Isotopic analysis of otolith carbonates using laser heating: A fast method for obtaining high-resolution climate...

The stable isotopic compositions of biogenic carbonates like fish otoliths (ear bones) are widely used for palaeoclimatic reconstruction. The conventional method using acid-digestion of micro-milled samples is a multi-step time-consuming process. Here we report a fast method based on laser heating of otolith carbonates to obtain accurate and high-resolution stable isotopic compositions.

Otoliths of catfish from the Gulf of Kutch were analysed to check the precision, accuracy and time-resolution of the isotope ratios. The CO₂ , generated by heating otoliths with a 50 W CO₂ laser, was analysed for its oxygen and carbon isotope ratio [δ¹⁸ O and δ¹³ C, with precision: 0.12 and 0.17‰ (1σ), accuracy: 0.13 and 0.25‰, respectively] using a continuous-flow isotope ratio mass spectrometer. The effect of laser power (0.7-2 W) was assessed for reproducible data. Samples were roasted and analysed to account for the effect of the inherent organic matter on the isotopic values.

Roasting did not alter the δ¹⁸ O of the otoliths but increased the δ¹³ C slightly. High-resolution (125 μm) analysis of the right and left otolith of a fish yielded similar δ¹⁸ O and δ¹³ C values, suggesting the suitability of either of them for deriving the climate signal. An increase in δ¹⁸ O values from ~ -2‰ to ~ -1‰, observed across the ontogeny, is consistent with the known migratory behaviour of the catfish between freshwater and the sea.

The otolith δ¹⁸ O value of an adult fish records the sea surface temperature (with ~3°C uncertainty) on a monthly scale. The otolith δ¹³ C values, with the knowledge of dietary δ¹³ C, provide the mean annual δ¹³ C value of dissolved inorganic carbon. The study provides a rapid method for retrieving high-resolution seasonal climate data from otoliths found aplenty in geological/archaeological records.