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Abstract Understanding the properties of time averaging (age mixing) in a stratigraphic layer is essential for properly interpreting the paleofauna preserved in the geologic record. This work assesses the age and quantifies the scale and structure of time averaging of land snail-rich colluvial sediments from the Madeira Archipelago (Portugal) by dating individual shells using amino acid racemization calibrated with graphite-target and carbonate-target accelerator mass spectrometry radiocarbon methods. Gastropod shells of Actinella nitidiuscula were collected from seven sites on the volcanic islands of Bugio and Deserta Grande (Desertas Islands), where snail shells are abundant and well preserved in Quaternary colluvial deposits. Results show that the shells ranged in age from modern to ~48 cal ka BP (calibrated radiocarbon age), covering the last glacial and present interglacial periods. Snail shells retrieved from two of the colluvial sites exhibit multimillennial age mixing (>6 ka), which significantly exceeds the analytical error from dating methods and calibration. The observed multimillennial mixing of these assemblages should be taking into consideration in upcoming paleoenvironmental and paleoecological studies in the region. The extent of age mixing may also inform about the time span of colluvial deposition, which can be useful in future geomorphological studies. In addition, this study presents the first carbonate-target radiocarbon results for land snail shells and suggests that this novel, rapid, and more affordable dating method offers reliable age estimates for small land snail shells younger than ~20 cal ka BP.
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This content will become publicly available on December 1, 2025
Rapid growth of a carbonate island over the last millennium
Abstract Low‐lying islands in tropical regions are vulnerable to near‐term sea‐level rise and hurricane‐induced flooding, with substantial human impact. These risks motivate researchers to elucidate the processes and timescales involved in the formation, growth and stabilization of coastlines through the study of Holocene shoreline dynamics. Little Ambergris Cay (Turks and Caicos Islands) is a low‐lying carbonate island that provides a case study in the nucleation and growth of such islands. This study investigates the sedimentology and radiocarbon chronology of the island's lithified sediments to develop a model for its history. The island's lithified rim encloses a tidal swamp populated by microbial mats and mangroves. Preliminary radiocarbon data supported a long‐standing inference that the island is Holocene in age. This study integrates petrographic, sedimentological and new radiocarbon data to quantify the age of the island and develop a model for its evolution. Results indicate that the ages of most lithified sediments on the island are <1000 cal yrbp, and the generation and lithification of carbonate sediment in this system supports coastline growth of at least 5 cm/year. The lithification of anthropogenic detritus was documented, consistent with other evidence that in recent centuries the lithified rim has grown by rates up to tens of centimetres per year. A unit of mid‐Holocene age was identified and correlated with a similar unit of early transgressive aeolianite described from San Salvador, The Bahamas. It is proposed that this antecedent feature played an important role in the nucleation and formation of the modern island. Results extend an established Bahamian stratigraphic framework to the south‐western extreme of the Lucayan archipelago, and highlight the dynamism of carbonate shorelines, which should inform forward‐looking mitigation strategies to increase coastal resiliency to sea‐level rise. These results inform interpretation of the palaeoenvironmental record of carbonate environments, underscoring their geologically rapid pace of lithification.
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- PAR ID:
- 10562987
- Publisher / Repository:
- International Association of Sedimentologists
- Date Published:
- Journal Name:
- Sedimentology
- Volume:
- 71
- Issue:
- 7
- ISSN:
- 0037-0746
- Page Range / eLocation ID:
- 2119 to 2143
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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