508155 3613508155The HoloceneMoreira et al. 2013
HOL231210.1177/095968
Research paper
Palaeohydrological controls on sedimentary organic matter in an Amazon floodplain lake, Lake Maracá (Brazil) during the late Holocene
The Holocene 23(12) 1903–1914 © The Author(s) 2013 Reprints and permissions: sagepub.co.uk/journalsPermissions.nav DOI: 10.1177/0959683613508155 hol.sagepub.com
Luciane S Moreira,1 Patricia Moreira-Turcq,2 Bruno Turcq,2 Renato C Cordeiro,1 J-H Kim,3 Sandrine Caquineau,2 Magloire Mandeng-Yogo,2 Kita D Macario1 and Jaap S Sinninghe Damsté3
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 14C accelerator mass spectrometry (AMS) and studied by x-ray, mineralogical composition, total organic carbon 13 (TOC) and total nitrogen (TN) contents, stable isotopic composition of TOC and TN ( δ C OC and δ15N) 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 C3-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 C4 macrophytes as well as C3 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.
Keywords Amazon floodplain lakes, glycerol dialkyl glycerol tetraethers, Holocene, palaeohydrology, sedimentary organic matter Received 21 January 2013; revised manuscript accepted 2 September 2013
Introduction The Amazon Basin covers more than one-third of the South American continent, and its discharge contributes almost onefifth of the total discharge of all rivers of the world (Mollinier et al., 1997). Due to the flat topography, the high rainfall and the pronounced seasonality of precipitation, large areas of the Amazon Basin are periodically flooded during rainy seasons (Junk, 1997). The flooded areas cover 44% of the entire Amazon Basin (Guyot et al., 2007) and comprise one of the largest wetlands in the world (Melack et al., 2004). The Amazonian floodplains contain thousands of lakes, which are connected temporarily or permanently to the main river channel. These lakes are formed by river water-level fluctuations, which cause the formation of bars and the accumulation of river-transported sediments by diffusive overbank flows and channelized flows (Dunne et al., 1998). Hence, large quantities of sediments (Maurice-Bourgoin et al., 2007; Mertes, 1994) and associated organic matter (OM; MoreiraTurcq et al., 2004) are accumulating in the floodplain lakes. High-resolution climate records are essential in order to understand the driving forces of past climate changes (Jones et al., 2009). Lake sediment records with annual laminations are one of the prime candidates for obtaining such records (Brauer et al., 2008). Sediment cores recovered in the Amazonian lakes have also been widely used to study palaeoenvironmental and
palaeoclimatological changes in the Amazon Basin during the Holocene (e.g. Absy, 1979; Behling and Hooghiemstra, 1999; Bush et al., 2007; Cordeiro et al., 1997; Mayle and Power, 2008; Sifeddine et al., 2001; Turcq et al., 1998). Such studies have provided evidence that the Amazon Basin repeatedly experienced relatively dry periods during the Holocene. Most of these studies have, however, been conducted in the lakes disconnected from the influence of the hydrological dynamics of the Amazon River. In contrast, little is known about the functioning of the Amazon floodplain lakes in association with climate change impacts on hydrology and thus sedimentary OM during the Holocene. Therefore, the aim of this study is to investigate changes in the hydrodynamics of the Amazon River as a
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Universidade Federal Fluminense (UFF), Brazil Institut de Recherche pour le Développement (IRD), France 3 NIOZ Royal Netherlands Institute for Sea Research, The Netherlands 2
Corresponding author: Luciane S Moreira, Departamento de Geoquímica, Universidade Federal Fluminense (UFF), Niterói, RJ 24020-141, Brazil. Email:
[email protected]
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Figure 1. A satellite image showing (a) the lower Amazon basin with the location of Lake Maracá and (b) detailed sediment core sites.
response to climate changes through the study of the sedimentary OM composition during the late Holocene.
Study area Lake Maracá is situated near the city of Monte Alegre on the southern bank of the Amazon River at 500 km from the mouth of the estuary (Figure 1). This lake represents an area of 50 km2, and during periods of high water-level, it is connected to the river main channel and with Comprido Lake (Moreira et al., 2013). During the period of low water-level, the lake is connected to the Amazon River only by small channels and no connection exists with the other lakes (Figure 1). The bedrock of the Terra Firme (i.e. unflooded upland) in the Maracá catchment area is the Cretaceous Alter do Chão Formation (Latrubesse et al., 2009), which has been subjected to intense long-term weathering processes (Irion, 1984). The main clay mineral delivered by Terra Firme creeks is predominantly kaolinite (Amorim, 2010; Behling et al., 2001; Guyot et al., 2007). The catchment area is characterized by a humid tropical climate without long dry periods. The annual mean precipitation is about 2200 mm, and the annual mean air temperature is about 27°C (Projeto Radambrasil, 1974). The lake is surrounded by a dense tropical rain forest (Terra Firme forest) in the southern bank, and a forest– savanna transition in the northern bank (Projeto Radambrasil,
1974). Around the lake, there are also pioneer formations (grasslands) with the predominance of Paspalum fasciculatum, Paspalum repens, Echinochloa polystachya (C4 plants) and Eichornia crassipes (a C3 plant).
Material and method Sediment cores Three sediment cores (MAR3, MAR1 and MAR2) along a transect from the Amazon River main channel to the inland were collected manually in Lake Maracá in January 2007 (Figure 1) at a water depth of approximately 2 m at all three core sites. Apparently, no differences in the depth occur along the transect. The cores were opened, described, photographed and submitted to x-ray analyses with the SCOPIX x-ray equipment in the Environnements et Paléoenvironnements Océaniques et Continentaux (EPOC) laboratory at the University of Bordeaux I (France). The x-ray images of the cores allow identification of sedimentary structures with high resolution. SCOPIX uses classical x-ray equipment (x-ray source: 160 kV, 19 mA), coupled with new radioscopy instrumentation (charge-coupled device (CCD) camera 756 × 581 resolution), connected to a computer for data acquisition and processing (Migeon et al., 1998). The processing software of this equipment displays the greyscale intensity logs, corresponding to x-ray densities.
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Radiocarbon (14C) analysis The 14C measurements were performed on total organic carbon (TOC) by an Artemis accelerator mass spectrometry (AMS) system based on a 3MV Pelletron from National Electrostatics Corporation (NEC; Middleton, WI, USA) at ‘Laboratoire de Mesure du Carbone 14’ (LMC14) – UMS 2572 (CEA/DSM CNRS IRD IRSN – Ministère de la Culture et de la Communication). Microscopic analyses are performed to remove possible contaminants. After that, the samples are treated with HCl 0.5 N at 80°C for 1 h to remove carbonates. The calibrated ages were obtained using the CALIB 6.0 available at http://radiocarbon.pa.qub.ac.uk/calib (Stuiver et al., 1998). The calibration curve used was IntCal09.14C. In order to obtain age–depth models, the software CLAM (Blaauw, 2010; current version 2.10.1) was used based on the linear interpolation (Blaauw, 2010).
Clay mineral analysis Clay mineralogy was determined on the
