LITHOSTRATIGRAPHY

Site 1098

Sediments recovered at Site 1098 consist of alternating massive muddy diatom oozes, laminated mud-bearing diatom oozes, and diatom-bearing silty clays and clayey silts with a generally low sand content. Sediment color varies between olive (5Y 4/3), olive gray (5Y 4/2), and dark olive gray (5Y 3/2) in the diatomaceous muds and between orange brown (10YR 5/6), dark brown (10YR 4/3 and 3/3), dark grayish brown (2.5Y 4/2), and dark gray (5Y 4/1) in the laminated oozes. Different degrees of bioturbation are present; burrows are mainly bedding-parallel (Planolites type), although a few long, open vertical burrows were observed. Diatom preservation is generally good. Diatom species composition varies between laminated and bioturbated intervals, and also from top to base of the hole. Graded sands and silts associated with structureless intervals above and slumped material below are interpreted as turbidites. Most of the sequence at Site 1098 is horizontally bedded, but slump features and inclined bedding were observed at the base of Holes 1098B and 1098C. Ice-rafted pebbles increase in number downhole. At the base of Holes 1098A and 1098C, a further increase in coarse clastic material and pebbles is observed. Depositional processes included (1) pelagic/hemipelagic settling and (2) sediment gravity flows from the steep sides of the Palmer Deep basin. Sponge debris was found in Section 178-1098C-5H-2 at 98 cm, and fish debris (vertebrae and teeth) at 105 cm. We describe the entire section as one lithostratigraphic unit divided into Subunits IA and IB (Table T2).

Unit I

Subunit IA
Intervals: Core 178-1098A-1H through Section 6H-3, 60 cm; Core 178-1098B-1H through Section 5H-CC; Core 178-1098C-1H through Section 5H-4, 50 cm
Depth: 0-43.5 (mbsf) in Hole 1098A; 0-43.0 mbsf in Hole 1098B; 0-42.2 mbsf in Hole 1098C
Age: Holocene

Description

Lithostratigraphic Subunit IA consists mainly of alternations of massive, bioturbated, muddy diatom ooze with laminated, mud-bearing diatom ooze (Figs. F7, F8 ). Diatoms, including resting spores, form 45%-80% of the sediment (estimated from smear slides), with sponge spicules averaging 5% in the three holes. Minor amounts (<4%) of silicoflagellates, radiolarians, and benthic foraminifers are present in certain intervals.

Massive, muddy diatom ooze ranges from olive (5Y 4/3) to olive gray (5Y 4/2) and dark olive gray (5Y 3/2) in color. Although no primary structures are preserved, faint burrow mottling is visible as slightly grayer burrow fills. Burrows are mainly bedding-parallel (Planolites type), although a few long, open, vertical burrows 3-6 mm wide were observed (e.g., intervals 178-1098B-1H-2, 13-65 cm, and 1H-3, 110-142 cm, and Sections 3H-2 and 3H-3). Diatom preservation is good at the top of the hole and moderate in the lower part (below ~25 mbsf), and the assemblage is diverse.

Laminated muddy and mud-bearing diatom oozes are quite variable in color and species composition. Colors include olive (5Y 4/3, 5Y 5/3), olive gray (5Y 4/2), dark olive gray (5Y 3/2) orange brown (10YR 5/6), dark brown (10YR 4/3, 10YR 3/3), dark grayish brown (2.5Y 4/2), and dark gray (5Y 4/1). In the upper part of the hole, the laminae are 5-20 mm thick and have diffuse tops and bases, locally with minor bioturbation (Fig. F9). No consistent color sequence was observed within laminated units. In the lower part of the hole (below 40 mbsf), the laminae are sharply defined, 3-40 mm thick, are not bioturbated, and may show weak grading. Individual laminae are each composed of a very restricted diatom assemblage. They are dominated by Chaetoceros spp. resting spores (the most common, particularly toward the base of the hole; orange brown, dark brown, and dark gray laminae); Corethron criophilum (the lighter olive laminae, most common near the top of the hole); and Eucampia antarctica (dark gray laminae). Diatom preservation in the laminae is excellent, with some samples resembling fresh plankton hauls.

The downhole occurrence of massive (bioturbated) and laminated sediments is shown in the lithology column in Figure F7. Laminated sediments predominate from 8 to 23 mbsf.

Sand-size particles are present throughout the sequence in low amounts (<10%; Fig. F8). Moderate to intense bioturbation is common in the massive muddy diatom oozes and seems to be more distinct in the upper part of the holes (Cores 178-1098A-1H through 3H; 1098B-1H through 3H; 1098C-1H through 3H).

Subunit IB
Intervals: Sections 178-1098A-6H-3, 60 cm, through 7H-CC; Sections 178-1098C-5H-4, 50 cm, through 5H-CC
Depth: 43.5-45.9 mbsf in Hole 1098A; 42.2-46.7 mbsf in Hole 1098C
Age: Holocene

Description

Lithostratigraphic Subunit IB is characterized by dark gray (5Y 4/1) silty clays, an increase in coarse clastic material, and a high abundance of pebbles at the base of Holes 1098A and 1098C (Fig. F7).

Turbidites and Mass Flows

The lithostratigraphy at Site 1098 shows intervals of graded silt or sand to mud. Three prominent intervals occur within the sequence between ~23 and 40 mbsf. Each appears entirely homogeneous, with no bioturbation except at the very top, and all three have low and uniform values of chromaticity parameter a* (Fig. F8). The uppermost is nearly 4 m thick (Sections 178-1098A-4H-2, 79 cm, through 4H-5, 27 cm; 178-1098B-3H-6, 40 cm, through 4H-2, 45 cm; 178-1098C-3H, 5-7 cm, to the base of Core 3H; not seen at the top of Core 178-1098C-4H). This bed shows a well-developed normal grading from diatom-bearing sandy muds to ooze and corresponds to a high in magnetic susceptibility (Fig. F7). The second is 1.2 m thick (Sections 178-1098B-4H-4, 30 cm, through 4H-5, 4 cm; 178-1098C-4H-2, 90 cm, through 4H-1, 60 cm; poorly recovered between Cores 178-1098A-3H and 4H), including 10-25 cm of graded medium to coarse sand at its base. In Section 178-1098B-4H-4, the sand contains mollusk fragments with one small articulated bivalve. The third was noted as a graded fine sand lamina just below 39 mbsf (Sections 178-1098A-5H-6, 81 cm, and 178-1098B-5H-4, 25 cm), with very high magnetic susceptibility (Fig. F7). These three graded layers are interpreted as turbidites and are numbered T1, T2, and T3 from the top down (Figs. F7, F8).

Most of the sequence at Site 1098 is horizontally bedded, but slump features and inclined bedding were observed at the base of Holes 1098B and 1098C (Fig. F10). Below a conspicuously laminated, nonbioturbated interval of mud-bearing diatom ooze, the lowest intact sediments at Hole 1098A (interval 178-1098A-6H-3, 130 cm, to the base of the core) consist of 1.8 m of sandy, pebbly, gray mud (diamict; 5Y 5/1) with thin, graded, fine sand-silt beds. Bedding is horizontal, except for moderate coring disturbance. Hole 1098B terminated within the laminated ooze interval without recovering the graded sands; the upper part of the laminated interval dips at 60º beneath a meter-thick slumped bed. In Hole 1098C, the entire section below the uppermost turbidite dips at ~20º. Beneath the laminated ooze interval is 3.3 m of massive diatom-bearing diamict.

Pebbles

Pebbles >0.5 cm in diameter occur throughout the sequence except in the upper 6 m (Fig. F8; Table T3); abundance ranges from two to four pebbles in Cores 178-1098A-2H through 5H, 178-1098B-2H and 4H, and 178-1098C-2H through 4H. As many as 18 pebbles were identified in Cores 178-1098B-5H and 6H and in Core 178-1098C-5H, which indicates an increase in coarse clast material downhole.

Site 1099

Sediments recovered at Site 1099 consist of alternating massive muddy diatom oozes, laminated mud-bearing diatom oozes, and diatom-bearing silty clays and clayey silts with a generally low sand content. Hole 1099A sediments vary in color between olive (5Y 4/3) and dark olive gray (5Y 3/2) in the diatomaceous muds; colors are orange brown (10YR 5/6), dark brown (10YR 4/3 and 3/3), dark grayish brown (2.5Y 4/2), and dark gray (5Y 4/1) in the laminated oozes. Hole 1099B sediment color varies from dark greenish gray (5Y 4/1) to very dark gray (5BG 4/1), which may indicate a decrease in oxygen after deposition because of the decay of organic matter. Site 1099 sediments show varying intensities of bioturbation. Graded sands and silts associated with structureless intervals above and slumped material below are interpreted as turbidites. They commonly show pervasive bioturbation of bed tops by small Phycosiphon burrows. Steep burrows (Fugichnia) were made following deposition of thin (1 cm) laminations and record the upward escape of burrowing organisms. Diatom preservation is generally good. Diatom species composition varies between laminated and bioturbated intervals, and also from top to base of the hole. Ice-rafted pebbles show two maxima (in Cores 178-1099A-4H and 1099B-3H). Depositional processes included (1) pelagic/hemipelagic settling and (2) sediment gravity flows from the sides of the Palmer Deep Basin III. We describe the entire section as one lithostratigraphic unit.

Unit I

Intervals: Core 178-1099A-1H through Section 7H-CC; Core 178-1099B-1H through Section 5H-CC
Depth: 0.0-62.3 mbsf in Hole 1099A; 60.0-107.5 mbsf in Hole 1099B
Age: Holocene

Description

Lithostratigraphic Unit I consists dominantly of alternations of massive, bioturbated, muddy diatom ooze with laminated mud-bearing diatom ooze and very fine-grained graded beds interpreted as turbidites (Figs. F11, F12). Diatoms, including resting spores, form 35%-80% of the sediment (estimated from smear slides), with sponge spicules averaging 5% throughout the two holes. Diatom preservation is good at the top of the section and moderate in the lower part (below ~25 mbsf), and the assemblage is diverse. In Cores 178-1099A-4H and 178-1099B-3H, two minima of biogenic abundance (~20%) are revealed. Minor amounts (<4%) of silicoflagellates, radiolarians, and benthic foraminifers are present in certain intervals.

The color of the massive, muddy diatom ooze varies between olive (5Y 4/4, 4/3, 3/2), olive gray (5Y 4/2), and dark olive gray (5Y 3/2) at Hole 1099A (Fig. F13A). Colors of the massive, muddy diatom oozes at Hole 1099B are significantly darker (dark gray, 5Y 3/1, to very dark gray, 5BG 4/1) and may show faint color banding. The thickness of the massive diatom ooze layers varies downhole. From Core 178-1099A-1H through 3H, thickness averages 40 cm, whereas the thickness increases to as much as 19 m in Cores 178-1099A-5H through 7H (Fig. F11). This lithology is intensely bioturbated, and burrows of Planolites type, as well as longer vertical open burrows of Phycosiphon type, were observed.

The laminated mud-bearing diatom oozes and diatom-bearing clayey silts and silty clays vary between orange brown (10YR 5/6), dark brown (10YR 4/3 and 3/3), dark grayish brown (2.5Y 4/2), and dark gray (5Y 4/1) at Hole 1099A. They are generally darker, varying from dark greenish gray (5Y 4/1) to very dark gray (5BG 4/1), at Hole 1099B. In the upper part of Hole 1099A, the laminae are typically 5-20 mm thick and have diffuse tops and bases. Downhole, the thickness of laminae ranges between 1 and 10 mm (Fig. F13B). Individual laminae are each composed of a nearly monospecific diatom assemblage. They are dominated by Chaetoceros spp. resting spores (the most common, particularly toward the base of the hole; orange brown, dark brown, and dark gray laminae); Corethron criophilum (the lighter olive laminae, most common near the top of the hole); and Eucampia antarctica (dark gray laminae). Diatom preservation in the laminae is excellent, with some samples resembling fresh plankton hauls. Bioturbation of the laminated lithology is minor and involved trace fossils similar to those mentioned above. In the interval 178-1099B-4H-3, 103-104 cm, a calcium carbonate concretion was found in the laminated mud-bearing diatom ooze. The concretion is bedded in millimeter-thick planktonic foraminifer ooze.

The darker colors found in both lithologies of Hole 1099B correspond to a pronounced increase in pyrite to as much as 8% estimated from smear slides in Cores 178-1099B-2H through 5H. This may indicate a decrease in oxygen during or after deposition because of the decay of organic matter (see "Inorganic Geochemistry"). Color started to change to olive gray (5Y 4/2) immediately after the core was split and exposed to laboratory conditions.

Turbidites

Sediment sequences of graded fine sand and silt, millimeters to centimeters thick, occur throughout Cores 178-1099A-1H through 4H and 178-1099B-1H through 5H in both the massive and the laminated lithologies (Fig. F11). The graded turbidite beds (Fig. F13C, F13D) commonly show pervasive bioturbation of bed tops by small Phycosiphon burrows. Phycosiphon is a very common trace fossil in successions deposited in such environments (Wetzel, 1984).

Diamict and Pebbles

A clast-poor olive (5Y 4/3) to gray (5Y 4/1) diamict layer with a diatom clayey silt matrix (Fig. F11) was recovered in the interval from Sections 178-1099A-4H-6, 115 cm, through 5H-2, 20 cm. This diamict layer coincides with a maximum occurrence of 10 pebbles (as much as 2 cm), mainly basalts, observed in Core 178-1099A-4H (Fig. F12; Table T3). A second concentration of pebbles (as many as eight pebbles) occurs in Core 178-1099B-3H. Fewer pebbles are recorded in Cores 178-1099A-3H and 178-1099B-2H, 4H, and 5H.

Interpretation of Sites 1098 and 1099

Site 1098 is located within Basin I of Palmer Deep; the water depth is 1040 m and the width of the basin floor only ~1 km, with the site near one edge (Fig. F3). The basin sides slope at 16º-26º from the inner shelf, which is ~200 m deep off Anvers Island (Rebesco et al., 1998). Site 1099 is located within Basin III of Palmer Deep; the water depth is 1430 m. In comparison to Basin I, Basin III may be more open to water-mass exchange to the west. However, the basins act as a sediment trap, both for biogenic material sinking from the surface waters and for near-bottom suspended sediment transported by turbidites or mass flows derived from surrounding shallow water. The area is sea-ice free for at least 4 months/yr; surface waters are strongly affected by winter cooling, and diatom productivity is therefore very seasonal. Water masses below the 200-m sill depth in Palmer Deep may be influenced by periodic influx of the warm Circumpolar Deep Water (CDW) of adjacent oceanic regions.

Palmer Deep sediments are ponded in Basins II and III, but the fill of Basin I (Site 1098) is draped (Rebesco et al., 1998). Likely depositional processes include pelagic/hemipelagic settling and sediment gravity flows. The basin is considered to be bounded by active faults (Rebesco et al., 1998), and the presence of turbidite beds may reflect this setting.

The sequences interpreted as turbidites at Sites 1098 and 1099 are coarse- to fine-grained graded sand and silt. Graded turbidite beds commonly show pervasive bioturbation of bed tops by small Phycosiphon burrows and an increase of burrow size upward over a few centimeters. Phycosiphon is a very common trace fossil in turbidite successions and is a shallow burrower normally restricted to depths <10 cm from bed tops (Wetzel, 1984). At Site 1099, this trace fossil occurs in the upper laminated divisions of turbidites (e.g., Fig. F13C), which indicates the ability of the trace maker to keep pace with the high deposition rates suggested during phases of multiple turbidite events.

The style of lamination in the upper part of the section (indistinct, diffuse laminae showing minor bioturbation, alternating with strongly bioturbated intervals) is consistent with pelagic settling into a deep basin that was oxygenated enough to support a benthic fauna. Leventer et al. (1996) studied a 9-m piston core taken at Site 1098. They suggested that the laminated mud-bearing diatom oozes indicate very rapid biosiliceous sedimentation with strong seasonal fluctuation in diatom abundance and species composition, whereas the bioturbated muddy diatom oozes record a lower biosiliceous sedimentation rate. According to Leventer et al. (1996), the inferred fluctuations in productivity of biosiliceous material (200-yr periodicity in the upper 9 m of sediment) may be related to global climate fluctuations. We agree with this interpretation and suggest that similarly alternating environmental conditions prevailed during the time represented by the upper 23 m of the sequence above T1. Good preservation of the delicate frustules of Corethron criophilum implies rapid burial after sinking of each diatom bloom. During deposition of the lower part of the sequence, in addition to pelagic settling, gravity flows were important. In addition to turbidites T1, T2, and T3, which are clearly graded with coarse material at the base, many of the sharp-based, nonbioturbated laminae in the lower part of the hole may have originated as small downslope flows. An additional explanation for the lack of bioturbation may be that the bottom water was low in oxygen.

Leventer et al. (1996) measured a sedimentation rate of 260 cm/k.y. by radiocarbon dating (Basin I). If this sedimentation rate has been constant at Site 1098, then the base of the lowest laminated sediments would be at ~16 ka, earlier than the ~11 ka onset of marine conditions determined by Pudsey et al. (1994) on the outer shelf west of Anvers Island. However, the sedimentation rate at Site 1098 remains to be determined by 14C measurements postcruise.

The diamicts at the base of Site 1098 and from Hole 1099A in the interval 178-1099A-4H-6, 115 cm, through 5H-2, 20 cm, are thought to be of glacial origin, but their age is unknown. The lateral variation at the base of Holes 1098A, 1098B, and 1098C may result from downslope movement on irregular basin-floor topography (seismic Unit 4b of Rebesco et al., 1998).

Preliminary investigations of diatom species assemblages recovered from Sites 1098 and 1099 help explain environmental variation in the basins. In the upper 9 m of a piston core recovered at the Site 1098 location, Leventer et al. (1996) noted that laminated sediments (low magnetic susceptibility) and massive, bioturbated sediments (high magnetic susceptibility) were characterized by different diatom assemblages. Laminated layers included variable species composition, but massive layers were more uniform, at least in the top 9 m. At Sites 1098 and 1099, the diatom assemblages in both major lithologies (massive sediments and the most common types of laminae) were examined to determine long-term trends downcore. Selected species known as environmental indicators (Table T4) were recorded as abundant, common, or rare. These data were then assigned values based on their abundance (i.e., abundant = 50, common = 30, rare = 10). A plot of indicator species was generated by summing all the species data to 100% for each sample (Fig. F14). Note that several species are not included in the plot (e.g., Rhizosolenia, Thalassiosira lentiginosa, and Thalassiosira gracilis).

Results of relative species abundance in the major lithologies are shown in Figure F14. At both sites, the abundance of the open-ocean species Fragilariopsis kerguelensis and Thalassiothrix spp. decreases downward, and that of Thalassiosira antarctica remains roughly constant. Eucampia antarctica is rare near the surface and common below 15 mbsf, down to near the base of the sequence at each site. Fragilariopsis curta is present throughout, but more consistently at Site 1099. Chaetoceros spp. spores show a marked increase toward the base of the sequence at both sites.

The data suggest decreasing open-ocean influence and restriction of the basin downward at both sites. The higher variability in species composition at Site 1098 is attributed to the more laminated, thin-bedded style of sedimentation. Many of the samples at Site 1099 come from the thick turbidites where the record may be averaged.

Many of the distinctive colored laminae are composed of nearly monospecific assemblages of Chaetoceros spp. spores, Corethron criophilum, T. antarctica, or Rhizosolenia. Fragilariopsis curta is a common minor species, but the open-ocean forms F. kerguelensis and Thalassiothrix spp. are not common in any laminae. The presence of these laminae throughout the cored sequences indicates that unusual productivity events (diatom blooms), followed by rapid sedimentation, occurred repeatedly during the time represented. Variation in diatom production is most likely related to change in shallow water-mass properties (a result of regional climate change) through the Holocene.

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