PP51E-1358 0800h
What Role Play Brines in Levee Sedimentation in the Riiser Larsen Sea (Antarctica)?
During two expeditions to the Riiser Larsen Sea (RLS) we collected a number of long gravity cores and measured transects using a parametric echosounder system (Parasound). In the western part of the RLS Parasound and bathymetric data reveal that channels dominate the seafloor. None of the channels shows influences of lateral sediment input suggesting that presently the channels are active pathways of dense waters and/or turbidity currents. The channels merge to few large channels on the lower slope. They are usually flanked by levees that sometimes reveal more than 100 m of sound penetration. Granulometry, coarse-fraction analysis, TOC and carbonate, stable oxygen and carbon isotopes among other analyses were carried out on sediment cores from levee tops from the upper part of the continental slope. Results suggest that sediments from the channels feed the levees during flooding events (overbank deposition). Planktic foraminifer and IRD sedimentation occurs predominantly during periods in which flooding events were most energetic, indicated by the current-sorted terrigenous silt fraction being generally coarser than during other periods. High-energy periods coincide generally with warmer climate phases such as MIS 1, 3, and 5. Both, IRD and planktic foraminifer export from the sea surface was significantly reduced during the cold periods such as MIS 2 and 4 indicating severe sea-ice conditions. Channel-overspill events during the cold climate phases were clearly less energetic (finer current-sorted silt fraction). Today, sea-ice that covers about 15 degrees of latitude in winter forms within few weeks. It is suggested that at least part of the enormous quantity of sea ice is formed in coastal polynyas in which consequently also brines form. We suggest that these brines promote channel-flooding events and add to deepwater if they form on the upper slope (warm climate). Under cold climate brines may not reach the seafloor as they occur further north over deeper water, would not cause overspill, and also would not add to deepwater.
PP51E-1359 0800h
A Glimpse at Late Mesozoic to Early Tertiary Offshore Stratigraphy from Wilkes Land, East Antarctica: Results of Strategic Dredging of the Mertz-Ninnis Trough
As early as 1912 Sir Douglas Mawson demonstrated that pre-glacial sedimentary successions could be recovered by seafloor dredging of erosional troughs found offshore, East Antarctica. Since then, little systematic dredging has been undertaken in Antarctica despite indications of outcropping strata exposed along seaward flanks of glacially excavated troughs and the dire need to resolve the nature of preglacial and synglacial strata in this region. During cruise {\it NB Palmer 01-01}, three dredges were collected in echelon along the seaward flank of the Mertz-Ninnis Trough, parallel to the Mertz Ice Tongue, in water depths of 900 to 450 m. We combine biostratigraphic (palynologic) and lithologic analyses on sedimentary clasts with multi- and single-channel seismic reflection data collected by the WEGA cruise in 2000. 1359 pebble to cobble sized clasts were collected from three dredges. Of these 15% to 43%, within each dredge, were of sedimentary character, including carbonaceous sandstones with plant macrofossils, black sulfide-rich mudstones, siltstones, lignites, red quartz arenites, arkoses, and diamictites in various states of lithification. Palynomorphs were separated from these sedimentary rocks. We examined eleven individual lithologies, nine of which yielded useful palynological detritus. Of these samples, five yielded palynomorphs distinctive to the Paleogene (i.e. {\it Nothofagus flemingii, Tricolporites spp., Proteacidites spp.}); two samples contained only Lower Cretaceous palynomorphs, while three samples provided no stratigraphically useful palynomorph kerogen. We combine these results with multi-channel seismic and multibeam swath mapping to demonstrate that dredged materials represent seafloor outcrop or shallow subcrop of strata beneath a thin glacial till. Our stratigraphic model for these samples is consistent with 62 km of multichannel seismic reflection data (WEGA line W02) showing seaward dipping strata onlapping the basement to the southwest and partly infilling a rifted basin of late Cretaceous age. Seaward dipping reflectors above the syn-rift strata represent post-rift deposits ranging from Paleogene to Quaternary. Included within this stratigraphy are lithified diamictites containing Mesozoic palynomorphs in addition to palynomorphs of Early Tertiary age (including dinoflagellates). Seaward dipping reflectors in the deep axis of the Mertz-Ninnis Trough were not sampled directly by our dredges, but are believed to be Lower Cretaceous siltstones by extrapolation to core DF-79-38, 100 km along strike to the southeast (Domack et al., 1980). Furthermore, the thermal maturity of the lignite samples recovered in our collections suggests that the coal is of Early Tertiary age, as are numerous organic-rich mudstones, which contain Paleogene palynomorphs. These results indicate that sedimentary strata in this portion of the Wilkes Land Margin contain significantly thick (greater than 2.7 km) post-rift (drift phase) marine sequences of both pre- and synglacial character. Strategic dredging is a promising methodology by which to sample stratigraphic succession in a cost effective manner along the East Antarctic margin in the absence of, or preparation for, International Ocean Drilling Projects on the shelf. Domack, E. W., Fairchild, W. W., and Anderson, J. B. (1980) Lower Cretaceous sediment from the East Antarctic continental shelf, {\it Nature, 287}, 625-626.
PP51E-1360 0800h
Solar vs. Tidal Forcing of Centennial to Decadal Scale Variability in Marine Sedimentary Records from the Western Antarctic Peninsula
Prior studies on Holocene marine sediments from the Antarctic Peninsula, including ODP Site 1098 and USAP N. B. Palmer jumbo piston cores have revealed pronounced multi-century scale variations within a number of paleoenvironmental proxies. In order to fully understand the exact timing of this signal an ultra-high resolution jumbo piston core from the Schollaert Drift was correlated with the well-known Palmer Deep record. A precise and accurate radiocarbon chronology is now available from the former site that utilizes in-situ mollusks, rather than bulk organic matter. The resulting time series spans the last 5000 years over the 20 m length of core NBP99-03 JPC28 and the surface stratigraphy of kasten core NBP01-07 KC8. The corrected and calibrated ages (with an applied reservoir correction of 1170 years) of 10 mollusks dated by the National Ocean Sciences Accelerator Mass Spectrometry Facility and the University of Arizona TAMS facility were used to construct an age-depth profile for JPC28 and KC8. A linear trend (R$^{2}$ value of 0.993) of the age-depth profile was used to extrapolate the constant time interval between magnetic susceptibility measurements, which were analyzed every 1 cm. Dominant periods in the upper 10 m of the time series were identified using the Arand Spectral Analysis Package (Howell, 2001). This analysis revealed a single pronounced maxima at 160 years, which is inconsistent with the dominant periods found in time series of climate proxies from other sites on the western side of the Antarctic Peninsula. In particular, periods of 200 and 400 years, which are dominant in the Palmer Deep site (Warner and Domack, 2002), are not present in the Schollaert Drift. The spectral peaks derived from the two records are significantly different at the 95% confidence level. The cycles in the Palmer Deep record have been associated with solar variability, where as the 160 year cycle in the Schollaert Drift is close to a 180 year cycle in tidal forces (Keeling and Whorf, 2000). We discuss these alternative forcing mechanisms with respect to: contrasts in regional processes of glacial marine sedimentation, the mechanism whereby the tidal or solar signal is transferred to the sediment column and possible cryptic stratigraphy of the Palmer Deep record (ie. missing time, Nederbragt and Thurow, 2002). Resolution of the correct forcing factor is critical to our ability to hind cast the last 100 years of paleoenvironmental data within these cores and hence to our attempts at recognizing an anthropogenic climate signal in the Antarctic Peninsula region. Howell, P. (2001), ARAND time series and spectral analysis package for the Macintosh, Brown University, IGBP PAGES/World Data Center for Paleoclimatology Data Contribution Series #2001-044, NOAA/NGDC Paleoclimatology Program, Boulder, Colo. Keeling, Charles D., and Timothy P. Whorf (2000), The 1,800-year oceanic tidal cycle: A possible cause of rapid climate change, {\it Proc. Natl. Acad. Sci., 97} (8), 3814-3819. Nederbragt, A. J., and Thurow, J. (2002) Sediment color variation and annual accumulation rates in laminated Holocene sediments, Site 1098, Palmer Deep. In Barker, P. F., Camerlenghi, A., Acton, G. D., and Ramsay, A.T.S. (eds), Proc. ODP Sci. Results, 178: College Station TX (Ocean Drilling Program). Warner, Nathaniel R., and E. Domack (2002), Millennial-to decadal-scale paleoenvironmental change during the Holocene in the Palmer Deep, Antarctica, as recorded by particle size analysis, {\it Paleoceanography, 17} (3), 8004, doi:10.1029/2000PA000602.
PP51E-1361 0800h
Seismic and Sediment Data From Herbert Sound, Northwestern Weddell Sea, Indicate a Long-Lived Holocene Ice Canopy
Seismic data and sediment cores from Herbert Sound, northern James Ross Island, Antarctica, were examined in order to constrain the timing of advance and retreat of the Peninsula Ice Sheet into the region during and after the Last Glacial Maximum (LGM). Approximately 124 km of high-resolution seismic data and sixteen sediment cores were collected during a 1982 Deep Freeze cruise and a 1991 Polar Duke cruise. Core analysis included lithographic descriptions, magnetic susceptibility, density, and grain size analysis. Seismic facies analysis indicates regionally tilted Cretaceous bedrock draped with strongly laminated deposits thinning into condensed sections along the fjord walls. Combined core and seismic analysis suggests that there is no significant till present and that the laminated seismic facies consist of glacial-marine and hemipelagic deposits. Subcrops of Cretaceous bedrock indicates ice sheet grounding in the fjord in recent time. Preliminary analysis shows the presence of only volcanic terrigeneous ice rafted material from James Ross Island within thick sections of consolidated clast-rich unfossiliferous glacial-marine sediments, and an apparently thin diatomaceous drape. The combined evidence implies an expanded ice sheet that grounded in the sound followed by glacial-marine deposition from a long-lived ice canopy followed by hemipelagic sedimentation in seasonally ice-free seas. The data indicates that the ice canopy persisted through the late Holocene. Radiocarbon ages are pending on samples from the condensed sections. These dates will be used to better constrain the timing of glacial events in Herbert Sound and relate these events to those that occurred elsewhere in the Antarctic Peninsula.
PP51E-1362 INVITED 0800h
Tertiary Sedimentation Patterns on the Wilkes Land Margin, Antarctica (100-140E)
During the Austral summers of 2000-01 and 2001-02, Geoscience Australia (GA) acquired over 4000 km of seismic reflection profile data across the Wilkes Land margin, Antarctica. Two major unconformities are recognised in these data that can be traced for over 1500 km along the margin. The deeper of these unconformities is related to the breakup of Australia and Antarctica at $\sim$85-90 Ma. The upper unconformity is the base of an onlapping reflector sequence beneath the continental slope. On the basis of correlation with similar stratal character in southern Australian margin data, we infer a base Eocene (50 Ma) timing for this unconformity. Compilation of sediment thickness data provides the first regional sediment isopach for the Wilkes Land margin. A previously unrecognised sedimentary basin is identified off west Wilkes Land. This basin comprises a sediment thickness of $>$5 s two-way-time (TWT), $\sim$9 km. The volume of sediments within this basin is comparable to the major river deltas of other continents, such as the Niger and Amazon deltas. A distinct west to east asymmetry exhibited by the basin indicates that deep-water currents have been an important control on sediment transport and deposition on the East Antarctic margin since at least the onset of regional Antarctic glaciation, $\sim$34 Ma. If eastward flowing deep-water currents are the primary agent of sediment transport within this basin, a modern analogue current is not well established by current oceanographic data. The modern day geostrophic currents primarily flow westward on the Wilkes Land margin; and the southern boundary of the eastward flowing Antarctic Circum-polar Current (ACC) occurs at the northern limits of the basin. A change in current circulation patterns may explain the contrary observations of westward flowing currents and eastward distributed sediments. However, the sediment distribution may represent a more southerly circulation axis of the ACC prior to the formation of continent-wide, frozen based glaciers at $\sim$9 Ma. The locus of thickest sediments occurs offshore of the termination of the Totten Glacier, a major fast-flow feature in East Antarctica. This indicates that major ice-streams and/or flowing glaciers may be very long lived, $>$10 My, despite observations of shorter term changes in glaciomorphological features.
PP51E-1363 0800h
Ice-rafted Detritus, Diatoms and the Climate of the Atlantic Sector of the Southern Ocean During the Last 15,000 Years
The late deglaciation and Holocene of the Atlantic sector of the Southern Ocean appear to be periods of relatively stable climatic conditions, compared to the last glacial period. This smoothly changing climate seems only to have been interrupted by abrupt events during the Antarctic Cold Reversal and during the middle Holocene Neoglacial cooling. This is apparent in ice-core records, as well as in records of Ice Rafted Detritus (IRD) from deep-ocean sediment cores. However, IRD deposition can increase through increased ice-berg production during warming as well as increased ice-berg survivability during cold periods. As paleoclimate records differ on the timing and magnitude of the cold periods, the climate in periods of increased IRD deposition needs scrutinizing. Two decadal-resolution records of diatom-based sea-surface temperature and annual sea-ice duration from the region of the Antarctic Polar Front show very different climate changes in periods of increased IRD deposition. Together with oxygen isotope records from foraminifera and diatoms, as well as records of dust deposition from Antarctic ice-cores, they illustrate how IRD deposition may be only partly dependent on the marine climate. Conditions at the ice-berg producing glacial fronts may be equally important. Also, the records show that the Antarctic Polar Front is not just a climatic barrier for atmospheric circulation, but also controls the melting pattern for ice-bergs transported into the open ocean.
PP51E-1364 0800h
Evaluating the Silicic Acid Leakage Hypothesis in the Pacific Sector of the Southern Ocean
The Silicic Acid Leakage Hypothesis (SALH; Matsumoto et al., 2002; Glob. Biogeochem. Cycles 16, 10.1029/2001GB001442) posits that lower atmospheric carbon dioxide concentrations during glacial times resulted from a net transfer of dissolved silicic acid from the Southern Ocean to the tropics, coupled with a corresponding shift in phytoplankton taxa in tropical waters toward a greater dominance by diatoms (relative to today). If valid, then the SALH requires that opal burial rates in the Southern Ocean were lower than today during the Last Glacial Maximum (LGM), while contemporary opal burial rates in tropical regions were greater than today. Previous studies in the Indian and Atlantic sectors of the Southern Ocean have shown little to no net change between the LGM and the Holocene when opal burial rates are integrated across Subantarctic and Antarctic zones. This study aims to test a sub-hypothesis suggested by Chase et al. (Deep-Sea Research-II, v.50, 2003, 799-832) that silicic acid was exported only out of the Pacific sector. Piston cores from the SW Pacific sector of the Southern Ocean were analyzed to determine opal burial fluxes. Magnetic susceptibility and {\it Eucampia antarctica} abundances were used to determine glacial cycles and identify cores containing both LGM and Holocene sediment. Fluxes of opal, calcium carbonate, and lithogenic material were evaluated by normalizing to $^{230}$Th to correct for sediment focusing. Our results show opal flux at, and to the south of, the Antarctic Polar Front (APF) to have been lower during the LGM compared to the Holocene, consistent with previous results from throughout the Southern Ocean. North of the APF, two cores show higher opal fluxes during the LGM. However, due to a possible hiatus in one of the cores, the degree to which opal flux during the LGM was greater than during the Holocene has a large uncertainty. Additional analyses are planned to help reduce this uncertainty.