Palaeohydrological controls on sedimentary organic matter in an Amazon floodplain lake, Lake Maracá (Brazil) during the late Holocene

Abstract

In order to understand the impact of hydrological changes of the Amazon River on sedimentary organic matter (OM) composition in Amazonian floodplain lakes, three sediment cores were collected from Lake Maracá (eastern Amazonia) along a transect from the Amazon River main channel to inland. The cores were dated with 14 C accelerator mass spectrometry (AMS) and studied by x-ray, mineralogical composition, total organic carbon (TOC) and total nitrogen (TN) contents, stable isotopic composition of TOC and TN ( δ 13 OC Cand δ 15 N) and glycerol dialkyl glycerol tetraether (GDGT) distributions. Two distinctive sedimentary depositional phases were identified based on the mineralogical composition and the geochemical characteristics of sedimentary OM. During the early-mid Holocene (~13,000-3200 cal. yr BP), low values of TOC followed by a break in sedimentation suggest a complete drying of the lake caused by drier climatic conditions. Between 3600 and 3200 cal. yr BP, this lake received a reduced influence of the Amazon River main stem. This induced a predominant deposition of C 3 -plant-derived OM supplied by surface erosion and runoff of acidic soil. A distinct connection of Lake Maracá to the Amazon River began after 3200 cal. yr BP and became permanently established, with its modern characteristics, at 1880 cal. yr BP. This change provoked an increased contribution of phytoplankton and semi-aquatic C 4 macrophytes as well as C 3 plant derived more alkaline soil OM to the sedimentary OM pool. Consequently, our study demonstrates that the source of sedimentary OM in the Amazon floodplain lakes was strongly linked to the Amazon River hydrodynamics during the late Holocene.

The Volta Grande do Xingu: reconstruction of past environments and forecasting of future scenarios of a unique Amazonian fluvial landscape

“…Downcore variability in organic and inorganic sedimentary components are commonly paired with biological proxy information (e.g., from pollen, diatoms, benthic invertebrates, and fish fossils) in the study of evolving floodplain lake systems (McGlue et al, 2012;Moreira et al, 2013). Together, these data demonstrate how climate or human activities may influence water levels, hydrochemistry, marine connectivity, and ecological relationships.…”

Late-Holocene palaeoenvironmental reconstruction from a lake in the Amazon Rainforest-Tropical Savanna (Cerrado) boundary in Brazil using a multi-proxy approach

“…Unit-I presents a decrease in average δ 13 C values, indicating an increasing influence of C3 plants, typical of forests (−25.0‰ to −35.0‰, Moreira et al, 2013b) and phytoplankton (between −28.0‰ and −34.0‰, Araújo-Lima et al, 1986), The C:N ratio did not indicate any significant change, corresponding to a mixed OM origin, and is mainly autochthonous (Figures 3 and 5). TOC and sedimentary pigment values reached the highest values of the core in this unit, while CAR decreased due to slower sedimentation rates (Figure 3).…”

As gravuras rupestres da bacia do baixo rio Urubu: levantamento e análise gráfica do sítio Caretas, Itacoatiara - Estado do Amazonas. Uma proposta de contextualização

“…Nessa época o sítio Caretas ainda não era banhado pelas águas do Urubu e, com certeza, não se encontrava inserido na fita rede de furos e lagos que tornou esse local o contexto ideal para a ocupação humana sedentária, pelo menos a partir do primeiro milênio DC, época em que se deu a formação dos primeiros sítios de terra preta, tão frequentes nos arredores do pedral, suporte da arte rupestre. (Behling et al, 2001;Keim, 1995;Moreira et al, 2013b;Moreira et al, 2009). …”