Greenhouse Gas and Ice Volume Drive Pleistocene Indian Summer Monsoon Precipitation Isotope Variability

Abstract:

Orbital-scale Indian Summer Monsoon variability is often interpreted as a direct response to northern hemisphere summer insolation. Here we present a continuous (0-640 kyr) orbital scale precipitation isotope record from the core monsoon zone of India. We interpret leaf wax δD_precip as reflecting changes in large-scale Indian Summer Monsoon circulation associated with precipitation amount, moisture source, and moisture transport. The δD_precip record has strong variance across all orbital bands and is consistently in phase with greenhouses gas maxima, ice volume minima, and the two existing orbital-scale Indian speleothem δ¹⁸O records. Hence, both the leaf wax δD and speleothem δ¹⁸O proxies represent the terrestrial precipitation isotope signal. All three records have significant variance outside of precession precluding interpretation as a direct response to northern hemisphere summer insolation. Taken together, Indian Summer Monsoon precipitation isotopes are driven by large-scale internal forcing associated with changes greenhouse gas and high latitude ice-volume.