Recent and Postglacial Production Rates of a Marl Lake1

Pigment degradation products were analyzed spectrophotometrically in postglacial, 14 C‐dated sediments from the central depression and a marl lakemount of Pretty Lake in northeastern Indiana. Fluctuations in pigment products were examined in relation to the development of the productivity of the lake and to environmental parameters that might effect differential rates of preservation and deposition. Agreement was good between quantities of pigment degradation products and organic matter and other paleolimnological indices of productivity. There was a major peak of pigment products per gram of organic matter following the Boreal period, subsequent regression of productivity with several lesser peaks, especially during the millennium 6,500–5,500 B.P., and a slow increase in recent times. Contemporary rates of primary production were extrapolated to the inception of the lake. A general inverse relationship was evident between chlorophyll degradation products per unit organic content and carbonate levels. Changing levels of carbonate are discussed in relation to contemporary nutritional interactions and photosynthetic regulation. Rates of sedimentation at the central depression of the lake differed markedly from those of the littoral lakemount. While the rates in the central depression accelerated, those of the marl lakemount fluctuated sigmoidally, greatly accelerated during the interval 9,000– 7,000 B.P. The lakemount ceased accretion abruptly 1 m from the lake surface at 2,740 B.P. The probable ontogeny of marl lakemounts is discussed.