Evidence for differential pollen preservation in late quaternary sediments in Minnesota

The state of preservation of fossil pollen grains in a variety of sediment types can be described by observing the proportions of grains in each of six preservation classes: (1) corroded, (2) degraded, (3) crumpled and exine thinned, (4) crumpled but exine normal, (5) broken, and (6) well-preserved. The numbers of indeterminable grains in each of these categories, plus a class for concealed grains, give a valuable indication of the reliability of pollen-grain counts. Counts of identifiable grains in each of the classes, moreover, reveal significant variation in the preservation both of different taxa in the same sample and the same taxon in different samples. In Late Glacial and Early Postglacial sediment from a lake in Minnesota, the mode of preservation is correlated with lithology. Degraded pollen grains are common in silt, crumpled pollen grains are dominant in algal copropel, and corroded pollen grains predominate in moss peat. Populus pollen is relatively well preserved in samples in which many other types (Alnus, Betula, Ulmus, Fraximus) are highly corroded. In other samples, Populus, Ulmus, and Gramineae pollen grains are badly crumpled but other types are well preserved. The data suggest that Populus pollen grains disappear from sediments through a progressive thinning and crumpling of the exine, rather than through fragmentation or corrosion of the grains.