Speleothems (a.k.a. the tropical ice cores)

Cave deposits Laminated structure of CaCO 3 Laminated structure of CaCO 3 Evaporation-deposition Evaporation-deposition Slow CO 2 degassing == minimal disequilibrium with drip waters Slow CO 2 degassing == minimal disequilibrium with drip waters Rapid CO 2 degassing == disequilibrium precipitation Rapid CO 2 degassing == disequilibrium precipitation Possibility for water-soil or water-rock chemical equilibration == undermines reliability of speleothems as paleothermometers Possibility for water-soil or water-rock chemical equilibration == undermines reliability of speleothems as paleothermometers Schwartz (2007)

Geochemistry of speleothems T (°C) = (  18 O ct -  18 O w ) (  18 O ct -  18 O w ) 2 (Epstein et al. 1953) T (°C) = (  18 O ct -  18 O w ) (  18 O ct -  18 O w ) 2 (Epstein et al. 1953)  18 O ct would also reflect precipitation patterns  18 O ct would also reflect precipitation patterns Caveat: the “amount” effect in  18 O w (increase in  18 O during heavy rain events) Caveat: the “amount” effect in  18 O w (increase in  18 O during heavy rain events) Age control: Age control: 14C - have to correct for nonradioactive C introduced into DIC from dissolution of calcite in the soil zone 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 U/Th - have to account for detrital 230Th in ‘dirty’ calcite

Geochemistry of speleothems Have to estimate  18 O of drip water: Have to estimate  18 O of drip water: Close to annual average of  18 O ppt Close to annual average of  18 O ppt But local alteration due to surface evaporation, transpiration, selective recharge of seasonal precipitation (snow meltwater) But local alteration due to surface evaporation, transpiration, selective recharge of seasonal precipitation (snow meltwater) Some assumptions and ways to estimate  w: Some assumptions and ways to estimate  w: Secular variation in  18 O ppt = annual (seasonal) variation (assumes a particular rate of change with time) Secular variation in  18 O ppt = annual (seasonal) variation (assumes a particular rate of change with time) Analysis of fluid inclusions in the speleothems (  D of the water should be unchanged) Analysis of fluid inclusions in the speleothems (  D of the water should be unchanged)  D ppt = 8  18 O ppt +  0 (  0 is deuterium excess, 10permil)  D ppt = 8  18 O ppt +  0 (  0 is deuterium excess, 10permil) Using models for variation in  18 O ppt (derivation of water vapor from seawater with known variability in  18 O) Using models for variation in  18 O ppt (derivation of water vapor from seawater with known variability in  18 O)

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

But what about the carbon? Fractionation between DIC and calcite is very small; so,  13 C reflecting isotopic composition of drip water Fractionation between DIC and calcite is very small; so,  13 C reflecting isotopic composition of drip water  13 C in calcite affected by:  13 C in calcite affected by: C3/C4 plants balance; C4 - higher  13 C C3/C4 plants balance; C4 - higher  13 C Plant vegetation density above cave -- dissolution by reaction w/ atm = higher  13 C Plant vegetation density above cave -- dissolution by reaction w/ atm = higher  13 C  13 C increases with decreasing rainfall due to decrease in the contribution of HCO 3 - to the drip water (Frumkin et al. 1999,2000)