Holocene Variability in the Indian Ocean Monsoon: A Stalagmite- Based, High-Resolution Oxygen Isotope Record from Southern Oman

ing an absolute “ice-core calendar”, to permit direct comparison with contemporaneous isotopic and meteorological data, still persists: Although the data measured in the ice core provide unmistakable evidence for the passage of time, where does a particular month (or season or year) actually begin and end? Thus, in spite of the unusually high resolution, the Fiescherhorn ice-core chronology may not be able to support evaluation of isotope-climate relations with the same level of confidence or at temporal scales equivalent to that possible using data from sampling of monthly composite precipitation. Indeed, this limitation can be demonstrated by comparing the stacked Fiescherhorn δ18O record with the monthly composite precipitation δ18O record from Grimsel, a high-elevation station (1950 m) less than 25 km distant (lower panel, Fig. 1). Even extensive adjustment to achieve a best-fit with this monthly time-series yields only weak correlation, in spite of the visual impression imparted by the strong annual cycling in both records and the similar longterm trends. Subsequent comparison with the North Atlantic Oscillation index (not shown), which generally correlates significantly with composite monthly European isotope records, also reveals weak correspondence. The most likely explanation to account for the apparent deficiency in the ability of this glacier to archive paleoprecipitation is revealed clearly by the plot shown in Figure 2, which shows that no correspondence exists between the net annual accumulation at Fiescherhorn and the amount of annual precipitation at Grimsel. This probably reflects at least in part the influence of drifting and other postdepositional processes that remove or redistribute snow, reminding us that alpine glaciers (and certainly to some extent polar ice sheets) are dynamic, open systems for precipitation, and that due caution is required in attempting to decipher their isotopic records.