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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. [ABSTRACT FROM AUTHOR]

Mangrove sedimentary deposits are sensitive to changes in sea level and can be used to reconstruct mid- to late Holocene sea-level fluctuations in intermediate and far-field locations, distant to the former polar ice sheets. However, they can be difficult to date using 14C because mangrove sediment can contain mixtures of carbon of different ages. The two main potential causes of error are younger mangrove roots penetrating down through the sediment column and bioturbation by burrowing animals which moves carbon up and down the sediment column. Both processes may introduce carbon not representative of the age of deposition of the layer being dated. This study reports new 14C dates on organic concentrates (10–63 µm) from mangrove sediments from Makoba Bay on Zanzibar (Unguja) where previous bulk sediment 14C age–depth profiles contained inversions and were therefore less useful for relative sea-level (RSL) reconstruction. Dates on organic concentrates provide a more coherent sequence of 14C ages compared with those from bulk sediments. These new data provide an improved environmental history and mid- to late Holocene RSL record for this site. Our reconstructions show that RSL rose during the mid-Holocene and reached within −3.5 m of present by c. 7900 cal. yr BP. RSL slowed as it reached present at or shortly after c. 7000 cal. yr BP, with falling and/or stable RSL from c. 4400 cal. yr BP to present. We are not able to determine whether there was a RSL highstand above present on Zanzibar during the mid- to late Holocene. The RSL reconstruction agrees broadly with changes predicted by the ICE-5G geophysical model, which includes 4 m of ice equivalent sea-level rise between 7000 and 4000 cal. yr BP. Our new dating approach has the potential to provide improved chronologies with which to interpret sea level data from this and other mangrove environments. [ABSTRACT FROM AUTHOR]