PALEOCLIMATE RECONSTRUCTION USING δ18O AND δ13C OF HOLOCENE CARBONATES FROM THE FRAZIER MOUNTAIN PALEOSEISMIC SITE, SOUTHERN CALIFORNIA

Frazier Mountain (FM) is a paleoseismic site on the San Andreas Fault, ~80 km north of Los Angeles, CA. This site preserves fault traces of several recent paleoearthquakes, with charcoal (for ¹⁴C dating) and carbonate nodules (for isotope analysis). Since δ¹⁸O and δ¹³C variations recorded in carbonates can be used to reconstruct paleoclimate (assuming a known origin), we determined: (1) the formation mechanism of the FM nodules, (2) climate history using stable isotope analysis of the nodules, and (3) absolute ages of climate changes using ¹⁴C dating. Ideally, climate shifts inferred from stable isotopic data can be used to correlate events at paleoearthquake sites without radiocarbon data.

Thin section petrography confirms that the nodules, sampled from a 2 m deep trench, are phreatic. Isotope analysis indicates they formed in isotopic equilibrium with local groundwater and soil CO₂. Assuming a Rayleigh distillation at 15°C (the measured groundwater temperature), only 10% evaporation of the source water is necessary to obtain δ¹⁸O_calcite values recorded in the nodules (~ -9.9‰ V-PDB), indicating equilibrium precipitation with respect to oxygen. Modern vegetation at the FM site is dominated by willow (δ¹³C_organic ≈ -24‰) and assuming ~15‰ fractionation from δ¹³C_organic to δ¹³C_calcite, the nodules indicate equilibrium precipitation with respect to carbon as well (~ -9.5‰).

We propose that variation in δ¹⁸O and δ¹³C values from samples taken across individual nodules indicates climate change on the order of decades (e.g. formation time of one nodule). ¹⁴C data indicate over 1600 years of deposition. Thus, climate variations on the order of centuries are recorded in the change in average isotope values of nodules with depth. Assuming nodules form nearly penecontemporaneously with deposition, two distinct climate shifts occur within the last 1600 years: (1) climate becomes dry and cool from 1600 cal yr BP to 1359 cal yr BP, and (2) climate becomes wetter and temperatures rise by ~3˚C after 1359 cal yr BP. At neighboring paleoseismic sites, an increase in clastic sediment input coincides with the latter climate shift seen at FM and likely corresponds to increased rainfall. Therefore, this study illustrates that climate variations (inferred from isotope data) are indeed a potential correlation tool for paleoseismic sites.