Geology | 2019
The effect of diagenesis on carbon isotope values of fossil wood
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 (δCcell) rather than analyzing whole wood (δCwood) due to concerns that molecular variability across tree rings could influence δCwood values, and that diagenesis may preferentially degrade cellulose over lignin. However, the majority of deep-time researchers analyze δCwood without correcting for possible diagenetic effects due to cellulose loss. We measured δCcell, δCwood, 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 δCcell and δCwood values made on fossil and modern trees. We report that, on average, the apparent enrichment factor (ε) between δCcell and δCwood (ε = δCcell – δCwood) 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 δCwood and δCcell 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 δCwood values are compared to extant trees or across long time scales, where cellulose content may vary. INTRODUCTION 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). In the past 30 yr, at least 229 δ13C records have been developed for reconstructing past climate using annual growth rings in modern trees; 83% of these measured δ13C of cellulose (δCcell), a polysaccharide that can be chemically extracted from whole wood (Table DR1 in the GSA Data Repository1). However, for deep-time applications that utilized fossil wood across a wide range of preservation states (e.g., lignified [Bechtel et al., 2003] to mummified [Jahren and Sternberg, 2002]), only 23% of studies measured δCcell, compared to 77% that reported δ13C values of whole wood (δCwood; Table DR1). Despite clear differences in the use of whole wood versus compound-specific analyses, measurement of δ13C values of living, buried, and fossilized trees is routinely used to discern past hydroclimate (Livingston and Spittlehouse, 1996; Barber et al., 2000; Edvardsson et al., 2014), precipitation patterns (Schubert et al., 2012; Schubert and Timmermann, 2015; Schubert et al., 2017), plant life strategy and taxonomy (Bechtel et al., 2007; Jahren and Sternberg, 2008), and atmospheric chemistry (Hesselbo et al., 2000; Schubert and Jahren, 2013). As yet, the effect of decomposition on δ13C value is poorly constrained, but it is necessary for the quantification of atmospheric and climatic change within the deep-time geologic record. For example, 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. Wood offers a unique opportunity to study diagenetic alteration of organic matter, because it is primarily composed of two compounds: lignin (25%–35%) and cellulose (40%–50%), with lesser amounts of hemicellulose and extractives (Parham and Gray, 1984; Pettersen, 1984; Scott, 2009). Lignin is more recalcitrant than cellulose; therefore, the preservation state of wood can be modeled using these two end members (Loader et al., 2003), where the δ13C value of lignin is typically 0.5‰–2‰ lower than that of cellulose (Harlow et al., 2006). Several researchers have concluded that as cellulose preferentially degrades in fossil wood, δCwood will trend toward the δ13C value of lignin and result in up to 2‰ lower values (Benner et al., 1987; Spiker and Hatcher, 1987; Schleser et al., 1999; van Bergen and Poole, 2002). Consequently, the apparent enrichment (ε) between δCcell and δCwood (i.e., ε = δCcell – δCwood) should show a negative correlation with cellulose content of wood. In contrast, incubation experiments (Schleser et al., 1999) and some deep-time fossil wood assemblages (Bechtel et al., 2007) possibly indicate that higher ε values may result from enrichment of 13C during cellulose degradation. Numerous studies have reported a 1:1 correlation between δCcell and δCwood for living and exceptionally well-preserved, subfossil trees (Schleser, 1990; Leavitt and Long, 1991; Livingston and Spittlehouse, 1996; Borella and *E-mail: [email protected] 1GSA Data Repository item 2019354, descriptions of fossil wood localities and analytical methods, Figures DR1–DR3, and Tables DR1 and DR2, is available online at http://www.geosociety.org/datarepository/2019/, or on request from [email protected]. Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/47/10/987/4830588/987.pdf by Brian Schubert on 25 September 2019 988 www.gsapubs.org | Volume 47 | Number 10 | GEOLOGY | Geological Society of America Leuenberger, 1998; Macfarlane et al., 1999; Loader et al., 2003; Edvardsson et al., 2014). Testing of this correlation for deep-time wood samples has been inhibited by the paucity of well-preserved wood in the stratigraphic record relative to Quaternary strata, and by differences in analytical approaches between Quaternary and deep-time workers. Whereas δCcell and δCwood are widely measured on living and recently felled trees (see review in McCarroll and Loader, 2004), only δCwood is routinely measured on deep-time wood fossils (e.g., Gröcke et al., 1999; Hesselbo et al., 2000, 2002, 2003, 2007; Pearce et al., 2005; Yans et al., 2010; Table DR1). We sought to rectify this discrepancy and quantify the effects of diagenesis on δ13C values by measuring δCcell, δCwood, and cellulose content on 38 fossil wood specimens that ranged in age from early Eocene to late Miocene. We supplemented these analyses with a new compilation of published paired δCcell and δCwood values from modern trees and deep-time wood fossils to test the following hypotheses: (1) δCwood correlates with δCcell regardless of geologic age, and (2) deep-time wood fossils that have lost cellulose will have greater ε values. METHODS The fossil wood samples analyzed in this study were recovered from deltaic and lacustrine deposits in the Eureka Sound Formation on Banks Island, Northwest Territories, Canada; the Yongning Formation in Nanning, China; the Xiaolongtan Formation in Yunnan Province, China; and the Khapchansky locality in northeast Siberia (Fig. 1; Table DR1). Descriptions of these localities and details of laboratory sampling and analytical methods are provided in the Data Repository. Typically, δ13C values of tree-ring tissue are corrected for changes in the δ13C value of atmospheric CO2 and pCO2 prior to interpretation of climatic signals (McCarroll et al., 2009; Treydte et al., 2009; Wang et al., 2011; Schubert and Timmermann, 2015; Trahan and Schubert, 2016). Biosynthetic fractionation between different plant substrates, however, is not affected by atmospheric chemistry (e.g., Loader et al., 2003; Schubert and Jahren, 2012; Diefendorf et al., 2015); therefore, determination of net carbon isotope discrimination was unnecessary for interpretation of the carbon isotopic difference between cellulose and whole wood. RESULTS AND DISCUSSION Effect of Diagenesis on Fossil Wood δ13C Values Our new analyses extend the age range and more than double the number of reported deeptime fossil wood δCcell and δCwood pairs. The δCwood values of the 38 fossil samples ranged from −31.8‰ to −22.6‰ (δCcell = −30.2‰ to − 19.8‰), consistent with the wide range of environments, taxa, climates, and atmospheric compositions represented by these fossils. The δ13C values of paired wood and cellulose samples were highly correlated between substrates (Spearman’s ρ = 0.97), and δCcell values were higher than δCwood values in every pair (Fig. DR2). Cellulose content ranged from 0.4% to 44.5%. Calculated ε values were negatively correlated with cellulose content (Pearson’s r = −0.49, p = 0.003; Fig. 2). A linear regression model estimated that pure cellulose (i.e., δCwood = δCcell) would have an ε value (δCcell – δCwood) within error of zero (−0.4‰ ± 0.4‰). The difference in ε between a sample that has 45% cellulose—a typical value for living trees—and one that has no cellulose is 1.4‰ ± 0.4‰. Cellulose content broadly corresponds to geologic age, with older wood fossils tending to have less cellulose remaining (Fig. 2B). However, individual deposits (e.g., the Oligocene Nanning Lagerstätte; Table DR2) 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 δCwood values. Comparison of the δ13C Value of Whole Wood Versus Cellulose We augmented our da