Tracing carbon isotopes - From the atmosphere to the cave. Summer School on Speleothem Science Jens Fohlmeister

Transcription

1 Tracing carbon isotopes - From the atmosphere to the cave Summer School on Speleothem Science Jens Fohlmeister Heidelberg,

2 Villars d 13 C record d 13 C reflects vegetation signal warm, more vegetation less vegetation, cold Genty et al., 2003

3 St. Micheals d 13 C record d 13 C signal driven by cave ventilation strong ventilation weak ventilation Mattey et al., 2008

4 4 Stalagmites from Bunker Cave short term-trends and offset (between red and black signal) can be explained difficult to explain the long-term trend Fohlmeister et al., 2012

5 Himalayan stalagmites high d 13 C values (~+2 ) large amplitudes and fast changes unpublished data

6 Formation of stalagmites precipitation δ 13 C CO 2 enrichment of meteoric water (CO 2 + H 2 O H 2 CO 3 ) carbonate dissolution (H 2 CO 3 + CaCO 3 Ca HCO - 3) CO 2 degassing and carbonate precipitation

7 Evolution of carbon isotopes in soil and drip water d 13 C = ( 13 C/ 12 C sample / 13 C/ 12 C std - 1) * 1000 δ 13 C

8 Atmospheric/soil CO 2 exchange- d 13 C atmospheric d 13 C ~ -8 Photosynthesis, e g C3 plant ~ -17 respired soil CO 2 ~ root respiration microbial decomposition soil air d 13 C is heavier than respired C due to diffusion of soil CO 2 to the soil surface typical values for soil respiration of grassland during growing season: 6-9 mmol/m 2 /hr Cerling, 1984

9 13 CO 2 in the soil Frisia et al., 2011

10 Evolution of carbon isotopes in soil and drip water d 13 C = ( 13 C/ 12 C sample / 13 C/ 12 C std - 1) * 1000 δ 13 C

11 Carbon isotope fractionation during phase transitions (e.g. between liquid and vapour) or from one compound to another (CO 2 into plant organic matter) or between two compounds in chem. equations (CO 2,g HCO - 3) fractionation factor (ε) is given in [ ] for 13 C 13 ε Transition from HCO - 3 with known δ 13 C to CO 2,g : δ 13 C CO2,g = δ 13 C HCO ε HCO3 CO2,g

12 Carbon isotope fractionation during phase transitions (e.g. between liquid and vapour) or from one compound to another (CO 2 into plant organic matter) or between two compounds in chem. equations (CO 2,g HCO - 3) fractionation factor (ε) is given in [ ] for 13 C 13 ε e.g. transition from HCO - 3 with known δ 13 C to CO 2,g : δ 13 C CO2,g = δ 13 C HCO ε HCO3 CO2,g temperature dependent DI 13 C species dependent (pco 2 level) IAEA, 2000 Peytraube et al., 2013

13 Evolution of carbon isotopes in soil and drip water d 13 C = ( 13 C/ 12 C sample / 13 C/ 12 C std - 1) * 1000 δ 13 C

14 Host rock dissolution open system water in contact with soil air gas exchange carbon isotope equilibration closed system water without no contact to soil air limited (no) gas exchange no carbon isotope equilibration DIC vs gaseous CO 2 soil matrix carbon exchange carbonate dissolution

15 Evolution of carbon isotopes in soil and drip water d 13 C = ( 13 C/ 12 C sample / 13 C/ 12 C std - 1) * 1000 δ 13 C

16 Maximum d 13 C range Maximum, potential variation literature Vegetation type (C3 vs C4) Water stress (C3 and C4) Vegetation density - diffusion Vegetation density - chemical equilibrium (water) Carbonate dissolution e.g.: Wickman, 1952; Baertschi, 1953; Craig, 1963; Bender, 1968 e.g.: Bowling et al., 2002; Hartman and Danin, 2010; Buchmann et al., 1996 e.g.: Cerling, 1984 e.g.: Hendy, 1970; 1971 Temperature - fractionation (CO 2 HCO - 3 CaCO 3 ) cave processes (PCP; drip rate; pco 2 difference, evaporation) e.g.: Mühlinghaus et al., 2007;2009; Dreybrodt, 2008; Scholz et al., 2009; Dreybrodt and Scholz, 2011; Deininger et al., 2012

17 Evolution of carbon isotopes in soil and drip water d 13 C = ( 13 C/ 12 C sample / 13 C/ 12 C std - 1) * 1000 δ 13 C

18 Evolution of carbon isotopes in soil and drip water a 14 C = ( 14 C/ 12 C sample / 14 C/ 12 C std ) * 100 d 13 C = ( 13 C/ 12 C sample / 13 C/ 12 C std - 1) * 1000 δ 13 C

19 Evolution of carbon isotopes in soil and drip water a 14 C = ( 14 C/ 12 C sample / 14 C/ 12 C std ) * 100 d 13 C = ( 13 C/ 12 C sample / 13 C/ 12 C std - 1) * 1000 δ 13 C Dead Carbon Fraction (DCF): DCF = (1-14 C stal / 14 C atm )*100

20 Maximum d 13 C and 14 C range Maximum, potential variation [ ] literature Vegetation type (C3 vs C4) Water stress (C3 and C4) Vegetation density - diffusion Vegetation density - chemical equilibrium (water) Carbonate dissolution Temperature - fractionation (CO 2 HCO - 3 CaCO 3 ) cave processes (PCP; drip rate; pco 2 difference, evaporation) e.g.: Wickman, 1952; Baertschi, 1953; Craig, 1963; Bender, 1968 e.g.: Bowling et al., 2002; Hartman and Danin, 2010; Buchmann et al., 1996 e.g.: Cerling, 1984 e.g.: Hendy, 1970; 1971 e.g.: Mühlinghaus et al., 2007;2009; Dreybrodt, 2008; Scholz et al., 2009; Dreybrodt and Scholz, 2011; Deininger et al., 2012

21 coupled d 13 C- radiocarbon systematics ageing of the peat bog layer above the cave Genty et al., 2001

22 coupled d 13 C- radiocarbon systematics Rudzka et al., 2011 variations in carbon isotopes due to climate-driven changes in soil carbon dynamics

23 14 C in a stalagmite from Flores Griffiths et al., 2012 B 1,520 a BP 2,480 a BP Flores sensitive to ITCZ changes cave covered by tropical rainforest No major vegetation changes 25 C mean T (constant in tropics +/- 1K) 24 cm A 70 a BP 1,160 a BP 11,550 a BP 12,640 a BP 2,780 a BP Hiatus 3,710 a BP

24 Time [ka BP] 14 C in stalagmite LR06-B1 atmospheric 14 C a 14 C stal. init [pmc] stalagmite 14 C

25 14 C as a tracer for precipitation? 14 C (DCF) in comparison with geochemical proxies + DCF = (1-14 C stal / 14 C atm )*100% more precipitation less Mg/Ca (carbonate dissolution dynamics), lighter δ 18 O (amount effect) and lighter δ 13 C (less disequilibrium fractionation (HCO 3 CaCO 3 ) in cave) rainfall high DCF low DCF more precipitation less precipitation -

26 14 C as a tracer for precipitation? 14 C (DCF) in comparison with geochemical proxies + DCF = (1-14 C stal / 14 C atm )*100% more precipitation less Mg/Ca (carbonate dissolution dynamics), lighter δ 18 O (amount effect) and lighter δ 13 C (less disequilibrium fractionation (HCO 3 CaCO 3 ) in cave) rainfall high DCF low DCF more precipitation less precipitation -? What is responsible for the DCF variations in this stalagmite? disentangling the influence of the individual processes

27 Influence of vegetation

28 Total DCF total DCF = (1-14 C stal / 14 C atm )*100

29 DCF without vegetation total DCF = (1-14 C stal / 14 C atm )*100 DCF no veg = (1-14 C stal / 14 C soil air )*100

30 Fractionation in soil water total DCF = (1-14 C stal / 14 C atm )*100 DCF no veg = (1-14 C stal / 14 C soil air )*100 DCF no veg&fract = (1-14 C stal / 14 C initial DIC )*100

31 Host rock contribution and in-cave fractionation δ 13 C

32 14 C as a tracer for precipitation more rainfall more host rock contribution to CaCO 3 of stalagmite more closed carbonate dissolution conditions higher DCF + rainfall -

33 14 C as a tracer for precipitation more rainfall + more host rock contribution to CaCO 3 of stalagmite more closed carbonate dissolution conditions rainfall higher DCF -

34 fractionation within the cave stalagmite d 13 C data at 14 C sample depths difference of both radiocarbon and d 13 C provide insight into in-cave fractionation processes (PCP, drip rate, ) calculated drip water isotopic composition at Ca 2+ saturation state

35 Explaining the 14 C variability during the last century

36 Explaining the 14 C variability during the last century + rainfall -

37 summary multiple potential causes for changes in δ 13 C together δ 13 C and 14 C are mighty tracers to disentangle soil processes radiocarbon is sensitive to precipitation changes both C isotopes provide insight to cave related processes helpful for modelling