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IODP Expedition 323:
Bering Sea Paleoceanography
Site U1345 Summary
PDF file is available for download.
1 September 2009
Background and objectives
The primary objective of
drilling at Site U1345 was to study high-resolution Holocene-Late Pleistocene
paleoceanography at a proximal gateway location to the Arctic Ocean at a water
depth of ~1008 m. The drill site is located on an interfluve ridge near the large and broad head of Navarin Submarine Channel
off the Bering Sea Shelf. It is anticipated to receive ample supply of
terrigenous sediments from the shelf during the glacials. This is also in the
area of high biological productivity called "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. As going
further northwest, the higher primary productivity and % organic carbon is
observed at the sea-floor. Thus, the expectation of the impingement by the
dissolved oxygen minimum zone (OMZ) in the past is relatively high at this
site. Because the expected sedimentation rates are high with intermittently
laminated sediment of mm to sub-mm in thickness (see below) it should be
possible at this site to pursue reconstructing detailed climate changes of sub-millennial
time scales. Therefore, it is possible to compare the data from this site with
those from other pertinent high-resolution records from such places as the
Santa Barbara Basin, the Cariaco Basin, and GISP2. It is also important to
compare the vertical extent of water mass conditions in a basin wide scale
extent including the Bowers Ridge and gateway as well as Umnak sites.
Furthermore, this site is located
close to the maximum extent of the present day seasonal sea-ice cover. Thus, it
is expected that this site had been extensively covered by the seasonal or
perennial sea-ice during the glacial low sea-level stands. Due to the proximity
to the location of sea-ice formation, where cold and dense brine is expelled
when sea-ice is formed, this site, as well as Site U1344, provide crucial
information regarding the formation of the North Pacific Intermediate Water
This relatively shallow 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
The sedimentation rates at the site
location vary significantly from 14 cm/k.y. during the Holocene and 91 cm/k.y.
during the last glacial maximum to as much as 242 cm/k.y. during the
deglaciation. Prior to drilling, the recoveries of the Holocene to Late
Pleistocene sections were expected at this site.
Five holes were cored at this site. With the
exception of Hole U1345B that was cored down to 36.7
m DSF for microbiological sampling, four holes were cored to ~150 m DSF. Coring in general was routine except for biogenic
methane encountered in the cores which complicated the curation of all cores at
this site. Two complete splices were obtained at Site U1345 to approximately
150 m CSF. APC coring totals for Site U1345 include 68 cores, 632.1 m
penetrated, 648.35 m recovered, for 102.6 % recovery. The time spent on Site
U1345 was 2.2 days.
Hole U1345A was spudded at 0045 hr on 23 August. The first APC core barrel recovered 4.39 m of sediment establishing
a seafloor depth of 1019.1 m DRF. APC coring continued through Core U1345A-16H
to a depth of 146.9 m DSF using non-magnetic coring assemblies. Temperature
measurements were taken on Cores U1345A-5H, -8H and -12H with the APCT3
temperature tool. APC core recovery for Hole U1345A was 101.1% with 148.49 m
The ship was offset 20 m northwest
of Hole U1345A. Both per-fluoro-methyl-cyclohexane (PFTs) and microspheres were
deployed for contamination testing. A total of 4 APC cores were recovered to be
used as samples for microbiology to a depth of 36.7 m. Average core recovery
for the APC on Hole U1345B was 104.2 % with 38.24 m of sediment recovered.
The ship was offset 20 m northwest
of Hole U1345B. APC coring continued through Core U1345C-16H to a depth of
148.5 m DRF using non-magnetic coring assemblies. APC core recovery for Hole
U1345C was 102.9 % with 152.85 m recovered.
The ship was offset 20 m northwest
of Hole U1345C. APC coring continued through Core U1345D-16H to a depth of
150.0 m DRF using non-magnetic coring assemblies. APC core recovery for Hole
U1345D was 103.1 % with 154.62 m recovered.
The ship was offset 20 m northwest
of Hole U1345D. APC coring continued through Core U1345E-16H to a depth of
150.0 m DRF using non-magnetic coring assemblies. APC core recovery for Hole
U1345E was 102.8 % with 154.15 m recovered.
Only one lithostratigraphic unit
was recognized at Site U1345. Unit I includes the same time period as Unit I at
the other sites: the Middle Pleistocene to the Holocene. Site U1345 is distinct
among the near-shelf sites due to the abundance and generally coarser texture
of the siliciclastic component in the sediments as well as the higher frequency
of laminated intervals. Intervals characterized by >25% sand and by thin
sandy layers occur at all depths at all the holes drilled at this site. The
laminations and thin-bedded sediments are numerous and well correlated between
Site U1345 is located in the
central portion of the modern oxygen minimum zone (OMZ). The sediments that
were deposited at this site can provide important information concerning
Pleistocene to Holocene variability of bottom water oxygen concentrations. The
preservation of laminated and thinly bedded sediments (beds <10 cm
thickness) could be interpreted as the result of a reduction of the activity of
benthic macrofauna due to low oxygen concentrations in the bottom waters and
surface sediments. Laminations do not have a clear signature in the physical
properties or reflectance data as observed at Site U1342.
The laminated intervals can be
divided into two categories based on the abundance of biogenic grains: (1)
couplets or triplets of diatom oozes, mixed siliciclastic/biogenic sediments,
and siliciclastic sediments, or (2) couplets of siliciclastic sediments of
alternating textures that may include minor (<40%) amounts of diatoms.
Laminated sediments of the first category are similar to laminated intervals at
other sites, which are typically biogenic rich, olive green, dark olive gray,
and very dark greenish gray. This category of lamination seems to occur mainly
during interglacials. This relationship supports previous observations of
higher flux of diatoms during interglacial compared to glacial periods. The
second category of laminated sediments is mainly siliciclastic and unique to
Site U1345. This type of lamination occurs in sediments that are tentatively
identified as deposited during glacial conditions. Since these sediments are
not biogenic-rich, changes in intermediate water ventilation may have been the
controlling parameter for bottom water oxygen concentrations during these
We observed intermittent, finely
disseminated authigenic carbonates deeper than ~30 m CSF-A in all holes at this
site. The sulfatemethane transition zone (SMTZ) is at ~6.5 m CSF-A, the
shallowest observed during the Expedition 323. Calcium and magnesium
concentrations in the pore water decrease towards the SMTZ, suggesting active
authigenic carbonate precipitation at and below this depth today.
There are few ash layers observed
at Site U1345. The ashes that do appear are light colored, suggesting that
their source is explosive rhyolitic volcanism. This site is distant from the
nearest likely source of volcaniclastic grains, the Aleutian Arc, so the
transport mechanism must have been one capable of widespread dissemination.
The lithologies at Site U1345 are
sandier than at any other Expedition 323 site. Lithologies with >25% sand and
thin sandy layers occur throughout all holes. The presence of this coarse
material is probably related to the position of Site U1345 at the crest of an
interfluve at the mouth of Navarin Canyon. The siliciclastic grain sizes at
Site U1345 contrast even stronger with Site U1339, located on a submarine
plateau isolated from the continental shelf. At the latter site, there were
virtually no sand-sized grains recorded. This may be due either to less
transport of terrigenous material to the site or to a high biogenic flux to
Site U1339 sediments.
High frequency variations can be
seen in the abundance and composition of all microfossil groups. The decrease
in sea-ice diatoms and the increase in dinoflagellates, planktonic foraminifers
and calcareous nannofossils, % open ocean diatoms Neodenticula and Actinoticlus, and in the high productivity dinoflagellate Islandinium
minutum, associated with increases in the
low oxygen benthic foraminifer Bulimina aff. exilis indicates
the approximate depth intervals of distinct interglacials (at ~5, 40, 130 and
145 m CCSF). These intervals also coincide with low GRA bulk density, and are
consistent with the age model.
Overall, the distribution of calcareous nannofossils at Site U1345 seems to follow glacial/interglacial cyclicity
with higher numbers during interglacials. Changes in their abundance generally
reflect changes in environmental factors such as temperature and nutrients.
Calcareous nannofossils do not become dominant components of the biota in areas
of sea-ice coverage. Elevated content of subpolar planktonic foraminifer
species with G. bulloides appear at ~5,
40, 90, 130 and 145 m CCSF largely coinciding with the inferred interglacials.
This shows increased sea surface temperatures during these intervals. G.
bulloides is controlled by temperature
rather than food availability in the Bering Sea. These periods of elevated sea
surface temperatures are probably reflecting interglacial conditions.
As at the previous Bering Sea
sites, the benthic foraminifer assemblage faunal composition shows large
changes in species dominance. These changes are interpreted as shifts in local
oxygen concentrations associated with surface productivity and/or deep water
ventilation on Milankovich time scales. Bulimina aff. exilis is generally
regarded as a low oxygen/deep infaunal species and occurs in samples associated
with high productivity and low sea-ice. This suggests that higher productivity
during some interglacials may have caused an expansion and intensification of
the oxygen minimum zone.
The LO of Proboscia curvirostris and the LO of Thalassiosira jouseae were observed from 71.1 to 73.3 m CSF depending on
the holes, giving the age of 0.3 Ma. In general, diversity is lower for this
Site than at the other Gateway sites. The diatom assemblage for this Zone (NPD
11) is dominated by Thalassiosira antarctica spores, Fragilariopsis spp., Paralia sol, P. sulcata, Thalassiothrix
longissima, Thalassionema nitzschioides, T. latimarginta S. L., and to a lesser extent Neodenticula seminae, Bacteriosira fragilis and Actinocyclus curvatulus. The core
interval above the LO of P. curvirostris to the Recent is assigned to Neodenticula seminae Zone (NPD12). This Zone is dominated by T. antarctica spores, T. latimarginta S. L., P. sulcata, T. hyalina, B. fragilis and minor presence of N. seminae and A. curvatulus. In
general, this site reveals a higher proportion of coastal neritic diatoms
together with freshwater species compared to the other Gateway Sites U1343 and
U1344. Low proportions of sea-ice diatoms and high proportions of open water
diatoms correspond well with the interglacial horizons. The last occurrence of
silicoflagellate Distephanus octonarius (0.2-0.3 Ma) was observed in Core U1345A-9H (71.01-80.64 m CSF-A).
Radiolarian zones at Site U1345
could not be established due to the absence of Stylatractus universus. Four radiolarian datums derived in the subarctic
Pacific were identified at this site. The LO of Lychnocanomma
nipponica sakaii (50 ka) and Spongodiscus sp. (280-320 ka) were determined. The LOs of Amphimelissa
setosa (70-90 ka) and Axoprunum
aquilonium (250-410 ka) were supported only
by seldom occurrences, indicating uncertain top positions of the stratigraphic
age. Estimated sedimentation rates between LOs of L. nipponica sakaii and Spongodiscus sp. are ~25cm/kyr at each hole. Among all radiolarian species, C. davisiana shows high fluctuations in
abundance, possibly relating to ventilation changes with
glacialinterglacial cycles. The Sphaeropyle langii/robusta group, which were commonly found at Sites U1343 and
U1344, show very low abundances. Since abundances of Sphaeropyle
langii/robusta group at the shallower Sites
U1339, U1340, and U1342 were also very low, their dwelling depth might be in
deep water below 1000 m.
The dinoflagellates species Brigantedinium spp. is one of the most ubiquitous taxa among
protoperidinials, and its distribution in modern sediments is closely related
to primary productivity in temperate regions and also to polar and subpolar
regions of the North Atlantic and Arctic oceans with seasonal sea-ice coverage.
Islandinium minutum is one of the
principal, if not the dominant components of assemblages in the modern Arctic
Ocean. The overall abundance of dinocysts and particularly the above mentioned species
suggest high productivity and upwelling during prominent interglacials.
Extremely high abundance of dinocysts, especially in the mudline from Hole
U1345B and Samples U1345A-5H-CC (44.4 m CCSF) and U1345A-13A-CC (130.6 m CCSF)
suggests interglacial periods. This coincides with relatively low pollen and
No polarity reversal boundary was
observed in the cores at Site U1345, therefore the whole sequence is assigned
to the Brunhes normal polarity zone. The relative paleointensity pattern is
consistent with that seen at all other sites. Based on the correlations, marine
isotope stages (MIS) 1-12 were assigned. The significant changes in NRM
indicate notable effects of early sediment diagenesis, as this has also been seen
at the previous sites. Significant magnetic mineral dissolution starts within 5
m CSF-A due to processes related to anaerobic oxidation of methane (AOM) and
sulfate reduction. This is also evident at Sites U1344, U1343, and U1339. The
active zone of dissolution appears to be limited to the top 10 m, so that
magnetization does not change significantly at deeper depths.
Site U1345 shows the shallowest
SMTZ of all sites investigated, at ~6.25 m CSF-A. Likewise this site is
characterized by the steepest flux of methane into this zone and the highest
interstitial water hydrogen sulfide concentrations. Similar to the other shelf
Sites U1343 and U1344, the almost linear sulfate and methane profiles suggest
that AOM coupled to sulfate reduction accounts for most of the sulfate
consumption in the sediment. Preliminary modeling of the DIC profile suggest
that net DIC production in the SMTZ accounts for 70% of the DIC production in
the top sediment layers. The organic matter degradation products phosphate and
ammonium show accumulation in the pore water, the distinct minimum in phosphate
concentration between 22.25 m and 27.25 m CSF-A however, also indicates the
consumption of this species is most likely due to the formation of
phosphate-bearing minerals such as apatite.
The occurrence of high
concentrations of interstitial water hydrogen sulfide in the SMTZ can be
attributed to very high sulfate reduction rates at this depth and probably also
to a lack of sufficient pool of reactive Fe-mineral phases, e.g.
Fe-(oxhydr)oxides that can react with hydrogen sulfide on short time scales.
Distinct peaks in dissolved Fe and Mn concentration immediately below the SMTZ
are the result of microbial dissimilatory Fe reduction. Calcium and magnesium
profiles show depletion at the depth of the present SMTZ, suggesting the
formation of authigenic Mg-rich carbonate (e.g., dolomite) driven by the
production of DIC during AOM and an increase in pH leading to oversaturation of
the pore water with respect to carbonate. Interestingly, the dissolved calcium
profile shows a further decrease with depth and a minimum concentration at
around 40 m CSF-A. This depth corresponds to a dolostone layer found at 40.27 m
CSF-A. Sites U1343, U1344 and U1345 furthermore show high concentrations of
dissolved Ba in the pore water and indicate a sink of this ion just above the
SMTZ. The distribution of Ba at these sites can be attributed to diagenetic
remobilization of Ba deposited as biogenic barite into the sulfate-depleted
pore water. The upper end of the SMTZ where the
sulfate and dissolved Ba profiles overlap marks the present front of authigenic
Samples for abundance of
prokaryotes were collected adjacent to interstitial water whole-rounds. High
resolution sampling took place in the microbiological dedicated cores as well
as additional samples taken once per core to APC refusal. It is of interest to
examine the relationship between microbial productivity and diversity in the
upper 25 m of sediment dedicated for microbial ecology. A special focus 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 an 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. To obtain an estimate of active
subseafloor life samples were also taken in low resolution for CARD-FISH at all
aforementioned zones and at depth.
As indicators of productivity we
will rely on estimates generated by shipboard participants (cell counts,
geochemical profiles) and shorebased participants (amino acid and amino sugar
composition). We will examine overall bacterial and archaeal diversity by a
combination of conventional 16S rRNA clone libraries, as well as quantitative
polymerase-chain-reaction (qPCR) and/or a new quantitative community fingerprinting method involving automated ribosomal intergenic spacer analysis (ARISA; Ramette 2009).
bulk densities at Hole U1345A appear to be higher by 0.1 to 0.2 g/cm3
compared to those measured at Hole U1344A in the upper 150 m of the sedimentary
section; the higher densities of sediment at Hole U1345A probably reflect their
higher sand content.
to stratigraphic sections drilled at Beringian margin sites, Sites U1343 and
U1344, GRA density values at Hole U1345A also document rhythmic fluctuations.
MS measurements seem to have realistically recorded the downhole contour of
changing values that are functions of many factors. An explanation for the
higher counts for the Hole U1345A section is its coarser and higher content of
siliciclastic mineral debris. In downhole profile, the contour of NGR readings
is broadly similar to that of GRA bulk density. The downhole distribution of
thermal conductivity readings displays an overall trend of increasing
conductivity. Down-section profiles of MAD-measured porosity and water content
record a progressive decrease in average values. The downhole distribution of
water content and porosity is rhythmic. Little change is seen in average grain
density with depth. The higher average grain density (2.75 g/cm3) of
Unit I at Hole U1345A is interpreted to be a consequence of its greater
abundance of coarse siliciclastic grains.
The composite depth scale and
splice at Site U1345 is complete and continuous from 0.0167.6 m CCSF-A.
The splice ranges from the top of Core U1345A-1H to U1345D16H-7, 146.6 cm.
There are no appended intervals. Most of the splice points are clear and
convincing based on the multiple copies of the section recovered in five holes.
The splice tie point between U1345A-10H4, 50.0 cm and U1345C-10H1, 100.18 cm
(93.024 m CCSF-A) is uncertain because Core U1345A-10H contains disturbed
flow-in starting approximately in the middle of Section 4 and extends through
the bottom of Section 7. The
disturbed section is not included in the splice. The splice tie point between
U1345D-10H-7, 58.4 cm and U1345A-11H-3, 115.4 cm (103.284 m CCSF-A) and between
U1345D-13H-6, 138.4 cm, and U1345A-14H-3, 94.7cm (133.637 m CCSF-A) are
tentative because of low signal amplitudes in MS. The cumulative offset between
CSF-A and CCSF-D depth scales is roughly linear. The affine growth factor at
Site U1345 is 1.11.
The only downhole measurements made
at Site U1345 were three deployments of the APCT-3 tool in Hole U1345A. The
measured temperatures ranged from 4.92°C at 42.4 m DSF to 8.15°C at 108.9 m
DSF, indicating a local geothermal gradient of 48.5°C/km. A simple estimate of
the heat flow can be obtained from the product of the geothermal gradient by the
average thermal conductivity, which gives a value of 51.6 mW/m2, in
agreement with existing measurement in the area.
Based on the four holes that were
studied, the following two mean radiolarian biostratigraphic datums were
employed for the determination of sedimentation rates: the last occurrences
(LO) of Lychnocanomma nipponica sakaii;
and the LO of Spongodiscus sp..
Only one sedimentation rate of 28 cm/k.y. appears to be applicable to this
site. This sedimentation rate is lower than that of adjacent and deeper water
Site U1344 of ~39 cm/k.y.