Ke Lin

A reconstruction of sea surface temperature variability in the southeastern Gulf of Mexico from 1734 to 2008 C.E. using cross-dated Sr/Ca records from the coral Siderastrea siderea

This study uses skeletal variations in coral Sr/Ca from three Siderastrea siderea coral colonies within the Dry Tortugas National Park in the southeastern Gulf of Mexico (24°42′N, 82°48′W) to reconstruct monthly sea surface temperature (SST) variations from 1734 to 2008 Common Era (C.E.). Calibration and verification of the replicated coral Sr/Ca-SST reconstruction with local, regional, and historical temperature records reveals that this proxy-temperature relationship is stable back to 1879 C.E. The coral SST reconstruction contains robust interannual (~2.0°C) and multidecadal variability (~1.5°C) for the past 274 years, the latter of which does not covary with the Atlantic Multidecadal Oscillation. Winter SST extremes are more variable than summer SST extremes (±2.2°C versus ±1.6°C, 2σ) suggesting that Loop Current transport in the winter dominates variability on interannual and longer time scales. Summer SST maxima are increasing (+1.0°C for 274 years, σMC = ±0.5°C, 2σ), whereas winter SST minima contain no significant trend. Colder decades (~1.5°C) during the Little Ice Age (LIA) do not coincide with decades of sunspot minima. The coral SST reconstruction contains similar variability to temperature reconstructions from the northern Gulf of Mexico (planktic foraminifer Mg/Ca) and the Caribbean Sea (coral Sr/Ca) suggesting areal reductions in the Western Hemisphere Warm Pool during the LIA. Mean summer coral SST extremes post-1985 C.E. (29.9°C) exceeds the long-term summer average (29.2°C for 1734–2008 C.E.), yet the warming trend after 1985 C.E. (0.04°C for 24 years, σMC = ±0.5, 2σ) is not significant, whereas Caribbean coral Sr/Ca studies contain a warming trend for this interval.

Testing the annual nature of speleothem banding

Speleothem laminae have been postulated to form annually, and this lamina-chronology is widely applied to high-resolution modern and past climate reconstructions. However, this argument has not been directly supported by high resolution dating methods. Here we present contemporary single-lamina 230Th dating techniques with 2σ precision as good as ±0.5 yr on a laminated stalagmite with density couplets from Xianren Cave, China, that covers the last 300 years. We find that the layers do not always deposit annually. Annual bands can be under- or over-counted by several years during different multi-decadal intervals. The irregular formation of missing and false bands in this example indicates that the assumption of annual speleothem laminae in a climate reconstruction should be approached carefully without a robust absolute-dated chronology.

Multidecadal rainfall variability in South Pacific Convergence Zone as revealed by stalagmite geochemistry

Pacifi c decadal variability (PDV) causes widespread, persistent fl uctuations that affect climate, water resources, and fi sheries throughout the Pacifi c basin, yet the magnitude, frequency, and causes of PDV remain poorly constrained. Here we present an absolutely dated, subannually resolved, 446 yr stable oxygen isotope (δ 18 O) cave record of rainfall variability in Vanuatu (southern Pacifi c Ocean), a location that has a climate heavily infl uenced by the South Pacifi c Convergence Zone (SPCZ). The δ 18 O-based proxy rainfall record is dominated by changes in stalagmite δ 18 O that are large (~1‰), quasi-periodic (~50 yr period), and generally abrupt (within 5‐10 yr). These isotopic changes imply abrupt rainfall changes of as much as ~1.8 m per wet season, changes that can be ~2.5◊ larger than the 1976 C.E. shift in rainfall amount associated with a PDV phase switch. The Vanuatu record also shares little commonality with previously documented changes in the Intertropical Convergence Zone during the Little Ice Age or solar forcing. We conclude that multidecadal SPCZ variability is likely of an endogenous nature. Large, spontaneous, and low-frequency changes in SPCZ rainfall during the past 500 yr have important implications for the relative magnitude of natural PDV possible in the coming century.

Gradual onset and recovery of the Younger Dryas abrupt climate event in the tropics

Proxy records of temperature from the Atlantic clearly show that the Younger Dryas was an abrupt climate change event during the last deglaciation, but records of hydroclimate are underutilized in defining the event. Here we combine a new hydroclimate record from Palawan, Philippines, in the tropical Pacific, with previously published records to highlight a difference between hydroclimate and temperature responses to the Younger Dryas. Although the onset and termination are synchronous across the records, tropical hydroclimate changes are more gradual (>100 years) than the abrupt (10–100 years) temperature changes in the northern Atlantic Ocean. The abrupt recovery of Greenland temperatures likely reflects changes in regional sea ice extent. Proxy data and transient climate model simulations support the hypothesis that freshwater forced a reduction in the Atlantic meridional overturning circulation, thereby causing the Younger Dryas. However, changes in ocean overturning may not produce the same effects globally as in Greenland.

Replicated stalagmite-inferred centennial- to decadal-scale monsoon precipitation variability in southwest China since the mid Holocene

A lack of agreement between stalagmite records of the centennial- to decadal-scale Asian summer monsoon (ASM) variability since the mid Holocene limited our understanding of forcings and the possible impacts on culture evolution. Here we present replicated high-resolution stalagmite δ18O profiles from Dark Cave in Guizhou province, southwest China, established with 512 oxygen isotope data and 28 230Th ages, providing a continuous history of the ASM precipitation from 6100 to 300 years before present (yr BP, before ad 1950). Consistency of stalagmite δ18O-inferred long-term decreasing ASM trends since the mid Holocene between caves at this site and others in the Indian and East Asian monsoon realms on orbital timescales supports the effect of primary orbital solar forcings on monsoonal precipitation. The Dark Cave records display significant centennial- to decadal-scale variations, more evident than previous stalagmite records. Spectral analysis of the Dark Cave δ18O time series yields periodicities of 731–723, 281, 91–83, 67, and 55–54 yr, implying ASM intensity for the last 6 kyr could be associated with solar activity and variability of El Niño-Southern Oscillation.

High-quality single-crystal thulium iron garnet films with perpendicular magnetic anisotropy by off-axis sputtering

High-quality single-crystal thulium iron garnet (TmIG) films of 10-30 nm thick were grown by off-axis sputtering at room temperature (RT) followed by post-annealing. X-ray photoelectron spectroscopy (XPS) was employed to determine the TmIG film composition to optimize the growth conditions, along with the aid of x-ray diffraction (XRD) structural analysis and atomic force microscope (AFM) for surface morphology. The optimized films exhibited perpendicular magnetic anisotropy (PMA) and the saturation magnetization at RT was ∼99 emu/cm3, close to the RT bulk value ∼110 emu/cm3 with a very low coercive field of ∼2.4 Oe. We extracted the H⊥ of 1734 Oe and the peak-to-peak linewidth ΔH of ferromagnetic resonance are only about 99 Oe, significantly lower than that of PLD grown TmIG film and bulk single crystals. The high-quality sputtered single-crystal TmIG films show great potential to be integrated with topological insulators or heavy metals with strong spin-orbit coupling for spintronic applications.

High-quality thulium iron garnet films with tunable perpendicular magnetic anisotropy by off-axis sputtering – correlation between magnetic properties and film strain

Thulium iron garnet (TmIG) films with perpendicular magnetic anisotropy (PMA) were grown on gadolinium gallium garnet (GGG) (111) substrates by off-axis sputtering. High-resolution synchrotron radiation X-ray diffraction studies and spherical aberration-corrected scanning transmission electron microscope (Cs-corrected STEM) images showed the excellent crystallinity of the films and their sharp interface with GGG. Damping constant of TmIG thin film was determined to be 0.0133 by frequency-dependent ferromagnetic resonance (FMR) measurements. The saturation magnetization (Ms) and the coercive field (Hc) were obtained systematically as a function of the longitudinal distance (L) between the sputtering target and the substrate. A 170% enhancement of PMA field (H⊥) was achieved by tuning the film composition to increase the tensile strain. Moreover, current-induced magnetization switching on a Pt/TmIG structure was demonstrated with an ultra-low critical current density (jc) of 2.5 × 106 A/cm2, an order of magnitude smaller than the previously reported value. We were able to tune Ms, Hc and H⊥ to obtain an ultra-low jc of switching the magnetization, showing the great potential of sputtered TmIG films for spintronics.