Uranium and strontium isotope study of late-stage speleothems from lava tube caves in El Malpais National Monument, New Mexico

—Strontium isotope values measured for six calcite and opal speleothem samples from five lava tube caves, formed in three different >15,000 year-old basalt flows in El Malpais National Monument, western New Mexico, reveal a combined soil- and bedrock-related origin. All speleothem growth was several thousand years after the lava tube caves formed. Uranium-series ages of all six samples shows that speleothem growth likely coincided with wetter periods of the Holocene. Four of the six samples formed during the late Holocene between 4,000 and 1,000 years ago, consistent with previous recon- structions of a wetter Late Holocene in the southwestern United States. Growth of a middle Holocene sample is coincident with a brief wetter period in northern Mexico and central New Mexico defined by lacustrine sediment studies, and growth of an early Holocene sample crudely matches a wetter period defined by Carlsbad area stalagmite growth.