Fire, Work with Me: A PAH record from a Southwestern US speleothem

Polycyclic aromatic hydrocarbons (PAHs) are molecules produced during incomplete combustion of organic matter and have been increasingly utilized as paleo-proxies for wildfires. More recently, their incorporation from drip water into speleothems has been utilized in conjunction with the stable isotopic and trace elemental measurements of host carbonate and fluid inclusions in order to assess a coupled record of fire and hydroclimate. Numerous studies have focused on cave systems in the Southwestern U.S., which has experienced highly variable hydroclimate and massive wildfires with past climate changes. Here, we present a PAH record covering ~19-11.5 ka obtained from a precisely dated and well-studied ML-1 stalagmite obtained from McLean’s Cave in the central Sierran foothills, CA. Total concentrations of four-ring PAHs reach maximum values from ~16.8-15 ka, associated with the first stage (1a) of Heinrich Stadial 1 (HS1) interval – this is interpreted as increased levels of soil PAHs produced from regional wildfires. Covariance of isomeric diagnostic ratios with total concentration indicates a shift in the nature of the associated fires, separating effects of PAH mobility in altered soils as well as shifts in soil water transport, stalagmite growth rates, and precipitation amounts. Paired climate signals from independent regional proxies are discussed, as well as factors affecting the interpretation of PAH signals in speleothems. Considerations and methods using small (~1g) speleothem samples are presented, with a focus on simultaneous extraction of useful paleoenvironmental information from other molecular biomarkers entombed within speleothems.