Precipitation Isotopes Reveal Intensified Indonesian Monsoon Circulation During the Dry Last Glacial Maximum

Abstract:

The Indo-Pacific Warm Pool (IPWP) invigorates the oceanic-atmospheric circulation in the tropics, with far-reaching climate impacts that extend into the high latitudes. A growing number of deglacial proxy reconstructions from the region have revealed the importance of both high- and low-latitude climate processes to IPWP rainfall during the late Pleistocene. Many of these proxies reconstruct the oxygen and hydrogen isotopic composition of rainfall (δ¹⁸O_precip, δD_precip), a powerful tool for understanding changes in climate. However, an increasing number of studies from the region have highlighted the tendency for δ¹⁸O_precip and δD_precip to reflect regional and/or remote circulation processes rather than local rainfall amounts, complicating the reconstruction of IPWP hydroclimate. To better understand high- and low-latitude drivers of late Pleistocene hydroclimate in the IPWP, precipitation isotopic reconstructions must be constrained with both modern observations and independent proxies for rainfall amount.

We present a reconstruction of δD_precip using leaf wax compounds preserved in the sediments of Lake Towuti, Sulawesi, from 60,000 years before present to today. We interpret our proxy record with the aid of a new precipitation isotopic dataset from our study site, with daily rainfall isotope measurements to constrain the processes controlling δD_precip. Our Lake Towuti δD_wax record is strikingly similar to a speleothem δ¹⁸O record from southern Indonesia (Ayliffe et al., 2013) and shares features with other nearby records spanning the Last Glacial Maximum to present. Together, these records indicate that monsoon circulation was intensified in central and southern Indonesia during the glacial period. However, other independent rainfall proxies from Lake Towuti indicate that dry conditions accompanied the intensified monsoon. Regional-scale isotopic depletion during the dry glacial period may have arisen from dynamical and other fractionating processes that are evident in our modern precipitation isotopic data during the monsoon season. We use these findings to reconcile some key differences among proxy reconstructions from the region, and to examine the influence of high-latitude and glacial processes on IPWP glacial climate.