Seasonal variations in the origin of river sediments (Baker River, Chile): A pre-requisite for climate and hydrological reconstructions

 

Fjord sediments are increasingly recognized as high-resolution recorders of past climate and hydrological variability. Using them as such, however, requires a comprehensive understanding of the variables that affect their properties and accumulation rates. 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 hydrodynamics of Baker River, Chile's largest river in terms of mean annual discharge. We apply end-member modeling to particle-size distributions of: (i) river suspended sediments, (ii) surface sediments collected along a proximal-distal transect at the fjord head, and (iii) fjord sediments collected in a sequential sediment trap at 15-day resolution during two consecutive years. Results show that the river suspended sediments and fjord sediments are consistently composed of two grain-size subpopulations. The finest end member (EM₁; mode 4.03 μm) reflects the meltwater contribution, which dominates in all but the winter season. The coarser end member (EM₂; mode 18.7 μm) dominates in winter, when the meltwater contribution is reduced, and is associated to rainfall events. We propose that log(EM₁/EM₂) can be used to reconstruct temperature in the lower Baker River watershed (r = 0.81, p < 0.001). We also show that the fluxes of EM₁ and EM₂ provide quantitative estimates of baseflow (r = 0.82, p < 0.001) and quickflow (r = 0.90, p < 0.001), respectively. These results support the use of fjord sediments for quantitative reconstructions of climate and hydrological changes in glacierized watersheds.