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doi:10.2204/iodp.proc.318.105.2011

Lithostratigraphy

Three holes were cored at Site U1357. In Hole U1357A, Cores 318-U1357A-1H through 21X penetrated to 186.6 mbsf, recovered 183.87 m of diatomaceous ooze, and penetrated the underlying last glacial diamict. The “official” recovery in Hole U1357A was calculated as 99%; however, expansion of the sediment section was significant as a result of gas evolution, mostly H₂S and CH₄, throughout the core. In Cores 318-U1357A-1H and 2H, tens of centimeters of sediment were lost before sections could be cut and capped. Minor amounts of additional material were lost from individual sections when several end caps blew off while cores sat on the rack in the laboratory. We reduced sediment loss by drilling holes in the core liner as soon as it arrived on the catwalk, but some material was typically lost from the ends of the core and was delivered to the micropaleontology laboratory. Several cores from Hole U1357A ruptured in the middle from gas pressure while sitting on the cutting rack on the catwalk. Sections that were disturbed by this process are identifiable by the presence of an outer liner and the shattered condition of the original liner. Care was taken to retain as complete a section as possible, but the interval from ~60 to 80 mbsf in Holes U1357A and U1357C will be examined carefully for disturbance. Cores from Hole U1357B did not rupture. The strategy of triple coring with a different starting depth for each hole will help in the production of a composite section for this site. The last APC core in Hole U1357A (Core 318-U1357A-20H) was an incomplete stroke, resulting in less than the attempted 9.5 m. We collected a single XCB core (318-U1357A-21X) from 185.6 to 186.6 mbsf. This core bottomed out in diamict, recovering ~11 cm of highly lithified carbonate-cemented gravelly mudstone. Above the diamict, the lowermost 15 m in Hole U1357A contains increasing amounts of terrigenous debris, mostly silt, toward the base, with some sand and gravel near the base of the hole.

After offsetting the ship 50 m east of Hole U1357A, we cored Hole U1357B. Cores 318-U1357B-1H through 19H penetrated to 170.7 mbsf and recovered 172.44 m (101%) of sediment. The loss of sediment because of gas evolution and section expansion was minimized throughout this core by drilling small gas release holes at regular intervals along the core liner, as was done for Hole U1357A below Core 318-U1357A-3H. The section recovered from Hole U1357B appears to provide an intact Holocene sedimentary sequence with the small apparent excess recovery (1%) representing gas expansion. We did not attempt to core into the diamict in Hole U1357B.

Hole U1357C, offset 25 m west of Hole U1357A, produced Cores 318-U1357C-1H through 11H, penetrated to 103.8 mbsf, and recovered 110.7 m (107%) of sediment. Again the excess recovery appears to be due to gas expansion, even with gas release holes drilled into the liners of every core. A portion of Core 318-U1357C-8H ruptured on the catwalk, similar to that observed for the core at this depth from Hole U1357A. Coring operations for Hole U1357C were terminated before reaching the underlying diamict because of weather and ice conditions. Hole U1357C was not logged, again because of time and weather constraints.

All cores from Hole U1357A were split and described after running them through the Whole-Round Multisensor Logger (WRMSL). As part of the description processed, the archive half of each section was photographed using the high-resolution Section Half Imaging Logger. The archive halves were also analyzed using the color reflectance and point source magnetic susceptibility scanner on the Section Half Multisensor Logger at 1 cm intervals for Sections 318-U1357A-1H-1 through 9H-1 and at 2 cm intervals for the remainder of the hole (Sections 318-U1357A-9H-2 through 20H-CC). Normal shipboard sampling of the working halves of Hole U1357A core included small amounts of material taken for physical property measurements, carbonate and organic carbon content, and diatom analysis. The working halves of all Hole U1357A cores were sampled at 10 cm intervals for foraminifer analysis and preliminary age-dating. All other sampling was deferred until after the expedition. Foraminifer samples were taken shipboard to ensure good preservation, as dissolution of calcareous microfossils during core storage and transport is a known phenomenon in highly organic reducing sediments from the Antarctic margin. After sampling and describing, working- and archive-half cores were double wrapped, flushed twice with nitrogen gas, sealed in heat-shrink tubing with oxygen absorber packs, and curated in D-tubes in cold storage.

The sedimentology and lithostratigraphy sections of this site report are based solely upon information from Hole U1357A. Cores from Hole U1357B were not split but rather were sealed as whole-round cores in shrink wrap with oxygen absorbers after running the sections through the WRMSL. Cores 318-U1357C-1H and 2H were mostly sampled shipboard as a series of contiguous whole-round cores dedicated to microbiology, interstitial water analysis, optically stimulated luminescent age-dating, and ³²Si age-dating. The remainder of Hole U1357C cores were not split and were curated as for the Hole U1357B cores after running the sections through the WRMSL. Splitting, description, and sampling of these cores will occur after the expedition.

Based on visual core descriptions and smear slide analyses, Hole U1357A is divided into three lithostratigraphic units (from top to bottom): 170 m of laminated Holocene diatom ooze, a 15 m thick transitional unit of sand and silt-bearing diatom ooze, and a subglacial diamict.

Unit descriptions

Unit I

• Interval: 318-U1357A-1H-1, 0 cm, through 19H-1, 115 cm

• Depth: 0–170.25 mbsf

• Age: Holocene, probably spanning the last 10,000 y

Unit I consists of dark olive-brown to light greenish brown diatom ooze. The entire unit exhibits centimeter-scale laminations defined by color changes between lighter greenish brown versus darker brown intervals (Fig. F4), which are occasionally bowed downward toward the edge of the core liner. These laminations are often not clearly visible on a freshly split core surface. In fact, most freshly split sections from cores that have not yet been exposed to air are initially a uniform dark brown color (Fig. F5, left panel). Initially, some laminations are visible on the basis of millimeter-scale textural features consisting of alternating “cotton candy” fibrous versus smoother surfaces or through differences in the size and shape of gas expansion microcracks along bedding surfaces. After exposure to air, faint color differentiation becomes apparent. The colors develop more fully over a period of hours. This color development upon surficial oxidation is illustrated in Figure F5. The figure shows images of Section 318-U1357A-6H-4 taken after 10 min, 1 h, and 3 h of exposure of the freshly split core to air at room temperature. The general lightening of the sediment surface and emergence of laminations defined by light–dark color variability can be seen in the three panels from left to right. Laminations are visible throughout Unit I and range from <1 cm thick to as thick as 4 cm. Laminations from Section 318-U1357A-18H-7, typical of the base of Unit I, are shown in Figure F6. The time available for description and layer counting did not allow for uniform exposure times to air, so real-time counting was not done on a section by section basis.

The entire core exhibits slight to moderate disturbance related to the release of hydrogen sulfide and methane. Gas release produces small pockets and fractures (Fig. F7) that reduce the utility of the shipboard track scanning systems as well as the reliability of shipboard physical property measurements. The maximum amount of gas release appeared to occur in Core 318-U1357A-8H.

Forty-four smear slides were described from Unit I (see Site U1357 smear slides in “Core descriptions”). The diatom ooze is unusually pure, with diatom contents estimated from smear slide description as ranging from 80% to 99% and a mean of 91% (Fig. F8). Rare intervals with minor clay or fine silt grains account for up to 10%–20% of the smear slide grains. Other rare (<3%) microscopic biogenic grains observed in smear slides from Unit I include silicoflagellates, sponge spicules, radiolarians, and foraminifers. Other rare (<5%) identifiable mineral grains observed in some smear slides from Unit I include quartz, feldspar, and pyrite.

The largest particles observed in Hole U1357A were fish remains, predominantly millimeter-size vertebrae, but also fish teeth. Fish fragments are found throughout Unit I and appear well preserved (Fig. F9). Intervals of concentrated vertebrae, similar in appearance, are listed in Table T2 as a guide to the most abundant macrofossil observed at Site U1357. Fish bones generally consist of 60%–70% hydroxyapatite crystals embedded in a fibrous collagen matrix (Newesely, 1989; Schenau and De Lange, 2000). The abundance of fish vertebrae throughout the core suggests they may be used for accelerator mass spectrometry age-dating of the embedded proteinaceous carbon.

A possible bivalve shell was observed at interval 318-U1357A-3H-3, 117 cm. Rare bryozoan grains were reported at several depths. Foraminifers were visible at the split core surface throughout Unit I and include Neogloboquadrina pachyderma, Globigerina bulloides, and the benthic foraminifer Globocassidulina subglobosa.

Siliceous microfossils are unusually well preserved throughout Unit I. Shipboard micropaleontologists report the absence of reworked older diatoms as well as a diverse array of Holocene species. Radiolarians (Fig. F10), sponge spicules, and silicoflagellates are also well preserved. Organic-walled dinoflagellates cysts are present, as well as motile stages, tintinnid loricae, copepod remains, and pollen.

A translucent 1 mm thick tabular mineral layer was observed at interval 318-U1357A-8H-8, 54–55 cm, as well as at several deeper levels in Hole U1357A. The mineral contains inclusions of the surrounding sediment and appears to be authigenic. It was first thought to be gypsum on the basis of hardness and texture, but X-ray diffraction (XRD) analysis shows it to be the mineral struvite (see “Geochemistry and microbiology”). Struvite was subsequently observed at multiple locations below Core 318-U1357A-8H. It likely occurs at higher levels as well; core describers initially thought the fragments were bits of plastic core liner. The struvite fragments are most evident when they occur in tabular layers 1–2 mm thick and aligned with laminations. The crystals can be several centimeters across (Fig. F11). Struvite is NH₄MgPO₄·6H₂O and is thought to form by bacterial biomineralization processes in anoxic sediments with abundant amounts of ammonium.

Interpretation

We interpret Unit I as a Holocene sequence of laminated diatom ooze representing the product of annual biogenic sedimentation focusing within the 1000 m deep Adélie Basin. This part of the Wilkes Land coastal zone exhibits a large and recurrent polynya (a region of open water surrounded by sea ice) within which intense diatom blooms form during the austral spring and summer. This material is deposited during the austral autumn on the Wilkes Land continental shelf, where it is remobilized and transported until it reaches the protected deep-basin setting of the Adélie site depositional basin. Slightly less biogenic, organic-rich material falls during the austral winter. The alternating high-productivity deposition during summer and less organic-rich sedimentation during winter produces the observed centimeter-scale laminations (e.g., Stickley et al., 2005; Maddison et al., 2006; Denis et al., 2006). The observation of hydrogen sulfide gas at very shallow depth (uppermost meter) in all holes and the presence of persistent laminations throughout the sequence indicate anoxic conditions that prevent larger benthic organisms from disrupting the laminations. The observation of benthic foraminifers, however, suggests that bottom waters contain sufficient amounts of oxygen to sustain small organisms living at the sediment/water interface.

Unit II

• Interval: 318-U1357A-19H-1, 115 cm, through 20H-CC, 85 cm

• Depths: 170.25–185.60 mbsf

• Age: earliest Holocene

Unit II consists of clay-bearing to clay-rich olive-green diatom ooze with distinct laminations (Fig. F12) as well as layers of structureless diatom-rich sand and sandy silt. The boundary between Units I and II is marked by an abrupt downward increase in clay, silt, and sand content. Sand layers and pockets, gravel, and faceted pebbles occur in the lower part of Core 318-U1357A-19H. Core 20H is similar in appearance to the lowermost portion of Core 19H but exhibits signs of drilling disturbance. This core likely hit the diamict during the APC process, did not fully stroke-out, and possibly sucked in additional sediment. Vertical flow-in features are observed in all of Section 318-U1357A-20H-2 and in interval 318-U1357A-20H-3, 0–130 cm. Sections 318-U1357A-20H-4 through the base of 20H-CC show well-defined centimeter-scale color banding ranging from reddish brown to greenish gray to yellowish gray (Fig. F13). These bands are more strongly bowed near the margins of the core liner than those from Cores 19H and above. The frequency of silt-bearing layers (Fig. F14) and abundance of sand and gravel increases toward the base of Core 318-U1357A-20H. Nevertheless, well-preserved diatoms are continuously observed to the base of Unit II (Fig. F15).

Interpretation

We interpret this unit as forming immediately after ice retreat from the Adélie Basin during deglaciation at the end of the last glacial interval. Meltwater from the rapidly receding ice margin, some of it likely introduced subglacially to the calving margin, carried in larger amounts of terrigenous debris. This unit contains diatoms, and the thickness of the layering, relatively high degree of compaction, and high biosiliceous content suggests an extremely high productivity environment. The multicolored layered interval observed in Core 318-U1357A-20H is similar in appearance to the siliceous mud oozes defined by Leventer et al. (2006) as representative of the initial sediments accumulated in shelf basins of the East Antarctic margin during and immediately following initial ice margin retreat. The model of a calving bay reentrant proposed by Domack et al. (2006) and Leventer et al. (2006) may well be applicable for explaining the production and preservation of laminated (possibly varved) and rapidly deposited deglacial biogenic sediments at the Adélie site. In this model, as the Antarctic ice sheet melts and sea level rises, grounded ice on the continental shelf lifts off first from deep basins and troughs while remaining pinned at banks and ridges. Differential ice sheet flow and calving then creates embayments in the retreating ice margin that manifest as ice-walled fjords. The 1000 m deep Adélie Basin cored at Site U1357 is surrounded by relatively shallow banks, so ice liftoff and breakout to form an ice-walled fjord seems likely during the initial phases of sea level rise and ice sheet melting. Only after the embayment is removed by continued ice-margin retreat to the south does normal pelagic, polynya-fueled sedimentation resume. The centimeter-scale brightly colored layers in this unit are similar to those described as early ice-retreat biogenic facies by Stickley et al. (2005) and Maddison et al. (2006).

Unit III

• Interval: 318-U1357A-21X-CC, 0–14 cm

• Depths: 185.60–185.71 mbsf

• Age: latest glacial to earliest Holocene

Unit III consists entirely of a highly lithified, carbonate-cemented, gravelly mudstone recovered from the core catcher of Core 318-U1357A-21X (Figs. F16, F17). The mudstone contains clasts of different lithologies including rock fragments of diorite, metasediment, quartzite, and volcanic debris. Larger grains are supported by a matrix composed of silt and clay minerals with subordinate carbonate. Carbonate cements are present as both vein fill and grain coatings.

Interpretation

Unit III is interpreted as a portion of the uppermost last glacial diamict. It is not known exactly how much of Unit II is missing from the section recovered above the diamict, but we estimate <1 m based on the downhole core barrel positions.

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