South West Pacific climate at c. 6600-6000 BP inferred from giant clam Tridacna maxima and massive coral Porites sp. records

International audienceIn the last three decades, the El Niño Southern Oscillation (ENSO) has been characterized by more frequent and stronger El Niño events. Recent paleo-climate studies demonstrated that ENSO periodicity was lower during the Mid-Holocene and that it increased towards present-day. While a better knowledge on such past changes is necessary to reliably predict future climate, the origin of the Mid-Holocene ENSO regime shift remains controversial. The present study is focused on the climate in the Southwest Pacific c. 6 600-6 000 BP. This period precedes the Mid-Holocene ENSO-shift, and is essential to understand the ENSO evolution. The Vanuatu archipelago (South West Pacific; 15° 40'S; 167°00' E) is characteristic of an open ocean context and climatic conditions are strongly linked to the ENSO variability in relation with the position of the South Pacific Convergence Zone (SPCZ). The Sea Surface Salinity (SSS) and Sea Surface Temperature (SST) are highly correlated to the Southern Oscillation Index (SOI) and the Niño 3.4 index. Consequently, high-resolution SSS and SST records from massive corals and giant clams provide reliable tools to track ENSO variability back in time in this region. Two massive Porites sp. coral colonies, dated at 6758 - 6544 14C cal. BP, and one giant Tridacna maxima clam dated at 6257-6056 14C cal. BP, were collected on Espiritu Santo Island. These archives were sampled at monthly resolution and analyzed for Sr/Ca ratio and δ18O. SST was reconstructed from corals Sr/Ca and the giant clam δ18O. The δ18O composition of seawater (δw), calculated from the coral δ18O record, was used as a proxy for evaporation/precipitation (E/P) budget. Compared to present-day conditions, reconstructed SST are 2.5°C lower at ~6600 BP and 1°C lower at ~6100 BP, and both archives present a slightly higher (+0.5°C) seasonal amplitude. Fossil Porites sp. from Vanuatu showed that the E/P budget was positively correlated to the SST c. 6600BP whereas the inverse relationship is observed today. Fossil coral δ18O recorded lower than actual ENSO-frequency band variations. The climate c. 6600-6000 BP in the Southwest Pacific was thus cooler than today, possibly linked to the progressive warming of the Pacific after the Last Glacial Maximum and/or the contraction of the West Pacific Warm Pool. Changes in the E/P balance ~6600 BP suggest that some changes occurred in the regime or position of the Pacific main climatic features. Compared to the ENSO-band variations of a 1928-1992 coral δ18O record (Kilbourne et al., 2004), the ~6600 BP ENSO activity appeared similar to the 1930's and 1960's low-ENSO activity phases. Additional analyses are currently in process to interpret reliably these results. This study is supported jointly by the French INSU-EC2CO program (Holbeco project), ANR EL Paso and the IRD