Mean annual temperature of New Zealand during the last glacial period derived from dissolved noble gases

Abstract:

Determining the magnitude of warming since the last glacial maximum (LGM) is fundamental to our understanding of glacial-interglacial climate dynamics, but is currently poorly quantified in certain regions of the world. In the southern mid-latitudes, temperature reconstructions based on paleoceanographic and terrestrial proxies vary widely and the mean annual temperature during the LGM of the region remains an ongoing debate.

This study presents a reconstruction of LGM mean annual surface temperature in New Zealand using groundwater noble gas paleothermometry. The concentration of dissolved noble gases in groundwater depends on a dissolved-gas phase equilibrium reached at a distinct temperature, pressure, and salinity. Measurement of dissolved neon, argon, krypton, and xenon concentrations in paleogroundwater enables the reconstruction of mean annual surface temperature at the time of recharge.

Initial results show an average last glacial period (LGP) mean annual surface temperature of about 4°C to 6°C cooler than today. Because the wells sampled have long (often >100 meters) screened intervals allowing for inflow of groundwater across a wide vertical profile, ‘mixing’ between LGP and modern waters influences the noble gas temperatures of LGP paleogroundwater samples. We therefore interpret the lowest temperature measured in this study, ~6°C cooler than today, as our best LGM mean-annual temperature estimate in New Zealand.

This value is larger than that of ~3°C, based on a number of paleoceanographic LGM temperature reconstructions in the vicinity of Australia and New Zealand derived from foraminiferal assemblages. However, this reconstructed mean annual temperature falls within the high end of a range of estimates from alkenones and foraminifera Mg/Ca-based studies, which have suggested LGM temperatures of 4-7°C cooler than present, and agrees closely with glacial-snowline based mean summer temperature estimates of ~6-7°C.