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.

巴西南部沿海地区拥有南美洲最大的湿地系统之一，对于为人类提供的动植物生物多样性和生态系统服务非常重要。为了了解这些湿地的发展，我们分析了在南里奥格兰德州沿海平原发现的硅藻泥和硬化硅藻土样品。在样品中发现的硅藻壳的碎片状态仅允许在属水平上识别大多数硅藻，但是今天在研究区域的淡水体中报告了 16 个属的组合。已确定的分类群包括浮游的Aulacoseira和Cyclotella以及几个底栖属，主要是Eunotia、Pinnularia和罗帕洛迪亚。硅藻土主要由被碱性水溶解的硅藻壳腐蚀的不规则形状的二氧化硅颗粒组成，但已鉴定出五个硅藻属。这些硅藻土聚集在最近更新世和中全新世之间干燥环境和季节性气候中的黄土沉积物上的临时池塘中。硅藻渗出液和相关的富含有机质的粉质沉积物积聚在具有丰富集水植被和大量溶解有机碳的浅湖中，由于流体动力学和低 pH 值，含有破碎但微溶解的硅藻壳，还有 Porifera 的巨核、金藻科的包囊、和丰富的禾本科植硅体。硅藻泥底下的沉积物年代为 4.6-4.4 cal ¹⁴C ka BP 并包含以底栖附生和嗜酸形式为主的硅藻组合，例如桉树属、羽属植物属和Gomphonema 属。覆盖在它上面的软泥和沉积物包含一个以浮游的Aulacoseira和Cyclotella为主的组合, 表明 ≥1 m 深的水柱受中等风力驱动的水动力和可变地下水位的影响。这一演替表明，在全新世中期，现代湿地的发展伴随着植被的扩张，是由于全新世低温湿热区的潮湿和温暖气候导致的高海平面和降雨量增加以及生产力增加的共同作用。硅藻泥中硅藻壳的密集堆积表明大约 4 ka BP 附近发生了更干燥的事件。结果表明，保存为硅藻土的硅藻组合可用于了解巴西南部湿地对环境变化的起源和响应，这可能有助于解决人为影响的程度和对未来气候变化的响应，