Intertropical convergence zone and El Niño Southern oscillation controls on late Holocene rainfall variability in Palau

Abstract

The Tropical West Pacific is an understudied region in environmental and archaeological studies when compared to continental regions. The unique geologies, climate systems, and ecological dynamics in the Tropical West Pacific greatly limit the transferrability of palaeoenvironmental proxies established from studies outside of this region. To address this gap, we present a ~5600 yr multi-proxy record of environmental change from a marine lake in Belau (Palau in English). The Belau archipelago is located in the Tropical West Pacific, a region with links to global atmospheric, oceanic, and hydrological climate systems. Additionally, this archipelago is central to debates of early human mobility in the Tropical Pacific ~3500 to 3000 years ago. By combining high-resolution data (XRF) with low-resolution proxies (pollen, fern spore, charcoal, TOC, δ13C, TN, δ15N, TOC:TN) we are able to reconstruct the response of a coastal Belauan system to changing Holocene climate dynamics and identify the primary processes driving these changes. Two distinct climate signatures are identified in the multi-proxy record. Drier and warmer conditions during Phase I (~ 5600 to 3250 cal yr BP, 689–400 cm) are consistent with local records of a southern displacement of the Intertropical Convergence Zone (ITCZ) and regional records of long-term cooling trends in the Northern Hemisphere. Phase II (~ 3250 to − 63 cal yr BP) marks a transition to wetter and cooler conditions in Belau, associated with the northward migration of the ITCZ and intensification of El Niño Southern Oscillation (ENSO). The Phase II transition coincides with a shift in catchment vegetation, as mangroves retreat and the rainforest expands, and evidence of rising local sea level. A brief vegetation disturbance event ~1250 to 500 cal yr BP does not align with documented changes in regional climate systems, suggesting that this was a localised signature. Currently it is unclear if an enrichment in bulk sediment δ15N values at ~1250 cal yr BP was driven by rising sea levels or local ecological dynamics and future studies should seek to clairfy the drivers of this signal.