Toward a Multi-Proxy Database of Pleistocene Water Isotope Records; Differentiating Precipitation Isotopic Composition from Precipitation Amount in the Asian Monsoon System

Monsoon rainfall is critical to billions of people living within South Asia and East Asia. It is thus important to understand monsoon variability and how and why monsoons may change over time. This project will analyze a core from the Indian Ocean continental shelf to produce records of the monsoons over the last 640,000-years.

The records will include changes in winds, rainfall, and river runoff. These records will be compared with records of known changes in temperature, greenhouse gases, and other potential monsoon forcing mechanisms. That comparison will help to understand the influence of climate on monsoon variability.

Those records will also provide real-world tests for the numerical climate models used to predict future climate change. The project will support a woman graduate student. It also will provide research experiences for undergraduates recruited from groups typically underrepresented in the earth sciences.

Two parameters are key to understanding climate change - temperature and rainfall. Temperature reconstructions are now quantitative, accomplished by a number of independent means, and directly comparable to climate simulations. Rainfall is a difficult parameter to simulate even in the most advanced climate models because, for example, most rainfall happens at spatial scales far smaller than models can accommodate now and for the foreseeable future.

Reconstructing rainfall in the past is similarly difficult and, at this point, qualitative. This research is part of a longer-term objective that focuses on resolving a key shortcoming in reconstructing rainfall, the capacity to differentiate changes in the isotopic composition of rainfall from rainfall amount in water isotope proxies commonly used to reconstruct rainfall (i.e. speleothem δ18O, sea water δ18O, and leaf wax δ2Η).

Both the isotopic composition of the rainfall and the rainfall amount are important. The former yields information on source areas, transport paths, and dynamics taking place as moisture travels from source to sink (large-scale circulation). The latter provides information on changes in rainfall amount at the local to regional scale.

Usefulness of these proxies depends on knowing what scale is expressed in the reconstruction. This research builds on past work, focusing here on leaf wax δ2Η. A 640,000-year record of leaf wax δ2H from the South Asian summer monsoon at Andaman Sea Site U1448 will be reconstructed at a nominal 2,000-year sample resolution.

Supporting records, benthic δ18O (chronology), and leaf wax δ13C (vegetation structure) will be generated at the same resolution. The resulting records will be quantitatively assessed using time-series analytical techniques to determine the sensitivity to insolation, ice volume, sea level, and greenhouse gases as well as their timing (phase) relative to existing seawater δ18O, speleothem δ18O and leaf wax δ2Η records from the South- and East-Asian monsoon systems.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.