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Speleothem Evidence from Oman for Continental Pluvial Events During Interglacial Periods

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Speleothem Evidence from Oman for Continental Pluvial Events During Interglacial Periods Speleothem evidence from Oman for continental pluvial events during interglacial periods Stephen J. Burns Dominik Fleitmann Albert Matter Institute of Geology, University of Bern, CH-3012 Bern, Switzerland Ulrich Neff Augusto Mangini Heidelberg Academy of Sciences, D-69120 Heidelberg, Germany ABSTRACT Growth periods and stable isotope analyses of speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods extending back over the past four of Earth's glacial-interglacial cycles. Rapid speleothem growth occurred during the early to middle Holocene (6±10.5 ka B.P.), 78±82 ka B.P., 120±135 ka B.P., 180±200 ka B.P., and 300±325 ka B.P. The speleothem calcite deposited during each of these episodes is highly depleted in 18O compared to modern speleothems. The d18O values for calcite deposited within pluvial periods generally fall in the range of 24½ to 28½ relative to the Vienna Peedee belemnite standard, whereas modern speleothems range from 21½ to 23½. The growth and isotopic records indicate that during peak interglacial periods, the limit of the monsoon rainfall was shifted far north of its present location and each pluvial period was coincident with an interglacial stage of the marine oxygen isotope record. The association of continental pluvial periods with peak interglacial conditions suggests that glacial boundary conditions, and not changes in solar radiation, are the primary control on continental wetness on glacial-interglacial time scales. Keywords: speleothems, stable isotopes, Oman, monsoon, uranium-series method. INTRODUCTION rine and continental records show that its in- such as the extent of glaciation on the Hima- How climate in Earth's tropical regions var- tensity has varied considerably in the recent layan plateau or sea-surface temperatures? ied over the course of Earth's glacial-intergla- past. For example, an early Holocene inten- cial cycles of the past several hundred thou- si®cation of the southwest summer monsoon, GEOLOGIC SETTING AND METHODS sand years remains largely unknown. Even followed by its dramatic reduction in the mid- Hoti Cave is a through cave in northern more acute is the lack of knowledge of con- dle to late Holocene is recorded by changes in Oman, in the foothills along the southern mar- tinental, as opposed to marine, climate varia- upwelling indicators in marine sediments gin of the Oman Mountains (Fig. 1). This 4.5- tion. The most commonly used archive of con- (Clemens et al., 1991; Sirocko et al., 1993; km-long cave is hosted by the Natih Forma- tinental climate change, lakes, are generally Overpeck et al., 1996), by variations in lake not long-lived enough to provide information levels across the Sahel region of Africa (Gasse on glacial-interglacial time scales, nor are and Street, 1978; Ritchie et al., 1985), in Ara- lacustrine sediments easily dated beyond the bia (McClure, 1976), and in India (Bryson and range of the radiocarbon method. Speleo- Swain, 1981), and by speleothem growth in thems, calcium carbonate deposits in caves, northern Oman (Burns et al., 1998). On longer offer the potential for considerably longer, time scales, knowledge of monsoon variation interpretable records of continental climate comes almost entirely from marine sediments variation (e.g., Dorale et al., 1992) includ- (Anderson and Prell, 1993; Rosteck et al., ing reconstruction of paleorainfall intensity 1997). Yet, these records primarily register (Bar-Matthews et al., 2000). Here, we at- changes of wind intensity and may or may not tempt to develop a record of continental climate re¯ect changes in available moisture on the variation for southern Arabia, a region affected continents. As a result, they leave unanswered by one of the most important components of some fundamental questions concerning cli- tropical climate, the Indian Ocean monsoon. mate variations in continental regions affected The record covers the past four glacial-inter- by the monsoon. In particular, does continen- glacial cycles, and is based on observed periods tal pluviality vary in concert with marine up- Figure 1. Map showing location of Hoti Cave of rapid growth and stable isotope data in spe- welling and/or wind strength, and are periods (star) and generalized modern, summer, sur- face wind pattern. Dashed line is approxi- leothems from Hoti Cave in Oman. of increased continental wetness in the region mate location of intertropical convergence The Indian Ocean monsoon is one of the primarily controlled by changes in solar in- zone, the present northern limit of monsoon major weather systems on Earth, and both ma- solation or by glacial boundary conditions rainfall. q 2001 Geological Society of America. For permission to copy, contact Copyright Clearance Center at www.copyright.com or (978) 750-8400. Geology; July 2001; v. 29; no. 7; p. 623±626; 3 ®gures; Data Repository item 2001069. 623 tion, a Cretaceous limestone. The present climate is arid to semiarid; average rainfall is ;200±250 mm/yr (Food and Agriculture Or- ganization, 1987). Groundwaters in the region are recharged primarily by cyclonic low-pres- sure systems moving from the Mediterranean Sea southeast across the Arabian Peninsula during the winter months (Weyhenmeyer et al., 2000). The northern limit of the summer monsoon, the intertropical convergence zone, is located generally south of the Arabian land- mass (Hastenrath, 1985). Oxygen isotopic val- ues of rainfall in the vicinity of Hoti Cave range from ;0to22½ (Macumber et al., 1997; Weyhenmeyer, 2000). We collected whole or sampled by drilling seven large fossil stalagmites and one large ¯owstone from the cave. These inactive sta- lagmites ranged from ;30 cm to 3 m in height; the ¯owstone is 2.7 m thick. In addi- tion, we collected drip waters from several lo- cations in the cave together with small active Figure 2. Composite record of speleothem growth periods and oxygen isotope ratios for stalagmites beneath the drips. Only one active Hoti Cave. Samples H2 and H3 are modern; all other ages are based on U/Th data (see footnote 1). Also shown is SPECMAP (Imbrie et al., 1990) marine oxygen isotope curve and stalagmite taller than 30 cm was found. More marine oxygen isotope stage numbers. typically, active stalagmites are ;10 cm in height. Temperature and humidity measure- ments were taken at numerous locations in the analyzed with an on-line, automated, carbon- dle Holocene; four different stalagmites yield- cave. ate preparation system linked to a VG Prism ed ages between 6.2 and 10.5 ka. Prior periods The ages of deposition of the samples were ratio mass spectrometer. Results are shown as of growth of large speleothems took place determined by measuring their U/Th ratios by the per mil difference between the sample and during the intervals 78±82 ka B.P., 117±130 thermal ionization mass spectrometry; we the Vienna Peedee belemnite (VPDB) stan- ka B.P., 180±210 ka B.P., and 300±325 ka B.P. made a total of 51 measurements for all sam- dard in delta notation. Reproducibility of stan- (Fig. 2). An important feature of one sample, 18 ples (Table 11). Analytical procedures for the dard materials is 0.08½. Drip water d O stalagmite H-13, is that it records three sepa- separation and puri®cation of thorium and ura- values were measured using an on-line, au- rate growth periods. As shown on the age ver- nium were as described by Ivanovich and Har- tomated equilibration system. The values are sus depth plot in Figure 3, the stalagmite grew mon (1993). U/Th measurements were per- reported against the Vienna standard mean rapidly during three relatively short periods of formed on a multicollector mass spectrometer ocean water (VSMOW) standard, and have a about 10±30 k.y.; intervening long periods of (Finnigan MAT 262 RPQ) with a double ®l- reproducibility of 0.08½. nondeposition are marked by thin, reddish, ament technique. Uranium and thorium were clay mineral±rich layers. Growth phases oc- measured in semi-peak-jump mode and peak- RESULTS curred during times equivalent to MIS (marine jump mode, respectively. Calibration of Far- Active stalagmites in Hoti Cave are small [oxygen] isotope stages) 5e, 7a, and 9. Note aday cup to ICM ef®ciency was made adopt- in comparison to fossil stalagmites and have that although two samples from the last 18 ing the natural 238U/235U of 137.88. To d O values that range from about 21½ to growth period in H-13 date beyond MIS 9, the 18 determine the uranium and thorium concentra- 23½ (Fig. 2). Measured d O values of drip ages of these samples have large errors and tions, de®ned quantities of a 233U/236U double waters associated with growing stalagmites are bracketed above and below by better- spike and a 229Th spike were added. Th/U vary mainly between 0½ and 22½, very determined ages (Fig. 3). Because growth was 18 ages were corrected for detritus following Iva- similar to rainfall d O values in the region of reactivated multiple times within a single sam- novich and Harmon (1993), assuming a 232Th/ the cave despite high evaporation rates in arid ple, the intervening hiatuses are taken to have 238U isotope ratio of 3.8. The reproducibility climates. For the measured cave temperatures been continually dry. Each of the growth pe- 18 of the isotope ratio of 234U/238U and the con- of 25±27 8C, the modern speleothem d O val- riods of large stalagmites is coincident with centration of 232Th of standard materials is ues are, thus, very close to expected equilib- an interglacial stage of the marine isotope re- 0.3% and 0.8% (2 s), respectively. For details rium calcite values and well re¯ect modern cord; growth took place during times equiva- about measurements of standard material see rainfall in the area.
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