Novel molecular proxies for inferring pyrogenic black carbon oxidation state using thermally assisted hydrolysis and methylation (THM-GC-MS) with 13C-labeled tetramethylammonium hydroxide (TMAH)

Analytical techniques to assess the degree of alteration of pyrogenic black carbon (C) in soils and sediments are needed to gauge the role of environment and time in the dynamics of this stable carbon form. The benzenecarboxylic acids (BnCA, with n = number of carboxylic groups) and naphthalene analogues (NnCA) released by thermally assisted hydrolysis and methylation (THM) of black C have the potential to yield information about black C oxidation state but it remains unclear to what extent the carboxylic groups in BnCA and NnCA are native to the sample or form as analytical artefacts during the THM reaction. In an effort to clarify the source of analytical products and investigate long term black C oxidation in a soil environment we applied THM gas chromatography mass spectrometry (THM-GC-MS) using 13C-labeled tetramethylammonium hydroxide (13C-TMAH) to oak-derived black C samples isolated from a colluvial soil profile dating to 50-8100 cal BP. The results showed that the BnCA and NnCA originate from the black C and are potentially usable as oxidation proxies, with the exception of B1CA and hydroxyl-B1CA which have a significant source in lignin. The cumulative abundance of BnCA (excluding B1CA) and NnCA increased with age of the black C (P < 0.05) from ca. 0.05-0.20 mg/100 mg organic C. Furthermore, the ratios B3CA/B2CA and (B3CA + B4CA)/B3CA increased with age, and are proposed as novel proxies of black C oxidation state. The results support previous studies showing that carboxyl functionalities dominate over hydroxyl and that molecular markers of lignin are selectively depleted within 1000 yrs after wildfire. The accumulation of hydroxyl and especially carboxyl functional groups on black C structures seems to be a gradual process that continued at least until the end of the studied ageing window of 8100 yrs.