A CONTEMPORARY MASS-BALANCE APPROACH TO SMALL-CATCHMENT DENUDATION RATES IN GLACIATED NEW ENGLAND

The glaciated, post-orogenic, metamorphic terrain of western New England is evolving slowly in the modern cool, moist climate. Most sediment and solutes transported from upland catchments likely originate in the mantle of glacial debris. In small catchments, water and sediment transit times are short, near-channel storage is minimal and biologic uptake and sequestration may approach a steady state. Thickening soil profiles, however, suggest that erosion is not keeping pace with chemical weathering. We use long-term hydrologic, chemical and sediment data from Hopkins Memorial Forest (HMF; Massachusetts/New York) and catchments at Hubbard Brook (HB; New Hampshire) and Sleepers River (SR; Vermont) to estimate sediment transport and solute export rates for small catchments that support old-growth forests. Annual export of inorganic sediment (3 to 7 T km^-2 yr^-1) and base cations + Si (~5 T km^-2 yr^-1) is low from silicate rocks. For carbonate-rich material (HMF; SR), base cations + Si export is ~10 to 15 T km^-2 yr^-1. Relatively long annual records at HMF and HB suggest the variability of export and hint at the importance of rare events. Measured values for chemical denudation do not vary substantially and reflect changes in annual runoff, modified over the past century by anthropogenic acid deposition. Annual sediment transport is highly variable and depends mainly on excess stream power and sediment availability. The legacy of recent glacial erosion and deposition is profound in North America and northern Europe. In New England a mantle of till and other glacial sediment 3 to 5 m thick covers bedrock in most places and headwater streams and hillslopes are developed in glacial deposits. Calculated bedrock lowering rates < 5 cm kyr^-1 are small compared to estimated glacial erosion during Quaternary time.