Early- to Late-Holocene Indian Ocean Monsoon Variability Recorded in Stalagmites From Southern Oman
by
Dominik Fleitmann
Geological Institute, University of Bern
Coauthors: Stephen J. Burns, Manfred Mudelsee, Ulrich Neff, Albert Matter, Augusto Mangini

Until now there has been a lack of long high-resolution terrestrial records monitoring Indian Ocean monsoon variability in Southern Arabia and the neighboring regions. Most of the existing records in North Africa, India and the Arabian Peninsula have low time resolution and large 14C age dating uncertainties. A potential source for information about Indian Ocean monsoon variability is stalagmites from caves in Southern Arabia. This region is still affected by the Indian Ocean monsoon and receives more than 80 % of total annual precipitation during the monsoon months June-September. Two Uranium-series dated stalagmites, sampled in two caves in Southern Oman, provide a continuous high-resolution terrestrial record of Indian Ocean monsoon variability from 10.6 to 2.8 kyrs BP. Oxygen and carbon isotopes as well as thickness of annual layers are used as an indicator for the amount of monsoon precipitation. Thick annual layers as well as depleted 18O and 13C indicate higher monsoon precipitation. Sampling interval varies between 1 and 5 years throughout the Holocene, The high-resolution delta 18O record (1400 stable isotope measurements) of stalagmite Q5 indicates that monsoon precipitation was considerably higher than today between 9.9 and 5.5 kyrs BP. This 4500 year long period consists of three humid intervals (9.9-8.6 kyrs BP, 8.0-6.3 kyrs BP and 6.1-5.8 kyrs BP), which are separated by three short dry periods occurring at 9.2-9.1 kyrs BP, 8.6-8.0 kyrs BP and 6.3-6.2 kyrs BP. After 5.8 kyrs BP monsoon precipitation decreased gradually, and reached a present day level at around 4.1 kyrs BP. Stalagmite growth ceased after 2.8 kyrs BP. Stalagmite S4, sampled in a different cave, covers the period between 10.6 and 9.1 kyrs BP and shows a good correlation to stalagmite Q5, indicating that sample or site specific noise did not overprint the climatic signals.

Both stalagmite-based oxygen isotope records show that the abrupt increase in monsoon precipitation at approximately 10 kyrs BP is in phase with the temperature increase in Greenland and the final melting of Northern Hemisphere ice sheets. This fact suggests that glacial boundary conditions and not solar forcing largely control early-Holocene monsoon intensity. After 10 kyrs BP monsoon intensity is mainly controlled by changes in solar insolation, which is indicated by an almost linear response of monsoon precipitation to changes in solar radiation in the Northern Hemisphere. Further, results of spectral analyses of all proxy records show statistically significant cycles of 1000 yr, 420 yr, 220 yr, 136 yr, 100 yr, 45 yr, 13-10 yr and 8-3 yr. These cycles support or hypothesis that monsoon variability during the Holocene is closely tied to solar activity.

Date received: April 29, 2001

Copyright © 2001 by the author(s). The author(s) of this document and the organizers of the conference have granted their consent to include this abstract in Atlas Conferences Inc. Document # cahi-49.