Seasonal Variations in Fjord Sediment Grain Size: A Pre‐requisite for Hydrological and Climate Reconstructions in Partially Glacierized Watersheds (Baker River, Patagonia)
Benjamin Amann, Sébastien Bertrand, Camila Álvarez-Garretón, Brian Reid
Abstract
Abstract Fjord sediments are increasingly recognized as high‐resolution archives of past hydrological and climate variability. Using them as such, however, requires a comprehensive understanding of the variables that affect their accumulation rates and properties. Here, we conduct a spatial and temporal study of sediment samples collected at the head of Martínez Channel (Chilean Patagonia, 48°S), to understand how the fjord's sediments register changes in the hydrology of Baker River, Chile's largest river in terms of mean annual discharge. We apply end‐member modeling to particle‐size distributions of: (a) river suspended sediments, (b) surface sediments collected along a proximal‐distal transect at the fjord head, and (c) fjord sediments collected in a sequential sediment trap at 15‐day resolution during two consecutive years. We then validate the use of the grain‐size end members for hydrological and climate reconstructions, using a sediment core that covers the last 35 years. Results show that the river suspended sediments and fjord sediments are consistently composed of two grain‐size subpopulations. The finest end member (EM 1 ; mode 4.03 μm) reflects the meltwater contribution, which dominates in all but the winter season. The coarser end member (EM 2 ; mode 18.7 μm) dominates in winter, when meltwater contribution is reduced, and is associated with rainfall. We show that the fluxes of EM 1 and EM 2 provide quantitative estimates of baseflow ( r = 0.87, p < 0.001) and quickflow ( r = 0.86, p < 0.001), respectively. Additionally, we propose that log (EM 1 /EM 2 ) can be used to reconstruct meltwater production ( r = 0.67, p < 0.001) and temperature ( r = 0.81, p < 0.001) in the lower Baker River watershed. These results support the use of fjord sediments for quantitative reconstructions of hydrological and climate variability in partially glacierized watersheds.