High Resolution Hydrologic Variability During ENSO Events in Modern, Mid-Holocene and Last Interglacial Periods Discerned from Coupled Isotope Time-Series in the Giant Clam T. gigas from the Raised Coral Reefs in Huon Pen., Papua New-Guinea

Abstract:

Understanding the past variability of El Niño-Southern Oscillation (ENSO) under contrasting climate states is of upmost importance because of its worldwide climate impact. The Western Pacific Warm Pool (WPWP) plays a dominant role in the development of ENSO but instrumental records do not go back far enough to determine the phase modes of the WPWP over intrannual and interannual intervals. Giant clams (T.gigas), associated with a flight of uplifted coral reef terraces on the Huon Peninsula, Papua New Guinea (PNG) along the Vitiaz Strait in the heart of the WPWP, are used in this study as highly resolved archives to investigate the intensity and frequency of ENSO events during modern, mid-Holocene and the Last Interglacial. Time-series analyses of stable oxygen and carbon isotopes at subweekly resolution, coupled with monthly resolution of AMS Δ¹⁴C determinations, has allowed documentation of hydrological changes occurring during ENSO events. Time series from modern T. gigas that overlap with instrumental rainfall and ENSO records, indicate that an El Niño event exhibits an increase of 5‰ above average in Δ¹⁴C accompanied by positive δ¹⁸O and δ¹³C excursions while during a La Niña event Δ¹⁴C falls 3‰ below average accompanied by negative δ¹⁸O and δ¹³C excursions. We show numerically that phase transitions in the three isotope proxies within the ENSO frequency band are compatible with excess seawater evaporation during severe El-Niño droughts and intensive freshwater discharge draining the Late Pleistocene coral terraces into the coastal reef environment during La-Niña wet events. The combined study of stable isotopes and Δ¹⁴C demonstrates that giant clams show high potential to serve as robust archives of seasonal and interannual hydrologic variability accompanying ENSO events. We applied modern δ¹⁸O threshold limits to fossil giant clams from the mid-Holocene (5.4 ka) and Last Interglacial (134 ka). The mid-Holocene time-series show a lower amplitude and an increased frequency of ENSO events (45 events/century) compared to the modern (38 events/century). In contrast, the Last Interglacial ENSO events were of higher intensity and frequency relative to present (45 events/century).