A method to measure Kr/N2 ratios in air bubbles trapped in ice cores and its application in reconstructing past mean ocean temperature

Abstract

We describe a new method for precise measurement of Kr/N2 ratios in air bubbles trapped in ice cores and the first reconstruction of atmospheric Kr/N2 during the last glacial maximum (LGM) ∼20,000 years ago. After gravitational correction, the Kr/N2 record in ice cores should represent the atmospheric ratio, which in turn should reflect past ocean temperature change due to the dependence of gas solubility on temperature. The increase in krypton inventory in the glacial ocean due to higher gas solubility in colder water causes a decrease in the atmospheric inventory of krypton. Assuming Kr and N2 inventories in the ocean‐atmosphere system are conserved, we use a mass balance model to estimate a mean ocean temperature change between the LGM and today. We measured Kr/N2 in air bubbles in Greenland (GISP2) ice from the late Holocene and LGM, using the present atmosphere as a standard. The late Holocene δKr/N2 means from two sets of measurements are not different from zero (+0.07 ± 0.30‰ and −0.14 ± 0.93‰), as expected from the relatively constant climate of the last millennium. The mean δKr/N2 in air bubbles from the LGM is −1.34 ± 0.37‰. Using the mass balance model, we estimate that the mean temperature change between the LGM ocean and today's ocean was 2.7 ± 0.6°C. Although this error is large compared to the observed change, this finding is consistent with most previous estimates of LGM deep ocean temperature based on foraminiferal δ18O and sediment pore water δ18O and chlorinity.