The 4He/U-Th dating method can be used to estimate the residence time of pore waters in low-permeable rocks and consolidated sediments, serving as a proxy for sediment deposition time. This residence time is inferred from the accumulation time of radiogenic 4He measured in the pore water being produced by the local decay of U and Th in the sediment matrix. We applied the 4He/U-Th method to date the pore waters of unconsolidated sediments from a glacial overdeepening in the Swiss Plateau (northern Alpine Foreland), where prior studies suggested sediments older than Marine Isotope Stage (MIS) 6 (191–130 ka). We show that compact and fine-grained (glaci)lacustrine sediments provide low-permeability conditions that allow 4He to accumulate in the pore water and be preserved in the pore space. The 4He/U-Th dating indicates that the sediments between 40 m and 140 m are 606 ± 122 ka. The dated infill was deposited in a glacial overdeepening eroded by a foreland glaciation larger than that of the Last Glacial Maximum (LGM). The results reveal extensive foreland glaciations and intense glacial overdeepening erosion during the early part of the Chibanian (i.e., Middle Pleistocene). This work highlights the potential of the 4He/U-Th method for dating sediments in similarly favorable hydrogeological settings.

In the Jurassic Opalinus Clay (Switzerland and southern Germany), the pore water composition still represents remnants of the marine formation water that was entrapped during sediment deposition and that was altered only by diffusive processes (Gimmi et al., 2007; Mazurek et al., 2011). The partial preservation of original Jurassic pore water signatures suggests a similar preservation may occur in younger dense glacial deposits. Indeed, low-permeable glacial tills from the last glaciation in Canada showed high radiogenic 4He (Herad) concentrations (Hendry et al., 2005). Dating based on the 4He/U-Th method has been used in the past to determine the residence time of pore waters in such glacial tills (Wassenaar and Hendry, 2000; Hendry et al., 2005; Hendry and Wassenaar, 2011). The 4He/U-Th method determines the time necessary for 4He produced by the α-decay of U and Th present in the sediment matrix (i.e., Herad) to accumulate in the pore water (Torgersen and Clarke, 1985). For a more precise and reliable 4He age estimation, processes such as fluid transport within the pore space (Strassmann et al., 2005), the He flux from deeper geochemical reservoirs (Tomonaga et al., 2020), or the interaction with confining aquifers (Torgersen and Clarke, 1985) may be considered.

If the 4He production rates are known or can be constrained by geological information, 4He accumulation times (or 4He ages) can be calculated from Herad. In low-permeability sediments, where transport processes barely separate Herad from the pore water, such accumulation times can be used to approximate the pore water residence times. For the above-mentioned Canadian glacial tills, the 4He ages (15–25 ka; Hendry et al., 2005) were in the range of the 14C ages (25–31 k.y. B.P.; Wassenaar and Hendry, 2000). Thus, pore water residence times were interpreted as the deposition time of the hosting sediments, allowing the timing of glaciations to be evaluated. These observations, along with studies on lacustrine sediments (Tomonaga et al., 2014, 2015), suggest that low-permeable sediments can preserve old pore waters. In these cases, the pore water ages can be interpreted as the minimum age of such sediments.

The 4He/U-Th method has the potential to date pore waters in sediments in the time range of hundreds of thousands to millions of years (i.e., an age range for which reliable dating methods for fluvial and glacial sediments are rare). Therefore, the method covers the expected age range of the sediments targeted by the Quaternary drilling program managed by the Swiss National Cooperative for the Disposal of Radioactive Waste (Nagra) to gain insights on the depositional history of the sediments covering the northern Alpine Foreland over past glacial periods. Indeed, the 4He/U-Th method can cover multiple glacial-interglacial cycles and may be a powerful tool to determine the timing and extent of extensive Pleistocene glaciations. Suitable sedimentary sequences with glacial and lacustrine deposits are preserved in glacial overdeepenings that were formed by deep-reaching subglacial erosion below foreland glaciers (Preusser et al., 2010; Cook and Swift, 2012). During glacier retreat, these overdeepened basins acted as primary sediment sinks and were rapidly infilled with subglacial to glacilacustrine, and eventually lacustrine, deposits. Dating such infills thus provides an estimate of the minimum age for many overdeepened basins. In contrast to more surficial sedimentary archives and landforms, sediments in overdeepenings are generally more complete and better preserved during subsequent glaciations (Buechi et al., 2018; Anselmetti et al., 2022).

Here we report the results of 4He/U-Th dating of sediments from the Nagra Quaternary Hochfelden-Strassberg drilling site (278-m-long core QHST) in Switzerland (Amschwand et al., 2020).

The Hochfelden-Strassberg drill site (8.507°E, 47.529°N) is in the lower Glatt Valley north of Zurich, Switzerland (Fig. 1). The region was repeatedly covered by foreland glaciers during the Pleistocene. The exact timing and extent of Chibanian glaciations is, however, debated (Graf, 2009; Preusser et al., 2011). A study of the regional Quaternary erosion and infill cycles (Buechi et al., 2018) indicates that the formation and infilling of the overdeepened trough east of the drill site occurred mainly during Marine Isotope Stage (MIS) 6 (191–130 ka). Buechi et al. (2018) hypothesized that the western Strass­berg Trough was eroded and infilled during a previous extensive foreland glaciation, which likely reached the Hochrhein area during the Chibanian. The sedimentary infill of this trough recovered by the Hochfelden-Strassberg drilling (Fig. 2) consists of three basin successions (units B, C–D, and E–F) possibly representing three ice-contact and retreat sequences emplaced during individual glacial cycles, or oscillation of the glacier. The infill is capped by fluvial deposits (unit A) and younger tills related to the last and the penultimate glacial cycles.

4He Age

As a first-order estimation, the 4He age (t in yr) can be interpreted as the water residence time and can be calculated as

with the radiogenic He concentration (Herad) in cm3STP/g (STP—standard temperature and pressure), the in situ 4He production rate (P) in cm3STP/g/yr, the porosity (φ), the water density (δH2O) in g/cm3, and the sediment density (δSED) in g/cm3. P = 0.2355 × 10−12 × U × [1 + 0.123 × (Th/U − 4)], where U and Th are the U and Th concentrations in ppm, respectively (Craig and Lupton, 1976; Zartman et al., 1961).

Noble Gases

Conventional methods (CM) were used to analyze noble gases by static mass spectrometry in a groundwater (GW) sample (CMGW; Beyerle et al., 2000) and in a pore water (PW) sample (CMPW; Tomonaga et al., 2011; Table 1).

The common procedure to acquire pore water samples (squeezing of bulk sediments into copper tubes; e.g., Brennwald et al., 2003; Tomonaga et al., 2011, 2014) failed in all but one case due to compact and/or coarse sediments. For these sediments, we developed a novel technique (the Headspace Method, HM) based on gas partitioning equilibrium between atmospheric air and the pore water in sediment chunks sealed in stainless-steel vessels. After reaching equilibrium, the noble-gas concentrations were determined using a portable mass spectrometer (HMPMS; Table 1; Brennwald et al., 2016; Tomonaga et al., 2019). Similar approaches have been shown to be successful in enabling noble gas analysis of solid rock samples (Rübel et al., 2002; Rufer et al., 2018). For selected samples, the results of our new method were compared to those from static mass spectrometry measurements (HMSMS; Table 1). For details see the Supplemental Material1.

U and Th

We analyzed sediment aliquots for their U and Th concentrations. Before the determination by γ-spectrometry, the samples were freeze-dried and finely ground. Vials containing ~10 g of sediments were put in a γ-spectrometer and their activity was measured for several days based on the emissions of the 232Th and 226Ra decay chains, the latter assuming secular equilibrium between U and Ra.

Noble Gases

We collected one groundwater sample and 28 sediment samples from the Nagra QHST borehole. The sediment sample at 122.75 m was acquired and processed using the conventional method. All other sediment samples were taken using the Headspace Method. The results are summarized in Table 1 and Figure 2.

The uppermost part of the sediment column (Fig. 2, unit A; down to 40 m), where the groundwater sample was collected, shows low Herad. The respective sediments are coarse (i.e., fluvial gravel and gravelly glaciogenic diamicts) and most likely allow the rapid transport of recently infiltrated groundwater.

The sediments between 40 m and 140 m show relatively high Herad. The matrix is fine-grained or includes fine-grained interbeds (Fig. 2, units B–D; possibly corresponding to the laminated silt and clay lithofacies of Buechi et al., 2018, with glaciogenic diamicts and tills between 120 m and 140 m). These sediments are very compact and thus are expected to have low permeabilities. Therefore, they have the potential to accumulate and preserve (in the pore water phase) the locally produced Herad.

Immediately below 140 m (Fig. 2, unit E) the Herad values are lower with respect to those in the overlying sediments. Such low Herad indicates the presence of younger pore water (i.e., groundwater with a shorter residence time, which limits Herad accumulation). Indeed, the sediments between 140 m and 200 m consist of glacilacustrine to lacustrine sand and silt, and those below 200 m consist of sandy and coarse-grained diamicts and gravels. These sediments are expected to be more permeable than the overlying fine-grained sediments.

Below 140 m, the Herad tends to increase with increasing depth, indicating emission of Herad from the bedrock (Ballentine and Burnard, 2002). The observed concentration gradient (Fig. 2, unit F, red arrow) suggests that the vertical transport of Herad is mainly of a diffusive nature, with a few isolated higher Herad values suggesting some degree of, for example, lateral heterogeneity in the transport regime. We hypothesize that young groundwater recharge and circulation through the strata below 140 m limits the vertical migration of upward-diffusing Herad from the bedrock, eliminating input into the low-permeability interval above. Such Herad removal by lateral groundwater flow provides a well-constrained boundary condition for the He transport in the pore space, and eliminates issues related to the migration of Herad from deeper strata into the overlying sediments that would affect the integrity of 4He/U-Th ages.

He Entrapment

While the concentration gradient in unit F indicates diffusive vertical transport, the similar sandy diamicts between 40 m and 80 m (Fig. 2, unit B) are characterized by almost uniformly high Herad. This suggests that no significant transport process is affecting the solutes in the pore space (similarly to the glacilacustrine to lacustrine fines in unit C). The only remarkable difference between the diamicts of units B and F is the presence of silty to clayey interbeds in unit B. Thus, we hypothesize that the retention of Herad in units B and C is controlled by the presence of fine-grained sediment layers.

Previous studies identified various mechanisms that effectively suppress diffusive exchange in unconsolidated sediments: mineral formation, mineral realignment, or the existence of “dead” pores (pores not connected to the main pore space through which diffusive transport occurs on macroscopic scales; Brenn­wald et al., 2013; Tomonaga et al., 2015). The sediments that show high Herad are extremely compacted, implying that the site was overrun by several glaciations. The coexistence of high Herad and strong compaction lets us speculate that the high load exercised by foreland glaciers modified the pore space geometry to such an extent that diffusive transport became negligible (i.e., by mineral re-alignment).

U and Th

The U and Th concentrations are rather homogeneous throughout the sediment column, with averages of ~2 ppm and ~5 ppm, respectively (Table 1; Fig. 2). The consistent U/Th ratios suggest a common origin. The two lowermost samples show slightly higher U and Th concentrations and lower U/Th ratios, suggesting the inclusion of reworked bedrock material (USM [Lower Freshwater Molasse]: U ~2.5 ppm, Th ~8.0 ppm; Pearson et al., 1991).

4He/U-Th Dating

The 4He/U-Th ages calculated using the Herad, U, and Th concentrations are listed in Table 1. The younger groundwater (almost free of Herad) in the aquifer below 140 m hampers the upward migration of 4He from greater depths. Thus, the Herad in the overlying sediments is most likely the result of in situ 4He production under nearly closed-system conditions. The difference between the 4He concentrations in the aquifer allowing relatively fast groundwater renewal and the Herad above 140 m can be interpreted as pure in situ 4He production (Fig. 2, yellow arrow). The lowest Herad determined by static mass spectrometry (Table 1, sample at 162.9 m) sets the representative aquifer concentration. This concentration was subtracted from Herad to determine the pore water ages. As the Herad values below 140 m are clearly affected by the emission of Herad from the bedrock, their conversion into ages is not meaningful and will not be discussed further.

Despite the rather large 4He/U-Th age uncertainties, the ages between 40 m and 140 m are significantly higher than a few hundred thousand years indicating that the time for 4He to accumulate in the pore space and, thus, the time since sediment deposition cover several glacial and interglacial cycles.

As our samples are numerous and spatially equally distributed, the interpretation of trends in the Herad profile provides crucial information on the processes affecting the fluid transport in the pore space that would not be assessable by single measurements only. The nearly constant Herad between 40 m and 140 m (Fig. 2) indicate that Herad is the result of in situ production and accumulation within the confining sediment layer. Averaging the derived ages helps to reduce the overall uncertainties, assuming that the sediment layers over which the ages are averaged were deposited during a time span that is negligibly small compared to the average age. This is supported by sedimentological evidence: The respective sediments belong to deposits emplaced during deglaciation of the valley with high sedimentation rates (Buechi et al., 2018). The average age of the uppermost seven pore water samples with consistently homogeneous Herad is 606 ± 122 ka.

We show the feasibility of dating Quaternary sediments from the northern Alpine Foreland based on the 4He/U-Th method. Such dating remains challenging, both in terms of analytical methods and of a conceptual frame to assess environmental conditions that allow meaningful and robust pore water dating.

The inferred average age of 606 ± 122 ka is, to our knowledge, the oldest radiometric dating of a Quaternary overdeepened glacial valley fill, and provides a minimum age for its erosion. The formation must have occurred during an extensive glaciation when the Alpine glaciers from the Rhine-Linth system reached ~60 km north of the northern Alpine front. Considering the dating uncertainty, this glaciation likely occurred during MIS 16 (676–621 ka), MIS 14 (ca. 563–524 ka), or MIS 12 (478–424 ka). A very extensive, or even the most extensive, glaciation in the region has been dated to MIS 12 and perhaps MIS 14 (Preusser et al., 2021; Dieleman et al., 2022). However, considering the minimum age character of our estimations, a formation prior to MIS 12 must be considered. MIS 16 was a major worldwide event with substantial ice volumes (Ehlers and Gibbard, 2007) and seems therefore also eligible.

Independent of the exact MIS attribution, our results document the formation of major distal overdeepenings during the first 100 ka cycle glaciations in the northern Alpine Foreland (after the Mid-Pleistocene Transition; Berends et al., 2021). This period coincides with an increase in erosion and relief in the main Alpine outlet valleys such as the Rhone and Aare valleys (Haeuselmann et al., 2007; Valla et al., 2011). Also, south of the Alps, the style and severity of glaciation increased with the onset of the Middle Pleistocene (Muttoni et al. 2003). For the distal Alpine Foreland, the morphological transition from an elevated, lower-relief landscape of the Deckenschotter Group to more incised valleys was tentatively associated with this period. Our results suggest that the formation of major foreland overdeepenings is part of this morphoclimatic transition.

In contrast to more surficial archives, overdeepenings and their infilling may be spared from erosion by later glaciations. The study location was reached by at least two other extensive foreland glaciers documented by ice-contact deposits. These glaciations did not, however, re-excavate the Strassberg Trough. Thus, the sedimentological archive in the Alpine Foreland might be richer and older than previously expected.

1Supplemental Material. Methods and further geological information. Please visit https://doi.org/10.1130/GEOL.S.27173118 to access the supplemental material; contact [email protected] with any questions.

We thank K. Benison (editor), D. Rufer, A. Seltzer, and an anonymous reviewer for their comments.