Extreme, abrupt Northern Hemisphere climate oscillations during the last glacial cycle (140,000 years ago to present) were modulated by changes in ocean circulation and atmospheric forcing. However, ...the variability of the Atlantic meridional overturning circulation (AMOC), which has a role in controlling heat transport from low to high latitudes and in ocean CO2 storage, is still poorly constrained beyond the Last Glacial Maximum. Here we show that a deep and vigorous overturning circulation mode has persisted for most of the last glacial cycle, dominating ocean circulation in the Atlantic, whereas a shallower glacial mode with southern-sourced waters filling the deep western North Atlantic prevailed during glacial maxima. Our results are based on a reconstruction of both the strength and the direction of the AMOC during the last glacial cycle from a highly resolved marine sedimentary record in the deep western North Atlantic. Parallel measurements of two independent chemical water tracers (the isotope ratios of (231)Pa/(230)Th and (143)Nd/(144)Nd), which are not directly affected by changes in the global cycle, reveal consistent responses of the AMOC during the last two glacial terminations. Any significant deviations from this configuration, resulting in slowdowns of the AMOC, were restricted to centennial-scale excursions during catastrophic iceberg discharges of the Heinrich stadials. Severe and multicentennial weakening of North Atlantic Deep Water formation occurred only during Heinrich stadials close to glacial maxima with increased ice coverage, probably as a result of increased fresh-water input. In contrast, the AMOC was relatively insensitive to submillennial meltwater pulses during warmer climate states, and an active AMOC prevailed during Dansgaard-Oeschger interstadials (Greenland warm periods).
Celotno besedilo
Dostopno za:
DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SBMB, SIK, UILJ, UKNU, UL, UM, UPUK
Continental dust input into the ocean‐atmosphere system has significant ramifications for biogeochemical cycles and global climate, yet direct observations of dust deposition in the ocean remain ...scarce. The long‐lived isotope thorium‐232 (232Th) is greatly enriched in upper continental crust compared to oceanic crust and mid‐ocean ridge basalt‐like volcanogenic material. In open ocean sediments, away from fluvial and ice‐rafted sources of continental material, 232Th is often assumed to be of predominantly eolian origin. In conjunction with flux normalization based on the particle reactive radioisotope thorium‐230 (230Th), 232Th measurements in marine sediments are a promising proxy for dust accumulation in the modern and past ocean. Here we present ThoroMap, a new global data compilation of 230Th‐normalized fluxes of 232Th. After careful screening, we derive dust deposition estimates in the global ocean averaged for the late Holocene (0–4 ka) and the Last Glacial Maximum (LGM, 19–23 ka). ThoroMap is compared with dust deposition estimates derived from the Community Climate System Model (CCSM3) and CCSM4, two coupled atmosphere, land, ocean, and sea ice models. Model‐data correlation factors are 0.63 (CCSM3) and 0.59 (CCSM4) in the late Holocene and 0.82 (CCSM3) and 0.83 (CCSM4) in the LGM. ThoroMap is the first compilation that is built on a single, specific proxy for dust and that exclusively uses flux‐normalization to derive dust deposition rates.
Plain Language Summary
Continental dust input into the ocean‐atmosphere system has significant ramifications for global climate and marine ecosystems. However, direct observations of dust deposition in the ocean remain scarce. Continental dust contains elevated concentrations of the trace element thorium, and dust input thus leaves a distinct geochemical signature in seawater and sea floor sediments. ThoroMap brings together thorium measurements from over 150 sediment core sites, unveiling a record of dust deposition to the global ocean in the recent geologic past.
Key Points
Continental dust input to the ocean is reconstructed using thorium isotopes
A new global data base of dust fluxes is presented for the late Holocene and the Last Glacial Maximum
Comparisons with recent coupled models identify regions of good model‐data agreement
The ductility dip cracking susceptibility of two Ni-base filler metals, Filler Metal 52 (59%Ni–29%Cr–9%Fe–1%Mn–1%Al) and Filler Metal 82 (72%Ni–20%Cr–1%Fe–3%Mn–3%Nb) was evaluated using the ...strain-to-fracture test. The analysis of these results and detailed weld metal microstructural characterization (SEM and TEM) was presented in Part I. Part II of this work provides new insight into the creep-like, grain boundary sliding mechanism that leads to elevated temperature intergranular cracking in these weld deposits. The effect of precipitation on grain boundary tortuosity and sliding, and its influence on ductility-dip cracking resistance is discussed in the context of current theories of high temperature creep. Finally, the effect of composition, including both precipitate forming elements and impurity elements, on ductility-dip cracking is discussed.
High-strength low-alloy or microalloyed steels are thermo-mechanically control processed (TMCP) to achieve desired microstructures and mechanical properties, and microalloy additions are critical for ...obtaining optimum properties. The steel of interest in this study is microalloyed with approximately 0.1 wt% Nb, levels that are typical in pipeline steels such as X70. Submerged arc welding (SAW) is a commonly utilized manufacturing method for constructing pipe segments via seam welding. There were two parts to this investigation. Part 1 analyzed controlled dilution levels consisting of commercially available SAW electrodes and Nb-bearing steel in proportions to simulate submerged arc weld metal. Part 2 of this investigation evaluated actual submerged arc welds on low-alloy steels. Bead-on-plate welds were deposited, and the Nb content of the weld metal was systematically increased by adding ferro-niobium directly to the flux. Two-run welds representative of line pipe production were also evaluated using the same ferro-niobium flux additions in order to increase the weld metal Nb content. The microstructural evolution and corresponding mechanical properties with increased Nb are evaluated based on this work and on the published literature.
High-energy density beam processes for welding, including laser beam welding and electron beam welding, are essential processes in many industries and provide unique characteristics that are not ...available with other processes used for welding. More recently, these high-energy density beams have been used to great advantage for additive manufacturing. This review of the literature serves to provide an overview of the evolution of the laser and electron beam processes including the fundamental nature of the beam itself and how such a high-energy density beam has been applied for welding. The unique nature of each process regarding weld bead formation and penetration, and metallurgical considerations are covered in detail. In addition, the evolution of monitoring systems for both characterization and control of these beam processes is reviewed. Over 500 references have been cited in this comprehensive review that will allow the reader to both understand the current state-of-the-art and explore in more detail the fundamental concepts and practical applications of these processes.
This paper celebrates the 75-year history of the OSU Welding Engineering program. It includes a brief history of the program, details regarding the academic mission, and current research activities. ...It also provides an outlook for the program into the future.
A detailed investigation of the early stages of secondary austenite precipitation in five duplex stainless steel (DSS) commercial alloys (UNS S32304, S32205, S32550, S32750, and S32760) has been ...conducted using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Based on this study, a model is proposed that describes the interaction between Cr^sub 2^N and austenite (intergranular and intragranular) precipitation in these alloys. Depending on nitrogen availability and interface mobility, Cr^sub 2^N precipitation along existing ferrire/austenite interfaces precedes intergranular secondary austenite growth. The low-energy interfaces formed between the Cr^sub 2^N, the ferrite, and the austenite, along with the coupled diffusion processes, are the factors controlling this phase transformation. Finally, in the case of the intragranular nitrides, a mechanism is proposed whereby the nitrides serve as sites for heterogeneous nucleation of intragranular secondary austenite. PUBLICATION ABSTRACT
Titanium alloys are commonly used in aerospace applications based on their superior strength-to-weight ratio and excellent corrosion resistance. One of the most common structural alloys is Ti-6Al-4V. ...Fusion welding of this alloy typically results in the formation of epitaxial nucleation and growth of coarse, columnar-shaped beta grains in the weld fusion zone. This coarse beta grain weld microstructure can negatively impact weld mechanical properties, and particularly the ductility of thin-sheet welds. High energy density welding processes, which exhibit a much lower net energy input than arc welding processes, have been shown to control and minimize the beta grain size and promote improved weld properties. The objective of this work was to quantify beta grain size and growth directions of laser welds in Ti-6Al-4V. Partial penetration, bead-on-plate laser welds were produced at a constant power and varying travel speed. Light optical microscopy was attempted to delineate beta grain boundaries, but insufficient boundary delineation resulted in exploring alternative methods. Prior-beta grains were reconstructed using electron-backscattered diffraction analysis, and this information was used to determine the size, orientation, and growth of the beta grains at different welding parameters. Beta grain growth during solidification was determined to be strongly influenced by the weld pool shape and generally occurred by columnar growth from the fusion boundary to the weld surface and/or weld centerline. The beta grain size decreased in the plan view orientation but showed a negligible change in grain width for the transverse and longitudinal sections.
The ductility dip cracking resistance of two Ni-base Filler metals, Filler Metal 52 (FM-52) (59%Ni–29%Cr–9%Fe–1%Mn–1%Al) and Filler Metal 82 (FM-82) (72%Ni–20%Cr–1%Fe–3%Mn–3%Nb) was evaluated using ...the strain-to-fracture (STF) test. The reinterpretation of previously reported STF test results for these filler metals, is discussed, based on detailed microstructural characterization performed using electron microscopy. In Part II of this paper, new insight into the creep-like, grain boundary (GB) sliding mechanism responsible for intergranular cracking in these filler metals is presented.
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