Spatial and seasonal contrasts of sedimentary organic matter in floodplain lakes of the central Amazon basin

Abstract

Abstract. In this study, we investigated the seasonal and spatial pattern of sedimentary organic matter (SOM) in five floodplain lakes of the central Amazon basin (Cabaliana, Janauaca, Canaçari, Mirituba and Curuai) which have different morphologies, hydrodynamics and vegetation coverages. Surface sediments were collected in four hydrological seasons: low water (LW), rising water (RW), high water (HW) and falling water (FW) in 2009 and 2010. We investigated commonly used bulk geochemical tracers such as the C : N ratio and the stable isotopic composition of organic carbon (δ 13 C org ). These results were compared with lignin phenol parameters as an indicator of vascular plant detritus and branched glycerol dialkyl glycerol tetraethers (brGDGTs) to trace the input of soil organic matter (OM) from land to the aquatic settings. We also applied the crenarchaeol as an indicator of aquatic (rivers and lakes) OM. Our data showed that during the RW and FW seasons, the surface sediments were enriched in lignin and brGDGTs in comparison to other seasons. Our study also indicated that floodplain lake sediments primarily consisted of allochthonous, C 3 plant-derived OM. However, a downstream increase in C 4 macrophyte-derived OM contribution was observed along the gradient of increasing open waters -i.e., from upstream to downstream. Accordingly, we attribute the temporal and spatial difference in SOM composition to the hydrological dynamics between the floodplain lakes and the surrounding flooded forests.

Source to sink: Evolution of lignin composition in the Madre de Dios River system with connection to the Amazon basin and offshore

“…In order to evaluate the Madre de Dios data in a broader context, we compiled published lignin data from the Amazon River system [ Ertel et al ., ; Hedges et al ., , ; Farella et al ., ; Kastner and Goñi , ; Bernardes et al ., ; Aufdenkampe et al ., ; Ward et al ., , ; Sobrinho et al ., ]. As noted previously, FPOM (>0.1 or 0.45 µm and < 63 µm) and CPOM (>63 µm) in the Amazon River system have distinct lignin content (Figure a), with higher Λ lignin values in the CPOM.…”

Mercury speciation and Hg stable isotope ratios in sediments from Amazon floodplain lakes—Brazil

“…Organic matter (OM) in the floodplain lakes of the central Amazon basin may potentially be derived from: (1) terrestrial Andean clay-bound and refractory suspended particulate OM, which may be transferred to the floodplain lakes via the Solimões-Amazon and Madeira rivers; (2) nonflooded soils and litters of the Amazonian lowland forests (nonflooded forests), which is transferred to the floodplain lakes via local streams; (3) wetland soils (flooded forests) and litters (leaves, grasses, woods, etc. ), transferred to the floodplain lakes during the receding waters (falling water season) or the rainy season (RW season); (4) wetland aquatic and semi-aquatic macrophyte vegetation of the floodplain lakes; and (5) phytoplankton from the river or produced in the lake itself (Sobrinho et al 2016).…”

“…Due to the absence of random variations, we propose that a single overriding mechanism may be responsible for the observed trend. Most likely, an increased contribution of C 4 macrophytes to the soil OM is responsible for the progressively less negative values (Sobrinho et al 2016). This explanation would be supported by other studies in the central Amazon basin, which reported that the abundance of macrophytes in this lakes increases in open water lakes and floodplains (Victoria et al 1992;Martinelli et al 2003;Sobrinho et al 2016).…”

“…Most likely, an increased contribution of C 4 macrophytes to the soil OM is responsible for the progressively less negative values (Sobrinho et al 2016). This explanation would be supported by other studies in the central Amazon basin, which reported that the abundance of macrophytes in this lakes increases in open water lakes and floodplains (Victoria et al 1992;Martinelli et al 2003;Sobrinho et al 2016). THg and monomethylmercury concentrations Hg species concentrations in lake sediments presented no significant difference when compared to soils and riverine sediments (Supporting Information Material 1).…”

Carbon accumulation in Amazonian floodplain lakes: A significant component of Amazon budgets?

“…During periods of intense rain and flooding, organic material is exported from catchments via leaching and runoff (Mayorga et al 2005;Ward et al 2013). Rivers within the Amazon Basin carry megatons of organic material, a large portion of which is deposited on the floodplains and in floodplain lakes when the seasonal flooding subsides (Hedges et al 1986;Aalto et al 2003;Moreira-Turcq et al 2004;Mortillaro et al 2012;Moreira-Turcq et al 2013;Sobrinho et al 2016). As a result, the floodplains and their lakes may be significant carbon sinks in the Amazon Basin because of slow organic material decomposition in mostly anaerobic sediments (Mertes 1994;Hamilton et al 2002;Dong et al 2012;Ferland et al 2014).…”