摘要 : Measurements of boron isotopes (delta B-11) in marine carbonates are and important tool for reconstructing past ocean acidity and atmospheric carbon dioxide levels, but are challenging to obtain due to low boron concentrations and... 展开 Measurements of boron isotopes (delta B-11) in marine carbonates are and important tool for reconstructing past ocean acidity and atmospheric carbon dioxide levels, but are challenging to obtain due to low boron concentrations and the presence of only two stable boron isotopes. Previous studies have observed differences in absolute delta B-11 values of marine carbonates measured via negative thermal ionization mass spectrometry (N-TIMS) and multicollector inductively coupled mass spectrometry (MC-ICP-MS) that are relatively large compared to magnitudes of delta B-11 variability in paleoceanographic studies. Here we investigate the cause of these delta B-11 offsets and implications for paleo-pH reconstructions by performing N-TIMS and MC-ICP-MS measurements within the same laboratory on a suite of eighteen homogenized samples, including inorganic calcites grown under controlled pH conditions and Quaternary planktic foraminifera. Results show that delta B-11 values between both measurement techniques are highly correlated (R-2 > 0.99), but calcite delta B-11 values are similar to 0.5 to 2.7 parts per thousand lower when measured by MC-ICP-MS compared to N-TIMS. The delta B-11 offset between techniques cannot be attributed to procedural blank contribution, but the magnitude of offset moderately correlates with indicators of sample chemical composition, including B/Ca, Mg/Ca and delta O-18 (R-2 = 0.39 to 0.50). N-TIMS delta B-11 does not change with calcium addition to a biogenic carbonate sample, suggesting that varying calcium abundance between samples and standards is not the reason for N-TIMS "matrix effects". Correlation between delta O-18 and the delta B-11 offset is particularly apparent for calcites (R-2 = 0.71); the origin of this relationship is not clear, but is not likely due to O-17 interference. Although identifying the origin of measurement offsets requires further study, we demonstrate that such offsets do not affect the validity of delta B-11 data: The sensitivity of delta B-11 in inorganic calcite to pH is the same when measured by either technique, and paleo-pH reconstructed from delta B-11 measurements in planktic foraminifera is equivalent within uncertainty when a technique-specific constant offset is applied. Our results strongly indicate that delta B-11 measurements from both measurement techniques (N-TIMS and MC-ICP-MS) can be employed for paleo-pH reconstructions with equal confidence. (C) 2016 Elsevier B.V. All rights reserved. 收起