Palaeoclimatic interpretation of high-resolution oxygen isotope profiles derived from annually laminated speleothems from Southern Oman
Introduction
Countries located in the Indian monsoon domain are densely populated with their economy depending to a large extent on rain-fed agriculture. Major departures from normal monsoonal rainfall cause serious famines and migration and politic instability. To better predict catastrophic monsoon failures, the monsoon dynamics and its linkages to other climate phenomena, such as the El Niño-Southern Oscillation (ENSO) in the Pacific Ocean and Eurasian snow cover, must be well understood. Our current knowledge of the Indian monsoon dynamics remains limited, mainly due to the lack of high-resolution and long-term monsoon records. The longest instrumental data of monsoon rainfall variability cover the last 150 years (Parthasarathy et al., 1994) and are, thus, too short to fully reveal monsoon variability on century-long time scales. On longer time scales only a few high-resolution geographically widely distributed paleomonsoon records, based on ice cores (Thompson et al., 1997), tree-rings (Bräunig, 1994) and marine sediments (von Rad et al., 1999; Agnihotri et al., 2002; Anderson et al., 2002), are currently available. Even more acute is the lack of information on the Arabian Peninsula and in Northern Africa. A source for information of Indian Ocean monsoon variability is annually laminated speleothems, such as stalagmites and flowstones, forming in caves in Southern Oman where still a monsoon-type climate exists (Burns et al., 2002; Fleitmann et al., 2003). To date, most speleothem-based palaeoclimate reconstructions rely either upon oxygen (δ18O) isotopic measurements of speleothem calcite or upon thickness of annual growth bands. The importance of both proxies as palaeoclimate variables is described hereinafter. When speleothems are deposited under conditions of isotopic equilibrium, δ18O of speleothem calcite reflect either variations in δ18O of the seepage water forming the speleothem and/or variations in cave air temperature. In the first case, the oxygen isotopic composition of the seepage water from which the speleothems are formed reflects the isotopic composition of mean annual rainfall (Yonge et al., 1985). In the second case, the temperature-dependent fractionation of δ18O between water and calcite (−0.24‰ per 1°C; O’Neil et al., 1969) can be used to reconstruct cave air temperatures (e.g. Lauritzen and Lundberg, 1999), which relates in many caves to mean annual surface air temperature (Wigley and Brown, 1976). However, in most caves the temperature-controlled variations in δ18O are obscured by changes in δ18O of the seepage water and surface precipitation, respectively. Generally, changes in δ18O of rainfall result from a variety of factors, including: (1) changes in δ18O of the oceanic source region (important on glacial–interglacial time scales), (2) changes in moisture sources or storm tracks, (3) changes in the proportion of rainfall (e.g. winter/summer rainfall), (4) air temperature, (5) amount of rainfall, and (6) evaporation. Particularly evaporation can alter the original oxygen isotopic composition of rainfall and seepage water, respectively (e.g. Bar-Matthews et al., 1996) when the cave is located in an arid or semi-arid regions, such as Southern Oman. To rule out the importance of evaporation processes, the oxygen isotopic composition of water along the pathway from precipitation to speleothem calcite must be traced. The knowledge of the principal controls on the oxygen isotopic composition of present-day stalagmites is essential before δ18O values of fossil stalagmites can be interpreted correctly in terms of palaeoclimate variability.
The thickness of annual growth bands in stalagmites is also a frequently used speleothem climate proxy, because band thickness is controlled by the drip rate, which relates to surface precipitation (Baker et al., 1993; Genty and Quinif, 1996; Holmgren et al., 1999; Qin et al., 1999). Qin et al. (1999) suggested that the thickness of annual growth bands is a proxy for the amount of drip water and surface precipitation respectively when the cave has a simple hydrological connection to the surface (thin overburden bedrock thickness). Furthermore, the stalagmite should have a columnar shape indicating that calcite precipitated at the top and no stalactite as a drip source.
Before speleothem-based time series can be used for palaeoclimate studies the following fundamental questions must be answered:
- 1.
Are the stalagmites deposited in isotopic equilibrium with the cave waters?
- 2.
Is the oxygen isotopic composition of speleothem calcite mainly controlled by the oxygen isotopic composition of rainfall or by air temperature?
- 3.
Do multiple proxies, in this case δ18O and annual band thickness, relate to the same climate variable?
- 4.
Are oxygen isotope profiles of individual stalagmites reproducible?
- 5.
Do the time series reflect local or regional climate variability?
In this paper, we present high-resolution stable isotope profiles derived from three contemporaneously deposited annually laminated stalagmites from Kahf Defore located in Southern Oman. In addition, we have sampled precipitation, cave drip waters and actively growing stalagmites to study the active water-carbonate system and to test for isotopic equilibrium. By comparing δ18O and thickness of annual bands of contemporaneously deposited stalagmites we test whether both proxies relate to the same climatic variable. To date speleothem studies combining both proxies at high-resolution are extremely rare. Finally, we compare our records with meteorological observations form the surrounding areas to validate and to put our palaeoclimate reconstructions into a broader context.
Section snippets
Site location and modern climatology
Kahf Defore (17°07′N, 54°05′E; ∼150 m above sea level) is located at the foothills of the Dhofar Mountains in the extreme SW of Oman (Fig. 1a and b). The cave is approximately 45 m long and has a narrow entrance (Fig. 1c). The thickness of the overlying bedrock, a karstic Eocene limestone of the Umm er Rhaduma formation varies between 15 and 20 m. Cave air temperature of 25.5°C at the sampling location closely reflects the mean air temperature of 25.7°C station at Salalah airport (1942–1998).
The Indian monsoon in southern Oman
Today, the Indian monsoon is the most dependable source of precipitation in Southern Oman, more that 80% of total annual precipitation falls during the summer monsoon months (June, July and August). The Indian monsoon is driven by two fundamental mechanisms. Different sensible heating between the Asian landmass and the southern Indian Ocean results in a low atmospheric pressure cell over the Tibetan Plateau and a high atmospheric pressure cell over the southern Indian Ocean (at about 30°S; Fig.
Sample description and methodology
All three columnar-shaped stalagmites, their lengths vary between 26 cm (S3), 7 cm (S6) and 20 cm (S9), were actively growing when sampled in November 1996 (stalagmite S3) and in August 99 (stalagmites S6 and S9) (Fig. 2a). The specimens were cut lengthwise parallel to their growth axis. The stalagmites are composed of white and grayish LMC calcite and show regular sub-millimeter lamination. No signs of secondary alternation, such as corrosion features and recrystalization, were found.
The ages of
Chronology
The chronology of all stalagmites is based on both Uranium-series dating and counts of annual growth bands. Uranium-series dating of stalagmites from Kahf Defore is difficult because Uranium contents are low (Table 1) and speleothem calcite contains high amounts of dust, as indicated by high 232Th concentrations. Because the 230Th ion-beams were too small and unstable to measure reliable 230Th/229Th ratios, it was not possible to determine ages for material above 150 mm for S3 and 90 mm for S9.
Palaeoclimatic significance of δ18O
Whether the studied stalagmites were deposited in isotopic equilibrium is of crucial importance for speleothem-based palaeoclimate reconstructions. Kinetic fractionation processes, such as kinetic loss of carbon dioxide and evaporation of water, can seriously affect the isotopic composition of speleothem and, thus, blur the climatic signal. This might be especially the case for caves located in arid and semi-arid areas, such as Kahf Defore. One robust test for isotopic equilibrium is to use δ18
Summary
Three annually laminated stalagmites from Kahf Defore in Southern Oman provide a millennial long and annually resolved record of Indian Ocean monsoon variability, which helps to fill the lack of historical climate records in Southern Arabia. The study of the active water-carbonate system in Kahf Defore clearly reveals that modern stalagmites do not grow in isotopic equilibrium and that δ18O calcite values are affected by evaporation occurring within the cave. However, despite kinetic
References (42)
- et al.
Evidence for solar forcing on the Indian monsoon during the last millennium
Earth and Planetary Science Letters
(2002) - et al.
Recent flowstone growth ratesfield measurements in comparison to theoretical predictions
Chemical Geology
(1995) - et al.
Carbon and oxygen isotope study of the active water-carbonate system in a karstic Mediterranean caveimplications for palaeoclimate research in semiarid regions
Geochimica et Cosmochimica Acta
(1996) Deposition of calcite from thin films of calcareous solutions and the growth of speleothems
Chemical Geology
(1980)- et al.
Warm period growth of travertine during the last interglaciation in southern Germany
Quaternary Research
(2000) The isotopic geochemistry of speleothemsthe calculations of the effects of different modes of formation on the isotopic composition of speleothems and their applicability as palaeclimate indicators
Geochimica et Cosmochimica Acta
(1971)Geochronology and isotope geochemistry in speleothems
- et al.
A 5000-yr record of climate change in varved sediments from the oxygen minimum zone off Pakistan, Northeastern Arabian Sea
Quaternary Research
(1999) - et al.
Stable isotope studies of cave seepage water
Chemical Geology
(1985) - et al.
Increase in the Asian southwest monsoon during the past four centuries
Science
(2002)
Annual growth bandings in a cave stalagmite
Nature
The effect of Eurasian snow cover on regional and global climate variations
Journal Atmospheric Science
Dendrochronology for the last 1400 Years in Eastern Tibet
GeoJournal
Interaction between the ENSO and the Asian monsoon in a coral record of the tropical climate
Science
Isotopic variations in meteoric waters
Science
Stable isotopes in precipitation
Tellus
Holocene forcing of the Indian monsoon recorded in a stalagmite from Southern Oman
Science
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