Research paperRobust chronological reconstruction for young speleothems using radiocarbon
Highlights
► We had problems in dating two young speleothems using the Th/U method. ▶ We built 14C-based chronologies for these spels using different age-depth models. ▶ Excellent agreement between the outputs of these models was observed. ▶ Reliable 14C-based chronologies for these speleothems were achieved. ▶ Radiocarbon can be used to build robust chronologies for young spels.
Introduction
The number of paleoclimate records based on speleothems has increased dramatically in recent years. These records have significantly contributed to a better understanding of climate variability during the Late Pleistocene (Wang et al., 2001; Cheng et al., 2009; Drysdale et al., 2009; Wagner et al., 2010), the Late Glacial/Early Holocene transition (Wang et al., 2001; Griffiths et al., 2009, 2010), and the Holocene (McDermott et al., 2001; Wang et al., 2005; Johnson et al., 2006). The Th/U dating method is usually employed to build precise and reliable chronologies for speleothems (Li et al., 1989; Richards and Dorale, 2003; Hellstrom, 2006; Hoffmann et al., 2007). However, for some speleothems U-series dates may not be useful due to low uranium concentrations (<10 ppb), insufficient 230Th, or because of multiple sources of 230Th. In such cases radiocarbon can be used as an alternative method for dating speleothems. It is well recognised that the radiocarbon age of speleothems is usually greater than that of other contemporaneous terrestrial samples drawing carbon from atmospheric CO2 (Vogel and Kronfeld, 1997; Goslar et al., 2000; Beck et al., 2001). This is due to the contribution of 14C-depleted material from bedrock and aged soil organic matter (SOM) leading to a radiocarbon reservoir age for speleothems, the magnitude of which is termed the dead carbon fraction (DCF; Genty and Massault, 1999; Genty et al., 2001). If DCF variations in a speleothem are well constrained, reliable chronological reconstruction for such speleothem based on radiocarbon can be achieved (Hua, 2009).
We have recently studied two young speleothems, SC4 from Smiths Cave (Christmas Island, eastern Indian Ocean) and WM7 from Wollondilly Cave (Wombeyan caves, SE Australia), with the aim of gaining a better understanding of past climate and rainfall variability beyond the record of instrumental weather measurements. In this paper, after discussing problems in the dating of these stalagmites by the Th/U method, we present our chronological reconstruction for SC4 and WM7 using radiocarbon based on different age-depth models. We then discuss the relevance of DCF values and the integrity of the models used in the reconstruction of reliable radiocarbon-based chronologies for these young speleothems.
Section snippets
SC4 stalagmite
SC4 was collected ∼150 m from the entrance of Smiths Cave (10°30ʹS, 105°34ʹE) on the north coast of Christmas Island in November 2004. This shallow cave system (∼20–30 m depth) has formed in Late Tertiary to Quaternary age limestone. Present-day mean temperatures vary between 22.5 and 27.8 °C with average precipitation and humidity of 2154 mm/yr and 80–90%, respectively. Vegetation above the cave consists mainly of open, semi-deciduous rainforest developed on shallow terra rossa soils. SC4 is a
Sample collection and analysis
U–Th analyses were undertaken by multi collector-inductively coupled plasma mass spectrometry (MC-ICP-MS) at The University of Melbourne. For WM7, carbonate prisms of 4–5 mm in thickness were collected for Th/U dating. For SC4 powdered dating samples were extracted by trenching using a 1 mm drill bit driven by a micromilling lathe, with the exception of the basal sample, which was a solid fragment removed from the fracture surface. Samples were dissolved in HNO3 and spiked with a mixed 229Th/233
Radiocarbon analysis
Carbonate powders along the growth axis of SC4 and WM7 were collected for accelerator mass spectrometry (AMS) 14C analysis using a Micromill 2000 LE-ER system and a 0.5-mm tungsten carbide milling bit. SC4 was continuously milled at 200-μm intervals for the top 12 mm, and at 1 mm intervals between 12 mm and 180 mm from the tip. A total of 44 carbonate samples were analysed for 14C. For WM7, powder samples were collected along tracks A and B at 100- and 200-μm resolution, respectively (Fig. 1b).
AMS 14C results
The AMS 14C results are presented in Table 1 and illustrated in Fig. 2. For SC4, 14C content increases from 87 pMC near to the base of the stalagmite at 169.5 mm in depth (distance from tip) to 92.5 pMC at a depth of 75.5 mm. The 14C content fluctuates around the latter value of 92.5 pMC between 19.5 mm and 9.1 mm, suggesting similar 14C values between 75.5 and 19.5 mm, as no samples in this portion of SC4 were collected for radiocarbon analysis. The 14C value then increases from 93 pMC at
Variations in DCF
Temporal variations in speleothem DCF were documented in the literature. According to Genty et al. (2001), these variations are mainly due to changes in the age of soil CO2 (in particular ages of SOM) and/or in the relative contribution of soil CO2 and bedrock carbonate to speleothems possibly associated with climatic and environmental changes. In addition, changes in soil-to-speleothem carbon transfer dynamics, such as temporal changes between open- and closed-system conditions, can result in
Conclusions
We have built chronologies for two young speleothems using dense sequences of radiocarbon dates with different age-depth models. Two different approaches were employed to estimate the DCF values for the pre-bomb period. For SC4, the pre-bomb DCF value of 5.8 ± 0.7% (1σ) was derived from the timing of 14C dates estimated by high-resolution δ18O recorded in the speleothem and the timing of the onset of bomb 14C. For WM7, a “maximum” range of pre-bomb DCF of 6.3–7.7% (1σ) was estimated. This range
Acknowledgements
AMS 14C measurements were supported by Ainse grants (08/046, 09/088 & AINSTU1007) and ANSTO's Cosmogenic climate Archives of the Southern Hemisphere (CcASH) Project. We would like to thank the Sydney Catchment Authority and the Australian Research Council Linkage Projects and Linkage Infrastructure, Equipment and Facilities scheme and the University of Newcastle for generous financial support. We also thank the staff at Wombeyan Caves and Parks Australia North (Christmas Island) for their
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