The History of the Monsoon in Arabia
A Speleothem-based Paleoclimate Study

Stephen J. Burns
Department of Geosciences
University of Massachusetts


The Indian Ocean Monsoon is one of the major weather systems of the earth, affecting the economies and agriculture, and fisheries of one of the most densely populated areas of the world. It is of obvious importance to attempt to better understand the nature of monsoon variability and mechanisms that drive the observed variation. This project utilizes a broad array of techniques to understand how monsoon-related climate signals are recorded by speleothems in southern Arabia. The overall goal is to produce high quality paleoclimatic reconstructions for the region on a variety of timescales, and to interpret those reconstructions in terms of climate forcing. The project is international in scope with collaborators in Switzerland, Germany, Oman and Yemen.

Speleothems as recorders of climate change

Speleothems, such as stalagmites, stalactites and flowstones, are formed when calcium carbonate precipitates from degassing solutions seeping into limestone caves. Paleoclimate information can be gained from such deposits in a several ways, including:

  1. Periods of rapid or more extensive deposition can be used to identify wetter vs drier climate intervals.
  2. The oxygen isotope ratios of speleothem carbonate primarily reflect the isotopic composition of regional precipitation. Where variation in the isotopic composition of groundwater is a function of climate, for example temperature or available moisture, that climate information is archived in speleothems. In southern Arabia, we have found that the direct effects of temperature on the speleothem isotopic records (via calcite water fractionation) are small in comparison to changes in the isootpic composition of drip water. Stalagmite d18O values are found to be inversely related to the amount of rainfall, primarily via an amount effect.
  3. The hydrogen isotopic composition of fluid inclusions incorporated in speleothems can yield a direct measure of the isotopic composition of paleogroundwaters (Schwarcz and Yonge, 1983), which may then be combined with isotopic analyses of speleothem carbonate to directly calculate paleotemperatures.
  4. In near surface caves in areas with a strong annual climate cycle, stalagmites may contain annual laminations. Variations in lamination thickness are also climate related (Baker et al, 1993b)
Thus far, our work in southern Arabia has concentrated on using speleothems from caves in Oman as a source of climate information for the region. The approach we have taken combines studies of modern caves and cave waters, dD analyses of water from fluid inclusions in modern and ancient speleothems, high resolution O and C stable isotopic analyses of recent and well-dated ancient speleothems and spectral analyses of the isotopic time series. We have worked on samples from two areas: Hoti Cave in northern Oman, and several caves in the Salalah region of southern Oman (Figure 1).

Figure 1. Location map of cave sites in Oman and Yemen: 1. Hoti Cave, 2. Salalah area caves, 3. Hadramaut in Yemen, 4. Socotra Island.

Soqotra Island

In October, 2000 our group from Bern, in collaboration with Prof. Abdul-Karim Al-Subary of Sanaa University, Yemen conducted an initial exploration and sampling expedition on Soqotra Island. Soqotra Island lies off the horn of Africa in the Indian Ocean at about 53E 12N (Fig. 2) and is a part of the Republic of Yemen.

Figure 2. Socotra Island.

Soqotra has been an island for an estimated 15 m.y. to 20 m.y. and has a highly endemic flora and fauna. Of 800 known plant species, more than 300 are found only on the island. The entire island is the site of an extensive biodiversity study funded by the United Nations (Mies and Beyhl et al., 1996).

Below are some photos of caves and sampling in Oman (Figs. 3-6)

Figure 3. Descending into Qunf Cave, near Salalah, Oman.

Figure 4. The Majlis al Jihn on the Salma Palteau, Oman.

Figure 5. Bottle tree (Adenium obesum) on Socotra Island, Yemen.

Figure 6. Drilling a large stalagmite in Hoti cave, Oman.