Determining the drivers of oxygen and carbon isotope fractionation in a monitored Bahamas cave

Abstract:

Traditionally, carbon and oxygen isotope analyses of speleothems have been used to unravel paleoclimate. Interpretation of the δ¹⁸O of the carbonate is complex and recent studies indicate the temperature dependent oxygen isotopic fractionation of cave calcites may be greater than previous laboratory studies indicated.

The goal of this study is to shed light on the environmental and kinetic drivers of the δ¹⁸O and δ¹³C kinetic isotope fractionation in a cave located on Eleuthera, Bahamas. The cave monitoring includes experiments in which the δ¹⁸O and δ¹³C of calcite precipitated from drip water could be studied by placing microscope slides and concave watch glasses on top of active stalagmites, in addition to analysis of temperature, relative humidity, geochemical analyses of the drip waters, cave air, and calcite. The precipitated calcite was deposited over ~three month period and could be considered a single layer as the calcite all precipitated approximately at the same time and under the same conditions.

Results demonstrate minimal fractionation for the calcite deposited from the microscope slides whereas the calcite on the watch glasses, intended to represent the geometry of a stalagmite, show a greater increase in δ¹⁸O and δ¹³C values with increasing distance from the center. Heavier isotopic values at the edges are driven by increased degassing and fractionation. Overall, the oxygen isotopes of the calcite demonstrate the best agreement with the temperature dependent fractionation equation from Tremaine et al. (2011. Geochim. Cosmochim. Ac.75,17 4929-4950). However, even samples which are thought to be precipitated in equilibrium are still offset, possibly driven by varying degrees of kinetic fractionation within the cave. Results demonstrate the portion of the cave which is the least ventilated is characterized by the highest growth rate and produces calcites which are closest to equilibrium for δ¹⁸O. This suggests that growth rate and ventilation may be primary drivers of the decreased fractionation within this cave. These results support previous findings that the temperature dependent oxygen isotopic fractionation of cave calcites may be greater than laboratory studies indicated and that stalagmite samples are best selected from deeper sections of the cave for geochemical analyses.