Post-Glacial Chemical Weathering and Landscape Development at Sargent Mountain Pond, Maine, USA: A Multiscale Investigation

Perry, Randall

'Sargent Mountain Pond, Maine, USA (SMP) was deglaciated ca. 16,600 Cal Yr BP, forming one of Maine's first lakes following Wisconsinan glaciation. The lake sediment provides one of the region's longest continuous records of post-glacial terrestrial development. Early chemical weathering of apatite (Cas(PO4)3(F,Cl,OH)) from the granitic till caused elevated pH (ca. 6.5 - 7.5) and high concentrations of dissolved calcium (Ca) and phosphorus (P) in the catchment runoff and lake water, relative to present-day conditions. High pH and low soil dissolved organic carbon (DOC) concentrations kept aluminum (Al) and iron (Fe) relatively insoluble and immobile in the catchment, permitting the P released from early apatite weathering to be biologically available. Catchment vegetation developed at SMP ca. 12,700 Cal Yr BP, producing organic acidity (in the form of DOC) that mobilized Al and Fe within the developing catchment soil, and transported them to the lake. The Al and Fe precipitated as oxyhydroxides in the catchment soil and lake sediment, adsorbing dissolved P, thus reducing P bioavailability. The Younger Dryas (ca. 12,200 - 11,500 Cal Yr BP) briefly interrupted the DOC-Al-Fe P-sequestration system. Decreased Al and P sedimentation suggest that the SMP catchment underwent a significant decrease in DOC production, due to decreased temperature, and the lake likely reverted to higher pH and higher bioavailable P. Upon climatic amelioration, ca. 11,500 - 11,000 Cal Yr BP, the SMP system again became dominated by Al hydroxide sedimentation and P adsorption. As apatite became depleted from the active weathering zone of the catchment, the lake progressively acidified (present-day pH ~ 5.0) and the sedimentation of P, nearly all of which was biologically unavailable, decreased. Sediment lead (Pb) isotopes and declining concentration of detrital apatite suggest that apatite became much less abundant in the catchment weathering zone ca. 13,500 Cal Yr BP, relative to its early abundance immediately following deglaciation. Continued P sequestration in the sediment by adsorption (maximum ca. 10,600 Cal Yr BP) indicates that catchment apatite, while much less abundant, was still weathering, after detrital apatite delivery had slowed. Laboratory chemical weathering experiments on the Cadillac Mountain Granite (the dominant SMP catchment till lithology) provided a mineral-scale test for the catchment-scale interpretations from the sediment core. Accessory mineral weathering, predominantly the weathering of apatite, exerted disproportionately strong control on the overall rock weathering for the first four weeks (of 16 total weeks) of pH 2 mineral acid batch reactor weathering. Later solution chemistry was indicative of a weathering environment that was apatite-depleted and strongly controlled by hornblende and feldspar weathering. Geochemical modeling of the SMP catchment runoff using ALLOGEN (Boyle, 2007a) successfully predicted the trajectories of base cation concentrations and Pb isotopes in solution, as inferred from the sediment core and measured in the weathering experiments. The modeled runoff became progressively more acidic, dilute, and less radiogenic as catchment minerals (assigned based on Cadillac Mountain Granite petrology) weathered (in the rate order of apatite > hornblende > feldspar > quartz) and organic acidity was introduced to the system according to estimates of the timing of vegetation of the catchment. The model predicted complete apatite depletion from the catchment weathering zone within 2,000 yr. However, the sediment record suggests that apatite was relatively abundant for at least 3,000 yr, and likely for as many as 6,000 yr. (Abstract continued in Notes).