Geobiology of stromatolites - GWDG

Keynotes International Kalkowsky-Symposium „Geobiology of Stromatolites“, October 4-11, 2008
stromatolitic lamination that consists of dense summer layers and porous winter
layers. The annual rhythm of the lamination can be converted to precise time-scale
of the tufas, which raises the potential as paleoclimatic archives (Andrews 2006). A
study at Shirokawa (Ehime Prefecture) hypothesized that the lamination has been
largely attributed to a seasonally variable inorganic precipitation rate of calcite
(Kano et al. 2003) that is generally high in summer and low in winter. This seasonal
pattern in precipitation rates is a common feature at many other tufa-depositing
sites (Kawai et al. 2006). However, clearness of the lamination varies depending on
sites, and the rule observed at Shirokawa was not always established in the other
sites.
Here, we examined the rate-controlled hypothesis by using two quantitative data
sources; 1) calcite packing-density (CPD) of measured by electron microprobe
analyser (EPMA) for the portions segmented in monthly resolution, and 2) the
precipitation rate of calcite (PWP rate) calculated from water chemistry. The results
for four tufa-depositing sites in SW Japan show a positive correlation between
CPD and PWP rate, but the correlation becomes less certain in one site where
PWP rate was generally high (Shimokuraida, Okayama Prefecture; Fig. 1). In the
temperature realm of SW Japan, tufas develop regular distinct seasonal change in
CPD when PWP rate is lower than 2.5 (x 10-10 M/sec.cm2) that roughly corresponds
to the Ca concentration less than 65 mg/L. On the other hand, even with
high PWP rate, the CPD of tufa deposits rarely exceeds 65 %, owing to pore space
between fine-grained calcite crystals and to openings derived from decomposed
cyanobacteria and other microorganisms. By increasing the Ca concentration to
more than 65 mg/L, the CPD often attains an upper limit and becomes insensitive
to seasonal changes in the PWP rate. Therefore, seasonal variation in CPD at sites
with a higher Ca content are unclear, as observed in two examples from tropical
islands in southern Japan and in one locality in temperate climate. Thus, the ratecontrolled
hypothesis was revised.
24
Fig. 1. Cross plot of CPD and PWP rate.
The dashed lines show the range of the
general trend. CPD increases with PWP
rates of