Compositional patterns of lignin derived phenols in bottom sediments and subsea permafrost from the Buor-Khaya Bay

Published: 20-01-2021| Version 1 | DOI: 10.17632/7dfh5v7k8b.1
Contributors:
Nikolay Belyaev

Description

Current set contains data on the molecular composition of lignin derived phenols and organic carbon content in bottom sediments and subsea permafrost rocks from five sediment cores obtained in the Buor-Khaya Bay (Laptev Sea). The substantially irregular distribution of lignin concentration and lignin-based molecular proxies revealed in the investigated samples reflect drastic environmental and depositional changes in the study area of the Laptev Sea. The increased OC content (2–5%) at some horizons and the highest concentrations (up to 23%) are related to the presence of vegetable residues such as wood and moss. Riverine flux and coastal thermoabrasion are considerable in lignin supply. Vanillyl and syringyl phenols dominate the lignin pool. Gymnosperm wood accounts for a significant fraction of the lignin. Concentration of lignin in sediments varies in five orders of magnitude. Strong share of lignin consists of biochemically unaltered compounds. Distribution of specific lignin phenols and related ratios coupled with lithology and grain size revealed that fluvial processes have been leading here. The substantial lignin stock revealed in this study represents an important portion of the terrestrial OC pool in subsea permafrost. The presence of biochemically intact OM in the gradually thawing subsea cryolithozone warrants further investigation.

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The OC content in the freeze-dried samples was measured by a EuroVector EA-3000 elemental analyser after HCl decarbonisation (Ulyantsev et al., 2017b). Lignin and soil phenols were derived from the samples by CuO oxidation (Hedges and Ertel, 1982) which was carried out in 10 mL stainless steel minibombs sealed with Teflon-lined screw caps. Dried sample (0.2–2 g, equal to 2–5 mg of TOC content) was placed into a bomb, then about 500 mg of pre-combusted powdered CuO, 100 mg of Fe(NH)4(SO4)2×6H2O, and 8 mL of 8% aqueous NaOH (wt./wt.) were added. Each minibomb was placed into a Branson 1210 bath for sonification and degassing with pure nitrogen for 20 min. CuO oxidation was carried out after N2 purgation using a programmable electric heating device by incubating at 170 °C for 3 h. Whereupon, minibombs were carefully cooled with running water and caps were displaced. The reacted contents of each bomb were put into a 10 mL centrifuge tube and rotated at 2500 rpm for 20 min to precipitate the remaining CuO. The supernatant was decanted with a graduated glass pipette into a 50 mL glass separating funnel. Each mixture was acidified with 6 M HCl by micropipette to pH 2, and HF was added to dissolve the precipitated SiO2. Acidic solutions were triply extracted with 20 mL volumes of distilled dichloromethane for 15 min. The combined extracts were dehydrated with anhydrous Na2SO4, quantitatively placed into brown glass vials, dried under pure N2 flow, and diluted in pyridine. Extracts were put into a refrigerator and held for GC analysis. The polar phenols of CuO oxidation were converted to the trimethylsilyl derivatives directly prior to GC procedures by rapid reaction with N,O-bis-(trimethylsilyl) acetamide in pyridine (1 : 1, v/v). The GC analysis of silylated phenols was carried out with a Shimadzu 2010 gas chromatograph fitted with a flame-ionization detector with an on-column injector and used in constant flow (1.5 mL/min) mode. A Supelco EQUITY-5 fused silica capillary column (30 m long × 0.25 mm i.d. × 0.25 µm film thickness) was used. The operating conditions were as follows: temperature held at 100 °C for 2 min, increased from 100 to 280 °C at a rate of 4 °C/min with a final isothermal hold at 280 °C for 20 min. Helium was used as the carrier gas. Peaks were identified by the retention time of individual components compared to standards (prepared mix of 11 commercially available compounds: p-hydroxybenzaldehyde (Pl), p-hydroxyacetophenone (Pn), p-hydroxybenzoic acid (Pd), vanillin (Vl), acetovanillone (Vn), vanillic acid (Vd), syringaldehyde (Sl), acetosyringone (Sn), and syringic (Sd), p-coumaric (pCd), and ferulic (Fd) acids). Trans-ethylvanillin was used as the external quantification standard.