The Region of Western Pacific Warm Pool (WPWP) is closely related to Global climate system. Previous studies indicate that the region of WPWP is affected by Walker circulation (WC) and Hadley cell (HC). WPWP expands when WC becomes stronger or when HC becomes weaker, and contracts when WC becomes weaker or when HC becomes stronger. In this study, records derived from core ODP1115B, including stable oxygen and carbon isotopes are used to reconstruct the long-term hydrological variations of WPWP over the past 2.2 Ma. We compared two species of foraminifera: Neogloboquadrina dutertrei and Globigerinoides sacculifer, for the reconstruction of differences between surface water and oceanic subsurface water. We try to find out the relationships between the thermocline depth in southern WPWP, the region of WPWP, HC and WC. However, our records indicate that the depth of thermocline in southern WPWP may be effected by the region of WPWP and the hydrology of southern WPWP. According to the Paleothermometry records of ODP806, ODP847, ODP1115 and MD063018, we can explain the relationship between WC, HC and the region of WPWP. Before 1.8 Ma, southern WPWP may not be affected by weak WC. During 1.8~1.2 Ma, WC becomes stronger and effect the region of WPWP. At the period of 1.2~0.9 Ma, southward migration of WPWP enhanced the influence of WC on the region of south WPWP and the depth of thermocline, then weakened HC in the southern hemisphere. After 0.9 Ma, the variation of hydrology in southern WPWP may be affected by stronge WC, not HC. We suggest that the influence of HC in southern WPWP is resulted in the southern region of WPWP.

In this study, specimens of Tridacna sp., which are reef dwelling bivalve molluscs and have been shown to live up to 60 years, collected from the Huon Peninsula, Papua New Guinea, were sampled for geochemical profiles. The Huon Peninsula is in the heart of the Western Pacific Warm Pool (WPWP), which plays a key role in ENSO dynamics. The uplifted reef terraces of the Huon Peninsula have been extensively studied, and are well dated, which gives the opportunity to reconstruct the local climate of this region at key intervals during the past. Previous work on Tridacna sp. has shown that they precipitate their aragonite shell in equilibrium with the surrounding seawater, and the δ18O profile of a modern T. gigas from the Huon Peninsula has been shown to correlate with precipitation and temperature anomalies, and the Niño 3.4 temperature anomaly record. Fossil samples from this region are therefore assumed to have the ability to capture changes in δ18O attributable to ENSO. Seasonally resolved δ18O measurements from Tridacna sp. from early Holocene and Marine Isotope Stage 3 (MIS3) reefs were used to reconstruct changes in mean climate, seasonality and inter-annual variability (e.g. ENSO). Reconstructions of the mean state tend to agree with previously published studies of Holocene and MIS3 climate, showing similar temperatures to today during the early Holocene, and an average cooling of 2- 3°C during MIS3. The early Holocene Tridacna sp. samples show a reduction in seasonality, consistent with the reduction in seasonal insolation at this time, while those from MIS3 show variable seasonality between 30-60ka. ENSO appears to have been supressed during the early Holocene by up to 50% compared with the late 20th century, which is consistent with coral data and modelling studies. During MIS3, ENSO appears to have been more variable with some records showing anomalous warm and cool events as strong as those seen in the modern T. gigas, used here as a benchmark. Trace element profiles derived from the Tridacna sp. used in this study show a tentative link with temperature and local productivity, but these relationships are subject to species specific and intra-shell effects.