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Holocene mangrove dynamics from Unguja Ukuu, Zanzibar

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JournalQuaternary International
DatePublished - 17 Jun 2013
Volume298
Number of pages16
Pages (from-to)4-19
Original languageEnglish

Abstract

Stratigraphical, pollen and charcoal data, set within a radiocarbon dated chronological framework, from Unguja Ukuu, Zanzibar are used to reconstruct mangrove ecosystem dynamics during the Holocene. Changes in mangrove ecosystem composition were driven by a combination of sea level and environmental change, anthropogenic interaction and geomorphological activity. The high occurrence of Rhizophora mucronata, accompanied by other mangrove species, suggests that the headland of Unguja Ukuu supported a mangrove community from about 7000 cal yr BP. Around 5300 cal yr BP mangroves migrated landward, probably in response to a mid Holocene sea level rise. After this period, a short term dramatic decrease in Sonneratia alba along with an increase in more mesic arid mangrove species, the appearance of Poaceae and increase in the quantity of charcoal occurred indicating a lower sea level and arid environmental conditions. This is likely to be coincident with the appearance of the earliest habitation of Zanzibar. Sea level rose again until around 1200 cal yr BP as indicated by increased S. alba. The increase in charcoal is likely to relate to drier conditions and may also relate to the earliest human settlement recorded at Unguja Ukuu. A recent decrease in R. mucronata, and an increase in S. alba, is likely to result from sea level rise and decreased moisture availability. Recent human-ecosystem interactions are characterized by a reduction in mangrove extent possibly associated with the mangrove pole trade and rising fuelwood consumption in Zanzibar, particularly from around 530 cal yr BP.

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