Strontium isotopes as a proxy for water-rock interactions: results from the Sierra Nevada foothills, California

Strontium isotope records from speleothems provide excellent proxies of climate-driven changes in groundwater interaction with host-carbonates, other rock units, and soils along the seepage flow-path. Sr isotopes compliment stable isotope proxies more commonly employed in speloethem paleoclimate studies due to their negligible fractionation during evaporation and degassing. This leads to speleothem carbonates that directly reflect the Sr content of their source waters

Speleothem-bearing caves are developed in the western Sierra Nevada foothills in marble units within the Calaveras Complex, a Permian-Triassic subduction melange that consists of metasedimentary rocks and basalts. Sr isotope analyses of soil leachates, overlying rock types, and host-marble from Moaning Cave reveal several potential sources of Sr to the modern cave system. Host-marble values of 0.70745 ±9 are consistent with their Permo-Carboniferous age and late Paleozoic to Mesozoic accretion and metamorphism. ⁸⁷Sr/⁸⁶Sr values of progressively stronger soil leachates reveal the presence of a less radiogenic Sr source to the Moaning Cave system (0.70351±5 to 0.70605±5), likely derived from local volcanic rocks.

Systematic variations in ⁸⁷Sr/⁸⁶Sr ratios of latest glacial through early Holocene speleothems from Moaning Cave record varying contributions from these two Sr end-members. Comparison with other proxy records (δ¹³C, δ¹⁸O, and trace elements) suggests increased contribution of marble Sr to the speleothem during drier periods. Less radiogenic, soil dominated ⁸⁷Sr/⁸⁶Sr ratios occur during colder wetter periods inferred from decreased δ¹⁸O, δ¹³C, and trace element concentrations. A transition to relatively constant ⁸⁷Sr/⁸⁶Sr values is recorded in early Holocene speleothem calcite and may reflect a significant change in hydrology. This is likely related to reduced soil thickness due to stripping of hill slopes over the cave as vegetation density declined in the dry early Holocene climate. Results from Moaning Cave demonstrate the utility of Sr isotopes as proxies of groundwater interactions in a complex metamorphic terrain.