Online

Kraft, Matthew P

Geology

2018

MS

Sediment accumulated in lakes stores valuable information about past environments and paleoclimatological conditions. Cores previously obtained from Saint Albans Bay, located in the Northeast Arm of Lake Champlain, VT record the transition from the Champlain Sea to Lake Champlain. Belrose (2015) documented the presence of a peat horizon separating the sediments of the Champlain Sea from those of Lake Champlain. Initially, this layer was thought to comprise the transition from the marine environment of the Champlain Sea to a freshwater wetland. However, based on the results from this study, the transition between marine and freshwater conditions is thought to be represented by an erosional unconformity, indicative of a lowstand at the end of the Champlain Sea period. For this study, five additional cores were collected from Saint Albans Bay along a transect following the long axis of the bay moving into progressively deeper water. These cores better constrain the spatial extent, thickness and age variability of the peat layer within the bay and allow us to better understand the environmental conditions that preceded the period of peat deposition. In each of the cores there is evidence of sediment reworking in the uppermost Champlain Sea sediments, indicated by the presence of coarse-grained sediment, which is suggestive of a lowstand at the end of the Champlain Sea period before the inception of Lake Champlain. This coarse-grained layer is immediately overlain by a thick peat horizon. The widespread occurrence of the peat layer points to a large wetland that occupied the entire inner portion of Saint Albans Bay, and lake level ~ 9 m lower than at present during the Early Holocene. Based on radiocarbon dating, this paleo-wetland existed in Saint Albans Bay from ~ 9,600-8,400 yr BP. The development of this wetland complex is time transgressive, reflecting rapidly increasing lake level during the Early Holocene. This hypothesis is supported by the basal peat radiocarbon dates, as well as by the composition of plant macrofossils recovered from the peat horizons. The shift from peat deposition to fine-grained, low organic content lacustrine sedimentation is believed to have occurred at ~8.6-8.4 ka and is likely the result of continued isostatically driven lake level rise coupled with a changing climate. Although it was not its primary focus, this study also seeks to address the variations in sediment composition in the Lake Champlain sections of the cores. Evidence from the Lake Champlain record in Saint Albans Bay indicates that there were notable fluctuations in sedimentation, which were likely linked to both climatic variations and a change in the morphology of the bay. The rebound in productivity from ~8-5 ka is likely the result of warmer conditions during the Hypsithermal period. An increase in terrigenous sedimentation during this same time suggests a change in the morphology of the bay in which the Mill River delta migrated towards the inner bay. Initially, the cooler conditions of the Neoglacial are reflected in Saint Albans Bay by a decrease in organic matter content from ~5-3 ka. During the latter part of the Neoglacial (~3-1 ka), increases in organic matter content and detrital input point to enhanced productivity in response to increased precipitation and runoff from the watershed. The most recently deposited sediments in Saint Albans Bay bear out the legacy of anthropogenic nutrient enrichment of the bay in the form of increased algal productivity.