Holocene floodplain soils along the Río Mamoré, northern Bolivia, and their implications for understanding inundation and depositional patterns in seasonal wetland settings

Abstract

The Llanos de Moxos (LM) in the Bolivian Amazon basin host one of the largest seasonally inundated savannah landscapes on Earth. Very little is known of this area with regard to sedimentary dynamics, soil formation, or their relationship to longer-term climatic and hydrological variability in this setting. Here we present a detailed study of three floodplain depositional sequences building up the inundation savannah along the Mamoré River in the central LM. Pleistocene sands and silts (unit 1) are overlain by thick overbank deposits of mostly Holocene age (unit 2) on both sides of the Mamoré River, and underline the importance of extensive flooding processes for the late Quaternary sedimentary and geomorphic evolution of the LM. The fine-grained distal overbank sediments show signs of strong modification by hydromorphic processes and reflect spatial and temporal variations in flood inundation patterns. In addition, widespread dark gray to blackish soil and paleosol horizons are intercalated with the overbank sediments, but lack evidence for significant weathering. These horizons are likely the result of cumulative incorporation of organic material under conditions of particularly low sedimentation rates. Therefore, their formation should be linked to processes such as channel migration, longer-term meanderbelt evolution, avulsive drainage reorganization, changes in sediment supply or climatically induced variations in flooding type, frequency or magnitude. The common occurrence of these floodplain soils in the early to mid-Holocene may thus reflect the combined effects of hydrological, geomorphic and sedimentary changes during a drier mid-Holocene.

Holocene land cover change in south-western Amazonia inferred from paleoflood archives

“…These are very similar to the 420 and 159 paleosols, which also show gypsum crystals, but very different from paleosol 480 (10486 ± 221 cal yr BP), which has by far the highest amount of diatoms and sponge spicules. Similar paleosols containing gypsum crystals, dated between ~8 and 6 kyr BP, have been described elsewhere along the Mamoré River banks and interpreted as backswamps of the Mamoré River (May et al, 2015) or as resulting from a downriver tectonic uplift and establishment of a lake-like environment (Lombardo, 2014). However, neither of these two hypotheses alone explain the formation of gypsum crystals or the establishment of Cerrado-like vegetation in the 499-2 paleosol.…”

“…The paleosols of profile 499, taken from an outcrop along a Mamoré River bank, are quite surprising. Paleosols along the Mamoré outcrops have been interpreted as cumulative soils formed by continued deposition of clay particles, but the geomorphological context under which these clays were deposited is still unclear (Lombardo, 2014;May et al, 2015). The two lowest paleosols, ca.…”

Alluvial plain dynamics and human occupation in SW Amazonia during the Holocene: A paleosol-based reconstruction

“…All the Mamoré's profiles have a shallow paleosol covered by clay. The paleosols are found at a depth between 60 and 120 cm below the surface and overlay clay layers with strong red oxidation and the presence of gypsum crystals, similar to other stratigraphic profiles studied along the banks of the Mamoré River (May et al, 2015). Profile 499, the Mamoré river bank outcrop, has a total of four paleosols.…”

“…Two other sub-regions, the Maniqui and the Sécure, have been defined based on the spatial distribution of the paleorivers observable in satellite imagery (Lombardo, 2016). The Mamoré sub-region is defined based on its geographical closeness to the Mamoré River and the similarity of its stratigraphy with published stratigraphy along the Mamoré river banks (May et al, 2015). Finally, the Río Grande sub-region is defined as the area covered by this river's paleochannels and the late Holocene sedimentary lobe, as described in Lombardo et al (2012) and in Plotzki et al (2015).…”

Holocene floodplain evolution in a central European loess landscape – geoarchaeological investigations of the lower Pleiße valley in NW Saxony

“…No clear sediment unconformities were detected in the overbank fines, although geomorphologically stable periods with less flooding often led to soil formation with the accumulation of organic material in floodplains of other regions (Zielhofer et al, 2009;May et al, 2015;von Suchodoletz et al, 2018b). Even at the Slavic level initial enrichment of organic material could not be detected (Fig.…”