A global model for cave ventilation and seasonal bias in speleothem paleoclimate records

Abstract

Cave calcite deposits (speleothems) provide long and continuous records of paleoenvironmental conditions in terrestrial settings. Typical environmental proxy measurements include speleothem growth rate and variations in elemental and isotope geochemistry. Commonly the assumption is made that speleothems grow continuously and at a constant rate throughout the year. However, seasonal variation of growth rate may be the rule in many caves. Here we apply observations of modern calcite growth and cave-air CO 2 concentrations and a model of factors controlling cave ventilation to construct a global model predicting where cave calcite growth may be seasonal. Previous models and measurements of calcite precipitation in caves demonstrate the retardation of speleothem growth by high cave-air CO 2 . Elevated CO 2 is commonly dissipated by ventilation driven by density differences between cave and surface air. Seasonal cycles in atmospheric temperature, pressure, and humidity commonly drive these density contrasts. Modeling these changes latitudinally and globally indicates a geographic control on seasonal cave ventilation and thus on a principal controlling factor of speleothem growth. The model predicts that given constant water, calcium, and CO 2 inputs, speleothems from temperate to boreal continental regions commonly accumulate more calcite in the cool season and less or none in the warm season. These models predict that proxies from temperate to boreal speleothems may be seasonally biased due to seasonal ventilation, whereas tropical and maritime records should not.

Late-glacial to late-Holocene shifts in global precipitation δ<sup>18</sup>O

“…There exist roughly twice as many groundwater reconstructions of late-glacial to late-Holocene precipitation δ 18 O shifts (n = 59) as the combined total of speleothem and ice core records (n = 27; where δ 18 O = ( 18 O / 16 O sample ) / ( 18 O / 16 O standard mean ocean water -1) × 1000). A recent global synthesis of paired precipitationgroundwater isotopic data demonstrated that modern annual precipitation and modern groundwater isotope compositions follow systematic relationships with some bias toward winter and wet-season precipitation (Jasechko et al, 2014). Systematic rainfall-recharge relationships shown by Jasechko et al (2014) support our primary assumption in this study that groundwater isotope compositions closely reflect meteoric water.…”

“…A recent global synthesis of paired precipitationgroundwater isotopic data demonstrated that modern annual precipitation and modern groundwater isotope compositions follow systematic relationships with some bias toward winter and wet-season precipitation (Jasechko et al, 2014). Systematic rainfall-recharge relationships shown by Jasechko et al (2014) support our primary assumption in this study that groundwater isotope compositions closely reflect meteoric water. Because groundwater records can only identify climate change occurring over thousands of years due to hydrodynamic dispersion during multi-millennial residence times (e.g., Davison and Airey, 1982;Stute and Deak, 1989), we limit the focus of this study to meteoric water isotope composition changes from the latter half of the last glacial time period to the late-Holocene.…”

Changes in the CO2 dynamics in near-surface cavities under a future warming scenario: Factors and evidence from the field and experimental findings

“…A similar seasonal pattern of the composition of cave atmosphere and its gas exchange with exterior has been previously study by James et al (2015). The process is also confirmed by the 222 Rn concentration, a good indicator of the connexion between the subterranean environment and the atmosphere (Valladares et al, 2014), which, simultaneously to the CO2, suffers a sharp decrease as consequence of the ventilation.…”

Consider a cylindrical cave: A physicist's view of cave and karst science

“…Seasonally alternating stability of the cave atmosphere, as produced by local convection (Fig. 4e), has recently been used to explain seasonal changes in CO 2 concentrations in caves and other subsurface voids that display low CO 2 concentrations in the winter and high concentrations in the summer (Banner et al 2007;Weisbrod et al 2009;Serrano-Ortiz et al 2010;Breecker et al 2012;James et al 2015). It is inferred that the voids have higher exchange rates with the surface atmosphere during winter than in summer.…”