Spatial variability of long-term chemical weathering rates in the White Mountains, New Hampshire, USA


Constraining the range of chemical weathering rates in soils is important because weathering can have a strong influence on surface and ground water chemistry, soil nutrient release, neutralization of acidity, and preparation of rock for physical erosion. Determination of chemical weathering rates in soils developed from glacial till can be advantageous relative to other soils, because soil ages are more easily determined. To allow measurement of real differences in weathering rates across sites, rigorous exclusion criteria need to be applied to soil profiles to reduce uncertainty that may arise from soil parent material heterogeneity and soil disturbance. Soil profiles used for weathering rate calculations in this study were required to have the following characteristics: a) an overall decreasing concentration of the immobile element (Ti) with depth; b) Ti concentrations of no more than 1 standard deviation lower in the B horizon than in the C horizon; and c) at least three mineral soil samples collected from above the C horizon. In the White Mountains (New Hampshire), we applied these criteria to data from 39 soil profiles developed in glacial till derived from granitic and high grade metamorphic parent material. Soils were excavated quantitatively, thus eliminating errors associated with measurement of bulk density. The resulting long-term weathering rates for individual soil pits range from 2 to 50 meq m− 2 year− 1. Average weathering rates for sites with two or three profiles within 50 m of each other range from 10 to 30 meq m− 2 year− 1. Averaging weathering rates from sites within 5 km of each other results in “local rates” ranging from 13 to 22 meq m− 2 year− 1. An inverse correlation is observed between site elevation and weathering rate, which may reflect differences in temperature, precipitation or vegetation type. Our best estimate of the regional average long-term weathering rate for the White Mountains is 17 meq m− 2 year− 1, considerably lower than the 35 meq m− 2 year− 1 previously reported for a 12 ha catchment at the nearby Hubbard Brook Experimental Forest. Weathering rates determined from granitic glacial deposits of similar age elsewhere in the world are of the same order of magnitude (2 to 62 meq m− 2 year− 1) as the rates determined in this study.

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