Site U1385 | Site U1386 | Site U1387 | Site U1388 | Site U1390 | Site U1391
IODP Expedition 339:
Site U1389 Summary
PDF file is available for download.
Background and objectives
Site U1389 (proposed site GC-11A)
is located on the southern Iberian Margin (36°25.515'
N and 7°16.683' W) about 90 km W of the Spanish city of Cadiz, in a
water depth of 644 m. This is one of two sites in the 'channels and ridges'
sector of the larger Cadiz Contourite Depositional
System (CDS). It is perched on a relative topographic high, which is currently
elevated 50-250 m above the flanking contourite channels, and lies approximately 4 km NW of the of the Guadalquivir >diapiric ridge.
We designate this as the Huelva sheeted/patch drift, which is in
fact a small remnant of a much larger middle-slope sheeted drift system that
has been dissected by several contourite channels.
Huelva Channel lies along the northern flank and Guadalquivir Channel along the
southern flank of Huelva drift. On seismic profiles it shows a well-layered
internal acoustic structure with laterally extensive, mainly >aggradational seismic depositional units, and widespread
discontinuities. There is a complex erosional-depositional relationship between
drift and flanking channel. Huelva drift has been developing in this region
over the past 4 million years, at least, and is presently under the influence
of the Lower Core of the Mediterranean Outflow Water (MOW).
Our primary objective at this
site was to recover a Pliocene, Pleistocene and Holocene sedimentary succession
formed under the influence of the Lower Core of MOW, and so to compare this
record with that found at Sites U1386 and U1387, which formed under the Upper
Core of MOW.
Site U1389 was occupied on 21 December 2011. Five holes were drilled and cored using the advanced piston corer (APC), the
extended core barrel (XCB), and the rotary core barrel (RCB) system, achieving
the target depth of 990 m in the fifth hole, U1389E. Downhole logging was
carried out at Holes U1389A and U1389E using the Triple Combo, FMS-Sonic and
VSI tool strings. Overall recovery for Site U1389 was 307 m (104%) with the
APC, 464 m (91%) with the XCB, and 352 m (54%) with the RCB. The total cored
interval for Site U1389 was 1463.4 m and total recovery was 1123.5 m (77%).
sedimentary succession at Site U1389 extends for 990 m from the early Pliocene
to Holocene. It is represented by a thick, rapidly accumulated, and very
uniform series of contouritic sediment, which has
been assigned to a single lithostratigraphic unit and
divided into five sub-units (Units IA to IE). Unit I is dominated by classic >contourite deposition, including calcareous mud, >silty mud, sandy mud and silty >bioclastic sand lithologies.
These are generally organized as bi-gradational sequences and partial
sequences, of which base-cut-out, normally-graded
sequences are more common that top-cut-out inversely-graded sequences. Carbonate content ranges from 21-35%, and total organic carbon from
division of Unit I into five subunits is based on subtle changes in the
relative abundance of the different lithologies and silt/sand intervals.
Calcareous microfossils (nannofossils, planktonic and benthic
foraminifera, and ostracods) are mostly common to
abundant, with moderate to good preservation throughout. Pteropod
fragments are more common than at any of the other sites, mainly within the
uppermost 30 m. The sedimentary record is continuous through the Holocene and Pleistocene to about 2.1
Ma, with a sedimentation rate of between 30-40 cm/ky.
A relatively short hiatus (2.1-2.4 Ma) occurs at around 640 mbsf, below which
the average sedimentation rate is 25 cm/ky. There is
some evidence from lithological, physical property and downhole logging data
for two other minor hiatuses, at around 0.4 and 0.9 Ma, but not yet confirmed
by the micropaleontological data. The deepest part of
the section is < 3.7 Ma.
There is a distinctive variability in benthic foraminifer assemblages, which reveals significant environmental
changes through the Pliocene-Holocene succession, closely comparable with that
observed at other sites. The upper 0.9 My of the Quaternary shows typical upper
bathyal assemblages indicative of increased organic
matter input and reduced ventilation. The remainder of the Pleistocene shows
lower nutrient supply, greater influence of the MOW, and significant mixing
with shelf-derived taxa. Pliocene assemblages suggest high-nutrient, low-oxygen
conditions, and generally warmer waters. Pollen and spores are abundant in most of the samples analyzed, together with microcharcoal and
dinocysts. Together, these indicate normal (fresh)
supply from Mediterranean forests and grasslands for the upper Pleistocene, a
transitional zone mixed with corroded, reworked forms, and no pines, then a
Pliocene succession with mostly corroded conifers.
Paleomagnetic measurements identified the >Brunhes/Matuyama polarity
transition (0.781 Ma), the top and bottom of the Olduvai Subchron
(1.778 and 1.945 Ma), the Matuyama/Gauss transition (2.581), the
Gauss/Gilbert transition, and potentially three minor excursions. These give
reliable confirmation of the biostratigraphic dating for Site U1389, although some of the inferred polarity boundaries need further confirmation.
Physical property data show relatively
close tracking of magnetic susceptibility and bulk density with the more
sand/silt-rich intervals within the Pleistocene succession, but a much more
complex pattern within the Pliocene. The downhole distribution of porosities
shows higher values in an interval of high interstitial water >chlorinity, arguing in favor of a lateral advection of
brine-related fluids through more permeable strata.
The pore water profiles at Site U1389 shows
distinct maxima in several elements at around 530 mbsf, with relatively sharp
transitions above and below. This suggests either a barrier to vertical
diffusion or enhanced fluid flux laterally. The increase in concentrations is
likely due to dissolution of minerals, most likely carbonates. There is also a
strong negative correlation between δ18O and >δD, which is characteristic of clay mineral
dehydration reactions that take place at temperatures greater than 50°C. This requires that the fresh signal is a
result of fluid migration from a deeper, higher temperature source.
Downhole measurements were
made in Hole U1389A to a depth of 355 mbsf and in Hole U1389E to a bridged-hole
depth of 568 mbsf. A good suite of FMS image logs was obtained in Hole U1389A. There is a distinct change in log characteristics at around 320 mbsf, which
correlates closely with a lithostratigraphic boundary
and with a zone of poor core recovery. This zone would appear to be more
sand-rich on the basis of borehole logs, although no sands were recovered by
coring. Distinct cyclicity is apparent in some parts
of the section, corresponding with both lithological and physical properties
data. Nine downhole temperature measurements were made in the top 100 m of
section, yielding a geothermal gradient of 20.9 °C/km, relatively lower than at other sites on this expedition
We recovered core to a total
depth of 990 mbsf at Site U1389, the deepest penetration of the expedition. The
site lies under the influence of MOW Lower Core and is perched on a topographic
high between contourite channels. For much of its
Pleistocene history, it has only received sediment from either bottom current
or hemipelagic processes. Especially notable is the extreme uniformity of the
succession and its rapid accumulation at rates of 25-40 cm/ky.
The sediments are distinctively contouritic in
character throughout, with mixed terrigenous-biogenic
composition and characteristic bi-gradational or partial contourite
sequences. Significantly, the long-duration hiatus observed at both Sites U1386
and U1387 on the Faro Drift under MOW Upper Core, and related to a phase of
highly active MOW, is reduced at Site U1389 to 2.1-2.4 Ma. Two other minor
hiatuses are inferred at around 0.4 and 0.9 Ma, and are also indicative of
enhanced MOW at these times.