Site U1385 | Site U1386 | Site U1387 | Site U1388 | Site U1389 | Site U1390
IODP Expedition 339:
Site U1391 Summary
PDF file is available for download.
Background and objectives
Site U1391 (proposed site WI-01B)
is located on the SW Iberian Margin (37°21.532' N and
9°24.656' W) about 50 km NW of the Sao Vicente Cape, in a water depth of
1085 m. This is the most distal of our sites under the influence of MOW. It
lies on the broad, gently inclined mid-slope region of the SW Portuguese
margin, on which the seismic data indicate an extensive plastered drift that
stretches alongslope for about 90 km between the Sao Vicente and Setubal
downslope-oriented submarine canyons.
We consider this plastered
drift as part of the larger Cadiz Contourite Depositional System (CDS). On
seismic profiles it shows a well-layered internal acoustic structure with
laterally extensive, mainly aggradational seismic depositional units, and gentle
thickening of the Quaternary succession towards the axial region of the drift.
Deeper within the section, there are one or more unconformities apparent in the
seismic profiles, and potentially some influence of tectonically-induced
downslope mass-movement. There is no separate designation of MOW Upper and
Lower Cores along the Portuguese margin, although in terms of water depth, Site
U1391 would be closer to the Lower Core.
Our primary objective at this site was to recover a Pliocene, Pleistocene and
Holocene sedimentary succession formed under the influence of the MOW, and so
to compare this record with that found at Sites within the Gulf of Cadiz that
are closer to the Gibraltar Gateway. Very little is known about contourite
deposition along this margin, so that all data will be new and significant in
this regard. It will also provide a direct comparison with hemipelagic
sedimentation at Site U1385, which is removed from contourite input and under
the influence of North Atlantic Deep Water.
Site U1391 was occupied on 8 January 2012. Three holes were drilled and cored using the advanced piston corer (APC), the
extended core barrel (XCB), and the rotary core barrel (RCB) system, achieving a
total depth of 672 m in the third hole, U1391C. Downhole logging was carried
out at Hole U1391C using the Triple Combo and FMS-Sonic tool strings. Overall
recovery for Site U1391 was 359 m (105%) with the APC, 331 m (91%) with the
XCB, and 269 m (81%) with the RCB. The total cored interval for Site U1391 was 1038.1 m and total recovery was 958.6 m (92%).
sedimentary succession at Site U1391 extends for 672 m from the mid Pliocene to
Holocene. It is represented by a thick, very uniform series of mud-rich contouritic sediment, with rapid rates of sedimentation through the later Quaternary. It has been divided into two lithostratigraphic
units (Units I and II), both of which are dominated by calcareous mud-rich
contourite deposition, with more minor lithologies including silty mud, sandy
mud, nannofossil mud and biosiliceous mud. Silty bioclastic sand is rare. These
lithologies are generally organized as bi-gradational sequences and partial
sequences, with bioturbated and gradational upper and lower contacts. Carbonate
content ranges from 18-48%, and total organic carbon from 0.5-1.8%.
Unit I is
subdivided into Sub-Unit IA, which has a greater number of silty (and sandy)
intervals, as well as a distinct alternation of greenish and reddish-colored
units. Sub-Unit IB is even more mud-rich and with mainly grey
to dark-grey color cyclicity. The Unit I to Unit II boundary occurs at a
possible minor hiatus, below which there is a slightly more mixed system,
including a thin debrite, some minor faulting, more biosiliceous material, and a 50 cm-thick, well-cemented,
dolomitic mudstone below a probable second minor unconformity. The mud-rich
sediments in Unit II have an intermediate contourite-hemipelagite aspect.
microfossils (nannofossils, planktonic and benthic foraminifera, and ostracods)
are mostly common to abundant, with moderate to good preservation throughout. The sedimentary record appears to be
relatively continuous through the Quaternary period, with an average
sedimentation rate of 27 cm/ky for the later Pleistocene and 17 cm/ky for the
early Pleistocene. However, there may be minor hiatuses present at around
0.7-0.9 Ma and 2.41-2.5 Ma. The average sedimentation rate for the Pliocene
section recovered is 13 cm/ky, although there is also evidence for a minor
hiatus at 3.0-3.19 Ma or, alternatively, for much reduced rates of
sedimentation at this time.
As found at the other CDS
sites, there is a distinctive variability in benthic foraminifer assemblages,
which reveals significant environmental changes through the Pliocene-Holocene
succession. In general, the later Quaternary shows typical upper bathyal
assemblages that are indicative of increased organic matter input and reduced
ventilation; this signature is also evident during some earlier intervals. The
remainder of the Quaternary and Pliocene succession shows lower nutrient supply
and improved or variable ventilation. Certain signature taxa show somewhat
lesser influence of the MOW on the Portuguese margin than in the Gulf of Cadiz.
Although nannofossils show common reworking in parts of the succession, it is
less evident than at the Cadiz sites, and there is no evident reworking of
planktonic foraminifers. Pollen and spores are abundant in most of the samples
analyzed, together with microcharcoal and dinocysts. They show a similar
assemblage and pattern to that found at the other sites on this expedition.
Paleomagnetic measurements showed
that the Brunhes/Matuyama polarity transition (0.781 Ma) occurs below 175 mbsf,
but is not clear perhaps due to the inferred hiatus across this boundary. Specific
identification was made of the top and bottom of the Olduvai Subchron (1.778 and 1.945 Ma),
and the Matuyama/Gauss transition (2.581). These give reliable confirmation of
the biostratigraphic dating for Site U1391, although some of the inferred
polarity boundaries need further confirmation.
As has been observed at the other sites drilled, physical property data show relatively close tracking of magnetic
susceptibility and bulk density in much but not all of the section. These may
correlate or anti-correlate with NGR values, and with color reflectance (L* and
a* values). Both larger-scale trends and smaller-scale cycles are evident, with
some correlation at the small-scale with lithology. In lithostratigraphic Unit
II, there is much lower NGR variability and very low MS values.
pore water profiles at Site U1391 show some significant distinction from those
of Site U1385, commensurate with their deposition under MOW rather than North
Atlantic Deep Water. Rates of sedimentation and hence of organic matter
accumulation are greater at Site U1391, which makes for a shallower zone of
measurements were made in Hole U1391C to near the bottom of the hole at a depth
of 668 mbsf. The borehole was very rugose with many narrow washouts that
affected log quality. There are minor changes in log characteristics at around 562
mbsf, which correlates closely with a lithostratigraphic boundary. The deeper
interval has generally lower natural gamma ray values, and includes two zones
with poor core recovery that may 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. Ten downhole temperature measurements were made in
the top 146 m of section, yielding a geothermal gradient of 14.2 °C/km, the lowest on this expedition.
The three holes cored provide
a complete composite stratigraphic section to the base of the APC cored section
at 171 mbsf, and a virtually complete section all the way down to 354 mbsf. The
section below this is cored only in Hole U1391C to a total depth of 671.5 mbsf,
with some short gaps between cores and larger gaps in the few instances where
core recovery was low.
We recovered core to a total
depth of 672 mbsf at Site U1391 on the SW Portuguese margin. The site lies
under the influence of MOW at the depth of the Lower Core and penetrates
through a relatively complete Quaternary and late Pliocene section of a
plastered contourite drift. This is the most distal of our MOW sites and is
distinctly more mud-rich throughout than those in the Gulf of Cadiz.
Nevertheless, sedimentation rates (27 cm/ky) for the later Quaternary are
equally as high as those of the Faro Drift, and the contouritic signature of
uniformity and bi-gradational sequences is ever present. Important similarities
with other sites include an upward increase in sedimentation rate, the number
of silt/sand intervals and organic matter supply, as well as reduced
ventilation. These all support enhanced MOW influence through the Quaternary.
Hiatuses, possible hiatuses or much reduced sedimentation rates are noted at
around 0.7-0.9, 2.4-2.5 and 3.0-3.2 Ma, all of which are recognized at one or
more of the Cadiz sites. These are interpreted as episodes of enhanced MOW and
bottom current activity.