3.4.1. Oxygen Stable Isotope Determination

For oxygen isotopes determination, on average, five *G. bulloides* tests from each sample were transferred into sample vials, crushed, and then dried in an oven at 50 ◦C. The vials were loaded into the carousel and analysed using a Thermo Kiel device attached to a Thermo MAT253 Mass Spectrometer in dual inlet mode at The Godwin Laboratory for Paleoclimate Research, Department of Earth Sciences, University of Cambridge. The preparation system operates automatically, analysing samples in sequence. Then, 100% orthophosphoric acid is dropped onto the evacuated vial and reacts with the calcium carbonate sample. The evolved carbon dioxide is cryogenically dried and then admitted to the dual inlet mass spectrometer for isotopic analysis by comparison with a reference gas. Each run of 30 samples was accompanied by 10 reference carbonates and 2 control samples. The results are reported with reference to the international standard VPDB and the precision is better than ±0.06 per mil for δ <sup>13</sup>C and ±0.08 per mil for δ <sup>18</sup>O.
