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

In this study, we investigated the seasonal andspatial pattern of sedimentary organic matter (SOM) in fivefloodplain lakes of the central Amazon basin (Cabaliana,Janauaca, Canaçari, Mirituba and Curuai) which have differentmorphologies, hydrodynamics and vegetation coverages.Surface sediments were collected in four hydrologicalseasons: low water (LW), rising water (RW), high water(HW) and falling water (FW) in 2009 and 2010.We investigatedcommonly used bulk geochemical tracers such as theC V N ratio and the stable isotopic composition of organic carbon(?13Corg/. These results were compared with lignin phenolparameters as an indicator of vascular plant detritus andbranched glycerol dialkyl glycerol tetraethers (brGDGTs) totrace the input of soil organic matter (OM) from land to theaquatic settings. We also applied the crenarchaeol as an indicatorof aquatic (rivers and lakes) OM. Our data showed thatduring the RW and FW seasons, the surface sediments wereenriched in lignin and brGDGTs in comparison to other seasons.Our study also indicated that floodplain lake sedimentsprimarily consisted of allochthonous, C3 plant-derived OM.However, a downstream increase in C4 macrophyte-derivedOM contribution was observed along the gradient of increasingopen waters – i.e., from upstream to downstream. Accordingly,we attribute the temporal and spatial difference inSOM composition to the hydrological dynamics between thefloodplain lakes and the surrounding flooded forests

Origin and fate of branched tetraether lipids in river drainage systems. Implications for the use of the MBT/CBP proxy as a continental palaeothermometer in marine sedimentary records

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids ofbacteria. The number of methyl groups of the alkyl chains in brGDGTs correlates with the mean annual air temperature (MAAT) and soil pH, while the number of cyclopentane moieties correlates with soil pH. Based on this observation the methylation index of branched tetraethers (MBT) and the cyclisation index of branched tetraethers (CBT) were introduced. Since brGDGTs are found in large quantities in soil, it was assumed that they are washed into rivers by erosion and transported to the ocean. Hence, the MBT/CBT proxy can be used in river influenced marine settings to reconstruct past MAAT and soil pH of the river basin. In this study the applicability of the MBT/CBT proxy in diverse river systems was assessed, by detailed studies of two river systems that are different in terms of river size and climate setting (Amazon River and Tagus River). BrGDGTs in drainage basin, soils and river suspended particulate matter (SPM) were compared to those in marine SPM and surface sediments. Core lipid (CL) and intact polar lipid (IPL)-derived brGDGTs were analyzed, in order to distinguish recently produced (IPL-derived) GDGTs from older (CL) GDGTs. The majority of brGDGTs in the Amazon River is derived from lowland basin soils. However, the distribution of CL and IPL-derived brGDGTs was slightly different to that of the soils, which showed that in-situ production of brGDGTs also played a role. The influence of seasonal variation on the brGDGT distribution is relatively small, but significant differences were detected. During the high water season the input of brGDGTs from soil increased, which was seen in increased brGDGT concentrations and a brGDGT distribution that was most similar to that of soil. During the other seasons the influence of in-situ production of brGDGTs increased. In the Amazon shelf and fan, the delivery of brGDGTs from the Amazon River was evident. However, further away from the river, the distributions of CL brGDGTs in marine SPM and sediments varied widely, since this difference was also found in IPL-derived brGDGTs it is most likely due to marine in-situ production. In the Tagus basin soils the brGDGT distributions were very variable and did not reflect the local MAAT and soil pH. The concentrations of brGDGTs in river SPM were substantially higher and the distributions were different compared to those of the drainage basin soils. This indicates that brGDGTs are mainly produced in the river itself. Therefore, the MAAT and soil pH cannot be reconstructed with the commonly used soil calibrations. Along the Portuguese margin the brGDGT concentrations rapidly decreased with increasing distance from the shore. The brGDGT distributions in marine SPM and surface sediments also changed, indicating that marine in-situ production takes place. In conclusion, this study emphasizes the importance of a detailed study of a river basin to interpret the MBT/CBT records for paleoreconstructions in adjacent marine setting. In general, it is recommendable to apply the MBT/CBT proxy only to sediment which was under strong river influence

Origin and fate of branched tetraether lipids in river drainage systems. Implications for the use of the MBT/CBP proxy as a continental palaeothermometer in marine sedimentary records

Branched glycerol dialkyl glycerol tetraethers (brGDGTs) are membrane lipids ofbacteria. The number of methyl groups of the alkyl chains in brGDGTs correlates with the mean annual air temperature (MAAT) and soil pH, while the number of cyclopentane moieties correlates with soil pH. Based on this observation the methylation index of branched tetraethers (MBT) and the cyclisation index of branched tetraethers (CBT) were introduced. Since brGDGTs are found in large quantities in soil, it was assumed that they are washed into rivers by erosion and transported to the ocean. Hence, the MBT/CBT proxy can be used in river influenced marine settings to reconstruct past MAAT and soil pH of the river basin. In this study the applicability of the MBT/CBT proxy in diverse river systems was assessed, by detailed studies of two river systems that are different in terms of river size and climate setting (Amazon River and Tagus River). BrGDGTs in drainage basin, soils and river suspended particulate matter (SPM) were compared to those in marine SPM and surface sediments. Core lipid (CL) and intact polar lipid (IPL)-derived brGDGTs were analyzed, in order to distinguish recently produced (IPL-derived) GDGTs from older (CL) GDGTs. The majority of brGDGTs in the Amazon River is derived from lowland basin soils. However, the distribution of CL and IPL-derived brGDGTs was slightly different to that of the soils, which showed that in-situ production of brGDGTs also played a role. The influence of seasonal variation on the brGDGT distribution is relatively small, but significant differences were detected. During the high water season the input of brGDGTs from soil increased, which was seen in increased brGDGT concentrations and a brGDGT distribution that was most similar to that of soil. During the other seasons the influence of in-situ production of brGDGTs increased. In the Amazon shelf and fan, the delivery of brGDGTs from the Amazon River was evident. However, further away from the river, the distributions of CL brGDGTs in marine SPM and sediments varied widely, since this difference was also found in IPL-derived brGDGTs it is most likely due to marine in-situ production. In the Tagus basin soils the brGDGT distributions were very variable and did not reflect the local MAAT and soil pH. The concentrations of brGDGTs in river SPM were substantially higher and the distributions were different compared to those of the drainage basin soils. This indicates that brGDGTs are mainly produced in the river itself. Therefore, the MAAT and soil pH cannot be reconstructed with the commonly used soil calibrations. Along the Portuguese margin the brGDGT concentrations rapidly decreased with increasing distance from the shore. The brGDGT distributions in marine SPM and surface sediments also changed, indicating that marine in-situ production takes place. In conclusion, this study emphasizes the importance of a detailed study of a river basin to interpret the MBT/CBT records for paleoreconstructions in adjacent marine setting. In general, it is recommendable to apply the MBT/CBT proxy only to sediment which was under strong river influence

Biological source and provenance of deep-water derived isoprenoid tetraether lipids along the Portuguese continental margin

There is increasing evidence that nitrifying Thaumarchaeota in the deep ocean waters may contribute to the sedimentary composition of isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs), impacting TEX86 paleothermometry. We investigated the potential effect of deep-water dwelling Thaumarchaeota in the warm and saline Mediterranean Outflow Water (MOW) on the distribution of isoGDGTs by analysing suspended particulate matter (SPM) and surface sediments collected along five land-ocean transects along the southern Portuguese continental margin. To this end, we directly compared for the first time the composition of intact polar lipid (IPL)-derived isoGDGTs of SPM with the diversity, abundance, and activity of Thaumarchaeota based on the genetic analysis of the genes coding for the archaeal ammonia monooxygenase (amo A) and the geranylgeranylglyceryl phosphate (GGGP) synthase involved in the isoGDGT biosynthetic pathway. Our results revealed a strong positive relationship between water depth and View the MathML source values for both SPM and surface sediments. The increasing View the MathML source trends for both core lipid (CL) and IPL-derived fractions were accompanied by increasing fractional abundances of GDGT-2 and crenarchaeol regio-isomer and decreasing fractional abundances of GDGT-1 and GDGT-3 with increasing water depth. Phylogenetic analyses based on the archaeal amoA and the GGGP synthase proteins showed that Thaumarchaeota populations detected at 1 m and 50 m water depth were different from those detected in 200 m and 1000 m water depth, which had an increased contribution of so-called ‘deep water’ Thaumarchaeota. The differences in the fractional abundances of isoGDGTs with water depth were compatible with the increasing contribution of ‘deep water’ Thaumarchaeota harboring a different GGGP synthase enzyme which has been suggested to relate to changes in the relative proportion of synthesized isoGDGTs. Accordingly, it appears that the sedimentary distribution of CL isoGDGTs used in View the MathML source along the Portuguese margin is primarily influenced by water depth due to the increasing contribution of the deep-water population of Thaumarchaeota residing in the MOW. Our study also reveals that the effect of deep water Thaumarchaeotal communities on sedimentary isoGDGT distributions should be considered globally

Examining the provenance of branched GDGTs in the Tagus River drainage basin and its outflow into the Atlantic Ocean over the Holocene to determine their usefulness for paleoclimate applications

The distributions of branched glycerol dialkyl glycerol tetraethers (brGDGTs), which are transported from the soils where they are predominantly produced to marine sediments via rivers, have been applied in reconstructing mean annual air temperature (MAT) and pH of soils. However, paleoclimate reconstructions using sedimentary brGDGTs have proven difficult in arid regions, including the Iberian Peninsula. Recently, six novel 6-methyl brGDGTs have been described using new analytical methods (in addition to the nine 5-methyl brGDGTs previously used for climate reconstructions), and so new pH and MAT calibrations have been developed that were shown to improve the accuracy of reconstructions in a set of global soil samples, especially in arid regions. Because of this we decided to apply the new method to separate the 5- and 6-methyl isomers along with the novel calibrations to a sample set from the Iberian Peninsula to determine whether it improves paleoclimate reconstructions in this area. This set includes samples that run in a transect from source to sink along the Tagus River and out to the deep ocean off the Portuguese margin spanning the last 6000 years. We found that although pH reconstructions in the soils were improved using the new calibration, MAT reconstructions were not much better even with the separation of the 5- and 6-methyl brGDGTs. This confirmed the conclusion of previous studies that the amount of aquatically produced brGDGTs is overwhelming the soil-derived ones in marine sediments and complicating MAT reconstructions in the region. Additionally, the new separation revealed a strong and until now unseen relationship between the new degree of cyclization (DC') of the brGDGTs and MAT that could be making temperature reconstructions in this and other arid regions difficult