Abstract
Abstract The carbon isotope (δ13C) value of modern and fossil wood is widely used as a proxy for environmental and climatic change. Many researchers who study stable carbon isotopes in modern and recently deceased trees chemically extract cellulose (δ13Ccell) rather than analyzing whole wood (δ13Cwood) due to concerns that molecular variability across tree rings could influence δ13Cwood values, and that diagenesis may preferentially degrade cellulose over lignin. However, the majority of deep-time researchers analyze δ13Cwood without correcting for possible diagenetic effects due to cellulose loss. We measured δ13Ccell, δ13Cwood, and cellulose content of 38 wood fossils that span ∼50 m.y. in age from early Eocene to late Miocene, using variability across such a large range of geologic ages and settings as a natural laboratory in diagenesis. For comparison with our measurements, we produced a literature compilation of 1210 paired δ13Ccell and δ13Cwood values made on fossil and modern trees. We report that, on average, the apparent enrichment factor (ε) between δ13Ccell and δ13Cwood (ε = δ13Ccell – δ13Cwood) is 1.4‰ ± 0.4‰ larger in deep-time samples than Holocene wood, and this can be explained by loss of cellulose during degradation, independent of atmospheric chemistry or climate conditions during growth. A strong linear correlation exists between δ13Cwood and δ13Ccell in both deep-time (r2 = 0.92) and Holocene (r2 = 0.87) samples, suggesting that either substrate can provide a reliable record of environmental conditions during growth. However, diagenetic effects must be corrected if δ13Cwood values are compared to extant trees or across long time scales, where cellulose content may vary.
Highlights
The stable carbon isotope composition (δ13C)of terrestrial plants is one of the primary means for tracking changes in Earth’s carbon cycle and climate before the instrument record (Nordt et al, 2016; Strauss and Peters-Kottig, 2003)
The δ13C values of paired wood and cellulose samples were highly correlated between substrates (Spearman’s ρ = 0.97), and δ13Ccell values were higher than δ13Cwood values in every pair (Fig. DR2)
Individual deposits can contain a large range of cellulose content among fossil samples. These findings indicate that diagenetic alteration of wood, which preferentially removes cellulose, can significantly bias deep-time δ13Cwood values
Summary
The stable carbon isotope composition (δ13C)of terrestrial plants is one of the primary means for tracking changes in Earth’s carbon cycle and climate before the instrument record (Nordt et al, 2016; Strauss and Peters-Kottig, 2003). Diagenetic alteration of organic substrates presents a fundamental limitation for interpreting δ13C signals from deep-time (i.e., pre-Quaternary) records (Jones, 1994; Tu et al, 2004; Baczynski et al, 2016). This challenge has been mitigated through the measurement of compound-specific isotope ratios using biomolecular substrates selected for their recalcitrance to diagenetic modification (e.g., Goni et al, 2000; Ververis et al, 2004). Recent work linking organic matter δ13C values to changes in atmospheric pCO2 have the potential to greatly increase the resolution of paleoclimate and paleoatmospheric reconstructions (Schubert and Jahren, 2012; Cui and Schubert, 2018). Correction of δ13C values for diagenesis will better constrain the nature and magnitude of pCO2 change and help to reduce uncertainty in δ13C-based paleoclimatic and paleoenvironmental proxies
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