A 1,000 year high-resolution hurricane history for the Boston area based on the varved sedimentary record from the Lower Mystic Lake (Medford/Arlington, Massachusetts)
The Lower Mystic Lake (Medford/Arlington, MA) is a 24 m deep, ectogenically meromictic, low elevation (1 m a.s.l.), coastal lake directly connected to Boston Harbor by the Mystic River. About 1,000 years ago, steadily rising sea level in the Boston area finally reached a point at which occasional marine water delivery events via the river channel could actually reach the lake basin. Since then, such events have continued with enough frequency to maintain the meromictic condition.
Meromixis has allowed the Lower Mystic Lake to accumulate an exquisitely laminated, annually resolvable (i.e. varved) archive of sedimentation in the lake over the last 1,000 years. A varve chronology was developed from this record. Multiple lines of robust evidence verify and validate the accuracy of the chronology.
A series of anomalous, graded beds was found in the stratigraphy, and they show excellent coordination with known historic hurricanes that have affected the Boston area. The graded beds appear to be the result of intense, hurricane-strength rains which cause erosive overland flow that entrains sediment which is carried into the lake where it is deposited as a graded bed. This is enhanced by hurricane-strength winds which disturb vegetation, and uproot trees to expose fresh, loose sediment. By analogy, similar graded beds in the prehistoric portion of the stratigraphy probably represent similar hurricane events.
This record of hurricane activity was compared to a record of sand layers in nearby Belle Isle Marsh (Boston Harbor) which are presumably the result of storm surge overwash events. Such sand layers in low resolution coastal archives are the main form of proxy evidence that paleotempestology studies have used to piece together longer term records of hurricane activity. Even when using multiple techniques, chronologic control in such archives is difficult, and linking sand layers to any particular storm is only tentative at best. Importantly, the Belle Isle Marsh record shows an apparent 600-year quiescent period. However, this is not due to a lack of hurricanes or storms, but to an unknown geomorphic change which affected the marsh, and it therefore serves as a cautionary note for the use of such archives.
0793: Freshwater ecology