About 40 samples are collected from the upper 250 cm of the sediment core GeoB 10053-7 offshore Java, covering the past 5,000 years (Mohtadi et al., 2011; doi:10.1038/ngeo1209). The average sample resolution is around 120 years. Here, we use markers for low intensity fires and soil erosion to reconstruct human activities in East Java (Indonesia) over the last 5,000 years. We use the accumulation rate of branched glycerol dialkyl glycerol tetraethers (brGDGTs), markers for soil-derived organic matter, to indicate levels of soil erosion in the catchment region. We also use the accumulation rate of levoglucosan to indicate past fire use in the catchment. Independent hydroclimate reconstruction that are not influenced by human activities is compared in order to differentiate the impact of human activities vs. hydroclimate on soil erosion in the catchment area. Specifically, the stable hydrogen isotope composition (δD) of leaf wax n-alkanes reflect changes in the monsoonal rainfall intensity in the catchment. In addition, the stable carbon isotope composition (δ13C) of leaf wax n-alkanes derived from our sediment core reflects the relative abundance of regional C3 versus C4 vegetation.
The analysis of levoglucosan was performed as described in detail by Schreuder et al. (2018; DOI: 10.1016/J.GCA.2018.02.020). The analyses of leaf wax n-alkanes and their isotopes were performed as described in detail by Ruan et al. (2019; doi:10.1016/j.quascirev.2019.06.015).For both leaf wax δD and δ13C, weighted average values are calculated based on three homologues (n-29, n-31 and n-33). The highest difference of two measurements based on three homologues are taken. GDGTs were extracted from a separate set of samples taken at 5 cm intervals from the sediment core. The branched GDGTs were extracted as described in Chen et al. (2014; doi:10.1016/j.dsr.2014.03.005) and analyzed using an Agilent 1260 Infinity II ultra-high-performance liquid chromatography-mass spectrometry (UHPLC-MS) system using the method as described by Hopmans et al. (2016; doi: 10.1016/j.orggeochem.2015.12.006) and Schouten et al. (2007; doi: 10.1021/ac062339v).
