A high-resolution oxygen-isotope record from a thorium-uranium-dated stalagmite from southern Oman reflects variations in the amount of monsoon precipitation for the periods from 10.3 to 2.7 and 1.4 to 0.4 thousand years before the present (ky B.P.). Between 10.3 and 8 ky B.P., decadal to centennial variations in monsoon precipitation are in phase with temperature fluctuations recorded in Greenland ice cores, indicating that early Holocene monsoon intensity is largely controlled by glacial boundary conditions. After approximately 8 ky B.P., monsoon precipitation decreases gradually in response to changing Northern Hemisphere summer solar insolation, with decadal to multidecadal variations in monsoon precipitation being linked to solar activity.

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Variations in the amount of solar radiation reaching the Earth are thought to influence climate, but the extent of this influence on timescales of millennia to decades is unclear. A number of climate records show correlations between solar cycles and climate, but the absolute changes in solar intensity over the range of decades to millennia are small and the influence of solar flux on climate is not well established. The formation of stalagmites in northern Oman has recorded past northward shifts of the intertropical convergence zone, whose northward migration stops near the southern shoreline of Arabia in the present climate. Here we present a high-resolution record of oxygen isotope variations, for the period from 9.6 to 6.1 kyr before present, in a Th-U-dated stalagmite from Oman. The delta18O record from the stalagmite, which serves as a proxy for variations in the tropical circulation and monsoon rainfall, allows us to make a direct comparison of the delta18O record with the Delta14C record from tree rings, which largely reflects changes in solar activity. The excellent correlation between the two records suggests that one of the primary controls on centennial- to decadal-scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation.

Oxygen isotope records of stalagmites from China and Oman reveal a weak summer monsoon event, with a double-plunging structure, that started 8.21 ± 0.02 kyr B.P. An identical but antiphased pattern is also evident in two stalagmite records from eastern Brazil, indicating that the South American Summer Monsoon was intensifi ed during the 8.2 kyr B.P. event. These records demonstrate that the event was of global extent and synchronous within dating errors of <50 years. In comparison with recent model simulations, it is plausible that the 8.2 kyr B.P. event can be tied in changes of the Atlantic Meridional Overturning Circulation triggered by a glacial lake draining event. This, in turn, affected North Atlantic climate and latitudinal position of the Intertropical Convergence Zone, resulting in the observed low-latitude monsoonal precipitation patterns. INTRODUCTION A prominent abrupt climate event ca. 8.2 kyr B.P. (the 8.2 kyr B.P. event) is evident in Greenland ice cores and other climate proxy records from across the Northern Hemisphere (Alley et al. , 1997). However, due to a distinct lack of highly resolved and precisely dated records, the timing, structure, and geographical extent of the event are not well documented (Alley and Ágústsdóttir, 2005;Rohling and Pälike, 2005). A key question is whether climatic anomalies ca. 8.2 kyr B.P. are indeed representative of one synchronous event with similar structure, different events, or of one time-transgressive event in different geographical regions (Alley and Ágústsdóttir, 2005). Precisely dated stalagmite records from Asia and Brazil can help to answer this question. In order to accurately characterize the 8.2 kyr B.P. event in Asian Monsoon (AM) and South American Summer Monsoon (SASM) records, we present new and revised sets of precisely dated stalagmite records from China, Oman, and Brazil. Improved 230 Th dating techniques with very small age errors of 15-45 years (2σ) for ca. 8.2 kyr B.P. allow us to: (1) correlate AM records from widely separate locations; (2) test the relationship between the AM and SASM and their correlations to Greenland ice core records on decadal time scales; (3) provide a benchmark for global correlation and age calibration of the event; (4) characterize the common structure of the 8.2 kyr B.P. event; and (5) probe the mechanism underlying the event in comparison with model simulations.

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Many palaeoclimate records from the North Atlantic region show a pattern of rapid climate oscillations, the so-called DansgaardOeschger events, with a quasi-periodicity of ,1,470 years for the late glacial period [1][2][3][4][5][6] . Various hypotheses have been suggested to explain these rapid temperature shifts, including internal oscillations in the climate system and external forcing, possibly from the Sun 7 . But whereas pronounced solar cycles of ,87 and ,210 years are well known [8][9][10][11][12] , a ,1,470-year solar cycle has not been detected 8 . Here we show that an intermediate-complexity climate model with glacial climate conditions simulates rapid climate shifts similar to the Dansgaard-Oeschger events with a spacing of 1,470 years when forced by periodic freshwater input into the North Atlantic Ocean in cycles of ,87 and ,210 years. We attribute the robust 1,470-year response time to the superposition of the two shorter cycles, together with strongly nonlinear dynamics and the long characteristic timescale of the thermohaline circulation. For Holocene conditions, similar events do not occur. We conclude that the glacial 1,470-year climate cycles could have been triggered by solar forcing despite the absence of a 1,470-year solar cycle.The onset of successive Dansgaard-Oeschger (DO) events, as documented in Greenland ice-cores 1,2 for example, is typically spaced by ,1,470 years or integer multiples thereof 13,14 . Because deviations from this cyclicity are small, often less than 100-200 years 15 , external forcing (solar or orbital) was suggested to trigger DO events 6,15,16 . However, neither orbital nor solar forcing shows a 1,470-year frequency. Spectral analysis performed on records of cosmogenic nuclides [8][9][10][11] , which are commonly used as proxies for solar variability 12 , indicates the possible existence of pronounced and stable 10,11 centennial-scale solar cycles (the DeVries-Suess and Gleissberg cycles with periods near 210 and 87 years 10,11 ) but does not reveal a 1,470-year cycle 8 . However, the DeVries and Gleissberg cycles are close to prime factors of 1,470 years (1,470/7 ¼ 210; 1,470/17 < 86.5). The superposition of two such frequencies could result in variability that repeats with a 1,470-year period.Here we propose that these two solar frequencies could have synchronized the glacial 1,470-year climate cycle. Support for the idea that a multi-century climate cycle might be linked with centuryscale solar variability comes from Holocene data: a multi-centennial drift-ice cycle in the North Atlantic was reported 17 to coincide with "rapid (100-to 200-year), conspicuously large-amplitude variations" in the production rates of the cosmogenic isotopes 14 C and 10 Be. To test our hypothesis, we force the coupled climate system model CLIMBER-2 (version 3) with the two solar frequencies. Earlier simulations with this model showed that, when forced by periodic and/or stochastic variations in the freshwater flux into the northern Atlantic, abrupt glacial warming events are triggere...

Speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods over the last 330,000 yr. Periods of rapid speleothem deposition occurred from 6000 to 10,500, 78,000 to 82,000, 120,000 to 135,000, 180,000 to 200,000, and 300,000 to 330,000 yr ago, with little or no growth during the intervening periods. During each of these five pluvial periods, δD values of water extracted from speleothem fluid inclusions (δDFI) are between −60 and −20‰ (VSMOW) and δ18O values of speleothem calcite (δ18OC) are between −12 and −4‰ to (VPDB). These values are much more negative than modern rainfall (for δD) or modern stalagmites (for δ18O). Previous work on the isotopic composition of rainfall in Oman has shown that northern and southern moisture sources are isotopically distinct. Combined measurements of the δD values of fluid-inclusion water with calculated δ18O values from peak interglacial speleothems indicate that groundwater was predominantly recharged by the southern (Indian Ocean) moisture source, when the monsoon rainfall belt moved northward and reached Northern Oman during each of these periods.

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