ENSO variability during MIS 11 (424-374 ka) from Tridacna gigas at Huon Peninsula, Papua New Guinea

Marine Isotope Stage 11 (MIS 11) from ~424,000 to 374,000 yrs ago included one of the longest and warmest interglacials of the last 800,000 yrs, and is a potential analogue for the Holocene due to the similarity of Earth's orbital configuration at this time. The question of how the El Niño-Southern Oscillation (ENSO) responds to warmer background climates remains unanswered and is critical to understand how the ENSO system will evolve under the influence of anthropogenic warming. In this study, we present a 35 yr-long, high-resolution record of MIS 11 climate variability in the Western Pacific Warm Pool (WPWP) based on coupled measurements of skeletal Mg/Ca and δ¹⁸O in giant Tridacna gigas clams from Huon Peninsula, Papua New Guinea. The δ¹⁸O of modern T. gigas from Huon Peninsula faithfully records sea surface temperature, salinity/rainfall and regional ENSO variability. The geochemical integrity of the MIS 11 T. gigas for recording paleo-ENSO events was established through trace element screening, detailed petrography and SEM analysis. The fossil T. gigas δ¹⁸O record indicates that ENSO was operating during a 35-yr window in MIS 11, but with fewer events of shorter duration compared to those experienced during the last 100 yrs. The suppressed ENSO variability in the MIS 11 T. gigas record corresponds with a reduction in the amplitude of the average annual cycle in δ¹⁸O values. Distinctive changes in local insolation seasonality, and T. gigas δ¹⁸O, brought about by changes in Earth's orbit, provide an additional geochronological constraint on the timing of reef growth at Huon Peninsula to around 402 ka during the MIS 11.3 sub-stage (~424-395 ka).