Peatlands in Southeast Asia: A comprehensive geological review

Peatlands are important carbon sinks, therefore their degradation mainly due to lowering of the water table, has an adverse effect to the carbon cycle and largely burden the atmosphere. Annually, extensive areas of these peatlands are affected by wildfires, therefore large peatland areas have been degraded, directly or indirectly related to anthropogenic activities, largely contributing to greenhouse gas emissions. Some of the most pristine tropical peatlands worldwide occur primarily at the coastal lowlands of Southeast Asia. In terms of their geological features and evolution, these sites are generally understudied despite covering more than half of the global area of tropical peatlands. This review compiles geological data from 52 peatlands from insular and continental Southeast Asia, providing a comprehensive geological dataset for future research. The Southeast Asian peatlands are mostly ombrogenous and hence poorly supplied by nutrients. During the Last Glacial Period (ca. 115,000–11,000 years ago), extensive areas were exposed because of the lowering of the seawater level, which caused a consequent lowering of the groundwater table landwards; the forests were under severe threat, mainly due to dry conditions, thus resulting in the retreat of the forest boundaries inland. This was an unfavourable environment for peatland formation and hence, most of the Southeast Asian peatlands were formed right after that period. Almost 40% of the reviewed sites are located on Borneo Island, highlighting the importance of Bornean peatlands, where many peatlands are already deforested and drained and converted to plantations. Overall, the available geological data from the Southeast Asian peatlands is incomplete and non-comparable to each other because each study has a different focus. Details, such as the type of peat-forming plants, age of peat, peat thickness, substrate type and the pH value are not reported systematically in approximately 30% of the reviewed sites, while other important geological data, such as the ash yield and the carbon content of peat are only reported in 30% and 10% of the reviewed sites, respectively. Characterisation of peatlands using data imputation and principal component analysis (PCA) is based on three physical parameters (maximum ash yield, maximum thickness and oldest age), and includes the study of their relation in terms of climatic periods, peatland type, region and substrate. It is observed that peatlands which were developed in warming periods share similar physical parameters (such as accumulation rates, ash yield, peatland type, and environment of the substrates). With better data reporting on these parameters, the PCA analysis can provide a more accurate reflection of peatland characteristics and their relationships. The study aims to raise awareness on the importance and vulnerability of the Southeast Asian peatlands and to highlight their role in the global climate fluctuations.