Research paperHydrological control of the dead carbon fraction in a Holocene tropical speleothem
Highlights
► We examine the dead carbon fraction (DCF) of a late Holocene tropical speleothem. ► We find a strong association between the DCF and above-cave hydrology. ► More dead carbon delivered to the speleothem during a stronger monsoon and vice versa. ► Soil-karst model reveals that dissolution of host-rock governs DCF variability. ► higher (lower) DCF corresponds with more closed (open)-system conditions.
Section snippets
Introduction and background
Cave carbonates, or speleothems, are rapidly becoming one of the most important and versatile palaeoclimate archives available to Quaternary researchers (e.g., Wang et al., 2008; Drysdale et al., 2009; Dorale et al., 2010; Hoffmann et al., 2010). Consequently, a highly productive and growing speleothem community has emerged, which in the last five years has resulted in a proliferation of new techniques and applications (e.g. Kluge et al., 2008; Blyth et al., 2011), substantial refinements to
Study site and sample description
The stalagmite used in this study (LR06-B1), previously reported in Griffiths et al. (2009, 2010a, 2010b), was collected from Liang Luar, a ∼1.7 km-long cave on the Indonesian island of Flores. The cave is situated ∼550 m above mean sea-level (a.m.s.l.) and has developed within reefal (mainly built by corals) carbonates of Late Miocene-Early Pliocene age, which are mantled in part by volcanics of Quaternary age (Monk et al., 1997). The limestone host-rock contains folded strata that dip between
U-Th dating
A total of 13 pieces of calcite (11 of which weighed ∼50 mg and two weighed ∼300 mg) were extracted from the central growth axis of stalagmite LR06-B1 (Fig. 1). Samples were extracted using a carbide dental burr fitted to an air drill or to a micromilling lathe. 230Th/234U age determinations were obtained using both thermal ionisation mass spectrometry (TIMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS). TIMS 230Th/234U analyses were carried out at the University
U-Th chronology
The stable-isotope, trace-element and DCF (see next section) time series for the late Holocene section of stalagmite LR06-B1, were anchored by a total of 11 U-Th dates (Fig. 2a, Table 1). In addition, the chronology for the upper section of LR06-B1, containing the 14C bomb pulse (i.e. the top ∼6 mm), was constrained by two U-Th dates (Fig. 2b, Table 1).
For all samples, U concentrations range from 189 to 463 ng/g, with the median being 315 ng/g. The 230Th/232Th ratios are relatively high in the
Discussion
Notwithstanding the fractionation effects inside the cave during calcite precipitation, the initial 14C activity of DIC in the soil-water, and hence the speleothem calcite, is typically dominated by two main end members: (i) the 14C of the soil CO2, which is controlled by the decomposition of SOM and plant respiration; and (ii) the C originating from the dissolution of the carbonate host rock, which is typically devoid of any 14C.
In regard to the first end member, the 14C of the total CO2,g is
Conclusions
In this study, we have assessed the dead carbon content of the late Holocene section of speleothem LR06-B1, which was recovered from the island of Flores, Indonesia. DCF values were established using coupled AMS 14C measurements and U-Th dating through an interval of the late Holocene (∼2.4–2.8 ka) marked by a prominent excursion in atmospheric radiocarbon content. In addition, 14C measurements through the pre- and post-bomb periods (∼1940–2000 A.D.) provided the input parameters to a soil-host
Acknowledgements
The authors wish to thank W.S. Hantoro, B. Suwargadi, N. Anderson, G. Smith, J. Rutledge, S. Lewis, E. St Pierre, E. Yulianto, and the Indonesian Institute of Sciences (LIPI) for logistical support and technical assistance with fieldwork, which was carried out under LIPI Research Permit number 2748/SU.3/KS/2007. We also thank members of the AMS team at the Australian Nuclear Science and Technology Organisation (ANSTO) for their help with sample preparation and AMS measurements. This research
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