scholarly journals Using Carbon Isotope Equilibrium to Screen Pedogenic Carbonate Oxygen Isotopes: Implications for Paleoaltimetry and Paleotectonic Studies

Geofluids ◽  
2018 ◽  
Vol 2018 ◽  
pp. 1-11 ◽  
Author(s):  
Nathan D. Sheldon
Keyword(s):  
Carbon Isotope ◽  
Fickian Diffusion ◽  
Pedogenic Carbonate ◽  
Southwestern Montana ◽  
Pedogenic Carbonates ◽  
Moisture Regimes ◽  
Theoretical Predictions ◽  
Elevation Changes ◽  
Geologic Record ◽  
Isotope Equilibrium

Stable isotope compositions of pedogenic carbonates (δ13Ccarb, δ18Ocarb) are widely used in paleoenvironmental and paleoaltimetry studies. At the same time, both in vertical stratigraphic sections and in horizontal transects of single paleosols, significant variability in δ18Ocarb values is observed well in excess of what could reasonably be attributed to elevation changes. Herein, a new screening tool is proposed to establish which pedogenic carbonate δ18Ocarb compositions reflect formation in isotopic equilibrium with environmental conditions through the use of the co-occurring δ13Corg composition of carbonate-occluded or in profile organic matter, where Δ13C = δ13Ccarb – δ13Corg. Based upon 51 modern soils from monsoonal, continental, and Mediterranean moisture regimes, Δ13C = +15.6 ± 1.1‰ (1σ), which closely matches theoretical predictions for carbonates formed at carbon isotope equilibrium through Fickian diffusion. Examples from both disequilibrium and equilibrium cases in the geologic record are examined, and it is shown that previous δ18Ocarb records used to infer Cenozoic uplift in southwestern Montana do not provide any constraint on paleoelevation because >90% of the pedogenic carbonate isotopic compositions are out of equilibrium. Guidelines for future paleoaltimetry studies include collection of both vertical stratigraphic sections and lateral transects, of at least three nodules per horizon, petrographic screening of nodules for diagenesis, collection of at least one independent proxy for paleoclimate or paleovegetation, and screening δ18Ocarb values using Δ13C measured for each paleosol.

Carbon isotope signatures of pedogenic carbonates from SE China: rapid atmospheric pCO2 changes during middle–late Early Cretaceous time

2013 ◽  
Vol 151 (5) ◽  
pp. 830-849 ◽  
Author(s):  
XIANGHUI LI ◽  
HUGH C. JENKYNS ◽  
CHAOKAI ZHANG ◽  
YIN WANG ◽  
LING LIU ◽  
...  
Keyword(s):  
Carbon Isotope ◽  
Early Cretaceous ◽  
Large Scale ◽  
Atmospheric Composition ◽  
Global Changes ◽  
Fossil Plants ◽  
Pedogenic Carbonates ◽  
Organic Carbon Burial ◽  
Cyclic Changes ◽  
Se China

AbstractLower Cretaceous pedogenic carbonates exposed in SE China have been dated by U–Pb isotope measurements on single zircons taken from intercalated volcanic rocks, and the ages integrated with existing stratigraphy. δ13C values of calcretes range from –7.0‰ to –3.0‰ and can be grouped into five episodes of increasing–decreasing values. The carbon isotope proxy derived from these palaeosol carbonates suggests pCO2 mostly in the range 1000–2000 parts per million by volume (ppmV) at S(z) (CO2 contributed by soil respiration) = 2500 ppmV and 25°C during the Hauterivian–Albian interval (c. 30 Ma duration). Such atmospheric CO2 levels are 4–8 times pre-industrial values, almost double those estimated by geochemical modelling and much higher than those established from stomatal indices in fossil plants. Rapid rises in pCO2 are identified for early Hauterivian, middle Barremian, late Aptian, early Albian and middle Albian time, and rapid falls for intervening periods. These episodic cyclic changes in pCO2 are not attributed to local tectonism and volcanism but rather to global changes. The relationship between reconstructed pCO2 and the development of large igneous provinces (LIPs) remains unclear, although large-scale extrusion of basalt may well be responsible for relatively high atmospheric levels of this greenhouse gas. Suggested levels of relatively low pCO2 correspond in timing to intervals of regional to global enrichment of marine carbon in sediments and negative carbon isotope (δ13C) excursions characteristic of the oceanic anoxic events OAE1a (Selli Event), Kilian and Paquier events (constituting part of the OAE 1b cluster) and OAE1d. Short-term episodes of high pCO2 coincide with negligible carbon isotope excursions associated with the Faraoni Event and the Jacob Event. Given that episodes of regional organic carbon burial would draw down CO2 and negative δ13C excursions indicate the addition of isotopically light carbon to the ocean–atmosphere system, controls on the carbon cycle in controlling pCO2 during Early Cretaceous time were clearly complex and made more so by atmospheric composition also being affected by changes in silicate weathering intensity.

scholarly journals The DIC carbon isotope evolutions during CO2 bubbling: implications for ocean acidification laboratory culture

2022 ◽  
Author(s):  
Hongrui Zhang ◽  
Ismael Torres-Romero ◽  
Pien Anjewierden ◽  
Madalina Jaggi ◽  
Heather Stoll
Keyword(s):  
Ocean Acidification ◽  
Carbon Isotope ◽  
Inorganic Carbon ◽  
Dissolved Inorganic Carbon ◽  
Carbon Isotope Ratio ◽  
Laboratory Culture ◽  
Total Alkalinity ◽  
Isotopic Tracers ◽  
Physiological Processes ◽  
Isotope Equilibrium

Ocean acidification increases pCO2 and decreases pH of seawater and its impact on marine organisms has emerged as a key research focus. In addition to directly measured variables such as growth or calcification rate, stable isotopic tracers such as carbon isotopes have also been used to more completely understand the physiological processes contributing to the response of organisms to ocean acidification. To simulate ocean acidification in laboratory cultures, direct bubbling of seawater with CO2 has been a preferred method because it adjusts pCO2 and pH without altering total alkalinity. Unfortunately, the carbon isotope equilibrium between seawater and CO2 gas has been largely ignored so far. Frequently, the dissolved inorganic carbon (DIC) in the initial seawater culture has a distinct 13C/12C ratio which is far from the equilibrium expected with the isotopic composition of the bubbled CO2. To evaluate the consequences of this type of experiment for isotopic work, we measured the carbon isotope evolutions in two chemostats during CO2 bubbling and composed a numerical model to simulate this process. The isotopic model can predict well the carbon isotope ratio of dissolved inorganic carbon evolutions during bubbling. With help of this model, the carbon isotope evolution during a batch and continuous culture can be traced dynamically improving the accuracy of fractionation results from laboratory culture. Our simulations show that if not properly accounted for in experimental or sampling design, many typical culture configurations involving CO2 bubbling can lead to large errors in estimated carbon isotope fractionation between seawater and biomass or biominerals, consequently affecting interpretations and hampering comparisons among different experiments. Therefore, we describe the best practices on future studies working with isotope fingerprinting in the ocean acidification background.

Aridity-driven decoupling of δ13C between pedogenic carbonate and soil organic matter

Geology ◽  
2020 ◽  
Vol 48 (10) ◽  
pp. 981-985 ◽  
Author(s):  
Jiawei Da ◽  
Yi Ge Zhang ◽  
Gen Li ◽  
Junfeng Ji
Keyword(s):  
Organic Matter ◽  
Soil Organic Matter ◽  
Pore Space ◽  
C4 Plants ◽  
Atmospheric Co2 ◽  
Chinese Loess Plateau ◽  
Pedogenic Carbonate ◽  
Chinese Loess ◽  
Pedogenic Carbonates ◽  
Soil Pore Space

Abstract Pedogenic carbonate is an invaluable archive for reconstructing continental paleoclimate and paleoecology. The δ13C of pedogenic carbonate (δ13Cc) has been widely used to document the rise and expansion of C4 plants over the Cenozoic. This application requires a fundamental presumption that in soil pores, soil-respired CO2 dominates over atmospheric CO2 during the formation of pedogenic carbonates. However, the decoupling between δ13Cc and δ13C of soil organic matter (δ13CSOM) have been observed, particularly in arid regions, suggesting that this presumption is not always valid. To evaluate the influence of atmospheric CO2 on soil δ13Cc, here we performed systematic δ13C analyses of paleosols across the Chinese Loess Plateau, with the sample ages spanning three intervals: the Holocene, the Late Pleistocene, and the mid-Pliocene warm period. Our paired δ13Cc and δ13CSOM data reveal broadly divergent trending patterns. Using a two-component CO2-mixing model, we show substantial incorporations of atmospheric CO2 (up to 60%) into soil pore space during carbonate precipitation. This result readily explains the enrichment of δ13Cc and its divergence from δ13CSOM. As a consequence, δ13C of pedogenic carbonates formed under semiarid and/or arid conditions are largely driven by regional aridity through its control on soil CO2 composition, and thus cannot be used to evaluate the relative abundance of C3 versus C4 plants. Nonetheless, these carbonates can be applied for atmospheric CO2 reconstructions, even for periods with low CO2 levels.

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