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Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. We cannot determine whether there was a RSL highstand above present on Zanzibar during the mid 2 to lateHolocene. 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.

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There is continued uncertainty regarding the rate, timing, duration and 12 direction of Holocene sea-level for the Indian Ocean, and indeed the wider 13 tropical realm. We present the first synthesis, and a new chronology, for 14 Holocene relative sea-level (RSL) using a range sediment cores retrieved from 15 mangrove ecosystems in three locations along coastal Tanzania. This study 16 applies the relationship of ratios between the key mangrove taxa of 17 Sonneratia:(Bruguiera/Ceriops) (S/BC) (ranging from 0-22.9) and 18 Sonneratia:Rhizophora (S/R) (ranging from 0-2.29), vegetation and altitude to 19 interpret mangrove dynamics and refine the vertical errors associated with relative 20 sea level change. The variations in mangrove taxa ratios in the sediment cores 21 obtained from each site shows mangrove development at different periods during 22 the Holocene from around 7900 cal yr BP. An early to mid-Holocene RSL rise 23 occurred from ~7900 to ~4600 cal yr BP that may have reached a higher level 24 than present. A lower RSL occurred after 4600 cal yr BP, resulting in mangroves 25 retreating seaward at all three study locations, before a low magnitude RSL rise 26 occurred between 4400 and 2000 cal yr BP. Another RSL rise is recorded at ~ 27 500 cal yr BP before falling to a level lower than present at ~100 cal yr BP. There 28 is evidence of a recent RSL rise recorded from mangrove ratios during the last 29 century. In addition, the sedimentation rates among sites are relatively different 30 due to different altitudinal ranges with freshwater input, sediment supply and progradation having significantly more effect in the Rufiji Delta (2.1-10.9 mm cal 32 yr-1) than at the Zanzibar sites (0.3-6.6 mm cal yr-1).

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