ALBERT

Description: Soil erosion plays a crucial role in transferring sediment and carbon from land to sea, yet little is known about the rhythm and rates of soil erosion prior to the most recent few centuries. Here we reconstruct a Holocene erosional history from central India, as integrated by the Godavari River in a sediment core from the Bay of Bengal. We quantify terrigenous fluxes, fingerprint sources for the lithogenic fraction and assess the age of the exported terrigenous carbon. Taken together, our data show that the monsoon decline in the late Holocene significantly increased soil erosion and the age of exported organic carbon. This acceleration of natural erosion was later exacerbated by the Neolithic adoption and Iron Age extensification of agriculture on the Deccan Plateau. Despite a constantly elevated sea level since the middle Holocene, this erosion acceleration led to a rapid growth of the continental margin. We conclude that in monsoon conditions aridity boosts rather than suppresses sediment and carbon export, acting as a monsoon erosional pump modulated by land cover conditions.

Description: Molecular fossils, like bacterial branched glycerol dialkyl glycerol tetraethers (brGDGTs), and the stable isotopic composition of biomarkers, such as δ2H of leaf wax-derived n-alkanes (δ2Hn-alkane) or δ18O of hemicellulose-derived sugars (δ18Osugar) are increasingly used for the reconstruction of past climate and environmental conditions. Plant-derived δ2Hn-alkane and δ18Osugar values record the isotopic composition of plant source water (δ2H/δ18Osource-water), which usually reflects mean annual precipitation (δ2H/δ18Oprecipiation), modulated by evapotranspirative leaf water enrichment and biosynthetic fractionation. Accuracy and precision of respective proxies should be ideally evaluated at a regional scale. For this study, we analysed topsoils below coniferous and deciduous forests, as well as grassland soils along a Central European transect in order to investigate the variability and robustness of various proxies, and to identify effects related to vegetation. Soil pH-values derived from brGDGTs correlate reasonably well with measured soil pH-values, but systematically overestimate them (ΔpH = 0.6 ± 0.6). The branched vs. isoprenoid tetraether index (BIT) can give some indication whether the pH reconstruction is reliable. Temperatures derived from brGDGTs overestimate mean annual air temperatures slightly (∆TMA = 0.5 °C ± 2.4). Apparent isotopic fractionation (εn-alkane/precipitation and εsugar/precipitation) is lower for grassland sites than for forest sites due to "signal damping", i.e. grass biomarkers do not record the full evapotranspirative leaf water enrichment. Coupling δ2Hn-alkane with δ18Osugar allows to reconstruct the stable isotopic composition of the source water more accurately than without the coupled approach (Δδ2H = ~-21 ‰ ± 22 and Δδ18O = ~-2.9 ‰ ± 2.8). Similarly, relative humidity during daytime and vegetation period (RHMDV) can be reconstructed using the coupled isotope approach (ΔRHMDV = ~-17 ± 12). Especially for coniferous sites, reconstructed RHMDV values as well as source water isotope composition underestimate the measured values. This can be likely explained by understory grass vegetation at the coniferous sites contributing significantly to the n-alkane pool but only marginally to the sugar pool in the topsoil. The large uncertainty likely reflect the fact that biosynthetic fractionation is not constant, as well as microclimate variability. Overall, GDGTs and the coupled δ2Hn-alkane-δ18Osugar approach have great potential for more quantitative paleoclimate reconstructions.

Description: Rapid and continuous analysis of radiocarbon (14C) concentration in carbonate samples at spatial resolution down to 100 µm has been made possible with the new LA-AMS (laser ablation accelerator mass spectrometry) technique. This novel approach can provide radiocarbon data at a spatial resolution similar to that of stable carbon (C) isotope measurements by isotope ratio mass spectrometry of micromilled samples and, thus, can help to interpret δ13C signatures, which otherwise are difficult to understand due to numerous processes contributing to changes in the C-isotope ratio. In this work, we analyzed δ13C and 14C on the Holocene stalagmite SPA 127 from the high-alpine Spannagel Cave (Austria). Both proxies respond in a complex manner to climate variability. Combined stable carbon and radiocarbon profiles allow three growth periods characterized by different δ13C signatures to be identified: (i) the period 8.5 to 8.0 ka is characterized by relatively low δ13C values with small variability combined with a comparably high radiocarbon reservoir effect (expressed as dead carbon fraction, dcf) of around 60 %. This points towards C contributions of host rock dissolution and/or from an “old” organic matter (OM) reservoir in the karst potentially mobilized due to the warm climatic conditions of the early Holocene. (ii) Between 8 and 3.8 ka there was a strong variability in δ13C with values ranging from −8 ‰ to +1 ‰ and a generally lower dcf. The δ13C variability is most likely caused by changes in C exchange between cave air CO2 and dissolved inorganic carbon in drip water in the cave, which are induced by reduced drip rates as derived from reduced stalagmite growth rates. Additionally, the lower dcf indicates that the OM reservoir contributed less to stalagmite growth in this period possibly as a result of reduced meteoric precipitation or because it was exhausted. (iii) In the youngest section between 3.8 and 2.4 ka, comparably stable and low δ13C values, combined with an increasing dcf reaching up to 50 % again, hint towards a contribution of an aged OM reservoir in the karst. This study reveals the potential of combining high-resolution 14C profiles in speleothems with δ13C records in order to disentangle climate-related C dynamics in karst systems.

Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 8 (2018): 11997, doi:10.1038/s41598-018-30091-8.

Description: The abundance of organic carbon (OC) in vegetation and soils (~2,600 PgC) compared to carbon in the atmosphere (~830 PgC) highlights the importance of terrestrial OC in global carbon budgets. The residence time of OC in continental reservoirs, which sets the rates of carbon exchange between land and atmosphere, represents a key uncertainty in global carbon cycle dynamics. Retention of terrestrial OC can also distort bulk OC- and biomarker-based paleorecords, yet continental storage timescales remain poorly quantified. Using “bomb” radiocarbon (14C) from thermonuclear weapons testing as a tracer, we model leaf-wax fatty acid and bulk OC 14C signatures in a river-proximal marine sediment core from the Bay of Bengal in order to constrain OC storage timescales within the Ganges-Brahmaputra (G-B) watershed. Our model shows that 79–83% of the leaf-waxes in this core were stored in continental reservoirs for an average of 1,000–1,200 calendar years, while the remainder was stored for an average of 15 years. This age structure distorts high-resolution organic paleorecords across geologically rapid events, highlighting that compound-specific proxy approaches must consider storage timescales. Furthermore, these results show that future environmental change could destabilize large stores of old - yet reactive - OC currently stored in tropical basins.

Description: We acknowledge funding support from the Agouron Institute Postdoctoral Fellowship (K.L.F), the US National Science Foundation (Awards: OCE-1333387 and OCE-13333826), the Investment in Science Fund given primarily by WHOI Trustee and Corporation Members, and the Swiss National Science Foundation (Award: 200020_163162).

Description: © The Author(s), 2018. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Nature Communications 9 (2018): 121, doi:10.1038/s41467-017-02504-1.

Description: Sediments in deep ocean trenches may contain crucial information on past earthquake history and constitute important sites of carbon burial. Here we present 14C data on bulk organic carbon (OC) and its thermal decomposition fractions produced by ramped pyrolysis/oxidation for a core retrieved from the 〉7.5 km-deep Japan Trench. High-resolution 14C measurements, coupled with distinctive thermogram characteristics of OC, reveal hemipelagic sedimentation interrupted by episodic deposition of pre-aged OC in the trench. Low δ13C values and diverse 14C ages of thermal fractions imply that the latter material originates from the adjacent margin, and the co-occurrence of pre-aged OC with intervals corresponding to known earthquake events implies tectonically triggered, gravity-flow-driven supply. We show that 14C ages of thermal fractions can yield valuable chronological constraints on sedimentary sequences. Our findings shed new light on links between tectonically driven sedimentological processes and marine carbon cycling, with implications for carbon dynamics in hadal environments.

Description: This study is supported by Doc.Mobility Fellowship (P1EZP2_159064) (R.B.) from the Swiss National Science Foundation (SNSF). This work is also supported by SNF “CAPS-LOCK” project 200021_140850 (T.I.E.), by SNSF grant (133481) (M.S.), and Austrian Science Foundation (P 29678-N28) (M.S.).