Geochemical approaches in tsunami research: current knowledge and challenges

Over the past decade or so, geochemical techniques have been applied to the study of modern and past tsunamis. Seawater incursions and the introduction of marine organic matter can be detected through geochemical analysis, providing strong evidence that an event deposit was formed by saltwater inundation. Furthermore, the marine geochemical signature of an event may reveal the full extent of tsunami inundation far more precisely than can be obtained from sediment alone. Based mainly on literature published during the last 4 years, this paper summarizes the latest advances in and some problems with tsunami geochemical research, and specifically addresses organic and inorganic proxies with high preservation potential, geochemical characteristics of invisible tsunami deposits, handling of data from core scanners, and offshore environmental impacts. Recent studies have proposed that some organic and inorganic geochemical proxies have high preservation potential, and sometimes such evidence can be recognized from invisible tsunami deposits. Quantitative assessments of biomarkers are also effective for detecting allochthonous materials. Organic and inorganic proxies can be applied both to identify tsunami deposits and to accurately reconstruct tsunami inundation areas; however, there are as yet no universal criteria for accurate reconstruction of tsunami inundation areas by detecting invisible tsunami deposits using geochemical approaches. For deeper understanding of the behavior of geochemical characteristics derived from tsunami events, additional knowledge of the geochemical substances associated with modern and paleo-tsunami events is required. Specifically, further work is required on assessment of the environmental selectivity of geochemical proxies and refinement of core-scanner analysis for both organic and inorganic substances.

Long-lasting impacts of a glacial lake outburst flood on the hydrology of a fjord-river system (Pascua River, Chilean Patagonia)

<p>Glacial Lake Outburst Floods (GLOFs) are an increasing threat to Patagonian environments and communities. Here, we investigate the geomorphological and hydrological impact of a recent GLOF from Pascua River, which discharges at the head of Baker Fjord (Chile, 48°S). To do so, a sediment core was taken ~4 km offshore of the Pascua River mouth at a water depth of 248 m. The coring site is located on the flank of a submarine channel incised trough the subaquatic delta of Pascua River, 30 m above the bottom of the channel. The sediment physical and chemical properties were analysed at high resolution with X-ray CT, MSCL and XRF core scanning, in combination with lower resolution grain-size and bulk organic geochemistry measurements, and a core chronology was established using downcore variations in <sup>137</sup>Cs activity. In addition, historical maps and satellite imagery of the past century were examined in combination with multibeam bathymetry of Baker Fjord to aid the interpretation of the sediment record.</p><p>Results show that the sediments are composed of two distinct units separated by a 5-cm thick event deposit dated 1945±9 CE. Below the event, the sediment consists of coarse silt and fine sand, likely representing sediment deposition from turbidity currents. Above it, it consists of very fine silts, likely representing settling from the surficial sediment plume. Historical evidence shows that the event deposit corresponds to a ~256 10<sup>6</sup> m<sup>3</sup> GLOF from Bergues Lake, the proglacial lake of Lucia Glacier that discharges directly into Pascua River. Before 1945, historical information shows that Pascua River drained via two active river branches that were most likely connected to the two submarine channels visible in the bathymetry of the subaquatic delta. After 1945, only the western river branch appears active, which likely caused the abandonment of the eastern submarine channel near which the sediment core was taken. Therefore, we hypothesize that the 1945 Bergues Lake GLOF caused the abandonment of the eastern river branch and submarine channel, which explains the absence of coarse-grained sediments in our sediment record after 1945±9 CE.</p><p>This study provides the first report of a GLOF from the northeastern part of the Southern Patagonian Icefield, and it demonstrates that GLOFs can have long-term impacts on the hydrology of fjord-river systems.</p>

A tsunamigenic delta collapse and its associated tsunami deposits in and around Lake Sils, Switzerland

Large lacustrine mass movements and delta collapses are increasingly being considered as potential tsunamigenic sources and therefore hazardous for the population and infrastructure along lakeshores. Although historical reports document tsunami events in several lakes in Switzerland, and although the propagation of lake tsunamis has been studied by numerical wave modeling, only little is known about on- and offshore lacustrine tsunami deposits. In Lake Sils, Switzerland, a large prehistoric mass-movement deposit originating from the Isola Delta with a minimum estimated volume of 6.5 × 106 m3 and a basinal thickness of > 6 m in the seismic record has been identified by previous studies and radiocarbon dated to around 700 Common Era. Here, we combine (i) comprehensive sedimentological investigation of sediment cores recovered from the on- and offshore settings, (ii) mineralogical fingerprinting of the inflows from key catchments to characterize sediment provenance, and (iii) numerical tsunami modeling, to test the hypothesis of a tsunamigenic delta collapse in Lake Sils. We observe a clastic event deposit consisting of coarse-grained, fining-upward sand overlying an organic-rich peat deposit in the shallow water. This layer thins and fines landward on the coastal plain. Toward the deeper water (20–40 m), the deposit transforms into a thicker and more heterogeneous sediment package with multiple sequences of fining-upward sand and a well-pronounced clay cap at the top. Radiocarbon dating of the peat underlying the event deposit yields a maximum age of 225–419 calibrated Common Era. The tsunami models, which indicate wave heights reaching up to 5 m, simulate areas of inundation that coincide with the location of event deposits. Based on our results, we propose that the historically undocumented Isola Delta collapse generated a basin-wide tsunami that inundated the lakeshore, transporting large amounts of unconsolidated sediment along the lakeshore toward the coastal plain and into the deeper lake basin.

Testing the veracity of turbidite paleoseismology using the Kaikōura earthquake-triggered turbidite, New Zealand

<p>To better understand earthquake reoccurrence and hazard, records of ancient earthquakes spanning millennia are routinely generated using seabed turbidite deposits, inferred to be synchronously triggered by strong ground motions over large geographic areas. However, the use of turbidites for paleoseismology is underpinned by untested hypotheses due to the dearth of verified earthquake-triggered turbidite deposits produced by well-characterised earthquakes. The aim of a RV <em>Tangaroa</em> voyage in October 2019 was to use the unique opportunity provided by the widespread and documented occurrences of the 2016 Mw7.8 Kaikōura earthquake-triggered turbidite to determine whether synchronous turbidite deposition can be reconstructed from the sedimentary record alone. Our presentation will summarise insights gleaned from precision short cores and high-resolution sub-bottom profiles collected along and across the axis of submarine canyons that preserve turbidite deposits triggered by the Kaikōura earthquake. Planned detailed laboratory characterisation of the turbidites will include high-resolution core imaging, texture, densitometry, down-core physical properties and geochemical characterisation combined with radioisotope-derived chronology. Through repeat coring at historical sites and future coring campaigns we hope to also quantify the impact of biological mixing on the Kaikōura event deposit to determine its likely preservation potential in the geological record. Our results have the potential to provide the first robust test of turbidite paleoseismology.</p>