Late-Holocene subtropical mangrove dynamics in response to climate change during the last millennium

Abstract

This work aims to study the impacts of climate change in the subtropical mangroves during the late-Holocene on a southern Brazilian coastal plain and discuss the environmental conditions to the mangrove establishment near the austral limit of South America mangroves. Samples were collected to study palynological groups, sedimentary facies, and geochemistry analyses (δ13C, δ15N, TOC, TN, C:N ratio, TS, and C:S ratio), synchronized with four radiocarbon ages. The main result of this research was the mangrove succession, divided into three palynological zones composing two facies association: (1) herbaceous tidal flat and (2) mangrove tidal flat. The first zone between at least ~1815 and ~1629 cal. yr BP was marked by the mangrove absence with marine particulate organic carbon, C3 terrestrial plants, and macrophytes organic matter influence with δ13C values between −26.7‰ and −20‰, δ15N values x̅ = 3.5‰ and C:N around 21.8. The second zone between ~1629 and ~853 cal. yr BP was marked by the mangrove establishment represented only by Laguncularia pollen under influence of estuarine organic matter (δ13C x̅ = −26.5‰; δ15N x̅ = 3.2‰, and C:N around 16.4). The third zone reveals an increase of Laguncularia and presence of Avicennia pollen since ~853 cal. yr BP. Near the surface (<10 cm) occurs Rhizophora pollen, indicating the establishment of this genus during the past decades, under the modern environmental condition (δ13C x̅ = −27.02‰; δ15N x̅ = 3.12‰, and C:N around 17.42). This mangrove vertical succession may be associated with the low-temperature tolerance, where the sequence Rhizophora, Avicennia, and Laguncularia occurs from the northern to the southern limits of Santa Catarina coast, respectively, reflecting the temperature gradient. Therefore, probably, the establishment of the mangrove assemblage identified along the studied pollen profile was influenced by a temperature increase of air and water during the late-Holocene and, considering the Rhizophora genus, only during the past decades.