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Phytoliths of the Barstow Formation through the Middle Miocene Climatic Optimum: preliminary findings Katharine M. Loughney 1,2 and Selena Y. Smith 1, 1 Museum of Paleontology, University of Michigan, 1109 Geddes Ave., Ann Arbor, MI 48109 2 Department of Earth & Environmental Sciences, University of Michigan, 1100 North University Ave., Ann Arbor, MI 48109 abstract—The Middle Miocene Climatic Optimum (MMCO) was an interval of signif- icant 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 indica- tors. 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 identi- fied in samples from the Owl Conglomerate and Middle members but were rare. These phytolith data provide evidence that grasses were present throughout deposition of the Barstow Formation, and that they coexisted with palms in mixed-vegetation habitats. While more sampling is needed to fill in stratigraphic gaps in the phytolith record of the Barstow Formation and few phytoliths have so far been recovered from samples, these preliminary data show that phytoliths can be used in the Barstow Formation to better understand Miocene ecosystems and how they may have changed through the MMCO. Introduction The fluvial and lacustrine sediments of the Global climate over the course of the Cenozoic Barstow Formation were deposited between 19.3 became generally drier and cooler but was punctu- and 13.3 Ma (MacFadden et al., 1990; Woodburne ated by several major and short-lived periods of et al., 1990), and the formation potentially records warming (Zachos et al., 2001). One of these warming the responses of the local fauna and flora to this intervals, the Middle Miocene Climatic Optimum warming interval. The mammalian fauna of the (MMCO), occurred between ~17.0 – 14.0 Ma as Barstow Formation has been well studied, and some recorded from both marine (Foster et al., 2012) and studies have focused on interpreting the lacustrine continental records (Retallack, 2007). In North paleoenvironments represented in the Barstow (e.g., America, few continental units capture this interval. Park, 1995; Park and Downing, 2001). The rarity of The Barstow Formation of the Mojave Desert, plant macrofossils has made it difficult to interpret eastern San Bernardino County, California, is one the terrestrial ecosystems in the formation, and the such unit that encompasses the MMCO. limited Barstow floral record does not currently span the MMCO. Phytoliths (plant silica), however, 2015 desert symposium 51 k. m. loughney and s. y. smith | phytoliths of the barstow formation can be sampled from sediment layers throughout the and compressions of vegetative and reproductive formation and may provide a record of ecosystem structures from woody dicots, palms, and juniper, change through this interval that is not dependent on as well as some unidentified remains (Reynolds and macrofossil preservation. Here, we present prelimi- Schweich, 2013). Petrified wood from the Solomon nary results of an investigation of paleoenvironments Skyline locality below the Dated Tuff marker unit and vegetation change in the Barstow Formation (14.8 Ma, MacFadden et al., 1990) has been identi- through the MMCO. fied as belonging to poison oak, juniper, and buck- thorn (Alf, 1970). The Barstow Formation has also Background been sampled for pollen without success (I. Browne, Multiple lines of evidence have been used to interpret personal commun., October 2014). Indirect evidence the timing and geography of the spread of grasslands for the presence of grasses comes from Feranec and in the Neogene, and fossils from the Barstow Forma- Pagnac (2013), who analyzed the carbon isotopic tion will add information to help refine our under- composition (δ13C) of herbivore teeth and demon- standing of when, where, why, and how this major strated that the diets of equids from the Barstow ecosystem change happened. Carbon isotopic data Formation included a small but significant (< 20%) from paleosol carbonates and fossil mammal teeth amount of C4 grasses. These isotopic data, then, capture this transition to open grassland habitats and suggests that grasses were present in Barstow ecosys- the adaptations of herbivores to a new food source tems, but it has been difficult to reconstruct paleoveg- during the Cenozoic (e.g., Koch, 1998). Likewise, etation from fossils that are highly localized in time plant macrofossils, pollen, and phytoliths constrain and space. Phytoliths could help address these issues the timing of these ecosystem changes and provide by providing a microfloral record of the vegetation. direct evidence of the types of plants in these increas- Phytoliths are sampled from fine-grained, non-clay- ingly open, grass-dominated habitats (Strömberg, rich sediments and paleosols in which plants are 2011). Macrofossils, however, are typically rare, inferred to have lived; although their preservation and pollen may be allochthonous or not regularly may be highly variable between or within lithologic preserved, so reconstructions of grassland expan- units, when present, they nevertheless record a sion are typically regional or local in scale. Phytoliths proportion of the paleoflora of a sampled locality. are amorphous silica bodies that are precipitated in As part of a broader study of Barstow paleoen- plant tissues; they are autochthonous and are either vironments, we sampled phytoliths through the taxonomically diagnostic for many groups, including formation in order to 1) test whether phytoliths are grasses and palms, or diagnostic of certain plant preserved in Barstow sediments, and 2) examine how functional types, such as woody plants (Piperno, paleovegetation as recorded by phytolith assemblages 2006). Phytoliths of certain taxa may also be useful changed through time. Vegetation and landscape as indicators of past climatic conditions; such indi- reconstructions provide context not only for the cator taxa have modern relatives that range within habitats of the Barstow fauna but also shed light on well-defined climatic or floral regimes (Chen and floral responses to the changing climate conditions of Smith, 2013; Strömberg, 2004). Therefore, phyto- the MMCO. liths are a potentially useful tool for understanding vegetation change through the MMCO due to their Methods ability to record different types of vegetation and key Sixty-eight sediment samples were collected at 20-m ecosystem/climatic indicator taxa. intervals through 12 stratigraphic sections measured The Barstow Formation is primarily known for its in the Mud Hills, Calico Mountains, and Daggett diverse and well-studied mammalian fauna, but few Ridge (Fig. 1). These sections include the entire thick- studies have been conducted on Barstow flora. Plant ness of the Barstow Formation, and all members macrofossils are known from only two localities in were sampled. Six samples were collected in the Owl the Middle Member of the Barstow Formation (Alf, Conglomerate Member, 30 samples were collected 1970; Reynolds and Schweich, 2013). The Rainbow in the Middle Member, and 31 were collected in the Loop Flora from the Mud Hills has been dated Upper Member (Table 1). Phytoliths were sampled between 16.3 – 15.8 Ma and preserves impressions from sediment ranging in grain size from clay to 52 2015 desert symposium k. m. loughney and s. y. smith | phytoliths of the barstow formation Figure 1. Locations of measured sections from which phytoliths were sampled in the Mud Hills, Calico Mountains, and Daggett Ridge. 1, 2, Truck Top Wash; 3, 4, 5, Falkenbach Wash; 6, Hell Gate Basin; 7, Cal-Uranium Prospect Canyon; 8, 9, Owl Canyon; 10, Copper City Rd; 11, Little Borate; 12, Daggett Ridge. Stratigraphic positions of phytolith-bearing samples are indicated with asterisks. Ages of tuffs are from MacFadden et al. (1990) and Miller et al. (2013), and age of tufa is from Cole et al. (2005). fine sand. In general, A and B horizons of paleo- Michigan, Ann Arbor, following the procedure for sols should have the highest chance of preserving extraction and heavy-liquid separation outlined by phytoliths and were typically targeted for sampling. Strömberg et al. (2007) for 1 g of sediment. Processed Paleosols in the Barstow Formation are restricted samples were mounted on slides using Cargille to the uppermost ~100 m of the Upper Member, Meltmount (refraction index 1.534) mounting exposed in Truck Top Wash (Fig. 1), and six samples medium and immersion oil (Cargille Non-Drying were collected in weakly to moderately developed Immersion Oil Type A) for rotation to characterize paleosols at this location. Given the overall dearth the 3D and surface morphology for initial phytolith of paleosols in the rest of the formation, sampling identification, and slides were examined on either a efforts were not restricted to any particular lithology, Leica DMP-1500 or a Nikon LV-100 transmitted light and rather, we sampled a variety of lithologies in microscope at 1000x. For this preliminary report, each member. We sampled siltstones, claystones, fine samples were qualitatively, rather than quantitatively, sandstones, marls, and ashes to test how phytolith assessed based on initial slides of sample extractions. preservation might differ with sediment type in the Phytolith identifications were made using Strömberg Barstow Formation.

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