Abstract

Patagonian fjord sediments are increasingly used as high-resolution archives of past climate and environmental change, including variations in glacier mass balance, flood frequency, and seismic activity. To accurately interpret these proxy records, it is crucial to comprehend modern day sedimentation processes and determine the provenance of the sediments. With this in mind, the main objective of this study is to identify parameters that can be used to reconstruct sediment provenance in the fjords of Chilean Patagonia. We focus on the Baker-Martinez fjord system, which is located between the Northern and Southern Patagonian Icefields and seems particularly sensitive to climate change. This fjord system connects the terrestrial ecosystems of Patagonia with the SE Pacific Ocean, and most of its sediment originates from glacier-fed rivers draining either the Patagonian Batholith (PB; Baker and Huemules rivers) to the north, or the Eastern Andes Metamorphic Complex (EAMC; Bravo and Pascua rivers) to the south. Eighteen surface sediment samples from across the Baker-Martinez fjord system and 44 suspended sediment samples from two sequential (Baker fjord) and one continuous (Steffen fjord) sediment trap were investigated. In addition, we analyzed suspended sediment samples collected at the mouths of the four main rivers that drain the PB and EAMC to define end-members. We focus on mass-specific magnetic susceptibility (MS) and inorganic geochemistry, which seem to be particularly promising in this fjord system dominated by lithogenic sediments (97–85 wt%). Our results indicate that sediments derived from the PB are characterized by high MS, Ti/Al, and Fe/Al values, reflecting the granodioritic nature of the batholith (rich in pyroxene and amphibole). In contrast, sediments from the southern EAMC-derived rivers have significantly lower MS, Ti/Al, and Fe/Al values. The sediment trap results reveal MS values that increase with increasing Baker river discharge, either during the summer melt season, or during high precipitation events (rain-on-snow) in winter. Likewise, the MS, Ti/Al and Fe/Al values of the fjord surface sediments are also directly related to sediment provenance. In fjords fed by only one river (e.g., Martinez channel), however, the MS is significantly correlated with mean grain size (r = 0.90; p < 0.01) and with the proportion of lithogenic particles (r = 0.73; p < 0.05). The latter observation means that future research is needed to correct the MS and geochemical data for grain-size before using them as quantitative provenance indicators. This study suggests that, after grain-size correction, MS and inorganic geochemistry (Ti/Al and Fe/Al) can be used to reconstruct sediment provenance within the Baker-Martinez fjord system. Ultimately, applying these provenance indicators to long sediment cores from the same fjord system will allow us to reconstruct variations in the behavior of outlet glaciers from both icefields independently.

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