EROSION AND DEPOSITION FUNCTIONS CALIBRATED TO CHRONOSTRATIGRAPHIC DATA PREDICT TRANSIENT HOLOCENE – PRESENT SEDIMENT AGE AND STORAGE TIME DISTRIBUTIONS IN MID-ATLANTIC RIVER CORRIDORS

Storage in alluvial deposits determines the timing of sediment delivery and influences geochemical transformations of stored carbon and other constituents. Here we quantify time-dependent changes in stored sediment mass and age distribution using a mass-conserving mathematical model calibrated with chronostratigraphic data from mid-Atlantic river corridors. Age distributions are reconstructed from synthetic stratigraphic sequences based on empirical distributions of pre-Settlement sediment thickness and age, post-Settlement sediment thickness, and depths to 1880 and 1950 chronostratigraphic horizons. The resulting stratigraphic sequences can be used to reconstruct age distributions for any time in the past by “removing” younger deposits. For example, the age distribution of sediment in storage 1000 years B.P. is approximately exponential, with a mean age of 3500 years. The age distribution of contemporary deposits is skewed towards younger sediments as a result of dramatic post-Settlement valley alluviation, but is heavy-tailed due to preservation of rare early Holocene deposits. We predict these changes by solving a mass-balance equation for alluvial storage, a deposition equation driven by sediment supply and the frequency of overbank flow, and an erosion equation. The erosion equation is a function of stored sediment by age category, its exposure to erosion, and an erosion rate constant. Our results suggest that stored sediment has an equal probability of erosion regardless of age, and that erosion rates have been low throughout the Holocene, indicating that changes in stored sediment and its age distribution have been primarily driven by changes in sediment supply, overbank frequency, and amount of stored sediment available to erode. Our mathematical model, calibrated to chronostratigraphic data, represents a data-driven approach for predicting transient sediment storage timescales for alluvial river corridors.