Ulrich Neff

Strong coherence between solar variability and the monsoon in Oman between 9 and 6 kyr ago.

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 δ18O 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 δ18O record with the Δ14C 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.

Speleothem evidence from Oman for continental pluvial events during interglacial periods

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 18 O compared to modern speleothems. The d 18 O values for calcite deposited within pluvial periods generally fall in the range of 24‰ to 28‰ relative to the Vienna Peedee belemnite standard, whereas modern speleothems range from 21‰ to 23‰. 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.

A 780-year annually resolved record of Indian Ocean monsoon precipitation from a speleothem from south Oman

[1] Meteorological records of monsoon rainfall in the Indian Ocean are generally less than 100 years long. The relative brevity of these records makes it difficult to investigate monsoon variation on decadal and centennial timescales, to determine what factors influence the intensity of rainfall on these timescales, or to place possible changes in the twentieth century into a broader historical context. Development of a geologic proxy for rainfall that records annual variation in the monsoon over much longer time periods than are covered by instrumental records would be a significant step forward. We have developed an annually resolved record of monsoon rainfall variation for the past 780 years based on annual layer thickness and stable isotope analyses of a laminated stalagmite from southern Arabia. Our results show that monsoon variation over the past century is not outside of the range of the past 800 years. Decreasing monsoon rainfall over the past century is related to increasing sea surface temperature in the Indian Ocean. Spectral analyses of the record are dominated by cycles that are similar to those observed in records of solar activity on centurial timescales. Decadal to interannual cycles in the record appear to originate in the tropical Pacific Ocean. INDEX TERMS: 3344 Meteorology and Atmospheric Dynamics: Paleoclimatology; 3354 Meteorology and Atmospheric Dynamics: Precipitation (1854); 1620 Global Change: Climate dynamics (3309); 1699 Global Change: General or miscellaneous; KEYWORDS: Monsoon, paleoclimate, rainfall, Indian Ocean, speleothem, oxygen isotopes

Changing moisture sources over the last 330,000 years in Northern Oman from fluid-inclusion evidence in speleothems

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.

EXPLAINING DISCREPANT DEPTH PROFILES OF 234U/238U AND 230THEXC IN MN-CRUSTS

Abstract Manganese encrustations are an important archive for the reconstruction of deep ocean circulation in the past. However, because of discordant growth rates derived from the decrease of the activity ratios of 234 U/ 238 U and of 230 Th exc with depth, their dating via the measurement of depth profiles of 230 Th and 231 Pa has been recently put into question (Chabaux et al., 1997) . In this study we present high precision depth profiles of uranium and thorium isotopes (TIMS) of a hydrogenous Mn-crust from the South China Sea. Indeed, the depth profiles of 234 U/ 238 U and 230 Th exc deliver very different growth rates of 8.80 ± 1.20 and 2.64 ± 0.12 mm/Ma, respectively. We solve this discrepancy with a simple model which assumes exchange of uranium adsorbed in the Mn-crust with uranium dissolved in the pore water. The best agreement to the data is obtained applying an exchange coefficient of 5 x 10 −6 [a −1 ]. Application of this model to the data set of Chabaux et al. (1997) , reproduces very well their profiles of the δ 234 U. We conclude that 234 U/ 238 U dating of Mn-encrustations is not reliable because of open-system conditions for uranium.

Holocene Records of Rainfall Variation and Associated ITCZ Migration from Stalagmites from Northern and Southern Oman

Oxygen isotope (δ18O) profiles of uranium-series-dated stalagmites from Oman provide a record of Holocene Indian Ocean monsoon intensity at sub-decadal resolution over the past 10,000 years. The δ18O values are a proxy for the amount of monsoon precipitation, which is controlled by the mean summer latitudinal position and convection intensity of the Intertropical Convergence Zone (ITCZ). The longest stalagmite record, derived from a stalagmite (Q5) from southern Oman, continuously covers the period from 10,300 to 2,700 BP (before present) with an average temporal resolution of 4 to 5 years. Additional contemporaneously deposited stalagmites from northern and southern Oman are used to confirm the results obtained from Q5 time series. All stalagmite records indicate that changes in monsoon precipitation between 10,300 and 8,000 BP are in phase with high-latitude temperature fluctuations recorded in Greenland ice cores during the glacial-to-interglacial transition; this result indicates that late glacial and early Holocene monsoon precipitation and tropical convection are largely controlled by glacial boundary conditions. After the final melting of Northern Hemisphere ice sheets, monsoon precipitation decreases gradually in near linear response to changing Northern Hemisphere summer solar insolation. Comparison with the Cariaco Basin precipitation record, off the Venezuelan coast, indicates that post-glacial to modern precipitation patterns in the northern tropics are controlled on a global scale by the gradual southward migration of the ITCZ due to orbitally induced changes in insolation. At the decadal and centennial scales, fluctuations in monsoon rainfall appear to be driven primarily by variations in solar irradiance, with higher solar irradiance being correlated with higher monsoon rainfall.