Microbialite Formation in Seawater of Increased Alkalinity, Satonda Crater Lake, Indonesia: Discussion

Arp et al. (2003) present data which confirm and refine the results of our long-time studies (field campaigns of 1986, 1993, 1996) on Satonda Crater Lake (summarized in Kempe and Kazmierczak 1993). They reiterate the importance of marine stratified water bodies in generating, through sulfate reduction, excess alkalinity, which in turns causes very high CaCO3 supersaturation levels. This geochemical process, called the alkalinity pump (Kempe 1990), was conceptually refined using the quasi-marine Satonda Lake as a geochemical model for some epicratonic marine basins from the geological past (Kempe and Kazmierczak 1994). The concept of an alkalinity pump is important for understanding the abundance of calcified benthic cyanobacterial mats in ancient shallow seas leading to the formation of a variety of marine calcareous microbialitic structures and fine-grained calcareous sediments accumulated on the seafloor (Kempe and Kazmierczak 1990; Kazmierczak et al. 1996). While principally agreeing with Arp et al.'s results, we strongly disagree with two of their major conclusions. The first major objection concerns Arp et al.'s scepticism with respect to the calcification potential of benthic cyanobacterial mats presently living in the lake. On the basis of samples gathered in 1993 and 1996, Arp et al. conclude that these mats calcify very weakly or, in some cases, do not calcify at all. This sharply contradicts our observations showing strong permineralization of the mats both with CaCO3 and silica (e.g., Kempe and Kazmierczak 1993, Pl. 6, figs. 3-6). Mat samples we took at the end of dry season 1986 demonstrate that calcification of the cyanobacteria occurred in vivo. Masses of minute grains of CaCO3 (predominantly Mg-calcite) were observed not only on the mat surface but also within the common mucilage sheaths (glycocalyx) surrounding aggregates of living cyanobacterial coccoid cells (classified as representatives of Pleurocapsales). Minute CaCO3 grains …