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IODP Expedition 318:
Wilkes Land Glacial History
Site U1361 Summary
PDF file is available for download.
6 March 2010
U1361 (WLRIS-05A) is located in
the continental rise at 3466 m water depth. Similar to Site U1359, the main
objective at Site U1361 was to provide a history of climate and
paleoceanographic variability record from the middle Miocene to the Pleistocene
and to test the stability of the East Antarctic Ice
Sheet during extreme warm periods (e.g.,
Miocene Climate Optimum, early Pliocene, and Pleistocene Isotopic Stages MIS 31
and MIS 11). Drilling at this site targeted the timing and nature of deposition
of the upper seismic units (i.e., above the WL-U6 unconformity) defined on the
Wilkes Land margin (De Santis et al., 2003; Donda et al., 2003). Within these
units, a shift in sedimentary depocenters from the continental rise to the
outer shelf is observed, possibly corresponding with the transition from a
dynamic wet-based to a more persistent cold-based EAIS (Escutia et al., 2002;
De Santis et al., 2003), which is inferred to occur during the late
Miocene–Pliocene (Escutia et al., 2005; Rebesco et al., 2006). At Site
U1361 unconformities WL-U6, WL-U7 and WL-U8 lie at about 5.13s, 5.03s, and
4.78s two way travel time (TWTT), respectively (about 385, 300 and 100 mbsf,
Site U1361 is located on the right (east)
levee of the Jussieau submarine channel downstream from Site U1359. The levee
relief (measured from the channel thalweg to top of the levee) at Site U1361 is
about 195 m. The fine-grained components of the turbidity flows traveling
through the channel and hemipelagic drape are inferred to be the dominant
sedimentary processes building these levees (Escutia et al., 1997; 2000; Donda
et al., 2003). Bottom-currents can further influence sedimentation in this setting (Escutia et al., 2002; Donda et al., 2003). The
record from Site U1361 should be complementary to the record from Site U1359.
Similar depositional environments were cored during ODP Leg 178 in the
Antarctic Peninsula (Barker, Camerlenghi, Acton, et al., 1999) and ODP Leg 188
in Prydz Bay (O'Brien, Cooper, Richter, et al., 2001).
Two holes were drilled at Site U1361.
Hole U1361A reached a total depth of 388.0 meters below sea floor (mbsf), using
the Advanced Piston Corer (APC) system to refusal at 151.5 mbsf, followed by
Extended Core Barrel (XCB) drilling to the bottom of the hole at 388.0 mbsf.
Hole U1361B reached 12.1 mbsf using the APC. Five lithofacies (designated A
through E) were identified at Site U1361 and, based on their distribution in
Hole U1361A, two lithostratigraphic units are defined. Facies A and B consist
of clays and silty clays with common diatoms and foraminifera, and rare
dm-scale sets of mm- to cm-scale silt and clay laminae. These facies are
restricted to the interval between 0.0 and 34.9 mbsf (Lithostratigraphic Unit
I). Facies A and B were deposited in hemipelagic depositional environments,
with isolated sets of silt and clay laminae indicating occasional sedimentation
from low-density turbidity currents or saline density flows in a distal levee
setting (Escutia et al., 2008). Facies C and D are strongly bioturbated silty
clays and diatom/nannofossil oozes with intervals containing dispersed clasts.
Facies E consists of laminated clays. Facies C through E are present between
34.9 and 386.3 mbsf (Lithostratigraphic Unit II) and are typical of contourite
facies associations, although down-slope currents possibly contributed sediment
as well. Samples U1361A-1H-CC through -41X-CC (1.5 to 386.31 mbsf) were
analyzed for microfossils. Diatoms and radiolarians provide good age-control
for Hole U1361A, resolving an uppermost Pleistocene through uppermost Middle
Miocene sedimentary succession with no major breaks in sedimentation.
Miocene diatom assemblages at Site U1361
are indicative of productive, seasonally variable, open marine conditions.
Fluctuations in the abundance of marine benthic and tychopelagic taxa such as Cocconeis spp., Diploneis spp., Paralia sulcata, stephanopyxids and Trinacria excavata may indicate
pulses of shelfal material to the drill site. The presence of well preserved
benthic foraminifers in Sample U1361A-34X-CC (321.07 mbsf) suggests that
depositional settings were favorable for calcite preservation (i.e., not
corrosive) for brief intervals in the Miocene. The persistent presence of
reworked Mesozoic/Paleozoic sporomorphs within the palynological associations
suggests ongoing erosion in the hinterland.
Late Neogene diatom assemblages from
sediments drilled at Site U1361 are typical Southern Ocean open water taxa,
with variable abundances of benthic, neritic and sea ice-associated diatoms,
indicating a high nutrient-high productivity, sea ice-influenced setting
throughout the late Neogene. High abundances of reworked sporomorphs within the
palynological associations indicate strong erosion in the hinterland. Dinocysts
are absent during this interval. The preservation of planktonic foraminifers in
the Pleistocene indicates that bottom waters were favorable to the preservation
of calcium carbonate.
investigations at Site U1361 document a complete section from the top of Chron
C2n to the top of Chron C3n. Below Chron C2n the recovered core was quite
disturbed and there is no complete analysis of the discrete samples as of yet.
The lower portion of Hole U1361A can plausibly be correlated to the bottom of
Chron C5n to C5An.
Forty samples from Hole U1361A were taken
for analyses of percent carbonate, carbon, nitrogen and sulfur content, as well
as major and trace element analyses. Due to technical problems with the ICP-AES
no major and trace element analyses could be obtained. CaCO3
contents for most samples are well below detection limit (<1 wt%). Between
313.96 and 342.04 mbsf, however, carbonate contents increase to 12.1 - 24.8
wt%. This matches the recognition of nannofossil-bearing clays constituting one
of three major facies below 313.2 mbsf (Unit IIb; see Lithostratigraphy
section). Carbon, nitrogen and sulfur contents were measured on 15 selected
samples covering the full range of CaCO3 contents (0–24.8
wt%). All concentration levels are very low (i.e., <0.5 wt% C, <0.03 wt %
N, <0.02 wt% S), except for the four samples with high calcium carbonate
contents. Taken together with the CaCO3 measurements, these samples
yield total organic carbon concentrations below 0.3 wt%, which is within the
error or the respective measurements.
physical properties program for Site U1361 include routine runs on the whole
round multi-sensor logger, which includes the gamma-ray attenuation density
(GRA), magnetic susceptibility, and P-wave velocity logger sensors, as well as natural gamma radiation measurements. P-wave
velocity measurements were also analyzed and samples were taken and analyzed
for moisture, density, and porosity. Thermal conductivity measurements were
made in one section of all cores. The magnetic susceptibility data exhibit
relatively high amplitude variations and this apparent cyclicity at several
scales occurs especially in the upper 165 mbsf and between 305 mbsf and the
bottom of the hole. There are two intervals with reoccurring relatively lower
magnetic susceptibility units between 165 to ~185 mbsf and between 265 to 305
mbsf. The variations in GRA density reflect the regular fluctuations in
lithology and porosity. The relative moisture content varies between 63 to 22
wt% and porosity from 82 to 42% with gradual decrease with increasing depth and
overburden pressure. A common feature of density, porosity, and water content
records of Site U1361 is a slight change to higher gradients below 330 mbsf
that occurs within lithostratigraphic Unit IIb.
Downhole Logging operations
started after a successful reentry of Hole U1361A, which had been left
temporarily to allow an iceberg to pass. Runs with the triple combo, followed
by the FMS-sonic were successful. The downhole logs at Hole U1361A have
high-amplitude 1-5-m-scale variability superimposed on a downhole compaction
trend. The character of the logs changes gradually downhole, with no major
steps in the base levels, so the entire logged interval was assigned to one
logging unit. It is likely that Milankovitch band variability at eccentricity
and possibly obliquity periods is recorded at Site U1361. The downhole
measurements at Site U1361 included four APCT-3 deployments in Hole U1361A. The
thermal resistance was calculated over the intervals overlying the APCT-3
measurements, and the resulting linear fit of the temperature gives the heat
flow value of 58.2 mW/m2.
Barker, P.F., Camerlenghi, A., Acton, G.D., et al., 1999. Proc.
ODP, Init. Repts., 178: College Station, TX (Ocean Drilling Program). doi:10.2973/odp.proc.ir.178.1999.
De Santis, L., Brancolini, G., Donda, F.,
2003. Seismic-stratigraphic analysis of the Wilkes Land continental margin
(East Antarctica). Influence of glacially-driven processes on the Cenozoic
deposition. Deep-Sea Research. Part 2. Topical Studies in Oceanography 50
Donda, F., Brancolini, G., De
Santis, L., Trincardi, F., 2003. Seismic facies and sedimentary processes on
the continental rise off Wilkes Land (East Antarctica). Evidence of bottom
current activity. Deep-Sea Research. Part 2. Topical Studies in Oceanography 50
Eittreim, S.L., Cooper, A.K., 1997. Cenozoic glacio- marine sequences on the
Wilkes Land continental rise, Antarctica. Proceedings Volume-VII International
Symposium on Antarctic Earth Sciences, pp. 791–795.
Escutia, C., Eittreim, S.L., Cooper, A.K., and Nelson, C.H., (2000).
Morphology and acoustic character of the Antarctic Wilkes Land turbidite
systems: ice-sheet sourced versus river-sourced fans. Journal of Sedimentary
Research, Vol. 70, No. 1, p. 84-93.
Escutia, C., Nelson, C.H., Acton, G.D., Cooper, A.K., Eittreim, S.L.,
Warnke, D.A., and Jaramillo, J. (2002) Current controlled deposition on the Wilkes Land continental rise. In D. Stow et al. (eds.): Deep-Water
Contourite Systems: modern drifts and ancient series, seismic and sedimentary
characteristics. The Geological Society of London, Memoirs, 22,
O'Brien, P.E., Cooper, A.K.,
Richter, C., et al., 2001. Proc. ODP, Init. Repts., 188: College Station, TX
(Ocean Drilling Program). doi:10.2973/odp.proc.ir.188.2001.
Rebesco, M., Camerlenghi, A., Geletti,
R., and Canals, M. (2006). Margin architecture reveals the transition to the
modern Antarctic Ice Sheet ca. 3Ma. Geology, 34, 301–304.