GRAIN BY GRAIN: HOLOCENE STORMS AND HILLSLOPE EROSION IN NEW ENGLAND

To determine the history of storms and hillslope erosion in New England, we collected 23 sediment cores from small (~ 0.3- 2.2 km²) post-glacial lakes. The cores contain discreet, terrestrially derived, inorganic deposits identified by physical changes in the sediment including visual character, Loss-On-Ignition (LOI), and Magnetic Susceptibility (MS). In order to estimate the energy available for sediment transport into the lakes, we have begun measuring the grain size of inorganic sediments cm-by-cm in these cores.

We have completed grain size measurements (1000) in two continuous sediment cores, 5.6 m and 5.7 m in length, taken from Lake Morey in Fairlee, VT. One core was collected offshore; the other was collected closer to the shore near a large delta. The basal age of the delta core is 12,800 cal yr. BP coincident with the draining of glacial Lake Hitchcock that once covered the site.

At several locations in the delta core (0-25 cm, 120-140 cm, 270-310 cm) all three techniques (grain size, LOI, and MS) detected changes that we attribute to storms. At other locations down the core (150-325 cm), only grain size analysis revealed event layers. Overall, grain size analysis was the most sensitive tool for detecting event layers (32), as opposed to LOI (9) and MS (16). Grain size analysis detects all events identified by LOI and MS.

Steady background grain sizes contrast with well defined peaks and make grain size data easier to interpret than LOI. For example, mean and median grain sizes averaged over the whole delta core (16 and 18 mm, respectively) contrast with maximum mean and median grain sizes (128.5 and 137.9 mm, respectively). Graphical analysis revealed sharp platykurtic peaks with a large volume of coarse grains (~100 mm) corresponding to the well-defined peaks in mean and median grain size.

We found good correlation between the shallower core and the deeper core, but the distal fining in the deeper core made event detection more difficult. Our results suggest that grain size is a powerful tool for detecting subtle changes in lake core sediment. Recognizing these changes yields a more complete record of storms than other methods, and may reveal temporal storm patterns not previously documented.