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doi:10.2204/iodp.proc.318.105.2011

Biostratigraphy

Systematic core catcher paleontology samples were not taken from the millimeter- to centimeter-scale laminated diatom oozes recovered at Site U1357 in order to preserve a more complete stratigraphy for postcruise investigations. Instead, whole-round samples were collected at section breaks only when core expansion pushed material out of the core liner, and selected samples from Hole U1357A were examined for siliceous microfossils, foraminifers, and palynomorphs. Additional targeted toothpick samples within the split core from Hole U1357A were taken at an average frequency of two samples per core through Core 318-U1357A-19H. Section break samples were collected from Cores 318-U1357A-1H through 9H (0–84.64 mbsf), 318-U1357B-2H through 6H (2.5–50.43 mbsf), and 318-U1357C-3H through 7H (18.30–66.41 mbsf).

All section-break samples yield diverse assemblages of well-preserved diatoms, common to abundant radiolarians, and rare to few silicoflagellates and sponge spicules. Ebridians, actiniscidians, and chrysophyte cysts were not recorded. Sieved sample splits contain benthic foraminifers and abundant, well-preserved planktonic foraminifers as well as fish skeletal debris. All samples processed for palynology yield rich, highly diverse palynomorph associations with very good preservation; palynomorph associations are dominated by dinoflagellate cysts and zooplankton remains. Noteworthy are thecate (motile) dinoflagellates, tintinnid loricae, copepod eggs, and other copepod remains. The fossil assemblages examined from Site U1357 sediments do not contain reworked constituents.

Available biostratigraphic datums do not provide age control within the Holocene (see “Biostratigraphy” in the “Methods” chapter). Therefore, shipboard micropaleontologic efforts were focused on (1) identifying foraminifer-bearing intervals for immediate sampling to prevent foraminifer dissolution (see “Lithostratigraphy”) and (2) characterizing microfossil assemblages to guide postcruise sampling.

Siliceous microfossils

Cores 318-U1357A-1H through 18H (0–170.25 mbsf) contain well-preserved and abundant biogenic silica (opal-A) dominated by diatoms with variable abundances of radiolarians, silicoflagellates, and sponge spicules. Filamentous diatoms and Chaetoceros spp. setae, sponge spicules, and radiolarian needles all contribute to a fibrous sediment texture. In Cores 318-U1357A-19H and 20H (170.25–185.45 mbsf), laminations comprising abundant, well-preserved biogenic silica (Fig. F18A–F18C) are interspersed with more siliciclastic laminations in which the biogenic constituents are common to abundant and moderately fragmented (Fig. F18D, F18E). Siliceous microfossils were not identified in the diamict (see “Lithostratigraphy”) below 185.45 mbsf.

Diatoms are the most diverse microfossil group examined at Site U1357, and the assemblages include many species for which modern environmental preferences have been characterized (Armand et al., 2005; Crosta et al., 2005b), making Site U1357 well suited for paleoenvironmental reconstruction.

Diatoms

A characteristic Holocene Southern Ocean diatom flora with varying contributions from the sea ice–associated and cool open-ocean taxa of Armand et al. (2005) and Crosta et al. (2005b) was identified in all samples analyzed through the biosiliceous sedimentary sequence (the upper 185.45 mbsf) of Hole U1357A (Table T3). In most samples diatoms are abundant with no evidence of dissolution and good to moderate preservation (Table T4). In samples taken from discrete siliciclastic laminations between 170.25 and 185.45 mbsf, diatoms are few to common with moderate to poor preservation caused by fragmentation, whereas samples taken from adjacent biosiliceous-rich laminations contain abundant well-preserved diatoms, sometimes as near-monospecific assemblages (e.g., the Thalassiothrix antarctica + Trichotoxon reinboldii–rich assemblage depicted in Figure F18C, which forms a white lamination).

More than 40 taxa were identified from section-break samples in Hole U1357A (see the “Appendix”); however, these samples were disturbed to such a degree that laminations could not be visually differentiated. Therefore, toothpick samples for diatom assemblage characterization were collected from pairs of discrete laminations in Cores 318-U1357A-1H through 9H (0–84.64 mbsf) and 13H through 19H (112.10–179.29 mbsf). Toothpick sampling is imprecise in comparison to the nondestructive backscattered electron imagery methodologies recently employed in detailed diatom lamination work on coeval sediments from the East Antarctic margin (e.g., Stickley et al., 2005; Denis et al., 2006; Maddison et al., 2006). Even so, distinctions between light and dark laminations are evident in our qualitative assessment (Table T4). Within light–dark lamination couplets (Fig. F6), the dark laminations generally contain a more diverse assemblage that is often more fragmented. In Cores 318-U1357A-18H through 20H (159.6–184.6 mbsf), centimeter- to millimeter-scale laminations that can easily be distinguished from the light–dark couplets by their unusual color and/or fibrous texture often comprise low-diversity assemblages with excellent preservation, suggesting that these represent single bloom events followed by rapid flux directly to the seafloor. For example, toothpick samples from intervals 318-U1357A-19H-2, 19 and 80 cm (thin white laminations), preserve assemblages dominated by of the open water oceanic needle-shaped pennate diatoms T. antarctica and T. reinboldii (Fig. F18C). Such fine-scale variability highlights the need for further detailed investigation to define the nature and temporal significance of the laminations at Site U1357.

Diatom assemblages characteristic of light and dark laminations vary from couplet to couplet, but with some consistent trends. When present, vegetative Chaetoceros dichaeta, vegetative Eucampia antarctica, Porosira glacialis, Proboscia alata, Proboscia inermis, Rhizosolenia antennata var. semispina, and T. antarctica are always more abundant within the light laminations. Fragilariopsis kerguelensis is often more abundant in the light lamina (Fig. F19A), although this distinction is one of many that is masked by the coarseness of the relative abundance scheme applied to Expedition 318 diatom micropaleontology (see “Biostratigraphy” in the “Methods” chapter), in which a species is categorized as “few” if 11–109 specimens are identified in two 22 mm traverses and “common” if 110–1099 specimens are identified. Chaetoceros spp. resting spores dominate the assemblage in all light laminations observed below 144 mbsf (Sample 318-U1357A-16H-3, 59 cm). Actinocyclus actinochilus, the cryophilic Fragilariopsis species of Armand et al. (2005), and Thalassiosira antarctica are generally, but not always, more abundant in dark laminations (Fig. F19B). In lamination couplet pairs at ~13, ~24.5, and ~120 mbsf, assemblages within light and dark laminations are similar.

Based on trends in assemblage successions in paired lamination couplets at Site U1357 and on previous work from the nearby Dumont d’Urville Trough (Denis et al., 2006) and other laminated diatom sections from the East Antarctic margin (e.g., Stickley et al., 2005; Maddison et al., 2006), we provisionally interpret each light–dark lamination couplet pair to reflect a single year of biogenic production and accumulation. In this scenario, production initiated during the spring sea ice retreat and persisted through summer in open water, concluding with regrowth of sea ice in autumn. The preservation of a seasonal diatom assemblage succession across the majority of lamination couplets investigated at Site U1357 requires that syndepositional rather than multiannual sediment focusing mechanisms dominate within the Adélie depositional basin. Given this seasonal accumulation model, it is likely that some of the differences in characteristic assemblages from couplet to couplet arise from sampling variability at a subseasonal scale, whereas other differences may be attributed to changing Holocene climate conditions.

Radiolarians

Radiolarians are common to abundant and well preserved in samples examined throughout Holes U1357A, U1357B, and U1357C. The assemblages consist of typical Antarctic radiolarians dominated by Antarctissa denticulata, Antarctissa strelkovi, and Spongotrochus glacialis. Some Phaeodarian radiolarians that comprise several species of the Family Challengeriidae and some species of Auloceros, Aulographis, and Aulospathis (Family Aulacanthidae) are preserved. Auloceros and Aulographis are known to have numerous thin and hollow needles up to 100 µm in length (Fig. F20). Along with diatom setae and spines and sponge spicules, these radiolarian needles create the fibrous, papery texture of the sediments.

Silicoflagellates and sponge spicules

The silicoflagellate Distephanus speculum speculum (Fig. F18B) occurs in common to trace abundance throughout Site U1357. Although additional silicoflagellate taxa were not identified during shipboard examination, postcruise investigation of the silicoflagellate assemblage may contribute valuable information for paleoenvironmental interpretation. Sponge spicules occur in few to trace abundance. Both groups exhibit good preservation.

Palynology

Three samples (318-U1357A-3H-1, 0–20 cm [17.20 mbsf]; 10H-CC [91.80 mbsf]; and 20H-CC [184.45 mbsf]) were processed to evaluate the potential for postcruise investigations. To obtain residues encompassing a particularly wide range of palynofacies, the residues were sieved through a 10 µm mesh, as opposed to the 15 µm mesh routinely used for the samples from the other Expedition 318 drill sites.

All processed samples yield abundant, well-preserved dinocysts and even motile stages of representatives of the heterotrophic genus Protoperidinium. In all samples, other organic microfossil remains were encountered, such as worm jaws, copepod eggs, other copepod remains, foraminifer test linings, tintinnid loricae, algal cysts other than those of dinoflagellates, chlorophytes, and ciliate remains. Terrestrial palynomorphs (pollen and spores) were not found. Amorphous organic matter is abundant in all samples.

Foraminifers

Reconnaissance shipboard examination of selected section break samples identified planktonic foraminifers to 79.99 mbsf (Sample 318-U1357A-9H-5, 0 cm) and benthic foraminifers to 37.02 mbsf (base of Core 318-U1357A-4H). A single planktonic foraminifer was also found in a strewn slide from a diatom-bearing terrigenous lamina at 177.04 mbsf (Sample 318-U1357A-19H-6, 45 cm), indicating that foraminifers are preserved at least to this depth. Constant carbonate content (see “Geochemistry and microbiology”) and visual identification in split-core halves (see “Lithostratigraphy”) provide further evidence that foraminifers are present to the base of the biosiliceous interval. These initial investigations indicate that foraminifers may be present throughout Hole U1357A. Therefore, >1800 samples were subsequently taken to preserve specimens for potential postcruise foraminifer, isotopic, and Mg/Ca ratio studies.

Planktonic foraminifers

Planktonic foraminifers are well preserved and present in relatively high abundances in low-diversity assemblages consisting of two species: Neogloboquadrina pachyderma and Globigerina bulloides. The former typically accounts for <70% of planktonic foraminifers in Hole U1357A. Such low diversity is typical for Antarctic waters (Berggren, 1992). Their high abundance and good preservation highlights the potential for high-resolution isotopic and Mg/Ca ratio analyses that will be performed postcruise.

Benthic foraminifers

Well-preserved specimens of the calcareous benthic foraminifer species Globocassidulina subglobosa and Triloculina frigida were found in reconnaissance checks of sieved samples from Core 318-U1357A-4H. These species are typical of bathyal to abyssal environments (van Morkhoven et al., 1986).

Fish skeletal debris

A pocket of fish skeletal debris was identified in a section-break sample from the top of Section 318-U1357A-8H-5 (68.79 mbsf) (Fig. F19A) and in samples sieved at 63 µm from 35.60, 54.53, and 68.79 mbsf. Additional intervals of concentrated fish remains identified in the split core are listed in Table T2.

Paleoenvironmental interpretation

Diverse phytoplankton assemblages were identified in the laminated diatom oozes recovered from Site U1357. In addition, abundant zooplankton, including protoperidinioid (i.e., heterotrophic) dinoflagellate taxa, zoobenthos, and nekton, provide evidence of a dynamic Holocene trophic structure within the Adélie depositional basin. Dinoflagellate assemblages comprise exclusively protoperidinioid (heterotrophic) taxa. The abundance of heterotrophic, diatom-consuming dinoflagellate taxa, in combination with fish remains, indicates seasonally high productivity in higher trophic levels.

Diatom assemblages from paired laminations include varying contributions from many of the sea ice–associated and open-ocean diatom taxa (Table T3) identified in the companion biogeographic surveys of Armand et al. (2005) and Crosta et al. (2005b), providing a framework for initial paleoenvironmental interpretation. Further paleoenvironmental studies, which will integrate data from other microfossil groups such as dinoflagellates, silicoflagellates, and radiolarians as well as from geochemistry, are planned for follow-up shore-based research.

Vegetative valves and resting spores of Chaetoceros, one of the most abundant diatom genera in the Southern Ocean, are present in the majority of samples examined in Hole U1357A. In samples between ~144 and 160 mbsf, Chaetoceros resting spores dominate the light lamination assemblage in abundances of <100 valves per field of view. In the review of sea ice biogeography by Armand et al. (2005), Chaetoceros resting spores are found in maximum abundance in conjunction with minimum annual sea ice duration of 7 months and February sea-surface temperatures (SSTs) between –0.5° and 1.5°C. Chaetoceros resting spores also dominate the biogenic flux to the sediments following massive monospecific bloom events in stratified water columns associated with spring sea ice melt (Leventer et al., 1993; Leventer et al., 1996). Porosira glacialis (minimum 7.5 months annual sea ice cover; maximum abundance with February SSTs from –1.3° to 2°C; Armand et al., 2005) commonly occurs in combination with maximum Chaetoceros resting spore abundance in these deepest couplets. The cold, stratified spring surface conditions inferred from these and other members of the assemblage have previously been recognized in quantitative diatom paleoenvironmental reconstructions from the Dumont d’Urville Trough (Crosta et al., 2008).

Rhizosolenia antennata var. semispina and R. antennata var. antennata (the Rhizosolenia pointed group of Crosta et al., 2005b) generally co-vary with F. kerguelensis in light laminations. Whereas both groups are included within the open ocean–related assemblage of Crosta et al. (2005b), the two Rhizosolenia species are categorized as cool open-ocean taxa (optimal SSTs = 1°–2°C; unconsolidated sea ice during winter, open-ocean conditions during summer; Crosta et al., 2005b) and F. kerguelensis is characterized as a pelagic open-ocean species (February SSTs = 1°–8°C; ice free in the summer with as much as 8 months annual sea ice cover; Crosta et al., 2005b). The alternating contributions of these diatoms, in concert with variable abundances of Thalassiothrix spp. (February SSTs = 2°–6°C; encountered north of the modern winter sea ice limit; Crosta et al., 2005b), may reflect fluctuations in polynya development and the sea ice regime throughout the middle and late Holocene. It is likely, however, that a component of this variability is an artifact of imprecise shipboard sampling methodology.

Despite the uncertainties inherent in the qualitative initial data set presented in this report, it is clear that the diverse diatom assemblage recovered from Site U1357 provides an excellent foundation for the development of robust, quantitative paleoenvironmental reconstructions from the East Antarctic margin on subseasonal to annual timescales. Future work will link these preliminary interpretations to other lines of evidence from other microfossil groups (notably dinocysts) as well as from geochemical data.

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