Far‐field tsunami deposits observed in the Kahana Valley, O‘ahu, Hawai‘i (USA), were investigated for their organic‐geochemical content. During short high‐energy events, (tsunamis and storms) organic and chemical components are transported with sediment from marine to terrestrial areas. This study investigates the use of anthropogenic based organic geochemical compounds (such as polycyclic aromatic hydrocarbons, pesticides and organochlorides) as a means to identify tsunami deposits. Samples were processed by solid–liquid extraction and analyzed using gas chromatography–mass spectrometry. A total of 21 anthropogenic marker compounds were identified, of which 11 compounds were selected for detailed analysis. Although the tsunami deposits pre‐date industrial activity in Hawai‘i by several hundred years, distinct changes were found in the concentrations of anthropogenic marker compounds between sandy tsunami deposits and the surrounding mud/peat layers, which may help in identifying tsunami deposits within cores. As expected, low overall concentrations of anthropogenic markers and pollutants were observed due to the lack of industrial input‐sources and little anthropogenic environmental impact at the study site. This geochemical characterization of tsunami deposits shows that anthropogenic markers have significant potential as another high‐resolution, multi‐proxy method for identifying tsunamis in the sedimentary record.
The Aleutian subduction zone is capable of generating magnitude ~9 earthquakes that have local impact and broadcast their destructive power across the Pacific through tsunamis. Field surveys of the tsunami from the 1957 Great Aleutian earthquake (reported M w 8.6) indicate a tsunami amongst the largest of the twentieth century. In the eastern half of the rupture zone, stranded logs record up to 18 m run-up in the Islands of Four Mountains (IFM) and 32±2 m on Unalaska Island. In conjunction with archaeological studies in the region, these observations show the potential impact of tsunamis on the ancient peoples in the IFM. Simulation of the near-field tsunami produced from the published slip distribution of 1957 is almost an order of magnitude smaller than all field observations. Increasing the earthquake magnitude and amount of eastern slip used in forward models of the tsunami demonstrate that run-up observations can be achieved throughout the eastern Aleutians if the earthquake was more than twice as large—at least M w 8.8 earthquake with 10–20 m of eastern slip. Additionally, up to five possible IFM paleotsunami deposits agree with the regional picture of regular large events, illustrating the circum-Pacific tsunami hazard from the east-central Aleutians.
Over the past 200 years of written records, the Hawaiian Islands have experienced tens of tsunamis generated by earthquakes in the subduction zones of the Pacific ‘Ring of Fire’ (for example, Alaska–Aleutian, Kuril–Kamchatka, Chile and Japan). Mapping and dating anomalous beds of sand and silt deposited by tsunamis in low‐lying areas along Pacific coasts, even those distant from subduction zones, is critical for assessing tsunami hazard throughout the Pacific basin. This study searched for evidence of tsunami inundation using stratigraphic and sedimentological analyses of potential tsunami deposits beneath present and former Hawaiian wetlands, coastal lagoons, and river floodplains. Coastal wetland sites on the islands of Hawai΄i, Maui, O΄ahu and Kaua΄i were selected based on historical tsunami runup, numerical inundation modelling, proximity to sandy source sediments, degree of historical wetland disturbance, and breadth of prior geological and archaeological investigations. Sand beds containing marine calcareous sediment within peaty and/or muddy wetland deposits on the north and north‐eastern shores of Kaua΄i, O΄ahu and Hawai΄i were interpreted as tsunami deposits. At some sites, deposits of the 1946 and 1957 Aleutian tsunamis are analogues for deeper, older probable tsunami deposits. Radiocarbon‐based age models date sand beds from three sites to ca 700 to 500 cal yr bp, which overlaps ages for tsunami deposits in the eastern Aleutian Islands that record a local subduction zone earthquake. The overlapping modelled ages for tsunami deposits at the study sites support a plausible correlation with an eastern Aleutian earthquake source for a large prehistoric tsunami in the Hawaiian Islands.
Salt marshes aggrade in quasi-equilibrium with sea level rise (SLR) via the accumulation of organic matter and mineral sediment, thereby maintaining marsh platform elevation within the tidal frame (e.g., Allen, 2000;Cahoon et al., 2019). External perturbations, such as an acceleration of relative SLR, can be compensated for by increased sediment delivery to the marsh platform. Increased inundation depth tends to augment sediment delivery as the associated longer flood duration increases time to trap suspended sediment (Day et al., 1999;Reed, 1990;Temmerman et al., 2003). In some cases, increased suspended sediment concentrations associated with land clearance has allowed marshes to recover from rapid SLR (Peck et al., 2020;Watson, 2004). In addition to increased mineral sediment delivery, there is some evidence that bioproductivity of low marsh grasses may increase with moderate increases in inundation, with subsequent vegetation drowning at higher levels of inundation (Morris et al., 2002;Voss et al., 2013). However, many studies have found declining biomass with increased
<p>Inorganic sediment supply is a critical component of a salt marsh&#8217;s ability to vertically aggrade in response to relative sea level rise, yet there remains significant uncertainty on the primary sources, timing, and rates of sediment delivery to marshes. This is particularly true for the Northeastern, U.S. Atlantic coastline where the magnitude and sourcing of sediment varies widely due to post-glaciated landscapes. Here we present results from a 3-year study between 2020 and 2023 designed to inform management and restoration decisions related to northeast marshes through the development of a scalable method for assessing the availability and distribution of inorganic sediment to and within marshes, including the identification of thresholds of inorganic sediment delivery required to maintain a stable marsh platform under various rates of sea level rise for the region. Field investigations involved instrumental observations, deployment and recovery of seasonal sediment traps, and the collection and analysis of marsh core samples. The study targets 12 marsh systems spanning environmental gradients for the region that allowed us to examine different sources and delivery mechanisms of sediment. Our compilation of existing data reveals spatial variability in marsh accretion rates, but also highlights regional trends and the general agreement among rates determined through a variety of different methodologies and time spans. Our instrumental observations and sediment trap deployments confirm differences in sediment delivery among marshes. Back barrier marshes with relatively small watersheds predominantly accumulated inorganic sediment during the fall in response to large storms and wave activity suspending coastal and offshore sediment deposits (marine sources) that are carried into marshes through tidal advection. In contrast, marshes proximal to large rivers (>10,000 km<sup>2 </sup>watersheds) have higher accumulation rates and receive the bulk of their inorganic sediment in response to fluvial delivery of terrestrial sediment during spring freshet events. Among our 12 study marshes, only one experienced its highest rate of sediment accumulation during summer months, which we attribute to substantially greater crab herbivory promoting internal recycling of sediment. Overall, we have measured sediment accumulation in over 450 individual traps across spring, summer and fall seasons in twelve marshes. The results from the analysis of these samples represents the largest dataset of its kind for the region and enable defining regionally appropriate input variables for modeling the spatial variations of sedimentation across marsh surfaces as a function of tidal inundation and distance to the nearest channel, as well as providing defined sedimentation limits needed to sustain healthy marsh growth under future sea level rise and various potential restoration pathways.</p>
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