Megan J. Kelly

A penultimate glacial monsoon record from Hulu Cave and two-phase glacial terminations

Oxygen isotope records of three stalagmites from Hulu Cave, China, extend the previous high-resolution absolute-dated Hulu Asian Monsoon record from the last to the penultimate glacial and deglacial periods. The penultimate glacial monsoon broadly follows orbitally induced insolation variations and is punctuated by at least 16 millennial-scale events. We confirm a Weak Monsoon Interval between 135.5 ± 1.0 and 129.0 ± 1.0 ka, prior to the abrupt increase in monsoon intensity at Asian Monsoon Termination II. Based on correlations with both marine ice-rafted debris and atmospheric CH 4 records, we demonstrate that most of marine Termination II, the full rise in Antarctic temperature and atmospheric CO 2 , and much of the rise in CH 4 occurred within the Weak Monsoon Interval, when the high northern latitudes were probably cold. From these relationships and similar relationships observed for Termination I, we identify a two-phase glacial termination process that was probably driven by orbital forcing in both hemispheres, affecting the atmospheric hydrological cycle, and combined with ice sheet dynamics.

The Asian monsoon over the past 640,000 years and ice age terminations

Oxygen isotope records from Chinese caves characterize changes in both the Asian monsoon and global climate. Here, using our new speleothem data, we extend the Chinese record to cover the full uranium/thorium dating range, that is, the past 640,000 years. The record’s length and temporal precision allow us to test the idea that insolation changes caused by the Earth’s precession drove the terminations of each of the last seven ice ages as well as the millennia-long intervals of reduced monsoon rainfall associated with each of the terminations. On the basis of our record’s timing, the terminations are separated by four or five precession cycles, supporting the idea that the ‘100,000-year’ ice age cycle is an average of discrete numbers of precession cycles. Furthermore, the suborbital component of monsoon rainfall variability exhibits power in both the precession and obliquity bands, and is nearly in anti-phase with summer boreal insolation. These observations indicate that insolation, in part, sets the pace of the occurrence of millennial-scale events, including those associated with terminations and ‘unfinished terminations’.

High-resolution absolute-dated Indian Monsoon record between 53 and 36 ka from Xiaobailong Cave, southwestern China

The oxygen isotopic record of stalagmite XBL-1 from southwestern China reveals millennial-scale variability of the Indian Monsoon between 53 and 36 ka, synchronous with changes in the East Asian Monsoon recorded at Hulu Cave and similar to Dansgaard-Oeschger cycles recorded in Greenland ice. Our record, in general, confirms the chronology of Hulu Cave. If our correlations between Greenland and the Xiaobailong Cave record are correct, both the Greenland Ice Sheet Project 2 and Greenland Ice Core Project (ss09sea) chronologies are accurate within quoted errors. A dry interval that we correlate with Heinrich Event 5 (H5) and the Greenland stadial preceding Greenland Interstadial 12 (GIS 12) is centered ca. 48.0 ka and a shift to drier conditions, correlated to the end of GIS 12, is ca. 43.5 ka. Overall, the variability of the Indian Monsoon, from XBL-1 data, on millennial scales is similar to and correlated with high-latitude ice core rec ords from the Northern Hemisphere. However, some Indian Monsoon characteristics more closely resemble, but are anticorrelated with, features in the Antarctic record, suggesting some link to climate of the high southern latitudes, in addition to the clear link to the climate of the high northern latitudes.

Variability of stalagmite-inferred Indian monsoon precipitation over the past 252,000 y

Significance This paper presents a new long speleothem δ18O time series from Xiaobailong cave in southwest China that characterizes changes in a major branch of Indian summer monsoon precipitation over the last 252 kyrs. This record shows not only 23-kyr precessional cycles punctuated by prominent millennial-scale weak monsoon events synchronous with Heinrich events in the North Atlantic, but also clear glacial–interglacial variations that are consistent with marine records but different from the cave records in East China. The speleothem records of Xiaobailong and other caves in East China show that the relationship between the Indian and the East Asian summer monsoon precipitation is not invariant, but rather varies on different timescales depending on the nature and magnitude of the climate forcing. A speleothem δ18O record from Xiaobailong cave in southwest China characterizes changes in summer monsoon precipitation in Northeastern India, the Himalayan foothills, Bangladesh, and northern Indochina over the last 252 kyr. This record is dominated by 23-kyr precessional cycles punctuated by prominent millennial-scale oscillations that are synchronous with Heinrich events in the North Atlantic. It also shows clear glacial–interglacial variations that are consistent with marine and other terrestrial proxies but are different from the cave records in East China. Corroborated by isotope-enabled global circulation modeling, we hypothesize that this disparity reflects differing changes in atmospheric circulation and moisture trajectories associated with climate forcing as well as with associated topographic changes during glacial periods, in particular redistribution of air mass above the growing ice sheets and the exposure of the “land bridge” in the Maritime continents in the western equatorial Pacific.