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IODP Expedition 318:
Wilkes Land Glacial History
Site U1357 Summary
PDF file is available for download.
26 February 2010
The primary objective at Site U1357
(ADEL-01B) was to recover a continuous ~200 meter Holocene sedimentary section
from the Adélie Basin located on the Antarctic continental shelf off the Wilkes
Land Margin. The Adélie Basin is a 1000 meter deep glacially-scoured trough
separated from the Adélie Depression (70 km to the east) by the 200 m deep
Adélie Bank. Previous piston- and kasten-coring of the
upper sediment column shows that sediments in the Adélie Basin are deposited as
annual-to-near-annual layers averaging 2 cm in thickness. The thickness of the
Holocene sedimentary section above the last glacial diamict (190 m) is consistent with this high rate of sedimentation being
maintained for the past 10,000 years.
The ultra-high resolution Adélie Basin
Holocene section will be used to produce the first annually-resolved time
series of oceanographic and climatic variability derived from a Southern Ocean marine sediment core. These data can be directly compared to annual ice core records from
Antarctica's coastal ice domes as well as other marine sediment cores from the
Antarctic Peninsula and other parts of the East Antarctic Margin. The site is
sensitive to drainage winds from Antarctica as well as the polar easterly
winds, sea ice extent, the Southern Annular Mode (SAM), and the position of the
southern boundary of the Antarctic Polar Frontal Zone in the Indian and Pacific
oceans. Little is known about past variability in these systems from marine
records and there are none with annual or near-annual resolution. The Adélie
Basin site lies directly downwind and down-current from the Mertz Glacier
polynya (Masson et al., 2001; 2003) and therefore collects biogenic materials
produced in one of Antarctica's major coastal polynyas. The Mertz Glacier
polynya and underlying Adélie Depression may produce as much as 25% of all
Antarctic Bottom Water (Rintoul, 1998; Marsland et al., 2004; Williams et al.,
2008). Given the known presence of benthic foraminifera in the Adélie Basin and
the substantial bottom water temperature anomaly associated with local High
Salinity Shelf Water, Site U1357 has the potential to yield information on
Antarctic Bottom Water production through time. Understanding Holocene climate
variability at this East Antarctic site will aid in determining the range and
characteristics of natural climate variability during a period of relatively
constant atmospheric carbon dioxide levels. This record will also aid in the
assessment of different forcing factors (solar, ocean-atmosphere interaction,
volcanic) responsible for climate change over the past 10,000 years.
Sediments accumulate in the 1000 m deep Adélie Basin
as a thick drape overlying a high amplitude reflector with no underlying
penetration. The strong reflector is interpreted as a glacial diamict. The East
Antarctic Ice Sheet expanded to the shelf edge during the last glacial maximum
(Domack, 1982; Barnes, 1987; Eittreim et al., 1995) and the Adélie Basin was
filled with ice. Ice lift-off and southward retreat from other deep shelf
basins of East Antarctica occurred between 10,000 and 11,000 years ago (Siegert
et al., 2008; Leventer et al., 2006); this is the expected age range of the
lowermost sediments recovered from Site U1357. The seismic line shows 190
meters of continuous, horizontal, parallel reflectors at the site, consistent
with a drape of Holocene sediment undisturbed by sea level change or glacial
Three holes were cored at Site U1357. In
Hole U1357A, Cores U1357A-1H to -21X penetrated to 186.6 mbsf, recovered 183.87
m of diatomaceous ooze and penetrated the underlying last glacial diamict.
After offsetting the ship 50 m to the east of Hole U1357A, we cored Hole
U1357B. Cores U1357B-1H to -19H penetrated to 170.7 mbsf and recovered 172.44 m
(101%) of sediment. Hole U1357C, offset 25 m west of Hole U1357A, produced
Cores U1357C-1H to -11H, penetrated to 103.8 mbsf, and recovered 110.7 m (107%)
of sediment. Cores from Hole U1357A were split and described during the
expedition. Cores from Holes U1357B and U1357C were preserved as whole round sections
for post-cruise splitting, describing, and sampling. All cores from this site
contained sediments that vigorously degassed methane and hydrogen sulfide upon
decompression to 1 atmosphere. Gas pressure caused expansion of the sediment
section resulting in the loss of some sediment from core breaks as well as
section breaks, particularly above 40 mbsf. This was minimized by drilling
small holes in the core liners at regular intervals.
The sediments from Site U1357 consist of
three lithologic units. The uppermost unit is 170 m of laminated Holocene
diatom ooze (Unit I). Unit I overlies a 15 m transitional unit of sand and
silt-bearing diatom ooze (Unit II), which in turn sits on a hard,
carbonate-cemented and poorly sorted gravelly siltstone (Unit III, a diamict).
Units I and II exhibit regular laminations defined by color (alternating dark
olive brown to light greenish brown layers) and textural variability. Individual
laminae range in thickness from 1 to 3 cm and extend throughout the entire 186
m thick section lying on top of the diamict.
Based on analysis of multiple samples
from core breaks and section breaks Site U1357 sediments contain a
well-preserved Holocene Southern Ocean diatom flora with varying contributions
from cool open-ocean and sea-ice associated taxa (Armand et al., 2005; Crosta
et al., 2005). Radiolarians, silicoflagellates, and sponge spicules are common
and well preserved. Organic-walled dinoflagellates cysts are present as well as
motile stages, abundant tintinnid loricae, copepod remains.
Light and dark laminae were sampled
throughout the Holocene section. Based on trends in diatom assemblage
succession within paired laminae as well as previous work from the Adélie Basin
region (Denis et al., 2006) and other laminated diatom sections from the east
Antarctic margin (e.g., Stickley et al., 2005; Maddison et al., 2006), each
light/dark laminae couplet is provisionally interpreted as a single season of
biogenic production and accumulation. It is assumed that diatomaceous sediments
begin to accumulate during spring sea ice retreat following the development of
early season phytoplankton blooms in the Mertz Glacier polynya. Blooms persist
through summer open water conditions, and conclude with the autumn regrowth of
sea ice and destabilization of the water column. The unusually high
accumulation rates (averaging 2 cm/year) are likely the result of
syndepositional focusing processes that sweep biogenic debris from the shallow
Adélie and Mertz banks into the deep shelf troughs of the Adélie Basin and
A low diversity assemblage of calcareous
planktic (Neogloboquadrina pachyerma and Globigerina bulliodes) and benthic foraminifera (Globocassidulina
subglobosa and Triloculina frigida) occurs in Site
U1357 Holocene sediments. Planktic foraminifera were observed throughout the
sedimentary section and benthic foraminifera were observed in several core
break samples that were sieved. The occurrence of well-preserved calcareous
foraminifera is unusual in Antarctic shelf basins as shelf bottom waters are
highly undersaturated with respect to calcite. High sedimentation rates likely
contribute to foraminiferal preservation in the Adélie Basin sediments.
In addition to abundant diatoms and
foraminifera, sediments from Hole U1357A contain large quantities of fish
debris, including at least 44 layers of concentrated fish vertebrae. With
abundant phosphatic, calcareous, opaline, and organic biogenic detritus, Site
U1357 sediments offer an unusually diverse array of assemblage-based and
geochemical environmental tracers for shore-based studies.
Site U1357 sediments posed a significant challenge for the analysis of physical properties. The sediments are so diatomaceous
that they exhibit extremely low magnetic susceptibility. Whole-round core
analysis of all three holes using a Bartington Loop sensor and split core
analysis using a point source magnetic susceptibility sensor on Hole U1357A did
not yield useful data for hole-to-hole correlation. The production of mm-scale
pockmarks from degassing of methane and hydrogen sulfide throughout the
sediment column makes bulk density determination difficult using discrete
sample analysis or with the Gamma-ray attenuation. Natural Gamma Ray (NGR)
levels are minimal because of the low concentration of terrigenous material in
Lithostratigraphic Unit I. Nevertheless, with sufficient background count
correction, NGR track scans yielded useful data for the correlation of cores
from all of Holes U1357B and U1357C and a portion of cores from Hole U1357A.
Additional information for hole-to-hole
correlation was provided by magnetic susceptibility determination of over 1800
discrete samples that had been taken through Hole U1357A at 10 cm intervals for
post-cruise foraminiferal analysis. Each dried sample was weighed and analyzed
with the Kappa Bridge KAY-4 magnetic susceptibility detector. The Kappa Bridge
has roughly 2 orders of magnitude greater sensitivity than the whole core loop
or split core point source sensors. The resulting data set suggests that
magnetic susceptibility measurements of discrete samples will be of use in
developing a robust hole-to-hole correlation.
Site U1357 is located close to the south
magnetic pole and we observed the expected high inclinations in the
paleomagnetic signature of split core sections. After processing and matching
paleomagnetic declinations across core breaks, Hole U1357A yielded a
paleomagnetic secular variability profile that appears to match a geomagnetic
secular variation model spanning the last seven thousand years (CALS7k.2 of
Korte and Constable, 2005) for 66oS, 144oE. The
age-depth relationship predicted by application of this model to the secular
variation signal obtained from Hole U1357A is consistent with that expected for
this site based on radiocarbon dating of the upper 50 m of the sediment column
(Costa et al., 2007) and the overall sediment sequence thickness.
of 96 sediment samples from Hole U1357A yielded CaCO3 contents
ranging from 1 to 3 wt% for most of the hole with one distinct carbonate-rich
layer (CaCO3 >9 wt%) at 126.34 mbsf. Organic C content is
uniformly high (for Antarctic shelf sediments) at between 1 and 2 wt%. C/N
ratios of between 7 and 12 are consistent with relatively well-preserved and
labile marine organic matter. SiO2 concentrations are high (76-91
wt%), and are accompanied by low concentrations of TiO2 (<0.3
wt%) and Al2O3 (<5.6 wt%), as expected for a nearly
pure diatom ooze with little terrigenous input. The authigenic phosphate
mineral struvite (NH4MgPO4 .
6H2O), which forms through bacterial biomineralization
in anoxic sediments in the presence of ammonium, was observed at several
Headspace methane concentrations varied by over an order of
magnitude down core, increasing from 5,000 to 43,000 ppm from 0 to 20 mbsf and
then declining to highly variable concentrations at greater depths, mostly
between 5,000 and 18,000 ppm. Significant concentrations of H2S were
detected as well, consistent with anoxic diagenesis of organic rich sediments.
An extensive microbiology program, focusing on phospholipid analyses and
molecular 16S rRNA sequencing, was completed in the upper 20 m of Hole U1357C.
Pore water samples collected from adjacent whole-round core samples show no
detectable SO4= except in core top samples suspected of
contamination with seawater. Ammonium increases almost linearly from near
surface values of 900 µM to 4500 µM at 18 mbsf. Total dissolved inorganic
carbon and alkalinity increase to 18 mbsf as well, to values of 79.6 mM and 88
mM, respectively, while pH drops to 7.5 at 20 mbsf. These profiles are
consistent with bacterially mediated diagenesis within anoxic pore waters.
Methane is derived from CO2 reduction following the removal of SO4=. Pore waters were not analyzed deeper than 20 mbsf in the core but samples for 16S rRNA sequencing were taken from core ends to the maximum depth of penetration at Hole U1357C (103.8 mbsf).
Armand, L.K., Crosta, X., Romero, O., Pichon, J.-J., 2005, The
biogeography of major diatom taxa in Southern Ocean sediments: 1. Sea ice
related species. Palaeogeography, Palaeoclimatology, Palaeoecology, 223, 93–126.
Crosta, X., Romero, O., Armand, L.K., Pichon, J.-J., 2005, The
biogeography of major diatom taxa in Southern Ocean sediments: 2. Open ocean
related species. Palaeogeography, Palaeoclimatology, Palaeoecology, 223, 66–92.
Barnes, P.W., 1987, Morphologic studies
of the Wilkes Land continental shelf, Antarctica: glacial and iceberg effects,
In: Eittrem, S.L., Hampton, M.A. (Eds.), The Antarctic Continental Margin:
Geology and Geophysics of Offshore Wilkes Land. Circum-Pacific Council for
Energy and Mineral Resources. Earth Science Series, vol. 5A, pp. 175–194.
Costa, E., Dunbar, R.B., Kryc, K.A.,
Mucciarone, D.A., Brachfeld, S., Roark, E.B., Manley, P.L., Murray, R.W., and
A. Leventer, 2007, Solar forcing and El Niño-Southern Oscillation (ENSO)
influences on productivity cycles interpreted from a late-Holocene
high-resolution marine sediment record, Adélie Drift, East Antarctic Margin, Online Proceedings of the 10th ISAES X, edited by A. K. Cooper and C. R. Raymond et al., USGS Open-File Report 2007-1047, National Academy of Sciences Press, Short Research Paper 036, 6 p.; doi:10.3133/of2007-1047.srp036.
Denis, D., Crosta, X., Zaragosi, S.,
Romero, O., Martin, B., and V. Mas, 2006, Seasonal and subseasonal climate
changes recorded in laminated diatom ooze sediments, Adélie Land, East
Antarctica, The Holocene, 16, 1137, doi: 10.1177/0959683606069414.
Domack, E.W., 1982, Sedimentology of
glacial and glacial marine deposits on the George-V-Adélie continental-shelf,
East Antarctica, Boreas, 11, 79–97.
Eittreim, S.L., Cooper, A.K., Wannesson, J., 1995, Seismic stratigraphic evidence of ice-sheet advances on the Wilkes Land margin of Antarctica, Sedimentary Geology 96 (1–2), 131–156.
Korte, M and Constable, C.G., 2005, The geomagnetic
dipole moment over the last 7000
years—new results from a global model, Earth
and Planetary Science Letters, 236, 348-358.
Leventer, A., Domack, E., Dunbar, R.,
Pike, J., Stickley, C., Maddison, E., Brachfeld, S., Manley, P., and C.
McClennen, 2006, East Antarctic Margin Marine Sediment Record of Deglaciation, GSA Today, 16, doi: 10.1130/GSAT01612A.1, 4-10.
Maddison, E., Pike, J., Leventer, A.,
Dunbar, R., Brachfeld, S., Domack, E., Manley, P., and McClennen, C., 2006,
Post-glacial seasonal diatom record of the Mertz Glacial Polynya, East
Antarctica, Marine Micropaleontology, 60, 66–88, doi: 10.1016/j.marmicro.2006.03.001.
Marsland, S. J., N. L. Bindoff, G. D.
Williams, and W. F. Budd, 2004, Modelling water mass formation in the Mertz
Glacier polynya and Adélie Depression, East Antarctica, J. Geophys. Res., 109, C11003, doi:10.1029/2004JC002441.
Massom, R. A., K. L. Hill, V. I. Lytle,
A. P.Worby,M. J. Paget, and I. Allison, 2001, Effects of regional fast-ice and
iceberg distributions on the behavior of the Mertz Glacier polynya, East
Antarctica, Ann. Glaciol., 33, 391– 398.
Massom, R. A., K. Jacka, M. J. Pook, C.
Fowler, N. Adams, and N. Bindoff, 2003, An anomalous late-season change in the
regional sea ice regime in the vicinity of the Mertz Glacier polynya, East
Antarctica, J. Geophys. Res., 109(C7), 3212, doi:10.1029/2002JC001354.
Rintoul, S., 1998, On the origin and
influence of Adélie Land Bottom Water, in Ocean, Ice and Atmosphere:
Interactions at the Antarctic Continental Margin, Antarct. Res. Ser., 75, edited by S. S. Jacobs and R. F. Weiss, pp. 151– 172, AGU, Washington, D.C.
Siegert, M.J., Barrett, P.J., DeConto,
R., Dunbar, R.B., Cofaigh, O., Passchier, S., and T.R. Naish, 2008, Recent
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C.E., Pike, J., Leventer, A. Dunbar, R., Domack, E.W., Brachfeld, S., Manley,
P. and C. McClennan, 2005, Deglacial ocean and climate seasonality in laminated
diatom sediments, Mac.Robertson Shelf, Antarctica, Palaeogeography,
Palaeoclimatology, Palaeoecology, 227,
Williams, G. D., N. L. Bindoff, S. J.
Marsland, and S. R. Rintoul, 2008, Formation and export of dense shelf water
from the Adélie Depression, East Antarctica, J. Geophys. Res., 113, C04039, doi:10.1029/2007JC004346.