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

Abstract

This study provides new data on the evolution of the landscape in southwestern Amazonia during the Holocene and the impact of climate change and fluvial dynamics on the region's ecosystems. Southwestern Amazonia is covered by an extensive seasonally flooded savannah, known as the Llanos de Moxos. Severe drought during the southern hemisphere winter, followed by months of permanent waterlogging, means that forests only grow on the most elevated parts of the landscape, mostly river and paleoriver levees and crevasse splays. Paleoclimate reconstructions from surrounding areas show that a shift to wetter conditions at around 4 kyr BP caused an increase in forest cover. However, the impact that this change in climate had on the landscape of the Llanos de Moxos is unknown. Published lacustrine archives from the area only cover the last 2 kyr. Here we present new data from the analysis of paleosols located along a 300 km transect across the central Llanos. The analyses of stable carbon isotopes, from 36 paleosols, and biogenic silica, from 29 paleosols, show that the patchwork of forests and savannahs that we see today was established after the 4 kyr BP climate change. During the dry period between 8 and 4 kyr BP, most of the central Llanos de Moxos, nowadays covered with seasonally flooded savannah, were covered by Cerrado-like savannah in the west and by forest in the east. However, results also suggest that, at both regional and local scales, vegetation cover has been influenced by changes in topography resulting from the region's river dynamics.

Persistent Early to Middle Holocene tropical foraging in southwestern Amazonia

“…Furthermore, the modern landscape bears no relation to the ancient one, which surrounded the study sites before the Late Holocene. Phytoliths and carbon stable isotope analyses suggest that, during the Middle Holocene, the central Llanos de Moxos was mostly covered by forest and cerrado-like vegetation ( 33 ). The occupation trajectory of these sites largely coincides with a macroregional episode of aridity during the Middle Holocene, roughly between 8000 and 5500 cal B.P., which has been linked to a weakening of the South American summer monsoon ( 34 ).…”

Early Holocene crop cultivation and landscape modification in Amazonia

“…ED1 ). This is probably far less than the original amount built in the early and mid-Holocene, as during the transition to the late Holocene, most of the rivers in the SW part of the LM became very active and many of the pre-existing soils and potential archaeological sites were covered by alluvial deposits, sometimes up to 5 m thick 15 . This explains the modern distribution of FIs and why 48% of the 6643 FIs we mapped are concentrated in a relatively small area in the north-western LM ( Fig.…”

“…A full scan of the slides was performed to detect the presence of squash, manioc and maize. Phytolith assemblages in SW Amazonia have been studied in modern soils 46 and 29 paleosols from the early and late Holocene 15 in different natural environments and land covers. In none of these natural contexts have phytoliths of Manihot or Curcubita been found, strongly suggesting that phytoliths of these two genera found in FIs are the direct result of human activity and not of the chance occurrence of wild relatives on these FIs…”

“…Paleoecological studies indicate that the LM was covered by cerrado-like savannah during the early and mid-Holocene 15 indicating that plant cultivation in the Neotropics also started in shrub savannahs along with seasonal tropical forest environments 4 . It does not come as a surprise that phytoliths derived from plants producing underground storage organs constitute an important part of the total phytolith assemblages from the LM’s FIs including Manihot , Calathea and other Marantaceae, Heliconia , Cyperaceae and Phenakospermum .…”

A arqueologia do alto Madeira no contexto arqueológico da Amazônia

“…Essa área é caracterizada por uma relativamente alta diversidade geológica, ao menos se comparada à calha do rio Amazonas e aos baixos cursos de seus principais afluentes (CPRM, 2007), manifestada em uma também rica diversidade paisagística, que inclui extensas planícies aluviais periodicamente alagadas, adjacentes a extensas áreas de terra firme e formações de serras, onde encontra-se o limite norte-ocidental do escudo brasileiro. Trata-se de uma área de ecótonos, no limite entre os ecossistemas de floresta e cerrado, cuja dinâmica, ao longo do Holoceno, evoluiu em resposta a mudanças climáticas de escalas milenares (Mayle et al, 2007;Carson et al, 2014;Lombardo et al, 2019).…”