MICROANALYSIS OF OXYGEN ISOTOPE RATIOS II: BIOCARBONATES (Invited Presentation)

For 75 years, oxygen isotope ratios (δ¹⁸O) in carbonates have provided critical evidence to understand the thermal, fluid and life history of low-temperature and sedimentary processes including the formation of diagenetic cements (Valley- I, this meeting), coral, foraminifera, mollusks, otoliths, pearls, speleothems, and stromatolites. Such samples are often zoned at micron-scale due to growth or alteration. Conventional analysis of larger samples by acid dissolution and gas-source mass spectrometry (GSMS) can thus result in average compositions, lost information, and misinterpretation.

SIMS (secondary ion mass spectrometry) analyses (1-10 micron spot size) can be correlated with microscopy and other in situ techniques to reveal otherwise unknown correlations and detail. Spot-to-spot δ¹⁸O precision of 0.2‰ (2SD) in 10 micron spots is attainable with care. For some studies, excellent precision is enough to elucidate trends/zoning and answer geologic questions. However, well-matched carbonate reference materials (RMs) are necessary for accurate calibration to VSMOW or VPDB scales. A suite of 30 inorganic Ca-Mg-Fe carbonate mineral RMs at WiscSIMS demonstrates that analytical bias for δ¹⁸O varies predictably with composition by up to 19‰. Correlated analysis of cations by electron microprobe (EPMA) or SIMS allows accurate values of δ¹⁸O ±~0.5‰. These matrix corrections must be calibrated by analysis of a suite of RMs during each SIMS session.

In contrast to inorganic carbonates, biocarbonates can have complex textures and contain additional components not amenable to EPMA including water and organics, making calibration by coarsely crystalline, inorganic RMs uncertain. SIMS analyses are often offset to lower values than GSMS data. Values of Δ¹⁸O(GSMS-SIMS) correlate with OH/O (SIMS) in young biocarbonates (Orland et al. 2015). Many biocarbonates have relatively constant OH/O and can be corrected with a constant offset (Wycech et al. 2018), but samples with variable OH/O (Helser et al. 2018) should be interpreted with caution.

Thus, no single technique is best for all applications. SIMS and GSMS each have specific advantages. Applications such as paleoclimatology, diet, migration, and growth kinetics often benefit from in situ analysis by SIMS. Some studies benefit from combination of both types of analysis.