143 research outputs found
Modern deep-water agglutinated foraminifera from IODP Expedition 323, Bering Sea: ecological and taxonomic implications
Despite the importance of the Bering Sea for subarctic oceanography and climate, relatively little is known of the
foraminifera from the extensive Aleutian Basin. We report the occurrence of modern deep-water agglutinated foraminifera
collected at seven sites cored during Integrated Ocean Drilling Program (IODP) Expedition 323 in the Bering Sea. Assemblages
collected from core-top samples contained 32 genera and 50 species and are described and illustrated here for the first time.
Commonly occurring species include typical deep-water
Rhizammina
,
Reophax
,
Rhabdammina
,
Recurvoides
and
Nodulina
.
Assemblages from the northern sites also consist of accessory
Cyclammina
,
Eggerelloides
and
Glaphyrammina
, whilst those of
the Bowers Ridge sites consist of other tubular genera and
Martinottiella
. Of the studied stations with the lowest dissolved
oxygen concentrations, the potentially Bering Sea endemic
Eggerelloides
sp. 1 inhabits the northern slope, which has the
highest primary productivity, and the potentially endemic
Martinottiella
sp. 3 inhabits Bowers Ridge, which has the lowest
oxygen concentrations but relatively low annual productivity.
Martinottiella
sp. 3, with open pores on its test surface, has
previously been reported in Pliocene to Recent material from Bowers Ridge. Despite relatively small sample sizes, ecological
constraints may imply that the Bering Sea experienced high productivity and reduced oxygen at times since at least the Pliocene.
We note the partially endemic nature of the agglutinated foraminiferal assemblages, which may at least in part be due to basin
restriction, the geologically long time period of reduced oxygen, and high organic carbon flux. Our results indicate the
importance of gathering further surface sample data from the Aleutian Basin
Micropaleontological characterization of Cenozoic deep-sea fan deposits, Congo Fan, offshore Angola
Abstrac
Palaeoclimatic implications of high-resolution clay mineral assemblages preceding and across the onset of the Palaeocene–Eocene Thermal Maximum, North Sea Basin
Understanding the composition of clay-rich sediments and their transportation into proximal marine basins allows us to better decipher hydroclimatic changes before and within the Palaeocene–Eocene Thermal Maximum (PETM). Only a limited number of such studies exists from the North Sea Basin, which was proximal to the volcanic activity and early rifting hypothesized to have triggered the PETM. The present study examines core material from well 22/10a-4, UK North Sea, as it exhibits an exceptionally expanded and almost stratigraphically complete fine-grained sedimentary sequence suitable for high-resolution analysis.
Quantitative Newmod-for-Windows™-modelled clay mineral assemblages, rather than traditional semi-quantitative estimates, are dominated by smectite-rich, interlayered illite-smectite that probably developed from volcanogenic deposits on continental landmasses. Soil development before the PETM is consistent with the existence of a seasonal tropical climate with a prolonged dry season. A striking rise and fall of kaolinite content within the PETM onset, prior to the principal carbon-isotope excursion, is reported here. This variation is interpreted as a signal of an enhanced hydrologic cycle producing an increase in erosionally derived kaolinite, followed by a dampening of this detrital source as sea-levels rose. Global variations in PETM kaolinite concentrations are consistent with a latitudinal shift in patterns of precipitation in models of global warming
Pathological macromolecular crystallographic data affected by twinning, partial-disorder and exhibiting multiple lattices for testing of data processing and refinement tools
Twinning is a crystal growth anomaly, which has posed a challenge in macromolecular crystallography (MX) since the earliest days. Many approaches have been used to treat twinned data in order to extract structural information. However, in most cases it is usually simpler to rescreen for new crystallization conditions that yield an untwinned crystal form or, if possible, collect data from non-twinned parts of the crystal. Here, we report 11 structures of engineered variants of the E. coli enzyme N-acetyl-neuraminic lyase which, despite twinning and incommensurate modulation, have been successfully indexed, solved and deposited. These structures span a resolution range of 1.45–2.30 Å, which is unusually high for datasets presenting such lattice disorders in MX and therefore these data provide an excellent test set for improving and challenging MX data processing programs
Evolution of South Atlantic density and chemical stratification across the last deglaciation.
Explanations of the glacial-interglacial variations in atmospheric pCO2 invoke a significant role for the deep ocean in the storage of CO2. Deep-ocean density stratification has been proposed as a mechanism to promote the storage of CO2 in the deep ocean during glacial times. A wealth of proxy data supports the presence of a "chemical divide" between intermediate and deep water in the glacial Atlantic Ocean, which indirectly points to an increase in deep-ocean density stratification. However, direct observational evidence of changes in the primary controls of ocean density stratification, i.e., temperature and salinity, remain scarce. Here, we use Mg/Ca-derived seawater temperature and salinity estimates determined from temperature-corrected δ(18)O measurements on the benthic foraminifer Uvigerina spp. from deep and intermediate water-depth marine sediment cores to reconstruct the changes in density of sub-Antarctic South Atlantic water masses over the last deglaciation (i.e., 22-2 ka before present). We find that a major breakdown in the physical density stratification significantly lags the breakdown of the deep-intermediate chemical divide, as indicated by the chemical tracers of benthic foraminifer δ(13)C and foraminifer/coral (14)C. Our results indicate that chemical destratification likely resulted in the first rise in atmospheric pCO2, whereas the density destratification of the deep South Atlantic lags the second rise in atmospheric pCO2 during the late deglacial period. Our findings emphasize that the physical and chemical destratification of the ocean are not as tightly coupled as generally assumed.We are grateful to I. Mather, J. Rolfe, F. Dewilde and G. Isguder for preparing and performing isotopic analyses, as well as C. Daunt, S. Souanef-Ureta and M. Greaves for technical assistance in performing trace element analysis. J.R. was funded jointly by the British Geological Survey/British Antarctic Survey (Natural Environment Research Council) and the University of Cambridge. J.G. was funded by the Gates Cambridge Trust. L.C.S. acknowledges support from the Royal Society and NERC grant NE/J010545/1. C.W. acknowledges support from the European Research Council grant ACCLIMATE/no 339108. This is LSCE contribution 5514. This work was funded (in part) by the European Research Council (ERC grant 2010-NEWLOG ADG-267931 HE). N.V.R. acknowledges support from EU RTN NICE (no. 36127). We thank the captain and crew of the RRS James Clark Ross for facilitating the collection of the marine sediment core GC528.This is the author accepted manuscript. The final version is available from PNAS via http://dx.doi.org/10.1073/pnas.151125211
Neodymium evidence for increased circumpolar deep water flow to the North Pacific during the Middle Miocene Climate Transition
Low salinity surface water inhibits local deepwater formation in the modern North Pacific.
Instead, southern-sourced Circumpolar Deep Water (CDW) fills the basin, which is the product of water
masses formed from cold sinking centers in the Southern Ocean and North Atlantic. This CDW is responsible
for transporting a significant amount of global heat and dissolved carbon in the deep Pacific Ocean. The
history of its flow and the broader overturning circulation are widely assumed to be sensitive to climate
perturbations. However, insufficient records exist of CDW presence in the deep North Pacific with which to
evaluate its evolution and role in major climate transitions of the past 23 Ma. Here we report sedimentary
coatings and fish teeth neodymium isotope values—tracers for water-mass mixing—from deepwater
International Ocean Discovery Program Site U1438 (4.7 km water depth) in the Philippine Sea, northwest
Pacific Ocean. Our results indicate the water mass shifted from a North Pacific source in the early Miocene to a
southern source by ~14 Ma. Within the age model and temporal constraints, this major reorganization of
North Pacific water mass structure may have coincided with ice sheet build up on Antarctica and is most
consistent with an increased northward flux ofCDWdue to enhanced sinking of cold water forced by Antarctic
cooling. The northward extent of this flux may have remained relatively constant during much of the past 14Ma
New Insights into the Nature of Transition Disks from a Complete Disk Survey of the Lupus Star-forming Region
Transition disks with large dust cavities around young stars are promising
targets for studying planet formation. Previous studies have revealed the
presence of gas cavities inside the dust cavities hinting at recently formed,
giant planets. However, many of these studies are biased towards the brightest
disks in the nearby star forming regions, and it is not possible to derive
reliable statistics that can be compared with exoplanet populations. We present
the analysis of 11 transition disks with large cavities (>20 AU radius) from a
complete disk survey of the Lupus star forming region, using ALMA Band 7
observations at 0.3" (22-30 AU radius) resolution of the 345 GHz continuum,
13CO and C18O 3-2 observations and the Spectral Energy Distribution of each
source. Gas and dust surface density profiles are derived using the
physical-chemical modeling code DALI. This is the first study of transition
disks of large cavities within a complete disk survey within a star forming
region. The dust cavity sizes range from 20-90 AU radius and in three cases, a
gas cavity is resolved as well. The deep drops in gas density and large dust
cavity sizes are consistent with clearing by giant planets. The fraction of
transition disks with large cavities in Lupus is ~11%, which is inconsistent
with exoplanet population studies of giant planets at wide orbits. Furthermore,
we present a hypothesis of an evolutionary path for large massive disks
evolving into transition disks with large cavities
Silicic Acid Cycling in the Bering Sea During the Mid‐Pleistocene Transition
The rate of deep-ocean carbon burial is considered important for modulating glacial-interglacial atmospheric CO2 concentrations and global climate during the Quaternary. It has been suggested that glacial iron fertilization and increased efficiency of the biological pump in the Southern Ocean since the Mid-Pleistocene Transition (MPT) was key in lowering atmospheric pCO2 and facilitating rapid land ice accumulation. There is growing evidence that a similar mechanism may have existed in the subarctic Pacific Ocean, although this has not yet been assessed. Here, the silicon isotope composition of diatoms (δ30Sidiatom) from the Bering Sea upwelling region is used to assess the role of nutrient cycling on the subarctic Pacific biological pump during the MPT. Results show that during and after the “900 kyr event,” the high productivity green belt zone was characterized by low silicic acid utilization but high supply, coincident with the dominance of diatom resting spores. We posit that as nutrient upwelling was suppressed following pack ice growth and expansion of glacial North Pacific Intermediate Water (GNPIW), primary productivity became nitrate-limited and enhanced opal remineralization caused a relative increase in silicic acid supply. However, preferential preservation and higher cellular carbon content of diatom resting spores, as well as increased supply of iron from expanded sea ice, likely sustained the net efficiency of the Bering Sea biological pump through the MPT. Remnant iron and silicic acid may also have propagated into the lower subarctic Pacific Ocean through GNPIW, aiding a regionally efficient biological pump at 900 kyr and during post-MPT glacials
Productivity and dissolved oxygen controls on the Southern Ocean deep‐sea benthos during the Antarctic Cold Reversal
Funding was provided by an Antarctic Bursary awarded to J.A.S., ERC and NERC grants awarded to L.F.R. (278705, NE/S001743/1, NE/R005117/1) and L.F.R. and J.W.B.R. (NE/N003861/1).The Antarctic Cold Reversal (ACR; 14.7 to 13 thousand years ago; ka) phase of the last deglaciation saw a pause in the rise of atmospheric CO2 and Antarctic temperature, that contrasted with warming in the North. A re-expansion of sea ice and a northward shift in the position of the westerly winds in the Southern Ocean are well-documented, but the response of deep-sea biota and the primary drivers of habitat viability remain unclear. Here we present a new perspective on ecological changes in the deglacial Southern Ocean, including multi-faunal benthic assemblage (foraminifera and cold-water corals) and coral geochemical data (Ba/Ca and δ11B) from the Drake Passage. Our records show that, during the ACR, peak abundances of thick-walled benthic foraminifera Uvigerina bifurcata and corals are observed at shallow depths in the sub-Antarctic (∼300 m), while coral populations at greater depths and further south diminished. Our ecological and geochemical data indicate that habitat shifts were dictated by (i) a northward migration of food supply (primary production) into the Subantarctic Zone and (ii) poorly oxygenated seawater at depth during this Antarctic cooling interval.Publisher PDFPeer reviewe
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