Chemical Geology, 81 (1990) 299-310 299 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands Chemistry and stromatolites of the sea-linked Satonda Crater Lake, Indonesia" A recent model for the Precambrian sea? STEPHAN KEMPE and JOZEF KAZMIERCZAK Institute of Biogeochemistry and Marine Chemistry, University of Hamburg, D-2000 Hamburg 13 (Federal Republic of Germany) Institute of Paleobiology, Polish Academy of Sciences, PL-02-089 Warsaw (Poland) (Received February 22, 1989; revised and accepted September 20, 1989 ) Abstract Kempe, S. and Kalmierczak, J., 1990. Chemistry and stromatolites of the sea-linked Satonda Crater Lake, Indonesia: A recent model for the Precambrian sea? Chem. Geol., 81: 299-310. The abundance of in situ calcified stromatolites in the Precambrian paleontological record and their absence from modern seas is still an enigma. We report on the discovery of the first recent, in situ calcified stromatolites growing in a sea-linked environment. The stromatolites are produced by coccoid cyanobacteria and started to grow 4000 yr. ago in the Crater Lake of Satonda Island, Indonesia. Compared to seawater, the lake has gained alkalinity and lost Ca. Its pH (8.45) as well as its saturation with respect to calcite is significantly higher than in seawater. The additional alkalinity is provided by biogenic C02 and subsequent weathering of volcanic silicates. Satonda Crater Lake may provide an analogue to the stromatolite-sustaining Precambrian marine environments which, therefore, may have been more alkaline than present-day seawater. This conclusion is congruent with the recently advanced hypothesis of an early "soda ocean". 1. Introduction The chemistries of the Precambrian ocean and atmosphere have been linked (e.g., Hol- land, 1984). The standard model suggests an early atmosphere of high Pco2 (Hart, 1978; Kuhn and Kasting, 1983). An ocean in equilib- rium with such an atmosphere should have been acidic or at least lower in pH than modern sea surface waters, having a pH close to 8.2 (Con- way, 1943; Wedepohl, 1963; Garrels and Mac- kenzie, 1971; Maisonneuve, 1982; Walker, 1983). Contrarily, we proposed that the early ocean was alkaline (pH >8.5) on the following grounds (Kempe and Degens, 1985; Kempe et al., 1989); (1) weathering of silicates rapidly consumes free CO2 in the presence of water (Urey reaction ) (Urey, 1951; Wollast and Chou, 1985); (2) rate of modern silicate weathering on continents amounts to 0.1-1015 g yr. -1 C (total atmospheric volume today is only 700-1015 g C) (Kempe, 1979); (3) modern al- kaline lakes ("soda lakes") occur in volcanic regions where both unweathered silicates and excess CO2 are abundant (e.g., Kempe, 1977; Eugster and Hardie, 1978; Kempe and Degens, 1985; Kempe et al., 1989); (4) Precambrian sediments including dolomite, limestone, jas- pilite and Fe- oxides and -silicates can form un- der alkaline conditions (Eugster, 1967, 1969; Tucker, 1982); and (5) earliest life forms in-