Giant Clams as Paleoclimate Archives of Holocene- age ENSO Variability

English Abstract:

Currently there is a lack of high resolution,datable paleoclimate data,for the Holocene interval to determine the onset of the modern El Niño-Southern Oscillation(ENSO)interannual variability. This study aims to ¹⁴C date and isotopically analyze a series of T.gigas,a clam species rife in the tropical Pacific coral reefs, that span the Holocene. The primary objective is to assess two contrasting hypotheses concerning ENSO’s past status:(i)ENSO was continuous throughout the last glacial cycle and(ii)ENSO was inactive during the last glacial cycle and restarted sometime during the mid-Holocene. If ENSO has been active since the last interglaciation then the interannual isotope profiles of Holocene-age T.gigas are predicted to show high d¹⁸O variabilities that are typical of today’s ENSO events. Conversely, if low seasonal amplitude and heavier d¹⁸O time-series are only present in the late Holocene samples then one would conclude that ENSO restarted sometime in the mid-Holocene.

Understanding the past variability of ENSO under contrasting climate states is of upmost importance in predicting future ENSO behavior because of its worldwide climate impact. Instrumental records go back only ~150 years, too short of an interval to assess ENSO variability under different climate states. The current global climate models vary on the effects that global warming will have on the frequency and intensity of ENSO. Paleoclimate records can offer improved parameters for these models in predicting the effects of rising greenhouse gasses. High resolution paleoclimate analyses are critical because they reveal patterns of climate variability that typically occur on interseasonal and interannual scales. T.gigas are an ideal high resolution paleoclimate archive for isotope time-series because of their(i)prominent annual growth bands;(ii)resistance to weathering because of a dense aragonitic shell and(iii)high growth rate.