Paleoecological indicators of the highstand sea level on the Amazonian supralittoral until the last two millennia

Abstract

Holocene sea-level changes are ubiquitous worldwide. The relative sea levels (RSL) of the Northeast and Southeast littoral zones of Brazil were higher than the current level during the middle Holocene. It has been assumed that the RSL was stable since 7 ka BP in the North Brazilian littoral, particularly in the Amazonian coast. Based on a multiproxy approach (pollen content, δ13C, 14C-dating, and sedimentary and geomorphological features) applied to a sediment core obtained from the Amazonian supralittoral zone (>50 km away from the modern coastline), our study investigated the signatures of the RSL highstand in this region. The results show that the marine influence was more intense from 8410 to 1490 cal yr BP than that of the present. During this period, a mangrove community represented by Rhizophora and Avicennia established and thrived on the Itacuruçá tidal flat, probably due to post-glacial sea-level rise, which surpassed the present level during the middle Holocene. The mangrove population declined after 1490 cal yr BP, while freshwater vegetation developed, with the dominance of various associations of taxa. The sudden deposition of organic debris in the sedimentary record is consistent with the decrease in mangrove communities. This pronounced environmental change is possibly related to the reduction of the RSL, which reached the present-day position before the last millennium. Climate influence has not been recognized as significant in this process. Irrespective of the curve model showing the position or values from the RSL and related causes, this investigation addresses the indicators of this phenomenon. The results presented here suggest that understanding the variable trends of the Late Holocene RSL and their influence on paleoecological dynamics and paleogeographical evolution requires further advancement.