Abstract:

Peatlands have been recognised as globally important carbon sinks over long timescales that produced a
global, net-climatic cooling effect over the Holocene. However, little is known about the role of tropical
peatlands in the global carbon cycle.We therefore determine the past rates of carbon storage and release
in the Indonesian peatlands of Kalimantan and Sumatra e the largest global concentration of tropical
peatlands e since 20 ka (kiloannum before present). Using a novel GIS (geographic information system)
approach we provide a spatially-explicit reconstruction of peatland expansion in a series of paleogeographic
maps.

Sea-level change is identified as the principal driver for peatland formation and expansion in western
Indonesia as it controls both atmospheric moisture supply and the hydrological gradient on the islands.
Initiation of inland peatlands in Kalimantan was coupled to periods of rapid deglacial sea-level rise with
rates of over 10 mm yr
1 whereas coastal peatlands could only form after 7 ka when the rate of sea-level
rise had slowed to 2.4 mm yr
1. Falling sea levels after 5 ka led to rapid peatland expansion in coastal
lowlands and a doubling of the total peatland area in western Indonesia to 131,500 km2 between 2.3 ka
and 0 ka. As a result of slow peatland expansion from 15 to 6 ka and rapid expansion afterwards the rate
of annual carbon storage of all western Indonesian peatlands remained <1 Tg C yr
1 until 6 ka and then
increased to 7.2 Tg C yr
1 by 0 ka. Associated with this rise in carbon storage was an exponential growth
of the peat carbon pool from 0.01 Pg C by 15 ka to 23.2 Pg C at present, of which 70% is stored in coastal
peatlands. In inland Kalimantan peatlands, falling sea levels together with increased El Niño activity
induced an annual carbon release of 0.15 Tg C yr
1 from aerobic peat decay since 2 ka. Cumulative carbon
losses from anaerobic decomposition do not seem to limit peat bog growth in the tropical peatlands of
Indonesia. Carbon losses from Holocene peat fires are only known from the Kutai basin since 4.4 ka with
an associated release of 0.1e3.6 Tg C per fire event, which never surpassed the contemporaneous annual
C storage. The peatlands of western Indonesia were thus a persistent carbon sink since 15 ka but this sink
was of global importance only over the past 2000 years when it likely contributed to a slower growth in
atmospheric CO2 concentrations. Currently, annual losses of carbon from peat drainage and fires are on
average 28 times higher than the pre-disturbance rate of uptake implying that this carbon reservoir has
recently switched from being a net carbon sink to a significant source of atmospheric carbon and is
currently in danger of eradication.

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