Reconstructing the rate of accumulation of lake sediment: The effect of sediment focusing

Use of accumulation rates of pollen or minerals to infer regional history is complicated by nonuniform deposition of lake sediment. Sediment focusing, direction of sediment to the deepest part of a basin, can introduce a discrepancy between changes in accumulation rates measured directly from sediment cores and actual changes in influx of sediment or pollen to a lake. This difference depends on values and temporal variation of the ratio of mean depth to maximum depth in a basin as it fills. Several models of sediment accumulation show how measurements from a single core can be transformed to yield basinwide influx rates, and how the distortion due to sediment focusing can be assessed. Basins shaped like hyperboloids or frustums may introduce much greater distortions than basins conforming to ellipsoid or sinusoid shapes.