Above: Two French scientists sample a vertical snow profile for a snow trench project at Concordia Station, Antarctica. The trench itself was cut with a snow plow, and the walls later cleaned and smoothed with snow shovels. A profile was sampled at least every 6 ft (2 m), with some additional profiles closer together. Some previously finished profiles can be seen as notches farther back along the wall; the irregular gaps between profiles is because the profile sampling order was random and not sequential. (All photos by Pete Akers unless otherwise noted).
Editor's note: Paleoclimate researcher Pete Akers (Institut des Géosciences de l’Environnement or IGE in Grenoble, France) is a participant in the 2019-20 East Antarctic International Ice Sheet Traverse (Project EAIIST). Pete is writing about the project in a special series for Category 6. See his Cat 6 contributor’s page for links to previous entries.
Due to financial, logistical, and time constraints, most snow sampling in Antarctica is done with a single core or pit at a given site. For example, all the revolutionary climate lessons learned from the EPICA Dome C ice core come from snow that originally fell on a patch of the ice sheet less than a square foot in area. However, as mentioned in my previous posts, the ice sheet surface is not uniform and instead is covered in sastrugi and snow drifts.
As the current surface of the ice sheet is steadily buried by future snow, the structure of the old surface will be destroyed as everything gets compressed into solid ice. However, if the chemistry of the snow varied on the original surface structure (for example, if nitrate levels are higher on ridges than the troughs), then the chemistry of the resulting ice may preserve some memory of the old surface. Generally when interpreting ice core data, we assume that the chemistry of these ‘ghost’ surfaces is completely blurred away when the snow is compressed into ice, or that any remnant variations are small enough to just be subtle ‘noise’. But in reality, not much hard data actually exists to support or challenge these assumptions.
The broad climate conclusions from the Dome C ice core, such as the CO2 record and glaciation cycles, are not in question as they have been repeatedly backed up by subsequent deep ice cores across Antarctica. However, all the chemical data records extracted from ice cores also have smaller variations superimposed on the broader climate patterns. It is still unclear if these smaller variations reflect actual important small climate changes, or if they are the signals of the ‘ghost’ surfaces (in which case they should be ignored for climate conclusions), or if they represent some other unexplained ‘noise’ in the record.
To help answer this question, Mathieu Casado, a postdoctoral researcher at the Alfred Wegener Institute in Germany and member of the EAIIST project, proposed a study at Concordia Station. His study was designed to determine how much the geochemical signals preserved in deep ice cores might vary over small distances due to different surface snow deposition patterns. For this project, a 160 ft (50 m) long and 5 ft (1.5 m) deep trench was excavated in the snow surface upwind from Concordia. The trench cut across sastrugi and snow drifts currently on the surface, and the full depth represented roughly 20 years of snow accumulation. Buried below the surface, you can physically feel harder and softer layers that represented old sastrugi or wind packed surfaces that are now buried. Every six feet (2 meters) along the length of the trench, a vertical snow profile would be sampled on the trench wall from top to bottom to develop a stratigraphic history of the snow.
Mathieu specializes in the stable isotopes of water and snow, and several French researchers from the Institut des Géosciences de l’Environnement (IGE) joined his project to add their own specialized expertise in snow physics and chemistry. Due to my recent change in EAIIST scheduling from the first leg of the traverse to the second, I was also able to join the project and personally help sample the trench.
The Dome C snow trench project
For each trench wall profile, we divided the wall into discrete sampling levels roughly every inch (2.5 cm) from the current snow surface down to bottom at the trench floor. At each sampling level, we took multiple samples of the snow for snow density, diameter and surface area of the individual snow grains, water isotopes, and deposited aerosols like nitrate and sulfate. Unlike reading air temperature from a thermometer these different observations don’t give us direct observation of a climate or environment variable. However, we know that the chemical or physical properties of each of these things we sample are related and controlled by climate or environmental variables.
For example, water isotopes can tell you clues about the history of the snow’s origin and evolution, such as the season it fell in and where its parent cloud moisture originated. In contrast, snow density can help identify buried surface structures because higher snow density is associated with wind-packed sastrugi features on the surface. Therefore, finding a layer of higher snow density buried below the surface indicated that sastrugi once existed at that point. All our observations thus serve as ‘proxies’ to actual climate information, and we can reconstruct past conditions after we ‘translate’ the proxy from its chemical or physical observation (like an isotope ratio) into the climate variable that originally controlled the proxy.
The big idea for our trench study was that for a given profile, we would have multiple stratigraphic histories from different proxies that each told a slightly different environment story. We could then see how the histories from the different proxies related or didn’t relate to each other. This type of study is known as a ‘multi-proxy study’, and it is a powerful tool for understanding the past. A climate record based on a single proxy can be very informative, but when you develop a climate record based on evidence that agrees across multiple proxies, you have a much stronger argument and often can reveal much more nuance and details of how the environment changed.
Each profile took two to three hours to completely prepare and sample, and we initially completed 25 profiles along the whole length of the trench. After completing these profiles, additional profiles were taken at 3 ft (1 m) and 1 ft (0.3 m) intervals in sections of the trench that had interesting buried snow structures, for a total of 35 profiles and over 1700 sampling points. This work required over two weeks of all-day work outside in -13°F (-25°C) conditions, but luckily our clothes and work kept us (mostly) warm enough!
This timelapse video covers roughly one hour of our sampling procedure.
Trench sample analysis
My focus was on collecting snow for the aerosol analysis of sulfate and nitrate. These aerosols naturally fall onto the snow surface and are at low concentrations in the snow (usually 20-300 parts per billion). This low concentration meant that large amounts of snow had to be collected from each sample layer in bags in order to get enough aerosols for analysis. After collection in the field, I melted these bags of snow in our Concordia chemistry lab, and each sample’s melted snow was run through a special resin that traps and concentrates anions like sulfate and nitrate.
Eventually, each roughly 1-lb (0.5-kg) bag of snow would be reduced into a 10-milliliter water sample with concentrated anions for shipment back to Grenoble for isotope analysis in our lab. A similar process will be performed for hundreds of similar snow and ice core samples taken on the EAIIST traverse for aerosol analysis.
The snow density measurements along the trench profiles were taken in the field by simply weighing a section of snow of known dimensions, while the snow surface area and grain size data was analyzed directly in the field as well with a device specifically designed to calculate such information in seconds from a sample. The water isotope samples will be shipped back to France and Germany for analysis, where we will analyze the samples with a state-of-the-art system that measures isotopic ratios through differences in how the water (after flash-boiled to vapor) absorbs an infrared laser. Although this is one of the fastest analytical methods for water vapor isotopes, it will still take several months to process all the samples.
All in all, it may be almost a year before all the basic results from the trench study are in, and analysis of the combined data will last for a year or more. Like many things in life, good science takes time but is worth the wait!
Return to EAIIST
After almost a month at Concordia that included some wonderful holiday meals and gatherings, I and the other member of the second leg of EAIIST were ready to join back up with the traverse. On January 3, we flew out to the Megadunes site to meet our traverse for the first time in a month.
This second leg would entail the return to Concordia and was planned to take about three weeks, split between ten days of travel and 15-20 days stopped for science. The bulk of this time stopped for science was a seven to ten day stint to core a 200 m ice core at a site dubbed Paleo2.
My next post will cover the science done on this final part of the EAIIST traverse as we slowly made our way back to Concordia and began wrapping up the main mission of EAIIST.