A 5000-yr Record of Climate Change in Varved Sediments from the Oxygen Minimum Zone off Pakistan, Northeastern Arabian Sea

Quaternary Research - Tập 51 - Trang 39-53 - 1999
Ulrich von Rad1, Michael Schaaf2, Klaus H. Michels1, Hartmut Schulz1, Wolfgang H. Berger3, Frank Sirocko4
1Bundesanstalt für Geowissenschaften und Rohstoffe, Stilleweg 2, D-30655 Hannover, Germany
2Shell International Exploration and Production B.V. P.O.Box 602280, AB Rijswijk, The Netherlands
3Scripps Institution of Oceanography, University of California San Diego, La Jolla, California, 92093-0524
4GeoForschungsZentrum Potsdam, Telegrafenberg A50, D-14473 Potsdam, Germany

Tóm tắt

AbstractThe upper Holocene marine section from a kasten core taken from the oxygen minimum zone off Karachi (Pakistan) at water depth 700 m contains continuously 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 AMS14C dating. Individual hemipelagic varve couplets are about 0.8–1.5 mm thick, with light-colored terrigenous laminae (A) deposited mainly during the 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). Precipitation and river runoff appear to control varve thickness and turbidite frequency. We infer that precipitation decreased in the river watershed (indicated by thinning varves) after 3500–4000 yr B.P. This is about the time of increasing aridification in the Near East and Middle East, as documented by decreasing Nile River runoff data and lake-level lowstands between Turkey and northwestern India. This precipitation pattern continued until today 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 intervening periods. As documented by spectral analysis, the thickness of varve couplets responds to the average length of a 250-yr cycle, a 125-yr cycle, the Gleissberg cycle of solar activity (95 yr), and a 56-yr cycle of unknown 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 band (ca. 3.5 and 5 yr) is only weakly expressed, our data do not support a straightforward interaction of the Pacific ENSO with the monsoon-driven climate system of the Arabian Sea.

Tài liệu tham khảo

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Quaternary Research

Tập 51

39-53

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