A 30 Myr Record of Late Triassic Atmospheric Pco2 Variation Refl Ects a Fundamental Control of the Carbon Cycle by Changes in Continental Weathering
Total Page：16
File Type：pdf, Size：1020Kb
A 30 Myr record of Late Triassic atmospheric pCO2 variation refl ects a fundamental control of the carbon cycle by changes in continental weathering Morgan F. Schaller1,†, James D. Wright1, and Dennis V. Kent1,2 1Department of Earth and Planetary Sciences, Rutgers University, Piscataway, New Jersey 08854, USA 2Lamont Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA ABSTRACT silicate weathering reactions on the continents. side processes that are functions of the spatial, Because CO2 exerts a fundamental control over temporal, and lithologic heterogeneity of conti- We generate a detailed ~30 Myr record temperature, and temperature infl uences pre- nental surface participation in weathering reac- of pCO2 spanning most of the Late Triassic cipi tation patterns (Manabe and Wether ald, tions. These effects are independent of changes (Carnian-Norian-Rhaetian) to earliest Ju- 1980) and determines the rate constants in in mid-ocean ridge degassing, but they can rassic (Hettangian), based on stable carbon conti nental hydrolysis reactions (for review, see impart variability of the same magnitude. isotope ratios of soil carbonate and preserved Kump et al., 2000), an increase in atmospheric The Late Triassic offers a unique opportunity organic matter from paleosols in the eastern pCO2 ultimately leads to an increase in chemi- to empirically test whether variations in the sink North American Newark rift basin. Atmo- cal weathering. Increased weathering more side (continental weathering) of the global car- spheric pCO2 was near 4500 ppm in the late rapidly consumes CO2, leading to an eventual bon cycle can drive long-term changes in atmo- Carnian, decreasing to below ~2000 ppm by negative feedback on atmospheric pCO2, thus spheric pCO2. This is because the rates of ocean the late Rhaetian just before the earliest Ju- constituting a “thermo stat” that keeps Earth’s crust production, based on inversion of sea-level rassic eruption of the Central Atlantic Mag- temperature within a stable range (Walker et al., records used in simple mass-balance models, matic Province, which triggered measurable 1981). One way to tip the balance in this rela- apparently changed little through the Late Tri- pulses of CO2 outgassing. These data are con- tionship is to change the rate of CO2 degassed assic (Fig. 1; Gaffi n, 1987), thereby severely sistent with published modeling results using from Earth’s mantle. This perspective has led limiting any pCO2 forcing incorporated in the the GEOCLIM model, which predict a de- to a generation of carbon-cycle mass-balance BLAG/GEOCARB models over this time inter- crease in pCO2 over the Late Triassic as a re- models (e.g., BLAG and GEOCARB [Berner, val, which indeed produce a fl at pCO2 profi le sult of the progressive increase in continental 1990, 2006; Berner and Kothavala, 2001; Ber- (see previously cited references). On the other area subject to the intense weathering regime ner et al., 1983], COPSE [Bergman et al., 2004]) hand, Goddéris et al. (2008) found that the of the tropical humid belt due to Pangea’s that force long-term (geologic scale) changes northward migration of Pangea during the Tri- northward motion. The fi ner-scale pCO2 in pCO2 by the cumulative effects of transient assic should have produced a two-thirds mod- changes we observe may be dependent on changes in the rate of degassing at mid-ocean eled reduction of atmospheric pCO2 by the latest the lithology introduced to the tropics, such ridges (Fig. 1), treating the weathering-induced Triassic, due to a steady increase in the amount as the dip to ~2000 ppm around 212 Ma and consumption of atmospheric CO2 as a simple of weatherable land area present in the tropical its rebound to ~4000 ppm at 209 Ma, which feedback response (Berner, 1991). humid belt. Therefore, given the lack of any can be accomplished by introducing a more Such source-side processes have dominated appreciable source-side forcing during the Late weatherable subaerial basaltic terrain. These attempts to explain the evolution of atmospheric Triassic we predict that any signifi cant observed observations indicate that the consumption pCO2 on time scales like the Phanerozoic, with change in atmospheric pCO2 through the inter- of CO2 by continental silicate weathering can the only substantial suggested sink-side pertur- val must be forced by another mechanism. force long-term changes in pCO2 compara- bations thought to be related to uplift during a In this paper, we empirically test the hypothe- ble to those driven by presumed changes in massive orogenic event such as the Hima layas sis that continental weathering can be the pri- mantle degassing. (e.g., Raymo and Ruddiman, 1992). How- mary control over the long-term evolution of ever, this simplifi ed view of the system does pCO2 through changes in the latitudinal dis- INTRODUCTION not account for tectonic-scale changes in the tribution of continental area, as modeled with distribution of continental area with respect GEOCLIM by Goddéris et al. (2008), by gener- On geologic time scales, equable climates are to the zones on Earth’s surface where weath- ating an atmospheric pCO2 record from the ~30 controlled primarily by a balance between the ering is most active (Donnadieu et al., 2006a, Myr continuous section of Late Triassic conti- CO2 outgassed at mid-ocean ridges and other 2006b; Goddéris et al., 2008), or the relocation nental strata of the Newark rift basin in eastern volcanoes and the consumption of that CO2 by or eruption of highly weatherable material into North America (Fig. 2). We use the stable carbon the equatorial humid belt (Dessert et al., 2001; isotope ratios of pedogenic carbonate and soil †Current address: Earth and Environmental Sci- Kent and Muttoni, 2008, 2013; Schaller et al., organic matter as a proxy for atmospheric pCO2 6 ence, Rensselaer Polytechnic Institute, Troy, New 2012). These alternate explanations for 10 - to (Cerling, 1999), which has been successfully 7 York 12180, USA; e-mail: [email protected]. 10 -yr-scale pCO2 variability constitute sink- used to estimate the transient pCO2 perturbations GSA Bulletin; May/June 2015; v. 127; no. 5/6; p. 661–671; doi: 10.1130/B31107.1; 5 fi gures; 1 table; published online 6 November 2014. For permission to copy, contact [email protected] 661 © 2014 Geological Society of America Schaller et al. δ δ13 Models vs. Assumed Crustal Production respiration of soil organic matter, s is the C 5.5 8000 δ δ13 GEOCARBSULF of soil CO2, ϕ is the C of soil-respired CO2, δ δ13 δ COPSE and a is the C of atmospheric CO2. All GEOCARB III 7000 values are relative to Vienna Peedee belem- 5 GEOCLIM nite (VPDB). Crustal Production 6000 pCO The carbon isotope ratio of soil carbonate /yr) 2 δ δ 2 ( cc) is used as a proxy for s, which involves 4.5 5000 (ppm) - Models a temperature-dependent equilibrium fraction- 4000 ation between calcite and CO2, described by 4 (Cerling, 1999): 3000 103 lnα=11.709 – 0.116( T) + Crustal Production (km 3.5 2000 2.16 ×10–4 (T),2 (2) 1000 α 3 where is the fractionation factor, and tempera- 0 ture (T, in °C) is fi xed at 25 °C (±5 °C), which 0 100 200 300 400 500 is assumed to be appropriate for the tropical Time (Ma) Newark basin. For the purposes of this work, Equation 2 gives results indistinguishable from Figure 1. The pCO2 output of several geochemical models over the Phanerozoic, GEOCARBIII (Berner and Kothavala, 2001); GEOCARBsulf (Berner, 2006); COPSE the Romanek et al. (1992) calibration at 25 °C. (Bergman et al., 2004); GEOCLIM (Donnadieu et al., 2006a; Goddéris et al., 2008), com- Because there is no kinetic or equilibrium car- δ pared to the record of assumed crustal production used in the GEOCARB models (from bon isotope fractionation due to respiration, ϕ Gaffi n, 1987), which is based on inversion of sea-level histories determined by sequence is related directly to the carbon isotopic ratio of soil organic matter (δ13C ) (see following stratigraphy. All models except GEOCLIM are based on the BLAG hypothesis. Shaded org band is Late Triassic (ca. 201–235 Ma). discussion). The carbon isotopic ratio of the atmosphere is calculated from the measured δ13 Corg by the following relationship (Arens et al., 2000): associated with the Central Atlantic magmatic (stratigraphically upward: Prince ton, Nursery, province (Schaller et al., 2011, 2012), situated Titusville, Rutgers, Somerset, Weston Canal, δ = δ13 in the upper portion of the eastern North Amer- and Martinsville) were spudded in the NW-dip- a ( Corg+18.67)/1.10, (3) ica section. The pCO2 estimates presented here ping strata for maximum overlap up section, and from the underlying ~4500 m of section show a even those sites that are as much as ~50 km geo- which assumes consistent fractionation by secular decrease between the Carnian and Rhae- graphically apart (e.g., the Rutgers and Titus- photosynthesis and builds carbon-cycle pertur- tian, consistent with the results of GEOCLIM, ville cores) show nearly identical stratigraphy bations directly into the model. We note that which modeled this decrease as a function of within the zones of overlap. there is a change in the degree of photosynthetic the amount and rate at which continental area The lacustrine facies are ordered in a rhyth- fractionation by terrestrial plants in response to is moved into the equatorial humid belt, where mic succession refl ecting periodic fl uctuations elevated atmospheric CO2 (Schubert and Jahren, atmospheric CO2 is most effi ciently consumed. in lake depth that have been related directly to 2012). Though this is an important effect, it Milankovitch orbital forcing of tropical precipi- is diffi cult to include in the model at this time ESTIMATES OF pCO2 FROM tation (Olsen, 1986; Olsen and Kent, 1996; Van because of its fundamental dependence on PALEOSOLS Houten, 1962).