Radiocarbon and stable isotope evidence of early to mid‐Holocene wet events from fluvial tufa deposits in Santa Cruz, CA

Abstract

It is increasingly important to document past records of hydrologic change in areas that are drought-prone to better predict the region's future vulnerability to recharge and water supply. Holocene spring-associated carbonate deposits serve as terrestrial records of water balance that can complement other local, high-resolution proxies that are moisture-sensitive. Here we examine two carbonate deposits (one inactive perched tufa site and one active fluvial tufa site) that form from ambient-temperature freshwater springs, as proxies of their depositional conditions. Radiocarbon (14C) analyses of charcoal fragments from the inactive perched tufa record depositional ages of 6.2 ± 0.06 (2σ) cal ka bp and 8.0 ± 0.04 (2σ) cal ka bp and agree with the age models from other proxies of past pluvial periods in the region (~16 to 5 ka). The active fluvial tufas date to 853 ± 0.4 cal bp, representing conditions similar to modern flow. Geomorphologic and radiocarbon results indicate the perched tufa reflects wetter conditions fed by a higher water table. Stable isotopic analyses of carbonate (δ13C, δ18O) reveal distinct isotopic values between modern and early–mid-Holocene tufa. This work underscores potential for the analysis of other moisture-sensitive tufa deposits in coastal central California.