No abstract

This study emphasizes the importance of rainstorm events in mobilizing carbon at the soil-stream interface from tropical rainforests. Half-hourly geochemical/isotopic records over a 13.5 h period from a 20 km 2 tropical rainforest headwater in Guyana show an order of magnitude increase in dissolved organic carbon (DOC) concentration in less than 30 mins (10.6-114 mg/L). The composition of DOC varies significantly and includes optically invisible dissolved organic matter (iDOM) that accounts for a large proportion (4-89%) of the total DOC, quantified using size exclusion chromatography (SEC). SEC suggests that iDOM is comprised of low molecular weight organic moieties, which are likely sourced from fresh leaf litter and/or topsoil, as shown in soils from the surrounding environment. Although poorly constrained at present, the presence of iDOM further downstream during the wet season suggests that this organic matter fraction may represent an unquantified source of riverine CO 2 outgassing in tropical headwaters, requiring further consideration.

Citation for published item:heingrossD tFF nd roviusD xF nd hellingerD wF nd riltonD FqF nd epshD wF nd hseD hF nd qr¤ okeD hFF nd iethErillerndD eF nd urowskiD tFwF @PHIWA 9reservtion of orgni ron during tive )uvil trnsport nd prtile rsionF9D qeologyFD RU @IHAF ppF WSVEWTPF The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-prot purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. CITATION: Scheingross, J.S., et al., 2019, Preservation of organic carbon during active fluvial transport and particle abrasion: Geology, v. 47, p. 958-962, https:// ABSTRACT Oxidation of particulate organic carbon (POC) during fluvial transit releases CO 2 to the atmosphere and can influence global climate. Field data show large POC oxidation fluxes in lowland rivers; however, it is unclear if POC losses occur predominantly during in-river transport, where POC is in continual motion within an aerated environment, or during transient storage in floodplains, which may be anoxic. Determination of the locus of POC oxidation in lowland rivers is needed to develop process-based models to predict POC losses, constrain carbon budgets, and unravel links between climate and erosion. However, sediment exchange between rivers and floodplains makes differentiating POC oxidation duringin-river transport from oxidation during floodplain storage difficult. Here, we isolated inriver POC oxidation using flume experiments transporting petrogenic and biospheric POC without floodplain storage. Our experiments showed solid phase POC losses of 0%-10% over ∼10 3 km of fluvial transport, compared to ∼7% to >50% losses observed in rivers over similar distances. The production of dissolved organic carbon (DOC) and dissolved rhenium (a proxy for petrogenic POC oxidation) was consistent with small POC losses, and replicate experiments in static water tanks gave similar results. Our results show that fluvial sediment transport, particle abrasion, and turbulent mixing have a minimal role on POC oxidation, and they suggest that POC losses may accrue primarily in floodplain storage.

1. Shrub encroachment has far-reaching ecological and economic consequences in many ecosystems worldwide. Yet, compositional changes associated with shrub encroachment are often overlooked despite having important effects on ecosystem functioning.2. We document the compositional change and potential drivers for a northern Namibian Combretum woodland transitioning into a Terminalia shrubland. We use a multiproxy record (pollen, sedimentary ancient DNA, biomarkers, compoundspecific carbon (δ 13 C) and deuterium (δD) isotopes, bulk carbon isotopes (δ 13 Corg), grain size, geochemical properties) from Lake Otjikoto at high taxonomical and temporal resolution.3. We provide evidence that state changes in semiarid environments may occur on a scale of one century and that transitions between stable states can span around 80 years and are characterized by a unique vegetation composition. We demonstrate that the current grass/woody ratio is exceptional for the last 170 years, as supported by n-alkane distributions and the δ 13 C and δ 13 Corg records. Comparing vegetation records to environmental proxy data and census data, we infer a complex network of global and local drivers of vegetation change. While our δD record suggests physiological adaptations of woody species to higher atmospheric pCO 2 concentration and drought, our vegetation records reflect the impact of broadscale logging for the mining industry, and the macrocharcoal record suggests a decrease in fire activity associated with the intensification of farming. Impact of selective grazing is reflected by changes in abundance and taxonomical composition of grasses and by an increase of nonpalatable and trampling-resistant taxa.In addition, grain-size and spore records suggest changes in the erodibility of soils because of reduced grass cover. | 963TABARES ET Al.

No abstract