Antarctic carbonate crusts date to periods of high southern summer sunlight

Why it matters

Geologic records of past surface melting in Antarctica are few. These crusts can be dated at many places and heights, so they offer a new way to test what drives melt. The authors suggest sunlight on darker surfaces near rock outcrops can cause melt even when air stays below freezing. They also say this may matter for reading ice surface histories from cosmogenic isotopes.

What they did

The team picked fourteen carbonate crusts from the Polar Rock Repository, from eight sites between the Amundsen Sea and the Ross Sea sector, at 365m to 2000m elevation by the authors’ site range. They measured carbon, oxygen and strontium isotopes to find the water and solute sources. They dated each crust with uranium-thorium methods, correcting for thorium picked up from dust. They compared the ages with ice core temperature records and with solar forcing, and used a simple surface energy balance model.

Key findings

• Carbon and oxygen isotopes are high and rise together, which fits evaporation of meltwater from local snow or ice, not freezing.
• Strontium isotopes follow the host rock type, so the dissolved salts come from weathering of local rock.
• Eleven of fourteen crusts are younger than 10ka and cluster between 2ka and 6ka. Three East Antarctic crusts at high elevation are older, with the oldest at 99.5ka.
• All crusts formed when peak southern summer insolation exceeded 540 W/m2, within age uncertainty. This covers three of the five most recent precession highs.
• A simple energy balance shows melt needs a surface albedo of about 0.35-0.45, which fits snow and ice mixed with rock debris.

Limitations

• Only three crusts are older than the Holocene, so the link to insolation rests on a small number of dates.
• The data are described as consistent with insolation among other possibilities. Other drivers may act on shorter timescales, and atmospheric rivers cannot be fully ruled out at every site.
• The energy balance model is simple and assumes a fixed surface temperature of 273.15K. Some age uncertainties are very large, up to 107.7%.

Glossary

• Nunatak: A rock peak or outcrop that sticks up through an ice sheet.
• U-Th dating: A method that dates carbonate from the decay of uranium into thorium.
• Southern Hemisphere summer insolation (SHSI): Sunlight reaching the southern high latitudes in summer, here the 21 December daily mean at 75°S.
• Albedo: The share of incoming sunlight a surface reflects.