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1. What plant is that? Chemotaxonomy from n-alkane molecular distributions of East African plants with implications for paleoecology

   Shi, S. C.; Cerling, T. E.; Uno, K. T. (December 2018, American Geophysical Union, Fall Meeting 2018, abstract #PP31D-1701)

   Plant wax n-alkanes serve as reliable biomarkers given their abundance, stability, and distribution in the sedimentary record. As a result, their utility as isotopic indicators of vegetation and hydroclimate is well-established. A less well studied aspect of plant n-alkanes is the use of their molecular distributions, or differences in the relative abundances of homologues, for chemotaxonomy. Limited plant n-alkane datasets from southern and western Africa suggest molecular distributions can differentiate C4 grasses from C3 woody vegetation. Here we examine a suite of plants from East Africa, where almost no plant biomarkers data exists from modern plants. In this study, over 100 samples of 19 species of plants were collected monthly from the Samburu National Reserve in Kenya from October 2001 to March 2003, across multiple growing seasons; n-alkane distributions and concentrations from both individual species and designated plant functional types (PFTs) - based on both photosynthetic pathway and growth form - were investigated. Previously published n-alkane data from western and southern Africa, or the "All Africa" dataset, were examined to further understand potential spatial differences in biomarker distributions. n-alkane distributions in both datasets vary in both individual species and within PFTs. Principal Components Analysis (PCA) was used to analyze distributions of n-alkanes in individual species and in PFTs, to determine the primary sources of variability. Results indicate that n-alkane distributions can be used to separate some individual species - namely, C4 grasses - and can be used to separate PFTs. C4 grasses and C3 woody vegetation were successfully separated in both datasets. Additionally, we found that n-alkane concentrations vary by four orders of magnitude across homologues and PFTs. A compiled African plant data set shows that C31 concentration is the most representative of the plant community for C4 grasses, C3 shrubs, and C3 trees and thus, is most ideal for stable isotope vegetation reconstructions. These data suggest that an organic geochemical approach to plant taxonomy is crucial to future biomarker applications for reconstructing vegetation distribution and structure in past ecosystems. 

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2. Modeling the influence of chain length on secondary organic aerosol (SOA) formation via multiphase reactions of alkanes

   Madhu, Azad; Jang, Myoseon; Deacon, David (January 2023, Atmospheric Chemistry and Physics)

   Abstract. Secondary organic aerosol (SOA) from diesel fuel is known to besignificantly sourced from the atmospheric oxidation of aliphatichydrocarbons. In this study, the formation of linear alkane SOA waspredicted using the Unified Partitioning Aerosol Phase Reaction (UNIPAR)model that simulated multiphase reactions of hydrocarbons. In the model, theformation of oxygenated products from the photooxidation of linear alkaneswas simulated using a nearly explicit gas kinetic mechanism. Autoxidationpaths integrated with alkyl peroxy radicals were added to the MasterChemical Mechanism v3.3.1 to improve the prediction of low-volatilityproducts in the gas phase and SOA mass. The resulting gas products were thenlumped into volatility- and reactivity-based groups that are linked to mass-basedstoichiometric coefficients. The SOA mass in the UNIPAR model is producedvia three major pathways: partitioning of gaseous oxidized products ontoboth the organic and wet inorganic phases, oligomerization in the organic phase,and reactions in the wet inorganic phase (acid-catalyzed oligomerization andorganosulfate formation). The model performance was demonstrated for SOAdata that were produced through the photooxidation of a homologous series oflinear alkanes ranging from C9–C15 under varying environments (NOxlevels and inorganic seed conditions) in a large outdoor photochemical smogchamber. The product distributions of linear alkanes were mathematicallypredicted as a function of carbon number using an incremental volatilitycoefficient (IVC) to cover a wide range of alkane lengths. The prediction ofalkane SOA using the incremental volatility-based product distributions,which were obtained with C9–C12 alkanes, was evaluated for C13and C15 chamber data and further extrapolated to predict the SOA from longer-chain alkanes (≥ C15) that can be found in diesel. The model simulationof linear alkanes in diesel fuel suggests that SOA mass is mainly producedby alkanes C15 and higher. Alkane SOA is insignificantly impacted by thereactions of organic species in the wet inorganic phase due to thehydrophobicity of products but significantly influenced by gas–particlepartitioning. 

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3. Variability of plant wax concentrations and carbon isotope values in surface lake sediments provide clues into their transport and deposition

   Bates, B.R.; Lowell, T.V.; Diefendorf, A.F.; Freimuth, E.J.; Stewart, A.K. (December 2017, AGU Fall Meeting 2017)

   Plant wax compounds preserved in lake sediments are used as proxies for paleohydrologic reconstructions. Despite their presence in lake sediments, little is known about their transport from plants to their deposition in lake sediments. By drawing on the leaf and pollen taphonomy literature combined with sediment focusing models, it is possible to develop several working hypotheses for the transport and deposition of plant waxes in lake sediments. An improved understanding of plant wax transport and deposition into lake sediments is necessary to increase the accuracy of paleohydrologic reconstructions. To better understand the controls on plant wax transport and deposition in lake sediment, we analyzed the sedimentary plant waxes from 3 lakes in the Adirondack Mountains of New York. These lakes were chosen to capture a range of basin-specific properties to evaluate their influences on the transport and deposition of plant wax compounds in surface sediments. We spatially characterized sediment properties with surface sediment samples and high-resolution underwater imaging, acoustically profiled the sub-bottom, and measured temperature profiles. From each site, we measured n-alkanes, bulk organic content (loss-on-ignition), bulk carbon and nitrogen concentrations, C:N ratios, and bulk carbon isotopes. Preliminary n-alkane concentrations and chain length distributions, as well as bulk carbon isotopes, are variable within each lake basin suggesting a mix of aquatic and terrestrial sources. The bulk carbon isotope values for two of the three lakes show a similar range of -2‰ compared to a range of -6.3‰ at the third lake. Likewise, the range of total n-alkane concentrations is much higher in the third lake suggesting that the controls on the distribution of n-alkanes and organic carbon are different between lakes. For terrestrial plant waxes, we find low n-alkane concentrations in sandy nearshore sediments relative to higher n-alkane concentrations in deeper fine-grained sediments. Combined, this information suggests that littoral processes focus organic compounds and fine sediments towards the main depo-center of the lake. These and other observations highlight important relationships between basin-specific properties and processes controlling the transport and deposition of plant wax compounds. 

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4. Transport and distribution of terrestrial plant biomarkers in modern lake sediments: implications for paleoenvironmental reconstructions in the Adirondack Mountains

   Bates, B.R.; Lowell, T.V.; Diefendorf, A.F.; Freimuth, E.J.; Stewart, A.K. (September 2017, 6th annual Midwestern Geobiology Conference)

   Terrestrial plant biomarkers preserved in lake sediments are commonly used in paleoenvironmental reconstructions. Basin-specific transport pathways and distribution controls of plant biomarkers, however, are poorly understood. This study mapped the distribution of sedimentary n-alkanes sourced from vascular plant waxes to delineate possible transport pathways and quantified the contribution of terrestrial and aquatic input. We combine these data with existing leaf and pollen taphonomy literature and sediment focusing models to develop a better understanding of the controls on plant biomarker transport within lake basins. Here, we report the spatial distribution of sedimentary n-alkanes, the carbon isotope values and C:N ratios of bulk sediment, and percent organic matter from three lakes in the Adirondack Mountains, NY. Preliminary carbon isotope data and n-alkane concentrations within each lake suggests a large terrestrial input. Bulk sediment carbon isotope values ranged from - 26‰ to -32‰ consistent with carbon isotope values of modern terrestrial vegetation. The concentrations of long-chain n-alkanes (indicative of higher land plants), moreover, are much higher than short-chain n-alkanes (indicative of aquatic and microbial activity) by almost two times. By contrast, C:N ratios range from 11-14 indicating a mix of aquatic and terrestrial contribution to the lake’s total organic matter. We combined high-resolution sonar data with the sediment analyses to identify basin- specific controls on the distributions of n-alkanes and bulk sediment carbon isotopes. The statistical categorization of sediment zones based on relative hardness and roughness along the lake bottom delineates where organic material is concentrated. For the terrestrially sourced plant waxes, we measured low n-alkane concentrations in sandy littoral sediments relative to deeper sediments towards the main depo-center. Together, this information validates sediment focusing models and suggests that terrestrial carbon and n-alkanes are preferentially transported to the main depo-center of the lake. These observations highlight important relationships between basin-specific sediment properties and processes controlling the transport and deposition of n- alkanes. 

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5. Organic contaminants and atmospheric nitrogen at the graphene–water interface: a simulation study

   https://doi.org/10.1039/d1na00570g  

   Thakkar, Ravindra; Gajaweera, Sandun; Comer, Jeffrey (March 2022, Nanoscale Advances)

   Ordered nanoscale patterns have been observed by atomic force microscopy at graphene–water and graphite–water interfaces. The two dominant explanations for these patterns are that (i) they consist of self-assembled organic contaminants or (ii) they are dense layers formed from atmospheric gases (especially nitrogen). Here we apply molecular dynamics simulations to study the behavior of dinitrogen and possible organic contaminants at the graphene–water interface. Despite the high concentration of N 2 in ambient air, we find that its expected occupancy at the graphene–water interface is quite low. Although dense (disordered) aggregates of dinitrogen have been observed in previous simulations, our results suggest that they are stable only in the presence of supersaturated aqueous N 2 solutions and dissipate rapidly when they coexist with nitrogen gas near atmospheric pressure. On the other hand, although heavy alkanes are present at only trace concentrations (micrograms per cubic meter) in typical indoor air, we predict that such concentrations can be sufficient to form ordered monolayers that cover the graphene–water interface. For octadecane, grand canonical Monte Carlo suggests nucleation and growth of monolayers above an ambient concentration near 6 μg m −3 , which is less than some literature values for indoor air. The thermodynamics of the formation of these alkane monolayers includes contributions from the hydration free-energy (unfavorable), the free-energy of adsorption to the graphene–water interface (highly favorable), and integration into the alkane monolayer phase (highly favorable). Furthermore, the peak-to-peak distances in AFM force profiles perpendicular to the interface (0.43–0.53 nm), agree with the distances calculated in simulations for overlayers of alkane-like molecules, but not for molecules such as N 2 , water, or aromatics. Taken together, these results suggest that ordered domains observed on graphene, graphite, and other hydrophobic materials in water are consistent with alkane-like molecules occupying the interface. 

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