Spring 2016

Pioneer Paleolimnology Lab

A Holocene Paleoenvironmental Reconstruction of a Unique Amazonian Landscape using Lake and Ria Sediments

Motivation

The operation of the 5km-wide Belo Monte Dam in Altamira (Brazil) could flood up to 6000 km2 and force the relocation of 20,000 people from 18 ethnic groups and indigenous communities. In addition to the social injustice, the Belo Monte project is expected to result in a tremendous loss of biodiversity downstream in the Volta Grande do Xingu, including many endemic fish and reptiles. This study was performed to examine the environmental sensitivity of the region using lake sediment cores. We can use insights from the Quaternary to better understand how the Volta Grande riverine landscape has responded to changes in climate and hydrology. These results may help us predict potential aquatic ecological changes associated with dam emplacement and altered tropical river dynamics in the future.

Background

This study is centered on a 122 cm sediment core that was extracted from a floodplain lake within the Volta Grande, a large bend in the Xingu River upstream of the vast Xingu ria. The core represents a depositional record of the mid-Holocene to present (~7000 yrs). The Xingu River is a bedrock-dominated anastomosing river in the Altamira region, draining a watershed of ~510,000 km2. Controlled by tectonics, the river flows over Archean gneisses, Devonian shales, siltstones, and sandstones, and Triassic-Jurassic diabase before entering the Amazon sedimentary basin. At this transition, the river forms a sharp bend and generates steep rapids which harbor substantial endemic species, including catfish and stingrays. Stable sandbars, large rapids, and a series of anastomosing channels together generate the hydrological conditions that gives the Xingu River its rich biodiversity.

Methods

All samples were analyzed in 2 cm intervals and routinely quality checked with standards. Geochronological and magnetic susceptibility values were provided by University of São Paulo.

• ED-XRF

  • Bruker Tracer IV-SD

• Carbonate Coulometry

• LECO

• Magnetic Susceptibility

• Geochronology

  • Radiocarbon (AMS)

  • Quartz OSL

Biodiversity

The Xingu River hosts a great variety of tropical freshwater fish; dam emplacement is predicted to set a record for biodiversity loss in this region. The Belo Monte Dam will impact the aquatic environment by blocking passages for migratory species, reducing flow velocity, and delaying seasonal flood pulses. Several endemic species of reophilic and armoured catfishes, including the cherished Hypancistrus zebra will be endangered as a result of the dam.

Results

The lake sediment core is interpreted to reflect three phases of depositional environment evolution in this upstream region of the Volta Grande do Xingu system. An integrated assessment of sedimentology, geochemistry, physical properties, and geochronological data indicates that topographic closure and lake formation began ~3900 yrs BP. Intriguingly, the inception of the lake phase immediately follows the famous 4.2 ka arid event that impacted much of tropical South America. Recovery of the monsoon in the late Holocene may have helped the basin maintain a positive water balance, as indicated by accumulation of organic matter-rich clays. Quartz OSL dating produced ages of 3090 ± 30 y BP at 15cm and 6840 ± 513 y BP at 120 cm. AMS produced a 4188 ± 378 y BP date for 84cm. General sedimentation rates were able to be calculated using these ages.

The core from the Xingu ria is much shorter temporal record (~1000 yrs in length), suggesting high sediment accumulation rates. Multi-indicator analysis has revealed the presence of high frequency “event layers” that stand out in sediment chemistry and magnetic properties. Our preliminary interpretation is that ria sedimentation is sensitive to large storms, and that the core may be one of the few records of late Holocene paleotempest activity in the lower Amazon. Future work on the age model will help clarify potential climatic forcings for these storms (e.g., solar activity, ENSO, etc).

Paleogeographical illustration of the Volta Grande near Altamira from ~7000 y BP - present. River appears blue, and the landscape appears green-brown (lighter, brown hues signify relative degree of aridity).

Conclusions

• Lithofacies and chemostratigraphic data indicate a three phase evolution (fluvial → transitional→lacustrine) of depositional environments at the island lake site. The lake phase is marked by clay and organic matter rich sediments, whereas the fluvial environment (channel center bar and/or unvegetated island) are sandy and organic lean.

• The inception of the floodplain lake occurs after the 4.2 ka arid interval in South America, suggesting that resumption of the summer monsoon may have been important for the evolution of this lentic ecosystem.

• Farther downstream, the Xingu ria enters the Amazon sedimentary basin forming a single wide slackwater channel. Preliminary analysis of the ria core suggests high sedimentation rates and the potential for a late Holocene paleotempest record that will be explored in the future.

• Natural factors influencing flow in the river clearly affect the surrounding environment, and changing the flow dynamics could alter ecological processes in severe ways.

References

Bertani, T. C., et al., (2015). Understanding Amazonian fluvial rias based on a Late Pleistocene–Holocene analog. Earth Surface Processes and Landforms, 40(3), 285-292.

McGlue, M. M., et al., (2012). Lacustrine records of Holocene flood pulse dynamics in the Upper Paraguay River watershed (Pantanal wetlands, Brazil). Quaternary Research, 78(2), 285-294.

Pérez, M. S. (2015). Where the Xingu Bends and Will Soon Break. American Scientist, 103(6), 395.

Sawakuchi, A.O., et al., (2015). The Volta Grande do Xingu: reconstruction of past environments and forecasting of future scenarios of a unique Amazonian fluvial landscape, Sci. Dril., 20,21-32, doi:10.5194/sd-20-21-2015, 2015.