Tag Archives: Deglaciation

The Sundaland Paleo-River System: Reconstructing the Submerged Drainage Networks of the Last Deglaciation

Book, , , , , , ,

A research by Dhani Irwanto, 13 October 2025

Abstract

This study reconstructs the paleo-drainage systems of the Sunda Shelf—now largely submerged beneath the Java, Karimata, and South China Seas—using high-resolution bathymetric and topographic datasets. Integration of GEBCO 2025 (15 arc-second), SRTM v3 (1 arc-second), and a deglacial inundation model reveals six major paleo-river systems and a large paleo-lake in the Gulf of Thailand. Watershed modeling was performed under consistent geomorphic thresholds (minimum watershed ≥ 1 000 km²; river length ≥ 10 km) after correcting for ship-passage artifacts in GEBCO data. The resulting networks portray an interconnected fluvial landscape that once linked the emergent landmasses of Sumatra, Java, Borneo, and the Malay Peninsula. These reconstructions illuminate the paleohydrological architecture that structured ecological corridors, sediment transport, and early human movement across Late Pleistocene Sundaland.

Keywords: Sundaland, paleo-river, deglaciation, GEBCO 2025, watershed modeling, Gulf of Thailand paleo-lake, Molengraaff River, Pleistocene hydrology

1. Introduction

During the Last Glacial Maximum (LGM) and the ensuing deglaciation, the Sunda Shelf constituted one of the world’s largest emergent plains, uniting the islands of Sumatra, Java, Borneo, and the Malay Peninsula. Rising sea level of more than 120 m since ~21 ka BP progressively drowned this continental platform, fragmenting it into the present Indonesian and Malaysian archipelagos. Reconstructing its paleo-river systems is essential for understanding patterns of freshwater and sediment routing, ecological and biogeographical connectivity prior to isolation, and the response of tropical fluvial systems to rapid post-glacial transgression. Earlier works (Molengraaff 1921; Voris 2000; Sathiamurthy & Voris 2006) outlined generalized drainage maps of Sundaland, but relied on coarse bathymetric data. With recent improvements in digital elevation models, it is now possible to delineate channels and basins at continental scale with greater realism. This paper extends previous reconstructions of relative sea-level change (Irwanto 2025a), sea-surface temperature evolution (Irwanto 2025b), and deglacial inundation rates (Irwanto 2025c) by mapping the paleo-hydrological network that organized the former Sunda landmass.

2. Data and Methods

2.1 Data Sources

To achieve a realistic reconstruction of the paleo-drainage framework across Sundaland, this study integrates the highest-resolution publicly available global terrain datasets. The bathymetric and topographic data were selected for their complementary spatial coverage—underwater and terrestrial—and for compatibility within a uniform geodetic framework. The sources are summarized below.

  1. GEBCO 2025 Bathymetry (15-arc-second grid), representing global ocean depth data compiled from multibeam surveys and satellite altimetry.
  2. SRTM Version 3 (1-arc-second grid), providing high-accuracy land elevations derived from radar interferometry.
  3. Sundaland Deglacial Inundation Dataset (Irwanto 2025), previously produced from sea-level modeling, supplying shoreline reference surfaces for paleo-hydrological interpretation.

All datasets were resampled and mosaicked into a continuous elevation model referenced to WGS 84 geographic coordinates to ensure consistent vertical and horizontal alignment.

2.2 Methodology

The reconstruction of the Sundaland paleo-river systems followed a sequence of geomorphometric and hydrological analyses within a GIS environment. A composite digital elevation model (DEM) was produced by mosaicking and resampling the GEBCO 2025 bathymetry (15 arc-second) and SRTM v3 topography (1 arc-second) into a uniform WGS 84 geographic grid. The surface was hydro-flattened to remove discontinuities along modern coastlines and ensure consistent flow routing across subaerial–submarine interfaces.

2.2.1 Artifact Minimization

Bathymetric trench-like anomalies—known as ship-passage artifacts—were visually identified as linear depressions aligned with survey tracks in the GEBCO grid. These were locally corrected through neighborhood median filtering and manual editing of aberrant grid nodes using bilinear interpolation. The objective was to suppress artificial cross-flow pathways that could distort hydrological connectivity while retaining genuine topographic variability. Complete removal of such artifacts was not feasible without the original multibeam soundings; however, their hydrological influence was minimized to a negligible level.

2.2.2 Hydrological Modeling

Watershed delineation and stream extraction were performed using standard flow-accumulation algorithms in the GIS software. Flow direction was derived from the corrected DEM using the D8 algorithm[1], followed by computation of flow accumulation and stream order. A minimum contributing area threshold of 1 000 km² was imposed for first-order streams, and a minimum channel length of 10 km was adopted to exclude spurious or ephemeral drainages. The resulting stream networks were then vectorized and topologically validated to ensure connectivity and realistic drainage hierarchy.

2.2.3 Integration with Modern Drainage

Modeled paleo-channels were spatially aligned with the outlets of modern rivers to maintain genetic continuity between subaerial and submarine catchments. The procedure involved adjusting terminal flow paths toward existing estuaries and delta fronts, based on hydrological gradients and sediment-transport direction inferred from slope and curvature analyses. This ensured that modeled paleo-drainage systems remained compatible with present river mouths and physiographic boundaries.

2.2.4 Hydrological Synthesis and Visualization

The final drainage mosaics were categorized by basin identity and exported as vector shapefiles for cartographic visualization. Six regional-scale systems were defined through hierarchical clustering of flow accumulation zones, corresponding to the Java Sea, Eastern Java Sea, Karimata Strait, Gulf of Thailand, Mekong Extension, and Strait of Malaka systems. The outputs were compared against regional bathymetric contours and deglacial shoreline reconstructions (Irwanto 2025a; 2025c) to validate drainage coherence under the −122 m sea-level surface (~22.5 ka BP), corresponding to the Last Glacial Maximum (LGM).

2.3 Limitations

The reconstructed paleo-river systems represent a first-order geomorphometric model constrained primarily by topography and bathymetry. Several geomorphic and dynamic processes were not explicitly incorporated because of the scarcity and inconsistency of regional datasets. Consequently, the results should be interpreted as generalized hydrological frameworks rather than exact paleochannel geometries.

  1. Sedimentary dynamics — Processes such as delta progradation, channel avulsion, floodplain aggradation, and littoral drift were not modeled. These factors can substantially modify valley morphology and estuarine geometry through time, especially during late-stage transgression.
  2. Subsurface and tectonic influences — Localized tectonic subsidence, fault reactivation, and differential uplift may have altered drainage gradients and basin shapes after initial channel formation. These effects remain unquantified at the shelf scale.
  3. Karst and dissolutional terrain — In regions underlain by carbonate lithologies (e.g., northern Java, western Borneo, and the Thai–Malay margin), subsurface drainage and sinkhole development may have influenced catchment connectivity in ways not captured by surface-based flow models.
  4. Sediment compaction and isostatic adjustment — Post-depositional subsidence and isostatic rebound following deglaciation were not integrated into the DEM corrections, introducing minor uncertainty in absolute elevation and relative base level.
  5. Bathymetric data quality — Despite artifact minimization, residual ship-passage anomalies in the GEBCO grid may still influence local flow routing. These artifacts—linear trench-like depressions created during data gridding—were reduced but cannot be wholly eliminated without access to original sounding lines. Their impact is minimized at the regional scale but may persist locally.
  6. Temporal simplification — The modeling assumes a quasi-static topography corresponding to a single reference sea-level surface (−122 m RSL, ~22.5 ka BP). Progressive shoreline migration, sediment redistribution, and hydrological reorganization through subsequent millennia are therefore beyond the scope of this reconstruction.

Overall, the uncertainties above are unlikely to alter the broad configuration of the six major drainage systems identified in this study, but they may affect local channel positions and tributary details. Future work integrating seismic stratigraphy, sediment cores, and higher-resolution bathymetry could further refine the paleo-hydrological realism of the Sundaland reconstruction.

3. Results

3.1 Major Paleo-River Systems

The hydrological modeling reveals a coherent network of six principal drainage systems that occupied the Sunda Shelf before Holocene flooding. Each system integrates numerous tributaries draining from the emergent landmasses of Sumatra, Java, Borneo, and the Malay Peninsula. Their relative magnitudes and contributing regions are summarized in Table 1, while their spatial configuration is illustrated in Figure 1.

Figure 1. Reconstructed paleo-river systems and major drainage basins across the Sunda Shelf. Pale-blue shading indicates the extent of deglacial inundation; darker blue marks the paleo-lake in the Gulf of Thailand. Black lines show modeled paleo-rivers, gray lines depict modern rivers. © 2025 Dhani Irwanto.

Table 1. Major Paleo-River Systems of Sundaland

System Principal Source Regions Approx. Watershed Area (km²)
Java Sea Southern Borneo, Northern Java, Southern Sumatra ≈ 570 000
Eastern Java Sea Southern Borneo (Barito, Kapuas-Murung, Kahayan) ≈ 180 000
Karimata Strait (Molengraaff) Eastern Sumatra, Western Borneo ≈ 630 000
Gulf of Thailand Eastern Malay Peninsula, Chao Phraya Basin ≈ 1 020 000
Mekong Extension Lower Mekong and adjacent South China Sea margin ≈ 690 000
Strait of Malaka Eastern Sumatra, Western Malay Peninsula ≈ 260 000

Table 2. Connectivity between Modern Rivers and Paleo-River Systems

Paleo-River System Modern River(s) — Borneo Modern River(s) — Sumatra Modern River(s) — Java Modern River(s) — Malay Peninsula/ Mainland
Java Sea Mendawai, Sampit, Pembuang Tulang Bawang Bengawan Solo, Serang, Cimanuk, Citarum –
Eastern Java Sea Barito, Kapuas-Murung, Kahayan – – –
Karimata Strait (Molengraaff) Kapuas Musi, Batanghari, Indragiri – –
Gulf of Thailand – – – Johor, Rompin, Endau, Kuantan, Kelantan, Tapi, Mae Klong, Chao Phraya, Bang Pakong
Mekong Extension – – – Mekong main stem and tributaries
Strait of Malaka – Kampar, Rokan, Barumun, Belawan – Malacca, Perak

Note: Table 2 illustrates the continuity between present river mouths and modeled paleo-channels, supporting the inference that many modern estuaries originated as terminal segments of these ancient systems.

3.2 Paleo-Lake in the Gulf of Thailand

A closed depression of approximately 93,000 km² with an outlet sill near −55 m relative sea level indicates the existence of a vast paleo-lake in the central Gulf of Thailand. The basin morphology suggests prolonged freshwater retention during the early deglacial stages before eventual overtopping and breaching into the South China Sea. This lacustrine phase is congruent with transitional sedimentary records documented in regional core studies (Horton et al., 2005; Chabangborn et al., 2020; Zhang et al., 2022)[2] that show shifts from freshwater-dominated facies toward more estuarine or marine-influenced depositional environments during rising sea levels.

For example, sediment cores along the western Gulf of Thailand (e.g., CP3, CP4, CP5) record stratigraphic transitions consistent with increasing marine influence around 7.9 ka BP, as evidenced by grain-size ratios, microfossil assemblages, and mangrove pollen influx (Chabangborn et al., 2020; Horton et al., 2005). These findings provide independent support for a broad freshwater-to-estuarine transformation compatible with the modeled paleo-lake hydrology of this study.

3.3 Effect of Artifact Reduction

Accurate delineation of flow paths across the shallow continental shelf requires correction of artificial trench-like depressions generated by gridding along ship-track data. After iterative smoothing of the GEBCO 2025 bathymetry, several improvements were achieved in the modeled drainage topology. These corrections produced morphologically consistent valley alignments and more realistic connectivity among adjacent basins. The most notable adjustments are outlined below:

  1. The Java Sea system expanded westward, integrating southern-Sumatran tributaries previously misrouted toward the Sunda Strait.
  2. A distinct Eastern Java Sea system emerged, isolating the Barito, Kapuas-Murung, and Kahayan catchments from the Java Sea basin.
  3. Linear transverse channels formerly produced by ship-track artifacts were removed, restoring natural curvilinear drainage.

4. Discussion

4.1 Paleogeographic Significance

The reconstructed networks demonstrate that the Sunda Shelf once functioned as a contiguous fluvial plain. Drainage convergence zones in the Java Sea, Karimata Strait, and Gulf of Thailand align with present depocenters identified in seismic surveys and sediment-core analyses (Horton et al., 2005; Chabangborn et al., 2020). These relationships clarify the shelf’s role as both a sediment sink and a corridor for freshwater discharge during deglaciation, providing the physical context for the rapid transgression and shoreline fragmentation patterns described in Irwanto (2025c). The Gulf of Thailand paleo-lake and its subsequent marine transgression exemplify this dynamic transition from terrestrial to marine environments, illustrating the sedimentary continuity between the ancient fluvial systems and the modern shelf basins.

4.2 Biogeographic Evidence

The modern distribution of freshwater and estuarine taxa implies historical continuity through these ancient waterways. The river threadfin (Polydactylus macrophthalmus), today restricted to the Kapuas (Borneo) and Musi–Batanghari (Sumatra) rivers, exemplifies vicariant separation of populations once joined by the Molengraaff River system (Motomura et al., 2001). Comparable disjunctions among mangrove species and aquatic mollusks reinforce the paleohydrological connections inferred from this model.

4.3 Comparison with Previous Models

Relative to Voris (2000) and Sathiamurthy & Voris (2006), the present reconstruction offers higher spatial fidelity and improved hydrological realism. GEBCO 2025’s finer resolution delineates meanders and tributary curvature previously unresolved, while artifact correction enhances drainage continuity. The resulting systems exhibit asymmetric basins and multi-branch deltas more consistent with tropical alluvial morphodynamics.

4.4 Hydrological Implications

The extensive low-gradient plains inferred from the model suggest slowly meandering rivers traversing broad floodplains, capable of sustaining vast wetlands and delta complexes. These channels likely transported large sediment loads toward the shelf edge, influencing near-shore nutrient dynamics and the eventual formation of submerged ridge sequences visible in present bathymetry.

4.5 Broader Implications for Human and Biotic History

During lowered sea levels, the integrated river corridors of Sundaland provided continuous freshwater, fertile soils, and navigable routes across the emergent shelf. Such corridors would have facilitated the dispersal of human groups, enabling occupation of interior basins and coastal margins long before marine transgression. The interconnected fluvial plains may have served as arteries for cultural and genetic exchange across what is now Island Southeast Asia.

Large alluvial tracts along the paleo-Kapuas–Musi–Batanghari (Molengraaff) and Gulf of Thailand systems possessed the ecological capacity to sustain proto-agricultural communities. These environments echo the environmental settings of later riverine civilizations elsewhere, suggesting that the Sunda Shelf offered similar opportunities for early food-producing and settlement behaviors, as discussed in the Riverine Civilizations section of Irwanto (2015).

Following progressive inundation, former trunk rivers evolved into coastal estuaries and deltaic plains, maintaining their roles as communication axes. The transformation from fluvial to estuarine transport networks likely fostered the emergence of hydraulic and navigational knowledge, promoting the transition from inland cultivation to maritime resource exploitation.

As shelf flooding severed continental routes, human communities adapted to rising waters by shifting toward littoral livelihoods. Former river valleys became sheltered bays and straits—natural conduits for early seafaring. This environmental forcing may have seeded the maritime orientation that later characterized Austronesian and other early Southeast Asian cultures.

Submergence of the Sunda Shelf fragmented once-continuous habitats, isolating freshwater and terrestrial species. The split distribution of Polydactylus macrophthalmus across Sumatra and Borneo (Motomura et al., 2001) typifies post-inundation vicariance. Similar processes likely affected elephants in Kalimantan (Fernando et al., 2003; Sharma et al., 2018), freshwater turtles, and riverine vegetation, producing the biogeographical mosaics evident today.

4.6 Empirical Evidence for Lacustrine-to-Estuarine Transition in the Gulf of Thailand

Multiple sediment-core studies from the Gulf of Thailand and adjacent coastal plains substantiate the interpretation of a paleo-lacustrine stage followed by progressive marine influence during the early to mid-Holocene. Cores from the western Gulf of Thailand (CP3, CP4, CP5; Chabangborn et al., 2020; Jiwarungrueangkul et al., 2022) reveal stratigraphic successions where fine-grained lacustrine and deltaic units are overlain by brackish to marine estuarine facies, accompanied by increases in mangrove pollen, foraminiferal abundance, and marine microfossils.

Similarly, Horton et al. (2005) documented analogous palaeoenvironmental transitions in coastal cores from the Malay–Thai Peninsula, with a clear evolution from freshwater swamp and fluvial deposits to tidal-flat and estuarine sediments synchronous with the mid-Holocene sea-level rise. Regional syntheses of shelf sedimentation (Zhang et al., 2022) further demonstrate that the Sunda Shelf experienced a widespread hydrological reorganization, wherein formerly subaerial basins became drowned estuaries and shallow marine embayments as sea levels rose rapidly between ca. 10 and 7 ka BP.

Collectively, these datasets reinforce the interpretation that the Gulf of Thailand depression functioned initially as a large freshwater basin and subsequently transitioned to a semi-enclosed marine embayment—a sequence consistent with the modeled topography and hydrological reconstruction presented in this study.

5. Conclusion

The integrated analysis identifies six major paleo-river systems and a large Gulf of Thailand paleo-lake that together shaped the hydrological framework of emergent Sundaland. By combining GEBCO 2025 bathymetry, SRTM v3 topography, and hydrological modeling, this study refines previous reconstructions and establishes a physically consistent depiction of the shelf’s drainage architecture. Beyond geomorphology, the findings elucidate how these fluvial networks structured ecological corridors and human pathways before Holocene transgression, laying groundwork for future interdisciplinary research.

References

Molengraaff, G.A.F. (1921). Modern Deep-Sea Research in the East Indian Archipelago.

Voris, H.K. (2000). Maps of Pleistocene Sea Levels in Southeast Asia. Journal of Biogeography, 27, 1153–1167.

Sathiamurthy, E., & Voris, H.K. (2006). Maps of Holocene Transgression and Pleistocene Coastlines, Southeast Asia.

Chabangborn, A., Phantuwongraj, S., Sinsakul, S., Choowong, M., & Nakagawa, T. (2020). Environmental changes on the west coast of the Gulf of Thailand during the Holocene. Quaternary International, 555, 3–16. https://doi.org/10.1016/j.quaint.2019.12.020

Horton, B. P., et al. (2005). Holocene sea levels and palaeoenvironments, Malay–Thai Peninsula. The Holocene, 15(8), 1189–1203. https://doi.org/10.1191/0959683605hl887rp

Zhang, H., Liu, S., Wu, K., Cao, P., Pan, H-J., Wang, H., … Shi, X. (2022). Evolution of sedimentary environment in the Gulf of Thailand since the last deglaciation. Quaternary International, 629, 36–43. https://doi.org/10.1016/j.quaint.2021.02.018

Jiwarungrueangkul, T., Jirapinyakul, A., Sompongchaiyakul, P., & Rattanakom, R. (2022). Response of sediment grain size to sea-level rise during the middle Holocene on the west coast of the Gulf of Thailand. Arabian Journal of Geosciences, 15, 167. https://doi.org/10.1007/s12517-022-09450-3

Motomura, H., et al. (2001). Redescription of a rare threadfin (Perciformes: Polynemidae), Polydactylus macrophthalmus (Bleeker, 1858), with designation of a lectotype and notes on distributional implications. Ichthyological Research, 48, 289–294.

Fernando, P., Vidya, T.N.C., Payne, J., Stuewe, M., Davison, G., Alfred, R.J., Andau, P., Bosi, E., Kilbourn, A., & Melnick, D.J. (2003). DNA analysis indicates that Asian elephants are native to Borneo and are therefore a high priority for conservation. PLoS Biology, 1(1), 110–115.

Sharma, R., Goossens, B., Heller, R., Rasteiro, R., Othman, N., Bruford, M.W., & Chikhi, L. (2018). Genetic analyses favour an ancient and natural origin of elephants on Borneo. Scientific Reports, 8, 880.

World Wildlife Fund (WWF). (n.d.). Borneo Pygmy Elephant. Retrieved from http://www.worldwildlife.org/species/borneo-pygmy-elephant

Irwanto, D. (2015). Sundaland: Tracing the Cradle of Civilizations. Sections “Riverine Civilizations” and “Kalimantan Elephants.”

Irwanto, D. (2025a). A Refined Relative Sea-Level Curve for Sundaland.

Irwanto, D. (2025b). Holocene and Deglacial Sea-Surface Temperatures in Sundaland.

Irwanto, D. (2025c). Deglacial Rapid Inundation and Land-Loss Rates of Sundaland.

Footnotes

[1] The D8 (Deterministic Eight-node) algorithm is a standard flow-direction model in hydrological GIS analysis. It assigns each raster cell a single downslope direction toward one of its eight neighboring cells—north, northeast, east, southeast, south, southwest, west, or northwest—based on the steepest descent gradient. This approach enables efficient computation of flow accumulation and watershed delineation across large terrain datasets.

[2] Direct core evidence for a continuous freshwater “lake” spanning the entire Gulf basin is limited; thus, the paleo-lake interpretation should be regarded as a working geomorphological hypothesis derived from modeled topography and hydrological potential, constrained by regional sedimentary analogs.

Deglacial Rapid Inundation and Land-Loss of Sundaland

Book, , , , , , , , ,

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.

Holocene and Deglacial Sea Surface Temperatures in Sundaland

Book, , , , , , ,

A research by Dhani Irwanto, 6 October 2025

Abstract

We present a regional synthesis of sea surface temperature (SST) evolution across Sundaland—the now-drowned continental shelf of Southeast Asia—using the Osman et al. (2021) LGMR global proxy–model assimilation. Two metrics were derived: a boundary-wide Sundaland mean and an inner-tropical mean (±6° latitude), both averaged at 100-year intervals from 22.5 ka BP to the present. The SST record shows a pronounced deglacial warming, with the coldest conditions centered at ≈ 19.7–19.0 ka BP rather than ≈ 21 ka BP, a locally expressed Younger Dryas-type slowdown between 14.1 and 12.1 ka BP, and a delayed Holocene thermal maximum centered at ≈ 5–3 ka BP. These phase offsets reflect tropical oceanic leads and lags relative to global benchmarks, shaped by monsoon feedbacks, shelf flooding, and smoothing inherent to LGMR data assimilation. The Sundaland series thus refines our understanding of Indo-Pacific thermal evolution and highlights the nuanced regional phasing of post-glacial climate recovery.

Keywords: Sundaland, Sea Surface Temperature, Holocene, Deglaciation, Osman 2021, LGMR, Tropical Climate, Younger Dryas

1. Introduction

During the Last Glacial Maximum (LGM), when global sea levels stood more than 120 m below their present level, the continental shelf connecting modern Indonesia, Malaysia, and surrounding seas formed a vast subcontinent known as Sundaland. Its low-latitude position at the heart of the Indo-Pacific Warm Pool (IPWP) made it a key region for both ocean–atmosphere interaction and early human dispersal. Reconstructing sea surface temperature (SST) variations across Sundaland is therefore crucial for understanding how post-glacial warming, monsoon variability, and sea-level rise transformed this once-emergent landscape.

Osman et al. (2021) introduced the Last Glacial Maximum Reanalysis (LGMR), a globally resolved temperature reconstruction that assimilates more than 700 paleoclimate proxy records—including marine sediments, ice cores, and terrestrial archives—into a climate-model framework. The data assimilation technique combines proxy constraints with model physics to produce spatio-temporally consistent fields of surface temperature and isotopic composition from 24 ka BP to the present. The LGMR achieves near-global coverage at approximately 2° spatial resolution and 120 time steps, validated against modern instrumental records and independent proxies. It therefore provides an unprecedented foundation for analyzing regional climate evolution within a globally coherent context.

Building on this dataset, the present analysis focuses on Sundaland’s SST trends within two complementary spatial masks: the full Sundaland boundary and a restricted inner-tropical belt (±6° latitude). This dual perspective allows evaluation of both regional mean conditions and tropical-core behavior, testing whether Sundaland’s thermal evolution followed global trajectories or exhibited unique Indo-Pacific dynamics.

2. Data and Methods

The analysis utilizes the LGMR (Osman et al., 2021) gridded sea-surface-temperature field (variable sst). The Sundaland boundary was delineated using a geographic shapefile representing the shelf area presently submerged under the Java, South China, and Sulu Seas. Two spatial subsets were defined: (1) all grid cells within the boundary and (2) those confined to ±6° latitude to represent the equatorial core. For each of the 120 chronological steps (spanning 24 ka BP → 0 ka BP), SST values were averaged using a simple arithmetic mean. Temporal aggregation at 100-year intervals reduced small-scale variability while preserving long-term structure.

Key climatic benchmarks were annotated according to established chronologies: the LGM (~21 ka BP), the Younger Dryas (12.9–11.7 ka BP), the Early Holocene warming (~11 ka BP), and the Mid-Holocene Thermal Maximum (8–6 ka BP). No area weighting was applied, as the objective was to maintain transparency and comparability with previous Sundaland-scale studies. Visualization employed a simple time-series overlay between the two means, emphasizing contrasts in amplitude and timing.

3. Results

The Sundaland-wide and inner-tropical SST series both display a strong deglacial warming trend from the Last Glacial Maximum through the early Holocene. The lowest mean SSTs occur at 19.7–19.0 ka BP, about 2 kyr later than the canonical global LGM, indicating a slightly delayed tropical temperature minimum. A marked warming followed after 18 ka BP, punctuated by a subdued but distinct slowdown between 14.1 and 12.1 ka BP—interpreted as a regional expression of the Younger Dryas event. SSTs then stabilized at elevated levels through the Holocene, reaching a thermal maximum at ≈ 5–3 ka BP, later than most Indo-Pacific records. Thereafter, a gradual decline persisted toward modern values, consistent with orbital forcing and monsoon realignment during the late Holocene.

This overall trajectory, encompassing early deglacial warming and a prolonged Holocene optimum, mirrors the large-scale evolution of tropical ocean systems. The inner-tropical (±6°) mean remains consistently warmer than the whole-region mean throughout the sequence, differing by roughly 0.4–0.6 °C on average. This offset reflects the latitudinal SST gradient within the Sundaland domain and confirms the relative thermal stability of the equatorial core. Both curves reproduce the timing of key deglacial transitions documented in coral proxy records (Gagan et al., 2004) and global temperature stacks (Shakun et al., 2012; Marcott et al., 2013).

Figure 1. Mean SST time series for Sundaland (whole boundary) and the inner tropics (±6°), with key climatic intervals highlighted

4. Discussion

The Sundaland SST evolution broadly parallels the global deglacial pattern yet reveals distinctive tropical phasing and amplitude. The coldest interval occurs around ≈ 19.4 ka BP—about two millennia later than the canonical global LGM—suggesting that tropical oceans reached their temperature minima slightly after maximum ice volume, possibly due to delayed deep-ocean mixing and greenhouse gas rise. The ensuing warming accelerated after 18 ka BP, interrupted by a modest slowdown between 14.1 and 12.1 ka BP that corresponds to a regionally expressed Younger Dryas-type event. Although muted compared with high-latitude signals, this episode marks the tropical imprint of global circulation perturbations transmitted through the Indo-Pacific Warm Pool (IPWP).

The mid- to late-Holocene evolution likewise departs subtly from global reconstructions. The thermal maximum appears at ≈ 5–3 ka BP rather than the canonical 8–6 ka BP, likely reflecting continued shelf flooding, monsoon realignment, and prolonged heat retention across the newly inundated Sunda shelf. Comparison with Osman et al. (2021) global composites indicates that Sundaland warmed broadly in phase with other tropical basins but maintained slightly higher absolute SSTs throughout the Holocene, consistent with its shallow-shelf setting and strong ocean–land coupling. The agreement with coral records from the western Pacific (Gagan et al., 2004) further demonstrates that the LGMR framework captures Indo-Pacific thermal evolution with realistic regional detail, reaffirming Sundaland’s role as a dynamically sensitive yet climatologically buffered component of the IPWP.

  1. LGM phase (~19.4 ka BP). The SST minimum appearing at ≈ 19.4 ka BP, slightly younger than the canonical ≈ 21 ka BP, is consistent with other tropical reconstructions showing that Indo-Pacific surface waters began to warm earlier than the global ice-volume maximum. This phase lead likely reflects tropical sensitivity to rising greenhouse gases and orbital precession, initiating equatorial convection before full glacial retreat.
  2. Regional expression of the Younger Dryas. The subdued warming between 14.1 and 12.1 ka BP represents a local manifestation of the Younger Dryas, shifted earlier by about one to two millennia. Such displacement may stem from regional feedbacks in the Indo-Pacific Warm Pool (IPWP), where ocean–atmosphere coupling and early resumption of overturning circulation produced a tropical lead relative to Northern Hemisphere cooling. Comparable leads have been reported in tropical SST syntheses (e.g., Tierney et al., 2020).
  3. Mid-Holocene peak timing (5–3 ka BP). The delayed maximum SST relative to the global mid-Holocene (8–6 ka BP) can be attributed to continued shelf inundation and regional monsoon asymmetry. As postglacial flooding transformed Sundaland into a mosaic of seas and islands, enhanced heat retention and sustained humidity may have extended warm conditions well into the middle Holocene. Additionally, the LGMR assimilation’s temporal smoothing likely distributed the Holocene thermal maximum over a broader interval, shifting the apparent peak toward later centuries.
  4. Chronometric and methodological factors. The Osman et al. (2021) LGMR dataset integrates multiple proxy types with varying age control, producing an estimated uncertainty of ±0.5–1 kyr for regional means. Its Kalman-filter approach dampens abrupt transitions but preserves long-term coherence; when averaged across Sundaland’s broad spatial domain, this smoothing can produce 1–2 kyr apparent offsets in peak or trough timing.

In summary, the phase shifts observed in the Sundaland SST curves do not contradict global reconstructions but rather highlight the spatial heterogeneity and lag–lead behavior of tropical oceans during deglaciation. The slow-warming interval at 14.1–12.1 ka BP likely represents a regional Younger Dryas signature modulated by tropical feedbacks, while the delayed thermal maximum at 5–3 ka BP reflects prolonged warmth associated with monsoon dynamics, shelf inundation, and model assimilation smoothing.

5. Conclusion

Analysis of the Osman et al. (2021) LGMR dataset reveals parallel SST histories for Sundaland’s full extent and its inner-tropical core. Both exhibit canonical deglacial transitions, but with regionally distinct phasing: the LGM minimum near ≈ 19.4 ka BP, an early Younger Dryas-like cooling at 14.1–12.1 ka BP, and a delayed Holocene peak at 5–3 ka BP. These offsets underscore Sundaland’s tropical sensitivity and the asynchronous yet coherent behavior of the Indo-Pacific Warm Pool relative to global climate evolution. They also emphasize how shelf flooding, monsoon feedbacks, and assimilation smoothing influence the apparent timing of climatic events. Together, these findings position Sundaland as a key indicator of tropical ocean variability and as a benchmark region for integrating paleoclimate, sea-level, and archaeological evidence of the late Quaternary transformation of Southeast Asia.

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