Inter-proxy evidence for the development of the Amazonian mangroves during the Holocene

Abstract

The dynamics of mangrove forest on the island of Marajo ´ (Ilha de Marajo ´) at the mouth of the river Amazon during the past *7,500 cal. B.P. were studied using multiple proxies, including sedimentary facies, pol- len, d 13 C, d 15 N and C/N ratio, related to 15 sediment samples by 14 C dating. The results allow us to propose a scheme of palaeogeographical development, with changes in vegetation, hydrology and organic matter dynamics. Today, the interior of the island is occupied by varzea freshwater herbaceous vegetation, but during the early to middle Holocene, mangroves with accumulations of estu- arine organic matter colonized the tidal mud flats. This spread of mangroves was caused by post-glacial sea-level rise, which combined with tectonic subsidence, produced a marine transgression. It is likely that the relatively greater marine influence at the studied area was favoured by reduced discharge from the river Amazon, which was itself caused by a dry period that occurred during the early and mid Holocene. During the late Holocene, there was a reduction of mangrove vegetation and the contribution of freshwater organic matter to the area was higher than during the early and mid Holocene. This suggests a decrease in marine influence during the late Holocene which led to a gradual migration of mangrove vegetation from the central region to the northeastern littoral zone of the island, and, consequently, its isolation since at least *1,150 cal. B.P. This was probably a result of lower tidal water salinity caused by a wet period that resulted in greater river discharge during the late Holocene. This work details the contraction of mangrove forest from the north- eastern part of the island of Marajounder the influence of Amazon climatic changes, chronologically and spatially. This allows us to propose a model of successive phases of sediment accumulation and vegetation change, according to the marine-freshwater influence gradient. As demon- strated by this work, the use of a combination of proxies is efficient for establishing a relationship between the chan- ges in estuarine salinity gradient and depositional envi- ronment/vegetation.