A dinâmica dos manguezais durante o Holoceno tardio na foz do rio Ceará-Mirim, Rio Grande do Norte

Abstract

This paper aims to debate the impacts of climate change on subtropical mangroves during late Holocene, on the coast of the state of Rio Grande do Norte (RN) and to discuss the environmental conditions for the establishment, expansion, and contraction of mangroves. Based on the samples collected from the cores (NAT 3 AND NAT 5) to study palynological groups, data from the optical space (Landsat), sedimentary facies and multi-proxy analysis (δ13C, δ15N, TOC, TN, C:N ratio), synchronized with three 14C dating ages. The main result of this research was the succession of mangroves, divided into three palynological phrases that suggest the development of three facies associations: (1) tidal channel and (2) vegetated plain - herbs/mangrove (3) herbaceous plain. The first phase between at least ~ 4500 and ~ 2915 cal yr BP was colonized by herbs, palms, and trees and shrubs on the bank of the estuary and by mangroves, of the Laguncularia type, followed by Avicennia and Rhizophora. With dissolved organic carbon, C3 terrestrial plants influence organic matter with δ13C values between -29.7 ‰ and -26.8‰, δ15N with x̅ = 3.8‰ and C:N values around 21.2. The second phase between ~2915 and ~660 cal yr BP characterized by the establishment of the mixed tidal plain dominated by herbaceous vegetation and by the expansion of mangroves represented by Rhizophora pollen between ~2915 cal yr BP and 2814 ± 29 cal yr BP with the increase of plants C3 and dissolved organic matter (COD) in fresh/estuarine water (δ13C x̅ = -26.9 and -29.4‰; δ15N x̅ = 3.86 and C:N around 12.3). The third phase is marked by a mangrove retraction evidenced by the decrease in Laguncularia, followed by Avicennia and Rhizophora, the dissolved organic matter had a greater influence of freshwater/estuary, and terrestrial plants (C3 plants) during the last 699 ± 35 cal yr BP. Near the surface (<15 cm) Rhizophora pollen occurs, indicating the establishment of this genus in recent decades, under modern environmental conditions (δ13C x̅ = -29 and -28.8‰; δ15N x̅ = 2.55 and C:N around 24.5 ). The data obtained in the isotopic and elementary analyzes indicate sedimentary organic matter of terrestrial origin, with the presence of C3 photosynthetic cycle plants, suffering a possible aquatic and estuarine influence, however, in the sedimentary sequences formed by lenticular heterolytic deposits to be correlated with the absence and decrease of mangrove vegetation in both cores. The data from the space-time analysis indicated an expansion of the area of mangroves, so this variation in the dynamics of mangroves can be directly linked to climate changes and the relative sea level at current levels and during the last three decades.