Abstract
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 CO 2 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”.
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