Hydrological control of the dead carbon fraction in a Holocene tropical speleothem |
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| Institution: | 1. Department of Earth System Science, University of California, Irvine, CA 92697-3100, USA;2. Heidelberg Academy of Sciences, c/o Institute for Environmental Physics, INF 229, 69120 Heidelberg, Germany;3. School of Land and Environment, The University of Melbourne, Parkville, VIC 2010, Australia;4. Australian Nuclear Science and Technology Organisation (ANSTO), Locked Bag 2001, Kirrawee DC NSW 2232, Australia;5. School of Earth Sciences, The University of Melbourne, Parkville, Victoria 2010, Australia;6. Research School of Earth Sciences, The Australian National University, Canberra, ACT 0200, Australia;7. Centre for Microscopy and Microanalysis, The University of Queensland, Brisbane, QLD 4072, Australia;1. Key Laboratory of Karst Dynamics, MLR & Guangxi, Institute of Karst Geology, Chinese Academy of Geological Sciences, Guilin 541004, China;2. School of Geography Sciences, Southwest University, Chongqing 400715, China;3. Sichuan Earthquake Administration, Chengdu 610041, China;1. ANSTO, Lucas Heights, Sydney 2234, Australia;2. Connected Waters Initiative Research Centre, UNSW Sydney, Kensington, Sydney 2052, Australia;3. Faculty of Geosciences, Tübingen University, Tübingen 72074, Germany;4. Institute of Earth and Environmental Science, University of Potsdam, Karl-Liebknecht-Strasse 24, D-14476 Potsdam, Germany;5. GFZ German Research Centre for Geosciences, Section Climate Dynamics and Landscape Development, Telegrafenberg, D-14473 Potsdam, Germany;6. School of Biological, Earth and Environmental Sciences, UNSW Sydney, Kensington, Sydney 2052, Australia;7. School of Civil and Environmental Engineering, UNSW Sydney, Kensington, Sydney 2052, Australia;1. Laboratoire des Sciences du Climat et de l''Environnement (LSCE/IPSL), Laboratoire CEA/CNRS/UVSQ, Gif-sur-Yvette, France;2. Faculty of Earth and Life Sciences, VU University Amsterdam, The Netherlands;3. Institute of Global Environmental Change, Xi''an Jiaotong University, Xi''an, China;4. Department of Earth Sciences, University of Minnesota, Minneapolis, MN, USA;1. Department of Geosciences, 611 North Pleasant Street, University of Massachusetts, Amherst, MA 01003, USA;2. Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA;3. Department of Anthropology, 240 Hicks Way, University of Massachusetts, Amherst, MA 01003, USA;4. Mention Bassins Sédimentaires, Evolution, Conservation (BEC), BP 906, Faculté des Sciences, Université d’Antananarivo, 101 Antananarivo, Madagascar;1. Max Planck Institute for Evolutionary Anthropology, Department of Human Evolution, Deutscher Platz 6, 04103 Leipzig, Germany;2. State Nature Conservancy of the Slovak Republic, Slovak Caves Administration, Hod?ova 11, 031 01 Liptovský Mikulá?, Slovakia;3. Catholic University in Ru?omberok, Pedagogical Faculty, Department of Geography, Hrabovská Cesta 1, 034 01 Ru?omberok, Slovakia;4. Department of Earth Sciences, University College London, WC1E 6BT, UK;5. MARUM, University of Bremen, Bremen 28359, Germany;6. Institute of Geological Sciences, Polish Academy of Sciences, Twarda 51/55 St., 00-818 Warszawa, Poland |
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| 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. |
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