Leaf wax biomarkers in transit record river catchment composition

Author Posting. © American Geophysical Union, 2014. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Geophysical Research Letters 41 (2014): 6420–6427, doi:10.1002/2014GL061328.Rivers carry organic molecules derived from terrestrial vegetation to sedimentary deposits in lakes and oceans, storing information about past climate and erosion, as well as representing a component of the carbon cycle. It is anticipated that sourcing of organic matter may not be uniform across catchments with substantial environmental variability in topography, vegetation zones, and climate. Here we analyze plant leaf wax biomarkers in transit in the Madre de Dios River (Peru), which drains a forested catchment across 4.5 km of elevation from the tropical montane forests of the Andes down into the rainforests of Amazonia. We find that the hydrogen isotopic composition of leaf wax molecules (specifically the C28 n-alkanoic acid) carried by this tropical mountain river largely records the elevation gradient defined by the isotopic composition of precipitation, and this supports the general interpretation of these biomarkers as proxy recorders of catchment conditions. However, we also find that leaf wax isotopic composition varies with river flow regime over storm and seasonal timescales, which could in some cases be quantitatively significant relative to changes in the isotopic composition of precipitation in the past. Our results inform on the sourcing and transport of material by a major tributary of the Amazon River and contribute to the spatial interpretation of sedimentary records of past climate using the leaf wax proxy.This work was supported by funding from the U.S. National Science Foundation award 1227192 to A.J.W. and S.J.F. V.G. was supported by the U.S. National Science Foundation award OCE-0928582.2015-03-2

Dual isotope evidence for sedimentary integration of plant wax biomarkers across an Andes-Amazon elevation transect

Author Posting. © The Author(s), 2018. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 242 (2018): 64-81, doi:10.1016/j.gca.2018.09.007.Tropical montane regions tend to have high rates of precipitation, biological production, erosion, and sediment export, which together move material off the landscape and toward sedimentary deposits downstream. Plant wax biomarkers can be used to investigate sourcing of organic matter and are often used as proxies to reconstruct past climate and environment in sedimentary deposits. To understand how plant waxes are sourced within a wet, tropical montane catchment, we measure the stable C and H isotope composition (δ13C and δD) of n-alkanes and n-alkanoic acids in soils along an elevation transect and from sediments within the Madre de Dios River network along the eastern flank of the Peruvian Andes, draining an area of 75,400 km2 and 6 km of elevation. Soils yield systematic trends in plant wax δ13C (+1.75 and +1.31‰ km−1, for the C29n-alkanes and C30n-alkanoic acids respectively in the mineral horizon) and δD values (−10 and −12‰ km−1, respectively) across a 3.5 km elevation transect, which approximates trends previously reported from canopy leaves, though we find offsets between δ13C values in plants and soils. River suspended sediments generally follow soil isotopic gradients defined by catchment elevations (δ13C: +1.03 and +0.99‰ km−1 and δD: −10 to −7‰ km−1, for the C29n-alkanes and C30n-alkanoic acids respectively) in the wet season, with a lowering in the dry season that is less well-constrained. In a few river suspended sediments, petrogenic contributions and depth-sorting influence the n-alkane δ13C signal. Our dual isotope, dual compound class and seasonal sampling approach reveals no Andean-dominance in plant wax export, and instead that the sourcing of plant waxes in this very wet, forested catchment approximates that expected for spatial integration of the upstream catchment, thus with a lowland dominance on areal basis, guiding paleoenvironmental reconstructions in tropical montane regions. The dual isotope approach provides a cross-check on the altitudinal signals and can resolve ambiguity such as might be associated with vegetation change or aridity in paleoclimate records. Further, the altitude effect encoded within plant waxes presents a novel dual-isotope biomarker approach to paleoaltimetry.This material is based upon work supported by the US National Science Foundation under Grant No. EAR-1227192 to A.J.W and S.J.F for the river work

Hydrologic change in New Zealand during the last deglaciation linked to reorganization of the Southern Hemisphere westerly winds

Millennial‐scale climate anomalies punctuating the last deglaciation were expressed differently in the Northern and Southern Hemispheres. While changes in oceanic meridional overturning circulation have been invoked to explain these disparities, the nearly synchronous onset of such events requires atmospheric mediation. Yet the extent and structure of atmospheric reorganization on millennial timescales remains unclear. In particular, the role of the Southern Hemisphere westerly winds (SHWW) and associated storm tracks is poorly constrained, largely due to the paucity of accessible archives of wind behavior. Here we present a new paleohydrologic record from a Lake Hayes, New Zealand (45° S) sediment core from ~17‐9 ka. Using two independent proxies for lake hydrology (Ca/Ti in sediments and δD values of aquatic plant biomarkers), we find evidence for a wetter Antarctic Cold Reversal (ACR, 14.7‐13.0 ka) and a drying trend during the Younger Dryas (YD, 12.9‐11.6 ka) and early Holocene (11.7 ka onward in this record). Comparisons of the Lake Hayes record with other Southern Hemisphere sites indicate coherent atmospheric shifts during the ACR and YD, whereby the former is wetter/cooler and the latter is drier/warmer. The wet/cool phase is associated with a northward shift and/or strengthening of the SHWW, whereas the drier/warmer phase indicates weaker mid‐latitude winds. These climatic trends are opposite to the Northern Hemisphere. There is a decoupling of climatic trends between Southern Hemisphere low‐ and mid‐latitude climates in the early Holocene, which could be explained by several mechanisms, such as the retreat of Antarctic sea ice

Comparison of three methods for the methylation of aliphatic and aromatic compounds

Rationale: Methylation protocols commonly call for acidic, hot conditions that are known to promote organic ^1H/^2H exchange in aromatic and aliphatic C—H bonds. Here we tested two such commonly-used methods and compared a third that avoids these acidic conditions, to quantify isotope effects with each method and to directly determine acidic-exchange rates relevant to experimental conditions. Methods: We compared acidic and non-acidic methylation approaches catalyzed by hydrochloric acid, acetyl chloride and EDCI (1-ethyl-3-(3-dimethylaminopropyl)carbodiimide) / DMAP (4-dimethylaminopyridine) respectively. These were applied to two analytes: phthalic acid (an aromatic) and octacosanoic acid (an aliphatic). We analyzed yield by gas chromatography flame ionization (GC/FID) and hydrogen and carbon isotopic composition by isotope ratio mass spectrometry (GC/IRMS). We quantified the ^1H/^2H exchange rate on dimethyl phthalate under acidic conditions with proton nuclear magnetic resonance (^1H-NMR) measurements. Results: The δ^2H and δ^(13)C values and yield were equivalent among the three methods for methyl octacosanoate. The two acidic methods resulted in comparable yield and isotopic composition of dimethyl phthalate; however, the non-acidic method resulted in lower δ^2H and δ^(13)C values perhaps due to low yields. Concerns over acid-catalyzed ^1H/^2H exchange are unwarranted as the effect was trivial over a 12-hour reaction time. Conclusions: We find product isolation yield and evaporation to be the main concerns in the accurate determination of isotopic composition. ^1H/^2H exchange reactions are too slow to cause measurable isotope fractionation over the typical duration and reaction conditions used in methylation. Thus, we are able to recommend continued use of acidic catalysts in such methylation reactions for both aliphatic and aromatic compounds

Sustained wood burial in the Bengal Fan over the last 19 My

Author Posting. © National Academy of Sciences, 2019. This article is posted here by permission of National Academy of Sciences for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences 116(45), (2019): 22518-22525, doi:10.1073/pnas.1913714116.The Ganges–Brahmaputra (G-B) River system transports over a billion tons of sediment every year from the Himalayan Mountains to the Bay of Bengal and has built the world’s largest active sedimentary deposit, the Bengal Fan. High sedimentation rates drive exceptional organic matter preservation that represents a long-term sink for atmospheric CO2. While much attention has been paid to organic-rich fine sediments, coarse sediments have generally been overlooked as a locus of organic carbon (OC) burial. However, International Ocean Discovery Program Expedition 354 recently discovered abundant woody debris (millimeter- to centimeter-sized fragments) preserved within the coarse sediment layers of turbidite beds recovered from 6 marine drill sites along a transect across the Bengal Fan (∼8°N, ∼3,700-m water depth) with recovery spanning 19 My. Analysis of bulk wood and lignin finds mostly lowland origins of wood delivered episodically. In the last 5 My, export included C4 plants, implying that coarse woody, lowland export continued after C4 grassland expansion, albeit in reduced amounts. Substantial export of coarse woody debris in the last 1 My included one wood-rich deposit (∼0.05 Ma) that encompassed coniferous wood transported from the headwaters. In coarse layers, we found on average 0.16 weight % OC, which is half the typical biospheric OC content of sediments exported by the modern G-B Rivers. Wood burial estimates are hampered by poor drilling recovery of sands. However, high-magnitude, low-frequency wood export events are shown to be a key mechanism for C burial in turbidites.This work was funded by National Science Foundation Grants OCE-1401217 and COL-T354A55 to S.J.F. and OCE-1400805 to V.G. Graduate student participation in the project received support from University of Southern California Provost’s Fellowship to H.L. Samples were provided by the International Ocean Discovery Program. We are grateful for the efforts of the Expedition 354 Science Party, Carl Johnson, and Zongguang Liu. C.F.-L. and A.G. were supported by IODP-France. We thank Colin Osborne and Maria Vorontsova for helpful discussions.2020-04-2

From Andes to Amazon: assessing branched tetraether lipids as tracers for soil organic carbon in the Madre de Dios River system

© The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Kirkels, F. M. S. A., Ponton, C., Galy, V., West, A. J., Feakins, S. J., & Peterse, F. From Andes to Amazon: assessing branched tetraether lipids as tracers for soil organic carbon in the Madre de Dios River system. Journal of Geophysical Research-Biogeosciences, 125(1), (2020): e2019JG005270, doi:10.1029/2019JG005270.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.The brGDGT analyses were supported by NWO‐Veni grant 863.13.016 to F.P. This material is based upon work supported by the US National Science Foundation under grant EAR‐1227192 to A. J. W. and S. J. F. for the river fieldwork and lipid purification. In Perú, we thank the Servicio Nacional de Áreas Naturales Protegidas por el Estado (SERNANP) and personnel of Manu and Tambopata National Parks for logistical assistance and permission to work in the protected areas. We thank the Explorers' Inn and the Pontifical Catholic University of Perú (PUCP), as well as the Amazon Conservation Association for the use of the Tambopata and Wayqecha Research Stations, respectively. For river fieldwork assistance, we thank M. Torres, A. Robles, and A. Cachuana. Soil samples were contributed by Andrew Nottingham and Patrick Meir. Logistical support was provided by Y. Malhi, J. Huaman, W. Huaraca Huasco, and other collaborators as part of the Andes Biodiversity and Ecosystems Research Group ABERG (www.andesresearch.org). We thank Dominika Kasjaniuk for technical support at Utrecht. Two anonymous reviewers have provided valuable comments that have helped to improve this manuscript. Geochemical and brGDGT data are available in the PANGAEA Data Repository (Kirkels et al., 2019) and can be accessed at https://doi.pangaea.de/10.1594/PANGAEA.90617

Mixing as a driver of temporal variations in river hydrochemistry: 1. Insights from conservative tracers in the Andes-Amazon transition

The response of hillslope processes to changes in precipitation may drive the observed changes in the solute geochemistry of rivers with discharge. This conjecture is most robust when variations in the key environmental factors that affect hillslope processes (e.g., lithology, erosion rate, and climate) are minimal across a river's catchment area. For rivers with heterogenous catchments, temporal variations in the relative contributions of different tributary sub-catchments may modulate variations in solute geochemistry with runoff. In the absence of a dense network of hydrologic gauging stations, alternative approaches are required to distinguish between the different drivers of temporal variability in river solute concentrations. In this contribution, we apportion the water and solute fluxes of a reach of the Madre de Dios River (Peru) between its four major tributary sub-catchments during two sampling campaigns (wet and dry seasons) using spatial variations in conservative tracers. Guided by the results of a mixing model, we identify temporal variations in solute concentrations of the mainstem Madre de Dios that are due to changes in the relative contributions of each tributary. Our results suggest that variations in tributary mixing are, in part, responsible for the observed concentration-discharge (C-Q) relationships. The implications of these results are further explored by re-analyzing previously published C-Q data from this region, developing a theoretical model of tributary mixing, and, in a companion paper, comparing the C-Q behavior of a suite of major and trace elements in the Madre de Dios River system

Source to sink: Evolution of lignin composition in the Madre de Dios River system with connection to the Amazon basin and offshore:Lignin evolution in Amazon

While lignin geochemistry has been extensively investigated in the Amazon River, little is known about lignin distribution and dynamics within deep, stratified river channels or its transformations within soils prior to delivery to rivers. We characterized lignin phenols in soils, river particulate organic matter (POM), and dissolved organic matter (DOM) across a 4 km elevation gradient in the Madre de Dios River system, Peru, as well as in marine sediments to investigate the source-to-sink evolution of lignin. In soils, we found more oxidized lignin in organic horizons relative to mineral horizons. The oxidized lignin signature was maintained during transfer into rivers, and lignin was a relatively constant fraction of bulk organic carbon in soils and riverine POM. Lignin in DOM became increasingly oxidized downstream, indicating active transformation of dissolved lignin during transport, especially in the dry season. In contrast, POM accumulated undegraded lignin downstream during the wet season, suggesting that terrestrial input exceeded in-river degradation. We discovered high concentrations of relatively undegraded lignin in POM at depth in the lower Madre de Dios River in both seasons, revealing a woody undercurrent for its transfer within these deep rivers. Our study of lignin evolution in the soil-river-ocean continuum highlights important seasonal and depth variations of river carbon components and their connection to soil carbon pools, providing new insights into fluvial carbon dynamics associated with the transfer of lignin biomarkers from source to sink