Holocene mangrove dynamics and relative sea-level changes along the Tanzanian coast, East Africa

There is continued uncertainty regarding the rate, timing, duration and direction of Holocene sea-level for the Indian Ocean, and indeed the wider tropical realm. We present the first synthesis, and a new chronology, for Holocene relative sea-level (RSL) using a range sediment cores retrieved from mangrove ecosystems in three locations along coastal Tanzania. This study applies the relationship of ratios between the key mangrove taxa of Sonneratia:(Bruguiera/Ceriops) (S/BC) (ranging from 0 to 22.9) and Sonneratia:Rhizophora (S/R) (ranging from 0 to 2.29), vegetation and altitude to interpret mangrove dynamics and refine the vertical errors associated with relative sea level change. The variations in mangrove taxa ratios in the sediment cores obtained from each site shows mangrove development at different periods during the Holocene from around 7900 cal yr BP. An early to mid-Holocene RSL rise occurred from ∼7900 to ∼4600 cal yr BP that may have reached a higher level than present. A lower RSL occurred after 4600 cal yr BP, resulting in mangroves retreating seaward at all three study locations, before a low magnitude RSL rise occurred between 4400 and 2000 cal yr BP. Another RSL rise is recorded at ∼500 cal yr BP before falling to a level lower than present at ∼100 cal yr BP. There is evidence of a recent RSL rise recorded from mangrove ratios during the last century. In addition, the sedimentation rates among sites are relatively different 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 yr−1) than at the Zanzibar sites (0.3-6.6 mm cal yr−1).