Aromatic acids from biomass burning in the WAIS Divide ice core

Abstract:

Biomass burning plays an important role in atmospheric chemistry, carbon cycling, and climate. The global long-term history of biomass burning is not well established, making it difficult to study the relationship between burning, climate change, and atmospheric chemistry. Here we present the Antarctic ice core records of vanillic acid and p-hydroxybenzoic from the WAIS Divide ice core covering the past 2.4-30 kyrs BP. These molecules are derived from incomplete combustion of plant lignin and transported/deposited as aerosols onto the ice sheet. Vanillic and p-hydroxybenzoic acids are associated with combustion of conifers and grasses, respectively, but are not uniquely derived from these plant types. Analyses were done using ion chromatography with electrospray MS/MS detection in negative ion SRM mode.

Vanillic p-hydroxybenzoic acid exhibited a range from baseline levels near 0.01 ppb (detection limit) to >0.5 ppb. Vanillic acid exhibited striking millennial scale variability during late glacial period, with 6 major peaks between 25 and 13.5 kyrs BP, with a spacing of 1.5-3 kyrs and durations of up to 1 kyr. There are no comparable peaks during the early-mid Holocene. A late Holocene vanillic acid peak starts at 3 kyrs BP. p-Hydroxybenzoic acid shares the same major peaks as vanillic acid from 25-13.5 kyrs BP, but exhibits additional variability of comparable magnitude throughout the WAIS Divide 2.4-30 kyr record.

These paleo records should be viewed as qualitative burning proxies because 1) a wide range of aerosol composition and sizes can be generated from combustion of various plant materials under different conditions, and 2) the ice core levels of aromatic acids may reflect changes in source regions, transport and atmospheric removal efficiency, and postdepositional mobilization. The major peaks in the WAIS Divide aromatic acid records most likely do not represent changes in global biomass burning emissions, because they are not highly correlated with variations in atmospheric methane. Such caveats notwithstanding, these records provide a surprising new picture of the long-term variability in the deposition of burning-derived aerosols to the Antarctic ice sheet.