09:30 〜 09:45
[MTT42-03] High-precision measurement of 17O-excess in phosphate in hydrosphere
キーワード：三酸素同位体異常、溶存無機体リン酸、水圏、生物圏
Since the pioneer work by Tudge (1960), the oxygen isotope ratio (δ18O) of inorganic phosphate (PO43–) has been widely used as a proxy for the sources of dissolved inorganic phosphate (DIP), an essential nutrient in hydrosphere. While phosphate assimilated into biosphere rapidly exchange oxygen atoms with those in H2O through enzymatic reactions, the P–O bonds in phosphate are always stable during the inorganic processes in hydrosphere. Therefore, DIP preserves isotopic signals that primary reflect the environment in which DIP had interacted with biosphere.
The δ18O value of DIP remineralized from organic phosphorus (Org-P), however, is a function of temperature and δ18O of H2O when DIP had been assimilated into biosphere and converted to Org-P. Besides, it also varies depending on the type of Org-P and enzymes which hydrolyzes Org-P into DIP. As a result, it was often difficult to fix δ18O values of remineralized DIP in general and to clarify the sources of DIP in hydrosphere only by using δ18O as a tracer.
Our aim in this study is to quantify the 17O-excess of DIP in hydrosphere (Δ'17O ≈ δ17O – 0.528 × δ18O) as an additional tracer to overcome the problems in using δ18O of DIP. Influences of the isotope fractionation process on 17O-excess should be negligible owing to mass-dependent fractionation between the corresponding δ17O and δ18O values during the progress of the enzymatic reactions. As a result, the 17O-excess in DIP remineralized from Org-P corresponds with that of ambient H2O when DIP had been assimilated into biosphere and converted to Org-P. Therefore, the 17O-excess in DIP can be a robust tracer to clarify the sources of DIP, such as biosphere or the other sources such as igneous rocks, which should be depleted in 17O relative to hydrosphere. This is the first study to report accurate and precise 17O-excess in DIP.
Firstly, we extracted phosphate from the samples of river water (Tempaku river, Aichi prefecture), igneous apatite, and chemical fertilizers and precipitate phosphate as Ag3PO4. The decomposition of Ag3PO4 to extract oxygen atoms as O2 was performed with a fluorination technique using BrF5 at 250℃. Extracted and purified O2 was analyzed using the dual inlet mode of a mass spectrometer with a cup configuration of m/z = 32, 33, and 34. The SD during multiple measurements of in-house Ag3PO4 standards showed 0.3 ‰ for δ17O, 0.6 ‰ for δ18O, and 0.02 ‰ for Δ'17O. The O2 yield relative to the ideal oxygen quantity in the decomposed Ag3PO4 was 119 % ± 7 %, implying that all oxygen in a sample was extracted quantitatively. Additionally, during the measurements on all samples including in-house Ag3PO4 standards, we could not find any significant correlations between the oxygen isotope ratios and the reciprocal of the O2 quantity, which implies that contribution of blank O2 was minimum. Thus, the δ17O and δ18O values were obtained as the mean of multiple measurements in this study.
While the Δ'17O value of DIP under the oxygen isotope exchange equilibrium with ambient river H2O was estimated to be +27 × 10–6, those of the riverine DIP determined in this study ranged from –71 to +3 × 10–6. It is difficult to explain this 17O-depleted DIP through remineralization of Org-P in hydrosphere. Besides to DIP remineralized from biosphere, we should assume some additional sources for the 17O-depleted DIP in this river. Because the Δ'17O value of the 17O-depleted DIP endmember coincided with that of chemical fertilizer (Δ'17O = –94 × 10–6) determined in this study, we concluded that direct input of chemical fertilizer into the DIP pool of this river might be highly responsible for the source of the 17O-depleted DIP. Based on the isotopic mass balance of the Δ'17O values, we estimated that 20–80 % of DIP in this river was derived from the chemical fertilizer.
The δ18O value of DIP remineralized from organic phosphorus (Org-P), however, is a function of temperature and δ18O of H2O when DIP had been assimilated into biosphere and converted to Org-P. Besides, it also varies depending on the type of Org-P and enzymes which hydrolyzes Org-P into DIP. As a result, it was often difficult to fix δ18O values of remineralized DIP in general and to clarify the sources of DIP in hydrosphere only by using δ18O as a tracer.
Our aim in this study is to quantify the 17O-excess of DIP in hydrosphere (Δ'17O ≈ δ17O – 0.528 × δ18O) as an additional tracer to overcome the problems in using δ18O of DIP. Influences of the isotope fractionation process on 17O-excess should be negligible owing to mass-dependent fractionation between the corresponding δ17O and δ18O values during the progress of the enzymatic reactions. As a result, the 17O-excess in DIP remineralized from Org-P corresponds with that of ambient H2O when DIP had been assimilated into biosphere and converted to Org-P. Therefore, the 17O-excess in DIP can be a robust tracer to clarify the sources of DIP, such as biosphere or the other sources such as igneous rocks, which should be depleted in 17O relative to hydrosphere. This is the first study to report accurate and precise 17O-excess in DIP.
Firstly, we extracted phosphate from the samples of river water (Tempaku river, Aichi prefecture), igneous apatite, and chemical fertilizers and precipitate phosphate as Ag3PO4. The decomposition of Ag3PO4 to extract oxygen atoms as O2 was performed with a fluorination technique using BrF5 at 250℃. Extracted and purified O2 was analyzed using the dual inlet mode of a mass spectrometer with a cup configuration of m/z = 32, 33, and 34. The SD during multiple measurements of in-house Ag3PO4 standards showed 0.3 ‰ for δ17O, 0.6 ‰ for δ18O, and 0.02 ‰ for Δ'17O. The O2 yield relative to the ideal oxygen quantity in the decomposed Ag3PO4 was 119 % ± 7 %, implying that all oxygen in a sample was extracted quantitatively. Additionally, during the measurements on all samples including in-house Ag3PO4 standards, we could not find any significant correlations between the oxygen isotope ratios and the reciprocal of the O2 quantity, which implies that contribution of blank O2 was minimum. Thus, the δ17O and δ18O values were obtained as the mean of multiple measurements in this study.
While the Δ'17O value of DIP under the oxygen isotope exchange equilibrium with ambient river H2O was estimated to be +27 × 10–6, those of the riverine DIP determined in this study ranged from –71 to +3 × 10–6. It is difficult to explain this 17O-depleted DIP through remineralization of Org-P in hydrosphere. Besides to DIP remineralized from biosphere, we should assume some additional sources for the 17O-depleted DIP in this river. Because the Δ'17O value of the 17O-depleted DIP endmember coincided with that of chemical fertilizer (Δ'17O = –94 × 10–6) determined in this study, we concluded that direct input of chemical fertilizer into the DIP pool of this river might be highly responsible for the source of the 17O-depleted DIP. Based on the isotopic mass balance of the Δ'17O values, we estimated that 20–80 % of DIP in this river was derived from the chemical fertilizer.