Diatom Evolution in Pliocene and Pleistocene Antarctic Shelf Sediments Charlotte Sjunneskog Louisiana State University, [email protected]
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University of Nebraska - Lincoln DigitalCommons@University of Nebraska - Lincoln ANDRILL Research and Publications Antarctic Drilling Program 2012 Fragilariopsis Diatom Evolution in Pliocene and Pleistocene Antarctic Shelf Sediments Charlotte Sjunneskog Louisiana State University, [email protected] Christina R. Riesselman Eastern Geology and Paleoclimate Science Center, U.S. Geological Survey, [email protected] Diane Winter Algal Analysis Reed Scherer Northern Illinois University Follow this and additional works at: http://digitalcommons.unl.edu/andrillrespub Part of the Oceanography Commons, and the Paleobiology Commons Sjunneskog, Charlotte; Riesselman, Christina R.; Winter, Diane; and Scherer, Reed, "Fragilariopsis Diatom Evolution in Pliocene and Pleistocene Antarctic Shelf Sediments" (2012). ANDRILL Research and Publications. 65. http://digitalcommons.unl.edu/andrillrespub/65 This Article is brought to you for free and open access by the Antarctic Drilling Program at DigitalCommons@University of Nebraska - Lincoln. It has been accepted for inclusion in ANDRILL Research and Publications by an authorized administrator of DigitalCommons@University of Nebraska - Lincoln. Micropaleontology (2012) 58(3): 273–289. U.S. Government work. Fragilariopsis diatom evolution in Pliocene and Pleistocene Antarctic shelf sediments Charlotte Sjunneskog1, Christina Riesselman2, Diane Winter3 and Reed Scherer4 1Corresponding author: Department of Geology and Geophysics, Louisiana State University, Baton Rouge, La 70803 Current address: AMGRF Florida State University, Tallahassee, FL 31306, Phone 850 644 2407 2Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305 Current address: Eastern Geology and Paleoclimate Science Center, U.S. Geological Survey, Reston, VA 20192 3Department of Geosciences, University of Nebraska-Lincoln, Lincoln, NE 68588 4Department of Geology and Environmental Geosciences, Northern Illinois University DeKalb, IL 60115 email: [email protected] ABSTRACT: The late Pliocene – early Pleistocene sediment record in the AND-1B core from the McMurdo Sound, Ross Sea, Antarctica, displays a rich diversity and high abundance of diatoms, including several new morphologies within the genus Fragilariopsis. These new morphologies exhibit similarities to the extinct late Miocene/early Pliocene species Fragilariopsis aurica Gersonde and Fragilariopsis praecurta Gersonde, as well as to the modern sea ice-associated species Fragilariopsis ritscheri Hustedt and Fragilariopsis obliquecostata van Heurck. From the diverse morphologies present, we use light microscopy and scanning electron microscopy to identify and describe the characteristics of three new taxa, Fragilariopsis laqueata Riesselman, Fragilariopsis bohatyi Sjunneskog et Riesselman, and Fragilariopsis robusta Sjunneskog, which are common in the diatom-bearing intervals from ~3.2 to 1.95 Ma. Comparisons with extant and extinct species are made to assess possible environmental affinities, evolutionary relationships, and potential for future biostratigraphic utility. This complex of new morphologies diversified as conditions cooled during the Pliocene, then went into decline as heavy sea ice conditions of the Pleistocene were established. Only the lineage of F. robusta appears to continue into the late Pleistocene, where it is interpreted to have evolved into F. obliquecostata. Key words: Fragilariopsis, Pliocene, Ross Sea, Antarctica, ANDRILL, sea ice INTRODUCTION than 45% in some AND-1B assemblages, and hence represents a significant part of the diatom record (text-fig. 2). Extant Antarctic representatives of the diatom genus Fragilari- opsis Hustedt include planktonic, benthic, and sea ice-associ- The aims of this paper are to (1) describe the most common spe- ated species (Hasle and Medlin 1990; Round et al. 1990). When cies in this assemblage, (2) establish biostratigraphic ranges and preserved in the sediment, these species are often used to infer assess the evolutionary trends of the species within this com- paleoenvironmental conditions, and especially to assess mid plex, and (3) assess environmental preferences for these species, Pleistocene through Holocene sea ice conditions (Zielinski and providing a basis for paleoenvironmental interpretation. Gersonde 1997; Bianchi and Gersonde 2002; Kunz-Pirrung et al. 2002; Crosta et al. 2004; Armand et al. 2005; Leventer et al. MATERIAL AND METHODS 2007). Materials The AND-1B core recovered from the Ross Sea inner continen- The present work is based on analyses of samples from the tal shelf, Antarctica, by the ANtarctic geological DRILLing AND-1B core collected from beneath the McMurdo Ice Shelf Program (ANDRILL) (text-fig. 1) comprises unparalleled suc- south of Ross Island, 77.8894°S, 167.0893°E, at a water depth cessions of Pliocene interglacial sediments characterized by di- of 936 meters (Falconer et al. 2007) (text-fig. 1). The upper 585 atomite and diatomaceous mud units, punctuated by sediments meters of the 1285-meter core comprises alternating diamictite characteristic of glacial advance (Naish et al. 2009). The inter- and diatomaceous sequences, with episodic volcanigenic rocks glacial intervals contain a rich diatom assemblage characterized (text-fig. 2). The initial report for AND-1B describes 13 by remarkable variability of previously undescribed, morpho- biostratigraphic diatomaceous units, DU-XIII through DU-I, logically similar specimens within the genus Fragilariopsis, representing interglacial conditions from the early Pliocene to which are collectively referred to as the “Fragilariopsis com- mid Pleistocene (Scherer et al. 2007). The Fragilariopsis com- plex” (Scherer et al. 2007). Fragilariopsis morphologies in- plex is abundant in six of these units, DU-IX through IV cluded in this complex are similar to the extant sea ice-affiliated (293-151 meters below sea floor (mbsf)). The assessed age for taxa F. obliquecostata Heurck and F. ritscheri Hustedt, as well this interval is <3.3 – 1.95 Ma (Naish et al. 2009) (text-fig. 2), as to the early Pliocene Fragilariopsis species F. praecurta which spans the late Pliocene to early Pleistocene following the Gersonde,F.auricaGersonde and F. arcula Gersonde de- recent revision of the Pliocene/Pleistocene boundary (Gibbard scribed from open-ocean sediment cores (Gersonde 1991). The et al. 2010). Smear slides from DSDP Leg 28 Site 274 (slides Fragilariopsis complex reaches a relative abundance greater 5R-5-50, 6R-5-50, 7R-2-50, 8R-CC and 9R-5-50) were exam- micropaleontology, vol. 58, no. 3, pp. 273–289, text-figures 1-4, table 1, plates 1-7, 2012 273 Charlotte Sjunneskog et al.: Fragilariopsis diatom evolution in Pliocene and Pleistocene Antarctic shelf sediments TABLE 1 Morphometrics of new Antarctic Fragilariopsis species, as well as established extinct (§) and extant (†) taxa. Morphometrics from Gersonde (1991) for extinct taxa, and from Scott and Marchant (2005) for extant taxa. ined for comparison of Fragilariopsis-rich Pliocene floras. ther consideration in this analysis. To compare the AND-1B These slides were provided by the Ocean Drilling Program Fragilariopsis assemblage with the modern flora, we primarily Micropaleontological Reference Center at the University of used the Fragilariopsis key of Hasle (1965) supplemented by Nebraska-Lincoln. the compilation of Scott and Marchant (2005), whereas the work of Gersonde (1991) was our main reference for late Diatom analysis Miocene/early Pliocene species comparisons. The descriptions are based on light microscopy (LM) with sup- port of scanning electron microscopy (SEM). Samples, 2 cc in RESULTS size, from the AND-1B core were collected at 25-30 cm inter- The abundances of the three species described in this section are vals from the diatomaceous units between 293-151 mbsf. These shown in Figure 2, together with the abundance of the extant sea semi-lithified samples were gently crushed and mixed to allow ice-related diatom F. curta, which appeared in the early Plio- representative sub-sampling. The subsamples were digested in cene and dominates modern southwestern Ross Sea sediments hydrochloric acid (10%) and hydrogen peroxide (20%) and today (Leventer and Dunbar 1988). The differentiation between then rinsed several times in distilled water. An aliquot of the species is based on morphometrics: length; width; number of sample was dried onto a cover slip, mounted on a slide using costae and poroids per unit of length; shape, distribution, and Norland Optical Adhesive 61 (refractive index 1.56), and cured regularity of costae; and presence or absence of hyaline, under UV light. The slides were examined using an Olympus non-perforated fields on the valve face. Specimen measure- BX51 microscope with Nomarski optics and equipped with ments for each species are summarized in Table 1. Figure 3 il- 100x and 63x oil-immersion objectives and a 40x dry objective, lustrates the morphometrics of these species and a selection of attached to an Olympus DP25 camera, and a Leica DM LB2 mi- extinct and extant taxa for comparison. Slides with type speci- croscope with Nomarski optics and equipped with 100x, 63x, mens are deposited at the California Academy of Sciences. and 40x oil-immersion objectives, attached to a SPOT Insight photomicrographic system. SEM images were obtained using Fragilariopsis laqueata Riesselman, sp. nov. JEOL 840A and FEI XL30 Siroin scanning electron micro- Plates 1 and 2 scopes. Many AND-1B samples contain moderately to heavily fragmented assemblages. Morphometric