Late Pleistocene-Holocene diatomites from the coastal plain of southern Brazil: Paleoenvironmental implications

Abstract

The southern Brazilian coastal area harbors one of the largest wetland systems of South America, important for its plant and animal biodiversity and ecosystems services provided for human populations. As an attempt to understand the development of those wetlands, samples from a diatom ooze and indurated diatomites found in the coastal plain of the state of Rio Grande do Sul were analyzed. The fragmented state of the frustules found in the samples allowed identification of most diatoms only to the genus level, nevertheless the assemblages are represented by 16 genera reported today in freshwater bodies in the studied region. The identified taxa include the planktonic Aulacoseira and Cyclotella and several benthic genera, mostly Eunotia, Pinnularia and Rhopalodia. The diatomites consist mostly of corroded irregular-shaped silica grains from frustules dissolved by alkaline water, but five diatom genera were identified. These diatomites accumulated in ephemeral ponds developed on loess sediments in dry environments and seasonal climate between the latest Pleistocene and Middle Holocene. The diatom ooze and associated organic-rich silty sediments accumulated in shallow lakes with abundant catchment vegetation and considerable dissolved organic carbon, and contain fragmented but slightly dissolved frustules as a result of hydrodynamics and low pH, and also megascleres of Porifera, cysts of Chrysophyceae, and abundant phytoliths of Poaceae. The sediment underlying the diatom ooze was dated as of 4.6–4.4 cal 14C ka BP and contains a diatom assemblage dominated by benthic epiphytic and acidophilous forms such as Eunotia, Pinnularia, and Gomphonema. The ooze and sediment overlying it contain an assemblage dominated by the planktonic Aulacoseira and Cyclotella, indicating ≥1 m-deep water column subject to moderate wind-driven hydrodynamics and variable water table. This succession shows that the development of the modern wetlands during the Middle Holocene, accompanied by the expansion of vegetation, resulted from a combination of high sea-level and increased rainfall and productivity driven by the wetter and warmer climate of the Holocene Hypsithermal, although the dense accumulation of frustules in the diatom ooze suggests a drier episode around ~4 ka BP. The results show that diatom assemblges preserved as diatomites can be used to understand the origins and response of the southern Brazilian wetlands to environmental changes, which may help address the extent of anthropogenic influence and response to future climate change, thereby contributing for the conservation and management of the coastal ecosystems.