RECORDS OF SEVERE DROUGHT IN CALIFORNIA DURING PLEISTOCENE INTERGLACIALS

As climates change, droughts have become common in many places in the world, including in the southwestern United States and particularly in the State of California, which is currently experiencing multi-year drought conditions. We often hear or read that such hyperaridity is extreme, exceptional, unprecedented, record-breaking, or that this drought is the "worst drought ever." However, the geological record clearly states otherwise. In California's Central Valley, platy caliche (calcrete) caps sedimentary sequences recognized as glacial-derived sediments deposited during melting of alpine glaciers in the adjacent Sierra Nevada. To determine the paleoenvironmental conditions under which this calcrete formed and to shed some new light on paleoclimatic conditions during Pleistocene interglacial periods, samples of calcrete were collected from several localities in the Central Valley and elsewhere in California and processed to recover acid-insoluble palynomorphs (pollen, spores, and other organic-walled microfossils). Palynomorphs recovered from these samples indicate that the calcrete was deposited under desert-like climates much drier than the current climate in the Central Valley and resembling the climate of southeastern California deserts. Thus, this calcrete and its enclosed palynomorphs record extreme/severe droughts during Pleistocene interglacial periods. In the past California has experienced far worse droughts than the state is currently experiencing. During Pleistocene interglacial periods, the Central Valley was most likely a true desert with less than ten inches (25 mm) of rain per year. In contrast, sediments immediately underlying and overlying the calcrete contain a more typical Pleistocene palynoflora, consisting of abundant indicators of cooler and wetter conditions. The extreme aridity recorded by calcrete deposits no doubt contributed to the extirpation and/or extinction of both flora (e.g., Carya, Pterocarya, Ulmus, Tilia, etc.) and fauna (e.g., Mammuthus, Mammut, camels, horses, ground sloths, etc.).