Flooding is a ubiquitous natural hazard, and climate change is likely to exacerbate the risks worldwide. Mountainous areas, such as the densely populated Alps, are of particular concern because topography and atmospheric conditions can lead to large, flash floods, and because they are experiencing a high rate of warming, which will likely result in more intense precipitation events.

In his latest publication, Bruno Wilhelm and his colleagues have compiled 33 paleoflood records to test the impact that these climate trends might have on the frequency and magnitude of floods in the Alps. The paleoflood records are derived from lake sediments, the only archive that guarantees continuity of records over long periods of time. The dataset passed a selection procedure of hydrological, sedimentary and geochronological controls, resulting in a final selection of 27 records that continuously cover the last 150 to 10,000 years, documenting a total of 7,792 floods. We perform three analyses with this dataset:

(i) analogous to modern projections that assess changes in climate variables between current and warmer future conditions, we determine changes in the occurrence of large (≥10 year) floods between cooler and warmer past subperiods.

(ii) following trend analysis techniques using modern streamflow data, we analyze trends in the occurrence of large (≥10-year) floods during warmer or cooler periods.

(iii) we replicate the first analysis by considering the occurrence of extreme floods (≥100 years) between colder and warmer past subperiods.

From these analyses, we show that a warming of +0.5-1.2°C led to a 25-50% decrease in the frequency of large floods (return period ≥10 years). This downward trend is not observed in records spanning less than 200 years (i.e., the maximum period of the instrumental series), but it is persistent in those ranging from 200 to 9000 years. In contrast, extreme floods (return period > 100 years) may increase with a similar degree of warming in some small Alpine catchments. This may result from a local intensification of extreme precipitation with higher temperature. The latest results of Bruno and his colleagues show how long and continuous paleoflood records can be used to unravel the complex relationships between climate and flooding.

How to cite: Rapuc, W., Wilhelm, B., Amann, B., Anselmetti, F. S., Arnaud, F., Blanchet, J., Brauer, A., Czymzik, M., Giguet-Covex, C., Gilli, A., Glur, L., Grosjean, M., Irmler, R., Nicolle, M., Sabatier, P., Swierczynski, T., and Wirth, S. B.: In Memory of Bruno Wilhelm: Impact of warmer climate periods on flood hazard in the European Alps, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16228, https://doi.org/10.5194/egusphere-egu23-16228, 2023.

On April 7^th 2022 we learnt the devastating news: our friend and colleague, Bruno Wilhelm, had passed in the mountain while ascending a steep slope in the aim of skiing it. One year after, our group, within which he came and grow to science up to the defence of his PhD thesis in 2012, is still graving. In that context, we are grateful to the organisers of this session for offering us the opportunity of reminding our scientific community the multiple advancements it owes to Bruno.

Bruno Wilhelm indeed did a hard and innovative work in the aim of reading the geological record of lakes. He focused himself on the recording of extreme events in lake sediments. In this keynote, we will emphasize how Bruno proposed original ways of characterising what we call “instant deposits”, mostly in the early stages of his too short career. He then particularly worked the question of distinguishing among triggering factors of those “alien” layers that often interbed within lake sediment calm sedimentation. Bruno also proposed a detailed reflexion aiming at quantifying or, at least, ranking the intensity of those triggering events. He hence proposed several ways of assessing past floods intensity. In the same spirit, he explicitly proposed a procedure to assess the “recordability” of past earthquake events within a given lake system, based on historical chronicles. He also led a common reflexion about the ability of lake sediment to record a regional flood signal. After his PhD he committed himself in the scientific animation of the PAGES’ “Floods” working group which led him to coordinate the establishment of a comprehensive database and to participate in the redefinition of a collaborative work to better understand past flood patterns in the iam of anticipating future changes due to global warming.

It is undeniable that Bruno Wilhelm had a brilliant and far too short career. However, with this speech we hope that the community will be even more aware about the important legacy he left us.

How to cite: Arnaud, F., Sabatier, P., Poulenard, J., Giguet-Covex, C., Jenny, J.-P., Messager, E., Delannoy, J.-J., Malet, E., Pignol, C., Allignol, F., and Wilhelm, B.: A tribute to Bruno Wilhelm’s legacy in reading the geological record of lakes, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-16653, https://doi.org/10.5194/egusphere-egu23-16653, 2023.

Mass transport deposits (MTDs) are common features in lakes that impose a significant natural hazard as they can be tsunamigenic (e.g., in Lake Geneva; Kremer et al., 2012). MTDs can be assigned to different categories, such as lateral-slope landslides, margin collapses, delta collapses and rockfalls (Sammartini et al., 2019). In 1996 AD, a MTD occurred in Lake Brienz, a 260 m deep Alpine lake at the frontal range of the Bernese Swiss Alps. The 1996 MTD extends over ∼8.5 km² and has a total volume of 2.7 10⁶ m³, which amounts to ∼8.7 years of the lake’s annual sediment input. The 1996 MTD had no specific trigger and occurred as a spontaneous failure due to sediment accumulation on the Aare Delta (Girardclos et al., 2007).

However, the 1996 delta collapse was not a unique event: a dense grid of 240 km of seismic lines reveals at least four older massive MTDs in the lake basin with similar dimensions. Our study aims to identify the ages and the processes behind these large deposits. For this purpose, we focus on long sediment cores with a cumulative length of about 40 m from four coring locations. A combined analysis of seismic profiles and sediment cores, coupled with an age model combining radionuclides, radiocarbon, coal and steamboat slag, and historical flood events, allows us to explore the feedback between climate and anthropogenic impact acting upon these MTDs. The MTDs are analysed regarding their source areas, either the Aare or the Lütschine deltas, both occurring at the opposite ends of the elongated lake basin. Moreover, we investigate whether earthquakes, spontaneous failures or a shift of the Lütschine River could act as potential triggers, as well as the role of sediment loading in the deltas related to climate changes or anthropogenic impacts in the watershed. Altogether, these factors have likely affected the recurrence rates of these significant sedimentological events.

References:

Girardclos, S., Schmidt, O.T., Sturm, M., Ariztegui, D., Pugin, A., Anselmetti, F.S., 2007. The 1996 AD delta collapse and large turbidite in Lake Brienz. Mar. Geol. 241, 137–154. https://doi.org/10.1016/j.margeo.2007.03.011

Kremer, K., Simpson, G., Girardclos, S., 2012. Giant Lake Geneva tsunami in AD 563. Nat. Geosci. 5, 756–757. https://doi.org/10.1038/ngeo1618

Sammartini, M., Moernaut, J., Anselmetti, F.S., Hilbe, M., Lindhorst, K., Praet, N., Strasser, M., 2019. An atlas of mass-transport deposits in lakes. Submar. Landslides Subaqueous Mass Transp. Depos. Outcrops Seism. Profiles 201–226. https://doi.org/10.1002/9781119500513.ch13 

How to cite: Girardclos, S., Gastineau, R., Kremer, K., and Anselmetti, F. S.: What are the causes of repeated mass-transport deposits in perialpine Lake Brienz (Switzerland)?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-6005, https://doi.org/10.5194/egusphere-egu23-6005, 2023.

To distinguish how climatic and human factors influence Holocene fire dynamics in arid, westerlies-dominated areas of Central Asia, we examined micro-charcoal and pollen records in well-dated sediment cores from Balikun lake, eastern Tianshan region, Central Asia. Our results show that vegetation biomass primarily controlled the suborbital-time-scale fire dynamics, with an increasing trend of fire intensity from the early to late Holocene. Evidence for substantial anthropogenic influence on fires is mainly related to the expansion of prehistoric agriculture during ~3.6-2.2 cal kyr BP in our study area. These human activities caused a departure from “natural” background fire levels that vary with climate change.

In contrast, the human-linked fire peak in the nearby Hexi Corridor, which lies in Asian summer monsoon dominated regions of eastern China, occurred before ~3.6 cal kyr BP. This may reflect differences in the timing and sequence of cultural development under regions dominated by different circulation patterns. The weakening of the Asian summer monsoon after ~3.6 cal kyr BP likely promoted the Hexi Corridor population to migrate to eastern Tianshan, as evidenced by the similar pottery from eastern Tianshan to those of the Siba culture (4.0-3.5 cal kyr BP) that originated from the Hexi Corridor. This migration facilitated the formation of the westward cultural exchange route known as the “Painted Pottery Road”, enhancing regional human activity and causing an increase of fires in eastern Tianshan.

How to cite: Zhao, Y., Miao, Y., Jia, X., Li, Y., Li, H., Dong, W., Zhao, J., Wang, X., Cao, X., and An, C.: Holocene fire-vegetation history and its relationship with climate and human activity revealed by lake sediments from eastern Tianshan, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17373, https://doi.org/10.5194/egusphere-egu23-17373, 2023.

Manicouagan, the ‘Eye of Québec’, is an annular-shaped impact structure located in northeastern Canada (51˚N). The completion of a large hydroelectric dam in 1970 CE flooded two 60-km long lakes that were located on east and west sides of the inner ring. The drowning of the lakes under 135 m of water marked the final stage of the Late Quaternary history of Manicouagan. The morpho-sedimentary record combining sismo-acoustic imagery, swath bathymetry, and sediment cores aims to (1) reconstruct the Late Quaternary history of Manicouagan; and (2) assess the impacts of damming, in this case the drowning of a large boreal lake that used to be an indigenous cultural habitat. At the eastern end, the flooded Lake Manicouagan, has a fjord-like setting characterized by a shelf representing the lowest lake-level, steep walls > 45˚, and deep environments. The glacial-postglacial evolution, linked to the northward retreat of the Laurentian Ice Sheet, is preserved in overdeepened basins reaching 455 m deep, that is 105 m below modern sea-level. To the north, these basins are buried under a postglacial drape resulting from strong river inputs. Downstream, sedimentary units build channel-and-levee systems along a deep canyon. At mid-lake, a sill connects to southern basins displaying a hummocky topography. Postglacial conditions are characterized by a lake-wide record of gravity events originating from the shelf and above. Core-scanning techniques along with dating information (¹⁴C, ²¹⁰Pb, ¹³⁷Cs) on a transect of short-cores highlight the role of natural and human-induced water-level fluctuations on the generation of slope hazards. Ongoing analyses expect to (1) precise the nature, timing, extent and impacts of natural and man-made hazards; (2) reconstruct environment-climate evolution from a remote boreal region where limnogeological records are scarce.

How to cite: Chassiot, L., Lajeunesse, P., L'Heureux-Houde, F.-X., Frigon, A., Lenz, K.-F., Gebhardt, C., and Francus, P.: The morpho-sedimentary record of impact crater Lake Manicouagan in northeastern Canada, from an overdeepened valley to a large hydroelectric reservoir, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10214, https://doi.org/10.5194/egusphere-egu23-10214, 2023.

The Tibet Plateau is the world’s highest inhabited terrain, with an area >2.5×10⁶ km² and an average elevation >4000 m asl. Its uplift has been regarded as the main factor driving the evolution of the Asian monsoon regime and alpine biodiversity. The Miocene epoch is now deemed critical for understanding the timing of the attainment of the highest paleoelevation of the Tibet Plateau, the development of the modern Asia monsoon regime, and the acceleration of the evolution of alpine biodiversity. However, the northern Tibet Plateau remains poorly investigated.

Tsuga, Podocarpus, Abies, and Picea are four typical montane conifer genera. Although influenced by factors such as precipitation, geographical location, and soil properties, these taxa have elevation-specific habitats in Asia: Tsuga and Podocarpus usually grow at middle elevations (300 to 3500 and 600 to 2000 m asl, respectively) whereas Abies and Picea grow at higher elevations (500 to 4700 and 300 to 4700 m asl, respectively). Therefore, the representation of these genera in pollen assemblages can be used to reconstruct paleoelevation.

We tested new pollen analysis from Miocene sedimentary sequences of Huaitoutala section, and compiled them with other three sites (Core Sino-German-1, Core KC-1, and Yahu section in the Qaidam Basin (paleo-Qaidam Lake), northern Tibet Plateau. We selected these pollen records of Tsuga, Podocarpus, Abies, and Picea to build a new paleoaltimetry to construct two parallel midrange paleoelevation sequences in the northern Tibet Plateau at 1332 ± 189 m and 433 ± 189 m, respectively, during the Middle Miocene (~15 Ma). They increased rapidly to 3685±87 m in the Late Miocene (~11 Ma) in the east, and to 3589±62 m at ~7 Ma in the west. Our estimated rises in the east and west parts of the northern Tibet Plateau during 15-7 Ma, together with data from other Tibet Plateau regions, indicate that during the Late Miocene the entire plateau may have reached a high elevation close to that of today, with consequent impacts on atmospheric precipitation and alpine biodiversity.

How to cite: Miao, Y., Fang, X., Sun, J., Xiao, W., Yang, Y., Wang, X., Farnsworth, A., Huang, K., Ren, Y., Wu, F., Qiao, Q., Zhang, W., Meng, Q., Yan, X., Zheng, Z., Song, C., and Utescher, T.: A new biologic paleoaltimetry applied to the paleo-Qaidam Lake sedimentary sequences indicating Late Miocene rapid uplift of northern Tibet Plateau, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17424, https://doi.org/10.5194/egusphere-egu23-17424, 2023.

Drainage evolution and source-to-sink history across the Tibetan plateau and the surroundings as the consequence of changes in landscape and climate, provide a window to understand the growth of the Tibetan Plateau and its environmental impacts. Previous studies focus mostly on the reconstruction of siliciclastic sediment, but less on the dissolved load that displays distinct transport dynamics. Here we provide a solute perspective for deciphering source-to-sink history in NE Tibet through the carbonate ⁸⁷Sr/⁸⁶Sr ratio. The modern observations around NE Tibet exhibit a remarkable contrast of the solute Sr isotopic regime. That is, the Qilian Shan supplies high ⁸⁷Sr/⁸⁶Sr ratios caused by the exhumation of high-grade metamorphic rocks, while the East Kunlun Shan and West Qinling provide low ⁸⁷Sr/⁸⁶Sr ratios.

Here, we reconstruct solute Sr evolution of basin paleowater using bulk carbonate from parallel lacustrine and fluvial sections in the Qaidam, Lanzhou, and Xining Basins, NE Tibet. In the Qaidam Basin, solute ⁸⁷Sr/⁸⁶Sr ratios of the paleo-Qaidam Lake and fluvial sediments remain constant at ~0.713 in most Cenozoic periods, indicating that the solute Sr sourced solely from the Qilian Shan. But two low ⁸⁷Sr/⁸⁶Sr ratio periods at ~44.5 - ~32 Ma and after ~16 Ma might be caused by hydrological connections between the northern source (Qilian Shan) and the southern source (dominantly from East Kunlun Shan) in the Paleogene western basin and Neogene eastern basin, respectively. The two time periods correspond to two stages of the paleo-Qaidam Lake that developed following a south-eastward migration of basin depocenter due to the tectonic evolution of the surrounding mountains.

In the Lanzhou and Xining Basins, ⁸⁷Sr/⁸⁶Sr ratios of the paleo-Longzhong Lake and fluvial sediments maintained similarly low values and turned into an increase after ~32 Ma, reflecting an elevated proportion of solute supply from high ⁸⁷Sr/⁸⁶Sr source area, i.e., Qilian material input. A diverse evolution of solute ⁸⁷Sr/⁸⁶Sr ratio in the Lanzhou and Xining Basins after ca. 25-23 Ma, suggests that there was a drainage reorganization in response to the growth of NE Tibet. Our study implies that radiogenic Sr is a useful proxy for tracing drainage adjustment in response to large-scale tectonics.

How to cite: Liu, Y., Yang, Y., Fang, X., and Song, B.: Deciphering drainage reorganization in the NE Tibet: Insights from 50 Ma-long solute Sr isotopic records of Cenozoic lacustrine-fluvial sequences, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17355, https://doi.org/10.5194/egusphere-egu23-17355, 2023.

The thick and continuous lacustrine-fluvial sediments in the Qaidam Basin (paleo-Qaidam Lake), NE Tibetan Plateau, serve as excellent archives to reconstruct the Asian inland climate evolution history and explore the driven factors involving Tibetan Plateau uplift, Paratethys retreat, and global climate change. However, the debated elevation history of the Tibetan Plateau and the overlapping climate effects of the Tibetan Plateau uplift and Paratethys retreat makes it difficult to assess the driving mechanism on regional climate change in a particular period. Fortunately, some recent progress suggests that precisely dated Paratethys transgression/regression cycles appear to have fluctuated over broad regions with low relief in the northern Tibetan Plateau before 26 Ma, when the global climate was characterized by generally continuous cooling, thus offering an opportunity to distinguish between the climatic effects of the Paratethys retreat and those of global cooling.

Here, we present detailed mineralogical and geochemical investigations of clay minerals collected from the Hongliugou section, northern Qaidam Basin, to reconstruct paleolake salinity variation and basin weathering history in the Paleogene (~54-26 Ma), aiming to obtain a complete regional climate change record and its controlling factors. The paleolake salinity, indicated by two clay-fraction paleosalimeters, equivalent boron and Couch’s salinity, collectively present a long-term lake salinization, from slightly mesohalobic environment (averaging ~4.4‰) in middle Eocene (42-~34 Ma) to mesohalobic/polysaline environment (averaging ~8.3‰) in late Eocene-early Oligocene (~34-~29 Ma). Regarding regional weathering history, the clay mineral assemblage, the clay-fraction geochemistry index (e.g., CIA and Mg/Al ratios) and monomineral indices involving illite chemistry index, beidellite content and chlorite chemistry index and Mg/Al ratios collectively suggest a long-term decreasing trend in 54-26 Ma. The history of both paleolake hydrological evolution (salinity) and basin weathering matches well with the global climate change as indicated by benthic oxygen isotope. We thus reasonably speculate that the secular trend of regional climate change is primarily controlled by global cooling, which regulates regional climate change by influencing the evaporation capacity in the moisture source of Qaidam Basin and the westerly wind strength. Superimposed on this trend, the Paratethys transgression/regression cycles served as an important factor regulating wet/dry fluctuations in the Asian interior.

How to cite: Ye, C., Yang, Y., Fang, X., Zhang, W., Guo, Z., and Liu, Y.: Global cooling controls Paleogene (54-26 Ma) paleolake hydrological evolution and regional chemical weathering processes in the Qaidam Basin, NE Tibet, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17427, https://doi.org/10.5194/egusphere-egu23-17427, 2023.