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

Abstract

The present study reconstructs Holocene fluvial dynamics in the southern Amazonian foreland basin through the analysis of 36 stratigraphic profiles taken along a 300 km long transect across the Llanos de Moxos (LM), in the Bolivian Amazon. Based on 50 radiocarbon ages from paleosols intercalated with fluvial sediments, the most important changes in floodplain dynamics on a millennial scale are reconstructed and the links between pre-Columbian cultural processes and environmental change in the region explored. Results show that the frequency of river avulsions and crevasses, as inferred from the number and age of the cored paleosols, is stable from 8k cal. yrs BP to 4k cal. yrs BP and increases significantly from 4k to 2k cal. yrs BP, following the strengthening of el Niño/la Niña cycle and an increase in average precipitation. Fluvial activity then decreases and reaches its minimum after 2k cal BP. A comparison between the stratigraphic record and the archaeological record shows a match between periods of landscape stability in SW Amazonia (low river activity) and periods of pre-Columbian human occupation. The first Amazonians lived in the LM until 4k yrs. BP, when an abrupt increase in the frequency of river avulsions and crevasses forced the abandonment of the region. After two thousand years of archaeological hiatus, which matches the period of highest river activity in the region, agriculturists reoccupied the Bolivian Amazon.

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

“…The environment of the Llanos de Moxos, in SW Amazonia, changed during the Holocene due to several factors: climate, tectonics, fluvial activity and human action (Carson et al, 2014;Dumont and Fournier, 1994;Lombardo, 2014;Lombardo et al, 2012;Mayle et al, 2000;Mayle and Power, 2008). It is extremely difficult to disentangle how and when each of these drivers affected the land cover, not only because of the implicit difficulties in reconstructing past vegetation per se, but also because we lack a chronological framework for neotectonics (Dumont and Fournier, 1994;Lombardo, 2014) and have a rather general one for climate and fluvial events (Lombardo et al, 2018). The lack of lacustrine archives from the region makes matters worse, as these would provide a more solid chronology for vegetation changes.…”

“…In most cases, humates and bulk fractions provided consistent ages, while humines were either present in insufficient quantity for dating or yielded anomalously old ages (Lombardo, et al, 2018). The humine fraction has been interpreted as resulting from the most recalcitrant part of the soil's organic matter, and thus more likely signifies the average age of the palesol rather than the time of its burial (Lombardo, et al, 2018). Therefore, the bulk fraction (or humate fraction if bulk was not available) was used to date the time of the burial of the paleosol.…”

“…Early to mid-Holocene paleosols have been found below the sediments of a distal part of the Grande River fan (Lombardo et al, 2012) and below the alluvia covering most of the south-western LM (Lombardo et al, 2018). The present study builds on previous research carried out in the region (Lombardo et al, 2018). Here, bio-geochemical analyses from the paleosols described in are used in order to reconstruct past vegetation dynamics.…”

“…The southern movement of the South American Summer Monsoon caused an increase in precipitations at about 4 to 3 kyr BP and the southward expansion of the Amazonian rainforest into north-eastern LM at the expense of the savannah (Carson et al, 2014;Mayle et al, 2000). Large scale river shifts, such as those of the Beni River (Dumont, 1996), the Mamoré (Plotzki et al, 2013), the Grande River (Lombardo et al, 2012;Plotzki et al, 2015) and several other tributaries of the Mamoré (Lombardo, 2014;Lombardo, 2016), formed fluvial distributary systems that covered most of the CSM (Lombardo et al, 2018). During the late Holocene, the combined action of strong winds and waves formed the hundreds of geometric and oriented lakes that dot the LM landscape (Lombardo and Veit, 2014).…”

“…To test this hypothesis and reconstruct the evolution of the land cover in the Llanos de Moxos during the Holocene, we analysed stable carbon isotopes and biogenic silica assemblages of paleosols found along a 300 km long stratigraphic transect across the LM ( Fig. 1) which included 36 stratigraphic profiles (Lombardo et al, 2018).…”