Extended Data Fig. 3: Model constraints on ²³⁴U ingrowth history of PRR39222.

The inset photograph shows sample PRR39222 under visible light with the location of δ²³⁴U measurements marked. The main plot shows the measured δ²³⁴U for PRR39222 at three horizons, revealing an increasing δ²³⁴U from top to bottom (Extended Data Table 1). Because of the high thorium (Th) contents, we cannot define a formation age and thus cannot identify a reliable δ²³⁴U_i. The purple curves in Fig. 3 represent the possible δ²³⁴U_i values for the top (higher δ²³⁴U) and bottom (lower δ²³⁴U) for any formation time. What we do not know is the absolute time at which this sample formed or the duration it formed over. However, we do know that: (1) the δ²³⁴U_i for the top and bottom of the sample must lie on these purple lines; (2) the sample must be younger than 1,500 kyr given that the measured δ²³⁴U is not in secular equilibrium. In addition to these known conditions, we can assume that the calcite in PRR39222 probably formed very rapidly as indicated by: (1) morphology, specifically radiating clusters of blade-like sparite; (2) lack of unconformities; (3) shared δ¹⁸O and δ¹³C composition with rapidly forming calcite from PRR50489, which is constrained by geochronology. Data from the literature as well as the geochronologic constraints presented in Extended Data Table 1 provide limits on the rate of sub-ice calcite formation (0.5 mm kyr⁻¹ is shallow and 5 mm kyr⁻¹ is steep). Given a known sample dimension of 4.5 cm, any assumed precipitation rate translates to a time duration for sample formation of 10–90 kyr. Assuming these durations, along with the requirement that the bottom and top of the sample intersects the purple curves in Fig. 3, permits us to define possible δ²³⁴U_i ingrowth histories (black arrows in Fig. 3). The rate of modelled ²³⁴U accumulation as recorded by PRR39222 is strongly controlled by assumed formation age with only a narrow time range yielding ²³⁴U ingrowth histories consistent with the other Wilkes Basin fluid histories. For example, if the sample were to have formed at 1,000 ka, we predict a change in δ²³⁴U_i of about 300% from the top to the bottom of this sample. Such rapid ingrowth histories result in δ²³⁴U compositions that would result in δ²³⁴U compositions that far exceeds anything observed in Antarctica (>6,000‰). If, however, the sample were to have formed at about 400 ka, the projected ingrowth histories would match both model projections and measured data for the Wilkes Basin. Only scenarios that place PRR39222 formation at roughly <500 ka yield projected ingrowth histories consistent with the blue curve. In addition to the above analysis, the occurrence of low δ²³⁴U (<500‰) in subglacial fluids is apparently rare, having been identified in this region only in samples older than about 300 ka. Collectively, this suggests that the ²³⁴U ingrowth history recorded by PRR39222 is at least consistent with formation at about 400 ka.