Cosmogenic burial dating based on measurements of the 26Al/10Be ratio is a well-established geochronological method that contributed to significant progress in several scientific disciplines, ...including fluvial geomorphology, Quaternary geology or palaeo-anthropology. Although it was originally developed to date buried alluvium in multi-level cave systems 25 years ago, merely ∼30 research papers focusing on deep cave environments worldwide, including the world-famous Mammoth Cave, have been published since then. It is all the more surprising considering that buried alluvium in multi-level cave systems perhaps is the most advantageous material to apply the 26Al/10Be dating method. This review manuscript aims to foster this approach given its demonstrated usefulness in reconstructing Late Cenozoic incision rates and fluvial landscape evolution in a wide variety of settings.
The manuscript first provides the state-of-the-art knowledge on endokarstic sediments deposited in large, well-developed multi-level epigenic cave systems, which have formed in response to regional base-level changes over long periods of time (105–106 years), and how to date them. It then presents the basic principles of the burial dating method based on depleted 26Al/10Be ratios. The first key section thoroughly answers following questions: (i) why is this geochronological tool suitable to date endokarstic fluvial deposits? (ii) What kind of material can be sampled and where to collect it? (iii) What are the main analytical and geomorphological issues? (iv) How can potential drawbacks and biases be identified and, if possible, be avoided? (v) How can this approach be combined to other dating methods? The second key section shows how 26Al/10Be numerical age estimates from multi-level cave systems are used to infer long-term incision rates and to unravel their variation through time as well as the drivers of incision. Benefits of comparing this chronological information with that obtained on subaerial fluvial markers, i.e., alluvium-mantled terraces, are addressed. Insights provided by palaeo-denudation rates into fluvial landscape evolution are discussed as well. This contribution finally presents three opportunities to enhance cosmogenic burial dating of endokarstic deposits and delivers useful recommendations to fully benefit from this approach.
Dating the transport/deposition time of supratidal coarse-clast deposits is difficult, limiting their value for inferring frequency-magnitude patterns of high-energy wave events. On Bonaire (Leeward ...Antilles, Caribbean), these deposits form prominent landforms, and transport by one or several Holocene tsunamis is assumed at least for the largest clasts. Although a large dataset of 14C and electron spin resonance (ESR) ages is available for major coral rubble ridges and ramparts, it is still debated whether these data reflect the timing of major events, and how these datasets are biased by the reworking of coral fragments. As an attempt to overcome the current challenges for dating the dislocation of singular boulders, three distinct dating methods are implemented and compared: (i) 14C dating of boring bivalves attached to the boulders; (ii) 230Th/U dating of post-depositional, secondary calcite flowstone and subaerial microbialites at the underside of the boulders; and (iii) surface exposure dating of overturned boulders via 36Cl concentration measurements in corals. Approaches (ii) and (iii) have never been applied to coastal boulder deposits so far. The three 14C age estimates are older than 40ka, i.e. most probably beyond the applicability of the method, which is attributed to post-depositional diagenetic processes, shedding doubt on the usefulness of this method in the local context. The remarkably convergent 230Th/U ages, all pointing to the Late Holocene period (1.0–1.6ka), are minimum ages for the transport event(s). The microbialite sample yields an age of 1.23±0.23ka and both flowstone samples are in stratigraphic order: the older (onset of carbonate precipitation) and younger flowstone layers yield ages of 1.59±0.03 and 1.23±0.03ka, respectively. Four coral samples collected from the topside of overturned boulders yielded similar 36Cl concentration measurements. However, the computed ages are affected by large uncertainties, mostly due to the high natural chlorine concentration. After correction for the inherited component and chemical denudation since platform emergence (inducing additional uncertainty), the calculated 36Cl ages cluster between 2.5±1.3 and 3.0±1.4ka for three of four boulders whilst the fourth one yields an age of 6.1±1.8ka, probably related to a higher inheritance. These 230Th/U and 36Cl age estimates are coherent with a suggested tsunami age of <3.3ka obtained from the investigation of allochthonous shell horizons in sediment cores of northwestern Bonaire. Whilst 230Th/U dating of post-depositional calcite flowstone appears to be the most robust and/or accurate approach, these results illustrate the potential and current limitations of the applied methods for dating the dislocation of supralittoral boulders in carbonate-reef settings.
We present the first experimental evaluation of the alpha efficiency value for electron spin resonance (ESR) dating of coarse quartz grains, which is used for the evaluation of the internal and ...external alpha dose rate components. Based on our results, we recommend the use of an a-value of 0.07 ± 0.01 (1σ) for both the Al and Ti centres. Although we acknowledge that quartz ESR alpha efficiency may be sample dependent, and could also be impacted by other sources of uncertainty, this potential variability is presently impossible to evaluate given the absence of other experimental a-values available in the ESR dating literature. Measured radioactivity of quartz grains from the Moulouya catchment (NE Morocco) provides an internal dose rate in the range of 50-70 µGy/a when using an a-value of 0.07. The use of this empirically derived a-value for the evaluation of the internal and external alpha dose rate has a limited overall impact on the final ESR age results: they change by <2% and <3%, respectively, in comparison with those obtained with an assumed a-value. However, the large variability observed among the broader sample dataset for quartz internal radioactivity and hydrofluoric acid (HF) etching rates underscores the potential importance of undertaking experimental evaluations of alpha dose rate parameters for each dated sample.
Depuis ses origines vieilles d’un siècle, la géochronologie du Quaternaire a connu un essor remarquable avec le développement de nombreuses méthodes de datation relative, corrélative et numérique. ...Comme les géomorphologues ont constamment tenté de quantifier les processus fluviaux à des échelles de temps variables couvrant tout le Quaternaire, la datation de morphologies fluviales et des matériaux associés constitua un long défi scientifique en parallèle. Cet article propose une double réflexion sur la manière dont la géochronologie du Quaternaire a permis la production de chronologies fluviales, tant au cours du siècle précédent qu’à l’heure actuelle. En premier lieu, un glissement méthodologique progressif est observé : alors que la majorité des études utilisèrent des méthodes de datation relative et corrélative jusqu’à la fin du xxe siècle, les méthodes numériques sont devenues de plus en plus omniprésentes depuis lors. Deux causes explicatives sont explorées. D’une part, on constate une évolution contrastée des méthodes relatives au cours des deux dernières décennies : alors que certaines d’entre elles ont connu des développements significatifs, comme la racémisation d’acides aminés, le principe de base d’autres méthodes, comme la lichénométrie, fut sévèrement remis en cause. D’autre part, l’avènement tardif des méthodes de datation numérique au cours de la deuxième moitié du xxe siècle est aussi abordé. En second lieu, le statut actuel de la géochronologie du Quaternaire est doublement questionné : (i) Certaines méthodes relatives sont-elles devenues définitivement obsolètes ? (ii) Les méthodes radiométriques peuvent-elles résoudre tous les problèmes chronologiques en contexte fluvial ? Dans cette optique, la datation par exposition de surfaces alluviales via les nucléides cosmogéniques est longuement abordée, tout en se focalisant sur certaines difficultés méthodologiques. Une réhabilitation de certaines méthodes relatives tombées en désuétude, comme celles basées sur des indicateurs d’altération superficielle, est suggérée pour venir en soutien des méthodes numériques. Enfin, l’utilisation d’approches géochronologiques multi-méthodes est vivement recommandée.
La paleotopografía de la ciudad de Gadir-Gades es un elemento fundamental para la comprensión del proceso histórico acontecido en este ámbito tan singular del entorno atlántico-mediterráneo. ...Recientes actividades arqueológicas y geoarqueológicas en la parte central del antiguo canal “Bahía-Caleta” (Edificio Valcárcel, 2018) han demostrado la existencia de un activo fondeadero ubicado entre 20 y 40 m circa de profundidad bajo el actual nivel del mar, muy fértil desde un punto de vista arqueológico, de cuya interpretación se infiere que este canal natural estuvo abierto entre época fenicia arcaica y al menos el Alto Imperio. Este singular hallazgo afecta a la reinterpretación de la paleotopografía de las islas gaditanas y al urbanismo de la ciudad, que se desarrolló en un entorno insular, frente a lo que se pensaba hasta la fecha. En este trabajo se presentan estos nuevos hallazgos y se reflexiona sobre las implicaciones histórico-arqueológicas que se derivan del mismo, abriendo sugerentes líneas inéditas de investigación para el futuro.
Geoarchaeological cores were retrieved in the centre of the old “Bahía-Caleta” palaeochannel located between the
Erytheia
and
Cotinusa
islands in the former Cádiz archipelago, in present-day ...southern Spain. The unprecedented coring depth (~ 35–50 m) allowed us to identify the bottom of a Phoenician–Punic and Roman harbour. Located at 20–40 m b.s.l. in the sedimentary sequence, silty sand deposits reveal a deep semi-protected shelter with abundant ceramic and archaeobotanical findings. Based on these new results, the palaeotopography of the islands of Cádiz is reinterpreted, demonstrating the presence of a harbour accessible from the west and possibly from the east until (at least) the Roman period. This major discovery opens meaningful perspectives for archaeological, geomorphological and palaeoenvironmental research.
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