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Phytoliths of the Barstow Formation through the Middle Miocene Climatic Optimum : preliminary findings

Introduction Global climate over the course of the Cenozoic became generally drier and cooler but was punctuated by several major and short-lived periods of warming (Zachos et al., 2001). One of these warming intervals, the Middle Miocene Climatic Optimum (MMCO), occurred between ~17.0 – 14.0 Ma as recorded from both marine (Foster et al., 2012) and continental records (Retallack, 2007). In North America, few continental units capture this interval. The Barstow Formation of the Mojave Desert, eastern San Bernardino County, California, is one such unit that encompasses the MMCO. The fluvial and lacustrine sediments of the Barstow Formation were deposited between 19.3 and 13.3 Ma (MacFadden et al., 1990; Woodburne et al., 1990), and the formation potentially records the responses of the local fauna and flora to this warming interval. The mammalian fauna of the Barstow Formation has been well studied, and some studies have focused on interpreting the lacustrine paleoenvironments represented in the Barstow (e.g., Park, 1995; Park and Downing, 2001). The rarity of plant macrofossils has made it difficult to interpret the terrestrial ecosystems in the formation, and the limited Barstow floral record does not currently span the MMCO. Phytoliths (plant silica), however, abstract—The Middle Miocene Climatic Optimum (MMCO) was an interval of significant warming between 17.0 – 14.0 Ma, and a record of the interval is preserved in its entirety in the type Barstow Formation (19.3 – 13.3 Ma) of southern California. In order to understand the biotic impacts of the MMCO, it is necessary to understand vegetation; however, macrofloral records from the middle Miocene in this region are rare and do not span the MMCO. Phytoliths (plant silica) can be preserved in continental sediments even when macrofossil or pollen remains are not, and they can be diagnostic of specific plant clades and/or functional groups, some of which are useful environmental indicators. Sixty-eight sediment samples were collected from 12 stratigraphic sections measured within the Barstow Formation in the Mud Hills, Calico Mountains, and Daggett Ridge, and 39 samples were processed for phytoliths. Ten samples yielded phytoliths, although phytoliths were rare in most of these samples. Paleosols from the uppermost part of the Barstow Formation yielded the most abundant and most diverse phytolith assemblages, including grass bilobates and echinate spheres of palms; grass phytoliths were also identified in samples from the Owl Conglomerate and Middle members but were rare. These phytolith data provide evidence that grasses were present …