In-situ boron isotopic analysis of modern and ancient marine carbonates by LA ICP-MS
Abstract
The boron isotopic composition of marine carbonates has been shown to be useful as a tracer of paleo-pH, and for the reconstruction of ancient pCO2 through time. Conventional wet chemical and TIMS techniques for the separation and measurement of δ 11B are hampered by complicated, tricky chemistry and boron contamination. Based on an in-situ, laser ablation inductively coupled plasma (LA ICP-MS) analysis of boron isotope compositions in MORB glasses (LeRoux et al., 2003), we have modified the technique to allow for the analysis of synthetic and natural carbonates with ±1‰ (2σ ) precision. A synthetic carbonate was prepared using nominally B-free calcite powder which was doped with a 1000 ppm NIST SRM 951 solution and fused in a Pt capsule at 1500oC and > 7 kbar for 10 min in a piston-cylinder apparatus. The resulting synthetic carbonate contained about 30 ppm boron. Analyses were done at the Department of Terrestrial Magnetism, Carnegie Institution of Washington with a CETAC LSX-200 Nd-YAG frequency quadrupled (266 nm) laser connected to a VG Axiom multi-collector magnetic sector ICP-MS. This instrument is equipped with a three electron multiplier detector which allows the simultaneous collection 10B and 11B in pulse counting mode. The technique interspersed repeat measurements of NIST SRM glasses (610 and 612 with independently measured compositions equivalent to SRM 951) with unknowns. The 951-doped synthetic carbonate gave a δ 11B = 0.50 ± 1.08‰ (n = 24) or within analytical uncertainty of pure SRM 951. A modern coral carbonate (Porites sp.) was similarly analyzed and gave a δ 11B value of 26 ± 0.38‰ (n = 3), which corresponds to the ocean pH of ~8.2 as predicted from seawater composition. Future analyses of a variety of coral species from Bermuda and the Florida Keys, collected at depths between 6 and 27 meters will allow the evaluation of species type, growth effects, and water depth on boron isotopic compositions. Ancient coral samples formed at intermediate to low sea levels from the Huon Gulf in the Pacific Ocean and from intermediate and high sea levels from Barbados in the Atlantic Ocean, which have been dated by U series disequilibria techniques to have grown during the last several glacial/interglacial cycles, will be studied to understand changes in oceanic pH associated with orbital cycles and other climatic perturbations.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2004
- Bibcode:
- 2004AGUFMPP51B1339K
- Keywords:
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- 4870 Stable isotopes;
- 4267 Paleoceanography;
- 1045 Low-temperature geochemistry;
- 1094 Instruments and techniques;
- 1635 Oceans (4203)