Gas isotope thermometry in the South Pole and Dome Fuji ice cores provides evidence for seasonal rectification of ice core gas records

Abstract

Abstract. Gas isotope thermometry using the isotopes of molecular nitrogen and argon has been used extensively to reconstruct past surface temperature change from Greenland ice cores. The gas isotope ratios δ15N and δ40Ar in the ice core are each set by the amount of gravitational and thermal fractionation in the firn. The gravitational component of fractionation is proportional to the firn thickness, and the thermal component is proportional to the temperature difference between the top and bottom of the firn column, which can be related to surface temperature change. Compared to Greenland, Antarctic climate change is typically more gradual and smaller in magnitude, which results in smaller thermal fractionation signals that are harder to detect. This has hampered application of gas isotope thermometry to Antarctic ice cores. Here, we present an analytical method for measuring δ15N and δ40Ar with a precision of 0.002 ‰ per atomic mass unit, a two-fold improvement on previous work. This allows us to reconstruct changes in firn thickness and temperature difference at the South Pole between 30 and 5 kyr BP. We find that variability in firn thickness is controlled in part by changes in snow accumulation rate, which is, in turn, influenced strongly by the along-flowline topography upstream of the ice core site. Variability in our firn temperature difference record cannot be explained by annual-mean processes. We therefore propose that the ice core gas isotopes contain a seasonal bias due to rectification of seasonal signals in the upper firn. The strength of the rectification also appears to be linked to fluctuations in the upstream topography. As further evidence for the existence of rectification, we present new data from the Dome Fuji ice core that are also consistent with a seasonal bias throughout the Holocene. Our findings have important implications for the interpretation of ice core gas records. For example, we show that the effects of upstream topography on ice core records can be significant at flank sites like the South Pole – they are responsible for some of the largest signals in our record. Presumably upstream signals impact other flank-flow ice cores such as EDML, Vostok, and EGRIP similarly. Additionally, future work is required to confirm the existence of seasonal rectification in polar firn, to determine how spatially and temporally widespread rectifier effects are, and to incorporate the relevant physics into firn air models.

A 2000-year temperature reconstruction on the East Antarctic plateau, from argon-nitrogen and water stable isotopes in the Aurora Basin North ice core

“…9b). However, the late Holocene conditions are unlikely to result in a strong rectifier effect at ABN, because this site is located on a slope where there is expected sustained surface winds, and even at South Pole where temperature is on average 7°C colder than ABN does not support a rectifier effect on the Holocene (Morgan et al, 2022).…”

“…In a recent study, Morgan et al (2022) suggest that the gas stable isotopes in the firn could be affected by seasonal rectification: in absence of mixing of air in the surface layer, the winter temperature inversion cools the snow surface and densifies the nearsurface firn air which could sink and advect the air column downward more efficiently than during summer. Winter advection of air down into the firn lowers the 15 Nexcess isotopic signal, which can result in an apparent colder ∆T.…”

“…On the other hand, strong katabatic winds induce a mixing of both the air above the snow surface where temperature inversion is the strongest, and the air in the uppermost layer of the firn, increasing the convection layer. Morgan et al (2022) hypothesize that the change in surface slope and curvature, which affect the strength of katabatic winds (Vihma et al, 2011), may in turn be responsible for some difference in the ∆T derived in the isotopes, along with snow accumulation and the lock-in depth. Surface topography changes are linked to the glacial flow both at South Pole (Morgan et al, 2022) and ABN, two Antarctic sites that are not on an ice divide.…”

“…Morgan et al (2022) hypothesize that the change in surface slope and curvature, which affect the strength of katabatic winds (Vihma et al, 2011), may in turn be responsible for some difference in the ∆T derived in the isotopes, along with snow accumulation and the lock-in depth. Surface topography changes are linked to the glacial flow both at South Pole (Morgan et al, 2022) and ABN, two Antarctic sites that are not on an ice divide.…”

“…can be achieved by inverse modelling of a vertical temperature diffusion model in the firn (Orsi et al, 2014). Inversion of past temperature gradients estimated from gases isotopes has been applied in Greenland (Kobashi et al, 2008;Orsi et al, 2014), and some places in Antarctica (Orsi, 2013;Morgan et al, 2022), but remains rare compared to temperature reconstructions based on water isotopes, given the longer processing time, the volume of ice needed, and the analytical precision required in inert gas analyses. Moreover, the accumulation rate controls the gases bubble enclosure speed, and restricts the locations where this method can be used to infer temperature changes: a too slow accumulation rate lets the time to the gases to diffuse out the temperature signals, while a too fast accumulation rate does not let the time for the gases to equilibrate with the firn, which is why this method is best used at sites within a 60-300 kg m −2 yr −1 accumulation range.…”