Natural attrition and growth frequency variations of stalagmites in southwest Sulawesi over the past 530,000 years
Introduction
The appeal of stalagmites as a long-term, precisely dated continental palaeoclimate archive has led to a proliferation of research in recent years (e.g., Wang et al., 2001, Genty et al., 2003, Cruz et al., 2005, Meckler et al., 2012, Jo et al., 2014). However, the removal of stalagmites from caves for scientific research can be a mixed blessing for local communities: palaeoclimate reconstructions from geochemical analyses of stalagmites can improve knowledge and adaptation strategies for climatic events and changes. But there is also significant monetary value in preserving the aesthetic value of cave systems in their natural state for tourism, which provides direct financial benefits to local communities. Recreational use of non-show caves also provides indirect benefits to local communities. Caves may represent important centres of local cultural heritage, such as religious spaces (e.g., Batu Caves, Malaysia). Therefore, caves of all types must be protected, and scientists should match the custodial care provided by the caving communities in preserving their natural conditions (Baker and Genty, 1998, Calaforra and Fernandez-Cortés, 2003, Cigna and Burri, 2012, Verheyden and Genty, 2013, Šebela and Turk, 2014).
In some cases stalagmites have grown continuously for tens of thousands of years; thus, they are irreplaceable and can be considered a finite resource. However, the value of stalagmites is uncertain because they are highly valuable in terms of their value to science and tourism and worthless because they have little or no direct economic value. Thus, questions of how to manage the competing interests of scientific research versus the present and the future value of stalagmites in their original locations remain an issue.
In this study, we show that these two ideals can be reconciled by extracting palaeoclimate information from a cave system without the removal of stalagmites, thus retaining the intrinsic value of the cave. We follow in the footsteps of stalagmite dating studies that reconstruct past climates using the age distributions of stalagmites harvested from caves (e.g. Hennig et al., 1983, Gordon et al., 1989, Baker et al., 1993, Ayliffe et al., 1998, Hercmann, 2000, Wang et al., 2004, Vaks et al., 2006, Vaks et al., 2010, Vaks et al., 2013, Williams et al., 2010, Jo et al., 2014). However, our approach utilises the ages of low-impact mini-cores extracted in situ from the bases of stalagmites, thus eliminating the need to remove specimens from their original locations. The focus of this work is twofold: (1) quantify the age distribution of populations of stalagmites to help improve sampling strategies for scientific research; and (2) determine if palaeoclimate information can be gained by analysing the growth frequency variations in populations of stalagmites.
Although the removal of stalagmites cannot be completely avoided using this approach (i.e., for isotopic and trace element studies), our approach reduces the overall number of stalagmites required for collection and provides an assessment of periods of time favourable to stalagmite growth. Where stalagmites cannot be removed because of cultural significance, this “first pass” may provide the only source of palaeoclimate information.
The simplest palaeoclimate proxy that can be provided by a stalagmite is its presence (Gascoyne et al., 1983). Carbonate stalagmite growth is generated by in-cave CO2 degassing that triggers the precipitation of calcium carbonate from a solution to an in-cave surface. For the occurrence of stalagmite formation, there must be a solvent (liquid water) and a solute (typically limestone or dolomite). High concentrations of dissolved CO2 in percolating waters are required for the dissolution of calcium carbonate host rocks and typically form via root and microbial respiration in the soil above the karst; however, they may also form by dissolution of soluble acidic minerals (Atkinson, 1983, Spötl and Mangini, 2007). The presence of a stalagmite is therefore dependent (in most cases) on the presence of soil and sufficient liquid water, which represents the simplest type of palaeoclimate information.
In an early study, Geyh (1970) used radiocarbon dating to compile histograms of speleothem growth density over the last 50 ka. Ayliffe et al. (1998) produced a 500,000-year precipitation record for southeastern Australia using speleothem growth as an indicator of wetter climates. Here, stadials and cool interstadials correspond to greater effective precipitation because of reduced evaporation rates. Similarly, Wang et al. (2004) found that periods of speleothem growth in a semi-arid region of Brazil correlated with local maxima in autumn insolation and millennial-scale Heinrich events over the last 200,000 years. They attributed speleothem growth to increased rainfall during southward displacements of the InterTropical Convergence Zone (ITCZ). Vaks et al., 2006, Vaks et al., 2010 mapped isohyets (lines of equal rainfall) in southern Israel to reveal wet periods in the area over the last two glacial cycles. Holocene speleothems in the region only grow where there is more than 300–350 mm of annual rainfall, and the age distribution of older stalagmites provides an estimate of the location of the 200–275 mm isohyet (Vaks et al., 2006). Vaks et al. (2013) repeated this experiment on Siberian stalagmites and inferred that periods of permafrost thawing allowed for flowing water and speleothem growth.
In wetter climatic settings where speleothem growth may not cease because of climate variability, the frequency distribution of stalagmites can yield similarly important information on climates of the past. Gordon et al. (1989) used 341 U–Th dates on stalagmites from the United Kingdom to constrain the timing of ten interstadials over the past 220,000 years. Hennig et al. (1983) compiled a global data set of 805 speleothem and travertine dates that showed high numbers of stalagmites during the last interglacial. Baker et al. (1993) compiled 520 U–Th dates to show that speleothem growth frequency is highly correlated with the marine oxygen isotope record over the past 160 kyr. In this case, the frequency of growing stalagmites was interpreted as resulting from a combination of changes in mobile groundwater supply and temperature modulating soil CO2 production.
The distribution of speleothems over time must be understood to improve our sampling “footprint” — only those stalagmites strictly necessary for scientific progress should be collected. Certain research groups take the commendable approach of only removing stalagmites that have been broken naturally (McDonnell, 2013), but this is rarely practicable for the production of a long and continuous palaeoclimate record.
As an alternative, Frappier (2008) outlined a four-step approach for identifying stalagmites that are likely to yield accurate proxy records of the desired palaeoclimate variable with a high signal-to-noise ratio. This method involves a step-wise screening process that gradually decreases in scale from the karst region to the cave and stalagmite required for analysis. Data gathered during this process, such as monitoring the geochemistry of cave drip waters, provide important additional information for the interpretation of geochemical proxies.
Stalagmites that satisfy all of the above criteria may still be rejected on ethical grounds if they are aesthetically important to cave tourism or the cultural heritage of the site. It is also possible, in certain cases, to reinstall stalagmites in their cave of origin in the event that they are not suitable for palaeoclimate study or after analysis if the stalagmites are cored rather than slabbed (Dorale et al., 1992). For example, high-resolution X-ray computed tomography can be used in the laboratory to assess the integrity of internal structures before opening the stalagmite (Mickler and Ketcham, 2004, Walczak et al., 2015). If the sample is suitable for geochemical analysis, this step helps to reduce wastage caused by misplaced cuts and improves results by ensuring that the central growth axis is accurately sampled.
Fairchild and Baker (2012) provide examples of good practice and suitable protocols for the archiving and protection of speleothem materials in the laboratory and storage using techniques largely borrowed from the archaeological community (Brown, 2011). Finally, resampling previously explored caves should be avoided by sharing archived material and storing data in online repositories, such as that of NOAA or the PANGAEA archive. The archived half of a speleothem should always be kept pristine so that non-destructive analyses can be conducted in the future.
In this study we propose an additional method by which speleothem researchers can minimise their impacts on cave systems. We utilise only the ages of minimally destructive mini-cores extracted in situ from the bases of stalagmites to recreate previous studies of speleothem growth history (e.g., Ayliffe et al., 1998, Vaks et al., 2010) without using data that require wholesale collection of stalagmites.
Section snippets
Materials and methods
Our case study is based on U–Th dates for 77 individual stalagmites drilled in situ in 2009 and 2011 in caves in the tower karst terrain of southwest Sulawesi. This study utilises basal cores from all stalagmites believed to be suitable for further palaeoclimate analysis found in 25 different caves located in and around Bantimurung-Bulusaraung National Park in the Maros Regency, South Sulawesi (4°54′S 119°45′E, Fig. 1a). In 12 of these caves, no specimens were found. Stalagmites were collected
Results
The basal U–Th ages of the 77 stalagmites span the last five glacial–interglacial cycles, with the oldest stalagmite dating to 529 ± 30 ka (thousand years before the present, where present is AD 1950) (Table S1). In the first instance, all of the dated samples are included in our results regardless of the amount of 230Th contamination and the size of the resulting age errors to avoid introducing bias. Under this scenario, the number of younger stalagmites greatly exceeds the number of older
Discussion
To explain the observed distribution of speleothem ages through time, we have introduced a simple model for exploring stalagmite natural attrition and variable growth phases. This model is applicable to the Sulawesi data set as well as to the data presented below for Korea (Jo et al., 2014) and Australia (Ayliffe et al., 1998).
Conclusions
We have shown that the distribution of stalagmites that survive to the present day shows a previously unrecognised first-order exponential relationship. This suggests that the combined effect of karst processes that govern the natural attrition of stalagmites is generally near-constant on > 100 kyr scales. Natural attrition of stalagmite populations is driven by downwards erosion, cave-collapse, burial of older material, dissolution by undersaturated groundwater and other weathering and erosional
Acknowledgements
We would like to thank David Heslop for his help with the statistics and for providing insight into the early versions of this paper. We thank the Indonesian Institute of Sciences (LIPI) for logistical support, Engkos Kosasih, Djupriono, and the staff of Bantimurung-Bulusaraung National Park (with special thanks to Saiful) for assistance in the field during the 2009 and 2011 expeditions. We are particularly grateful to Neil Anderson, Dan Zwartz, Garry Smith and Bambang Suwargadi, who provided
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