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IODP Expedition 323:
Bering Sea Paleoceanography
Site U1344 Summary
PDF file is available for download.
31 August 2009
Background and objectives
The primary objective of drilling
at Site U1344 (Site GAT-3C) was to study high-resolution Pliocene-Pleistocene
paleoceanography at a proximal gateway location to the Arctic Ocean at the
deepest water depth of Expedition 323. The site is located at ~3200 m along the
small summit of a canyon interfluve ~10-15 km southeast of Pervenets Canyon, a
large submarine canyon that deeply and widely incises the Beringian continental
slope. Pervenets Canyon, along with companion Zhemchug Canyon adjacent to Site
U1343, was discovered in the early 1960s by the Soviet fishing industry and
named after one of the canyonŐs discovering trawlers. At times of glacially
lowered sea level, the head of Pervenets Canyon is commonly presumed to have
been one of the outfall locations for the Anadyr River, which presently drains
the Russian far northeast and enters the Bering Sea at the Gulf of Anadyr. It
is anticipated to receive supply of terrigenous sediments from the shelf during
both the interglacials and the glacials.
This is also the area of high
biological productivity called the "Green Belt". The Green Belt is formed by the
Bering Slope Current (BSC), which has an origin in the incoming Alaskan Stream
water in the western Aleutians into the Bering Sea. The water that entered into
the Bering Sea moves eastward along the Aleutian Islands and consequently
encounters the shallow blocking Bering Shelf. The bottom depth of the BSC is
~300 m, therefore the BSC cannot flow eastward against the shelf any longer at
the shelf break and it upwells and its flow turns to the northwest along the
shelf break. Moreover, tidal mixing causes further vertical mixing of the water
masses along the BSC, enhancing biological productivity within the "Green
Belt", a zone adjacent to the northwest trending shelf break where high
primary productivity in the surface waters and high %organic carbon accumulation
at the seafloor are taking place. However, we anticipate finding less organic
carbon supply to the seafloor than at the other gateway sites or Site U1339 due
to deeper water depth than at the other sites. Thus, the expectation of the
impingement by the dissolved oxygen minimum zone (OMZ) in the past is
relatively small at this site. Nevertheless, it is important to compare the
vertical extent of water mass conditions in a basin wide scale extent including
this site. Hence, the records from the shallower drill sites on the Bowers
Ridge as well as the other gateway sites can be fully employed for the
comparison.
Site U1344 also is located close to
the maximum extent of the present day seasonal sea-ice cover. Thus, it is
expected that this site had been covered by the seasonal or perennial sea-ice
during the glacial low sea-level stands. Since it is adjacent to the Bering
Shelf, high amount of terrigenous sediment supply is expected especially during
the glacial low stands.
This relatively deep drill site in
the gateway region to the Arctic Ocean can also be used to study the impact of
subseafloor microbes on biogeochemical fluxes in the highest surface ocean
productivity areas of the drill sites in the Bering Sea. Organic-fueled
subseafloor respiration and its impact on biogeochemistry in such a highly
productive region have never been quantified previously. To do this, the
drilled sediments in the gateway region were used to determine subseafloor cell
abundance and to investigate the link between the mass and characteristics of
subseafloor microbes and the extent of export productivity from the surface
ocean.
Sedimentation rates at this site
have been estimated at 170-180 m/m.y. based on an earlier site survey piston
core studies. Neither of the piston cores taken in these studies recovered the
Holocene section, possibly indicating erosion during the recent past. Prior to
drilling, recovery of the Pleistocene to the Pliocene section was expected at
this site.
Operations
Five
holes were cored at Site U1344. The first used the APC/XCB systems to 745.0 m
DSF. Hole U1344A was then
successfully logged with the triple combo and FMS-Sonic logging tools. The
second hole, a planned dedicated microbiology hole, was cut short due to
contamination when the core liner split during the mudline coring. Therefore, the third hole was assigned
to microbiology. Hole U1344D was cored with the APC system to 286.5 m DSF. The
last hole, U1344E, was cored with the APC system to 202.8 m DSF. A complete set
of site specific tide tables provided by the science party were used to make
adjustments relative to initial mudline core and for each successive core on
each hole. Coring in general was routine except for biogenic methane
encountered in the cores which complicated the curation of all cores at this
site. APC coring totals for Site U1344 include 87 cores, 781.8 m penetrated,
791.1 m recovered, for 101.2% recovery. XCB coring totals for Site U1344
include 52 cores and 489.9 m penetrated, 384.1recovered for 78.4% recovery. Total
cored interval for Site U1344 was 1271.7 m, with 1175.19 m of core recovered
for a 92.4% total recovery. The time spent on Site U1344 was 8.5 days.
Hole U1344A
The first APC barrel recovered 9.1
m of core and an official seafloor depth was established at 3183.4 m DRF. Hole
U1344A was spudded at 0932 hr on 14 August. APC coring continued through Core
U1344A-26H to 245.6 m DRF using non-magnetic coring assemblies. The XCB system
was deployed and coring continued through Core U1344A-79X to a depth of 745.0 m
DSF. The coring tools were secured and the hole was swept clean and then
displaced with 200 barrels of prepared high viscosity logging mud. There was no
fill identified at total depth. The end-of-pipe (EOP) was placed at ~100.0 m
DSF. Two logging strings were deployed. The triple combo string reached total
hole depth of 745 m DSF and good quality logs were obtained. The second logging
string consisted of the FMS-sonic tool, which reached total depth of 745 m DSF
on the first pass but reached only 725 m DSF on the second pass as hole
conditions began to deteriorate. Good data were obtained on both passes. APC
core recovery for Hole U1344A was 103.5% with 264 m recovered. XCB core
recovery for Hole U1344A was 78.4% with 384.1 m recovered. Total core recovery
for Hole U1344A was 87% with 648.1 m recovered.
Hole U1344B
The vessel was offset 20 m north of
Hole U1344A. Hole U1344B was spudded with APC at 1335 hr on 19 August. One APC
core was recovered. Unfortunately,
Core U1344B-1H came to the surface with a split/damaged core liner and was
unfit for use for microbiological studies.
Hole U1344C
The first APC barrel recovered 7.12
m of core and an official seafloor depth was established at 3184.4 m DRF. Hole
U1344C was spudded at 1505 hr on 19 August. Both contamination testing methods,
per-fluoro-methyl-cyclohexane (PFTs) and microspheres were deployed. A total of
4 APC cores were recovered for microbiology to a depth of 35.6 m. Average core
recovery for the APC on Hole U1344B was 94.13% with 33.5 m recovered.
Hole U1344D
The ship was offset 20 m north from
Hole U1344C. The APC was deployed and Hole U1344C was spudded at 1940 hr on 19
August. The first mudline core recovered 2.73 m of sediment and the calculated
seafloor was 3185.8 m DRF. APC coring continued through Core U1344C-25H to
224.5 m DRF using non-magnetic coring assemblies. The non-magnetic coring
equipment was changed to the standard APC coring system and coring continued.
Coring with the APC system was suspended after Core U1344D-32H at a depth of
286.5 m DSF. Cores U1344D-30H and 32H required drill over. Overall recovery for
Hole U1344D using the APC coring system was 99.9% with 286.1 m recovered.
Hole U1344E
The ship was offset 20 m north from
Hole U1344D. The first APC barrel recovered 5.3 m of core and an official
seafloor depth was established at 3185.7 m DRF. Hole U1344E was spudded at 0750
hr on 21 August. APC coring continued through Core U1344E-23H using
non-magnetic coring assemblies. Coring was uneventful and continuous except for
a 3 m section at 3276.5 m DSF, which was drilled to maintain stratigraphic
overlap. APC core recovery for Hole U1344E was 101.4% with 202.7 m
recovered.
Lithostratigraphy
There was one lithologic unit
defined at this site spanning the early Pleistocene to the Holocene. Unit I at
Site U1344 encompasses a time period comparable to Unit I defined at the Bowers
Ridge Sites U1340 and U1341, and is in general very similar to Unit I at the
other Bering Sea margin Sites U1339 and U1343. However, Site U1344 is distinct
in that it has an even higher proportion of siliciclastic components and a
higher occurrence of sand-sized grains than Site U1343. This is probably
related to the location of this site on the continental slope and its relative
proximity to sources of terrigenous sediments from the continental margin.
Sandy lithologies are concentrated in three relatively distinct intervals at
Site U1344. They can be correlated not only between the holes at Site U1344,
but also to Site U1343 where three distinctly sandy intervals occur. While the
lithologies dominated by diatoms are associated with changes in color
reflectance and analogous to those at other sites, the lithology changes here
are more subtle due to the overall higher abundances of siliciclastic detritus.
Higher abundance of diatoms may reflect high diatom flux during interglacials,
as previously observed in the Bering Sea.
Only one diatom-rich laminated
interval was observed. All other laminations are defined by faint color changes
with gradational boundaries with the surrounding lithologies. It appears that
well-oxygenated bottom water conditions probably prevailed throughout most of
the Pleistocene, preventing preservation of laminations. Almost all dropstones
are well-rounded, indicating a period of reworking prior to incorporation in
the ice. The rounding therefore favors a coastal provenance and sea ice rafting
rather than icebergs. Unlike most other sites but similar to Site U1343,
volcaniclastic material is a minor component of the sediment at Site U1344
because it is more distant from the Aleutian arc. Authigenic carbonate occurs
throughout the sediment and is not constrained to deeper parts of the sequence
as it was at Site U1343. The shallowest appearance in Hole U1344A is at 63 m
CSF-A. The presence of gas in the sediments caused several types of coring
disturbance, mostly cracks up to several centimeters wide. In some cases, this
affected the stratigraphic integrity of the sediment sequence, similar to Sites
U1339 and U1343.
Biostratigraphy
The water depth at Site U1344 is ~ 3200 m has a potential to reconstruct
past deep-water changes because of it is presently located below the oxygen
minimum zone. Benthic foraminiferal faunas indicate high frequency changes in
the bottom water oxygen content over the entire section, probably related
primarily to surface water productivity, but possibly to bottom water
ventilation changes and methane seeps as well. There is a general increase in
abundance from ~300 m CSF to the top of the section as well as bottom water
oxygen variability. The low oxygen indicator Bulimina aff. exilis can be seen to be more abundant at
both Sites U1343 and U1344 after approximately 0.8 Ma, along with benthic
foraminifer abundance maxima. Both high abundance and low oxygen benthic faunas
were found to be common during the last deglacial at the Bowers Ridge , and the
increase in such characteristics from 0.8 Ma may mark the onset of more intense
deglacials, greater nutrient availability, and higher surface water
productivity. Similarly to Site U1343, the increases of planktic foraminifers
also coincide with the highest numbers of sea-ice diatoms and polar
dinoflagellate cysts after 1 Ma. The change from low to high abundances of
planktic foraminifers coincides with the increases of the abundances of
dinoflagellate cysts, calcareous nannofossils, benthic foraminifers and the
number of low-oxygen benthic foraminifers, analogous to previously published
data for the CaCO3 preservation peaks during the last deglaciation
in the Bering Sea.
This site is
characterized by very low abundance of calcareous nannofossils. Only samples
from the upper most cores to the base of Core U1344D-4H can be assigned to the
calcareous nannofossil Zone NN21, with an estimated age of less than 0.29 Ma.
The planktic foraminifer faunal assemblage found during the Late Pleistocene is
dominated by Neogloboquadrina pachyderma
(sinistral) throughout. Below 260 m CCSF-A N. pachyderma (sinistral) is reduced or absent from the assemblage
and the fauna is replaced by a subpolar assemblage dominated by G.
bulloides. The occurrence of this species
is mainly ruled by sea surface temperature, indicating that the Late
Pliocene–Early Pleistocene were warmer than the Late Pleistocene at this
site.
The drilled interval above the LO of diatom P. curvirostris (ranging from 107.2 to 122.3 m CSF depending on the holes) is assigned to the
Neodenticula seminae Zone (NPD12).
Due to the absence of Actinocyclus
oculatus at Holes U1344D and U1344E the
bottom of each hole is assigned to the Zone NPD 11. The FO datum of P.
curvirostris was defined at Sample
U1344A-56X-CC and assigned the age of 1.85±0.1 Ma in the A.
oculatus Zone. The LO of Pyxidicula horridus (1.9-2.0 Ma) was estimated at the base of Core
U1344A-63X. The LO of silicoflagellate Dictyocha subarctios was assigned to Cores U1344A-30H (270.25-280.31 m
CSF-A) and U1344D-26H (224.41-234.09 m CSF-A). The LO of ebridian Ammodochium
rectangulare appears to be located in Core
U1344A-78X (733.13-739.75 m CSF-A). The radiolarian ages at Site U1344 spans
from the Botryostrobus aquilonaris
Zone (Late Quaternary) to the Eucyrtidium matuyamai Zone (Middle Quaternary) in the subarctic Pacific.
Five radiolarian datums derived from the subarctic Pacific were identified at
this site. Estimated sedimentation rates in the upper 150 m at Holes
U1344A, U1344D, and U1344E are greater than 30 cm/k.y., which is slightly
higher than at the neighboring Site U1343 (~20 cm/k.y.). The LO of E.
matuyamai (0.9-1.5 Ma) was identified in
samples from Hole U1344A. The occurrence of dinoflagellate Filisphaera filifera at the base of Core U1244A-50X
(473.4 m CSF-A) suggest an age of 1.41-1.7 Ma, according to its last occurrence
datum in the North Pacific and North Atlantic. This species dominates the
assemblages in a few samples above this depth, quite similar to Site U1343. The
occasional occurrence of the autotrophic species Operculudinium centrocarpum may be related to oceanic
conditions with relatively low productivity.
Paleomagnetism
The Brunhes/Matuyama boundary is
clearly identified at ~280 m CSF-A depth. The Jaramillo, Cobb Mountain and
Olduvai subchrons might be correlatable with the extracted normal polarity
zones placed at about 380, 420 and 680 m CSF-A, respectively. The
paleointensity variation has quite large amplitude and obviously shows a
coherent change with the magnetic susceptibility, suggesting that the NRM
intensity has been largely influenced by environmental changes. The relative
paleointensity pattern seen at this site is consistent with those observed at
Sites U1340, U1341, U1342, and U1343. Based on the correlations, marine isotope
stages (MIS) 1-19 have been assigned to ~280 m CSF-A. The dramatic changes in
NRM indicate notable effects of early sediment diagenesis. Significant magnetic
mineral dissolution starts within 10 m CSF-A due to anaerobic methane oxidation
(AOM)–sulfate reduction processes, which is also evident at Sites U1343
and U1339.
Geochemistry
The rate of carbon turnover in the
sediment at Site U1344 is similar to or slightly higher than rates at Site
U1343, as evidenced by similar SO42-, DIC, PO43-
and NH4+ concentration profiles. Similar to Site U1343,
profiles of CH4 and SO42- at Site U1344
suggest that sulfate reduction is largely driven by CH4 diffusing
into the sulfate zone. The CH4 flux into the SO42-
zone, as calculated from the concentration gradient between 8-13 m CSF-A, is
approximately 70%-80% of the SO42- flux into the
sulfate–methane transition zone (SMTZ). The
importance of AOM for overall carbon turnover is also indicated by the
curvature in the DIC profile. The steepest concentration gradient in the upper
10 m CSF-A is observed directly above the SMTZ, suggesting that the highest DIC
flux occurs from this zone. Preliminary modeling of the DIC profile suggest
that net DIC production in the SMTZ accounts for 80% of the DIC production in
the upper 30 m CSF-A of the sediment. Hydrogen
sulfide is also at a maximum in the SMTZ most likely because sulfate reduction
rates are the highest and the content of oxidized iron is the lowest in this
zone. Magnetic susceptibility data obtained during fast scan of the cores
confirm a low content of oxidized iron in the SMTZ.
It is well known that AOM favors
the deposition of carbonates in the SMTZ.
At Site U1344 a relatively high flux of Ca2+ into the SMTZ is
observed that indicates the formation of calcium carbonate. There were also
indications for Mg2+ flux into the SMTZ, which may suggest dolomite
formation. The curvature of the NH4+ profile suggests
production from organic matter degradation throughout the sediment column.
Microbial mediated degradation is either conducted via a respiratory or via a
fermentative pathway. According to the classical reduction scheme in sediments,
only fermentation and hydrogenotrophic methanogenesis occurs below the SMTZ;
however, at this site the Fe profile suggests that Fe reduction occurs below
the SMTZ. Organic matter degradation also leads to the accumulation of DIC and
PO43- in the interstitial water. The accumulation of
these species; however, is much lower than predicted from the NH4+
profile assuming steady state and a constant ratio between C, N and P of
remineralized organic matter. This suggests both production and consumption of
DIC and PO43- in the sediment. Consumption of these
species is most likely due to formation of apatite and calcium carbonates
(e.g., dolomite). The interstitial water profiles suggest that rates of net consumption of PO43-
and DIC are the highest between 300-350 m CSF-A. Ca2+ and Mg2+ concentration profiles
likewise indicate net consumption of these species between 300-350 m CSF-A.
Microbiology
Samples for abundance of
prokaryotes were collected adjacent to interstitial water whole-rounds in
sections drilled using APC system. High resolution sampling took place in the
microbiological dedicated cores as well as additional samples taken once per
core to APC refusal in Hole U1344A. Additional samples were taken from Cores
U1344A-78X to -80X recovered using the XCB system to evaluate cell abundance
and community structure in the deepest portion of Hole U1344E. PFT analyses
performed on these cores show no contamination from the drill fluid. Samples
from all cores were fixed.
It is of interest to examine the
relationship between microbial productivity and diversity in the upper 25 m of
the sediment column. Special attention will be directed toward the function of
Archaea in the sulfate reduction zone, the SMTZ and the methanogenesis zone.
The sulfatemethane transition is a "hot spot" for microbial activity and abundance
within deep-sea sediments and we will expect and increase in the abundance and
activity of microbial life. While the remainder of the core should see a
significant decrease with depth in microbial life both active and benign.
Physical Properties
The downhole profile of density for
Hole U1344A is remarkably similar to that of Hole U1343E. The overall downward
increase in bulk density is interpreted to record compactive dewatering in a
generally lithologically uniform sequence of fine-grained sediment. Magnetic
susceptibility (MS), as measured by the wholeround multisensory logger (WRMSL)
exhibits little change in average value and character with depth. Based on what
has been learned at previous sites, it is presumed that the rhythmic
oscillations are a function of lithologic composition and patterns of in-situ
sediment alteration.
Except
for the upper three cores at Hole U1344A, P-wave velocity readings for the
sedimentary section penetrated at this hole were only collected by the
FMS-sonic downhole logging tool. Sonic P-wave velocity data reveal a profile
similar to that recorded at Hole U1343E in that the average velocity increases
down section in step-like sectors. Except for the upper ~80-100 m CSF-A, across
which NGR readings increase from a near-surface measurement of ~25 count/s to
about 34 counts/s, NGR values at deeper depths oscillate around this average to
the base of the Hole U1344A at 745 m. Presumably variations in counts/s reflect
downhole changes in content of clay and siliciclastic minerals.
Thermal
conductivity measurements can be grouped into an upper and lower sequence.
The
upper vertical sequence displays an estimated average reading of ~0.905 W/mK
and extends downward from the near surface to a depth of ~260 m CSF-A, below
which APC refusal caused a change to XCB coring and Vp-wave velocity shifts
abruptly to higher readings. At Hole U1344A, porosity decreases most rapidly in
the upper part of the drilled section, falling to an average value of ~60% at
80-100 m CSF-A. Below this depth, other than oscillating readings, little
detail tracking shifts in average value or changes in trend are exhibited. The
overall downward decrease in porosity tracked by MAD and logging data is
presumably a manifestation of compaction dewatering.
The upper group of dry grain density extends from the surface to a
depth of ~160 m CSF-A is ~2.70 g/cm3. The middle sequence, from ~160
to 620 m CSF-A, exhibits an average density of 2.65 g/cm3, and the
underlying basal group a lower density of ~2.62 g/cm3. It appears
that an overall upward increase in deposition of denser siliciclastic mineral
debris is recorded.
Stratigraphic Correlation
The complete and continuous
composite depth scale and splice at Site U1344 is constructed from 0.0–332.02
m CCSF-A. The continuous splice ranges from the top of Core U1344A-1H to Core
U1344A-31X Section 5 at 50 cm. The appended cores range from Core U1344A32X, to
U1344A79X (790.37 m CCSF-A) with a constant affine value of 43.78 m. All splice
points in the interval of 0-50 m CCSF-A are clear and convincing based on
multiple data types. The splice
tie point between U1344A-5H-7, 4.44 cm and U1344D-6H-3,79.88 cm (51.99 m
CCSF-A) is uncertain, and could be moved about 2.4 m shallower in Core
U1344D-6H with equal uncertainty.
This is a point to be checked with postcruise data. The splice tie
points between U1344E-6H-6, 112.47 cm and U1344A-6H-4, 122.06 cm (60.40 CCSF-A)
and between U1344E-14H-1, 10.92 cm and U1344D-14H-2, 136.02 cm (140.43CCSF-A)
and U1334A-16H-7, 1.04 cm and U1334D-17H-4, 1.34 cm (175.52 CCSF-A) are
uncertain because of low signal amplitude in MS.
Downhole Measurements
Two tool strings were deployed in
Hole U1344A: the triple combo and the FMS-sonic combination. Overall, the
caliper of the density sonde shows an irregular borehole, with a particularly
large interval between 170 and 260 m WSF, but with very good conditions in the
lower section. Deeper in the borehole, small enlargements regularly spaced
every ~9.5 m indicate where the bit was sitting whenever a core was recovered.
However, all the calipers show that the tools were making at least partial
contact with the formation over most of the interval logged, suggesting that
the overall quality of the data is good. Irregular hole size has an effect on
the measurements that require good contact with the formation, namely density
and porosity. The anomalously low density values between 230 and 250 m WSF,
within the 100-m interval with the largest hole size, are probably erroneous,
as well as most neutron porosity measurements in this entire interval. The
quality of the logs can also be assessed by comparison with the NGR and GRA
track data and with the moisture and density (MAD) measurements made on cores
recovered from Hole U1344A. Except for two short intervals with lower density
logging data (230-250 m WMSF and
420-430 m WSF), all density data sets are in good agreement, confirming
the overall good data quality despite the enlarged hole. Comparison of the
gamma ray logs measured during the main pass of the two runs shows an excellent
repeatability between the two runs.
Logging Unit 1 (100 - 330 m WMSF) is characterized mainly by a steady
increase with depth in Vp and Vs, while the other log
data remain mostly uniform despite some variability such as in the gamma ray. The
bottom of this unit is defined by a noticeable drop in Vp, Vs,
gamma ray, density and resistivity, immediately above by a sharp peak in these
measurements, particularly in Vs and resistivity, indicating a fine
stiff layer. This sequence corresponds to a core with poor recovery. Logging
Unit 2 (330-460 m WMSF) is almost uniquely defined by the Vp and Vs
log, both increasing steadily through the unit. Gamma ray and density also
increase with depth in this unit, in a more subdued manner. The top of Logging
Unit 3 (460-620 m WMSF) is defined by an inflection in the velocity profiles
which, combined with a decreasing trend in density, generates the strong
reflector at 4.83 s twtt. The variability with depth in gamma ray and in most
logs, displays a cyclicity more clearly defined than in the upper units.
Finally, the top of Logging Unit 43 (620 - 745 m WMSF ) is defined by a sharp
increase in Vp, Vs, gamma ray and density, as well as a
significant change in the trends of all the logs. As in the deepest unit of Site U1343, the gamma ray,
potassium, thorium, density, resistivity, Vp and Vs logs
all display a variability with depth of wider amplitude and lower frequency
than in the upper units, suggesting a significant change in the deposition
history and rates.
The APCT-3 tool was successfully
deployed three times in Hole U1344A. The measured temperatures ranged from 4.51
°C at 47.1 m DSF to 9.57 °C at 142.1 m DSF, and fit closely a linear geothermal
gradient of 53.3 °C/km. The temperature at the seafloor was 1.65 °C, based on
the average of the measurements at the mudline during all the APCT-3
deployments. A simple estimate of the heat flow can be obtained from the
product of the geothermal gradient by the average thermal conductivity (0.911
W/m°C), which gives a value of 48.5 mW/m2, within the range of
previous measurement in the area
Sedimentation Rates
Sedimentation rates observed at
Site U1344 are mostly similar to values within a narrow range of 29-50 cm/k.y.
throughout Holes U1344A, U1344D, and U1344E. One exception is the interval
between the top and the bottom of the Cobb Mountain Subchron (459.0-469.6 m
CCSF-A), which resulted in 89 cm/k.y. Based on the sedimentation rates the
bottom age of Hole U1344A was determined to be ~1.9 Ma.
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