ANTHROPOGENIC EFFECTS ON SEDIMENT BUDGETS MANIFESTED IN DETRITAL ZIRCON PROVENANCE SIGNATURES

U-Pb radiometric age dating of detrital zircons (DZ) provides a sediment provenance signature that is a function of 1) source U-Pb age distributions, 2) source zircon concentration and 3) relative volumetric contributions of sand-sized material from individual sources. Because calculating volumetric contributions requires knowledge of 1 and 2 for all source inputs, DZ data often proves difficult to use for quantitative sediment budgets. DZ data for the Monterey Bay depositional system of central California is sufficiently complete to use the system as a natural laboratory to demonstrate the application of DZ analysis to quantitative sediment budgets. Monterey submarine canyon, the system’s ultimate sediment sink, receives sediment from three regional rivers and minor coastal erosion. Sediment staged along the 250 km active canyon thalweg increases in age down system and represents on the order of 1,000 years of record. The largest sand-sized sediment input, the Salinas River, has decreased in relative contribution from 50-60% in natural state to approximately 20-30% over the last 100 years due to agricultural activity in the Salinas River Valley and sand mining at its mouth. A quantitative record of this decrease is seen in the detrital zircon U-Pb age distributions of 6 samples taken by submersible ROV along an axial transect of Monterey submarine canyon between 3,665 m and 0 m water depth. Because the DZ age distributions of all sediment inputs are known, statistically distinguishable, and of similar zircon concentration, weighting coefficients calculated from inverse Monte Carlo modeling of best fit source mixtures for canyon DZ age distributions describe relative contributions from sediment sources. Results suggest contribution of 52% Salinas River input for lower canyon samples representing the oldest active thalweg sediment. Calculated Salinas River input deceases systematically up canyon to an average of 17% for the upper canyon and canyon head beach samples, which represent modern sediment. This up canyon decrease in calculated Salinas River contribution closely matches the known decrease in Salinas River sediment input due to anthropogenic forcings, highlighting the utility of detrital zircons as markers in quantitative sediment budgets in systems with appropriate constraints.