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Floods are among the most destructive natural hazards on Earth. In paleohydrology, sediments are generally considered as one of the best archives to extend flood records to pre-historical timescales. Doing so requires being able to identify flood deposits from sediment archives and decipher between flood types. The latter is particularly important in glacierized regions, where meteorological floods frequently co-occur with glacial lake outburst floods (GLOFs). In Patagonia, results from a recent study suggest that GLOFs are recorded in downstream fjord sediments as fine-grained and organic-poor layers, representing the high amount of glacier rock flour transported during lake outbursts, whereas meteorological floods are represented by coarser and more organic deposits. However, not all fine-grained organic-poor deposits could be associated with historical GLOFs. Here, we reconstruct the provenance of these Baker River flood deposits using Sr-87/Sr-86 and epsilon Nd, taking advantage of the clear lithological differences that exist between both sides of the watershed. Our results show that both Sr-87/Sr-86 and epsilon Nd are suited to reconstruct sediment provenance in the Baker River watershed but that epsilon Nd is the most effective and the least affected by grain-size variations. Our provenance results confirm that the 21st-century fine-grained and organic-poor deposits represent GLOFs and that the largest winter meteorological flood on record has a distinct coarse and organic-rich signature. However, our results show that rain-on-snow events that occur in summer, and therefore primarily affect the western glacierized part of the watershed, have the same fine-grained organic-poor signature as GLOFs. Therefore, this study shows that the sedimentary signature of rain-on-snow floods in partially glacierized watersheds depends on the season during which they occur. We anticipate that our findings will contribute to a better interpretation of flood records from partially glacierized watersheds.

Glacial Lake Outburst Floods (GLOFs) constitute a major hazard in glacierized regions. They are particularly pronounced in the Baker River watershed (Chilean Patagonia, 48 degrees S), where 23 events occurred between 2008 and 2020. Although GLOF deposits have previously been studied in lake settings, how modern GLOFs are recorded in fjord sediments remains mostly unknown. To address this issue, ten sediment cores collected in the fjord immediately downstream of the Baker River (Martinez Channel) were investigated and compared to the recent GLOF history of the river. Results show that sediments accumulate at 2.0 to 3.4 cm year(-1) and that GLOF deposits can be distinguished from background sediments by their finer grain size (5.98 +/- 0.82 mu m) and lower organic carbon content (0.31 +/- 0.06%), reflecting the release and transport in suspension of high amounts of glacial rock flour during GLOFs. Although 21 GLOFs from Cachet 2 Lake occurred between 2008 and 2017, the first events left a stronger imprint in the sediment, suggesting that more sediment of glacial origin was released during those initial events, possibly due to lake-bed erosion. An older GLOF deposit was tentatively linked to the outburst of Las Lengas Lake in 1988. The sediment cores also contain fine-grained turbidites, especially in the prodelta area. These turbidites confirm recent channel activity, but most of them seem to have been triggered by processes other than GLOFs. Overall, the results of this study suggest that GLOF deposits are distinct from typical flood turbidites. They are best identified by their low grain size and total organic carbon content, and best archived on the delta slope, away from any submarine channel influence. Finally, these results highlight the potential of fjord sediment archives to establish pre-historical GLOF records and ultimately improve GLOF hazard assessments.

Turbidites embedded in lacustrine sediment sequences are commonly used to reconstruct regional flood or earthquake histories. A critical step for this method to be successful is that turbidites and their trigger mechanisms are determined unambiguously. The latter is particularly challenging for prehistoric proglacial lake records in high‐seismicity settings where both earthquake‐generated and flood‐generated turbidites interrupt the background varved sedimentation. This calls for a new method to allow efficient and objective identification and classification of turbidites. This study examined turbidites in five long (9 to 17 m) sediment cores from Eklutna Lake, a proglacial lake in south‐central Alaska, using standard core logging and grain‐size data. A novel statistical approach is presented, in which varve‐thickness distributions were first analyzed to objectively identify the thickest turbidites and distinguish them from background sedimentation. For each turbidite, a selection of variables were then measured, including: basal grain‐size, thickness, magnetic susceptibility and spectrophotometric variables. Triggering mechanisms were discriminated by a combination of principal component analysis and clustering, and by calibration with historical events. Using this approach, a 2250 year long lake‐wide event stratigraphy was constructed, with 94 prehistoric events, including 24 earthquake and 70 flood events. Basal grain‐size and thickness variables turn out to be the most effective proxies for discrimination. This statistical approach is a powerful and new method to identify turbidites and their triggering mechanisms in long prehistoric sediment records. It opens up new prospects for palaeoseismological, palaeohydrological and palaeoclimate studies in proglacial lakes worldwide.

Glacial Lake Outburst Floods (GLOFs) constitute a major hazard in periglacial environments. Despite a recent increase in the size and number of glacial lakes worldwide, there is only limited evidence that climate change is affecting GLOF frequency. In Patagonia, GLOFs are particularly common in the Baker River watershed (47°S), where 21 GLOFs occurred between 2008 and 2017 due to the drainage of Cachet 2 Lake into the Colonia River, a tributary of the Baker River. During these GLOFs, the increased discharge from the Colonia River blocks the regular flow of the Baker River, resulting in the inundation of the Valle Grande floodplain, which is located approximately 4 km upstream of the confluence. To assess the possible long-term relationship between GLOF frequency, glacier behavior, and climate variability, four sediment cores collected in the Valle Grande floodplain were analyzed. Their geophysical and sedimentological properties were examined, and radiocarbon-based age-depth models were constructed. All cores consist of dense, fine-grained, organic-poor material alternating with low-density organic-rich deposits. The percentage of lithogenic particles, which were most likely deposited during high-magnitude GLOFs, was used to reconstruct the flood history of the last 2.75 kyr. Results show increased flood activity between 2.57 and 2.17 cal kyr BP, and between 0.75 and 0 cal kyr BP. These two periods coincide with glacier advances during the Neoglaciation. Our results suggest that GLOFs are not a new phenomenon in the region. Although rapid glacier retreat is likely responsible for high GLOF frequency in the 21st century, high-magnitude GLOFs seem to occur more frequently when glaciers are larger and thicker.

Snow avalanches cause many fatalities every year and damage local economies worldwide. The present-day climate change affects the snowpack and, thus, the properties and frequency of snow avalanches. Reconstructing snow avalanche records can help us understand past variations in avalanche frequency and their relationship to climate change. Previous avalanche records have primarily been reconstructed using dendrochronology. Here, we investigate the potential of lake sediments to record snow avalanches by studying 27 &lt, 30-cm-long sediment cores from Kenai Lake, south-central Alaska. We use X-ray computed tomography (CT) to image post-1964 varves and to identify dropstones. We use two newly identified cryptotephras to update the existing varve chronology. Satellite imagery is used to understand the redistribution of sediments by ice floes over the lake, which helps to explain why some avalanches are not recorded. Finally, we compare the dropstone record with climate data to show that snow avalanche activity is related to high amounts of snowfall in periods of relatively warm or variable temperature conditions. We show, for the first time, a direct link between historical snow avalanches and dropstones preserved in lake sediments. Although the lacustrine varve record does not allow for the development of a complete annual reconstruction of the snow avalanche history in the Kenai Lake valley, our results suggest that it can be used for long-term decadal reconstructions of the snow-avalanche history, ideally in combination with similar records from lakes elsewhere in the region.

Glacial Lake Outburst Floods (GLOFs) constitute a major threat in glacierized regions. Despite a recent increase in the size and number of glacial lakes worldwide, there is only limited evidence that climate change is affecting GLOF frequency. GLOFs are particularly common in the Baker River watershed (Patagonia, 47°S), where 21 GLOFs occurred between 2008 and 2017 due to the drainage of Cachet 2 Lake into the Colonia River, a tributary of the Baker River. During these GLOFs, the increased discharge from the Colonia River blocks the regular flow of the Baker River, resulting in the inundation of the Valle Grande floodplain, which is located approximately 4 km upstream of the confluence. To assess the possible relationship between GLOF frequency and climate variability, four sediment cores collected in the Valle Grande floodplain were analyzed. Their geophysical and sedimentological properties were examined, and radiocarbon-based age-depth models were constructed. All cores consist of dense, fine-grained, organic-poor material alternating with low-density organic-rich deposits. The percentage of lithogenic particles, which were most likely deposited during high-magnitude GLOFs, was used to reconstruct the flood history of the last 2.75 kyr. Results show increased flood activity between 2.57 and 2.17 cal kyr BP, and between 0.75 and 0 cal kyr BP. These two periods coincide with Neoglacial advances that are coeval with periods of lower temperature and increased precipitation. Our results suggest that GLOFs are not a new phenomenon in the region. Although rapid glacier retreat is likely responsible for high GLOF frequency in the 21st century, high-magnitude GLOFs seem to occur more frequently when glaciers are larger and thicker.

Dissolved silica (DSi) plays an important biogeochemical role in the fjords of northern Chilean Patagonia (44–48°S), where it drives high biogenic productivity and promotes carbon burial. It is generally believed that the DSi riverine input to lakes and coastal environments is controlled by a combination of factors including lithology, climate, topography, vegetation, and meltwater input. In northern Chilean Patagonia several authors have proposed that the postglacial volcanic ash soils (andosols) may play a significant role in the high supply of DSi to the regional fjords. To assess the influence of andosols on DSi concentrations in north Patagonian rivers, we mapped andosol thickness and compared our results with river chemistry. The mineralogical and geochemical composition of three representative andosol profiles was also examined to evaluate the efficiency of weathering processes. The andosol thickness map clearly demonstrates that volcanic ash was predominantly deposited on the eastern side of the regional volcanoes, reflecting the influence of the prevailing westerly winds on the distribution of pyroclastic material. Mineralogical and geochemical results show that the andosol parent material has the typical andesitic basaltic signature of the regional volcanoes, i.e. high amounts of amorphous material, plagioclase, K-feldspar, and pyroxene. Down-profile variations in soil mineralogy and geochemistry indicate increased leaching of silica with depth, resulting from weathering of the volcanic parent material. For the five studied watersheds, a highly positive correlation (R2=0.98) was found between average andosol thickness and DSi concentrations, suggesting that andosol thickness is the main parameter affecting DSi concentrations in north Patagonian river systems. On seasonal timescales, increased precipitation (winter) and glacial meltwater (summer) input can significantly reduce DSi concentrations. We argue that the weathering of andosols constitutes the most important source of DSi to the lakes and fjords of northern Chilean Patagonia, explaining the particularly high regional rates of biogenic silica production. Copyright © 2015 John Wiley & Sons, Ltd.

Chilean fjord sediments constitute high-resolution archives of climate and environmental change in the southern Andes. To interpret such records accurately, it is crucial to understand how sediment is transported and deposited within these basins. This issue is of particular importance in glaciofluvial Martinez Channel and Steffen Fjord (48°S), due to the increasing occurrence of Glacial Lake Outburst Floods (GLOFs) originating from outlet glaciers of the Northern Patagonian Icefield. Hence, the bathymetry of the head of Martinez Channel and Steffen Fjord was mapped at high resolution and the grain-size and organic carbon content of grab sediment samples were examined. Results show that the subaquatic deltas of Baker and Huemules rivers at the head of Martinez Channel and Steffen Fjord, respectively, are deeply incised (up to 36 m) by sinuous channels. The presence of sediment waves and coarser sediments within these channels imply recent activity and sediment transport by turbidity currents. Although several triggering mechanisms are possible, we argue that elevated river discharge and the associated relatively high suspended sediment loads is the main cause of the occurrence of turbidity currents at the head of these fjords and are of key importance in shaping the fjord's subaquatic morphology. This study shows that the heads of Martinez Channel and Steffen Fjord are dynamic sedimentary environments with rapidly migrating channels. It highlights the importance of site selection and multi-coring in any future project aimed at reconstructing environmental change using the sediments of either fjord.