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\section{Methods}  (i) Sample \subsection{Sample  preparation and mass spectrometry. spectrometry}  Samples of vein calcite were prepared for clumped isotope analysis by either gently crushing small crystal fragments in an agate pestle and mortar, or by drilling using a dental bit in a Dremel drill and avoiding undue use of pressure so as to minimise frictional heating of the sample. Analyte CO2 is produced by reacting 6-8mg of each sample with 102% 102\%  ortho-phosphoric acid in vacuo at 25° for a period of 12 hours. The evolved CO2 is then dried, collected by cryo-distillation into a calibrated volume manometer to check reaction yields and then stripped of potential hydrocarbon contaminants before collection in a valved glass gas tube. The drying stage involves freezing the CO2 into a glass spiral trap at liquid nitrogen temperatures before sublimation at -120°C, passing the gas through a second trap at -120°C, and freezing with liquid nitrogen into the manometer. We strip any potential hydrocarbon contaminants from the CO2 by cryo-distillation into a gas tube via a 20cm glass tube packed with porapak Q ion exchange resin held at a temperature of -20°C. The sample gases were analysed for their clumped isotope values, δ45 - δ49 on the UEA MIRA dual-inlet isotope ratio mass spectrometer (Dennis, 2015). Samples were analysed at a major beam (m/z=44) intensity of 7.5×10-8A and data collected for each cardinal mass of the CO2 molecule (m/z = 44 - 49). Each sample measurement consists of 4 acquisitions each of 20 reference-sample gas pairs. Before analysis and between each acquisition the sample and reference gas volumes and signal strengths are balanced to within 1%. Each sample or reference cycle consists of a 10s ‘dead time’ after switching of the changeover valve followed by a 20s integration period. The total measurement time, including sample and reference gas balancing is approximately 90 minutes. The total integration time is 1600s each for the sample and reference gas. Internal precisions (±1σ) for δ45 and δ46 are better than 0.001‰, for δ47 better than 0.008‰, for δ48 better than 0.03‰ and for δ49 better than 10‰.  The reference gas used in MIRA is CO2 produced by reaction of BDH marble chips with 85% ortho-phosphoric acid and subsequently equilibrated with water at 20°C for a period of 1 month. This is to ensure that the Δ47 value of the reference gas is in equilibrium at the laboratory temperature. The nominal composition of the reference gas is: δ13C = 2.007‰VPDB; δ18O = 34.899‰VSMOW, and Δ47 = 0.94URF. To ensure a robust calibration of scale compression and transfer function between the local reference frame for Δ47 and the universal reference frame (URF) both 1000°C heated and 20°C water equilibrated reference gas samples are measured on a daily basis (Dennis et al., 2011). Data quality and long term stability of measured values is monitored by daily measurement of two laboratory standards that bracket the range of Δ47 values for samples in this study: UEACMST (Δ47 = 0.39±0.01, n= ) and UEAHTC (Δ47 = 0.56±0.01, n=).  MIRA is inherently linear with no variation in the calculated Δ47 and Δ48 values of samples as a function of their bulk isotopic composition as represented by the δ47 and δ48 values of samples (Huntington et al., 2010). Not-with-standing this we regularly check for linearity by measurement of 1000°C heated cylinder CO2 (BOC) that is depleted in δ47 with respect to the reference gas by approximately 65‰. We have not recorded any change in the linear behaviour of MIRA over a four year period since commissioning.