Abstract
Over the past decade, a number of speleothem studies have used radiocarbon (14C) to address a range of palaeoclimate problems. These have included the use of the bomb pulse 14C to anchor chronologies over the last 60 years, the combination of U-Th and 14C measurements to improve the radiocarbon age-calibration curve, and linking atmospheric 14C variations with climate change. An issue with a number of these studies is how to constrain, or interpret, variations in the amount of radioactively dead carbon (i.e. the dead carbon fraction, or DCF) that reduces radiocarbon concentrations in speleothems. In this study, we use 14C, stable-isotopes, and trace-elements in a U-Th dated speleothem from Flores, Indonesia, to examine DCF variations and their relationship with above-cave climate over the late Holocene and modern era. A strong association between the DCF and hydrologically-controlled proxy data suggests that more dead carbon was being delivered to the speleothem during periods of higher cave recharge (i.e. lower δ18O, δ13C and Mg/Ca values), and hence stronger summer monsoon. To explore this relationship, we used a geochemical soil-karst model coupled with 14C measurements through the bomb pulse to disentangle the dominant components governing DCF variability in the speleothem. We find that the DCF is primarily controlled by limestone dissolution associated with changes in open- versus closed-system conditions, rather than kinetic fractionation and/or variations in the age spectrum of soil organic matter above the cave. Therefore, we infer that periods of higher rainfall resulted in a higher DCF because the system was in a more closed state, which inhibited carbon isotope exchange between the karst water dissolved inorganic carbon and soil-gas CO2, and ultimately led to a greater contribution of dead carbon from the bedrock.
Highlights
Introduction and backgroundCave 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)
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)
The detrital-thorium corrected ages for the upper and lower portions of the Fifteen calcite cubes weighing w100 mg were extracted every w5 mm along the central growth axis of LR06-B1 between 345 and 422 mm (i.e. 2.4e2.8 ka) from the top of the stalagmite; of these 15 samples, 11 were splits of calcite used for U-Th dating
Summary
Introduction and backgroundCave 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). 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 Some of the earliest speleothem research employed 14C as a dating tool (Broecker et al, 1960; Hendy and Wilson, 1968), but up until recently it has played a very minor role in speleothem research because of the ‘dead carbon’ problem. This problem stems from the inclusion of bedrock-derived, ‘radioactively dead’ carbon into the total dissolved inorganic carbon (DIC) pool that is subsequently encoded into a growing speleothem.
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