Recrystallization of pedogenic and biogenic carbonates in soil: environmental controls, modelling and relevance for paleoenvironmental reconstructions
Physical Geography
Final Report Abstract
The reliability of paleoenvironment (paleoclimate, paleovegetation etc.) interpretations and radiocarbon ages based on isotopes of carbon (δ13C, 14 C), oxygen (δ18O) and clumped isotopes (Δ47) in soil carbonates types e.g. mollusk shells, eggshells, fruit carbonates, rhizoliths, nodules, clast coatings etc., depends on the recrystallization degree of these carbonate types after formation and/or embedding in soil. Soil CO2 concentration and its isotopic composition is in equilibrium with CO2 respired by roots and organisms. Dissolution of carbonate types in soil solution and further recrystallization re-equilibrate the δ13C and Δ14C of carbonate types with soil CO2. Hence, the δ13C in recrystallized carbonate will save fingerprints of dominant vegetation during the recrystallization phase and Δ14C will reflect the recrystallization time and most likely δ18O as well Δ47 will record the isotopic composition of oxygen in soil water and the recrystallization temperature, respectively. For example addition of only 5% modern C due to recrystallization to a 45,000-year old bone leads to more than 20,000 years underestimation of the age. Despite the known effects of recrystallization on paleoenvironmental interpretations and radiocarbon dating, the dynamics of this process and its affecting factors remain poorly understood. This is because of low solubility of calcium carbonate and low recrystallization rates 14 which complicate experimentally assessments. Recently, the sensitive technique of 14C labeling has been shown to help understand the recrystallization process. This technique is based on 14CO2 labeling of the soil atmosphere and subsequent tracing 14C activity in a carbonate sample. 14C labeling approach was used to determine the recrystallization of mollusk shells; one of the most common carbonate types in soils, sediment and cultural layers; under various environmental conditions. Recrystallization begins soon after embedding of shells in soil and increases exponentially with time. Shell carbonate recrystallization rates ranged between 1.0 10^-3 and 1.6 10^-2 % day^-1 depending on environmental conditions such as cation exchange capacity (CEC), presence of geogenic carbonates in soil i.e. loess carbonate and degree of shell fossilization i.e. degradation of organic compounds in shell structure. The recrystallization was one order of magnitude higher in soils with relatively low CEC e.g. sandy soils comparing to loamy soils with higher CEC. Presence of 30% geogenic carbonates intensifies shell carbonate recrystallization up to seven times because geogenic carbonate may also recrystallize and accumulates on shells. Recrystallization in fossils was 1.2 times higher than the fresh specimens due to higher porosity in shell structure and so more surface area contacting soil solution. Furthermore, The full recrystallization (i.e. when the whole shell materials would be recrystallized and the original isotopic signals are vanished completely) for shell particles in of 2-2.5 mm in diameter were determined as about 90 to 770 years depending on presence or absence of geogenic carbonates and fossilization stages.
Publications
- 2016. Cation exchange retards shell carbonate recrystallization: consequences for dating and paleoenvironmental reconstructions. Catena 142, 134–138
Zamanian, K., Pustovoytov, K., Kuzyakov, Y.
(See online at https://doi.org/10.1016/j.catena.2016.03.012) - 2016. Pedogenic carbonates: Forms and formation processes. Earth-Sci. Rev. 157, 1–17
Zamanian, K., Pustovoytov, K., Kuzyakov, Y.
(See online at https://doi.org/10.1016/j.earscirev.2016.03.003) - 2016. Recrystallization of shell carbonate in soil: 14C labeling, modeling and relevance for dating and paleoreconstructions. Geoderma 282, 87–95
Zamanian, K., Pustovoytov, K., Kuzyakov, Y.
(See online at https://doi.org/10.1016/j.geoderma.2016.07.013) - 2017. Carbon Sources in Fruit Carbonate of Buglossoides arvensis and Consequences for 14C Dating. Radiocarbon 59, 141–150
Zamanian, K., Pustovoytov, K., Kuzyakov, Y.
(See online at https://doi.org/10.1017/RDC.2016.123) - 2018. Nitrogen fertilization raises CO2 efflux from inorganic carbon: A global assessment. Glob. Change Biol. 24, 2810–2817
Zamanian, K., Zarebanadkouki, M., Kuzyakov, Y.
(See online at https://doi.org/10.1111/gcb.14148) - 2019. Contribution of soil inorganic carbon to atmospheric CO2: More important than previously thought. Glob. Change Biol. 25:e1–e3
Zamanian, K., Kuzyakov, Y.
(See online at https://doi.org/10.1111/gcb.14463)
