17:15 〜 19:15
[BCG06-P14] Developement of a novel pretreatment method for measuring the triple oxygen isotope compositions of phosphate associated with iron rocks
キーワード：三酸素同位体組成、リン酸、前処理手法、縞状鉄鉱層、古環境
Reconstructing the temperature of the ancient ocean is essential to clarify the scale of biogeochemical cycles and rock–water interactions in the Earth′s surface environment over the geologic time scale. Since the pioneering study by Longinelli and Nuti (1973), the 18O/16O ratio (the δ18O value) of phosphate in fossils of vertebrates, as well as in sedimentary rocks, has been used as a tracer to estimate the temperature of the ancient ocean. Phosphate taken by phytoplankton attains the temperature-dependent oxygen isotope equilibrium with surrounding H2O because oxygen atoms in phosphate rapidly exchange with those in H2O due to the enzymatic activities of the ubiquitous intracellular enzyme pyrophosphatase (PPase). Therefore, the δ18O value of phosphate co-precipitated with sedimentary rocks can be used as an isotope thermometer for estimating the temperature of the ancient ocean, if the original δ18O value of phosphate in sedimentary rocks has been preserved over the geologic time scale. However, in principle, it is difficult to verify whether the δ18O value preserves its original by using only the δ18O value of phosphate as a tracer.
Recent progress in the high-precision analytical techniques on the triple oxygen isotopes of various oxides enables us to use the Δ′17O value [= ln(δ17O+1) – 0.528 × ln(δ18O+1)] as an additional tracer to clarify the path of chemical reaction to produce each oxide. The Δ′17O value basically changes depending on the deviation in the equilibrium fractionation exponent for the triple oxygen isotopes [θEQ (= ln17αEQ/ln18αEQ)] from 0.528 and the degree of change in δ18O during the fractionation processes. In our previous work, we experimentally quantified the temperature-dependent variation of Δ′17O of dissolved inorganic phosphate (DIP) under the oxygen isotope equilibrium with H2O by the enzymatic activities of PPase (Sambuichi et al., under review). Therefore, the Δ′17O value of phosphate can be used as an additional tracer to clarify whether the triple oxygen isotope compositions are under the equilibrium, as well as diagenetic alteration processes that cause disequilibrium, by comparing both δ18O and Δ′17O values of phosphate in sedimentary rocks with those of the equilibrated DIP calculated using the 18αEQ and θEQ between DIP and H2O.
The final goal of this study is to determine the δ18O and Δ′17O values of phosphate associated with banded iron formations (BIF) to accurately estimate the temperature of the Precambrian ocean. Because BIF is a sedimentary rock co-precipitated with DIP in an oxic ocean during the Precambrian eon, those of phosphate in BIF can be used as an isotope thermometer for the Precambrian ocean. As a first step toward achieving this goal, we developed an optimal method for extracting DIP from an extremely low-pH solution in which a rock sample was dissolved. Specifically, zirconium-loaded activated carbon (ZrC) was adopted as the adsorbent for DIP in a low-pH solution. To assess potential changes in δ18O and Δ′17O of phosphate during the pretreatment, we prepared Ag3PO4 converted from phosphate in low-pH solutions with various Fe3+ concentrations. The δ18O and Δ′17O values of phosphate were then analyzed by using the fluorination method presented by Sambuichi et al. (2023).
We found that the δ18O value of Ag3PO4 prepared through the pretreatment was approximately 2‰ lower than that of the original phosphate. Considering the non-quantitative recovery of phosphate through adsorption on and desorption from ZrC (≈ 50%), isotopic fractionation during these processes should be responsible for this slight deviation in δ18O. By contrast, the Δ′17O values were in good agreement between them. As a result, we conclude that the Δ′17O value of phosphate in iron rocks can be quantified accurately without any correction, whereas the correction factor was needed to accurately determine the δ18O value. In the presentation, we will also present the isotopic results of phosphate associated with iron rocks, including BIF.
Recent progress in the high-precision analytical techniques on the triple oxygen isotopes of various oxides enables us to use the Δ′17O value [= ln(δ17O+1) – 0.528 × ln(δ18O+1)] as an additional tracer to clarify the path of chemical reaction to produce each oxide. The Δ′17O value basically changes depending on the deviation in the equilibrium fractionation exponent for the triple oxygen isotopes [θEQ (= ln17αEQ/ln18αEQ)] from 0.528 and the degree of change in δ18O during the fractionation processes. In our previous work, we experimentally quantified the temperature-dependent variation of Δ′17O of dissolved inorganic phosphate (DIP) under the oxygen isotope equilibrium with H2O by the enzymatic activities of PPase (Sambuichi et al., under review). Therefore, the Δ′17O value of phosphate can be used as an additional tracer to clarify whether the triple oxygen isotope compositions are under the equilibrium, as well as diagenetic alteration processes that cause disequilibrium, by comparing both δ18O and Δ′17O values of phosphate in sedimentary rocks with those of the equilibrated DIP calculated using the 18αEQ and θEQ between DIP and H2O.
The final goal of this study is to determine the δ18O and Δ′17O values of phosphate associated with banded iron formations (BIF) to accurately estimate the temperature of the Precambrian ocean. Because BIF is a sedimentary rock co-precipitated with DIP in an oxic ocean during the Precambrian eon, those of phosphate in BIF can be used as an isotope thermometer for the Precambrian ocean. As a first step toward achieving this goal, we developed an optimal method for extracting DIP from an extremely low-pH solution in which a rock sample was dissolved. Specifically, zirconium-loaded activated carbon (ZrC) was adopted as the adsorbent for DIP in a low-pH solution. To assess potential changes in δ18O and Δ′17O of phosphate during the pretreatment, we prepared Ag3PO4 converted from phosphate in low-pH solutions with various Fe3+ concentrations. The δ18O and Δ′17O values of phosphate were then analyzed by using the fluorination method presented by Sambuichi et al. (2023).
We found that the δ18O value of Ag3PO4 prepared through the pretreatment was approximately 2‰ lower than that of the original phosphate. Considering the non-quantitative recovery of phosphate through adsorption on and desorption from ZrC (≈ 50%), isotopic fractionation during these processes should be responsible for this slight deviation in δ18O. By contrast, the Δ′17O values were in good agreement between them. As a result, we conclude that the Δ′17O value of phosphate in iron rocks can be quantified accurately without any correction, whereas the correction factor was needed to accurately determine the δ18O value. In the presentation, we will also present the isotopic results of phosphate associated with iron rocks, including BIF.