Stable isotope composition of cave guano from eastern Borneo reveals tropical environments over the past 15,000 cal yr BP
Introduction
In caves throughout the world, bats roosting in large numbers produce copious amounts of faecal material (guano) (e.g., Mizutani et al., 1992, Mcfarlane et al., 2002), that can accumulate over millennia (e.g., Wurster et al., 2010a, Wurster et al., 2010b). Throughout southeast Asia, swiftlets (Aerodramus sp.) also inhabit caves in large numbers and contribute substantially to cave guano accumulations (Bird et al., 2007). Faecal input is modified by bacteria and fungi, reactions with cave material and authigenic inputs, (Shahack-Gross et al., 2004, Wurster et al., 2015), while also supporting a guano-based fauna (e.g., Ferreira et al., 2007, Iskali and Zhang, 2015). After processing and removal of much of the organic component, residual ‘rock’ guano at depth consists largely of clay and phosphate minerals, as well as detrital material such as quartz, and a trace metal signature enriched in transition metals (Shahack-Gross et al., 2004, Onac and Forti, 2011, Wurster et al., 2015). These deposits are often located in regions where there is a general lack of complimentary palaeoclimate records, and elucidate past environmental conditions where there are few other proxy records (Wurster et al., 2008, Wurster et al., 2010b, Onac et al., 2014).
Although several new studies have demonstrated that guano is a valid and powerful palaeoenvironmental archive, it is still relatively unexploited. Recent work has shown its suitability to provide past environmental proxies to a range of broad environments including tropical (Bird et al., 2007, Wurster et al., 2010a), semi-arid (Wurster et al., 2008, Wurster et al., 2010b) and temperate locations (Onac et al., 2014, Onac et al., 2015, Forray et al., 2015, Widga and Colburn, 2015). Pollen (Maher, 2006, Batina and Reese, 2011, Onac et al., 2015, Forray et al., 2015), geochemistry (Bird et al., 2007, Johnston et al., 2010, Wurster et al., 2015) and stable isotope ratios of carbon and hydrogen (Bird et al., 2007, Wurster et al., 2008, Wurster et al., 2010b, Onac et al., 2014, Onac et al., 2015, Forray et al., 2015), have been successfully exploited. Moreover, archaeological evidence is often found in caves that contain considerable guano deposits (e.g., Karkanas et al., 2002, Shahack-Gross et al., 2004, Campbell et al., 2017) potentially enabling a direct comparison between environmental change and human adaptation. Arguably, δ13C values are the most powerful proxy for past environments that can be obtained from tropical guano records.
In tropical environments, grasses dominantly utilise Hatch-Slack (C4) photosynthesis; whereas trees and woody vegetation exclusively utilise the Calvin cycle (C3) photosynthesis (Bird and Pousai, 1997, Ehleringer et al., 1997). These different enzymatic pathways of carbon fixation result in δ13C values of C4 plants (− 9 to − 16‰) that are distinct when compared with C3 plants (− 19 to − 34‰) (Ehleringer and Cerling, 2002). Moreover, significant variability in individual plant δ13C values are considerably reduced at the regional level (Randerson et al., 2005). Insect abundance is largely determined by available vegetation (Pinder and Kroh, 1987, Warren and Gaston, 1992), and insect δ13C values are determined by diet with little fractionation (Webb et al., 1998, Gratton and Forbes, 2006). An integrated measure of insect cuticle, δ13C value is therefore a strong indicator of vegetation type at the regional level. It has further been demonstrated that the δ13C values of guano from insectivorous bats largely consists of insect cuticles and directly reflect the relative abundance of C4 vegetation in a region via δ13C values (Sullivan et al., 2006, Wurster et al., 2007). Communities of bats and swiftlets generally forage within a 15 km range of the roost (e.g., Zahn et al., 2005), although long-range foragers including wrinkle-lipped bats (Chaerephon plicatus) may travel as long as 80 km (Stimpson, 2012), ensuring that a local to regional signal is captured in the guano deposit. Guano deposits from insular Southeast Asia have been demonstrated to provide important records of vegetation change via δ13C profiles in a region where few additional records currently exist (Bird et al., 2007, Wurster et al., 2010b, Wurster and Bird, 2016).
The nature of glacial to interglacial climate and vegetation change experienced in insular southeast Asia is currently a topic of debate, and not even the general vegetation type present during the Last Glacial Maximum (LGM, 23-19 cal kyr BP) are known unequivocally (Wurster and Bird, 2016). During the Last Glacial Period (LGP, 110-11.7 cal kyr BP), reduced global sea level exposed the continental shelf from south of Thailand to Sumatra, Java, and Borneo, revealing the contiguous continent ‘Sundaland’ (Bird et al., 2005), and some of these land connections remained until well into the Holocene (Bird et al., 2010). Forest reduction during the LGM has been demonstrated for Palawan and Peninsular Malaysia (Wurster et al., 2010a) in the north, and similarly drier conditions have been reported in the south during the LGM and Antarctic Cold Reversal (ACR, 15-12.9 cal kyr BP) (Dubois et al., 2014, Wurster and Bird, 2016). However, climate modelling (Cannon et al., 2009) and dipterocarp species distribution models (Raes et al., 2014) along with offshore pollen (Wang et al., 2009) have suggested that much of Sundaland remained tropical rainforest and humid during the LGM.
Eastern Borneo has relatively little environmental information over the Late Pleistocene and Holocene, despite it being reputed as a possible refuge for rainforest specialists (Wurster and Bird, 2016), and despite having a unique and rich archaeological context including prehistoric paintings dating to the upper Pleistocene (Plagnes et al., 2003, Grenet et al., 2016). The environment surrounding Niah Cave is thought to have remained forested throughout the LGP and beyond, and has been inferred to be a rainforest refugium, however the extent of this refugium is not known (Bird et al., 2005, Wurster et al., 2010a, Wurster and Bird, 2016). Archaeological work in East Kalimantan showed that lithic industries changed little over the Pleistocene/Holocene transition; one interpretation of this stability is that climate and environments remained stable in the region (Grenet et al., 2016). However, there are no well-dated and unambiguous environmental records of past environments during the Pleistocene/Holocene transition in eastern Borneo (Wurster and Bird, 2016), the nearest being in northern Borneo to the north (Wurster et al., 2010a, Dubois et al., 2014), or Sulawesi to the east (Russell et al., 2014).
Herein we present the stable isotope composition and geochemistry from two new cave guano profiles sampled in eastern Borneo: Gomantong in Sabah, Malaysia (5°32′60″N, 118° 5′60″E) and Bau Bau (0°55′0″S, 117°13′11″E) along the Bungalun River in East Kalimantan, Indonesia (Fig. 1). Both records have higher deposition rates than previously reported for insular Southeast Asian tropical guano (Wurster et al., 2010a), and provide a ~ 15 cal kyr BP history of tropical environments in these regions over the Pleistocene/Holocene transition.
Section snippets
Study area and sampling
Surface and ancient ‘rock guano’ (at depth) were collected from three sites along the eastern edge of Borneo (Fig. 1). From Sabah, Malaysia, a 1.8-m-deep guano profile derived from both insectivorous bats and Aerodramus sp. was collected from Simud Puteh Cave in the centre of Pandandanan Chamber (GOM-SP-P) (5°32′60″N, 118° 5′60″E) in Gomantong Forest Reserve. Gomantong caves are located on Gomantong Hill, an isolated limestone outcrop on the flood plain of the Menugai River (Lundberg and
Geochemistry
The geochemistry of selected elements is presented in Fig. 2. Both Gomantong and Bau Bau have similar abundances of many elements, including high contents of P, Cu, Zn, V, and Cr. Phosphorous abundances range between 4 and 12%, higher in Gomantong compared with Bau Bau, which range between 3 and 7%. Copper abundances are very high in both profiles, ranging between 800 ppm at the surface to over 4000 ppm at depth in Gomantong, and 1800–7300 ppm at Bau Bau. Other trace metals (Zn, Cr, V) have high
Geochemistry
Both deposits have a geochemical fingerprint indicative of ancient guano. High P, Zn, Cu, V and Cr contents are associated with ancient guano deposits (Wurster et al., 2015), and Gomantong has Cu values as high as 2538 ppm, while Bau Bau exceeds 7000 ppm. Similarly, high phosphorous content (up to 12%) and Zinc (up to 2300 ppm), and low pH values indicate that the profiles are derived from guano. Conspicuously high amounts of Ca and associated Sr and/or S likely has to do with inclusion of gypsum
Conclusions
We investigated environmental change over the last 15,000 cal yr BP surrounding two sites in Eastern Borneo where very little information currently exists. δ13C values of guano sampled in Gomantong, Sabah, strongly suggest the continuity of rainforest cover with little sensitivity to broader climate changes. However, despite rainforest being the dominant component of vegetation surrounding Bau Bau, East Kalimantan, a significant component was C4 vegetation, with estimated biomass contributions
Acknowledgments
We thank Mr. Roslee Martinis and Mr. Ismail Majid for help in the field at Gomantong, with additional help from Mr. Hussin Tukiman, from the Kinabatangan District Forest Officer. We thank Erni from Sriwijaya University for help in East Kalimantan and Tewet and sons for guiding us to and around Bau Bau. We thank A. Calder, J. Whan, and S. Askew for help in the laboratory. NERC Grants NE/D001501 and NE/D01185x/1, Australian Research Council Federation Fellowship (FF0883221) and Discovery (
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