Lake Reeve is a coastal back-barrier lagoon that has been deprived of significant amounts terrigenous sediment for up to 4000 yr. During most of this period there has been no significant tidal or marine influence, and the lagoon is now schizohaline. Rainfall is seasonal: more than 50% is during June-August. Annual evaporation is 2.5 times precipitation, causing the lagoon to be intermittent, drying up during the summer. These conditions, plus the abundance of photosynthetic organisms, lead to widespread carbonate mud accumulation throughout most of the lagoon. This mud, up to 30 cm thick in some parts of the lagoon, is typically in the form of < 2 mu m crystals. Unlike other carbonate-accumulating back-barrier lagoons, most of the CaCO 3 is low-Mg calcite with some skeletal aragonite. High-Mg calcite is present in two small lakes within the Pleistocene barrier immediately landward of Lake Reeve, and no dolomite was detected. We interpret the carbonate mud to have resulted from the activities of cyanobacteria, which are abundant through much of Lake Reeve.