Abstract
Suspended particulate matter (SPM) was collected along the Amazon River in the central Amazon basin and in three tributaries during the rising water (RW), high water (HW), falling water (FW) and low water (LW) season. Changes in the concentration and the distribution of branched glycerol dialkyl glycerol tetraethers (brGDGTs), i.e., the methylation index of branched tetraethers (MBT) and the cyclization of brGDGTs (CBT), were seen in the Amazon main stem. The highest concentration of core lipid (CL) brGDGTs normalized to particulate organic carbon (POC) was found during the HW season. During the HW season the MBT and CBT in the Amazon main stem was also most similar to that of lowland Amazon (terra firme) soils, indicating that the highest input of soil-derived brGDGTs occurred due to increased water runoff. During the other seasons the MBT and CBT indicated an increased influence of in situ production of brGDGTs even though soils remained the main source of brGDGTs. Our results reveal that the influence of seasonal variation is relatively small, but can be clearly detected. Crenarchaeol was mostly produced in the river. Its concentration was lower during the HW season compared to that of the other seasons. Hence, our study shows the complexity of processes that influence the GDGT distribution during the transport from land to ocean. It emphasizes the importance of a detailed study of a river basin to interpret the MBT/CBT and BIT records for paleo reconstructions in adjacent marine setting.
Highlights
Branched glycerol dialkyl glycerol tetraethers (Figure 1) are membrane-spanning lipids most likely of anaerobic (Weijers et al, 2006a,b) and heterotrophic (Pancost and Sinninghe Damsté, 2003; Oppermann et al, 2010) bacteria that are ubiquitous in peats (Weijers et al, 2006a; Huguet et al, 2010) and soils (Weijers et al, 2007a; Peterse et al, 2012)
Our results reveal that the influence of seasonal variation is relatively small, but can be clearly detected
The seasonal pattern of the core lipid (CL) Branched glycerol dialkyl glycerol tetraethers (brGDGTs) concentrations in μg gPOC−1 and to a lesser extent in ng L−1 was similar to the hydrological pattern of the Amazon main stem
Summary
Branched glycerol dialkyl glycerol tetraethers (brGDGTs) (Figure 1) are membrane-spanning lipids most likely of anaerobic (Weijers et al, 2006a,b) and heterotrophic (Pancost and Sinninghe Damsté, 2003; Oppermann et al, 2010) bacteria that are ubiquitous in peats (Weijers et al, 2006a; Huguet et al, 2010) and soils (Weijers et al, 2007a; Peterse et al, 2012). Crenarchaeol (Figure 1) is a membrane-spanning lipid of Thaumarchaeota, formerly known as Group I Crenarchaeota (Brochier-Armanet et al, 2008; Spang et al, 2010). It is abundant in aquatic environments: oceans (Schouten et al, 2002; Kim et al, 2010), lakes (Blaga et al, 2009; Sinninghe Damsté et al, 2009; Powers et al, 2010), and rivers (e.g., Herfort et al, 2006; Kim et al, 2007; Zell et al, 2013), and occurs in soils (Weijers et al, 2006b). The brGDGT and crenarchaeol concentrations are used to calculate the branched and isoprenoid tetraether (BIT) index, which www.frontiersin.org
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