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Pliocene Chrysophycean Stomatocysts from the Sonoma Volcanics, Napa County, California

1996; Micropaleontology Press; Volume: 42; Issue: 1 Linguagem: Inglês

10.2307/1485985

1937-2795

Barbara A. Zeeb, John P. Smol, Sam L. VanLandingham,

Microbial Community Ecology and Physiology

We use International Statospore Working Group (ISWG) guidelines to taxonomically describe the chrysophycean cyst flora from a deposit (ca. 2.5-4 m.y.) in the Sonoma Volcanics from Napa County, California. Cyst preservation was very good. We observed a total of 21 different cysts (19 with SEM, 2 with LM only), nine of which are described here for the first time. This represents a relatively low diversity flora when compared to cyst assemblages in Holocene sediments. Dominant cyst morphotypes that have previously been described in the literature indicate that the environment of deposition was probably shallow (littoral) and alkaline. We believe that this study is the first to describe pre-Holocene cysts using ISWG guidelines. At this time, it is unclear whether newly described morphotypes represent extinct or extant species. INTRODUCTION Stomatocysts (or statospores) are the endogenously formed siliceous resting stages of the Chrysophyceae and Synurophyceae (Sandgren 1988). Cysts vary in shape from spherical, to oval, to oblate, to obovate, to ovate, to pyramidal (Duff et al. 1994), and have diameters that range from 30ugm (Adam and Mahood 1981; Duff et al. 1994). All cysts have a single pore through which the germinating cell emerges; the pore may be surrounded by a collar. Mature cysts can be a single layer thick or consist of many layers, and the external wall may be smooth or ornamented with a variety of siliceous projections (i.e. scabrae, verrucae, conula, spines, ridges, circuli, or reticula), or indentations (i.e. psilae, depressions, fossae) (Duff et al. 1995). Chrysophyte cyst morphotypes possessing distinctive collar and ornamentation features appear to be species-specific (Cronberg 1986) and can be distinguished by their size, collar structure, and surface ornamentation. The use of cysts in paleolimnological research has great potential because, unlike chrysophyte scales, they reflect the complete record of past chrysophyte populations. Furthermore, in some ecosystems, such as the high Arctic (Smol 1983; Douglas and Smol 1994) and high altitude lakes (Rott 1988), scaled chrysophytes are very rare, while non-scaled chrysophytes may be abundant. In addition, many cysts are thickly silicified and are therefore relatively resistant to dissolution and fragmentation. Recently, shifts in cyst assemblages have been correlated with known changes in environmental conditions such as trophic status (e.g., Carney 1982; Carey and Sandgren 1983; Rybak 1986, 1987; Zeeb et al. 1990, 1994; Duff and Smol 1995a), pH (e.g. Rybak et. al. 1987; Duff and Smol 1991, 1995b), salinity (Pienitz et al. 1992; Zeeb and Smol 1995), and climate (Zeeb and Smol 1993b). The single greatest obstacle facing researchers at this time is chrysophyte cyst taxonomy. Although many papers deal with cyst floras, only the most recent of these include detailed descriptions and scanning electron micrographs which are critical for determining the species-specific features of cysts. A standardized system for describing new morphotypes did not become available until 1986 (Cronberg and Sandgren 1986), following the first meeting of the International Statospore Working Group (ISWG). Unfortunately, most older literature sources lack adequate descriptive detail, providing only a sketchy description and possibly a line drawing, making comparisons between cyst morphotypes difficult, and sometimes impossible. Although current studies (mostly on Late Pleistocene/Holocene deposits) have contributed considerably to our knowledge of North American chrysophycean stomatocysts, very little is known about stomatocysts from pre-Holocene deposits. According to Cornell (1969), as late as 1969 only Tynan (1960) and VanLandingham (1964) had published studies of pre-Pleistocene chrysophyte cysts from North America, and only the latter dealt with non-marine cysts. Tynan (1960) described fifteen cyst morphs from the Calvert formation of Maryland, a Miocene deposit. He used the systematics of Deflandre (1932), placing all fifteen morphotypes into the Archaeomonadaceae, an artificial family of cysts produced by unidentifiable fossil, marine forms. VanLandingham (1964) found thirty-one siliceous cyst 'types' in diatomaceous sedimentary rocks from localities of the Yakima Basalt (Miocene) in Washington state. Both of the above studies used line drawings to illustrate cyst morphotypes, and in addition, VanLandingham (1964) has some light micrographs. More recently, Srivastava and Binda (1984) described 13 chrysomonad cyst species from the Maastrichtian Battle Formation, a nonmarine lacustrine deposition (ca. 65 m.y.) of Alberta, Canada, and their study is the only one we know of to date that has scanning electron micrographs of pre-Holocene cysts. The objectives of this study are two-fold: 1) to describe, using ISWG g idelines, the fossil cysts recovered from the Sonoma Volcanics formation; and 2) to compare these morphotypes to those described from other deposits and regions to determine if our Pliocene-aged flora was markedly different from present-day floras. SITE DESCRIPTION All of the cysts described are from sample 1461, which was collected by Sam VanLandingham on July 11, 1982, in the diatomaceous member (of Kunkel and Upson 1960) of the Sonoma Volcanics from a road cut along the northeast side of the Silverado Trail, 300 meters northwest of its intersection with Zinfandel Road, near centre SW1/4 of sec. 33, T. 8 N., R. 5 W., Napa County, California. Beds in the diatomaceous member strike NW-SE and micropaleontology, vol. 42, no. 1, pp. 79-91, text-figures 1-2, plates 1-4, 1996 79 This content downloaded from 207.46.13.85 on Tue, 23 Aug 2016 05:52:07 UTC All use subject to http://about.jstor.org/terms B. A. Zeeb, J. P Smol and S. L VanLandingham: Pliocene chrysophycean stomatocysts from the Sonoma volcanics, Napa County, California dip up to about 55-60?SW. Microfaulting is present and there is a distinctive rhombohedral jointing-fracture network in the diatomite (text-fig. 1). K-Ar and fission-track dates from the Sonoma Volcanics in the nearby Mark West Springs 71/2' Quadrangle range from 9.1 ?4.5 to 2.6 ?0.3 m.y. (Wagner and Bortugno 1982). The dominant species in sample 1461 is Stephanodiscus carconensis, and this diatom is often dominant at approximately 3 m.y. in known diatom assemblages of the world. The Sonoma Volcanics range in age from late-Miocene to late-Pliocene (VanLandingham 1990). First occurrences, extinctions, and dominances of the diatoms in sample 1461 suggest an age of about 2.5 to 4 m.y. The fine-grained, massive diatomaceous clay and tuff of sample 1461 is lithologically and chronologically near the top of the Sonoma Volcanics complex. The Sonoma Volcanics and the underlying Petaluma Formation contain one of the most complex non-marine diatomaceous sequences in the world. There are at least six diatom bearing units scattered throughout the Sonoma Volcanics except in the uppermost rhyolitic portions. All six of these diatom bearing units have associated chrysophycean stomatocysts. In most units the stomatocysts are common, even abundant (as in sample 1461). MATERIALS AND METHODS The sample (#1461) used in this study was cleaned with repeated decantings with deionized water only; no acids or caustic material were used. For light microscopy, an aliquot of the cleaned siliceous material was resuspended and evaporated on glass coverslips, and placed on a GCA/Precision Scientific slide warmer set at 25C. The dried coverslips were then permanently mounted in Hyrax, and examined under oil immersion using a 100X objective lens (N.A. = 1.32) on a Leitz Dialux 20 light microscope. For electron microcopy, an aliquot of the cleaned siliceous material was evaporated onto a smooth piece of aluminum foil. Double-sided tape was used to affix the foil to aluminum scanning electron microscope (SEM) stubs. Each stub was sputter-coated with gold and examined at 20kV, at a working distance of 15-20gm, using a Hitachi S-2500 SEM equipped with a 35mm camera for photography.