Stratigraphy of fossil pigments as a guide to the postglacial history of Kirchner Marsh, Minnesota

Successional developments in the postglacial ecology and limnology of Kirchner Marsh revealed by concentration and diversity of sedimentary plant pigments include a peak of aquatic production (indicated by maximum pigment concentrations in sedimentary organic matter) reached during the early postglacial warm, humid interval at about 10,000 B.P. Production levels then declined. Very low levels of aquatic production (indicated by low pigment concentrations) seem indicated by about 6,000 B.P., in the middle part of a warm, dry interval following which pigment concentrations remained low (except for a brief interval about 5,000 B-P.), but with a marked rise in the ratio of chlorophyll derivatives to carotenoids, indicating an increase in allochthonous organic input accompanying the return of cooler moister conditions. These resulted in an encroachment of oak forest on the prairie, followed by the succession to reed-swamp on the lake. A return to more truly aquatic conditions was reflected by an increase in sedimentary pigments from about 3,000 B.P. to a peak at 1,700 B.P., accompanied by a decline in the ratio of chlorophyll derivatives to carotenoids. Subsequently pigment concentrations fell, and their ratio increased sharply, as emergent aquatics became well established and caused a rapid evolution of pond to grass-sedge meadow. The pigment data are generally in harmony with the ecological conclusions from pollen and macrofossils, but they provide much more information on changes in aquatic productivity at the site. Ratios of chlorophyll derivatives to carotenoids, along with measurements of pigment diversity, reveal changes in the balance between autochthonous and allochthonous organic contributions to the sediment and provide substantial evidence for the presence or absence of open-water conditions.