Diatom Evolution in Pliocene and Pleistocene Antarctic Shelf Sediments Charlotte Sjunneskog Louisiana State University, [email protected]

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 data were collected Description: The valve outline is linear to elliptic, isopolar to from all whole Fragilariopsis complex specimens encountered slightly heteropolar with obtusely rounded apices. The length of and imaged in several slide traverses of 23 samples with good the apical axis ranges from 14.4 to 39.2µm with an average of preservation. Subjective characteristics such as valve outline, 24µm; the transapical axis ranges from 8.5-11.8µm averaging costae branching, and perceived poroid fineness were used to 10.1µm. Transapical costae are ribbon-like with low external divide most documented specimens into three groups. The relief. Costae are straight in the middle portion of the valve face morphometric characteristics of these groups are sufficiently and curve slightly towards the apices, at a density of 8 to 14 in unique to distinguish each as a new species; specimens that fall 10µm (average 10; Table 1). At the apices, some costae branch outside of these species concepts have been excluded from fur- multiple times; in rare specimens, branching or forking contin-

274 Micropaleontology, vol. 58, no. 3, 2012

TEXT-FIGURE 1 Map and detail of sites discussed in the text. Panel a also indicates major oceanographic boundaries: SSI=average summer sea ice edge, SIZ=seasonal sea ice zone, WSI=average winter sea ice edge, POOZ=permanent open ocean, PF=Polar Front, SAZ=Sub Antarctic Zone. Panel b shows the detailed loca- tion of the AND-1B site in the western Ross Sea.

ues down the lateral margin (Plate 1, figs. e, k, l, u). Striae com- correlation between length and width, (R2=0.03; text-fig. 3a), monly comprise two rows of circular poroids with regular to which are strongly correlated features of F. aurica (Gersonde irregular spacing, frequently decreasing to one row near the api- 1991); however, there is a weak correlation between number of ces. In a single row, poroids number 19 to 35 in 10µm (average costae in 10 mm and length, (R2=0.44; text-fig. 3b). We attribute 24), although in rare specimens they are too fine to count in this correlation to the presence of single rows of poroids near LM. A single exceptional specimen from 288.76 mbsf preserves the apices; costae numbers are measured in the valve center, and a perforated poroid membrane (Plate 2, figs. g-i). As no other measurements of shorter valves extend into the apical area specimens with this degree of preservation were encountered, where costae are spaced more closely together. The epithet we are unable to establish whether the hymen structure dis- Fragilariopsis laqueata refers to the ornamental aspect of the played by this specimen is consistent throughout the species. multiply branching apical costae and is derived from the Latin On most specimens, hyaline, non-preforated areas interrupt the verb “laqueo” – “to adorn with a paneled or fretted ceiling.” regular distribution of poroids and costae, often forming two This species is not previously documented in the literature. small, semi-regular poleward fields on either side of the valve face center (Plate 1, figs. a, s-t; Plate 2, figs. a-b; d-e). Valve Distribution in AND-1B: Fragilariopsis laqueata is consis- face to mantle transition is smooth, and the mantle is shallow. tently present in low abundance in DU-IX to -VI, 293-182 mbsf The raphe is located at the junction between valve face and (text-fig. 2). A few fragmented specimens also occur between mantle and terminates near the apex (Plate 2, fig. c). The num- 160 and 153 mbsf (in DU-V and -IV). These may be reworked, ber of fibulae is equal to or slightly less than the number of as the common reworking indicators Paralia spp. and costae. Type specimen: Plate 1, fig. e, accession #627360, slide Stephanopyxis spp. are also present (Scherer et al. 2007). #222082, California Academy of Sciences. Type locality: McMurdo Sound, Ross Sea, Antarctica core AND-1B 220.11 Fragilariopsis bohatyi Sjunneskog et Riesselman, sp. nov. mbsf. Plates 3 and 4 Synonym: Fragilariopsis sp. of Bohaty et al. (1998), Plate 1, figs. 11 and Comments: Fragilariopsis laqueata differs from F. praecurta in 13. its overall coarser structure (text-fig. 3), and by obtusely rounded apices that are much broader than those of F. praecurta Description: The valve outline is linear to oblong, commonly (Plate 3, fig. 11). It differs from F. aurica in its overall coarser with parallel margins although some specimens have slightly structure, broader apices, and linear valve outline. The finer convex or concave margins; the smaller specimens especially structure, higher poroid density, more pronounced hetero- tend to be sub-rounded. Valves are dominantly isopolar with ob- polarity, and more developed branching of the costae at the api- tusely- to broadly-rounded apices; in most specimens, one apex ces separate it from F. bohatyi (see below). There is no is more angular than the other. The length of the apical axis is

275 Charlotte Sjunneskog et al.: Fragilariopsis diatom evolution in Pliocene and Pleistocene Antarctic shelf sediments

TEXT-FIGURE 2 Relative abundance of the new Fragilariopsis species compared to F. curta in the late Pliocene and early Pleistocene sections of the AND-1B core. The ages of diatomaceous units are indicated in the right column (Naish et al. 2009).

13.8-77.1µm with an average of 31.0µm, and the transapical face center (Plate 3; Plate 4, figs. a and d). Valve face to mantle axis is 7.0-11.6µm with an average of 8.5µm. Transapical transition is smooth; the mantle is shallow. The raphe is located costae are regularly distributed, flat on the valve exterior and at the junction between valve face and mantle and terminates thickened on the valve interior. Costae are straight in the middle short of the apex (Plate 4, fig. b). The number of fibulae is equal portion of the valve face at a density of 5-11 per 10µm (average to or slightly less than the number of costae. Type specimen: 7), and curve towards the apices. Apical costae are branched; Plate 3, fig. m, accession #627358, slide #222080, California typically, branching in both apices feathers out from one margin Academy of Sciences. Type locality: McMurdo Sound, Ross (Plate 3, figs. c, f, g, i, r, s). Striae commonly comprise two rows Sea, Antarctica core AND-1B 219.40 mbsf. of circular poroids, paired in the middle of the valve and alter- nating or decreasing to a single row near the apices (Plate 4, fig. Comments: In AND-1B material, F. bohatyi encompasses a e). In a single row, poroids number 15-29 per 10µm with an av- wide range of sizes and morphologies, and there is no correla- erage of 19. Multiple specimens from samples at 287.75 and tion between width and length (R2=0.00) or between number of 288.76 mbsf preserve a poroid membrane with one to two rings costae and length (R2=0.10) (text-fig. 3). The number of costae, of perforations (Plate 4, figs. f, h-i), which form “flaps” as the poroids, and the valve length and width overlap substantially perforations succumb to dissolution (Plate 4, fig. h). As this with those of F. ritscheri and F. linearis (Castracane) Frenguelli characteristic was identified in multiple valves from multiple (text-fig. 3; Table 1). F. bohatyi differs from F. ritscheri in its samples, we infer that it is consistent throughout the species. In isopolar valve outline at all valve lengths, curving and branch- most specimens, hyaline areas occlude poroids on the valve ing apical costae, bluntly rounded apices, and possession of face, commonly forming paired fields on either side of the valve hyaline fields that occlude pores in the middle of the valve face.

276 Micropaleontology, vol. 58, no. 3, 2012

TEXT-FIGURE 3 Fragilariopsis morphometrics, comparing the three new species to select extinct and extant taxa. F. laqueata sp. nov. is indicated by tan triangles, F. bohatyi sp. nov. by red circles, and F.robusta sp. nov. by blue squares. Boxes enclose the size ranges of F.obliquecostata (dotted), F.ritscheri (solid) and F. praecurta (dashed).

It differs from F. linearis in its lower overall poroid density and and the transapical width is 4.8-9µm with an average of 7.3µm. lack of visible fibulae under LM. Fragilariopsis bohatyi is Transapical costae are strongly silicified, externally raised, ir- coarser and larger compared to both F. praecurta and F. aurica, regularly spaced, and of variable width. They extend in a which it most closely resembles. The epithet honors Dr. Steven rib-like fashion into the valve interior (Plate 6, figs. f and i). The Bohaty, who first documented this species from Cape Roberts number of costae per 10µm ranges from 6 to 8, averaging 6. Project 1 (CRP-1) material (Bohaty et al. 1998), and for his Costae are generally straight throughout the length of the valve, contributions to Antarctic diatom biostratigraphy. sometimes with a slight deflection at the apices; a few speci- mens display slightly oblique costae. Fibulae are present in the Distribution in AND-1B: This species occurs in high abun- same number as costae. The striae are comprised of one to three dance, up to >40%, from 293-192 mbsf, then remains in low rows of circular poroids, sometimes in pairs, but often randomly abundance through 151 mbsf (text-fig. 2). distributed close to the costae. There are 16-23 poroids per 10µm with an average of 20. A hyaline membrane was not ob- Fragilariopsis robusta Sjunneskog, sp. nov. served in the poroids of this species under SEM; it is unknown Plates 5 and 6 whether this absence is an artifact of preservation or a primary F.obliquecostata sensu Mahood and Barron (1996), Plate 7, figs. 10-15 characteristic. Elongate hyaline fields are common, are fre- quently arranged parallel to the margin, and occlude some Description: The valve outline is lanceolate to linear with poroids (Plate 5, figs. b, e, h; Plate 6, fig. f). Valve mantle is broadly rounded apices, and is dominantly isopolar. The length deep and the raphe terminates just short of the apex (Plate 6, of the apical axis is 29.6-91.2µm with an average of 50.4µm, figs. d and e). Type specimen: Plate 5, fig. h, accession #627359

277 Charlotte Sjunneskog et al.: Fragilariopsis diatom evolution in Pliocene and Pleistocene Antarctic shelf sediments slide #222081, California Academy of Sciences. Type locality: Sea continental shelf (text-fig. 1) (Flemming and Barron 1996). McMurdo Sound, Ross Sea, Antarctica core AND-1B 152.80 A re-examination of a selection of slides from this site within mbsf. the age interval 3.17-2.0 Ma revealed that both F. robusta and F. bohatyi comprise a substantial part of the F. ritscheri group Comments: The outline and size range of F. robusta are similar (Plate 7). Finally, F. robusta has recently been identified in pis- to those of F. obliquecostata, however, the expansion in the ton cores penetrating Early Pleistocene sediment of the T. kolbei middle of the valve is less developed. The hyaline fields, irregu- Zone (2.2-2.8 Ma) in the North Basin, Ross Sea (Bart et al., larly spaced and dominantly straight costae give this species an 2011). We conclude that the Fragilariopsis complex is overall disorganized appearance. The morphological variability widespread beyond the AND-1B site, but has been grouped converges towards F. bohatyi in smaller and wider specimens, with modern taxa in previous investigations. in which minor branching of costae at the apices is sometimes observed. We found that some specimens in the youngest sam- Age constraints: The three species described here appear simul- ples of AND-1B (153-151 mbsf) could not be differentiated as taneously in the record during the transition between DU-X, this species and were consequently identified as F. oblique- where they are absent, and the base of DU-IX, 293 mbsf costata. There is no correlation between width and length (text-fig. 2). This corresponds to a First Occurrence Datum (R2=0.10) and only a weak negative correlation between length (FOD) of 3.2 Ma based on chronostratigraphic datums (Naish et and number of costae (R2=-0.21) (text-figs. 3a, b). The epithet al. 2009). The upper range for F. robusta and F. bohatyi is trun- is descriptive of the robust form and coarse ornamentation that cated by a glacial unconformity at ca. 1.95 Ma. Pleistocene sed- distinguishes this species from the extant F. obliquecostata and iment from AND-1B ca 1.6 Ma (DU-III) does not contain these from the other members of the AND-1B Fragilariopsis species, but instead the modern F. obliquecostata and F. complex. ritscheri, suggesting that the Last Occurrence Datum (LOD) for these two new species is between 2.0 and 1.6 Ma. We propose a Distribution in AND-1B: Fragilariopsis robusta occurs in the LOD for F. laqueata of 2.58 Ma, which coincides with the AND-1B record from 293-151 mbsf. This species occurs in Gauss/Maruyama boundary. The punctuated appearance of F. trace abundance between 293-250 mbsf, but increases in abun- bohatyi at high relative abundance in the sedimentary record dance of up to 30% in the two younger units from 180-151 mbsf provides a particularly good biostratigraphic marker (Winter et (text-fig. 2). al. in press).

DISCUSSION Relationships to previously described species: The late Pliocene part of this record includes numerous Fragilariopsis morph- Fragilariopsis complex in Antarctic coastal sediments ologies that appear to be transitional or “hybrid” forms, and Regional distribution: Pliocene and Pleistocene sediments were which may reflect a rapid diversification from one or a few spe- recovered near the Antarctic continent from DSDP Leg 28 Sites cies to many short-lived Fragilariopsis varieties. The gradual 271 and 274 and CRP-1 in the Ross Sea; from ODP Leg 119 transitions between these forms highlight both the challenge of Site 742 and ODP Leg 188 Site 1165 in Prydz Bay; and from defining species within a diverse assemblage and the challenge ODP Leg 178 along the Antarctic Peninsula (text-fig. 1): we ex- of establishing lineages based on morphology alone. Lineages pect the AND-1B Fragilariopsis complex to be present at these that include extinct diatom species can only be inferred from coastal sites. In a review of existing Plio-Pleistocene material, morphological similarities, which provide the basis for the com- we find that the new species presented here are only represented parison of these new species with modern and extinct species. by trace occurrences at Site 271 (D. Winter, unpublished data) Fragilariopsis praecurta has been proposed as the ancestor to F. and Leg 178 sites (Winter and Iwai 2002). F. bohatyi is identi- curta by Gersonde (1991). This was based on morphological fied in near-shore site CRP-1, where it is reported as similarities such as the branching of apical costae, and may be Fragilariopsis sp. from the A. ingens zone and assigned an age supported by the fact that there is a slight stratigraphic overlap of 1.15-0.86 Ma. However the only CRP-1 sample from which between the LOD of the former and the FOD of the latter F. sp. is reported (32.05-.15 mbsf) also includes reworking indi- (text-fig. 4a). Our observation of species that morphologically cators (Bohaty et al. 1998). Therefore, we cannot determine appear to be transitional between F. praecurta and the modern whether the CRP-1 occurrence represents a later LOD than that flora as a whole suggests there may have been a Fragilariopsis recovered from AND-1B. F. robusta is illustrated from Site 742 radiation in the late Pliocene (text-fig. 4a, b). However the (Plate 7, 10-15 in Mahood and Barron 1996), where the species stratigraphic gap between the LOD of F. praecurta (4.28-4.19 is documented as “strongly silicified, well preserved specimens Ma; Cody et al., 2008) and the FOD of our species, possibly due of Fragilariopsis… found to closely resemble F. oblique- to glacial sediment erosion, prevents establishment of a direct costata.” Although Mahood and Barron elect to report the taxon morphological link at this time. as F. obliquecostata, they acknowledge differences from that extant species and highlight the need for further investigation of Overall morphological similarity, with finely structured valve this morphology. The Site 742 sediment sequence in which it face elements, ovoid outline, and extensive apical branching of occurs is dated within the T. kolbei Zone (2.2-1.8 Ma), which costae, suggest that F. aurica (LOD is between 4.35-4.21 Ma) corresponds to the peak occurrence of F. robusta in the may have been a precursor to F. laqueata. As with the species AND-1B core. Site 1165 yields F. cf. obliquecostata, which po- discussed above, we have neither directly observed the link be- tentially could be equivalent to F. robusta in sediment from the tween these two species, nor have we observed any continuation T.insigna/vulnifica biostratigraphic Zone (3.5-2.4 Ma) and in a of this lineage that terminated at 2.58 Ma (text-fig. 4a). younger un-zoned sequence (Whitehead and Bohaty 2003). Fi- nally, an unusually high abundance of F. ritscheri (30%, as op- Morphologically, F. bohatyi is a coarser, larger form of F. posed to 3% in modern sediments) is reported in the praecurta, with similar valve outline, apical branching of Pliocene-Pleistocene section from Site 274, a rise off the Ross costae, and hyaline fields on the valve face. Hence, we identify

278 Micropaleontology, vol. 58, no. 3, 2012

TEXT-FIGURE 4 Age ranges of extinct, extant, and new Fragilariopsis species discussed in this paper. Panel a provides age ranges from previous diatom biostratigraphic literature (light grey) and from two independent constrained optimization (CONOP) models of Cody et al. (2008) (mid grey; black). The reader is re- ferred to Cody et al (2008) for a comprehensive discussion of the distinctions between total (mid grey) and average (black) CONOP range models. New species occurrences in AND-1B are shown in dark grey. Panel b presents a proposed lineage for new Fragilariopsis species based on morphology, utiliz- ing the age axis from panel a.

F. praecurta as a possible phylogenetic precursor to F. bohatyi. gests a continuous lineage. Because these two species co-occur Fragilariopsis ritscheri is of similar size to F. bohatyi, however throughout the record, with F. bohatyi relative abundance de- the absence of hyaline fields, distinct curvature of unbranched creasing through time as F. robusta abundance increases, we in- apical costae, and sub-round poroids in F. ritscheri are suffi- fer that F. robusta likely branched off from F. bohatyi at an early ciently dissimilar to F. bohatyi that we do not believe these two stage of the Fragilariopsis radiation (text-fig. 4b). Specimens of species follow a direct lineage. We have observed co-occur- F. robusta in the lower Pleistocene sections of AND-1B are rence of F. bohatyi and F. ritscheri at Site 274 and at AND-1B. transitional to and ultimately indistinguishable from F. Given the FOD for F. ritscheri at 3.44-2.82 Ma (Cody et al. obliquecostata, implying a direct lineage between these two and 2008) with high abundance at Site 274 and trace occurrence at a transition to the extant morphology around 2.0 Ma. The com- AND-1B, it is possible that F. ritscheri arose in the open ocean prehensive Antarctic diatom biostratigraphic synthesis of Cody during the same Fragilariopsis radiation that gave rise to our et al. (2008) finds a FAD of 1.73-1.66 Ma for F. obliquecostata, three new marginal marine species. With the possible exception using constrained optimization to generate an average range age of F. robusta (see below), we have not observed any clear de- model. This FAD is consistent with our data, and the reader is scendant of F. bohatyi in the AND-1B sediment record, which referred to Cody et al. (2008) for a detailed discussion of suggests that this line ended by 2.0-1.6 Ma (text-fig. 4). The constrained optimization as a biostratigraphic tool. species F. linearis, which is morphologically intermediate be- tween F. bohatyi and F. ritscheri, is rare both in modern and Environment: The evolution and establishment of new species older sediments and is consequently not a good base for is commonly associated with environmental change, and it is comparison. likely that the diversification we observe within the genus Fragilariopsis is related to Pliocene cooling. The F. praecurta - Finally, the Pliocene-Pleistocene AND-1B record includes tran- F. aurica - F. arcula group evolved during the late Miocene, sitional forms between F. robusta and F. bohatyi, which sug- which represents a significant step in the late Cenozoic cooling

279 Charlotte Sjunneskog et al.: Fragilariopsis diatom evolution in Pliocene and Pleistocene Antarctic shelf sediments trend (Zachos et al. 2001). Their decline in the early Pliocene and Pleistocene, following the mid-Pliocene climate optimum may then be related to an interval of relative warmth culminat- when Fragilariopsis were rare (Naish et al. 2009, Winter et al. ing in the mid-Pliocene climatic optimum ca. 3.6 Ma (Naish et in press), and represents a transition from limited sea ice cover al. 2009; Winter et al. in press). Possibly this group continued to to the modern seasonal sea ice belt and a progressive cooling. exist in small numbers in refugia, and the onset of cooling in the Fragilariopsis bohatyi, F. robusta, and F. laqueata appear for a late Pliocene brought about more successful forms, which were relatively short duration in the AND-1B biostratigraphic record able to adapt to the changing environment. and may reflect floral response to cooling following the Plio- cene climatic optimum and prior to establishment of the Pleisto- Of the species described here, only F. robusta appears to persist cene sea ice dominated environment that continued into the across the Pleistocene cooling, as it transitions into the modern Holocene. No direct modern analog exists for this environment. sea ice-affiliated species F. obliquecostata. The abundance of F. robusta generally increases in concert with F. curta in AND-1B (text-fig. 2). This supports our interpretation of a general cool- SUMMARY ing trend. Fragilariopsis curta is strongly associated with the ·Three new species of Fragilariopsis are described: F. laqueata modern summer sea ice edge and is observed in highest abun- Riesselman, F. bohatyi Sjunneskog et Riesselman, and F. dance close to melting sea ice, whereas F. obliquecostata is robusta Sjunneskog. sometimes associated with sea ice (Crosta et al. 2004, Armand et al. 2005) but also with cold open water and polynyas of the ·First and last occurrence dates are: F. laqueata <3.3 to 2.58 Antarctic Oceans (Taylor et al. 1997; Cunningham and Ma; F. bohatyi <3.3 to 2.0-1.6 Ma; F. robusta <3.3 to 2.0-1.6 Leventer 1998). These are also the species most commonly Ma. used for tracking paleo sea ice conditions (Zielinski and Gersonde 1997; Bianchi and Gersonde 2002; Kunz-Pirrung et ·These species occur together but in different proportions al. 2003; Crosta et al. 2004; Armand et al. 2005; Leventer et al. throughout the stratigraphic column, which suggests different 2007). environmental preferences for each. From the relatively low abundance of F. curta in the AND-1B · record, we infer that the Pliocene and early Pleistocene seasonal The genus Fragilariopsis underwent a radiation on the conti- sea ice in the McMurdo Sound was neither as extensive nor as nental shelf during the Pliocene. long in seasonal duration as in the Holocene. We suggest F. robusta is not primarily related to the sea ice coverage and the ·The evolution of the Fragilariopsis flora might be an environ- summer sea ice edge environments that characterize the region mental expression of increasingly colder conditions with an in- today, but rather with increased pack ice and dispersed sea ice. creasingly ice-influenced environment as the Pliocene cooled. By the same line of reasoning, the very low abundance of F. Fragilariopsis bohatyi is negatively correlated with F. curta. curta and the coincident peak abundance of F. bohatyi suggest even less association with sea ice for this species. ·These species are circumantarctic in distribution, although at lower abundances than observed in AND-1B. Previous reports We propose therefore that the Fragilariopsis flora described on Pliocene Antarctic diatoms have grouped them with known here developed as conditions grew colder through late Pliocene modern taxa.

PLATE 1 Light microscope images of Fragilariopsis laqueata Riesselman. Scale bar is 10ìm for all specimens.

a, f 251.76 mbsf; e from type slide at 220.11 mbsf; b, i, k, m, r 288.76 mbsf; g, w 288.50 mbsf; c, t, v 275.90; h 220.90 mbsf; d, j, l, n-q 289.75 mbsf; s 291.26 mbsf.

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ACKNOWLEDGMENTS Palaeoclimatology, Palaeoecology, 260:92–121, doi:10. 1016/j. The authors thank Dr. John Barron, Dr. Sherwood Wise, and palaeo. 2007. 08. 020. two anonymous reviewers for their comments and insights, CROSTA, X., STURM, A., ARMAND, L. and PICHON, J. –J., 2004. which have significantly improved this manuscript. The Late Quaternary sea ice history in the Indian sector of the Southern ANDRILL Program is a multinational collaboration between Ocean as recorded by diatom assemblages, Marine the Antarctic Programs of Germany, Italy, New Zealand and the Micropaleontology, 50: 209–223. doi:10. 1016/S0377–8398(03) United States. Scientific studies are jointly supported by the 00072–0. U.S. National Science Foundation, NZ Foundation for Re- search, the Italian Antarctic Research Program, the German CUNNINGHAM, W. and LEVENTER, A., 1998. Diatom assemblages Science Foundation and the Alfred Wegener Institute. This in surface sediments of the Ross Sea: relationship to present oceano- study is based upon work supported by the National Science graphic conditions. Antarctic Science, 10: 134–146. Foundation and the ANDRILL Science Management Office un- FALCONER, T., PYNE, A., LEVYR., OLNEY,M., CURRENT, M. and der Cooperative Agreement No. 0342484. Samples for this in- the ANDRILL–MIS Science Team, 2007. Operations Overview for vestigation were provided by the Antarctic Marine Geology the ANDRILL McMurdo Ice Shelf Project, Antarctica. Terra Research Facility, Florida State University, Tallahassee, FL. Antartica, 14: 131–140.

REFERENCES FLEMMING, F. and BARRON, J., 1996. Evidence of Pliocene Nothofagus in Antarctica from Pliocene marine sedimentary deposits ARMAND, L. K., CROSTA, X., ROMERO, O. and PICHON, J. –J., (DSDP Site 274). Marine Micropaleontology, 27: 227–236. 2005. The biogeography of major diatom taxa in Southern Ocean sediments: 1. Sea ice related species. Palaeogeography, GERSONDE, R., 1991. Taxonomy and mrphostructure of late Neogene Palaeoclimatology, Palaeoecology, 223: 93–126. doi:10. 1016/j. diatoms from Maud Rise (Antarctic Ocean). Polarforschung, 59: palaeo. 2005. 02. 015 141–171.

BIANCHI, C. and GERSONDE, R., 2002. The Southern Ocean surface GIBBARD, P., HEAD, M. J., WALKER, M. J. C. & THE between Marine Isotope Stages 6 and 5d; Shape and timing of cli- SUBCOMMISSION ON QUATERNARY STRATIGRAPHY, mate changes. Palaeogeography, Palaeoclimatology, Palaeoecol- 2010. Formal ratification of the Quaternary System/Period and the ogy, 187: 151–177. Pleistocene Series/Epoch with a base at 2.58 Ma. Journal of Quater- nary Science, 25: 96–102. BOHATY,S. M., SCHERER, R. P. and HARWOOD, D. M., 1998. Qua- ternary diatom biostratigraphy and palaeoenvironments of the HASLE, G., 1965. Nitzschia and Fragilariopsis species studied in the CRP–1 Drillcore, Ross Sea, Antarctica. Terra Antarctica,5: light and electron microscopes, III Fragilariopsis. Det Norske 431–453. Videnskaps–Akademi i Oslo, Matematisk–Naturvidenskapelig Klasse. Ny Serie, 21: 1–49. BART, P. J., SJUNNESKOG, C. and CHOW, J. M., 2011. Piston–core based biostratigraphic constraints on Pleistocene oscillation of the HASLE, G. and MEDLIN, L., 1990. Family Bacillariaceae: the genus West Antarctic Ice Sheet in western Ross Sea between North Basin Nitzschia section Fragilariopsis. In: Medlin, L. K. and Priddle, J., and AND–1B drillsite. Marine Geology, 289: 86–99, doi:10. 1016/j. (Eds), Polar Marine Diatoms, 181–196. Cambridge: British Antarc- margeo. 2011. 09. 005 tic Survey

CODY, R., LEVY, R., HARWOOD, D. and SADLER P., 2008. Think- KUNZ–PIRRUNG, M., GERSONDE, R. and HODELL, D. A., 2002. ing outside the zone: High–resolution quantitative diatom Mid–Brunhes century scale diatom sea surface temperature and sea biochronology for the Antarctic Neogene. Palaeogeography, ice records from the Atlantic sector of the Southern Ocean (ODP Leg

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PLATE 2 SEM images of Fragilariopsis laqueata Riesselman. All specimens are from a well-preserved diatomite at 288.76 mbsf. Black boxes group multiple images from a single specimen; scale bars as noted.

a-c valve face exterior view; d-i valve face interior view, including g interior view of an unusually well-preserved valve fragment with h-i detail of perforated poroid membrane.

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177, sites 1093, 1094 and core PS2089–2), Palaeogeography, TAYLOR, F., MCMINN, A. and FRANKLIN, D., 1997. Distribution of Palaeoclimatology, Palaeoecology, 182: 305–328. diatoms in surface sediments of Prydz Bay, Antarctica. Marine Micropaleontology, 32: 209–229. LEVENTER, A. and DUNBAR, R., 1988. Recent diatom record of McMurdo Sound, Antarctica: Implications for the history of sea ice WHITEHEAD, J. M. and BOHATY, S. M., 2003. Data report: Quater- extent, Paleoceanography, 3:259–274. nary–Pliocene diatom biostratigraphy of ODP Sites 1165 and 1166, Cooperation Sea and Prydz Bay. In: Cooper, A. K., O’Brien, P.E. and LEVENTER, A., ARMAND, L., HARWOOD, D., JORDAN, R. and Richter, C., Eds., Proceedings of the Ocean Drilling Program, Sci- LIGOWSKI, R., 2007. New approaches and progress in the use of ence Results, 188: 1–25. College Station TX: Ocean Drilling Pro- polar marine diatoms in reconstructing sea ice distribution, In: Coo- gram. per, A. K., Raymond, C. R. et al., Eds., Antarctica: A keystone in a changing world – online proceedings of the 10th ISAES X, 1-4, WINTER, D. and IWAI, M., 2002. Data report: Neogene diatom Washington DC: US Geologival Survey. Open–File Report biostratigraphy, Antarctic Peninsula Pacific margin, ODP Leg 178 2007–1047, Extended Abstract 005 rise sites. In: Barker, P. F., Camerlenghi, A., Acton, G. D. and Ramsay, A. T. S., Eds., Proceedings of the Ocean Drilling Program, Science Results, 178: 1–25. College Statiom TX: Ocean Drilling MAHOOD, A. D. and BARRON, J. A., 1996. Late Pliocene Diatoms in Program. a Diatomite from Prydz Bay, East Antarctica. Micropaleontology, 42(3):285–302. WINTER, D., SJUNNESKOG, C., SCHERER, R., MAFFIOLI, P., RIESSELMAN C., and HARWOOD, D., 2010. Pliocene–Pleisto- NAISH, T., POWELL, R., LEVY,R., et al., 2009. Obliquity–paced Plio- cene diatom biostratigraphy of nearshore Antarctica from the cene West Antarctic Ice Sheet oscillations. Nature, 458: 322–328. AND–1B drillcore, McMurdo Sound. Global and Plantetary Change. 10.1016/j. gloplacha.2010.04.004 ROUND, F. E., CRAWFORD, R. M. and MANN, D. G., 1990. The dia- toms: Biology and morphology of the genera: Cambridge: Cam- ZACHOS, J., PAGANI, M., SLOAN, L., THOMAS, E. and BILLUPS, bridge University Press, 747 pp. K., 2001. Trends, rhythms, and aberrations in global climate 65 Ma to present. Science, 292: 686–693. SCHERER, R., HANNAH, M., MAFFIOLI, P., PERSICO, D., SJUNNESKOG, C., STRONG, P., TAVIANI, M., WINTER, D. and ZIELINSKI, U. and GERSONDE, R., 1997. Diatom distribution in the the ANDRILL–MIS Science Team, 2007. Palaeontologic characteri- Southern Ocean surface sediments (Atlantic sector): Implications for sation and analyses of the AND–1B Core, ANDRILL McMurdo Ice paleoenvironmental reconstruction. Palaeogeography, Palaeo- Shelf Project, Antarctica. Terra Antartica, 14: 297–316. climatology, Palaeoecology, 129: 213–250.

SCOTT, F. J. and MARCHANT, H. J., 2005. Antarctic marine protists: Received April 11, 2011 Canberra and Hobart: Australian Biological Resources Study and Accepted March 17, 2012 Australian Antarctic Division, 579 pp. Published July 25, 2012

PLATE 3 Light miscroscope images of Fragilariopsis bohatyi Sjunneskog et Riesselman. Scale bar is 10ìm for all specimens. The panel marked 11 is F. praecurta from Gersonde (1991) Friedrich-Hustedt Collection No. Zu 4/4, Sample PS1467-1, 790cm.

a-g, i-j, r 288.76 mbsf; n, t 219.90 mbsf; h, s 284.90 mbsf; p 220.90 mbsf; k, o, q, l, u 251.76 mbsf; v, x 197.70 mbsf; m from type slide at 219.40 mbsf; w 191.95 mbsf.

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PLATE 4 SEMimagesofFragilariopsis bohatyi Sjunneskog et Riesselman. All specimens are from a well-preserved diatomite at 288.76 mbsf. Black boxes group multiple images from a single specimen; scale bars as noted.

a-b valve face exterior view; detail of perforated poroid membrane and g) interior view of a valve in which h) perforated membranes are c-i valve face interior view, including c) interior view of beginning to detach. an unusually well-preserved valve fragment with f, i)

286 Charlotte Sjunneskog et al. Plate 5

PLATE 5 Light microscope images of Fragilariopsis robusta Sjunneskog. Scale bar is 10ìm for all specimens.

a, d 151.86 mbsf; f 189.36 mbsf; b 185.10 mbsf; g 191.40 mbsf; c 185.32 mbsf; h from type slide at152.80 mbsf; e 165.33 mbsf; i 153.80 mbsf.

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PLATE 6 SEM images of Fragilariopsis robusta Sjunneskog. All specimens are from a well-preserved diatomite at 153.30 mbsf. White boxes group multiple images from a single specimen; scale bars as noted. a-b, d-e, g valve exterior including f internal valve detail showing hyaline regions which interrupt the semi-regular distribution of poroids on d-e detail of raphe ending; the valve face. c, f, h-i valve interior including

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PLATE 7 Specimens from DSDP Site 274. Scale bar is 10ìm for all specimens. Fragilariopsis robusta Sjunneskog.

a 5-3-50; d, i 8-CC; b 8-CC; and e 5-3-50; c 5-3-50. Note the irregular spacing of costae and g-h 9-5-50. These specimens are distinct from the modern hyaline, non-perforated fields on the valve face that F.ritscheri in the branching of costae at the apices, and occlude poroids, but similar outline as compared to in the presence of hyaline, non-perforated fields on the modern F. obliquecostata. Fragilariopsis bohatyi valve face that occlude some poroids. Sjunneskog et Riesselman;

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