By Edward Lo, Ph.D. Candidate and NSF Graduate Research Fellow
A broad expanse of river channels snakes towards the horizon, interspersed with a glimmering patchwork of water bodies—you can’t even begin to count them. Spotted jaguars roam the floodplain as jabiru storks glide over the South American caimans, and anteaters, giant armadillos, and marsh deer graze in the tall grasses. These animals live in a vast, open plain occasionally punctuated with rounded ridges of ancient Precambrian bedrock. Traditional pantaneiro fishing communities and Indigenous tribes sustain themselves with local resources that depend wholly on the abundant water supply.
This is the Pantanal, the world’s largest wetland. It is seven times larger than the Florida Everglades, spanning western Brazil into Bolivia and Paraguay and changing seasonally from ponds and marshes to a drier savanna. Like the Everglades, it hosts an incredibly diverse floral and faunal community. The slope of the wetland is so minimal that floodwaters linger in shallow pools, slowly draining to the rivers in the months following the wet season, creating a season of flooding. Most rainfall comes in the wet season from October to March, delivering in six months 1330 mm of rain that Jacksonville, Florida receives in a year. The Pantanal hosts fisheries, controls water and nutrient cycles, buries carbon, and provides a means of transportation for industries and traditional fishing communities that depend on the water level of the Paraguay River. But we are still working to understand what controls that all-important water.
Despite providing these critical services and unique habitats, the Pantanal is an under-studied environment in South America. With the climate becoming hotter and drier, the wetland is also one of the most fragile environments in the area and is at risk of being lost completely. The Pantanal is especially sensitive because most of its ecosystem services depend on seasonal water availability and a precariously-balanced sediment supply. The relentless wildfires that swept through the Pantanal in 2020—which stemmed from a prolonged drought—left behind exposed soils, sediment, and organic material that were flushed into the streams and ponds as the wet season returned. The river channels became overwhelmed with sediment, destabilizing their precarious balance between sediment and water. To improve our ability to predict how this ecosystem might respond to climate change, I study how different environmental variables affect the supply of the rivers’ sediments.
Changes to the sediment supply in the Pantanal, like an influx triggered by wildfires, can interrupt these inland river ecosystems by triggering changes to light penetration in the water, nutrient cycling, and water quality. Cascading biological changes follow as microorganisms, aquatic plants, and fisheries must adapt or perish. Too much sediment can accelerate the disappearance of floodplain lakes, filling up the lake basins with sand instead of water. Tributary rivers respond to more sediment by alternating their channels more frequently, which can lead to major diversions of water and sediment. In floodplains, a diverted river channel floods a new location and abandons its old riverbed, which redistributes local ecosystems; what was a grassland in one year can become a lake a few years later. Wetlands sink where sediment supply decreases, making them easier to flood; conversely, increased sedimentation raises the surface elevation until the land is above the water table and seasonal flood zones. The mineral composition of those sediments matters because it influences compaction rates (stability), soil fertility, water quality, and carbon burial, all of which affect where environments suitable for ecotourism, agriculture, cattle ranching, and fisheries occur. To predict these environmental changes, it’s important that we understand how the tropical plateau environment affects sediment supply and distribution to the lowlands.
The Pantanal lies in a 2.5-million-year-old geological depression filled with 500 meters of sediment, and that number is growing: the basin is still being filled, with sediments washing in from the broad plateau surrounding the wetland. A primary goal of my research is to answer two deceptively simple questions: where are the sediments coming from and what are they made of?
Thanks to how compositionally distinct different parts of the surrounding plateau are, I can use the sediments’ mineralogy to track where they’re coming from (sediment provenance) and potentially link that to long-term changes of the basin topography. Up in the hills surrounding the wetlands, the rock type varies in every direction. Along the eastern margin, sandstones outcrop in densely vegetated hills with jagged escarpments. The northern and southern margins expose both carbonate and metamorphic rocks (like schist and phyllite). As these plateau rocks get broken down by the hot temperatures and heavy rainfall, they turn into sediments that wash into the wetlands below, and then to the Paraguay River.
Because it’s so large, the northern Pantanal actually has a slightly different climate from the southern end; the north receives more total precipitation and a longer dry season than the south. In both regions, vegetation in the highlands further enhances chemical and physical breakdown of rock due to roots wedging their way into tiny cracks and generating potent organic acids. The localized rock outcrops and climate gradient make the Pantanal an ideal natural setting to evaluate how distinct lithologies, weathering effects, and topography influence sediment composition and transport. I expect that sediment composition is more affected by provenance—the type of rock it originally was— than by a dominant weathering intensity.
To determine how local characteristics control sediment generation, I sampled surface sediments from ~60 sites throughout the basin during 2019 plus remaining samples from my collaborators in 2021. I worked with Dr. Aguinaldo Silva at the Federal University of Mato Grosso do Sul in Brazil for ten days, logging over 5000 km in a pickup truck. At most sampling locations, I descended the river bank to collect sediment, making sure first that jaguars and other predators were nowhere in sight. In the lab, I used a petrographic microscope to visually identify and count 500 sand-sized grains from each sample and I then identified what kinds of clay minerals were present. I am now assembling a snapshot of the sediments and evaluating how variations in land cover, plateau rocks’ lithology, and hydrology affect sediment composition downriver, in the wetlands of the Pantanal.
The first 60 samples analyzed thus far are very homogeneously quartzose, independent of the exposed lithologies. Only the samples near the parent rocks have more diverse mineral compositions and larger average grain size. Because sandstones in the plateau highlands are major sources of the quartz content, the compositions reflect a mix of both intense physicochemical weathering and recycled quartz grains from earlier weathering episodes. When the analysis is completed, this study will help identify the factors that control sedimentation in the basin. This has broader implications to land use and conservation management policies that enable the community stakeholders to better predict shifting ecosystems and water resources.
As land use and climate change continue to intensify, the tropics will experience novel environmental pressures unlike that of any current place on Earth. Tropical lowlands like the Pantanal are particularly vulnerable to climate and land use change. These fragile environments provide a diverse range of ecosystem services that we have only just begun to appreciate. More comprehensive studies of tropical floodplain environments are urgently needed to understand how low-latitude landscape change differs from high-latitude floodplains. I am determined to continue addressing questions with such urgent global relevance and helping communities become more resilient and sustainable.
Edward Lo is a fourth-year NSF Graduate Research Fellow studying for a PhD in geological sciences at the University of Kentucky under the supervision of Dr. Michael McGlue. Edward received a GSA Graduate Student Research Grant in 2016 entitled “Documenting lake formation on a distal distributive fluvial system in the heart of South America.” He received a 2020 NSF/GSA Graduate Student Geoscience Grant entitled “Source-to-sink analysis in the Pantanal Basin.” Born in the US to Taiwanese immigrants who were raised in Brazil, his complex identity fuels his efforts to increase equity and inclusion in geoscience. He is keen to continue studying fluvio-lacustrine processes in the tropics and contribute to improved well-being of local communities.
GSA will begin accepting applications for 2022 Graduate Student Research Grants on 1 December 2021 at www.geosociety.org/gradgrants. The application deadline will be 2 February 2022. Edward’s 2020 grant was supported by the National Science Foundation (NSF) under Grant No. 1949901.