Advection and non-climate impacts on the South Pole Ice Core

Abstract

Abstract. The South Pole Ice Core (SPICEcore), which spans the past 54 300 years, was drilled far from an ice divide such that ice recovered at depth originated upstream of the core site. If the climate is different upstream, the climate history recovered from the core will be a combination of the upstream conditions advected to the core site and temporal changes. Here, we evaluate the impact of ice advection on two fundamental records from SPICEcore: accumulation rate and water isotopes. We determined past locations of ice deposition based on GPS measurements of the modern velocity field spanning 100 km upstream, where ice of ∼20 ka age would likely have originated. Beyond 100 km, there are no velocity measurements, but ice likely originates from Titan Dome, an additional 90 km distant. Shallow radar measurements extending 100 km upstream from the core site reveal large (∼20 %) variations in accumulation but no significant trend. Water isotope ratios, measured at 12.5 km intervals for the first 100 km of the flowline, show a decrease with elevation of −0.008 ‰ m−1 for δ18O. Advection adds approximately 1 ‰ for δ18O to the Last Glacial Maximum (LGM)-to-modern change. We also use an existing ensemble of continental ice-sheet model runs to assess the ice-sheet elevation change through time. The magnitude of elevation change is likely small and the sign uncertain. Assuming a lapse rate of 10 ∘C km−1 of elevation, the inference of LGM-to-modern temperature change is ∼1.4 ∘C smaller than if the flow from upstream is not considered.

Reconstruction of Temperature, Accumulation Rate, and Layer Thinning From an Ice Core at South Pole, Using a Statistical Inverse Method

“…, which is both the observed modern surface isotope-temperature relationship at the site (Fudge et al, 2020) and the value commonly used in the literature for Antarctica (e.g., Jouzel et al, 2003), for which Masson-Delmotte et al ( 2008) report a 1 s.d. error of 0.01‰°C −1 .…”

“…Recall that while we used diffusion length determined from the δ 18 O power spectrum in our reconstruction, we do not use the absolute δ 18 O values; hence, these comparisons serve as an independent calibration of the traditional water‐isotope thermometer, similar to what has been done previously with borehole thermometry (Cuffey et al., 1995, 2016) but maintaining higher‐frequency information. The red curve in Figure 7 uses a scaling of = 0.8‰°C −1 , which is both the observed modern surface isotope‐temperature relationship at the site (Fudge et al., 2020) and the value commonly used in the literature for Antarctica (e.g., Jouzel et al., 2003), for which Masson‐Delmotte et al. (2008) report a 1 s.d.…”

“…For ages older than 10.2 ka, there is an insignificant linear trend in the accumulation rate along the 100 km flowline such that Fudge et al. (2020) suggest no long‐term advection correction. Thus we make no correction to the ice‐core reconstruction for ages older than 10.2 ka.…”

“…(2020) measured δ 18 O values using 10‐m firn cores at 12.5 km intervals along the flowline to determine an appropriate correction for δ 18 O. Fudge et al. (2020) also measured 10‐m firn temperatures, and while the results were inconclusive, they were consistent with a typical 10°C km −1 lapse rate (dry adiabatic). Using this information, we apply corrections to the “ice core” reconstructions described above to produce “site” reconstructions of accumulation rate and temperature.…”

Core handling, transportation and processing for the South Pole ice core (SPICEcore) project

“…We estimate the firn bubble close-off depth near the SPC14 site to be in the 120–123 m range based on previous firn studies at the South Pole (Battle and others, 1996; Severinghaus and Battle, 2006). Modern ice velocities at the core site are 10 m a −1 along 40° W (IceCube Collaboration, 2013) and the deepest ice recovered at SPICEcore (1751 m is 54 302 ± 519 BP (years before 1950) (Winski and others, 2019)) likely originates ~150 km away at Titan Dome (Lilien and others, 2018; Fudge and others, 2020). SPICEcore was intentionally not drilled to bedrock due to the drill site location being away from an ice-flow divide and this increases the likelihood that deeper ice is stratigraphically disturbed.…”