Seasonal Variations in Fjord Sediment Grain Size: A Pre‐requisite for Hydrological and Climate Reconstructions in Partially Glacierized Watersheds (Baker River, Patagonia). Issue 2 (6th February 2022)

• Record Type: Journal Article
• Authors: Amann, Benjamin; Bertrand, Sebastien; Alvarez‐Garreton, Camila; Reid, Brian
• Is Part Of: Journal of geophysical research. Volume 127:Issue 2(2022)
• Page Start: n/a
• Page End: n/a
• Publication Date: 2022-02-06
• Subjects: end‐member analysis -- sediment fluxes -- river suspended sediments -- fjord sediments -- meltwater -- temperature; Geomorphology -- Periodicals; 551.3
• DOI: 10.1029/2021JF006391
• Languages: English
• ISSNs: 2169-9003

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 (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 meltwater contribution is reduced, and is associated with rainfall. We show that the fluxes of EM1 and EM2 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 (EM1 /EM2 ) 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. Plain Language Summary: Sediments transported by rivers and deposited in fjords retain information about river discharge conditions and about the climate that prevails in the fjord watersheds. Fjord sediments are therefore frequently used to reconstruct past river discharge and climate conditions. Here, we show that the sediments deposited in the fjord in which Chile's largest river discharges are composed of a mixture of two grain size populations: fine particles originating from the melting of snow and glaciers, particularly in summer, and coarser particles representing sediments eroded during rainfall events. Our results reveal that the amount of fine particles deposited each day in the fjord represents the amount of water that melts from snow and glaciers. Similarly, the amount of coarse particles deposited in the fjord each day reflects the rain that fell in the watershed. To reconstruct changes in water originating from the melting of snow and glaciers in the past, we suggest measuring grain size on fjord sediment cores and using the ratio between the amount of fine and coarse particles. Since the melting of snow and glaciers depends on temperature, this ratio can also be used to reconstruct past changes in air temperature. Key Points: The grain size of fjord sediments deposited at the head of Martínez Channel reflects Baker River discharge Fluxes of the coarse and fine end members represent rainfall‐driven discharge and meltwater production, respectively Temporal variations in the proportions of the grain‐size end members can be used to reconstruct meltwater production and temperature