speleothems a k a the tropical ice cores n.
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Speleothems (a.k.a. the tropical ice cores)
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  1. Speleothems (a.k.a. the tropical ice cores)

  2. Cave deposits • Laminated structure of CaCO3 • Evaporation-deposition • Slow CO2 degassing == minimal disequilibrium with drip waters • Rapid CO2 degassing == disequilibrium precipitation • Possibility for water-soil or water-rock chemical equilibration == undermines reliability of speleothems as paleothermometers Schwartz (2007)

  3. Geochemistry of speleothems • T (°C) = 16.5 - 4.3 (d18Oct - d18Ow) + 0.14 (d18Oct - d18Ow)2 (Epstein et al. 1953) • d18Oct would also reflect precipitation patterns • Caveat: the “amount” effect in d18Ow (increase in d18O during heavy rain events) • Age control: • 14C - have to correct for nonradioactive C introduced into DIC from dissolution of calcite in the soil zone • U/Th - have to account for detrital 230Th in ‘dirty’ calcite

  4. Geochemistry of speleothems • Have to estimate d18O of drip water: • Close to annual average of d18Oppt • But local alteration due to surface evaporation, transpiration, selective recharge of seasonal precipitation (snow meltwater) • Some assumptions and ways to estimate dw: • Secular variation in d18Oppt = annual (seasonal) variation (assumes a particular rate of change with time) • Analysis of fluid inclusions in the speleothems (dD of the water should be unchanged) • dDppt = 8 d18Oppt + d0 (d0 is deuterium excess, 10permil) • Using models for variation in d18Oppt (derivation of water vapor from seawater with known variability in d18O)

  5. Paleoclimate studies • Matching changes in North Atlantic climate and South China • Greenland - cooling; coincidentally, enhanced summer monsoon rains in South China, reflected in the isotopes Wang et al. 2001

  6. But what about the carbon? • Fractionation between DIC and calcite is very small; so, d13C reflecting isotopic composition of drip water • d13C in calcite affected by: • C3/C4 plants balance; C4 - higher d13C • Plant vegetation density above cave -- dissolution by reaction w/ atm = higher d13C d13C increases with decreasing rainfall due to decrease in the contribution of HCO3- to the drip water (Frumkin et al. 1999,2000)