Preliminary scientific assessment

The overall objective of Expedition 318 was to obtain long-term record of Antarctic glaciation and its relationships with global paleoclimate and paleoceanographic and eustatic sea level changes by drilling the Antarctic margin along an inshore–offshore transect. Of particular interest was testing the sensitivity of the EAIS to episodes of global warming and detailed analysis of critical periods in Earth's climate history (i.e., the Eocene–Oligocene and Oligocene–Miocene glaciations, late Miocene, Pliocene, and the last deglaciation) during which the Antarctic cryosphere evolved in a step-wise fashion to ultimately assume its present-day configuration, characterized by a relatively stable EAIS. These records were obtained by coring and analyzing sedimentary records along the inshore–offshore transect to constrain the age, nature, and environments of deposition, until now only inferred from seismic surveys of the Wilkes Land continental shelf, rise, and abyssal plain.

The principal goals were

  1. To obtain the timing, nature, and consequences of the first major phase of EAIS growth and arrival of ice at the Wilkes Land margin (onset of glaciation) inferred to have occurred during the earliest Oligocene. In marine records elsewhere, this event is thought to correlate to a steep increase in oceanic δ18O values widely referred to as the Oligocene isotope Event 1 (Miller et al., 1985).
  2. To obtain the nature and ages of the changes in the geometries of the progradational wedges interpreted to correspond with large fluctuations in the extent of the EAIS and possibly coinciding with the transition from a wet-based to a cold-based glacial regime;
  3. To obtain a high-resolution record of Antarctic climate variability during the Oligocene, Neogene, and Quaternary; and
  4. To obtain an unprecedented ultrahigh resolution (i.e., seasonal to decadal) Holocene record of climate variability.

Expedition 318, January–March 2010 (Wellington to Hobart), occupied seven sites (Figs. F6, F7) that produced ~2000 m of high-quality upper Eocene–Quaternary sediments (Fig. F25) at proposed Sites WLRIS-6A, WLRIS-7A, WLRIS-4A, and WLRIS-5A (Sites U1355, U1356, U1359, and U1361) on the Wilkes Land rise and proposed Sites WLSHE-8A, WLSHE-9A, and ADEL-01B (Sites U1358, U1360, and U1357) on the Wilkes Land shelf at water depths between ~400 and 4000 mbsl. Together, the cores represent ~53 m.y. of Antarctic history (Fig. F25). The cores reveal the history of the Wilkes Land Antarctic margin from an ice-free "greenhouse Antarctica," to the first cooling, to the onset and erosional consequences of the first glaciation and the subsequent dynamics of the waxing and waning ice sheets, all the way to thick, unprecedented "tree ring style" records with seasonal resolution of the last deglaciation that began ~10,000 y ago (Fig. F26). They also reveal details of the tectonic history of the so-called Australo-Antarctic Gulf (at 53 Ma), the onset of the second phase of rifting between Australia and Antarctica (Close et al., 2009), ever-subsiding margins and deepening, all the way to the present ocean/continent configuration. Tectonic and climatic change turned the initially shallow, broad subtropical Antarctic Wilkes Land offshore shelf into a deeply subsided basin with a narrow ice-infested margin (Fig. F26). Thick Oligocene and notably Neogene deposits, including turbidites, contourites, and larger and smaller scaled debris mass flows, witness the erosional power of the waxing and waning ice sheets and deep-ocean currents. The recovered clays, silts, and sands and their microfossils also reveal the transition of subtropical ecosystems and a vegetated Antarctica into sea ice–dominated ecosystems bordered by calving glaciers (Fig. F26).

"Preglacial" regional Unconformity WL-U3 and the timing, nature, and consequences of the first major phase of EAIS growth

Strata above and below Unconformity WL-U3, interpreted precruise to separate preglacial strata from glacial-influenced deposits, was drilled and dated at continental rise Site U1356. We confirmed that this surface represents major erosion related to the onset of glaciation at ~34 Ma (early Oligocene), with immediately overlying deposits dated as 33.3 Ma. We sampled strata overlying the unconformity at the inshore shelf Site U1360 and dated it as early early Oligocene (~33.3 Ma) (Figs. F25, F26). Below Unconformity WL-U3, we recovered a late early to early middle Eocene record from peak greenhouse conditions, likely including some of the early Eocene hyperthermals, at Site U1356. Subtropical shallow-water depositional environments are indicated by dinocysts and the chemical index of alteration, among other indicators (i.e., clay mineralogy; see "Site U1356" in "Site summaries"). A hiatus spanning 2 m.y. separates the lower Eocene from the middle Eocene record at Site U1356 according to dinocyst evidence. This hiatus may be related to tectonic activity related to the commencement of rapid seafloor spreading in the Australian Antarctic Basin (AAB), reported to initiate around the same time (~50 Ma) (Close et al., 2009). Combined Site U1356 and Leg 189 dinocyst distribution patterns suggest earliest through-flow of South Pacific Antarctic waters through the Tasmanian Gateway to also be coeval with this tectonic phase. Sedimentological and microfossil information from this interval from Hole U1356A also suggest progressive deepening during the early middle Eocene.

The upper middle Eocene to the basal Oligocene is conspicuously missing in a ~19 m.y. hiatus at ~890 mbsf (~47.9–33.3 Ma) based on dinocyst and paleomagnetic evidence in sediments below regional Unconformity WL-U3. Despite ongoing tectonic reorganizations, it appears likely that the erosive nature of Unconformity WL-U3 is notably related to the early stages of EAIS formation. The combination of concomitant internal dynamics, sea level fall, ice sheet growth, and related erosion is proposed as the principal mechanism underlying the formation of regional Uniformity WL-U3. This is supported by the abrupt steep increase in oceanic δ18O values (Oi-1) and coeval sea level change globally recorded in marine successions (Oi-1; Miller et al., 1985). Nevertheless, progressive subsidence, the large accommodation space created by erosion in the margin (300–600 m of missing strata) (Eittreim et al., 1995), and partial eustatic recovery allowed sediments of early early Oligocene age to accumulate above Unconformity WL-U3.

Microfossil contents, sedimentology, and geochemistry of the Oligocene sediments from Site U1356, at present occupying a distal setting (i.e., lowermost rise-abyssal plain) and immediately above Unconformity WL-U3, unequivocally reflect icehouse environments with evidence of iceberg activity (dropstones) and at least seasonal sea ice cover (Figs. F25, F26). The sediments, dominated by hemipelagic sedimentation with bottom current and gravity flow influence, as well as biota, indicate deeper water settings relative to the underlying middle Eocene environments. These findings imply significant crustal stretching, subsidence of the margin, and deepening of the Tasman Rise and the Adélie Rift Block between 47.9 and 33.3 Ma (Fig. F26). Furthermore, combined paleoenvironmental data indicate significantly cooler, high-productivity, and sea ice–influenced surface waters, with only occasional incursions of warmer conditions. We surmise that as Antarctic-Australian separation progressed and deepening took place, influence from the warmer waters of the Proto-Leeuwin Current gradually diminished, whereas Antarctic Counter Current flow and deep-water formation along the Wilkes Land margin strengthened.

Record of EAIS variability and the nature and ages of the changes in the geometries of progradational wedges

Drilling at continental rise Site U1356 also recovered a thick section of Oligocene to upper Miocene sediments indicative of a relatively deep water, sea ice–influenced setting (Fig. F26). Oligocene to upper Miocene (Fig. F25) sediments are indicative of episodically reduced oxygen conditions either at the seafloor or within the upper sediments prior to ~17 Ma. From the late early Miocene (~17 Ma) onward, progressive deepening and possible intensification of deep water flow and circulation lead to a transition from a poorly oxygenated low-silica system (present from the early to early middle Eocene to late early Miocene) to a well-ventilated silica-enriched system akin to the modern Southern Ocean. This change coincides with one of the major regional unconformities in the Wilkes Land margin, Unconformity WL-U5, which represents a ~3 m.y. hiatus in the early Miocene. This unconformity marks a change in the dominant sedimentary processes at this site, which are dominated by mass transport processes below the unconformity and by hemipelagic, turbidity flow, and bottom-current deposition above. Further studies are needed to determine to what extent the observed changes are related to relevant steps in the evolution of the EAIS, changes in the continental shelf morphology (i.e., shelf overdeepening), or both.

A complete record with good recovery of late Miocene to Pleistocene deposits was achieved at continental rise Sites U1359 and U1361 (Fig. F25), drilled on levee deposits bounding turbidity channels. We successfully dated the seismic units between Unconformities WL-U6 and WL-U8, and the sedimentological, logging, and magnetic susceptibility data exhibit relatively high amplitude variations, indicating strong potential for shore-based analysis revealing EAIS dynamics down to orbital timescales (100 and 40 k.y. cyclicity). This cyclicity likely documents the successive advances and retreats of the ice sheet and sea ice cover, as well as the varying intensity of cold saline density flows related to bottom water production at the Wilkes Land margin (e.g., high-salinity shelf water flowing from the shelf into the deep ocean to form ABW). In general, typical Southern Ocean open cold-water taxa, with variable abundances of sea ice–associated diatoms, indicate a high-nutrient, high-productivity sea ice–influenced setting throughout the Neogene. Combined sedimentological and microfossil information indicates the ever-increasing influence of typical Antarctic Counter Current surface waters and intensifying ABW flow (Figs. F26). Furthermore, the preservation of calcareous microfossils in several intervals indicates times when bottom waters were favorable to the preservation of calcium carbonate. These observations point to a very dynamic ice sheet/sea ice regime during the late Miocene through the Pleistocene. Detailed postcruise studies in sediments from the late Neogene will provide a history of glacial–interglacial climate and paleoceanographic variability, including a history of Antarctic Bottom Water production, that can be linked to sea ice variations in this margin.

Continental shelf Site U1358 (Fig. F25) recovered a record from the early Pliocene to Pleistocene (with a small hiatus at ~2.5 Ma). Although targeted Unconformity WL-U8 was not reached, glacial and glacial-marine sediments recovered at this site are dated early early Pliocene. Therefore, the change in the geometries of the progradational wedge from low-angle progradational strata above to very steep foresets is older than 4.5 Ma. In addition, the record from Site U1358 in combination with those from Sites U1359 and U1361 will provide the link between ice sheet behavior on the continental shelf, including times of ice sheet instability (e.g., early Pliocene warmth) and the history of sea ice and paleoceanographic changes recorded at Sites U1359 and U1361.

Ultrahigh resolution (seasonal to decadal) Holocene record of climate variability

Coring at Site U1357 yielded a 186 m section of continuously laminated diatom ooze as well as a portion of the underlying Last Glacial Maximum diamict. Based on much shorter piston cores recovered from adjacent basins and banks, the onset of marine sedimentation during the deglacial interval began between 10,400 and 11,000 y ago. The site was triple cored, providing overlapping sequences that will aid in the construction of a composite stratigraphy spanning at least the last 10,000 y. The Site U1357 sediments are unusual for Antarctic shelf deposits because of their high accumulation rate (2 cm/y), lack of bioturbation, and excellent preservation of organic matter as well as calcareous, opaline, phosphatic, and organic fossils. The sediments are profoundly anoxic, with levels of H2S as high as 42,000 ppm at 20 mbsf. Larger burrowing organisms are completely excluded from this ecosystem, yet the regular occurrence of benthic foraminifers suggests that some oxygen is present at the sediment/water interface. The mineral struvite, a hydrous ammonium phosphate phase, forms in nitrogen-rich pore waters and has never before been reported in Antarctic sediment. These sediments provide an excellent sample set for geomicrobiology and sedimentary geochemistry studies. In fact, the upper 20 m of one of the three holes was completely consumed for pore water and sediment studies.

The greatest achievement from a paleoclimatic standpoint was the retrieval of a continuously layered deposit (Fig. F27). Spot checks of laminae from top to bottom of the split Hole U1357A sections suggests that paired light–dark laminae sets range in thickness from ~1 to 3 cm. Based on radiocarbon dating of a piston core taken earlier from this site (Costa et al., 2007), our own preliminary secular paleomagnetic findings, and the thickness of the deposit combined with the expected age at its base, it is very likely each laminae pair represents 1 y. If supported by our shore-based research, this will be the first varved sedimentary sequence extending through the Holocene recovered from the Southern Ocean. Analysis at the annual timescale will permit us to examine decadal to subdecadal variability in sea ice, temperature, and wind linked to the SAM, Pacific Decadal Variability, and possibly ENSO. We will also be able to address questions regarding rates of change during the Hypsithermal Holocene neoglacial events and the time immediately following the first lift-off and pull-back of ice at the end of the last glacial interval. In addition, we now have an excellent opportunity for ultrahigh resolution correlation to the nearby Law Dome Ice Core, one of the most important Holocene ice cores in Antarctica.

Microscopic analyses of smear slides, micropaleontological slides, and palynological preparations reveal the presence of an unusually well preserved and diverse assemblage of both soft and hard biotic remains, including abundant fish, copepods, and euphausiids. In fact, these sediments may well have captured the most complete record of any ancient ecosystem structure and its variability through the Holocene yet recovered from the Southern Ocean.