A depth transect of sediment cores is a powerful tool to overcome the limitation of an individual sea-level index point13. This approach assumes that two cores with different paleo-water depths at the same age may be explained by a single sea-level curve plus age uncertainty. Uncertainty in paleo-water depth may be constrained by evaluating the consistency between deep- and shallow-water indices (Supplementary Fig. S5). If a shallower sea-level point cannot be explained, water-depth uncertainty should be revised. We applied this method to our sea-level curve and re-evaluated previous work (Methods and Supplementary Fig. 4b,c). The depth transect indicates that lagoonal/estuarine and intertidal facies occur within +15 m water depth using points from KH11-1-GC03 (point X in Fig. 4b) and GC06 (point Y in Fig. 4b) at 20.5 cal kyr BP. This range is consistent with the present water depth of the estuarine environment in the Bonaparte Gulf (Supplementary Fig. S6). Here, a water-depth uncertainty of lagoonal/estuarine and intertidal facies of +15 m encompasses contemporaneous sea-level indices.

Figure 4 Illustration of sea-level change with observations and predictions for the Bonaparte Gulf and other MIS 2 sea-level records. Dashed lines correspond to ice volume equivalent sea level, and black lines to predictions at elastic lithosphere of 70 km, upper mantle viscosity of 4 × 1020 Pa s, and lower mantle viscosity of 5 × 1022 Pa s. (a) RSL data in the Bonaparte Gulf based on the depth-transect approach. The isostatic differences among cores are corrected to the sea-level equivalent value at KH11-1-GC06. Red symbols indicate data from this study. Whites correspond to data from previous work13,14. The light blue bar is the duration of sea-level minimum. The dark blue bar is the duration of sea-level plateau. The depth transect indicates that marginal marine facies13,14 occur within +25 m water depth and shallow marine13,14 facies occur over +25 m water depth. (b) Reconstruction limited 22 to 18 cal kyr BP. Symbols same as panel (a). The depth transect indicates that lagoonal/estuarine and intertidal facies occur within +15 m water depth using points from KH11-1-GC03 (point X) and GC06 (point Y) at ca. 20.5 cal kyr BP. (c) Reconstruction limited 12 to 18 cal kyr BP. The point α cannot be explained by a single sea-level curve deduce from point β if paleo-water depth uncertainty is +5 m, suggesting that the uncertainty of the marginal marine environment (inverted triangles) should be revised to +25 m. The brackish environment is observed at ca. 13 cal kyr BP (point γ)13,14. However, this core was not reevaluated in ref. 14. Other sea-level data from corals6 and sediments8 show the sea-level position above −80 m, suggesting that the paleo-water depth of this brackish facies is underestimated. (d) RSL reconstruction from the Huon Peninsula10,11,12. (e) RSL reconstruction from the Sunda Shelf8,9. Gray shades show radiocarbon ages of acid-insoluble and leachable organic matter in the same horizon. Uncertainties of age derive from the effect of old carbon30. (f) RSL reconstruction from Barbados6,7. Full size image

RSL can be deduced from the ages and depths of significant facies changes in this transect. The age-depth model of K11-1-GC10 indicates that sea level was above −110 m from 25.9 to 23.6 cal kyr BP (Fig. 4; modeled median age probability), which is consistent with KH11-1-GC11 indicating sea level above −110 m at 25.7 cal kyr BP. Little lithological (massively bedded clay matrix) and geochemical variation suggest only a minor variation of sea level at this time (Figs 2 and 4, Supplementary Fig. S2). While previous work26 reports a single datum at ca. 22 cal kyr BP with −94 m sea level, this may reflect reworking due to strong bottom currents during sea-level highstands. The age-depth model of KH11-1-GC03 shows that sea level was above −114 m from 21.3 to 20.4 cal kyr BP (Fig. 4b), which is consistent with the lagoonal/estuarine facies observed in KH11-1-GC06. The lagoonal/intertidal facies in KH11-1-GC03, GC10 and, GC11 indicate a sea-level plateau from 25.9 to 20.4 cal kyr BP, which is shallower than previously reported MIS 2 sea level27 (Supplementary Fig. S7). RSL above −117 m during 20.8 to 19.7 cal kyr BP is indicated by GC07 and consistent with sea level reconstructed from KH11-1-GC03, GC06, GC08 and GC09 (Fig. 4b). Previous work in the Bonaparte Gulf13,14 reports a brackish environment at −120 ± 2 m sea level during 19.8 to 19.1 cal kyr BP in core RS176-GC5 (Fig. 1). This agrees with the end of lagoonal/estuarine facies in KH11-1-GC07 (2σ age range). A rapid sea level rise13,14 indicates that the LGM sea-level minimum terminated at 19.1 cal kyr BP, corresponding to the demise of lagoonal/estuarine facies in KH11-1-GC06. Lagoonal/estuarine facies in KH11-1-GC06 and GC07, combined with the brackish environment in RS176-GC513,14, support the interpretation that the LGM sea-level minimum in the Bonaparte Gulf occurred from 19.7 to 19.1 cal kyr BP (modeled median probability). Sea level then rose to above −98 m by ca. 17 cal kyr BP as indicated by the presence of an intertidal facies in KH11-1-GC13 (Fig. 4a,c). The intertidal facies observed in KH11-1-GC14 and GC19 further indicate a rapid sea-level rise at ca. 14 cal kyr BP, corresponding to Meltwater Pulse 1A interval28.

Our constraints on paleo-tidal amplitude indicate that tidal amplitude is reduced below −70 m sea level due to a shift of an amphidromic area (i.e., where tidal amplitudes take minimum values) and the changes in the basin configuration of the Bonaparte Gulf (Fig. 3). The Bonaparte Gulf was largely isolated from the open ocean due to lowered sea level with restricted connections between exposed carbonate platforms16,17 (Fig. 1). The location of submerged platforms could also be identified in the −70 m model results because they roughly coincide with a band of K1 amplitude minimum (Fig. 3) or an offshore limit of M2 amplitudes lower than 0.5 m (Fig. 3). The semi-enclosed environment during sea-level lowstands below −90 m could enhance deposition due to the decreased tidal influences in the Bonaparte Gulf and lack of bottom current scouring, suggesting that the environment is suitable to reconstruct RSL change during the exposure of carbonate platforms and terraces.

The 500-km width of the Bonaparte Gulf continental shelf causes a sea-level gradient across this shelf to appear due to a local difference in isostatic effect13 and bathymetry (Supplementary Fig. S8), which can be calculated using a GIA model. The facies-based observational RSL records described above can then be corrected to sea-level equivalent values at KH11-1-GC06 (Fig. 4) to consider the offset of GIA predictions among sites29,30. This global ice volume history model consistently explains observational RSL, indicating that a sea-level plateau occurred from 25.9 to 20.4 cal kyr BP, prior to the LGM (19.7 to 19.1 cal kyr BP; Fig. 4, Supplementary Fig. S8).

Considering the calculated regional isostatic effect, the Bonaparte Gulf RSL can be compared with other RSL records. Observational RSL records during MIS 2 and 3 using uplifted terraces in the Huon Peninsula10,11,12 are consistent with predicted RSL of this study’s ice model (Fig. 4d), supporting the previous-suggested uplift rate of the Huon Peninsula. A MIS-2 RSL record was previously created through sedimentary environmental reconstruction and radiocarbon dating of near-shore cores on the Sunda Shelf8,9 (Fig. 4e). Because the radiocarbon dating was performed on organic matter with leachable and insoluble components, it exhibits ca. 1,000 years difference within the same horizon, likely due to the effect of old carbon31. The Sunda Shelf record is also consistent with our global ice volume history within age uncertainties derived from the effect of old carbon31. Additionally, MIS 2 RSL has been reconstructed through precise U-series dating of Barbados corals (Fig. 4f)6,7. However, inconsistent results prior to 19 cal kyr BP suggest differential uplift rates between sites, perhaps caused by faulting32. There is also large uncertainty in the growth positions (20–50 m) of particular species (Montastrea annularis, P. asteroids, and Diploria)7.

Rapid ice-sheet growth of approximately 10 m ice-volume equivalent sea-level between the sea-level plateau prior to the LGM and the time of minimum RSL is observed in our RSL records and the GIA modeling (Fig. 4a,b). This would primarily be ice in the Northern hemisphere because the maximum volume added to the Antarctic ice sheet during the LGM is reported to be less than 10 m33. The extremely short duration of the global ice volume maximum, revealed by our results, has implications for the usage of LGM boundary conditions for modeling MIS 2 climate since continental ice sheets likely never reached isostatic equilibrium. This could lead to overestimating the maximum volume of each continental ice sheet, which in total may be closer to that inferred from far-field sea-level records34. However, additional MIS 2 records are needed to more fully explore these implications. In particular, near-field sites with highly-resolved age dating are required to understand the discrepancy between reconstructed global ice volume and the total estimated from individual ice-sheets by near-field dataset34. We conclude that a sea-level plateau occurred from 25.9 to 20.4 cal kyr BP with a subsequent 10 m sea-level fall in ~1,000 years, suggesting that continental ice sheets during MIS 2 were less stable than previously believed27 (Supplementary Fig. S7). Future work reconstructing paleoclimate during glacial periods should consider the instability in sea level and ice volume revealed by this research.