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Monsoonal rivers play an important role in the land-to-sea transport of soil-derived organic carbon (OC). However, spatial and temporal variation in the concentration, composition, and fate of OC in these rivers remains poorly understood. We investigate soil-to-sea transport of OC by the Godavari River in India using branched glycerol dialkyl glycerol tetraether (brGDGT) lipids in soils, river suspended particulate matter (SPM), riverbed sediments, and in a marine sediment core from the Bay of Bengal. The abundance and composition of brGDGTs in SPM and sediments in the Godavari River differs between the dry and wet season. In the dry season, 6-methyl brGDGTs dominate SPM and riverbed sediments in the whole basin. Currently, mobilisation and transport of soils from the upper basin is limited due to deficient rainfall and damming. This promotes aquatic brGDGT production in this part of the basin, which is reflected by a high relative abundance of 6-methyl brGDGTs in both seasons. In the wet season, brGDGT distributions in SPM from the lower basin closely resemble those in soils, mostly from the North and East Tributaries, corresponding to precipitation patterns. The brGDGT composition in SPM and sediments from the delta suggests that soil OC is only effectively transported to the Bay of Bengal in the wet season, when the river plume extends beyond the river mouth. The sediment geochemistry indicates that also the mineral particles exported by the Godavari River primarily originate from the lower basin, similar to the brGDGTs. River depth profiles of brGDGTs in the downstream Godavari reveal no hydrodynamic sorting effect in either season, indicating that brGDGTs are not associated with certain minerals. The similarity of brGDGT distributions in bulk and fine-grained sediments (≤63 μm) further confirms the absence of selective transport mechanisms. Nevertheless, the composition of brGDGTs in a Holocene, marine sediment core near the river mouth appears substantially different from that in the modern Godavari basin, suggesting that terrestrial-derived brGDGTs are rapidly lost upon discharge into the Bay of Bengal and/or overprinted by marine in situ production. The change in brGDGT distributions at the river-sea transition implies that this zone is key in the effective transfer of soil OC, as well as for the interpretation of paleorecords based on brGDGTs in coastal marine sediment archives.

Summary Glycerol Dialkyl Glycerol Tetraethers (GDGTs) are a group of molecules that have been successfully used for paleoclimate reconstructions in a wide range of environments. GDGTs have shown to be useful biomarkers as they are widely available, and their structure is relatively resistant to diagenetic alteration. Specific GDGTs are associated with particular environments, which suggests that they could potentially be used to classify ancient depositional conditions in cases where the exact paleoenvironment is not known. In this work we present a comprehensive branched and iGDGT dataset from published and novel analyses, with more than 1000 modern samples from marine, lake and river sediments as well as soils and peat samples. Unsupervised and supervised machine learning techniques were applied to this dataset to generate GDGT and environmentally based clusters and train a classification algorithm. The trained algorithm has the potential to be applied to samples formed in unclear or transitional depositional environments, allowing us to identify the potential environment in which they were formed. Furthermore, the classification model can help determine which environmental calibration is most appropriate for the setting, thereby improving paleoenvironmental reconstructions.

RATIONALE: Traditionally, dissolved organic carbon (DOC) stable isotope analysis (SIA) is performed using either offline sample preparation followed by elemental analyzer/isotope ratiomass spectrometry (EA/IRMS) or a wet chemical oxidation (WCO)-based device coupled to an isotope ratio mass spectrometer. The first method is time-consuming and laborious. The second involves the risks of underestimation of DOC concentration and isotopic fractionation due to incomplete oxidation. The development of an analytical method for accurate and sensitive DOC SIA is described in this study. METHODS: A high-temperature combustion (HTC) system improves upon traditional methods. A novel total organic carbon (TOC) system, specially designed for SIA, was coupled to an isotope ratio mass spectrometer. An integrated purge and trap technique (peak focusing), flexible injection volume (0.05-3 mL), favorable carrier gas flow, modified ash crucible, new design of combustion tube and optimized drying system were used to achieve the necessary performance. RESULTS: The system can reliably measure concentrations up to 1000 mgC/L. Compounds resistant to oxidation, such as barbituric acid, melamine and humic acid, were analyzed with recovery rates of 100 +/- 1% proving complete oxidation. In this initial testing, the delta C-13 values of these compounds were determined with precision and trueness of CONCLUSIONS: The novel HTC system coupled to an isotope ratio mass spectrometer resulted in significantly improved sensitivity. The system is suitable for salt-containing liquids and compounds that are resistant to oxidation, and it offers a large concentration range. A second paper (which follows this one in this issue) will present a more comprehensive assessment of the analytical performance with a broad set of solutions and real samples. This highly efficient TOC stable isotopic analyzer will probably open up new possibilities in biogeochemical carbon cycle research. Copyright (C) 2014 John Wiley & Sons, Ltd.

RATIONALE: Dissolved organic carbon (DOC) plays an important role in carbon cycling, making precise and routine measurement of delta C-13 values and DOC concentration highly desirable. A new promising system has been developed for this purpose. However, broad-scale application of this new technique requires an in-depth assessment of analytical performance, and this is described here. METHODS: A high-temperature combustion Total Organic Carbon analyzer was interfaced with continuous flow isotope ratio mass spectrometry (TOC/IRMS) for the simultaneous analysis of the bulk DOC concentration and delta C-13 signature. The analytical performance (precision, memory effects, linearity, volume/concentration effects, accuracy) was thoroughly evaluated, including realistic and challenging conditions such as low DOC concentrations and natural DOC. RESULTS: High precision (standard deviation, SD predominantly ≤0.15 ‰) and accuracy (R2 = 0.9997) were achieved for the δ13C analysis of a broad diversity of DOC solutions. Simultaneously, good results were obtained for the measurement of DOC concentration. Assessment of natural abundance and slightly 13C-enriched DOC, a wide range of concentrations (~0.2–150 mgC/L) and injection volumes (0.05–3 mL), demonstrated minor/negligible memory effects, good linearity and flexible usage. Finally, TOC/IRMS was successfully applied to determine low DOC concentrations (