A 5000-yr record of climate change in varved sediments from the oxygen minimum zone off Pakistan, northeastern Arabian sea

The upper Holocene marine section from a kasten core taken from the oxygen minimum zone off Karachi (Pakistan) at water depth 700 m contains continuou sly laminated sediments with a sedimentation rate of 1.2 mm/yr and a unique record of monsoonal climatic variability covering the past 5000 years. Our chronostratigraphy is based on varve counts verified by conventional and A MS C-14 dating. Individual hemipelagic varve couplets are about 0.8-1.5 mm thick, with light-colored terrigenous laminae (A) deposited mainly during t he winter monsoon alternating with dark-colored laminae (B) rich in marine organic matter, coccoliths, and fish debris that reflect deposition during the high-productivity season of the late summer monsoon (August-October). P recipitation and river runoff appear to control varve thickness and turbidi te frequency. We infer that precipitation decreased in the river watershed (indicated by thinning varves) after 3500-4000 yr B.P. This is about the ti me of increasing aridification in the Near East and Middle East, as documen ted by decreasing Nile River runoff data and lake-level lowstands between T urkey and northwestern India. This precipitation pattern continued until to day with precipitation minima about 2200-1900 yr B.P., 1000 yr B.P., and in the late Middle Ages (700-400 yr B.P.), and precipitation maxima in the in tervening periods. As documented by spectral analysis, the thickness of var ve couplets responds to the average length of a 250-yr cycle, a 125-yr cycl e, the Gleissberg cycle of solar activity (95 yr), and a 56-yr cycle of unk nown origin, Higher frequency cycles are also present at 45, 39, 29-31, and 14 yr. The sedimentary gray-value also shows strong variability in the 55- yr band plus a 31-yr cycle. Because high-frequency cyclicity in the ENSO ba nd (ca. 3.5 and 5 yr) is only weakly expressed, our data do not support a s traightforward interaction of the Pacific ENSO with the monsoon-driven clim ate system of the Arabian Sea. (C) 1999 University of Washington.