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Reliability of mangrove radiocarbon chronologies: A case study from Mahé, Seychelles

Sefton, Juliet; Woodroffe, Sarah; Ascough, Philippa; Khan, Nicole

Reliability of mangrove radiocarbon chronologies: A case study from Mahé, Seychelles Thumbnail


Authors

Juliet Sefton

Philippa Ascough

Nicole Khan



Abstract

Mangrove sediments are valuable archives of paleoenvironmental and relative sea-level changes. The most widely applied method to obtaining chronologies of past changes in mangrove sediments is radiocarbon dating, because mangroves produce large amounts of organic matter in situ. However, there are many challenges to obtaining reliable radiocarbon chronologies because bioturbation processes from roots and crabs can rework mangrove sediments, resulting in ages that are not in stratigraphic order. Previous studies have suggested that methods that isolate specific sediment size fractions may yield ages closer to the age of the paleo depositional surface by removing younger carbon contamination from fine roots. This study examines which sample types are more likely to yield reliable radiocarbon ages using shallow cores from a mangrove environment on Mahé, Seychelles, in the Indian Ocean. We compare radiocarbon ages from bulk sediment, sieved organic concentrates and above-ground macrofossils collected from the same stratigraphic depths. Bulk sediment and organic concentrate ages are comparable, which suggests that methods that separate out different size fractions do not sample different carbon sources in Seychelles mangrove cores. Identifiable above-ground macrofossils are rare in Seychelles mangrove cores, but yield older radiocarbon ages than comparable bulk sediment or organic concentrate ages. We suggest that in Seychelles, limited accommodation space over the late-Holocene, determined by relatively stable relative sea levels, has resulted in poor preservation of above-ground macrofossils for radiocarbon dating due to low rates of burial and sediment accretion. Low accretion rates have likely resulted in a mangrove sediment sequence that is highly bioturbated and degraded, meaning both bulk sediment and organic concentrate samples are impacted by contamination from younger roots. We argue that the availability of accommodation space and sediment composition controls the reliability of mangrove radiocarbon chronologies, which has implications for sample choice and site selection.

Citation

Sefton, J., Woodroffe, S., Ascough, P., & Khan, N. (2022). Reliability of mangrove radiocarbon chronologies: A case study from Mahé, Seychelles. Holocene, 32(6), 529-542. https://doi.org/10.1177/09596836221080756

Journal Article Type Article
Acceptance Date Jan 2, 2022
Online Publication Date Feb 23, 2022
Publication Date Jun 1, 2022
Deposit Date Mar 1, 2022
Publicly Available Date Mar 31, 2022
Journal The Holocene
Print ISSN 0959-6836
Electronic ISSN 1477-0911
Publisher SAGE Publications
Peer Reviewed Peer Reviewed
Volume 32
Issue 6
Pages 529-542
DOI https://doi.org/10.1177/09596836221080756

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Copyright Statement
Sefton, Juliet, Woodroffe, Sarah, Ascough, Philippa & Khan, Nicole (2022). Reliability of mangrove radiocarbon chronologies: A case study from Mahé, Seychelles. The Holocene 32(6): 529-542. Copyright © 2022 (The Authors). DOI: 10.1177/09596836221080756






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