THE EFFECT OF SEDIMENT TRANSPORT IN GLACIATED CATCHMENTS ON MULTIMINERAL DETRITAL GEOCHRONOLOGY: DECIPHERING CONTRASTING ZIRCON AND APATITE U-PB DATES

Detrital geochronology and thermochronology are widely used to study erosion of Quaternary landscapes. In glaciated regions, these have shown erosion to be highly heterogeneous; however, these studies have typically only used apatite as a heavy mineral indicator, a mineral prone to mechanical and chemical breakdown, and methods generally restricted by lower precision or data dispersion. As such, it is unclear if the detrital apatite signal represents derivation from the entire catchment. In this study, we present both zircon and apatite U-Pb dates from a modern detrital sample of the Saco River, New Hampshire, USA. The river flows through a catchment that was highly modified during the last glacial maximum. The underlying geology is characterised by two asynchronous and contrasting groups; Palaeozoic metasediments and granites, and Mesozoic granitoids. Zircon U-Pb dates are consistent with near-uniform erosion of the catchment, encompassing both Proterozoic–Palaeozoic and Jurassic–Cretaceous grains consistent with the catchment’s geology. However, detrital apatite U-Pb dates are predominantly Palaeozoic. Detrital U-Pb modelling shows that zircon grains are indeed likely derived from erosion of the entire catchment, while apatite likely only originates from areas at high elevation. This stark contrast in dates is likely an indicator of apatite’s fragility, breakdown during sediment transport, storage, and dissolution due to acidic soils in the river’s catchment. As such, the detrital apatite signal appears to only record erosion from the modern fluvial system, while zircon represents a collective uniform erosion of the catchment from both glaciation and from the modern system. These findings imply the detrital apatite signal is prone to significant modification in areas of sediment storage and acidic soils with implications for sediment generation in other formerly glaciated landscapes worldwide.