From Andes to Amazon: Assessing Branched Tetraether Lipids as Tracers for Soil Organic Carbon in the Madre de Dios River System

Abstract

We investigate the implications of upstream processes and hydrological seasonality on the transfer of soil organic carbon (OC) from the Andes mountains to the Amazon lowlands by the Madre de Dios River (Peru), using branched glycerol dialkyl glycerol tetraether (brGDGT) lipids. The brGDGT signal in Andean soils (0.5 to 3.5 km elevation) reflects air temperature, with a lapse rate of −6.0°C/km elevation (r 2 = 0.89, p < 0.001) and −5.6°C/km elevation (r 2 = 0.89, p < 0.001) for organic and mineral horizons, respectively. The same compounds are present in river suspended particulate matter (SPM) with a lapse rate of −4.1°C/km elevation (r 2 = 0.82, p < 0.001) during the wet season, where the offset in intercept between the temperature lapse rates for soils and SPM indicates upstream sourcing of brGDGTs. The lapse rate for SPM appears insensitive to an increasing relative contribution of 6-methyl isomer brGDGTs produced within the river. River depth profiles show that brGDGTs are well mixed in the river and are not affected by hydrodynamic sorting. The brGDGTs accumulate relative to OC downstream, likely due to the transition of particulate OC to the dissolved phase and input of weathered soils toward the lowlands. The temperature-altitude correlation of brGDGTs in Madre de Dios SPM contrasts with the Lower Amazon River, where the initial soil signature is altered by changes in seasonal in-river production and variable provenance of brGDGTs. Our study indicates that brGDGTs in the Madre de Dios River system are initially soil derived and highlights their use to study OC sourcing in mountainous river systems.

Late Paleocene – early Eocene Arctic Ocean Sea Surface Temperatures; reassessing biomarker paleothermometry at Lomonosov Ridge

“…Their occurrence in modern peats (Naafs et al, 2018a), lake sediments (Baxter et al, 2019) and Paleogene lignites (Inglis et al, 2019) might also imply transport from land to marine sediments. A soil-derived source is currently unsupported, as they were most often below detection limit in recent studies of geothermally heated soils (De Jonge et al, 2019) and a soil transect from the Peruvian Andes (Kirkels et al, 2020). The brGMGT abundances we record are close to the maximum abundance found in modern peats (Naafs et al, 2018a).…”

Late Paleocene–early Eocene Arctic Ocean sea surface temperatures: reassessing biomarker paleothermometry at Lomonosov Ridge

“…In our record, the high GDGT-2 / GDGT-3 ratios are associated with normal marine conditions and the dinocyst assemblages are not indicative of upwelling conditions (Sluijs et al, 2009). Unfortunately, the available nitrogen isotope record (Knies et al, 2008) does not cover this interval in sufficient resolution to assess a relation with diazotroph activity. The increase in the GDGT-2 / GDGT-3 ratio correlates with a strong drop in BIT index values and an increase in normal marine dinocyst species (Sluijs et al, 2009), but a shift to more open marine environment does not explain the high ratio values.…”

Molecular Tracing of Riverine Soil Organic Matter From the Central Himalaya

“…The relative abundance of different brGDGT isomers (here referred to as brGDGT distributions) depends on environmental parameters such as mean annual temperature (MAT) and soil pH (Weijers, Schouten, et al, 2007), which makes them useful paleo‐climate proxies (e.g., Sinninghe Damasté et al, 2012; Weijers et al, 2007). Despite evidence for in situ brGDGTs production in some aqueous environments (e.g., De Jonge et al, 2014), it has recently been shown that brGDGT signatures in suspended sediments can be used for tracing SOC inputs in riverine systems (Kirkels et al, 2020). Here, we use the MAT calculated from brGDGT isomer distributions as a proxy for the elevation from which SOC is mobilized and entrained in riverine suspended sediments of the Central Himalaya.…”