Soil cation exchange capacity: main factor of shell carbonate diagenesis

Shell carbonate diagenesis occurs in interaction with soil solution, where the concentration of Ca2+ is in equilibrium with exchangeable Ca2+ and weathering of Ca-bearing minerals. While the exchange process takes place within seconds, the dissolution equilibrium with Ca-bearing minerals achieves after months. We hypothesized that shell carbonate diagenesis proceeds slower in soils with high cation exchange capacity (CEC) than those with low CEC. The goal of this study was to determine the effects of soil CEC and exchangeable cations on shell carbonate diagenesis. Shells of Protothaca staminea were mixed in glass bottles with 1) carbonate-free sand (CEC = 0.37 cmol+kg-1) (S), 2) a native loamy soil (CEC = 16 cmol+kg-1) (LS) and 3) the same loamy soil saturated with KCl (replacing exchangeable cations with K+) (LSK). Samples were incubated at room temperature for 5, 20, 60 and 120 days. Bottles air was labeled with 14CO2 at the beginning and day 55. 14C was measured at sampling dates in bottles air, soil solutions, bulk soils and shells. Dissolved and exchangeable cations were measured. Shell carbonate diagenesis in LS and LSK (0.016 and 0.024 mg CaCO3, respectively) was one order of magnitude lower than in S (0.13 mg CaCO3). Shell carbonate dissolution and consequently recrystallization decreased at high amounts of exchangeable Ca2+ because exchange is faster than dissolution. Therefore, soil CEC and composition of exchangeable cations is a determinant factor of shell carbonate diagenesis and it should be considered by radiocarbon dating. Because shells in soils with lower CEC undergo more intensive diagenesis, they need further pretreatments before dating. Soil CEC should be also included in shell carbonate diagenesis models. Furthermore, 14C labeling can be used to investigate the rates of minerals weathering - at least for Ca-bearing minerals - and soil acidification

Carbon sources in fruit carbonate of buglossoides arvensis and consequences for¹⁴C dating

© 2017 by the Arizona Board of Regents on behalf of the University of Arizona.Fruit carbonate of Buglossoides arvensis (syn. Lithospermum arvense) is a valuable dating and paleoenvironmental proxy for late Quaternary deposits and cultural layers because CaCO3 in fruit is assumed to be accumulated from photosynthetic carbon (C). However, considering the uptake of HCO3- by roots from soil solution, the estimated age could be too old depending on the source of HCO3- allocated in fruit carbonate. Until now, no studies have assessed the contributions of photosynthetic and soil C to the fruit carbonate. To evaluate this, the allocation of photo- assimilated carbon and root uptake of HCO3- was examined by radiocarbon (14C) labeling and tracing. B. arvensis was grown in carbonate- free and carbonate- containing soils (sand and loess, respectively), where14C was provided as (1)14CO2 in the atmosphere (5 times shoot pulse labeling), or (2) Na214CO3 in soil solution (root- labeling; 5 times by injecting labeled solution into the soil) during one month of fruit development. Distinctly different patterns of14C distribution in plant organs after root- and shoot labeling showed the ability of B. arvensis to take up HCO3- from soil solution. The highest14C activity from root labeling was recovered in roots, followed by shoots, fruit organics, and fruit carbonate. In contrast,14C activity after shoot labeling was the highest in shoots, followed by fruit organics, roots and fruit carbonate. Total photo- assimilated C incorporated via shoot labeling in loess- grown plants was 1.51mg lower than in sand, reflecting the presence of dissolved carbonate (i.e. CaCO3) in loess. Loess carbonate dissolution and root- respired CO2 in soil solution are both sources of HCO3- for root uptake. Considering this dilution effect by carbonates, the total incorporated HCO3- comprised 0.15% of C in fruit carbonate after 10 hr of shoot labeling. However, if the incorporated HCO3-during 10 hr of shoot labeling is extrapolated for the whole month of fruit development (i.e. 420- hr photoperiod), fruit carbonate in loess- grown plants incorporated approximately 6.3% more HCO3- than in sand. Therefore, fruit carbonates from plants grown on calcareous soils may yield overestimated14C ages around 500 yr because of a few percentage uptake of HCO3-by roots. However, the age overestimation because of HCO3- uptake becomes insignificant in fruits older than approximately 11,000 yr due to increasing uncertainties in age determination

Cation exchange retards shell carbonate recrystallization: Consequences for dating and paleoenvironmental reconstructions

© 2016 Elsevier B.V.The radiocarbon method has been frequently used to date mollusk shell carbonate. The accuracy of estimated ages, however, depends on the degree and completeness of shell carbonate recrystallization. Although the effect of contamination of the shell CaCO3 with environmental carbon (C) is well known, the role of Ca2+ in diagenetic processes remains unclear. Addition of young C to shells during diagenesis occurs in soil solution, where the Ca2+ concentration is in equilibrium with exchangeable Ca2+ and/or weathering of Ca-bearing minerals. While the exchange process takes place within seconds, the dissolution equilibrium requires longer timescales (on the order of months). It has therefore been hypothesized that the dissolution and recrystallization of shell carbonate in soils with higher cation exchange capacity (CEC) should proceed slower compared to those with low CEC. The objective was to determine the effects of soil CEC and exchangeable cations on shell carbonate recrystallization using the 14C labeling approach. Shell particles of the bivalve Protothaca staminea were mixed with carbonate-free sand (CEC = 0.37 cmol+ kg-1) (Sand), a loamy soil (CEC = 16 cmol+ kg-1) (Loam) or the same loamy soil saturated with KCl, where exchangeable cations were replaced with K+ (Exchanged). The high-sensitivity 14C labeling/tracing approach was used to determine carbonate recrystallization rates. Shell carbonate recrystallization after 120 days in Loam and Exchanged (0.016 and 0.024 mg CaCO3, respectively) showed one order of magnitude lower recrystallization than in Sand (0.13 mg CaCO3). A high level of soil exchangeable Ca2+ decreased the solubility of shell carbonate and consequently its recrystallization because the exchange is faster than dissolution. Therefore, soil CEC and cation composition are determinant factors of shell carbonate recrystallization. Shells in soils with low CEC may undergo more intensive recrystallization; hence they may need further pretreatments before the dating procedure

Cation exchange retards shell carbonate recrystallization: Consequences for dating and paleoenvironmental reconstructions

© 2016 Elsevier B.V.The radiocarbon method has been frequently used to date mollusk shell carbonate. The accuracy of estimated ages, however, depends on the degree and completeness of shell carbonate recrystallization. Although the effect of contamination of the shell CaCO3 with environmental carbon (C) is well known, the role of Ca2+ in diagenetic processes remains unclear. Addition of young C to shells during diagenesis occurs in soil solution, where the Ca2+ concentration is in equilibrium with exchangeable Ca2+ and/or weathering of Ca-bearing minerals. While the exchange process takes place within seconds, the dissolution equilibrium requires longer timescales (on the order of months). It has therefore been hypothesized that the dissolution and recrystallization of shell carbonate in soils with higher cation exchange capacity (CEC) should proceed slower compared to those with low CEC. The objective was to determine the effects of soil CEC and exchangeable cations on shell carbonate recrystallization using the 14C labeling approach. Shell particles of the bivalve Protothaca staminea were mixed with carbonate-free sand (CEC = 0.37 cmol+ kg-1) (Sand), a loamy soil (CEC = 16 cmol+ kg-1) (Loam) or the same loamy soil saturated with KCl, where exchangeable cations were replaced with K+ (Exchanged). The high-sensitivity 14C labeling/tracing approach was used to determine carbonate recrystallization rates. Shell carbonate recrystallization after 120 days in Loam and Exchanged (0.016 and 0.024 mg CaCO3, respectively) showed one order of magnitude lower recrystallization than in Sand (0.13 mg CaCO3). A high level of soil exchangeable Ca2+ decreased the solubility of shell carbonate and consequently its recrystallization because the exchange is faster than dissolution. Therefore, soil CEC and cation composition are determinant factors of shell carbonate recrystallization. Shells in soils with low CEC may undergo more intensive recrystallization; hence they may need further pretreatments before the dating procedure

Carbon sources in fruit carbonate of buglossoides arvensis and consequences for¹⁴C dating

© 2017 by the Arizona Board of Regents on behalf of the University of Arizona.Fruit carbonate of Buglossoides arvensis (syn. Lithospermum arvense) is a valuable dating and paleoenvironmental proxy for late Quaternary deposits and cultural layers because CaCO3 in fruit is assumed to be accumulated from photosynthetic carbon (C). However, considering the uptake of HCO3- by roots from soil solution, the estimated age could be too old depending on the source of HCO3- allocated in fruit carbonate. Until now, no studies have assessed the contributions of photosynthetic and soil C to the fruit carbonate. To evaluate this, the allocation of photo- assimilated carbon and root uptake of HCO3- was examined by radiocarbon (14C) labeling and tracing. B. arvensis was grown in carbonate- free and carbonate- containing soils (sand and loess, respectively), where14C was provided as (1)14CO2 in the atmosphere (5 times shoot pulse labeling), or (2) Na214CO3 in soil solution (root- labeling; 5 times by injecting labeled solution into the soil) during one month of fruit development. Distinctly different patterns of14C distribution in plant organs after root- and shoot labeling showed the ability of B. arvensis to take up HCO3- from soil solution. The highest14C activity from root labeling was recovered in roots, followed by shoots, fruit organics, and fruit carbonate. In contrast,14C activity after shoot labeling was the highest in shoots, followed by fruit organics, roots and fruit carbonate. Total photo- assimilated C incorporated via shoot labeling in loess- grown plants was 1.51mg lower than in sand, reflecting the presence of dissolved carbonate (i.e. CaCO3) in loess. Loess carbonate dissolution and root- respired CO2 in soil solution are both sources of HCO3- for root uptake. Considering this dilution effect by carbonates, the total incorporated HCO3- comprised 0.15% of C in fruit carbonate after 10 hr of shoot labeling. However, if the incorporated HCO3-during 10 hr of shoot labeling is extrapolated for the whole month of fruit development (i.e. 420- hr photoperiod), fruit carbonate in loess- grown plants incorporated approximately 6.3% more HCO3- than in sand. Therefore, fruit carbonates from plants grown on calcareous soils may yield overestimated14C ages around 500 yr because of a few percentage uptake of HCO3-by roots. However, the age overestimation because of HCO3- uptake becomes insignificant in fruits older than approximately 11,000 yr due to increasing uncertainties in age determination

Carbon sources in fruit carbonate of buglossoides arvensis and consequences for¹⁴C dating

© 2017 by the Arizona Board of Regents on behalf of the University of Arizona.Fruit carbonate of Buglossoides arvensis (syn. Lithospermum arvense) is a valuable dating and paleoenvironmental proxy for late Quaternary deposits and cultural layers because CaCO3 in fruit is assumed to be accumulated from photosynthetic carbon (C). However, considering the uptake of HCO3- by roots from soil solution, the estimated age could be too old depending on the source of HCO3- allocated in fruit carbonate. Until now, no studies have assessed the contributions of photosynthetic and soil C to the fruit carbonate. To evaluate this, the allocation of photo- assimilated carbon and root uptake of HCO3- was examined by radiocarbon (14C) labeling and tracing. B. arvensis was grown in carbonate- free and carbonate- containing soils (sand and loess, respectively), where14C was provided as (1)14CO2 in the atmosphere (5 times shoot pulse labeling), or (2) Na214CO3 in soil solution (root- labeling; 5 times by injecting labeled solution into the soil) during one month of fruit development. Distinctly different patterns of14C distribution in plant organs after root- and shoot labeling showed the ability of B. arvensis to take up HCO3- from soil solution. The highest14C activity from root labeling was recovered in roots, followed by shoots, fruit organics, and fruit carbonate. In contrast,14C activity after shoot labeling was the highest in shoots, followed by fruit organics, roots and fruit carbonate. Total photo- assimilated C incorporated via shoot labeling in loess- grown plants was 1.51mg lower than in sand, reflecting the presence of dissolved carbonate (i.e. CaCO3) in loess. Loess carbonate dissolution and root- respired CO2 in soil solution are both sources of HCO3- for root uptake. Considering this dilution effect by carbonates, the total incorporated HCO3- comprised 0.15% of C in fruit carbonate after 10 hr of shoot labeling. However, if the incorporated HCO3-during 10 hr of shoot labeling is extrapolated for the whole month of fruit development (i.e. 420- hr photoperiod), fruit carbonate in loess- grown plants incorporated approximately 6.3% more HCO3- than in sand. Therefore, fruit carbonates from plants grown on calcareous soils may yield overestimated14C ages around 500 yr because of a few percentage uptake of HCO3-by roots. However, the age overestimation because of HCO3- uptake becomes insignificant in fruits older than approximately 11,000 yr due to increasing uncertainties in age determination

Cation exchange retards shell carbonate recrystallization: Consequences for dating and paleoenvironmental reconstructions

© 2016 Elsevier B.V.The radiocarbon method has been frequently used to date mollusk shell carbonate. The accuracy of estimated ages, however, depends on the degree and completeness of shell carbonate recrystallization. Although the effect of contamination of the shell CaCO3 with environmental carbon (C) is well known, the role of Ca2+ in diagenetic processes remains unclear. Addition of young C to shells during diagenesis occurs in soil solution, where the Ca2+ concentration is in equilibrium with exchangeable Ca2+ and/or weathering of Ca-bearing minerals. While the exchange process takes place within seconds, the dissolution equilibrium requires longer timescales (on the order of months). It has therefore been hypothesized that the dissolution and recrystallization of shell carbonate in soils with higher cation exchange capacity (CEC) should proceed slower compared to those with low CEC. The objective was to determine the effects of soil CEC and exchangeable cations on shell carbonate recrystallization using the 14C labeling approach. Shell particles of the bivalve Protothaca staminea were mixed with carbonate-free sand (CEC = 0.37 cmol+ kg-1) (Sand), a loamy soil (CEC = 16 cmol+ kg-1) (Loam) or the same loamy soil saturated with KCl, where exchangeable cations were replaced with K+ (Exchanged). The high-sensitivity 14C labeling/tracing approach was used to determine carbonate recrystallization rates. Shell carbonate recrystallization after 120 days in Loam and Exchanged (0.016 and 0.024 mg CaCO3, respectively) showed one order of magnitude lower recrystallization than in Sand (0.13 mg CaCO3). A high level of soil exchangeable Ca2+ decreased the solubility of shell carbonate and consequently its recrystallization because the exchange is faster than dissolution. Therefore, soil CEC and cation composition are determinant factors of shell carbonate recrystallization. Shells in soils with low CEC may undergo more intensive recrystallization; hence they may need further pretreatments before the dating procedure

Carbon sources in fruit carbonate of buglossoides arvensis and consequences for¹⁴C dating

© 2017 by the Arizona Board of Regents on behalf of the University of Arizona.Fruit carbonate of Buglossoides arvensis (syn. Lithospermum arvense) is a valuable dating and paleoenvironmental proxy for late Quaternary deposits and cultural layers because CaCO3 in fruit is assumed to be accumulated from photosynthetic carbon (C). However, considering the uptake of HCO3- by roots from soil solution, the estimated age could be too old depending on the source of HCO3- allocated in fruit carbonate. Until now, no studies have assessed the contributions of photosynthetic and soil C to the fruit carbonate. To evaluate this, the allocation of photo- assimilated carbon and root uptake of HCO3- was examined by radiocarbon (14C) labeling and tracing. B. arvensis was grown in carbonate- free and carbonate- containing soils (sand and loess, respectively), where14C was provided as (1)14CO2 in the atmosphere (5 times shoot pulse labeling), or (2) Na214CO3 in soil solution (root- labeling; 5 times by injecting labeled solution into the soil) during one month of fruit development. Distinctly different patterns of14C distribution in plant organs after root- and shoot labeling showed the ability of B. arvensis to take up HCO3- from soil solution. The highest14C activity from root labeling was recovered in roots, followed by shoots, fruit organics, and fruit carbonate. In contrast,14C activity after shoot labeling was the highest in shoots, followed by fruit organics, roots and fruit carbonate. Total photo- assimilated C incorporated via shoot labeling in loess- grown plants was 1.51mg lower than in sand, reflecting the presence of dissolved carbonate (i.e. CaCO3) in loess. Loess carbonate dissolution and root- respired CO2 in soil solution are both sources of HCO3- for root uptake. Considering this dilution effect by carbonates, the total incorporated HCO3- comprised 0.15% of C in fruit carbonate after 10 hr of shoot labeling. However, if the incorporated HCO3-during 10 hr of shoot labeling is extrapolated for the whole month of fruit development (i.e. 420- hr photoperiod), fruit carbonate in loess- grown plants incorporated approximately 6.3% more HCO3- than in sand. Therefore, fruit carbonates from plants grown on calcareous soils may yield overestimated14C ages around 500 yr because of a few percentage uptake of HCO3-by roots. However, the age overestimation because of HCO3- uptake becomes insignificant in fruits older than approximately 11,000 yr due to increasing uncertainties in age determination