MICHAEL SCHAAF1 & JLRGEN THUROW
Department of Geological Sciences, University College, Gower Street, London WC1E 6BT, UK (e-mail. j. thurow[@ucl. ac. uk)
1Present address: Shell International Exploration and Production, RTS, EPT-HM, Volmerlaan 8, PO Box 60, 2280 AB Rijswijk, The Netherlands
Abstract: We present a review of digital imaging techniques to obtain highest resolution colour/greyvalue time-series from geological material. From suitable sediments these methods can yield time-series with subannual resolution. They can be utilized for the detection of high-frequency (<300 years) climate change over long periods of time. Automated laminae counting produces a high-precision stratigraphy and data on the laminae thickness. The application of the methods to drill cores from the Santa Barbara Basin demonstrates the power of the techniques in detecting rapid climate change. El Nino/Southern Oscillation cycles as well as solar and lunar cycles could be detected from both, the greyvalue and the laminae thickness time-series. Evolutionary power spectra, calculated from the colour/greyvalue time-series, reveal dramatic variations in the power and frequency of rapid climate oscillations during the Holocene. Keywords: Santa Barbara Basin, Holocene, digital image analysis, climate, sediments.
For the human observer climate changes with two modes: those changes recorded during human lifetime or during written human history and those changes recorded only in natural archives, e.g., geological records. Examples for the latter, low-frequency changes, are tectonically (periods of 106 to 107 years) or orbitally induced changes, such as the Milankovitch cycles (periods of 104 to 106 years). Examples for the first, high-frequency changes, are the El Nino/Southern Oscillation cycles (Quinn & Neal 1992) or (possible) climatic impacts of variable solar activity (Stuiver & Braziunas 1992). Whilst the low-frequency changes have been the subject of scientific research since the 1940s, the superimposed highfrequency changes, which strongly affect human lifes and wealth, have only recently received strong scientific and public interest.
To understand the occurrence and mode of operation of high-frequency climatic change it is necessary to investigate long records of their past occurrence. Since instrumental and written records of climate are too short, such information has to come from geological archives. The best are annually laminated sediments. Lacustrine or marine laminated sediments are abundant world-wide. They can be easily retrieved from many lakes and coastal basins. Yet, long records from such sediments are scarce, due to two main problems: laminae are thin (usually 1 mm) and thus are very difficult to sample and if sampling of individual laminae is successful, a huge number of samples has to be processed to obtain long continuous records (>2000 years).
Digital image analysis techniques have recently become available that allow the fast and easy extraction of long time series of climatic variability from laminated sediments, with sub-annual resolution (Ripepe et al 1991; Bond et al. 1992; Schaaf & Thurow 1994, 1995; Merrill & Beck 1995). This paper aims to give an introduction into these techniques and, utilizing laminated sediments from the Santa Barbara Basin (SBB), demonstrate their use in the reconstruction of past climatic change. Stratigraphy
The study of high-frequency sedimentary cycles with periods of 100 years and less is only feasible if the stratigraphy for the studied material is excellent. The age error of the studied portion of the time-series has to be only a fraction (better then 1/3) of the length of the shortest cycle analysed. This error, however, is the 'internal' error for the portion of data for which a particular FFT should be calculated, not the 'external' error, that is, the absolute age (in 14C or calendar years) of this …