Science - USA (2021-11-12)

rules out degradation mechanisms in which
methoxyl groups are indiscriminately removed
in concert with neighboring carbons (materials
and methods, section S4) and instead suggests
targeted O-demethylation. Four known catalysts
could be responsible—heat, acids, oxidants,
or enzymes. To distinguish between them, we
consider the isotopic consequences of each.

Isotopic fractionations during thermal mat-

urationincoalareoftenwelldescribedby

Rayleigh distillation ( 16 ). To test whether this

model applies specifically to thermally acti-

vated O-demethylation, we pyrolyzed poplar

woodinadry,opensystemat300°Cand

analyzed the remaining wood sample (table

S4; fig. S5; and materials and methods, section

S1.1). Methoxyl concentration andd^13 Cmethoxyl

conformed to a Rayleigh process with an

isotope effect of^13 e=−12.2 ± 0.4‰[2s;^13 e=

(^13 Rprod/^13 Rreact)−1] (fig. S6). This^13 eis similar

to theoretical predictions for O-demethylation

through homolytic O–C cleavage at this tem-

perature [−16 to− 17 ‰( 17 )]. At temperatures

of the environmental samples studied here

SCIENCEscience.org 12 NOVEMBER 2021•VOL 374 ISSUE 6569 895

Fig. 1. Evolution of methoxyl concentration and carbon isotopic
composition during maturation from lignin to subbituminous coal.
(AandB) Methoxyl data are compared with two types of degradation
models: abiotic O-demethylation mechanisms (A), which all follow Rayleigh
fractionation, and biologic mechanisms (B), which can conform to

Rayleigh models (solid lines and semitransparent regions) or to enzymatic

O-demethylation with a potential mass transport limitation (dashed lines)

( 21 ). Fractionation factors for each mechanism are maximum ranges

previously documented (table S5). Existing wood methoxyl data are shown

as semitransparent squares (table S6). Ro, vitrinite reflectance.

Fig. 2. Conceptual model of O-demethylation in relation to major coalification
processes in humic materials.(A) Schematic generation of carbonaceous
gases as a function of maturity [( 29 ); all values approximate]. (Band
C) Concentration (B) and carbon isotopic composition (C) of methoxyl groups

and O-demethylation reaction products during the diagenesis of humic materials.

Additional Belchatow lignite data (semishaded diamonds) are from ( 32 ).

Relationship between methoxyl carbon fraction and reflectance used to project

O-demethylation model to these axes given in fig. S13. sub-bit., subbituminous.

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