Chemical Characteristics of Macroscopic Pyrogenic Carbon Following Millennial-Scale Environmental Exposure

Abstract

Pyrogenic Carbon (PyC) is ubiquitous in global environments, and is now known to form a significant, and dynamic component of the global carbon cycle, with at least some forms of PyC persisting in their depositional environment for many millennia. Despite this, the factors that determine the turnover of PyC remain poorly understood, as do the physical and chemical changes that this material undergoes when exposed to the environment over tens of thousands of years. Here, we present the results of an investigation to address these knowledge gaps through chemical and physical analysis of a suite of wood PyC samples exposed to the environment for varying time periods, to a maximum of >90,000 years. This includes an assessment of the quantity of resistant carbon, known as Stable Polyaromatic Carbon (SPAC) vs. more chemically labile carbon in the samples. We find that, although production temperature is likely to determine the initial "degradation potential" of PyC, an extended exposure to environmental conditions does not necessarily mean that remaining PyC always progresses to a "SPAC-dominant" state. Instead, some ancient PyC can be composed largely of chemical components typically thought of as environmentally labile, and it is likely that the depositional environment drives the trajectory of preservation vs. loss of PyC over time. This has important implications for the size of global PyC stocks, which may have been underestimated, and also for the potential loss of previously stored PyC, when its depositional environment alters through environmental or climatic changes.

2013–2014 Survey of Chars Using Raman Spectroscopy

“…These samples include a carbonaceous residue from tire pyrolysis carbon and an anthracite coal standard. A sample of ancient oak charcoal from an Irish archaeological site (dated to around 3700 years old) was included along with two samples of thermally modified wood recovered from volcanic ash deposits from Rotomahana tephra/mud in New Zealand (~129 years old, [22]) and Noname beach tephra in Australia (~92,000 years old [23]).…”

Assessing Modern Calluna Heathland Fire Temperatures Using Raman Spectroscopy: Implications for Past Regimes and Geothermometry

“…For charcoals held or deposited terrestrially, high aromaticity due to high temperatures of formation (e.g., Ascough et al, 2008), contributes not only toward microstructural maturation but subsequent increases in charcoal stability and resistance to oxidative degradation (Ascough et al, 2018;, growing increasingly susceptible with time and exposure (Ascough et al, 2011a;Ascough et al, 2020;Smidt et al, 2020), particularly in alkaline environments (Braadbaart et al, 2009;Ascough et al, 2011b). This suggests that low-temperature charcoals are in turn more susceptible to alteration, degradation, and preferential loss from the palaeofire record, including as a result of sample acquisition and analysis .…”

“…The application of charcoal reflectance analyses to modern wildfire charcoals-with the intention of better understanding temperature, intensity, and severity-may be found within several examples (e.g., Scott et al, 2000;Hudspith et al, 2015;New et al, 2018;Belcher et al, 2021). Similarly, Fourier-Transform Infra-Red spectroscopy (FTIR) has shown considerable potential in understanding chemical and microstructural change in charcoals (e.g., Guo and Bustin, 1998;Ascough et al, 2020). This has developed into applications in the assessment of wildfire intensity and severity (Gosling et al, 2019;Maezumi et al, 2021).…”

“…Discrepancies in natural vs. experimental fire thermometry may also be perpetuated by variable environmental conditions, and the inter-and intraheterogeneity of fuels (Ishimaru et al, 2007a;Guedes et al, 2010). Furthermore, in the context of palaeowildfires, the nature of archaeological and geological records with regards to charcoal suggest considerable potential for alteration of wildfire evidence, through diagenetic processes such as humification and oxidation (Cohen-Ofri et al, 2006;Braadbaart et al, 2009;Ascough et al, 2008;Ascough et al, 2010;Ascough et al, 2011a;Ascough et al, 2018;Ascough et al, 2020;de Sousa et al, 2020;Smidt et al, 2020), and taphonomic bias (Scott and Damblon, 2010 and references therein). Such natural variability, particularly with regards to the influence of energy flux may bring into question the validity of historic assessments of palaeowildfire temperature.…”

Infrared spectroscopy refines chronological assessment, depositional environment and pyrolysis conditions of archeological charcoals

“…Some fundamental facts can be deduced from numerous experimental set-ups: feedstock composition, production temperature/carbonization degree and environmental conditions, e.g. soil properties 16 , 17 are the main influencing factors for black carbon longevity or short-term degradation. These findings were compiled in review articles 18 , 19 , including uncertainties regarding the stocks and stability of BC, especially in boreal regions 20 .…”