Utilizing Salt Accumulation and Organic Carbon as Proxies for Exposure Age Dating in Beacon Valley, Antarctica
ABSTRACT
The McMurdo Dry Valleys (MDV) of Antarctica contain some of the oldest exposed surfaces on Earth, making them an incredibly important setting for testing methods of reconstructing past climate and glacial history. This study examines Anion accumulation and organic carbon (OC) preservation as tools for estimating surface exposure age in Mullins Valley, a sub-valley of Beacon Valley. Soils were collected from four moraines in Mullins Valley (12 ka, 200 ka, 600 ka, and 1.2 Ma) and tested for anion accumulation and OC in assessing their potential as proxies for surface exposure age and environmental history. Soluble anions (chloride, nitrate, sulfate) derived from atmospheric salt deposition were extracted from particles under 63 microns and measured using Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES), while OC was quantified through loss-on-ignition. Chloride concentrations increased significantly with age (p < 0.05), supporting it as an exposure-age proxy in Antarctic settings. Sulfate and nitrate displayed broadly similar age-related increases, but variability across sites reduced statistical significance, suggesting that local preservation and redistribution processes may skew long-term trends. Organic carbon patterns were less consistent and more variable. While high levels at the youngest site align with recent biological input, unexpectedly high concentrations at the oldest site suggested long-term preservation or possible reintroduction of carbon under unusually stable conditions, challenging the expectations that organic carbon steadily declines with time. These findings reinforce chloride’s utility for dating Antarctic surfaces while also supporting that nitrate, sulfate, and OC preservation are additionally shaped by environmental factors, not just age.
INTRODUCTION.
Earth has undergone various climate cycles over thousands of years, shaped by both internal and external forces. In Antarctica, the behavior of glaciers has been influenced by major geological events like the breakup of Pangaea and the movement of tectonic plates, but more significantly by long-term climate change [1-2]. One key driver of these changes is the variation in the amount of solar energy reaching the region, which is primarily affected by the Earth’s orbital cycles. Orbital changes, including eccentricity and obliquity, influence solar energy distribution throughout the world. [3-4].
Past studies support that atmospheric salt buildup can reliably indicate exposure age, especially in cold and dry places like the McMurdo Dry Valleys (MDV) [5]. Because this area is very dry with very little precipitation, salt deposited by aerosols stays on the surface and builds up rather than being washed away. When leaching is minimal, higher salt amounts usually indicate longer exposure times. This method’s reliability is supported by research comparing salt levels with ages found using cosmogenic nuclide dating [5]. Those studies showed a strong link between salt concentration and exposure age. While cosmogenic dating is very precise and trusted, it can be expensive and hard to access. Salt-based dating, on the other hand, could provide a practical and fairly accurate alternative for estimating surface ages in Antarctic soils.
This study focuses on the McMurdo Dry Valleys (MDV), the largest ice-free area in Antarctica. Beacon Valley (BV), located within this region, is unusual. While most dry valleys are mainly influenced by eccentricity-driven climate cycles, Beacon Valley seems to respond more to obliquity-driven changes [4]. This distinction is particularly intriguing, as geological features such as moraines visibly reflect these periodic changes throughout cycles. Notably, moraines in Beacon Valley dated at approximately 1.2 million (1.2 Ma, Oldest site), 600,000, 400,000, 200,000, and 12,000 years (12 Ka, Youngest site) appear to decrease in size with age, suggesting a possible reduction in obliquity cycles, although the full meaning is still unclear in Antarctic climate studies.
In addition to salts, Antarctic soils also contain biological input, also known as organic carbon (OC). Research shows that OC tends to break down and decay over time, so older surfaces generally have lesser amounts than younger ones [6]. This is likely because younger surfaces have had more recent biological activity and less time for organic matter to degrade. In this study, OC levels were measured in the samples to see if this pattern holds true in Beacon Valley.
Determining exposure-age in Antarctic soils helps reconstruct past glacial behavior and long-term climate variability. These insights improve models predicting sheet response to global warming, with implications for global sea level rise beyond Antarctica [2].
MATERIALS AND METHODS.
Sample Sites.
Dr. Morgan and his field team collected sediment samples from four different moraines in Mullins Valley, consisting of glacial till deposited from Mullins Glacier. The estimated surface ages of these sites range from 1.2 million, 600,000, 200,000, and 12,000 years (Moraines arranged chronologically from youngest (12 Ka) to oldest (1.2 Ma). At each of these sites, the field team dug two pits, collecting samples from a section near the top (removing top aeolian layer), and from a deeper part of the pit. After collection, samples were later transported by ship to Vanderbilt University in April 2025.
Lab Prep.
29 samples were used from Beacon Valley for this study. All samples were first oven dried on a low heat setting (drying times ranged from 24-72 hours depending on moisture content) for 24-72hrs. After completely drying, samples were sieved <2mm prior to further processing, as the focus of the study is more concentrated in smaller grain sizes.
Salt Extractions.
To extract soluble salts, each sample was sieved to keep particles smaller than 63 µm, since salts tend to collect in finer sediment. One gram of this fine material was placed into a centrifuge tube, and then 40 mL of Milli-Q water was added. The tubes were sonicated for two hours to help dissolve the salts, and then they were centrifuged for 20 minutes to separate the solid parts from the liquid. The liquid was extracted using a syringe and divided into two different tubes, each containing 10 mL of solution. One tube was sealed using Parafilm. The other tube was treated with 0.145 mL of nitric acid for stabilization before being sealed too [5]. All of the prepared samples were transported to Featheringill Hall at Vanderbilt University, where they underwent Inductively Coupled Plasma Optical Emission Spectroscopy (ICP-OES) analysis.
Organic Carbon (OC).
Organic carbon was measured using a gravimetric loss-on-ignition method [7]. Before filling, crucibles were heated in the oven to remove any moisture that might have been absorbed from the lab environment. Crucibles (porcelain cups) were first weighed empty, then filled with 45 to 50 grams of glacial till and weighed again. (The weight of the sample was determined by subtracting the crucible’s weight from the total.) After adding the till, the crucibles were placed back in the oven and heated at 500°C for six hours to burn off any organic carbon. Once cooled, they were weighed again. The difference between the weight before and after heating was used to calculate the total amount of organic carbon that had burned off. All samples were tested again to confirm accuracy, and there were no significant differences found between the initial and repeated runs.
RESULTS.
Sulfate Concentrations
Sulfate levels (Fig. 1) were generally higher at older sites. The 1236 ka moraine had the highest median value. But there was a slight variation noted in the mid-aged moraines, especially at 636 ka where one unusually high outlier was recorded. However, the age of the moraines did not have a significant impact on the number of sulfates in the sample (p = 0.7).
Nitrate Concentrations.
Nitrate (Fig. 2) stayed relatively low across the three younger sites (12.5, 225, and 636 ka). But at 1236 ka, both the median and overall spread increased. One sample was especially high and skewed the range a bit. However, the age of the moraines did not have a significant impact on nitrate concentrations (p= 0.14).
Chloride Concentrations.
Chloride concentrations (Fig. 3) were found to increase with age. The highest levels were found at the 1236 ka site. ANOVA and Tukey HSD tests (p < 0.05) show this site was significantly different from the others. There was a significant relationship between chloride and age. Specifically, the oldest site, which was found to be statistically different from younger sites, reported higher salt concentrations.
Organic Carbon Content.
Organic carbon content Fig. 4) was highest at the youngest (12.5 ka) and oldest (1236 ka) sites. The ANOVA and Tukey HSD tests (p < 0.05) showed a significant difference between the oldest site to the younger sites, having had a significantly higher organic carbon [6][8].
DISCUSSION.
Salt Concentrations.
Sulfate concentrations (Fig. 1) showed a general increase with age, with the oldest moraine containing the highest values. However, statistical tests found no significant differences among sites, suggesting that while age does play a role, local environmental factors such as leaching, wind redistribution, or preservation differences may obscure clear age-related trends.
Nitrate (Fig. 2) followed a similar pattern, remaining low at younger sites, but increasing at the oldest moraine. Although not statistically significant, this trend may reflect long-term accumulation under extremely arid and cold conditions. The delayed buildup of nitrate could indicate that its signal only becomes apparent over very long exposure periods, hence the lack of statistical significance [6].
Chloride (Fig. 3) displayed the clearest relationship with age, with significantly higher concentrations at the oldest moraine compared to younger sites. This supports chloride as the strongest proxy among soluble anions for dating Antarctic soils, consistent with its expected stability and relatively linear buildup in such dry environments. Chloride increased with moraine age and may be a useful dating proxy in older, hyper-arid settings. More intermediate-age samples are needed to confirm a consistent trend.
Organic Carbon Preservation.
Organic carbon (Fig. 4) patterns were less predictable. High concentrations at both the youngest and oldest sites challenge the assumption of steady decline with age. The elevated OC at the youngest moraine likely reflects recent biological input with minimal degradation, while the persistence of high OC at the oldest site may suggest unusual preservation under stable conditions or reintroduction through environmental processes.
Limitations.
Limitations of this study include relatively limited sample sizes, variability in site-specific conditions, and the possibility of external influences such as wind-driven redistribution or favorable conditions for preservation. These factors may mask or complicate otherwise clear signals but do not undermine the broader patterns observed. Despite these limitations, the findings support the use of chloride as a dependable exposure-age indicator, while also pointing to the potential of other soluble anions and OC as tools for understanding Antarctic environmental history.
CONCLUSION.
Overall, the results show a clear relationship between chloride concentration and age. The oldest site, 1236 ka, had significantly higher levels than the others. This supports chloride as an effective proxy for exposure age when dating Antarctic soils. Sulfate and nitrate didn’t show statistically significant differences, though their trends visually paralleled chloride’s pattern, with increases at the oldest moraine. This suggests that their trends still reflect long-term buildup, but the signals might be overshadowed by variability or environmental factors.
Organic carbon gave a less expected result. High concentrations appeared both at the youngest and oldest sites, which challenges the common idea that organic carbon steadily declines with time. These findings point to possible increased preservation under unusually stable conditions, potential reintroduction of organic matter, or effects from changes in climate, especially those related to obliquity cycles.
Improving surface exposure dating strengthens climate forecasts, as Antarctica ice-sheet stability plays a key role in global sea level and ocean circulation patterns affecting not just polar regions, but mid-latitude regions as well.
Although a respective portion of the data aligned with expectations, some anomalies were observed that suggest external factors may be influencing the results. These irregularities hint at processes affecting organic carbon preservation or salt accumulation beyond age alone. To better understand these effects, we plan to analyze grain size, which will provide further insight into particle size and shape [9], as well as examine other elemental compositions. This additional data may help explain the unexpected patterns seen in sulfate, nitrate, and organic carbon, offering a clearer picture of the environmental history and processes at these sites.
ACKNOWLEDGMENTS.
This project was funded by the National Science Foundation. Thank you to the Department of Earth and Environmental Sciences and the Department of Chemistry at Vanderbilt University. Thank you to the School for Science Math at Vanderbilt. Thank you to Dr. Daniel Morgan, Sophie Lopez, Lauren Lamson, and Dr. Rebekah Stanton.
REFERENCES
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Posted by buchanle on Thursday, May 14, 2026 in May 2026.
Tags: Antarctica, Exposure-Age Dating, McMurdo Dry Valleys, Polar Sciences