Mixed diatom assemblages in glacigenic sediment from the central Ross Sea, Antarctica

Palaeogeography, Palaeoclimatology, Palaeoecology 218 (2005) 287 – 300 www.elsevier.com/locate/palaeo

Mixed diatom assemblages in glacigenic sediment from the central Ross Sea, Antarctica Charlotte Sjunneskog, Reed P. Scherer* Department of Geology and Environmental Geosciences, Northern Illinois University, DeKalb, IL, USA Received 25 August 2003; received in revised form 25 October 2004; accepted 21 December 2004

Abstract We have investigated diatom assemblages and their distribution in piston and trigger cores from six sites in the central Ross Sea, including analysis of absolute abundance of diatom valves and diatom fragments, with the aim of characterising different glacigenic sediment facies and sediment packets. These data provide a new perspective on the current texturally based classification of diamicton, mud, and diatomaceous mud. All samples investigated include diatom taxa characteristic of the modern Ross Sea flora, thus all deposits evaluated are interpreted to be upper Quaternary in age. Reworked diatoms are ubiquitous, but they vary in terms of species content and concentration. Sediment units classified as diamicton are enriched in robust reworked valves of long-ranging taxa such as Paralia spp., Stephanopyxis spp., and Stellarima microtrias, but different diamicton packets can be distinguished based on variable concentrations of biostratigraphically constrained taxa including Denticulopsis spp., Thalassiosira spp., and Actinocyclus spp. Absolute abundance of diatoms and diatom fragments provide another useful criterion for distinguishing sedimentary processes. Together, these criteria permit interpretations of past glacial and glacial marine processes with greater certainty than allowed by textural analyses alone. We are able to distinguish between tills deposited beneath ice streams from those emplaced by slow-moving ice—information that is important in ice sheet reconstruction. D 2005 Elsevier B.V. All rights reserved. Keywords: Antarctica; Ross Sea; Diatom assemblages; Diatom abundance; Marine sediment; Diamicton

1. Introduction Domack et al. (1999) correctly stated that, bdiamicton is the most difficult sediment to interpret, primarily because it has characteristics of both * Corresponding author. Tel.: +1 815 753 7951; fax: +1 815 753 1945. E-mail address: [email protected] (R.P. Scherer). 0031-0182/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.palaeo.2004.12.019

marine and glacial processes.Q These deposits are further complicated by (1) the fact that they are formed by differing glacier bed processes, (2) they incorporate material from multiple source beds, and (3) successive glacial advances remobilise previous glacial deposits. Diamictons of the Ross Sea incorporate sedimentary materials from the extensive Cenozoic stratigraphic record of the Ross Embayment. These source beds include abundant age-

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diagnostic fossils, notably diatoms, which provide reliable tracers of sediment provenance and mixing in glacial diamictons (Harwood et al., 1989; Scherer, 1992). Domack et al. (1999) and Domack and Harris (1998) constructed a stratigraphic model based on radiocarbon dating and descriptive and textural analysis of multiple cores from the central Ross Sea. Licht (1999) and Licht and Andrews (2002) also analysed some of these cores, and drew some slightly different stratigraphic conclusions. These cores provide a wealth of information. Using diatoms as a means of inferring sediment mixing and transport we can directly compare and further evaluate these complex records, expanding on and strengthening the interpretations of earlier workers.

2. Setting The seafloor morphology in the study area, the central Ross Sea, consists of a broad trough, the Challenger Basin, and the Ross and Pennell Banks (Fig. 1). The seafloor in the inner trough, near the Ross Ice Shelf barrier, is characterised by drumlinlike features, which lead into mega-scale lineations and gullies (Shipp et al., 1999). These features are interpreted as indicators of past ice streaming (Shipp et al., 1999). Washboard moraines are present on the surface and flanks of the Pennell Bank (Shipp et al., 1999), indicating a stepwise withdrawal of the ice sheet from this region. The upper few metres of sediment in the central Ross Sea is recognised as diamicton (generally interpreted as till), covered by

Fig. 1. Generalised map for the Ross Sea embayment showing the location of investigated sites. Squares indicate the location of cores analysed, circles indicate the location of sediments recovered from beneath the West Antarctic Ice Sheet and Ross Ice Shelf. Elevation and bathymetric contours are given in metres. Ice Stream B has been renamed the Whillans Ice Stream.

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transitional glacial marine to hemipelagic sediments, typically muds and diatomaceous muds (Kellogg et al., 1979; Domack et al., 1999). Domack et al. (1999) and Domack and Harris (1998) recognise a muddy pelletised or granulated unit between the diamicton and the mud, interpreted as representing decoupling of grounded ice. This overall stratigraphy is widespread, although these sediment units vary in thickness, and are discontinuous.

3. Material and methods Trigger cores (TC) and piston cores (PC) from the polar research ship Nathaniel B. Palmer (NBP) cruises of 94-01 and 95-01 in the Ross Sea were selected for analysis (Fig. 1; Table 1), following the sedimentologic and stratigraphic work of Domack et al. (1999). In our descriptions we use the descriptive term diamicton, rather than the genetic term till, to describe texturally mixed glacigenic sediment. Diatom microscope slides were prepared for quantitative analyses of diatom abundance following the method of Scherer (1994). Diatom abundance is calculated by counting diatom valves, which include fragments large enough for unequivocal identification to species, or in some cases genus level. We emphasise that the sediments investigated differ from standard deep-sea diatom oozes in that a significant proportion of the diatoms identified is reworked. Consequently we are especially cognizant of the fact that most of the diatoms reflect diverse postdepositional sedimentary processes. In addition to diatom valve counts, diatom fragments, 2–5 Am in size, were counted to quantify taphonomic effects.

Table 1 Location and depth of cores used in the study Core RISP-14 NBP94-01 NBP94-01 NBP95-01 NBP95-01 NBP95-01 NBP95-01 NBP95-01

PC/TC31 TC33 PC/TC11 TC13 TC16 TC18 KC39

Latitude

Longitude

Water depth (m)

82.22S 75.165S 75.300S 76.453S 76.726S 76.943S 77.333S 74.473S

68.38W 178.548W 179.615W 179.086W 178.630W 179.823W 179.536W 173.512E

177 473 603 659 677 712 819 557

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To aid in identification of fragmented diatoms, selected diamicton and mud samples were sieved, using a 20 Am sieve, in order to concentrate wellpreserved specimens. Sieved samples were not used in the quantitative analysis. The diatom counts were performed with a 1000 magnification using oil immersion (minimum of 300 specimens or 500 fields of view). Additionally the slides were scanned at 600 magnification (oil), to encounter the youngest stratigraphic markers present, which might have been missed during standardised counting. Diatom abundance data, N2%, were square root transformed, and processed by principal component analyses (PCA), and plotted using the Canoco 4.0 software (terBraak and Smilauer, 1998), to identify the most important similarities between samples. Nondiatom siliceous microfossils (ebridians and chrysophyte cysts) were included in a supplementary run thus do not directly influence the diatom results.

4. Results 4.1. Diatom valve and fragment abundance Our analyses of diatom absolute abundance generally corroborate the Domack et al. (1999) stratigraphic descriptions, however we find that diatom valve abundance in sediment described as diatomaceous mud (Domack et al., 1999) varies by up to three orders of magnitude, from a maximum of 250106 to a minimum of 0.4106 valves/gram dry sediment (v/gds) (Fig. 2, Plate Ia, and b). Similarly, in the diamicton and mud sequences, diatom abundance is found to vary by two orders of magnitude, from 10106 to 0.1106 v/gds (Fig. 2), overlapping with diatomaceous mud, with the highest diatom valve abundance in diamicton unit of core PC31 and mud of TC33 (Plate Ic and e). The low overall diatom abundance in diamictons necessitated the use of diatom fragment counts to permit statistically significant and reproducible results. The lowest concentration of 2–5 Am fragments is found in diamictons from TC/PC11, TC13 and TC16 (Plate If). High diatom fragment abundance is recognised in diatomaceous mud and mud units of TC33 and TC18 (Fig. 2; Plate Ib, c and d).

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Fig. 2. Absolute abundance of diatom valves and fragments (2–5 Am) in millions per gram dry sediment, and the sum of percent bmodernQ diatom species, as defined in the text. The abundance scale is selected to span most of the represented abundances, but several samples from diatomaceous mud of PC31 and TC33 contain diatom abundances beyond the displayed axes, as indicated by numbers 1–6 on the graph. The actual values for these samples (106 v/gds) are follows: #1=221, #2=73, #3=97, #4=71, #5=209 and #6=164. Samples from TC18 in the interval 3–20 cm were unavailable for study.

4.2. Species distribution We identify a late Quaternary (postglacial) diatom assemblage component by the occurrence of typical modern Ross Sea taxa, including Fragilariopsis curta, Fragilariopsis obliquecostata, Fragilariopsis kerguelensis, Thalassiosira antarctica, Thalassiosira gracilis, Thalassiosira lentiginosa, and/or Thalassiosira

tumida. Although many of these taxa have their first occurrence in the Pliocene (Table 2), we interpret the assemblage as reflecting, at least in part, a late Pleistocene component. All sediments include occurrences of diatoms characteristic of the modern flora, although Pleistocene specimens are generally rare in diamicton units. Among the most ubiquitous and abundant diatom groups are the tychopelagic Paralia

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Plate I. Typical microscopical views of sediment types discussed in the text, following the stratigraphical descriptions of Domack et al. (1999). The images illustrate the difficulty in establishing widely applicable descriptive terminology for Antarctic glacigene sediments. All micrographs were taken at 630 magnification and printed at the same scale. Scale bar (lower right) is 50 Am. The sample mass used in slide preparation is shown in parentheses to allow visual normalization of sample density. (a) bDiatomaceous mudQ from TC33, 12 cm (0.01493 g); (b) bDiatomaceous mudQ from TC16, 28 cm (0.03934 g); (c) bMudQ from TC33, 55 cm (0.01490 g); (d) bMudQ from TC18, 43 cm (0.03636 g); (e) bDiamictonQ of PC31, 55 cm (0.04275 g); (f) bDiamictonQ from TC11, 67 cm (0.04966 g). Note the concentrations of identifiable diatoms and matrix among bmud,Q bdiatomaceous mud,Q and bdiamictonQ samples. Whole diatoms in mud and diatomaceous mud samples include a variable concentration of modern taxa, whereas whole diatoms in diamicton samples are dominantly extinct Tertiary forms.

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Table 2 Diatom taxa with abundance N2%, and their stratigraphic range

Table 2 (continued) No.

Species

Age range

No.

Species

Age range

35

1 2 3

Fragilariopsis ritscherii Thalassiosira antarctica Comber Fragilariopsis kerguelensis (O’Meara) Hasle Fragilariopsis obliquecostata (Van Heurck) Hasle Thalassiosira gracilis (Karsten) Hustedt Thalassiosira tumida (Janisch) Hasle Corethron criophilum Castracane Chaetoceros rsa Fragilariopsis angulata (O’Meara) Hasle Fragilariopsis curta (Van Heurck) Hasle Fragilariopsis cylindrus (Grun?) Hasle Actinocyclus actinochilus (Ehrenb.) Simonsen Eucampia antarctica (Castracane) Mangin Asteromphalus spp. Nitzschia grossepunctata Shrader Denticulopsis spp. Thalassiosira vulnifica (Gombos) Fenner Actinocyclus ingens Rattray Denticulopsis lauta (Baily) Simonsen Thalassiosira fraga Schrader Denticulopsis maccollumii Simonsen Trinacria spp.b Paralia spp. Actinoptychus spp.c Stephanopyxis spp. Coscinodiscus spp. Thalassiosira cf. nansenii Scherer and Koc¸ Aulacodiscus browneii McCollum Denticulopsis delicata Yangasawa and Akiba Rhabdonema spp. Trochosira spp Fossil sporesd Kisseleviella spp. Actinocyclus spp.

Pliocene–Recent Quaternary Pleistocene–Recent

Middle Miocene– Lower Pliocene Eocene–Oligocene Long range Upper Miocene– Pleistocene Long range Pliocene

4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34

Pliocene–Recent

39 40

Pliocene–Recent

41

Long range

42

Long range Upper Pliocene– Recent Lower Pliocene– Recent Lower Pliocene– Recent Pliocene–Recent

43

Denticulopsis hustedtii Simonsen and Kanaya Hemiaulus spp. Rhizosolenia spp. Actinocyclus octonarius Ehrenberg Thalassiothrix spp. Thalassiosira inura Gersonde Stellarima microtrias (Ehrenb.) Hasle and Sims Thalassiosira oliverana var. sparsa Harwood and Maruyama Rouxia spp.

44

Thalassiosira torokina Brady

45

Pterotheca spp.

36 37 38

Pliocene–Recent

Miocene–Recent Long range Middle Miocene Miocene Pliocene Upper Miocene– Lower Pleistocene Middle Miocene Lower Miocene Middle Miocene Oligocene–Miocene Long range Long range Cretaceous–Pliocene Long range Oligocene–Lower Miocene Middle Miocene Upper Miocene– Lower Pliocene Long range Eocene–Oligocene Long range Eocene–Oligocene Long range

Ebridianse Chrysophyte cysts

Cretaceous–Recent Upper Miocene Eocene–Lower Pleistocene Upper Miocene– lower Pleistocene Cretaceous–Lower Miocene Eocene–Miocene Cretaceous–Recent

The number in the first column refers to the PCA plot shown on Fig. 5. a Chaetoceros Hyalochete resting spores with modern morphology. b T. pileolus, and T. excavata. c Mainly A. undulata. d Including heavily silicified Chaetoceros, Chasea, Lyradiscus, Xanthiopyxis, etc. e Dominantly Pseudammodochium lingii Bohaty and Harwood.

spp. (dominantly P. sulcata) and various species of the neritic genus Stephanopyxis. Paralia spp. represent between 1 and 30% of the diatom assemblage in diatomaceous mud units, and between 10 and 55% in the diamictons (Fig. 3). Both Paralia and Stephanopyxis include extant species, but in the Ross Sea their presence is almost certainly due to reworking from Tertiary strata. Both genera are robust, thus are most likely to survive glacial transport. The occurrence of ebridians and chrysophyte cysts (non-diatom siliceous microfossils) varies greatly among cores, with the highest concentrations recorded in diamicton from PC31. We describe the diatom results from the cores from the top down. PC31: The upper 31 cm has a typical post-glacial diatom assemblage and high diatom abundance, averaging 60106 to 200106 v/gds (Figs. 2, 3), which is in the same order of magnitude as high

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Fig. 3. (a) Down-core relative abundance (%) diatom distribution of selected diatom species in cores PC31, TC33, and TC11. Species displayed were selected on PCA-score and biostratigraphic importance. (b) Down-core relative abundance (%) diatom distribution of selected diatom species in cores TC13, TC16, and TC18.

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productivity sites in the Antarctic Peninsula and the Ross Sea (Leventer et al., 1993, 1996; Sjunneskog and Taylor, 2002). Samples collected from between

40 and 175 cm share a similar diatom composition, comprising ca. 25–30% early–mid-Miocene species (e.g., Denticulopsis lauta, Denticulopsis maccollumii,

Fig. 3 (continued).

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Nitzschia grossepunctata) and ca. 25% Paralia spp. (Fig. 3). Age-diagnostic taxa such as Actincyclus ingens increase below 55 cm, and Thalassiosira vulnifica occurs from 60 cm to the bottom of the core. Chaetoceros resting spores of modern morphology occur in abundance between 40 and 70 cm depth. Chrysophyte cysts and ebridians are common in the lower section of PC31. TC33: The upper ca. 25 cm has a typical postglacial diatom flora, which is underlain by a stepwise increase in the percentage of Paralia spp., with lower concentration of Chaetoceros spores and Fragilariopsis obliquecostata (Fig. 3). The diatom abundance ranges from 0.5 to 2108 v/gds in the upper 40 cm of the core and 1–3107 v/gds in the lower sections of the core (Fig. 2). Age-diagnostic taxa (N2%) including A. ingens, T. vulnifica, and Denticulopsis maccollumii occur below 33 cm. TC11: The diatom flora in the upper 15 cm is dominated by Fragilariopsis obliquecostata with ca. 10% contribution of Paralia spp. Typical post-glacial species are rare below 20 cm (Figs. 2, 3), and there is a gradual down-core change between 15 and 35 cm toward a higher concentration of Paralia spp., ca. 50%, and a reduction in F. obliquecostata. The diatom abundance is 5–30106 v/gds (Fig. 2). PC11 is dominated by Paralia spp. and Stephanopyxis spp., and the diatom abundance is only 4106 v/gds (not shown). Age-diagnostic taxa (N2%) Thalassiosira torokina and Denticulopsis maccollumii occur in the upper ca. 40 cm of the core. TC13: Only the upper ca. 10 cm contains the typical post-glacial diatom assemblage (Fig. 2). Below this, the diatom flora gradually changes to a dominance of Paralia spp., Stellarima microtrias, and a variety of fossil spores (Fig. 3). The diatom abundance is ca. 2107 v/gds in the upper few centimetres and 2106 v/ gds in the remainder of the core (Fig. 2). Agediagnostic taxa Denticulopsis maccollumii, Thalassiosira oliverana var. sparsa, and Thalassiosira inura (N2%) occur randomly throughout the core, whereas T. torokina occurs below 30 cm. TC16: The upper 20 cm of the core is dominated by Paralia spp., with a very small component of the postglacial diatom flora (Figs. 2, 3). The diatom abundance is 0.4–20106 v/gds (Fig. 2). Below 20 cm, the diatom assemblage changes toward a lower percentage of the tychopelagic Paralia spp. and higher holoplanktonic

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taxa such as Rhizosolenia spp., and Thalassiothrix spp. (Fig. 3). Post-glacial diatoms are present, in low abundance, throughout the core (Fig. 2). The diatom abundance is very low, only ca. 1106 toward the bottom of the core (Fig. 2). Age-diagnostic taxa Denticulopsis maccollumii, Denticulopsis lauta and Denticulopsis delicata occur in the upper ca. 30 cm, T. torokina and T. inura are common below 30 cm. TC18: This core is dominated by Paralia spp. and a variety of fossil spores, together making up 30–35% of the diatom assemblage in the upper 30 cm of the core (Fig. 3). The diatom abundance is 2107 v/gds in the upper 3 cm, declining to ca. 2106 v/gds below 3 cm (Fig. 2). As in TC16, a small fraction of post-glacial diatoms are present throughout the core (Figs. 2, 3), and age-diagnostic reworked taxa occur throughout the core. The diatom percentage data were analysed using PCA to compare the diatom distribution between samples. Diatom species encountered and their published biostratigraphic ranges are listed in Table 2. PCA was performed to identify differences between sites as well as down-core shifts in diatom assemblage. No attempt to quantify or test significance between groups identified in the PCA was made at this stage. The eigenvalues of axes 1 and 2 are 0.434 and 0.102 respectively, and four axes explain 66% of the data (cumulative percentage variance of species data) (Figs. 4, 5). Diatom species typical of a Quaternary and post-glacial assemblage are located along the positive first axis, whereas diatoms characteristic of Tertiary strata are situated along the second axis. No distinct age distribution is evident among the diamictons, suggesting that the pre-Quaternary diatom component is well mixed. Non-age-specific diatoms are spread throughout the plot, although those common in the post-glacial diatom flora plot accordingly. The samples clustered along the first axis to the far right in the diagram (Fig. 4) share a typical post-glacial diatom flora, and represent diatomaceous mud and mud from PC/TC31 and TC33 (Domack et al., 1999) (Plate I, Fig. a). Closer to the centre of the plot, along the first axis, are samples dominated by a post-glacial diatom assemblage, but with a considerable contribution of diatoms typical of older strata (e.g., up to 20% Paralia spp.). Diatom species that are characteristic of preQuaternary deposits and non-age-specific taxa that are rare in modern sediment are represented in the left side

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Fig. 4. PCA plot showing relationship among samples analysed, labelled by core and depth (cm). Holocene diatomaceous muds plot along the axis of the right quadrants. The upper left quadrant includes ubiquitous long-ranging planktonic diatoms, and the lower left quadrant includes Tertiary age coastal diatom dominated assemblages. The plot demonstrates how diatom assemblages can distinguish among sedimentary packets formed by different processes, with particles derived from different source beds. See Fig. 5 for the same data plotted by species.

of the PCA plot. The lower left quadrant is dominated by coastal, non-age-specific taxa (Paralia spp. and Stephanopyxis spp.) The upper left quadrant is dominated by non-age-specific, taxa such as Rhizosolenia spp., Thalassiothrix spp. and Stellarima microtrias. Located in the upper left quadrant are all samples from TC13, all but one of TC18, and all except two of TC16. Represented in the lower left quadrant are all diamicton samples from PC31, the diatomaceous mud of TC16 and all but two samples of TC/PC11, and mud of TC33 (Fig. 4). The down-core succession in PC31 and TC33 follows a trend that might be expected from the lithological description, where the major shifts in diatom assemblage coincide with lithological changes. In PC31 the diatomaceous mud is rich in post-glacial diatom species, and has a diatom abundance that is compatible with other post-glacial deposits in and around the Antarctic (Leventer et al., 1993, 1996; Sjunneskog and Taylor, 2002). The shift in diatom assemblage coincides with the transition from diatomaceous mud to diamicton over a sequence of mud

and clast-rich diamicton. TC33 has a different lithology: a diatomaceous mud in the upper 21 cm, followed by mud that is divided into an upper and lower unit, separated by laminated mud. The diatomaceous mud is high in diatom abundance and rich in post-glacial diatom species. The upper mud is similar to that of the diatomaceous mud but with a higher contribution of Paralia spp. The lower mud unit contains a high proportion of mixed-age diatom species and the composition is more similar to that of diamictons (Figs. 4, 5). Sieving of samples from TC33 results in an enrichment of Eucampia antarctica (from 45 cm and down), this species is often associated with ice shelf/sea ice retreat and/or winnowing (Kellogg et al., 1979; Cunningham et al., 1999; Taylor et al., 2001). Diatom stratigraphic changes in TC/PC11, TC13, TC16 and TC18 are less clear, without apparent changes following the lithology provided by Domack et al. (1999). The core-tops and the upper few centimetres of diatomaceous mud, described to 30 cm depth, plot among the post-glacial species of the PCA plot. Samples from further down-

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Fig 5. PCA plot of species distribution. The numbered vectors (1– 46) represent diatom species listed in Table 2, along with their biostratigraphic ranges. The modern diatom flora plots along the first positive axis. Species dominating the upper left quadrant include long-ranging forms such as Rhizosolenia spp., Thalassiothrix spp. and Stellarima microtrias. The lower left quadrant is dominated by Tertiary forms including Paralia spp. and Trinacria spp. Ebridians and chrysophyte cysts are not included in the diatom PCA analyses, but are plotted here based on a supplementary analysis that includes these groups.

core but still within Qdiatomaceous mudQ plot among the diamicton samples dominated by pre-Quaternary species (Figs. 4, 5), indicating that the diatom composition of diatomaceous mud is more similar to diamicton in those cores.

5. Discussion From our data it is evident that not only diamictons, but also muds and diatomaceous muds, have complex compositions of diatom species, reflecting mixing from source rocks of different ages and environments of

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initial deposition, with transport and mixing due to differing glacial processes and depositional mechanisms. The diatom data are shown to provide a powerful means of subdividing sedimentary packets in the Ross Sea that may be texturally indistinguishable. Because nomenclature and application varies among authors, we choose to specifically define our use of the nomenclature. Diatomaceous mud, defined as possessing a diatom component of between 15 and 30% (Janecek, 1997), usually implies nearly or actually din situT deposition. Sections described as diatomaceous mud sensu Domack et al. (1999) are shown to contain highly variable diatom abundance ranging from high values of 2.5108 v/gds to very low abundance of 0.4106 v/gds (Plate Ia and b, Fig. 2). The lower abundance clearly represents less than 15% diatom component and we find that much of the diatom content in those sediments is reworked, with a concentration of late Quaternary diatom species as low as 10% (Fig. 2). Consequently, diatom valve abundance cannot be interpreted as a reflection of primary productivity or hemipelagic (open marine) sedimentation. The relative abundance of Quaternary to Tertiary diatom species, the state of diatom preservation, and the absolute abundance of diatom valves are all useful criteria for interpreting depositional mechanisms and source beds. Diatombearing sediment of post-glacial age (radiocarbon dated) with a major component of post-glacial diatom flora can be considered representative of din situT hemipelagic deposition. A high proportion of reworked diatom valves in the post-glacial age sediment indicates reworking by currents, dissolution of Holocene components, or possibly slumping or iceberg scouring. Sites PC31 (0–35 cm) and TC33 (0–21 cm) are deposits from a true open marine setting, following the criteria above and in agreement with the model by Domack et al. (1999). These core sections are similar to post-glacial high productivity areas of the Antarctic Peninsula and Ross Sea (Leventer et al., 1993, 1996; Sjunneskog and Taylor, 2002). Based on the diatom evidence, the uppermost 30 cm of sites TC11, TC13, TC16, and TC18, located in drumlinised areas in bathymetric lows, are strongly influenced by processes of reworking, and should be classified as mud. Our interpretation is more similar to Licht (1999) and Licht and Andrews (2002) who describe the sediment as mud and/or stratified diamicton. The central Ross Sea has been described as a low or non-accumulation area (Cunningham and

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Leventer, 1998; Truswell and Drewry, 1984), and our findings corroborate these results. Mud, defined by grain-size (N70% silt and clay, and b15% diatoms) is a detrital terrigenous or volcanic deposit (Janecek, 1997). Two of the sites included here, TC33 and TC18, comprise expanded sequences of mud. We interpret the stratigraphically mixed diatom flora of the lower mud unit of TC33 (60–92 cm) as deposition of a glacial/post-glacial diatom flora beneath or close to an ice shelf, with a contribution of pre-Quaternary diatoms carried englacially or by subglacial melt water, or advected by sub ice shelf currents. In the upper mud unit, increased concentration of Thalassiosira antarctica and Eucampia antarctica might indicate a break-up of the nearby ice shelf (Cunningham et al., 1999; Taylor and Sjunneskog, 2002). This is in agreement with the findings of Shipp et al. (1999) who suggested that this site was not covered by grounded ice at the Last Glacial Maximum (LGM). The diatomaceous mud of TC33 is radiocarbon dated, with age range from 3.4 ka (uncorrected) at the top to 8.1 ka just above the transition to mud (Domack et al., 1999; Licht, 1999). The upper mud unit is dated to 12.7 ka, which would appear as a likely time for ice sheet recession at this site. The outer continental shelf was glaciated prior to 14 ka and the grounded ice had retreated by 11 ka (Domack et al., 1999). In this context, the deposition of mud could be interpreted as a transitional sequence following ice shelf retreat. TC18 contains the most fine-grained sediment succession (Sjunneskog, 2002) and also is the core with the most abundant small diatom fragments. These observations indicate a lowenergy setting and enhanced preservation of small fragments, probably due to rapid burial. Our findings are in accord with Licht and Andrews (2002) who describe site TC18 as mud with anomalous old radiocarbon ages, suggesting old carbon being deposited by subglacial melt water. The reported age at 26– 28 cm depth is 20.5 kyr and 24.7 kyr at 62–64 cm depth (Licht and Andrews, 2002). We do not know enough regarding sub-ice-shelf/near ice shelf deposition to explain the differences observed in diatom composition between the different mud sections represented in our selection of cores, although the apparent differences suggest that complex depositional settings and processes are involved. Our results do suggest less open marine contribution than indicated

by Domack et al. (1999). Between RISP and the Ross Ice Shelf barrier lies ca. 500 km of sea floor that has not been sampled. Sub-ice-shelf oceanographic study and additional sediment coring via access holes through the ice shelf will help constrain sub-ice-shelf transport distances and mechanisms. Diamicton units can be characterised by diatom content, especially when consideration is given to three independent parameters: diatom abundance, preservation, and stratigraphic mixing. Absolute abundance of diatom valves provides an objective estimate of biogenic versus terrigenous components. Diatom preservation, including absolute and relative abundance of small diatom fragments, is influenced by dissolution, predation, compaction, and subglacial shearing (Scherer et al., 2004). Degraded diatom assemblages show a marked increase in relative abundance of heavily silicified forms. Fragmentation of Antarctic continental shelf diatoms by subglacial shearing has been shown to have a distinctive signature, characterised by depletion in large diatoms and, especially, pennate forms (Scherer et al., 2004). Abundance of fragments over complete valves can also suggest particle size sorting, as seen in TC18. Recognition of age-specific diatom markers is an especially powerful tool for gauging stratigraphic mixing and sediment provenance, as previously shown by Harwood et al. (1989) and Scherer (1992). The diatom signature that characterises the diamicton unit of PC31, a strong mid-Miocene component and relatively little stratigraphic mixing, coupled with higher diatom abundance, can be compared with sediment samples recovered from Crary Ice Rise, an ice shelf pinning point near the southern margin of the Ross Ice Shelf (Fig. 1). The sedimentary record at Crary Ice Rise is characterised by a dominant upper Miocene diatom assemblage, with relatively little stratigraphic mixing (Scherer et al., 1988). The unusually high diatom abundance and relatively low stratigraphic mixing in diamicton of PC31 is interpreted as the result of less pervasive reworking than is evident in the Challenger Basin diamictons (e.g., sites TC11 through TC16). Fragmentation is considerable, compared to the other cores investigated (Plate Ie and f), although at least some of this fragmentation is likely an artefact of fragmentation in the source beds. Our results from PC31 support the model by Domack et al. (1999) suggesting a subglacial deposit, and the

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hypothesis of Shipp et al. (1999) who suggested, based on geophysical investigations, that the Pennell Bank acted as a pinning point during the last glacial advance. It is possible, based on diatom stratigraphy, to subdivide the diamicton of PC31 into two parts. The common occurrence of modern Chaetoceros resting spores delineates the upper 40 cm. Miocene diatoms characterise the section from 40 to 70 cm, which coincides with the bgranulatedQ facies described by Domack et al. (1999 and the unpublished cruise report), as well as within the underlying diamicton. Domack et al. (1999) suggest that the granulated facies reflects the decoupling of the grounded ice. Our data indicate that the granulated facies is composed of sorted material derived from underlying diamicton which is consistent with the interpretation of Domack et al. (1999). Similar to PC31, TC/PC11 is dominated by Paralia spp. but PC/TC11 lacks the early–midMiocene component and high diatom abundance of PC31 (Fig. 5). TC/PC11 is located in a fluted area (Shipp et al., 1999), indicating presence of active ice or ice streaming at the time of deposition of these bedforms, making the setting much different from PC31. The low diatom abundance and high percentage of heavily silicified, non-age-specific taxa, as well as the low diversity assemblage, is the likely result of pervasive subglacial reworking and long-distance transport of sediment particles. Late Quaternary diatom species, such as Thalassiosira antarctica, are encountered in the bottom of PC11, suggesting that the scouring of the seafloor was a late Quaternary event, which is in accord with the radiocarbon dates obtained from TC/PC11, 22.7–27.1 kyr (Licht and Andrews, 2002). The near absence of whole pennate diatoms further suggests subglacial shearing (Scherer et al., 2004), but the diatom record of TC/PC11 is in general too poor to draw any further conclusions. Diamicton of sites TC13, TC16, and T18 is dominated by Stellarima microtrias, Rhizosolenia spp., and Thalassiothrix spp. but as with TC/PC11, overall diatom abundance is poor.

6. Conclusions We find considerable variation in diatom composition and absolute diatom abundance among sediment units of described similar lithology, which might

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have implications for interpretation of depositional environments and processes. There is a distinct difference in post-glacial diatom deposition between sites located at the outer Ross Sea Embayment (TC33 and PC31) and the sites in the Challenger Basin (TC11 through 18). TC33 and PC31 have a dominance of post-glacial diatom flora and high diatom abundance that continues throughout the diatomaceous mud unit. The Challenger Basin sites have lower diatom abundance and a relatively high contribution of reworked diatom species, gradually increasing with depth and within the diatomaceous mud. Cores TC33 and TC18 both have extended sequences of mud, but display significant differences in diatom flora and abundance, whereas TC33 has a higher diatom abundance and enrichment of Paralia spp. TC18 has a low diatom abundance and enrichment in Stellarima microtrias and Rhizosolenia spp., indicating different depositional processes and/or source beds. Diamicton samples can be described in three categories. Diamicton in PC31 has relative high diatom abundance and low stratigraphic mixing, suggesting an ice rise or pinning point characterised by slow moving ice during the LGM. The diamicton of TC/PC11 and TC16 shows low diatom abundance with a dominance of reworked Paralia spp. and Stephanopyxis spp., as compared with the dominance by Stellarima microtrias and Rhizosolenia spp. seen in TC13. The reason for this difference might be attributed to source beds, degree of reworking, and/ or transport distance. A difference in diatom preservation is observed between the Pennell Bank site and the Challenger Basin sites, with a higher diatom and fragment abundance at the Pennell Bank. This is likely related to grounded ice flow and/or compaction from ice sheet overburden within local diatomaceous source beds. In general, we interpret Pennell Bank sediments as having been deposited beneath thick, slow moving ice, whereas the upper metres of sediment in the Challenger Basin were emplaced beneath a low profile ice sheet, probably under ice streaming conditions. These sediment units may have formed during the same broad time interval, as suggested by the fact that all samples contain Quaternary diatom components, but the differences seen between sites suggest that these sediment units were formed from different source beds and

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emplaced under significantly different glacial and glacial–marine regimes, resulting in complex sets of facies. Diatoms are shown to be powerful tracers for analysing the complex glacigenic marine sediment facies of the Antarctic continental shelf. We have demonstrated that analysis of fossil assemblages provides an important addition to traditional lithostratigraphic description for distinguishing glacial sedimentary packets and interpreting past ice sheet processes. These interpretations would be further improved by a better understanding of sedimentary processes under ice streams and slow-moving ice. We encourage further direct observation and sampling of the sub-ice and sub-ice-shelf environment.

Acknowledgements This work was funded by NSF grant OPP9980364 to Reed Scherer. We thank Eugene Domack, Jason Whitehead, Matthew Olney, and an anonymous reviewer for their helpful comments.

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