THE EVOLUTION OF WALDEN POND, MASSACHUSETTS SINCE DEGLACIATION: PALYNOLOGICAL EVIDENCE

Pollen and non-pollen palynomorphs preserve exceptionally well beneath the stratified waters of the small, deep lake that achieved fame through the writings of Thoreau. These organic-walled microfossils provide insights into the evolution of Walden Pond and the surrounding catchment since ice sheets retreated from northeastern Massachusetts. Spruce-rich sediments dominated by cysts of the dinoflagellate Parvodinium umbonatum record deep, oligotrophic conditions when meltwater filled the kettle. Through the Early Holocene, lake level fell in response to climate warming and peak aridity that produced pine parkland in the small catchment, and green algae (charophytes and chlorophytes) began to proliferate. A sharp increase in cyanobacteria marks influx of limiting nutrients via runoff and increasing water levels during the warm, wet conditions of the Middle Holocene are recorded by spores of the shoreline lycopod Isoetes, reworked during transgression. The dominant conifer in the resulting mixed forest was hemlock, except during the well-documented hemlock decline at the end of the Middle Holocene, and the drier conditions saw an increase in cysts of Peridinium and colonies of Botryococcus and other coccal chlorophyte algae. The wettest conditions of the Holocene are recorded by a resurgence of hemlock during the Late Holocene, and rapid shoreline transgression is recorded by abundant Isoetes spores. The sharp increase in non-nitrogen-fixing cyanobacteria such as Microcystis at the expense of nitrogen-fixing taxa (e.g. Anabaena) confirms that nutrients were not limiting during the moist Late Holocene. The correlation between algal palynomorph assemblages and climate change, recorded by pollen assemblages and quantitatively reconstructed using transfer functions, is evidence of the utility of the acid-resistant remains of primary producers in ‘pollen slides’.