Browsing by Author "Rifai, H"

Now showing 1 - 2 of 2
  • No Thumbnail Available
    Item
    Stable isotope composition of cave guano from eastern Borneo reveals tropical environments over the past 15,000calyrBP
    (Elsevier, 2017-03-24) Wurster, CM; Rifai, H; Haig, J; Titin, J; Jacobsen, GE; Bird, MI
    Insular southeast Asia is a key driver for global atmospheric and oceanic circulation, is a hotspot for biodiversity and conservation, and is likely to have played a unique and important role in early human dispersals. Despite this, partially due to its vast size and remote tropical location, very few continuous palaeoenvironmental records exist, especially in eastern Borneo. Therefore, we investigated δ13C and δ15N values, and geochemistry of two cave guano deposits to reconstruct palaeoenvironments in eastern Borneo. Firstly, a profile was recovered from Gomantong caves, Sabah, reflecting a continuous deposit over ~ 15 cal kyr BP. Secondly, a profile was recovered from Bau Bau cave, East Kalimantan, that ranged from ~ 15–5 cal kyr BP. The geochemical signature of each deposit confirmed the material to be ancient guano. δ13C values revealed that a continuous dense rainforest persisted over at least the last 15 cal kyr BP around the Gomantong site that was relatively insensitive to regional climate change. By contrast, δ13C values at Bau Bau indicate that, although rainforest remained dominant in the record, a significant drying occurred between 7.7 and 6.3 cal kyr BP, with up to 25% grasses present. Although most regional models suggest that sea-level rise and increased Holocene insolation led to an increase in monsoonal moisture, we find some evidence of more regional variability, and that a reduction in monsoonal precipitation could have occurred. However, we cannot discount the implementation of an anthropogenic fire regime that opened the canopy allowing more grasses to occur. © 2017 Elsevier B.V. All rights reserved.
  • No Thumbnail Available
    Item
    What drives vegetation changes in South Sulawesi during the MIS 5e transition?
    (Copernicus Publications, 2024-04-17) Kimbrough, A; Gagan, MK; Dunbar, GB; Treble, PC; Hantoro, WS; Zhao, JX; Edwards, RL; Shen, CC; Suwargadi, B; Wong, HKY; Rifai, H
    Sulawesi speleothem carbon isotopes (δ13C) are found to co-vary with deglacial warming and atmospheric CO2 measured from Antarctic ice cores. This co-variation has thus far been attributed to speleothem δ13C recording changes in vegetation productivity and microbial activity in the soils overlaying caves as vegetation and microbes respond to glacial-interglacial changes in temperature and atmospheric CO2 (Kimbrough et al., 2023; Krause & Kimbrough et al., in press). However, the relationship between speleothem δ13C and regional environmental change is complex and deconvolving the effect of different environmental drivers is difficult. To further investigate the ecosystem response in the Indo-Pacific Warm Pool to substantial warming and CO2 rise during the penultimate deglaciation/marine isotope stage 5e (~127 kyrs ago) we use complimentary geochemical proxies extracted from stalagmite CaCO3. These proxies include phosphorus and sulphur which respond to nutrient uptake by forest biomass above the cave (Treble et al., 2016). The relative abundance of metals such as copper, iron, zinc, and lead are assessed as another means to track biomass/soil regeneration via selective element delivery to the stalagmites by organic colloids flushed from the soil zone (Borsato et al., 2007). These vegetation proxies are compared with the speleothem δ13C and δ18O records and corresponding high-resolution fluorescence mapping of organics via confocal laser scanning (fluorescence) microscopy (Sliwinski & Stoll, 2021). The comparison of transition metals to stable isotopes (δ18O, δ13C) in the Sulawesi speleothem records makes it possible to distinguish between periods in the record where vegetation productivity increased in response to a rise in temperature and CO2 verses periods where changing hydroclimate played a more dominant role. Characterising the appropriate drivers and proxy response is critical to accurately interpret tropical paleoclimate records where interpretations rely on assumptions that rainfall is the primary driver of vegetation change. © Author(s) 2024. This work is distributed under the Creative Commons Attribution 4.0 License.