First Dating of an Early Chibanian (Middle Pleistocene) Glacial Overdeepening in the Alpine Foreland Using the 4He/U-Th Method

Academic Background

Glacial overdeepenings are deep valleys formed by glacial erosion, typically filled with glacial sediments. These sediments record the history of glacial activity and are crucial for understanding the expansion and retreat of Pleistocene glaciers. However, due to the lack of reliable dating methods, the ages of these sediments have been difficult to determine accurately. Traditional radiocarbon dating (14C) is only applicable to younger sediments (usually no more than 50,000 years), while other methods such as uranium-series dating (U-Th) have limitations when applied to glacial sediments.

To address this issue, researchers developed the 4He/U-Th dating method. This method estimates the age of sediments by measuring the accumulation of radiogenic helium (4He) in pore water. 4He is produced by the decay of uranium (U) and thorium (Th) in the sediment matrix, and its accumulation time can reflect the deposition time of the sediments. This method is particularly suitable for low-permeability sediments, as they effectively preserve 4He in the pore water.

Source of the Paper

This paper was co-authored by Yama Tomonaga, Marius W. Buechi, Gaudenz Deplazes, and Rolf Kipfer, affiliated with the Swiss Federal Institute of Aquatic Science and Technology (Eawag), the University of Basel, the University of Bern, and the Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra). The paper was accepted on October 4, 2024, and published in the journal Geology with the DOI 10.1130/G52544.1.

Research Process

1. Study Site and Sample Collection

The study site is located at the Hochfelden-Strassberg drilling site (8.507°E, 47.529°N) north of Zurich, Switzerland. This region was repeatedly covered by glaciers during the Pleistocene, forming several glacial overdeepenings. Researchers collected 28 sediment samples and one groundwater sample from a 278-meter-long borehole.

2. Application of the 4He/U-Th Dating Method

The core of the 4He/U-Th dating method is to infer the age of sediments by measuring the concentration of 4He in pore water. The specific steps are as follows: - 4He Concentration Measurement: The concentration of 4He in pore water was measured using static mass spectrometry and portable mass spectrometry. - Uranium and Thorium Concentration Measurement: The concentrations of uranium and thorium in the sediments were determined by γ-spectrometry. - 4He Age Calculation: Based on the 4He concentration, uranium and thorium concentrations, and the porosity and density of the sediments, the accumulation time of 4He, i.e., the age of the sediments, was calculated.

3. Data Processing and Analysis

Researchers conducted detailed data analysis on the collected samples, including: - 4He Concentration in Pore Water: Measurements using static mass spectrometry and portable mass spectrometry showed higher 4He concentrations in sediments between 40 and 140 meters, indicating that these sediments are older. - Uranium and Thorium Concentrations: The concentrations of uranium and thorium in the sediments were relatively uniform, with average values of 2 ppm and 5 ppm, respectively. - 4He Age Calculation: Based on the 4He concentration and uranium and thorium concentrations, the age of sediments between 40 and 140 meters was calculated to be 606 ± 122 ka (thousand years ago).

Main Results

1. Distribution of 4He Concentration

The results showed that the 4He concentration was higher in sediments between 40 and 140 meters, indicating that these sediments are older. In contrast, the 4He concentration was lower in sediments below 140 meters, suggesting that these sediments are younger.

2. Sediment Age

Using the 4He/U-Th dating method, researchers calculated the age of sediments between 40 and 140 meters to be 606 ± 122 ka. This result indicates that these sediments were formed during the early Chibanian (Middle Pleistocene), significantly earlier than the Last Glacial Maximum (LGM).

3. History of Glacial Activity

The results suggest that the Alpine foreland experienced extensive glacial expansion and erosion during the early Chibanian, forming multiple glacial overdeepenings. These overdeepenings were rapidly filled with sediments after the retreat of the glaciers, forming the present-day sediment layers.

Conclusions and Significance

1. Scientific Value

This study successfully applied the 4He/U-Th dating method for the first time to determine the age of glacial overdeepenings in the Alpine foreland, providing a new tool for understanding the history of Pleistocene glacial activity. This method is particularly suitable for low-permeability sediments and can cover a time range of millions of years, filling the gap left by traditional dating methods.

2. Application Value

The 4He/U-Th dating method can be used not only for dating glacial sediments but also for other low-permeability sediments, such as lacustrine and deep-sea sediments. This provides geologists with a powerful tool for studying major events in Earth’s history.

3. Research Highlights

• Innovative Method: The 4He/U-Th dating method is a novel approach, particularly suitable for low-permeability sediments.
• Significant Findings: The results indicate that the Alpine foreland experienced extensive glacial expansion and erosion during the early Chibanian, which is of great significance for understanding the history of Pleistocene glacial activity.
• Broad Application Prospects: This method can be applied to the dating of other low-permeability sediments, offering wide-ranging application prospects.

Other Valuable Information

The study also found that sediments in glacial overdeepenings were well-preserved during subsequent glacial activities, providing a rich sedimentary record for studying the history of Pleistocene glacial activity. Additionally, the study revealed geomorphological changes in the Alpine foreland during the early Chibanian, offering new insights into the geological evolution of the region.

This research not only provides a new method for dating glacial sediments but also offers important scientific evidence for understanding the history of Pleistocene glacial activity.