Post-speleogenetic Biogenic Modification of Gomantong Caves, Sabah, Borneo

The Gomantong cave system of eastern Sabah, Malaysia, is well-known as an important site for the harvesting of edible bird’s-nests and, more recently, as a tourist attraction. Although the biology of the Gomantong system has been repeatedly studied, very little work has been done of the geomorphology. Here we report on the impact of geobiological modification in the development of the modern aspect of the cave, an important but little recognized feature of tropical caves. Basic modelling of the metabolic outputs from bats and birds (CO2, H2O, heat) reveals that post-speleogenetic biogenic corrosion can erode bedrock by between ~3.0 mm/ka (1 m/~300 ka) and ~4.6 mm/ka (1 m/~200 ka). Modelling at high bat densities yields corrosion rates of ~34 mm/ka (or 1 m/~30 ka). Sub-aerial corrosion creates a previously undescribed speleological feature, the apse-flute, semicircular in cross-section, ~80 cm wide, vertical regardless of rock properties, developing in parallel but apparently completely independently, and often unbroken from roof to floor. They end at a blind hemi-spherical top with no extraneous water source. Half-dome ceiling conch pockets are remnants of previous apse-fluting. Sub-cutaneous corrosion creates the floor-level guano notch formed by organic acid dissolution of bedrock in contact with guano. Speleogenetic assessment suggests that as much as 70-95% of the total volume of the modern cave may have been opened by direct subaerial biogenic dissolution and biogenically-induced collapse, and by subcutaneous removal of limestone, over a timescale of 1-2 Ma.