The last mangroves of Marajó Island — Eastern Amazon: Impact of climate and/or relative sea-level changes

Abstract

The dynamics, over the last 7500years, of a mangrove at Marajó Island in northern Brazil were studied by pollen and sedimentary facies analyses using sediment cores. This island, located at the mouth of the Amazon River, is influenced by riverine inflow combined with tidal fluctuations of the equatorial Atlantic Ocean. Herbaceous vegetation intermingled with rainforest dominates the central area of the island, while várzea is the main vegetation type along the littoral. In particular, the modern northeastern coastal zone is covered by a mosaic of dense rainforest, herbaceous vegetation, mangroves, várzea, and restinga. The integration of pollen data and facies descriptions indicates a tidal mud flat colonized by mangroves in the interior of Marajó Island between ~7500calyr BP and ~3200calyr BP. During the late Holocene, mangroves retracted to a small area (100–700m in width) along the northeastern coastal plain. Mangrove expansion during the early and mid Holocene was likely caused by the post-glacial sea-level rise which, combined with tectonic subsidence, led to a rise in tidal water salinity. Salinity must have further increased due to low river discharge resulting from increased aridity during the early and mid Holocene. The shrinking of the area covered by mangrove vegetation during the late Holocene was likely caused by the increase in river discharge during the late Holocene, which has maintained relatively low tidal water salinity in Marajó Island. Tidal water salinity is relatively higher in the northeastern part of the island than in others, due to the southeast–northwest trending current along the littoral. The mixing of marine and riverine freshwater inflows has provided a refuge for mangroves in this area. The increase in flow energy during the last century is related to landward sand migration, which explains the current retraction of mangroves. These changes may indicate an increased exposure to tidal influence driven by the relative sea-level rise, either associated with global fluctuations or tectonic subsidence, and/or by an increase in river water discharge.