Deglacial Rapid Inundation and Land-Loss of Sundaland

A research by Dhani Irwanto, 9 October 2025

Abstract

This study quantifies the rate and magnitude of continental inundation across the Sundaland region, the now-submerged subcontinent connecting mainland Southeast Asia with the islands of Sumatra, Java, Borneo, and their adjacent shelves. Using the GEBCO 2025 bathymetric grid integrated with a Relative Sea Level (RSL) curve previously developed for the Indo-Pacific Warm Pool, temporal changes in exposed land area were reconstructed from 22.5 ka BP (Last Glacial Maximum) to the present. Calculations indicate a total land loss of ≈ 2.53 million km², with mean inundation rates of ≈ 112 km² yr⁻¹ and peak rates exceeding 1,260 km² yr⁻¹ during the major meltwater pulses between 16–11 ka BP. The inundation pattern was spatially heterogeneous: gradual transgression over the western Sumatra Shelf contrasted with abrupt inundation along the Java Sea–Karimata Strait–Gulf of Thailand–South China Sea corridor, where shallow basins and sills controlled rapid inundation. These results define the first high-resolution, time-continuous estimate of Sundaland’s deglacial transgression history, providing a quantitative baseline for interpreting past sea-level dynamics, ecological transitions, and human dispersal pathways in maritime Southeast Asia.

Keywords: Sundaland, deglaciation, sea-level rise, land loss rate, continental shelf inundation, Indo-Pacific Warm Pool, Holocene transgression, meltwater pulse, paleogeography, human migration.

Land-loss at 15.0, 11.6, 8.0 and 6.0 ka BP

1. Introduction

At the Last Glacial Maximum (LGM, ~22–21 ka BP), sea levels stood more than 120 m below present, exposing a vast continental shelf—Sundaland—that connected Indochina with the western Indonesian islands. This subcontinent formed a continuous tropical landmass hosting lowland rainforests, river networks, and coastal plains that supported Pleistocene megafauna and early human populations.

Understanding the tempo and magnitude of Sundaland’s inundation is crucial for reconstructing paleo-environments and migration corridors that later became submerged beneath the Java and South China seas. Previous studies have described qualitative patterns of sea-level rise, but few have attempted quantitative, time-resolved area–loss estimation using consistent bathymetric datasets. This study addresses that gap by calculating the progressive reduction of Sundaland’s exposed area and corresponding inundation rates from 22.5 ka BP to the present.

2. Data and Methods

2.1 Data Sources

1. Bathymetry: Global 15-arc-second GEBCO 2025 grid (WGS 84 reference).
2. Sea-level data: A Refined Relative Sea-Level Curve for Sundaland (Irwanto, 2025), harmonized with global reconstructions (Lambeck et al., 2014; Siddall et al., 2003).
3. Spatial extent: The Sundaland domain bounded northward by 19.9378° N, encompassing the Sunda Shelf, Java Sea, and South China Sea margins.

2.2 Analytical Procedure

1. The maximum (22.5 ka BP) and minimum (0 ka BP) land extents were generated from GEBCO 2025 grid.
2. Intermediate shoreline positions were interpolated along the RSL curve, producing a series of sea-level stands and corresponding exposed-area estimates. Islands with less than 500 km2 area were ignored.
3. The inundation rate (R) was derived as: R = (At – Δt – At)/Δt, where A is land area (km²) and t is time (yr BP).
4. Results were compiled into a CSV time series for visualization and statistical analysis.

2.3 Limitations

Geomorphic and dynamic processes such as sedimentation, scouring, limestone dissolution, tectonic movement, delta progradation, littoral drift, meandering, and river-regime changes were not incorporated due to sparse and inconsistent regional datasets. The resulting inundation curve thus represents purely hydrostatic transgression—a first-order approximation of areal submergence driven by sea-level rise alone.

3. Results

3.1 Total Land Loss

From 22.5 ka BP to the present, Sundaland’s emergent area decreased from ≈5.38 million km² to ≈2.85 million km², yielding a net loss of ≈2.53 million km² (≈47% of the original landmass).

3.2 Inundation Rates

The mean inundation rate across the full deglacial period (22.5–0 ka BP) was approximately 1.12 × 10⁵ km² kyr⁻¹, with two distinct accelerations associated with global meltwater pulses.

A focused examination of the principal transgressive phase (16.65–6.5 ka BP) reveals a substantially higher mean rate of ≈ 2.41 × 10⁵ km² kyr⁻¹, indicating sustained and regionally extensive submergence of low-lying plains. Within this interval, an extreme subphase (13.15–8.9 ka BP) maintained inundation rates above 0.25 × 10⁶ km² kyr⁻¹, averaging ≈ 3.97 × 10⁵ km² kyr⁻¹, corresponding to the culmination of Meltwater Pulse 1A and 1B.

A pronounced spike at 12.05 ka BP (≈ 1.26 × 10⁶ km² kyr⁻¹) marks the abrupt inundation of a large paleo-lake system across the Gulf of Thailand, producing one of the fastest shelf-transgression events recorded in the sequence. Following this, rates gradually declined toward Holocene stabilization after ~6 ka BP, when sea-level rise largely ceased and the modern shoreline configuration was established.

Figure 1. Sundaland land area vs. time (22.5–0 ka BP)

Figure 2. Inundation rate vs. time (22.5–0 ka BP)

Figure 3. Land-loss at 15.0, 11.6, 8.0 and 6.0 ka BP

4. Discussion

4.1 Deglacial Sea-Level Dynamics

The shape of the Sundaland inundation curve mirrors global deglacial sea-level reconstructions, with distinct rapid-rise intervals associated with the disintegration of the Laurentide and Antarctic ice sheets. The Phase I surge (18–14 ka BP) corresponds to Meltwater Pulse 1A (~14.6 ka BP), when rates reached nearly 1 cm yr⁻¹ globally and >1 × 10⁶ km² kyr⁻¹ regionally. Phase II aligns with the Younger Dryas termination and early Holocene stabilization (~11–8 ka BP). This suggests strong coupling between global eustatic forcing and regional shelf exposure across Southeast Asia.

4.2 Environmental and Biogeographic Implications

The contraction of Sundaland fragmented continuous lowland ecosystems into emergent island cores, catalyzing genetic divergence among flora and fauna and driving the island biogeography patterns observed today. Major paleoriver networks (e.g., Siam, Malacca, North Sunda, and East Sunda rivers) were progressively drowned, reshaping sediment transport and nutrient pathways that sustained early coastal wetlands.

4.3 Cultural and Archaeological Implications

For human populations, rapid shoreline retreat likely compressed habitable zones and forced adaptive migration toward the new coastal margins. These transgressive episodes correspond temporally to pulses of technological and cultural innovation recorded in regional lithic and shell-midden sites. The timing also aligns with hypothesized dispersal corridors of Austroasiatic and proto-Austronesian populations, reinforcing the role of Sunda shelf inundation in shaping maritime Southeast Asian prehistory.

4.4 Regional Differentiation

Western Sundaland (Sumatra Shelf) experienced a relatively gradual and continuous transgression owing to its broad, gently sloping morphology. In contrast, the Java Sea–Karimata Strait–Gulf of Thailand–South China Sea corridor underwent more abrupt inundation, governed by the inundation of structural depressions and bathymetric thresholds that linked a series of shallow basins. These abrupt transitions produced stepwise drowning events and rapid lateral shoreline migration, particularly where narrow sills controlled hydrodynamic exchange between basins.

Local subsidence around Borneo and the Makassar Strait further modulated the timing and pattern of submergence, creating spatial heterogeneity in shelf inundation across the broader Sundaland domain.

5. Conclusion

Sundaland lost approximately 2.5 million km² of land since the Last Glacial Maximum, with two principal pulses of rapid submergence linked to global meltwater events. The mean transgression rate of ≈ 112 km² yr⁻¹ underscores the dynamic nature of post-glacial sea-level rise in the equatorial zone. The transgression pattern, however, was regionally variable—gradual over the western Sumatra Shelf but abrupt along the Java Sea–Karimata Strait–Gulf of Thailand–South China Sea corridor, reflecting the interplay between bathymetric thresholds and structural basins. This continuous inundation record provides a critical geospatial foundation for evaluating environmental shifts, cultural adaptations, and continental-shelf geomorphology throughout the Holocene.

References

1. GEBCO Compilation Group. (2025). GEBCO 2025 Grid — A continuous bathymetric dataset.
2. Hanebuth, T. J. J., Stattegger, K., & Grootes, P. M. (2011). Rapid flooding of the Sunda Shelf: A late-glacial sea-level record. Science, 288, 1033–1035.
3. Lambeck, K., Rouby, H., Purcell, A., Sun, Y., & Sambridge, M. (2014). Sea level and global ice volumes from the Last Glacial Maximum to the Holocene. PNAS, 111(43), 15296–15303.
4. Siddall, M., Rohling, E. J., Almogi-Labin, A., et al. (2003). Sea-level fluctuations during the last glacial cycle. Nature, 423, 853–858.
5. Irwanto, D. (2025). A Refined Relative Sea-Level Curve for Sundaland.