The Lesser Antilles are a densely populated region where local populations and industrial facilities are concentrated at the coastlines, and are therefore exposed to many rapid-onset hazards such as ...hurricanes and tsunamis. However, the historical catalog of these events is too short to allow risk assessment and return period estimations, and it needs to be completed with long-term records of washover deposits in coastal sedimentary environments such as lagoons. In this study, two sediment cores were taken in March 2018 in a small coastal lagoon on Scrub Island (northeastern Caribbean). Sedimentological, geochemical, microfaunal and chronological analyses enabled us to identify 25 sandy layers resulting from high-energy-marine floods. Two of these layers were interpreted as tsunami deposits based on sedimentological (rip-up clast of the underlying cohesive substrate and internal mud laminae), and geochemical evidence. The most recent deposit is associated with the transatlantic tsunami triggered by the 1755 CE Lisbon earthquake. The older one is the thickest sandy layer recorded in the lagoon, with an age range of 1364–1469 cal. CE, as determined using 14C dating. This event was recorded in sedimentary archives of both the northern and the southern part of the Caribbean, with its large spatial extent, supporting a distant tsunamigenic origin. The 23 remaining sandy layers were interpreted as storm deposits, based on sedimentological and chronological data, with the three most recent layers being correlated with historical hurricanes. This new 1600 year-long record displays similarities with that of the Bahamas, with the periods of intense hurricane activity being in antiphase with those of the north-eastern US coast. This regional comparison may provide evidence for a latitudinal forcing of hurricane tracks through time in relation to climate fluctuations.
Sedimentary, geochemical, geomorphological, radiocarbon and numerical modelling data were used to examine the nature, extent and age of a boulder scatter at Okawa Point, Chatham Island, New Zealand. ...Boulders up to 98 t were traced around 800 m inland and comprised both a landward and seaward grouping on either side of a mid-Holocene high-stand storm ridge dated to around 4840–4810 cal BP. The landward boulder scatter was linked with an enigmatic coarse sand/gravel layer that extends up to 1100 m inland and has been dated to around 3500–4500 cal BP.
Numerical modelling indicated that while the seaward boulders that mainly rest upon the Chatham Island Schist shore platform could have been emplaced by either storm or tsunami waves, those landward of the mid-Holocene storm ridge were most probably transported by a tsunami. There are several near-contemporaneous palaeotsunami deposits reported from mainland New Zealand, Australia, SW Pacific and the wider Pacific region. If some or all of these are associated with the same basin-wide palaeotsunami then it appears likely to have been one of the largest Holocene Pacific palaeotsunamis. An analysis of historical and numerically modelled data suggests that the most likely candidate is from within the northern Chile seismic gap, with early evidence suggesting that a large palaeoseismic and palaeotsunami event may have occurred around 4000 yr BP. If correct, this has important implications for assessing the largest possible magnitude earthquakes in the northern Chile seismic gap and the size of the tsunamis they generate.
•Boulders up to 98 t scattered over 800 m inland.•Associated sand and gravel layer extends over 1100 inland.•Numerical modelling indicates northern Chilean source most likely.•Largest palaeotsunami in past 4000 years from trans-Pacific source
Sedimentary, geochemical, microfossil and geochronological analyses were carried out on 10 sediment cores across the south-eastern corner of Mataora-Wairau Lagoon (M-WL), South Island, Aotearoa-New ...Zealand. This multi-proxy approach provides strong evidence for three late Holocene palaeotsunamis (two previously unreported) and a historical tsunami generated by rupture of the Wairarapa Fault in 1855CE. The main scientific evidence for these events consists of anomalous, high-energy marine sediment layers that overlie material of contrasting composition. These layers coincide with peaks in planktonic and benthic marine to brackish-marine diatoms and geochemical signatures indicative of sudden changes in environmental conditions. Palaeotsunami I (maximum date 2095cal.BP) and Palaeotsunami II (maximum date 915cal.BP - previously identified by Clark et al., 2015) appear to represent large events contemporaneous with major earthquakes inducing tectonic uplift and coseismic subsidence around the Cook Strait region. Palaeotsunami II is close to the date of the earliest settlement of the M-WL area by Māori, and a traditional narrative that references loss of life following waves over-washing the Wairau boulder bank. Given the boulder bank was likely to have been seaward of its current position during events I and II, we surmise that the inundation distances would have been greater than the most landward extent (340m) inferred from the deposits found in this study. Palaeotsunami III (maximum date 525cal.BP) was associated with tectonic subsidence across the study site, most likely associated with rupture of the southern Hikurangi subduction zone. We expect the ongoing refinement of the methods used in this study, and the results from future studies of analogue sites and offshore fault systems, will improve our understanding of the significant tsunami hazard facing this region.
•Māori oral tradition refers to experience with extreme environmental disturbance.•Detailed record of past tsunamis preserved within a salt marsh environment•Multi-proxy evidence for three late Holocene palaeotsunamis and one tsunami in 1855CE•Sources capable of inducing tectonic uplift, coseismic subsidence, submarine landslides•Benefits to cross-disciplinary research in tsunami hazards science
Internationally, a key problem in reconstructing chronologies of coastal hazards is the ability to distinguish between storm and tsunami deposits. This situation has been exacerbated by the low ...number of known locations where both types of deposit occur along the same stretch of coastline. The sedimentological characteristics of a tsunami and a storm deposit laid down on the same stretch of coastline on the southeast coast of the North Island, New Zealand, are distinctly different. The 15th-century tsunami was probably caused by fault rupture in the Cook Strait region, whereas the Easter 2002 storm was generated by a meteorological depression centred some 900 km to the southeast. The differences include areal extent, thickness, and grain-size characteristics. The tsunami deposit thins abruptly at the margins, fines inland, is more poorly sorted, has entrained rip-up clasts, and has an erosional lower contact, often with a buried soil. The storm deposit has a highly variable grain-size distribution with a marked coarsening at its landward extent, is better sorted, coarser, and has a sharp, non-erosional lower contact associated with buried vegetation and soil. The coarser grain size is probably the result of differences in sampling regime as opposed to wave energy. The storm deposit extends about 40 m inland as opposed to about 200 m for the tsunami. Variations in the preservation of evidence are a reflection of the age of deposition. Records of tsunamis and storms in New Zealand indicate that there are probably several coastal sites where both types of deposit can be compared.
Late Holocene environmental changes were examined in Dee Why Lagoon, New South Wales, based on the sedimentological, geochemical and geochronological analysis of six cores collected from the fluvial ...delta and lagoon basin. The lagoon formed about 7300-7200 cal yr BP, following the post-glacial marine transgression and establishment of a sand barrier. Infilling of the lagoon occurred at a rate of 0.09-0.15 mm/yr until about 3300-3200 cal yr BP, when the barrier closed, resulting in mostly freshwater conditions, as evidenced by a change from pyrite-rich units to sediment containing little sulfur and a lack of foraminifera. Sedimentation rates increased to 0.23 then 0.43 mm/yr until about 100 years ago. Post-European land clearance led to an increase in sedimentation rates to 1.0-1.3 mm/yr on the fluvial delta, which are lower than those from other wave-dominated estuaries in New South Wales as well as those estimated by previous studies in Dee Why Lagoon. Our study shows that the fluvial delta started forming much earlier than originally thought, based on results of radiometric dating, and confirmed using sedimentological and geochemical data, as well as a critical examination of historic aerial photographs. Human impact has resulted in an increase in heavy metal (Cu, Pb, Zn) and metalloid (As) concentrations in the recent sediment, most likely attributed to stormwater discharge. Pb and As concentrations are above the ANZECC high sediment quality guideline values at the site closest to the stormwater outlet, with As-based pesticides one of the possible sources. Using the mean enrichment quotient, which is based on normalised Cu, Pb and Zn concentrations over their normalised background concentrations, we show that the surface sediments in Dee Why Lagoon are severely enriched, reflecting the recent anthropogenic impact that has also led to an increase in sedimentation rates.
The composition of tsunami deposits is variable and governed by source material. Many unusual items have been recorded from tsunami deposits. For example, during the 2004 Indian Ocean Tsunami, a ...dolphin was reported to have been transported 1400 m inland, a shark was found in a hotel swimming pool, and sea turtles were stranded kilometers inland. Reports and papers from early settlers and explorers have often highlighted similar unusual finds. We briefly discuss an example from New Zealand where intact cetacean skeletons were found elevated on land adjacent to the coast. The validity of this find as evidence for tsunami emplacement is considered. It is also noted that such old reports should be treated with respect and re-evaluated in the light of more recent findings.
Sediment cores collected in a coastal lagoon a few kilometres east of Wairoa, northern Hawke's Bay, New Zealand, were examined using sedimentological, geochemical, palynological and ...micropaleontological analyses.
A distinct short-lived catastrophic saltwater inundation (CSI) about 6300 years BP and possibly other minor marine incursions are preserved in the coastal estuarine to lagoonal freshwater sedimentary sequences, which have been deposited in the last 6500 years. The CSI is characterised by a gravel unit that thins landward and decreases in particle size to sand, within a sequence consisting mainly of brackish estuarine muds. Diatom assemblages indicate a marked change from the shallow brackish estuarine muds to marine gravels and sands to brackish estuarine muds. The marine influence in the gravel and sand is also shown by the presence of marine dinoflagellates and a peak in Na/Rb. Sedimentological, chemical and paleontological (in particular diatoms) evidence indicates it is a CSI. We conclude that this was a tsunami and propose the most likely propagating mechanisms.
Marine influence decreases upcore and totally freshwater conditions are evident in the upper section of the cores. The geochemistry of the sediments mainly reflects the change in stratigraphy, with distinct signatures for tephra (Na, Fe, Cr), organic-rich and peat units (As, Br) and the coarse gravel-sand CSI unit (Na/Rb, Cr, Fe), but it is also indicative of changes in depositional environment. The change in chemistry (Na/Rb) in the CSI event is indicative of a saltwater influence, whereas a marked change in S content suggests a sudden change from brackish to freshwater conditions shortly after 4800 years BP. Another peak in S and Br content about 3200 years BP may indicate another temporary change to brackish conditions.
The origin of two sand layers buried in Pololū wetland, on the northeast coast of the island of Hawai'i, was investigated using a multi-proxy approach, including radiometric dating, sedimentology, ...geochemistry, micropalaeontology, palynology and historical records. Based on 210Pb, 137Cs and pollen data, the lower sand unit extending 250 to 350m inland is attributed to the 1946 Aleutian tsunami, while the upper sand unit was laid down by the 1957 Aleutian tsunami. Chronological information does not appear to support any link between these deposits and historical storm events. Furthermore, sedimentological, geochemical, diatom and pollen results suggest deposition of marine and near-beach sediments under high energy conditions typically associated with tsunami inundation. This work presents the first conclusive sedimentary evidence for distantly-generated tsunamis on the Hawai'ian Islands, opening the way for more comprehensive historical and palaeotsunami studies. Indeed, a brief review of archaeological data suggests that the Hawai'ian coast may have experienced similar large magnitude events in the past, from both local and distant sources.
► First conclusive sedimentary evidence for distantly-generated tsunamis in Hawai’i. ► Multi-proxy approach includes geochemistry, diatoms, pollen, historical records. ► Highlights potential for evidence of trans-Pacific palaeotsunamis.
The sedimentary record of known natural catastrophic events and anthropogenic activity in an estuarine environment is assessed using sedimentological, chemical and geochronological techniques. ...Shallow cores collected from intertidal and salt marsh sediments in Ahuriri Estuary, Hawke's Bay, reveal a variety of signatures of natural and human disturbance. Evidence for the 1931 Hawke's Bay earthquake, which resulted in an uplift of one to two metres in the Napier area, is given by a change from silt- to sand-dominated sediment in the lower estuary, which is consistent with a shift toward higher energy depositional conditions following uplift. However, based on physical properties of sediments, the 1931 uplift event does not appear to have caused major changes in depositional conditions in the upper estuary. Similarly, no changes were recorded at one site in the lower estuary, which seems to represent an area of low energy depositional environment. Although the 1960 Chilean tsunami resulted in structural damage in Napier, it did not produce any recognisable sedimentological and geochemical signature in the sedimentary record, suggesting that the study sites were possibly beyond the limit of sedimentation of the tsunami.
Post-European settlement impact is mainly restricted to the lower estuary, where increased concentrations of Zn, Cr, Pb and Cu are attributed to industrial discharges. Evidence of agricultural runoff is shown by an increase in Cu concentrations within a fine-grained depositional environment that is distal from industrial sources in the town of Napier. Chemical data (Cl and S) suggest a change in the depositional environment in the upper estuary due to increased freshwater influx and/or decrease in seawater influence. Dating by
210Pb suggests that this occurred around the middle part of the 19th century, and might be attributed to river flooding in the region at that time.
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