Carbon isotope compositions of terrestrial C3 plants as indicators of (paleo)ecology and (paleo)climate

Abstract

A broad compilation of modern carbon isotope compositions in all C3 plant types shows a monotonic increase in δ(13)C with decreasing mean annual precipitation (MAP) that differs from previous models. Corrections for temperature, altitude, or latitude are smaller than previously estimated. As corrected for altitude, latitude, and the δ(13)C of atmospheric CO(2), these data permit refined interpretation of MAP, paleodiet, and paleoecology of ecosystems dominated by C3 plants, either prior to 7-8 million years ago (Ma), or more recently at mid- to high latitudes. Twenty-nine published paleontological studies suggest preservational or scientific bias toward dry ecosystems, although wet ecosystems are also represented. Unambiguous isotopic evidence for C4 plants is lacking prior to 7-8 Ma, and hominid ecosystems at 4.4 Ma show no isotopic evidence for dense forests. Consideration of global plant biomass indicates that average δ(13)C of C3 plants is commonly overestimated by approximately 2‰.

Fig. 1.

(A) Histogram of MAP values for isotopically charact.erized C3 plants, showing emphasis on relatively arid ecosystems (MAP ≤ 500 mm/yr) and tropical rainforests (spikes at MAP ∼ 2,000, 3,000 mm/yr). (B) Histogram of δ¹³C values of modern C3 plants. Data compiled in this study average -27.0‰, excluding analyses from the understory of closed-canopy forests. Estimated global average composition, based on global trends in precipitation and vegetation, is approximately -28.5‰, significantly lower than typically assumed. An accurate average δ¹³C value for C3 plants is needed for accurate models of carbon fluxes, atmospheric CO₂ compositions, and soil organic matter. (C) δ¹³C values vs. MAP showing increasing δ¹³C with aridity. Data sources are listed in SI Text. White dots are average compositions of data from a large collection made in a single month during a wet year (35).

Fig. 2.

(A) Averaged δ¹³C (Table S1) vs. MAP and models of carbon isotope compositions. Averaged data are provided for clarity; thin lines are published models; thick curve is preferred regression from this study. Solid portions of lines represent MAP range over which models were developed; dotted lines show extrapolations. D = Diefendorf et al. (8); G1 = Gouveia and Freitas (54); G2a,b = Guo and Xie (6); H = Hatté et al. (55); L1 = Leffler and Enquist (56); L2 = Liu et al. (57); M1 = Miller et al. (58); M2 = Macfarlane et al. (59); R = Roden et al. (, averaged from two similar regressions); S = Song et al. (61); Y = Youfeng et al. (62). Original line of Stewart et al. (4) is best linear model, but deviates from data at large values of MAP; model of Diefendorf et al. (8) fits high MAP data but does not predict low MAP data well. (B) Carbon isotope residuals for a model that omits latitude and altitude, showing significant correlation with altitude. “D” indicates model of Diefendorf et al. (8). Inset shows raw δ¹³C values (uncorrected for any parameter) vs. altitude. (C) Carbon isotope residuals for a model that includes altitude but omits latitude, showing small but significant correlation. Inset shows raw δ¹³C values (uncorrected for any parameter) vs. latitude; high values centered at approximately 30° latitude reflect dry ecosystems on Earth. (D) Carbon isotope residuals for a model that includes latitude and altitude vs. MAT showing no significant trend. Inset shows raw δ¹³C values (uncorrected for any parameter) vs. MAT. Numbers are values for slopes, and errors are ± 2σ. X-axes on insets are same as in encompassing panel.

Fig. 3.

Paleodietary compositions corrected for altitude and latitude contoured for MAP; this plot permits interpretation of paleoenvironments from carbon isotope compositions of fossil tooth enamel or collagen. Tooth enamel and collagen compositions are averaged across species and corrected for δ¹³C of atmospheric CO₂, physiological fractionations, altitude, and latitude. Most data plot at MAP ≤ 800 mm/yr, i.e., relatively dry environments, although wetter environments are also represented. b1 = Bibi (51); b2 = Bocherens and Drucker (63); c1 = Chritz et al. (30); c2, c3 = Coltrain et al. (, 20–25 ka and 12 ka); c4 = Cerling et al. (2); d = DeSantis and Wallace (26); e = Eberle et al. (21); f1, f2 = Fox-Dobbs et al. (, caribou, equid); f3 = France et al. (64); i = Iacumin et al. (65); k = Koch et al. (19); m1 = Morgan et al. ( at 15.5 Ma); m2 = Merceron et al. (52); m3 = MacFadden and Higgins (20); n = Nelson (66); p1 = Passey et al. (49); p2 = Palmqvist et al. (27); s1 = Ségalen and Lee-Thorp (67); s2 = Secord et al. (25); t1 = Tütken et al. (68); t2 = Tütken et al. (69); v = van Dam and Reichart (34); w = Wang et al. (29); z = Zanazzi and Kohn (33).