Non-pollen palynomorphs and black carbon in sediments from Walden Pond (Massachusetts, USA): evidence of sudden onset events and human impact over the past millennium.

Abstract

Walden Pond (Concord, MA, USA) has long held important value from a cultural, ecological and geological perspective. It consists of three distinct subbasins where large-scale changes within the catchment and the lake are recorded in the acid-resistant organic fraction of the sediments. Palynological data from sediment cores in each of the three basins in Walden Pond record anomalous sedimentation events that can be correlated across the lake over the last 1000 years, and differences in the palynological signature of these events between sub-basins provide insights into depositional triggers. The youngest event, marked by abundant charcoal in palynological preparations, is attributed to late 19th – early 20th century CE fires, consistent with peak abundance of Ambrosia in the pollen assemblage. The non-pollen palynomorph (NPP) assemblage in this unit differs from that in the earlier three intervals of anomalous sedimentation in this deep, closed basin that pollen data help constrain to the mid- 18th, 14th–15th, and 12th–13th centuries CE, with additional potential redeposition events around 1000 CE and predating the turn of the second millennium. Earlier events are marked by increases in palynomorphs that record resedimentation from shallow parts of Walden Pond into deeper parts of the lake. The mid-18th century event is attributed to the 1755 Cape Ann Earthquake (estimated magnitude MLg 6.2 ± 0.5), supported by the first appearance of fly ash/ spheroidal carbonaceous particles (SCPs) produced by smelting of bog iron by early European settlers in the late 17th century as well as the rapid increase in Ambrosia /nonarboreal pollen (i.e., ‘agricultural weeds’) at a time of rapid population growth. Although less well constrained chronologically, the two earlier events appear to have been of even greater magnitude based on palynological and sedimentological characteristics that are consistent with seiche action in Walden Pond, and thus can be used to refine estimates of paleoseismic risk in this heavily populated region characterised by infrequent historic high magnitude earthquakes and suggestions of earlier events in oral histories.