The influence of climate on clay genesis (rates and mineralogy) in the southern blue ridge mountains: geochemical mass balance and implications for mudrock mineralogy as an indicator of paleoclimate.

Disparity exists between researchers who find detrital clays as an accurate proxy for continental (source area) paleoclimate interpretations, and those who express caution with respect to the use of clay mineralogy in retrospective analysis of climate. A means of gaining insight into this debate is by actually calculating rates of clay genesis, and identifying and quantifying clay mineral assemblages, in present-day terrestrial weathering environments where climate is known. Source areas for terrigenous clay sediment may be placed along a continuum between two end-members;source areas are characterized by steep slopes where transport processes are more rapid than the weathering processes generating the clay, and transport-limited on gentle slopes whereby the maximum weathering rate exceeds the ability of transport processes to remove sediment. The latter pose the greatest concern for paleoclimatologists trying to use clay mineral assemblages from mudrocks and sediments to in erpret source area climate changes through geologic time. These concerns arise because clay detritus eroded from a transport-limited landscape may originate from paleosols, and not reflect the climatic conditions during erosion and transport of the clay to the depositional basin. A transport-limited environment may require several million years to develop a deep mature profile, and under such circumstances only long-term (1 or 2 Ma) paleoclimatic changes may be recorded in the clay mineral assemblages of the sedimentary deposits. | It is hypothesized that for the study area of the U.S. Forest Service Coweeta Hydrologic Laboratory (CHL), that the mass of a given clay produced per unit volume of saprolite per time is climate dependent. Furthermore, the weathering profiles developed on the hillslopes of CHL will possess characteristic clay mineral assemblages that also reflect climate and hydrology. | Coweeta Hydrologic Laboratory provides a unique opportunity to evaluate the temporal resolution of clay mineral assemblages in paleoclimatic reconstructions, as CHL is characterized by relatively high rainfall (200 cm/year), substantial erosion rates (3.8 cm/1000 years), a thick (6 m) modern (100 Ka) regolith, and extensive hydrochemical data for use in mass balance determinations of clay genesis rates. CHL also permits the calculation of rates of clay genesis in saprolite developed under different climatic regimes (i.e., catchments located at significantly different elevations), but with other variables held constant (e.g., bedrock, aspect, vegetation, weathering duration, etc.). Most important is that weathering (and clay genesis) at CHL is occurring at the present time. | In terms of the weathering continuum outlined above, CHL provides intermediate data between the two end-members, and, through calculation of clay genesis rates, will permit determination of the resolution of the paleoclimatic record in deposits of clay minerals derived from source areas similar to CHL. This study will calculate rates of clay formation, and identify clay mineral assemblages, for weathering profiles (saprolite) which only vary by climate. Such data are essential if the clay mineral assemblages of ancient mudrocks and sediments are to permit accurate paleoclimatic interpretations.Coweeta Hydrologic Laboratory was chosen for the study because it maintains a continuous collection of data needed for the mass balance equations used for calculating clay genesis rates, including precipitation data (with solute chemistry), biomass data, stream discharge, and stream solute data. This study will utilize two pairs of watersheds. For a given pair, one watershed will be located at a significantly different elevation (and hence climate) than the other with all other variables being identical. Watershed (WS) 34 (low flushing rate and lower temperature) will be paired with WS 2, as both overlie Tallulah Falls Formation bedrock. WS 36 (high flushing rate and lower temperature) will be paired with WS 18, as both straddle the Shope Fork Fault, and hence overlie both Tallulah Falls Formation and Coweeta Group bedrock. This investigation will evaluate watersheds of all combination of temperature and flushing rate; that is, low temperature and low flushing rate (WS 34), higher temperature and higher flushing rate (WS 2), very low temperature and very high flushing rate (WS 36), and higher temperature and low flushing rate (WS 18). | Field sampling will be performed within the applicable watersheds on as complete weathering profiles as possible. Ideally, bedrock, a thick sequence of saprolite, and soil will all be present. Samples may be collected from outcrops, road cuts, and/or trail cuts, possibly also including use of a hand auger or soil probe. When vertical profiles are available, regolith may be sampled as frequently as every 10-20 cm through the section. Samples will be collected by driving a sampling tube (approximately 4 cm in diameter and 8 cm long) into the weathering profile. The bedrock underlying watersheds 2, 18, 34, and 36 is heterogeneous, and hence sampling will be performed spatially in such a manner that allows weathering profiles on similar (hopefully identical) bedrock to be compared. Lithologic variability will be evaluated using geologic maps and bedrock descriptions published by Robert Hatcher. | Based on the above discussion, a small quantity of vegetation at each sample location will need to be removed in order to gain access to the regolith and bedrock. Penetration into a vertical exposure will likely be no deeper than 30 cm, and will be performed one time only. Sampling performed by hand coring will only disrupt an area the diameter of the probe. The cored hole will be backfilled upon completion of sampling. Based on previous experience in the southern Appalachians, even vertical exposures of saprolite and soil revegetate quickly. No hazardous materials are involved during sampling, nor any special safety consideration, other than basic field safety