|
General geology |
1 Graduate School of Environmental Studies, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464–8601, Japan (e-mail: katsuta{at}eps.nagoya-u.ac.jp)
2 Faculty of Education, Gifu University, 1–1 Yanagido, Gifu 501–1193, Japan
3 Institute for Geo-Resources and Environment, National Institute of Advanced Industrial Science and Technology, Central 7, 1–1–1 Higashi, Tsukuba, 305–8567, Japan
4 The Kyoto University Museum, Kyoto University, Yoshida Honmachi, Sakyo-ku, Kyoto 606–8501, Japan
Environmental changes were flagged by lamination within Precambrian sedimentary rocks sequences using a high-resolution method. A continuous sample was collected and embedded in epoxy resin to form sample plates in order to investigate 2D laminations on sample surfaces. Elemental distributions in laminated samples were determined and recorded as XRF profiles, using the scanning X-ray analytical microscope (SXAM) and an image processing lamination tracer.
These methods were used to study a rhythmite sequence at the base of a Neoproterozoic cap carbonates, which immediately overlies glacial deposits in Namibia, to determine profiles of Si, Ca, Mn, Ti, Fe, and Sr. At the base and top of each interval, the concentrations of six elements were relatively constant. In the middle of the interval, Ca–Sr and Mn, which reflect presence of calcite and dolomite respectively, fluctuated. This reflects seventeen cycles. The cyclic fluctuations of Ca–Sr and Mn were interpreted as calcite–dolomite cycles. Strontium also recorded two cycles. Furthermore, the fluctuations of Ca–Sr and Mn in calcite–dolomite cycles were laterally traceable. Possibly, these calcite–dolomite cycles were deposited synchronously and regionally. New methods allow the extraction of data that record cyclic- and event-phenomena from long sequences of laminated sedimentary rocks; this is not isolated to the Neoproterozoic but also throughout Earth's history.
