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Abstract: C25 highly branched isoprenoids (HBIs) are potentially valuable indicators of diatom-derived organic matter (OM) inputs to sediments and the C25 alkane has been used previously as a biomarker in paleoenvironmental studies. In this work, a suite of C25 HBI monoenes and dienes was detected in a sediment core from Russell Key, central Florida Bay, USA. Hydrogenation proved all these compounds to be acyclic alkenes with the parent structure of the C25 HBI alkane 2,6,10,14-tetramethyl-7-(3′-methylpentyl)pentadecane. The tentative double bond positions and geometry of three monoenes and one diene were also established on the basis of comparison of retention indices (RI) and mass spectra with those published for synthetic or isolated compounds. The abundance of individual alkenes showed significantly different depth profiles. The concentrations of dienes decreased rapidly with increasing depth, but this trend was not observed for the monoenes. The highest concentration of total C25 HBIs was observed at mid depth in the core, suggesting strong historical inputs of diatom-derived sedimentary OM during that period. In fact, the depth profile of C25 HBIs reflected quite well historical variations in diatom abundance and variations in diatom species composition in central Florida Bay, based on the results of fossil diatom species analysis using microscopy. This study provides further evidence that, with care, some C25 HBIs can be applied as biomarkers for diatom inputs in paleoenvironmental studies although our data did not allow the identification of one specific diatom group as the main HBI contributor. [Copyright &y& Elsevier]

Abstract: Ester bound, long chain fatty acids (e-LFAs), hydroxy fatty acids (FAs) and di-acids are key components of the hydrolysable aliphatic lipids in soil organic matter (SOM). Cutin and suberin are important sources of these SOM components, which may contribute to the recalcitrant soil carbon pool. However, their carbon isotopic composition and turnover rates are largely unknown. This study represents the first compound specific isotopic analysis (CSIA) of major hydrolysable lipids in the surface mineral soil horizons from the Duke Forest, North Carolina, USA. The investigated lipids include hydroxy FAs and di-acids that only originate from cutin and suberin (i.e., cutin- and suberin-derived compounds) and e-LFAs that originate from plant waxes as well as cutin and suberin. These compounds were 2–7‰ more depleted in 13C as compared to the bulk organic matter in plant tissues as well as in soils. Furthermore, they were more depleted in 13C in plant tissues produced under the elevated versus ambient CO2 concentration in the free air CO2 enrichment (FACE) experiment. Under a steady-state condition, this isotopic signature was incorporated into the hydrolysable lipids in the surface soil at a rate of 0.029–0.031year−1. This turnover rate corresponds to a mean residence time (MRT) of 32–34year, which is longer than that of bulk SOM but much shorter than that expected for recalcitrant SOM components. We suggest that a significant fraction of cutin and suberin is transformed into non-hydrolysable SOM, which may not be accounted for in the hydrolysable fraction of SOM. [Copyright &y& Elsevier]

This study investigates the effect of photodissolution on the production of dissolved black carbon (DBC) from particulate charcoal and a fire-impacted soil. A soil sample and a char sample were collected within the burn vicinity of the 2012 Cache La Poudre River wildfire and irradiated in deionized water with artificial sunlight. Photoexposure of the suspended char and soil significantly enhanced production of DBC after 7 days continuous exposure to the simulated sunlight. The increase was coupled with an increase in the DBC polycondensed character. In agreement with this, characterization using Fourier transform-ion cyclotron resonance-mass spectrometry (FT-ICR-MS) showed an increase in the number of BC molecular formulae detected and in their average molecular weight, suggesting that increasing photoexposure is required for dissolution of larger, more polycondenced DBC compounds. An increase in molecular signatures with lower H/C ratio and higher O/C ratio after 7 days photoexposure suggested increasing functionality of newly produced DBC with irradiation time, and therefore photooxidation as a potential mechanism for the photodissolution of BC. The photoproduced DBC was also strongly coupled with the photoproduced bulk dissolved organic carbon (DOC). The results suggest that photodissolution may be a significant and previously unrecognized mechanism of DBC translocation to aquatic systems. [ABSTRACT FROM AUTHOR]