Dynamics of the Pantanal Wetland in South America

Underneath the wetlands of the Brazilian Pantanal are hidden key ecological and geological events of the history of our planet. In this chapter we show that Precambrian rocks forming the hills and mountains surrounding the Pantanal floodplains record (a) the cyclic process of supercontinents assembling, (b) the origin of complex life forms on Earth, and (c) the past global climate changes. It further unveiled the most recent geochronological data and paleontological and tectonic discoveries for modeling the evolution of the Pantanal basement rocks. Various questions are also addressed, including the formation time of the Rodinia and Gondwana supercontinents, the triggering factor leading to animal skeleton biomineralization, and the “Snowball Earth Hypothesis.”

What is an inlier sedimentary basin? What are the main mechanisms of sedimentary infilling? How do the depositional systems behave? And last, but certainly not the least, what geological events occurred in the last million years and continue to take place in the Pantanal area today? These issues are considered in this chapter, based on available geological, geomorphological, and geochronological datasets. The Pantanal is an active sedimentary basin with numerous faults and associated earthquakes. Movements along these faults cause subsidence on blocks within the basin, generating depressions that are highly susceptible to flooding, and also create accommodation space for sediment storage. One hypothesis on the origin of the Pantanal Basin relates the processes of subsidence with tectonic activity in the Andean orogen and foreland system during the Quaternary. Alternatively, the lack of geochronological data leaves open the possibility that the basin formed much earlier, perhaps during an interval of widespread tectonism in Brazil during the Eocene. The modern Pantanal depositional tract is composed of the Paraguay River trunk system, numerous fluvial megafans and interfan floodplains, and thousands of lakes, many of them integral to the Nhecolândia landscape. The Pantanal’s geomorphology is most likely the product of climatic fluctuations and environmental changes that have been occurring since the Late Pleistocene. Relict morphologic features like paleochannels have been preserved on the surfaces of abandoned lobes on several large fluvial megafans. After a period dominated by arid conditions in the Late Pleistocene, the Pantanal area experienced an episode of humidification and increasing fluvial discharge in the Early Holocene. This process promoted important modifications in the extant drainage system, for example, the avulsion of the Paraguay River that caused the Nabileque paleomeander belt to be abandoned. The landscape and sedimentary deposits of Pantanal Basin are “living” geologic records of changing rivers, avulsions, floods, and climate changes that occurred in the last several thousand years. Understanding the dynamics of these transitions is critical for unveiling the geologic history of the world’s largest tropical wetland.

In spite of its global significance to biodiversity and biogeochemical cycles (e.g., as a methane source and carbon dioxide sink), the Pantanal of western Brazil remains underexplored from the perspective of Quaternary paleoecology, paleogeography, and paleoclimatology. Long in the scientific and cultural shadow cast by the Amazon Basin, recent research using lake sediment cores from different sites across the Pantanal lowlands has provided a glimpse at the sensitivity of this savanna floodplain wetland to climate-driven perturbations in the hydrologic cycle. Understanding the controls and feedbacks associated with this sensitivity is important, as the Pantanal is a critical freshwater resource situated in the headwaters of the immense Río de la Plata Basin. Published lake sediment archives have adopted a multi-indicator analytical approach, focusing on physical sedimentology, geochemistry, palynology, and siliceous microfossils. Such studies extend in time from the late Pleistocene to the present day, with the greatest emphasis placed on reconstruction of the Holocene environmental history. Several important transitions in effective precipitation have been inferred for the Holocene, which appear to be dominantly linked to variability in insolation and the South American Summer Monsoon system. By contrast, evidence of aridity in the Pantanal during the Last Glacial Maximum suggests that the wetlands also respond in a complex manner to Northern Hemisphere ice volume and that insolation forcing alone fails to fully explain patterns of environmental change. The great diversity of lacustrine ecosystems in the Pantanal warrant additional study and hold the potential to broaden our understanding of the response of tropical wetlands to global change. Such insights will be valuable for conservation planning, resource security, and sustainable management.

This chapter presents and discusses the avulsive nature of the Pantanal rivers and shows how the ever-changing drainage network influences the surface hydrology and ecology. Besides, the systemic portrait here outlined provides new insights concerning the Pantanal hydrodynamics, in its particularities and as a whole system. A simple model of the avulsion process is illustrated, and several realistic examples of the processes leading to river avulsions are shown and discussed. The north-to-south flood-pulse wave due to the presence of bottlenecks is further described in detail. This systemic approach allows identifying that the fluvial “avulsive and bottleneck” dynamics seasonally affects both local and regional ecohydrological processes. Moreover, it shows that avulsive processes are commonplace in Pantanal, and changes in land use, particularly in river headwaters in the highlands, accelerate the avulsions, making the sustainable use of the Pantanal lowland areas difficult.

Scaling invariance in living systems emerges from complex interactions of organisms with the physical world. According to the Metabolic Scaling Theory (MST), the way that energy and materials are distributed generally follows an invariant power law scaling with the body mass, independent on the species and the environment. Such generalization has been defined universal or ubiquitous, which is however not broadly accepted. For native woody savanna species in the Nhecolândia landscape, the scaling between trunk diameter and the whole plant body mass as d ~ m ^3/8 follows MST prediction. Nevertheless, empirical data and model suggest biomass allocation beyond 50% to branches for trunk diameters above 18 cm, whereas root–trunk ratio does not vary significantly with plant size. The elevated water table explains such biomass allocation by limiting vertical root growth while enhancing branch growth to cope with evapotranspiration. Therefore, empirical deviations from MST scaling exponents of biomass partitioning for these plants can be understood as ecohydrological adaptations to conspicuous physical constraints.

Alkaline lakes are diversified ecosystems of the Nhecolândia landscape in the southern Taquari Megafan, Pantanal. By 1985 they covered an approximate area of 1,060 km² distributed in nearly 12,761 shallow water bodies. Historical Landsat-5 image analysis from 1985 to 1998 indicates a reduction of 24% in the area and 28% in the number of these shallow lakes. The alkaline lakes in the region can be generally arranged in three major ecological typologies: macrophyte lakes (autotrophic above waterline, lower electron conductivity (EC), and pH), bacterial lakes (heterotrophic, moderate EC, and pH), and saline lakes (autotrophic below waterline, elevated EC, and pH). During 1985–1998, the relative area of macrophyte lakes and bacterial lakes decreased 22 and 40%, respectively, while saline lakes increased 53%. Alkaline lakes may interchange their ecological typologies as a function of short- and long-term water rainfall, surface water flows, and interplay with the aquifer. The decline of lake area and number, and the particular increase in saline lakes, can be associated to highland deforestation and its impact on the regional rainfall distribution. Ancillary data of water chemistry and greenhouse gases allowed sketching a general biogeochemical model of macrophyte and saline typologies and deriving a gross estimation of carbon exchanges with the atmosphere from these alkaline lakes in the Nhecolândia landscape.

Worldwide wetlands contribute to the global carbon cycle by emitting about a third of the global methane (CH₄) emissions. However, CH₄ and carbon dioxide (CO₂) dynamics remain poorly understood in the largest tropical wetland on Earth, the Pantanal. In this chapter, we aim to characterize the CH₄ and CO₂ biogeochemistry in the floodplain of the Paraguay River, near Corumbá, during the course of annual anoxia phenomena locally known as dequada. The strong anoxia is associated to the flooding of terrestrial habitats that enhances respiration, dissolved oxygen (DO) consumption, and methanogenesis. The extremely low DO also leads to high fish mortality in the region. CH₄ and CO₂ concentration in surface waters and diffusive water–air fluxes were measured in the oxbow Tuiuiú Lake and in the Paraguay River main stem in order to identify temporal and spatial patterns. The whole dataset shows that, for instance, dissolved CH₄ and diffusive CH₄ fluxes increased dramatically during the dequada. In the study area, CH₄ emissions can reach 9–85 mg CH₄ m⁻² h⁻¹ during dequada climax. Riverine anoxic waters steadily penetrate the oxbow Tuiuiú Lake, indicating water inflow from the river main stem, whereas small reminiscent patches of oxbow waters not mixing with anoxic river waters may function as survival refuges to the aquatic wildlife. Clearly, the DO distribution during several dequadas in major rivers of the Pantanal highlights the importance of geomorphology on the biogeochemistry in the riverine floodplains of the Pantanal wetland.

This chapter presents a brief discussion on the potential for contamination of the Pantanal by pesticides and reviews the studies, which evaluated these substances in different matrices in the Pantanal and its surroundings. Agricultural activities occur mainly in the highlands around the floodplain and represent the main source of pesticides to the Pantanal. It is evident that they can be transported to the lowlands, but the pesticides were detected in low frequency and concentrations so far. There is no monitoring program, and a few studies regarding this contamination are limited in sampling frequency and spatial distribution as well as in analyzed substances that considered the large spectrum of pesticides used. Moreover, only water and sediment have been subject to analysis, and no biological indicators have been studied. Also, to the moment, no risk analysis has been carried out indicating the need for more studies in the region in order to prevent future damages to this important ecosystem.

The state roads MS-184 and MS-228 constitute the Estradas Parque Pantanal. Located in the northwestern portion of Mato Grosso do Sul (Brazil), in the hydrographic basin of the Paraguai River, and in the subbasins of the Negro, Miranda, and Taquari rivers, the roads cross an important area of the Pantanal in the State of Mato Grosso do Sul, in the subregions of Paraguai, Paiaguás, Nhecolândia, Abobral, Nabileque, and Miranda. Besides the strategic transportation aspect, there is ecological, cultural, scenic, and recreational potentials leading to tourism. It is an area with special interests, and the lack of data regarding environmental features motivated the study by GIS algebra mapping to consider jointly geology, geomorphology, soil and vegetation maps, information on climate, flooding areas, and land use and occupation. The obtained results are synthesized in natural and environmental vulnerability maps that indicate moderately stable/vulnerable areas tending to moderately vulnerable. The Estradas Parque Pantanal is undergoing ecological pressure, which means that further changes applied to the landscape can alter its “status” to vulnerable, increasing the risk of soil losses by erosion.

The South America Pantanal is a large floodplain wetland in the center of the Upper Paraguay River Basin, which has a total area of around 360,000 km². Large sectors of the Pantanal floodplain are submerged from 4 to 8 months each year by water depths from a few centimeters to more than 2 m. Changes in rainfall and temperature and also on land use can affect significantly the flood season with severe consequences for downstream inhabitants. However, impact of climate change on wetlands is small so far compared to the damage caused by the lack of management at the local level due to land-use change. In this chapter we assess climate and hydrology variability for the present and projections of climate change using the global climate models from the Fifth Assessment Report (AR5) from the Intergovernmental Panel on Climate Change (IPCC). Projections show that by the end of the century, temperatures can increase up to 7°C and rainfall can decrease in both summer and particularly winter. The possibility of longer dry spells and increased evaporation may affect the water balance in the region. However, uncertainties on climate projections are still high, particular for rainfall.