Linkages between continental hydrology, vegetation and biomass burning in East Java since the Last Glacial

Abstract

The Maritime Continent is the vast region between the tropical Indian Ocean and Pacific Ocean, consisting of the Indo-Pacific warm pool and thousands of islands of various sizes. It is the primary region of strong atmospheric convection and widespread land-sea interaction on Earth. Today, the tropical rainforest ecosystem and millions of residents in this region are facing great threats such as droughts, floods and wildfires in the context of ongoing global warming and continuing greenhouse gas emission. The understanding of interactions between rainfall, vegetation and fire is essential but limited. Satellite-based monitoring of rainfall, vegetation and fire occurrence, which only exists in the past few decades, are too short to understand their long-term feedbacks with each other. An accurate reconstruction of the past changes in hydro-climate (rainfall), vegetation and fire in this region is therefore a prerequisite for understanding tropical ecosystems and for projections into the future.

With a focus on East Java, Indonesia, a monsoonal region of the Maritime Continent, this thesis applies multiple proxies to the same climate archive (a sediment core retrieved off the southern shore of East Java) in order to accurately reconstruct the regional vegetation, rainfall and fire regime (frequency and intensity of fires) over the past 22,000 years.

Leaf wax δ13C based on multiple n-alkane homologues (n-C29, n-C31 und n-C33) together with palynological data are used to reconstruct vegetation types, while leaf wax δD based on the multiple homologues is used to reconstruct the rainfall intensity during the wet season. The results show that different homologues predominantly reflect distinct ecosystems: n-C29 mainly reflects montane rainforest during the Last Glacial Maximum (LGM, around 21,000 years ago) and lowland rainforest during the Holocene (since 10,000 years ago), while n-C31 and n-C33 predominantly reflect lowland vegetation through the record. The results further suggest that in East Java, evergreen rainforest remained the dominant vegetation type in montane regions since the rainfall seasonality there remained relatively unaltered over the past 22,000 years. In contrast, the East Javanese lowlands were characterized by C4 grass expansion and an extended dry season but a wetter rainy season, thus stronger seasonality, during the LGM.

The past fire regime is reconstructed using micro-charcoal in combination with two molecular markers of burning, i.e. levoglucosan and polycyclic aromatic hydrocarbons. While the micro-charcoal accumulation rate reflects general fire occurrence in East Java, the ratio of levoglucosan vs. polycyclic aromatic hydrocarbons indicates fire intensity. The results show that both fire occurrence and intensity were high during the LGM but low during the Heinrich Stadial 1 (17,500-14,600 years ago), the Younger Dryas (12,900-11,600 years ago) and the early Holocene (10,000-3,000 years ago). A comparison of the fire history with the regional lowland vegetation and hydro-climate shows that both fire regime and vegetation were primarily controlled by rainfall seasonality. However, fire additionally stabilized the savannah (rainforest)-dominated ecosystem during the LGM (early Holocene) but caused transitions between the two vegetation types during the deglaciation (17,500-11,600 years ago) and the late Holocene (3,000 years ago till now).

The impact of human activities need to be considered in assessing the vegetation cover and fire history of East Java during the late Holocene. Therefore, the history of land use/land cover (vegetation cover, fluvial erosion and fire disturbance) in East Java over the past 5,000 years is reconstructed. To assess the climatic vs. human impact on it, regional rainfall reconstructions and archaeological records are evaluated respectively. The results show that the fluvial erosion co-varied with the regional annual rainfall amount throughout the record; the highest erosion between 2,800 and 1,800 years ago occurred with a gradual increase in both C4 vegetation and high intensity fire occurrence. The results suggest a primary hydro-climatic impact on the East Javanese fluvial system over the past 5,000 years. In the meanwhile, the prehistoric human society of East Java potentially caused deforestation by swidden cultivation using fire as a tool, which further enhanced soil erosion. Such a human impact became step-wise profound through the record reflected by the increasing occurrence of high intensity fires.