analyzed using singular spectrum analysis (SSA) which revealed pronounced interannual cycles, interpreted as reflecting past ENSO events, as well as other interdecadal cycles and a long term trend. Comparison of the ENSO components (period between 3 and 5 years) with the historic records of ENSO and SST shows that at Clipperton coral d18O is generally sensitive to ENSO variability, although it also shows some local characteristics. Comparison with coral isotopic records from other regions in the Pacific and Indian Oceans further corroborates other studies which indicate that ENSO has occurred persistently for at least 100 years over the entire tropical ocean and that it has oscillated in the same characteristic 3- to 6-year frequency band. The interdecadal cycles in the d18O record may also be related to ENSO events, while the long-term trend maybe related to the global temperature rise as the result of the CO2 "greenhouse effects". Coral d13C also has a long-term trend. Comparison with a model of variation of d13C in dissolved inorganic carbon (DIC) at Clipperton based on measurements of atmospheric O2 and CO2 concentration shows that this trend may be due to the input of 13C depleted CO2 and the "Suess" effect.

Ren, L., 1998. Identification of past climate variability of the eastern Pacific Ocean using both d13C and d18O records in corals from Clipperton Atoll (1994-1906). Unpublished MSc. thesis, State University of New York at Albany. 105 pp., +xi
University at Albany Science Library call number:  SCIENCE Oversize (*) QC 869 Z899 1998 R46

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