The Fate of Carbon in Sediments of the Xingu and Tapajós Clearwater Rivers, Eastern Amazon

Abstract

The Xingu and Tapajós rivers in the eastern Amazon are the largest clearwater systems of the Amazon basin. Both rivers have "fluvial rias" (i.e., lake-like channels) in their downstream reaches as they are naturally impounded by the Amazon mainstem. Fluvial rias are widespread in the Amazon landscape and most of the sedimentary load from the major clearwater and blackwater rivers is deposited in these channels. So far, little is known about the role of Amazon rias as a trap and reactor for organic sediments. In this study, we used organic and inorganic geochemistry, magnetic susceptibility, diatom, and pollen analyses in sediments (suspended, riverbed, and downcore) of the Xingu and Tapajós rias to investigate the effects of hydrologic variations on the carbon budget in these clearwater rivers over the Holocene. Ages of sediment deposition (∼100 to 5,500 years) were constrained by optically stimulated luminescence and radiocarbon. Major elements geochemistry and concentration of total organic carbon (TOC) indicate that seasonal hydrologic variations exert a strong influence on riverine productivity and on the input and preservation of organic matter in sediments. Stable carbon isotope data (δ 13 C from −31.04 to −27.49‰) and pollen analysis indicate that most of the carbon buried in rias is derived from forests. In the Xingu River, diatom analysis in bottom sediments revealed 65 infrageneric taxa that are mostly well-adapted to slack oligotrophic and acidic waters. TOC values in sediment cores are similar to values measured in riverbed sediments and indicate suitable conditions for organic matter preservation in sediments of the Xingu and Tapajós rias at least since the mid-Holocene, with carbon burial rates varying from about 84 g m −2 yr −1 to 169 g m −2 yr −1 . However, redox-sensitive elements in sediment core indicate alternation between anoxic/dysoxic and oxic conditions in the water-sediment interface that may be linked to abrupt changes in precipitation. The variation between anoxic/dysoxic and oxic conditions in the water-sediment interface controls organic matter mineralization and methanogenesis. Thus, such changes promoted by hydrological variations significantly affect the capacity of Amazon rias to act either as sources or sinks of carbon.

Integrating carbon emission, accumulation and transport in inland waters to understand their role in the global carbon cycle

“…For example, long‐term C accumulation rates are not typically measured in streams and rivers as they are often assumed to be negligible compared to the other fluxes. Yet, C can accumulate in sediments of streams (Jones, 1997; Webster & Meyer, 1997), large rivers (Bertassoli et al, 2017) and river floodplains (Lininger et al, 2018; Wohl & Pfeiffer, 2018), but these rates have never been integrated with emissions and transport. Likewise, C transport from lakes and reservoirs is rarely estimated because the focus has been mostly on C retention due to long water residence times (WRT) that favour emission and accumulation.…”

Provenance of heavy and clay minerals in bottom sediments of Green Lake, an Amazonian fluvial lake in Brazil

“…Nevado et al 2010), and carbon in sediments (e.g. Bertassoli Jr. et al 2017). However, little is known about the sources and the transport of sand as bed loads or its relationship with suspended sediments in Amazonian rivers (Sawakuchi et al 2018).…”

Mudanças climá¡ticas e antrópicas no leste da Amazônia durante o Holoceno tardio

“…As the Xingu River mainly drains Proterozoic and Archean rocks of the Central Brazilian Shield or their derived-sedimentary cover (Almeida et al, 1981), topography, weathering conditions, denudation patterns (6-15 × 10 -3 mm/yr, Wittmann et al 2011 and geological substrates of the Xingu watershed are markedly different from Andean-draining tributaries of the Amazon River (Konhauser et al, 1994;Guyot et al, 2007;Sawakuchi et al, 2018). These distinctions are manifested in the relatively low particulate load and high proportions of stable and conservative elements in both suspended and riverbed sediments of the Xingu River (Filizola & Guyot, 2009;Bertassoli et al, 2017). Thus, it is possible to geochemically trace the influence of Amazon River sediments in the Lower Xingu Ria.…”

“…Variations in grain size, major elements, and χfd% in core XC-03 ( Figure 3b Fe/K values have been used as a provenance proxy for distinguishing sediments from the Amazon River main stem and lowland-draining tributaries (Govin et al, 2014;Häggi et al, 2016). Modern riverbed and suspended sediments from the Amazon River present Fe/K values around 2, while sediments from the Xingu River range from 3 to 27, depending on season (Bertassoli et al, 2017;Zhang et al, 2017;Sawakuchi et al, 2018). Thus, the increase in Fe/K values, frequency-dependent magnetic susceptibility and silt and clay content in XC-03 indicate diminished relative proportions of coarser Amazon River sediments reaching the studied site (Figure 3b-d We interpret that periods of reduced contribution of Amazon River sediments to the XC-03 site can be caused by two processes: (i) a diminished efficiency of the backwater effect in the confluence of the Amazon and Xingu rivers; and (ii) a more downstream position of the mixing zone of Amazon and Xingu waters, which would increase the distance of the XC-03 site from the confluence.…”