Jack Rieley

The geochemistry of major and selected trace elements in a forested peat bog, Kalimantan, SE Asia, and its implications for past atmospheric dust deposition

Abstract Biogeochemical processes in a forested tropical peat deposit and its record of past atmospheric dust deposition were assessed using the vertical distribution of lithophilic and plant essential elements in a dated core profile from Borneo, SE Asia. Peat formation started ∼22,120 14C yr before present (BP), and Ca/Mg mass ratios of the solid peat and very low ash contents indicate a strongly ombrotrophic character throughout the deposit, implying that most of the inorganic fraction has been supplied exclusively by atmospheric inputs. Concentration profiles of Mn, Sr, and Ca suggest a very minor influence of chemical diagenesis in the underlying sediments. Silicon, Ca, Mg, P, S, and K show a strong and extended zone of enrichment in the top 200 cm of the profile, indicating that biological accumulation mechanisms are much more extensive than in temperate peat bogs. In the lower core sections, where the element distribution is dominated solely by past atmospheric deposition, average Al/Ti ratios are similar to the upper continental crust (UCC), whereas Fe is slightly enriched and Si is strongly depleted: this condition favors highly weathered tropical soil dust as the main inorganic mineral source. Significant correlation of Al, Fe, Si, S, Ca, and Ti with the lithophilic elements Y and Zr suggests that the distribution of these elements is controlled by sources of atmospheric mineral dust. The Ca/Mg, Ca/K, and Mg/K ratios of the collected rainwater samples are similar to the global average of continental rainwater and suggest a continental character for the site. This is supported by the similarity of the average concentration of Br, Mg, Ca, and S to that in temperate continental and maritime bogs in Switzerland and Scotland. The concentration profiles of Si, Fe, Al, and Ti show distinct peaks within the profile, implying enhanced dust deposition, reduced rates of peat accumulation, or possibly both owing to climatic changes during the Holocene. Enhanced dust deposition between ∼10,830 and 9060 14C yr BP is tentatively interpreted as a Younger Dryas–like event with dust fluxes of ∼10.8 mg/m2/yr. The variations in Al/Ti and Fe/Ti profiles suggest that mineral dust sources have been changing constantly during the Holocene, with local sources being dominant between ∼7820 and 9500 14C yr BP and long-range transport (derived most likely from China) being important during the late Pleistocene and early Holocene and from ∼7820 14C yr BP to the present.

Tropical peatland fires in Southeast Asia

Extensive tropical peatlands are located in the Malaysian and Indonesian lowlands, particularly in Borneo, Sumatra, West Papua, and Peninsular Malaysia. In an undisturbed condition, these peatlands make a significant contribution to terrestrial carbon storage, both in terms of their aboveground biomass (peat swamp forest) and thick deposits of peat. Occasional forest fires, including peatland fires, have occurred in Southeast Asia over several millennia but, in recent years, they have become a more regular feature. The most severe fires have been linked with the El Nino phase of ENSO which causes extended periods of drought, particularly across the peatland areas of southern Sumatra and southern Kalimantan. During the last 20 years, rapid land use change, exacerbated by climatic variability, has led to an increase in fire frequency, as the remaining peat swamp forests come under pressure from increased illegal logging, development for plantations and agriculture-based settlement, and, where economic development has failed, land abandonment. A case study of fire occurrence in Borneo illustrates that peat swamp forests are much more prone to fire than any other forest type, largely as a result of the high pressure being put on these last remaining forested lands. From studies in central Kalimantan (southern Borneo), we demonstrate the relationships between peat drainage, vegetation change, and increased fire frequency, including the role that peat combustion and subsidence play in an increased incidence of surface flooding. Tropical peatland fires, and the changes in vegetation that they bring about, have significant impacts on the atmosphere, the carbon cycle, and various ecosystem services; they also cause wide-ranging social and economic impacts. Fires on peatlands usually affect both the surface vegetation and the underlying peat layer and, as a result, they release much larger amounts of C02 into the atmosphere than forest fires on mineral soils. In 1997, peatland fires in Indonesia resulted in the release of between 0.81 Gt and 2.57Gt of carbon into the atmosphere, equivalent to 13% to 40% of mean annual global carbon emissions from fossil fuels, and over the last ten years a conservative estimate of total carbon emissions from peatland fires in Southeast Asia is of the order of 2Gt to 3Gt. Future climate changes may place further pressure on the tropical peatland ecosystem and are likely to lead to enhanced carbon emissions from both peat degradation and fire.

Lowland tropical peatlands of Southeast Asia

[Extract] Peatlands are terrestrial wetland ecosystems in which the production of organic matter exceeds its decomposition and a net accumulation results. Several factors influence peat formation and preservation, including a positive climatic moisture balance (precipitation minus evaporation), high-relative humidity, topographic and geological conditions that favor water retention, and low substrate pH and nutrient availability. The majority of the world’s peatlands occur in boreal and temperate zones where they have formed under high-precipitation, low-temperature climatic regimes. In the humid tropics, however, regional environmental and topographic conditions have enabled peat to form under a high-precipitation, high-temperature regime (Andriesse, 1988) and, as a consequence, extensive peatlands occur in southeast Asia, mainland east Asia, the Caribbean and Central America, South America and southern Africa. Most of these are located at low altitudes where rain forest vegetation grows on a thick mass of organic matter accumulated over thousands or tens of thousands of years, to form deposits up to 20m thick (Anderson, 1983). In the tropics, these lowland peatlands are almost exclusively ombrogenous (the peat surface only receives water from precipitation), whereas geogenous peatlands, that are fed additionally by water that has been in contact with the mineral bedrock and soils, are of more limited distribution, being confined to the edges of coastal lagoons, the banks and flood zones of rivers, and the margins of upland lakes. Undisturbed, lowland ombrogenous peatlands support peat swamp forest; freshwater swamp forests are associated with geogenous peatlands.