Trade winds drive pronounced seasonality in carbonate chemistry in a tropical Western Pacific island cave—Implications for speleothem paleoclimatology

Abstract

Carbon dioxide concentrations in caves are a primary driver of rates of carbonate dissolution and precipitation, exerting strong control on speleothem growth rate and geochemistry. Long-term cave monitoring studies in midlatitude caves have observed seasonal variability in cave pCO 2, whereby airflow is driven by temperature contrasts between the surface and subsurface. In tropical settings, where diurnal temperature cycles are larger than seasonal temperature cycles, it has been proposed that caves will ventilate on daily time scales, preventing cave pCO 2 from increasing substantially above atmospheric pCO 2 . By contrast, the relatively small temperature difference between the surface and subsurface may be insufficient to drive complete ventilation of tropical caves. Here we present results of an 8 year cave monitoring study, including observations of cave pCO 2 and carbonate chemistry, at Jinapsan Cave, Guam (13.48N, 144.58E). We find that cave pCO 2 in Jinapsan Cave is both relatively high and strongly seasonal, with cave pCO 2 ranging from 500 to 5000 ppm. The seasonality of cave pCO 2 cannot be explained by temperature contrasts, instead we find evidence that seasonal trade winds drive cave ventilation and modulate cave pCO 2 . Calcite deposition rates at seven drip sites in Jinapsan Cave are shown to be seasonally variable, demonstrating that speleothem growth rates in Jinapsan Cave are strongly affected by seasonal variations in cave pCO 2 . These results highlight the importance that advection can have on cave ventilation processes and carbonate chemistry. Seasonality in carbonate chemistry and calcite deposition in this cave affect the interpretation of speleothem-based paleoclimate records.

Dripwater and Calcite Geochemistry Variations in a Monitored Bahamas Cave

“…On shorter timescales, we note that the periods of lower δ 13 C CO2 were observed in June 2012, December 2012, November 2013, and April 2014. Such periods of decreased ventilation could be driven by variations in soil δ 13 C CO2 value, temperature, and wind direction (James et al, ; Noronha et al, ). Studies have also shown that seasonally, ventilation can be impacted by the temperature difference between the cave and the outside.…”

“…Within the cave, atmospheric CO 2 ventilation may drive variations in the δ 13 C values of the CO 2 and degassing processes can affect the δ 13 C values of the speleothem calcite similar to oxygen isotopes (Bergel et al, ; Meyer et al, ). Monitoring studies show that cave ventilation can vary seasonally (Mattey et al, , ; Noronha et al, ; Vieten et al, ).…”

Pipes to Earth's subsurface: the role of atmospheric conditions in controlling air transport through boreholes and shafts

“…Our results indicate that changes in atmosphere barometric pressure determine the advective airflow direction. This is presumably due to the difference between the barometric pressure and the cavity pressure caused by the vertical propagation of the barometric pressure Neeper and Stauffer, 2012;Perina, 2014;You et al, 2011). In the case of positive barometric pressure changes (i.e., increase in barometric pressure over time), the barometric pressure will be greater than the cavity pressure and thus inflow of air will develop.…”

“…Understanding air transport between the Earth's subsurface and the atmosphere is a key element in the study of surface and near-surface biological, chemical, and physical processes. Air transport between the Earth and the atmosphere is commonly driven by diffusive and advective mechanisms (Choi and Smith, 2005;Ganot et al, 2014;Hillel, 1998;Kawamoto et al, 2006;Kuang et al, 2013;Noronha et al, 2017;Sánchez-Cañete et al, 2013) and has mainly been studied within soils (e.g., Allaire et al, 2009;Bayer et al, 2017;Choi and Smith, 2005;Zeng et al, 2017). However, as different types of natural or artificial boreholes and shafts also exist (e.g., Berthold and Börner, 2008;Kang et al, 2014Kang et al, , 2015, understanding the mechanisms that govern air and green-house gas (GHG) transport in such conduits is also important (Berthold and Börner, 2008;Perrier et al, 2005).…”

“…These underground cavities can be more than 1 order of magnitude larger than the connecting conduit. Advective air transport in such boreholes or shafts can be governed mainly by (1) barometric pumping (BP), which is the inflow and outflow motion of subsurface air due to pressure gradients governed by changes in barometric pressure (Ellerd et al, 1999;Neeper, 2002;Neeper and Stauffer, 2012;Perina, 2014;Perrier and Le Mouël, 2016;Rossabi and Falta, 2002;Thorstenson et Published by Copernicus Publications on behalf of the European Geosciences Union. 1142 E. Levintal et al: Pipes to Earth's subsurface: the role of atmospheric conditions in controlling air transport al., 1998), and (2) density-induced convection (DIC), which develops when there are unstable density gradients resulting from temperature and air composition differences between the atmospheric air and the borehole or underground cavity (Ganot et al, 2012;Nachshon et al, 2008;Perrier et al, 2002;Weisbrod et al, 2009;Weisbrod and Dragila, 2006).…”