Stratigraphy around the Cretaceous-Paleogene boundary in sediment cores from the Lord Howe Rise, Southwest Pacific

During Deep Sea Drilling Project (DSDP) Leg 21, Cenozoic and latest Cretaceous sediments were recovered at Site 208 on the Lord Howe Rise, Southwest Pacific. We provide new biostratigraphic, magnetostratigraphic and chemostratigraphic data from Site 208 to constrain the stratigraphy around the Cretaceous-Paleogene (K-Pg) boundary and to determine the depth of the K-Pg boundary more precisely. Biostratigraphic data from calcareous nannofossils indicate a near-continuous succession of sediments from the mid-Maastrichtian (Late Cretaceous) to lowermost Thanetian (Paleocene) at depths of 540−590 m below seafloor (mbsf). The biostratigraphic data suggest that the K-Pg boundary corresponds to a siliceous claystone at the base of an interval of silicified sediments (576.0−576.8 mbsf). Carbonate carbon isotopic composition (δ^{13}_{Ccarb}) reveals a negative shift across this interval, which is consistent with global patterns of δ^{13}C across the K-Pg boundary. Osmium concentration and Os isotopic composition ({187}^Os/{188}^Os) can also be used to identify the K-Pg boundary interval, as it is marked by a peak in Os concentration and a drop in 187^{Os}/{188}^Os values to 0.12−0.15, both of which are the result of the Chicxulub impact event. Our {187}^Os/{188}^Os data show trends similar to those of coeval global seawater with the lowest value of 0.12−0.16 in the siliceous claystone (576.8 mbsf). However, the concentration of Os is low (<80 pg g^{−1}) in this sample, which suggests that this siliceous claystone was deposited around the K-Pg boundary but may not include the boundary itself. Although the sedimentary record across the K-Pg interval at Site 208 may not be completely continuous, it nevertheless captures a time interval that is close to the Chicxulub impact event

Diazotrophy drives primary production in the organic-rich shales deposited under a stratified environment during the messinian salinity crisis (Vena Del Gesso, Italy)

Density stratification between freshwater and brine is periodically formed during massive evaporation events, which often associates deposition of organic-rich sediments. Here, we investigated phototrophic communities and nitrogen cycle during the deposition of two organic-rich shale beds of gypsum\u2013shale alternation, representing the initial stage of the Messinian salinity crisis (Vena del Gesso, Northern Apennines, Italy). The structural distributions and the carbon and nitrogen isotopic compositions of geoporphyrins show a common pattern in the two shales, indicating the predominance of a particular phototrophic community under freshwater\u2013brine stratified conditions. The 3c6\u2030 difference in \u3b413C of total organic carbon between PLG 4 and 5 shales was associated with similar shift in \u3b413C of the porphyrins derived from chlorophyll c, suggesting that the eukaryotic algae producing chlorophyll c were the major constituent of the phototrophic community. Importantly, these porphyrins show \u3b415N values (-7.6\u2013-4.7\u2030) indicative of N2-fixation. We suggest that nitrate-depletion in the photic zone induced the predominance of diazotrophic cyanobacteria, which supplied new nitrogen for the chlorophyll c-producing eukaryotic algae. The large difference in the \u3b413C values of porphyrins and total organic carbon between PLG 4 and 5 shales are interpreted to reflect the depth of the chemocline, which fluctuates in response to changes in the regional evaporation\u2013precipitation balance. Such variation in the chemocline depth may have dynamically changed the mode of the nitrogen cycle (i.e., nitrification\u2013denitrification\u2013N2-fixation coupling vs. phototrophic assimilation of ammonium) in the density-stratified marginal basins during the Messinian salinity crisis

Biomarker records and mineral compositions of the Messinian halite and K–Mg salts from Sicily

The evaporites of the Realmonte salt mine (Sicily, Italy) are important archives recording the most extreme conditions of the Messinian Salinity Crisis (MSC). However, geochemical approach on these evaporitic sequences is scarce and little is known on the response of the biological community to drastically elevating salinity. In the present work, we investigated the depositional environments and the biological community of the shale–anhydrite–halite triplets and the K–Mg salt layer deposited during the peak of the MSC. Both hopanes and steranes are detected in the shale–anhydrite–halite triplets, suggesting the presence of eukaryotes and bacteria throughout their deposition. The K–Mg salt layer is composed of primary halites, diagenetic leonite, and primary and/or secondary kainite, which are interpreted to have precipitated from density-stratified water column with the halite-precipitating brine at the surface and the brineprecipitating K–Mg salts at the bottom. The presence of hopanes and a trace amount of steranes implicates that eukaryotes and bacteria were able to survive in the surface halite-precipitating brine even during the most extreme condition of the MSC.This work was performed with the support of Japan Society for the Promotion of Science (JSPS) Research Fellowship (16 J07844) to YI and JAMSTEC President Fund to NO

Temperature-Dependent Site Control of InAs/GaAs (001) Quantum Dots Using a Scanning Tunneling Microscopy Tip During Growth

Site-controlled InAs nano dots were successfully fabricated by a STMBE system (in situ scanning tunneling microscopy during molecular beam epitaxy growth) at substrate temperatures from 50 to 430°C. After 1.5 ML of the InAs wetting layer (WL) growth by ordinal Stranski–Krastanov dot fabrication procedures, we applied voltage at particular sites on the InAs WL, creating the site where In atoms, which were migrating on the WL, favored to congregate. At 240°C, InAs nano dots (width: 20–40 nm, height: 1.5–2.0 nm) were fabricated. At 430°C, InAs nano dots (width: 16–20 nm, height: 0.75–1.5 nm) were also fabricated. However, these dots were remained at least 40 s and collapsed less than 1000 s. Then, we fabricated InAs nano dots (width: 24–150 nm, height: 2.8–28 nm) at 300°C under In and As4 irradiations. These were not collapsed and considered to high crystalline dots

A radiocarbon-based assessment of the preservation characteristics of crenarchaeol and alkenones from continental margin sediments

Author Posting. © Elsevier B.V. , 2008. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Organic Geochemistry 39 (2008): 1039-1045, doi:10.1016/j.orggeochem.2008.02.006.Crenarchaeotal glycerol dibiphytanyl glycerol tetraether (GDGT) lipids and alkenones are two types of biomarkers derived from planktonic marine micro-organisms which are used for reconstruction of sea-surface temperatures. We determined the radiocarbon contents of the archaeal GDGT crenarchaeol and of alkenones isolated from continental margin sediments. Systematic differences were found between the two biomarkers, with higher radiocarbon contents in crenarchaeol than in the phytoplankton-derived alkenones. These differences can be explained by variable contributions of pre-aged, laterally advected material to the core sites. Crenarchaeol appears to be more efficiently degraded during transport in oxygen-replete environments than alkenones. Whether this reflects the influence of chemical structure or mode of protection (e.g., particle association) is not yet known.This work was funded by a Spinoza grant of NWO to J.S.S.D. and by NSF-grant OCE-0327405 to T.I.E.

Efficient recycling of nutrients in modern and past hypersaline environments

The biogeochemistry of hypersaline environments is strongly influenced by changes in biological processes and physicochemical parameters. Although massive evaporation events have occurred repeatedly throughout Earth history, their biogeochemical cycles and global impact remain poorly understood. Here, we provide the first nitrogen isotopic data for nutrients and chloropigments from modern shallow hypersaline environments (solar salterns, Trapani, Italy) and apply the obtained insights to δ15N signatures of the Messinian salinity crisis (MSC) in the late Miocene. Concentrations and δ15N of chlorophyll a, bacteriochlorophyll a, nitrate, and ammonium in benthic microbial mats indicate that inhibition of nitrification suppresses denitrification and anammox, resulting in efficient ammonium recycling within the mats and high primary productivity. We also suggest that the release of 15N-depleted NH3(gas) with increasing salinity enriches ammonium 15N in surface brine (≈34.0‰). Such elevated δ15N is also recorded in geoporphyrins isolated from sediments of the MSC peak (≈20‰), reflecting ammonium supply sufficient for sustaining phototrophic primary production. We propose that efficient nutrient supply combined with frequent bottom-water anoxia and capping of organic-rich sediments by evaporites of the Mediterranean MSC could have contributed to atmospheric CO2 reduction during the late Miocene.This study was partly supported by a Japan Society for the Promotion of Science (JSPS) Research Fellowship (16J07844) to YI, Grants-in-Aid (16H02236) and the JAMSTEC President Fund to NO

Electronic structure of Ga1−x_{1-x}Crx_{x}N and Si-doping effects studied by photoemission and X-ray absorption spectroscopy

The electronic structure of the magnetic semiconductor Ga$_{1-x}$Cr$_{x}$N and the effect of Si doping on it have been investigated by photoemission and soft x-ray absorption spectroscopy. We have confirmed that Cr in GaN is predominantly trivalent substituting for Ga, and that Cr 3$d$ states appear within the band gap of GaN just above the N 2$p$-derived valence-band maximum. As a result of Si doping, downward shifts of the core levels (except for Cr 2$p$) and the formation of new states near the Fermi level were observed, which we attribute to the upward chemical potential shift and the formation of a small amount of Cr$^{2+}$ species caused by the electron doping. Possibility of Cr-rich cluster growth by Si doping are discussed based on the spectroscopic and magnetization data.Comment: 5 pages, 3 figure

Reconstruction of the past compound-specific N and C isotopic analyses of sedimentary porphyrins

Porphyrins are thought to have derived from biomoleculessuch as chlorophylls and heme, among which eoxophylloerythroetioporphyrins(DPEP) and its analogues arestructurally related to chloropigments. These compounds thuspreserve stable isotopic compositions of N and C ofphototrophs of the past environment. We have developedmethods for precise determinations of N and C isotopiccompositions of individual sedimentary porphyrins andmaleimides, which include isolation and purification ofindividual alkylporphyrins and porphyrin acids by dual-stepHPLC preparation (Kashiyama et al., 2007a). We haveanalyzed various alkylporphyrins and porphyrin acids fromorganic-rich Miocene sediments of the paleo-Japan Sea(Onnagawa Formation) as well as Cretaceous black shales(Livello Selli and Livello Bonarelli, Italy) deposited in thewestern Tethys during the Ocean Anoxic Events (OAEs). TheN isotopic composition of DPEP, which should have derivedin chlorophylls in general, ranged from -6.9 to -3.6‰ (n=7) inthe Miocene shale and -6.6 to -3.9‰ (n=5) in the OAE blackshales, indicating that the N2 fixation was a major process forN assimilation hence the dominance of diazotrophiccyanobacteria in primary production in these paleo-oceans.Furthermore, DPEP were relatively enriched in 13C in bothenvironments (-17.9 to -15.6‰ in the Miocene shale and -20.5to -17.9‰ in the OAE black shales), which suggests relativelysmall carbon isotopic fractionation during photosynthesis andsupports significant contribution of cyanobacteria-derivedchloropigments. We also determined isotopic compositions ofsource-specific porphyrins such as 17-nor-DPEP (derivedfrom chlorophyll-c), 8-nor-DPEP (possibly derived fromdivinylchlorophylls; Kashiyama et al., 2007b) as well asDPEP with extended alkyl side chains and their equivalentsamong porphyrin acids (derived from bacteriochlorophyll c, d,and e; analyzed as maleimides). The latter two porphyrins hadvariable and rather unique isotopic compositions compared toDPEP. Such an approach should allow reconstruction ofcommunity structures of phototrophs and associatedbiogeochemical processes associated with the photosynthesisin the past oceans.abstractの

Influence of hydrodynamic processes on the fate of sedimentary organic matter on continental margins

Understanding the effects of hydrodynamic forcing on organic matter (OM) composition is important for assessment of organic carbon (OC) burial in marginal seas on regional and global scales. Here we examine the relationships between regional oceanographic conditions (bottom shear stress), and the physical characteristics (mineral surface area and grain size) and geochemical properties (OC content [OC%] and carbon isotope compositions [13C, 14C]) of a large suite of surface sediments from the Chinese marginal seas to assess the influence of hydrodynamic processes on the fate of OM on shallow continental shelves. Our results suggest that 14C content is primarily controlled by organo‐mineral interactions and hydrodynamically driven resuspension processes, highlighted by (i) positive correlations between 14C content and OC% (and surface area) and (ii) negative correlations between 14C content and grain size (and bottom shear stress). Hydrodynamic processes influence 14C content due to both OC aging during lateral transport and accompanying selective degradation of OM associated with sediment (re) mobilization, these effects being superimposed on the original 14C characteristics of carbon source. Our observations support the hypotheses of Blair and Aller (2012, https://doi.org/10.1146/annurev‐marine‐120709‐142717) and Leithold et al. (2016, https://doi.org/10.1016/j.earscirev.2015.10.011) that hydrodynamically driven sediment translocation results in greater OC 14C depletion in broad, shallow marginal seas common to passive margin settings than on active margins. On a global scale, this may influence the extent to which continental margins act as net carbon sources and sinks. Our findings thus suggest that hydrodynamic processes are important in shaping the nature, dynamics, and magnitude of OC export and burial in passive marginal seas