Holocene coastal stratigraphy, coastal changes and potential palaeoseismological implications inferred from geo-archives in Central Chile (29–32° S)

Coastal geomorphology and the stratigraphy of coastal geoarchives record past coastal and fluctuations of coastal environments. In addition, these archives potentially store traces of past extreme events such as earthquakes and tsunamis, severe storms, and major flfl oodings of the coastal hinterland, e.g. due to El Niño conditions. Studying their characteristics may thus improve the knowledge of past frequency and magnitude patterns of such extreme events. For instance, large scaled spatial information about past earthquakes is needed for the understanding and estimation of seismo-tectonic processes. Misinterpretations in the size of preceding earthquakes may lead to incorrect strain balance estimations along megathrusts. Thus, fundamental research on the occurrence of past earthquakes is needed, which can be reflected in sudden or long-term coastal changes. Using sedimentological, geomorphological and microfaunal evidence, coeval geomorphodynamic and palaeoenvironmental changes at four different locations between 29° 50′ and 32° 20′ S in Central Chile were identififi ed in estuary systems, coastal swamps and coastal plains. The results may represent possible indirect evidence for palaeoseismicity, affecting the coastal system by vertical tectonic movements. Changes of coastline elevation, morphodynamic activity and/or coastal environments, as well as the formation of a liquefaction layer took place during the last c. 400 years. Moreover, major flfl ooding events related to strong El Niño conditions are assumed to have influenced the coastal stratigraphy by depositing high energy fluvial deposits. Our results suggest that the coastal environment, geomorphology and stratigraphy are considerably inflfl uenced by tectonic processes in the study area; a relation of the presented fifi ndings to the 1730 Great Valparaíso Earthquake is assumed. In general, the findings may encourage the implementation of comparable detailed studies, which may ultimately contribute to a better understanding of the Holocene coastal evolution and its relation to palaeoseismicity in Central Chile

Coastal hazards from tropical cyclones and extratropical winter storms based on holocene storm chronologies

Aware of past and future climate changes, the question arose whether modern instrumental data adequately reflect the chronology of tropical cyclones and extratropical winter storms for the period of the present eustatic sea level highstand (approx. the past 6,000 years). For pre-instrumental times, geological and sedimentological methods have been applied at geo- and bioarchives such as coastal marshes, lagoons, washover features or beach ridges, showing a frequency of strong cyclones roughly every 100–300 years, which is in contrast to the high number of major cyclones recorded recently. Many of these palaeotempestological records are discontinuous or contain hiatuses and it may be difficult to evaluate whether these sections of the record represent quiet phases without major cyclones or simply erosion or fluctuations in the ability of an archive to record the signature of cyclones. Manifold questions are still unanswered: as the potential number of former cyclones may be stored in landforms and sediments, how can the intensity of these cyclones be identified? Is the crest height of beach ridges a good indicator for storm surge heights, air pressure, and cyclone categories? This paper reviews important achievements in palaeotempestology and discusses open questions of cyclone distribution, frequency and energy (i.e., hazard potential) in the last few decades and reconstructions of these parameters back into Mid-Holocene times

A prehistoric tsunami induced long-lasting ecosystem changes on a semi-arid tropical island--the case of Boka Bartol (Bonaire, Leeward Antilles)

The Caribbean is highly vulnerable to coastal hazards. Based on their short recurrence intervals over the intra-American seas, high-category tropical cyclones and their associated effects of elevated storm surge, heavy wave impacts, mudslides and floods represent the most serious threat. Given the abundance of historical accounts and trigger mechanisms (strike-slip motion and oblique collision at the northern and southern Caribbean plate boundaries, submarine and coastal landslides, volcanism), tsunamis must be considered as well. This paper presents interdisciplinary multi-proxy investigations of sediment cores (grain size distribution, carbonate content, loss-on-ignition, magnetic susceptibility, microfauna, macrofauna) from Washington-Slagbaai National Park, NW Bonaire (Leeward Antilles). No historical tsunami is recorded for this island. However, an allochthonous marine layer found in all cores at Boka Bartol reveals several sedimentary criteria typically linked with tsunami deposits. Calibrated (14)C data from these cores point to a palaeotsunami with a maximum age of 3,300 years. Alternative explanations for the creation of this layer, such as inland flooding during tropical cyclones, cannot entirely be ruled out, though in recent times even the strongest of these events on Bonaire did not deposit significant amounts of sediment onshore. The setting of Boka Bartol changed from an open mangrove-fringed embayment into a poly- to hyperhaline lagoon due to the establishment or closure of a barrier of coral rubble during or subsequent to the inferred event. The timing of the event is supported by further sedimentary evidence from other lagoonal and alluvial archives on Bonaire