Classification of hyperspectral images by exploiting spectral-spatial information of superpixel via multiple kernels

For the classification of hyperspectral images (HSIs), this paper presents a novel framework to effectively utilize the spectral-spatial information of superpixels via multiple kernels, termed as superpixel-based classification via multiple kernels (SC-MK). In HSI, each superpixel can be regarded as a shape-adaptive region which consists of a number of spatial-neighboring pixels with very similar spectral characteristics. Firstly, the proposed SC-MK method adopts an over-segmentation algorithm to cluster the HSI into many superpixels. Then, three kernels are separately employed for the utilization of the spectral information as well as spatial information within and among superpixels. Finally, the three kernels are combined together and incorporated into a support vector machines classifier. Experimental results on three widely used real HSIs indicate that the proposed SC-MK approach outperforms several well-known classification methods

Stalagmite evidence for early Holocene multidecadel hydroclimate variability in Ethopia

A multiproxy oxygen and carbon isotope (d13C and d18O), growth rate and trace element stalagmite paleoenvironmental record is presented for the Early Holocene from Achere Cave, Ethiopia. The annually laminated stalagmite grew from 10.6 to 10.4 ka, and from 9.7 to 9.0 ka with a short hiatus at ~9.25 ka. Using oxygen and carbon isotopic, and cave monitoring data, we demonstrate that the stalagmite deposition is out of isotopic equilibrium, yet trace element and isotope geochemistry is sensitive to hydroclimate variability. Variogram analysis of annual growth rate data suggests that this proxy can only contain hydroclimate information over less than 28-year timescales. Statistically significant and coherent spectral frequencies in d13C and d18O are observed at 15-25 and 19-23 years respectively. Combined with compelling evidence for deposition out of isotope equilibrium, the observed ~1 ‰ amplitude variability in stalagmite d18O is likely forced by non-equilibrium deposition, likely due to kinetic effects during the progressive degassing of CO2 from the water film during stalagmite formation. These frequencies are similar to the periodicity reported for Holocene stalagmite records from Ethiopian caves, suggesting that multidecadal variability in stalagmite d18O is typical. We hypothesise that a hydroclimate forcing, such as runs of one or more years of low annual rainfall, is likely to be the primary control on the extent of the partial evaporation of soil and shallow epikarst water, which is subsequently modulated by karst hydrology, and the extent of in-cave non-equilibrium stalagmite deposition. Combined with possible recharge-biases in drip water d18O, modulated by karst hydrology, these processes can generate multidecadal d18O variability which can operate with opposite signs. Comparison of Early Holocene d18O stalagmite records from the monsoon regions of Ethiopia, Oman and central China show different multi-decadal d18O signals, implying regional difference in climate forcing. Seismic activities due to the active tectonics in the region control the frequency of growth gaps (hiatuses) by changing the water flow paths to the stalagmite

Evaluating model outputs using integrated global speleothem records of climate change since the last glacial

Although quantitative isotopic data from speleothems has been used to evaluate isotope-enabled model simulations, currently no consensus exists regarding the most appropriate methodology through which to achieve this. A number of modelling groups will be running isotope-enabled palaeoclimate simulations in the framework of the Coupled Model Intercomparison Project Phase 6, so it is timely to evaluate different approaches to use the speleothem data for data-model comparisons. Here, we illustrate this using 456 globally-distributed speleothem δ18O records from an updated version of the Speleothem Isotopes Synthesis and Analysis (SISAL) database and palaeoclimate simulations generated using the ECHAM5-wiso isotope-enabled atmospheric circulation model. We show that the SISAL records reproduce the first-order spatial patterns of isotopic variability in the modern day, strongly supporting the application of this dataset for evaluating model-derived isotope variability into the past. However, the discontinuous nature of many speleothem records complicates procuring large numbers of records if data-model comparisons are made using the traditional approach of comparing anomalies between a control period and a given palaeoclimate experiment. To circumvent this issue, we illustrate techniques through which the absolute isotopic values during any time period could be used for model evaluation. Specifically, we show that speleothem isotope records allow an assessment of a model’s ability to simulate spatial isotopic trends. Our analyses provide a protocol for using speleothem isotopic data for model evaluation, including screening the observations to take into account the impact of speleothem mineralogy on 18O values, the optimum period for the modern observational baseline, and the selection of an appropriate time-window for creating means of the isotope data for palaeo time slices

The SISAL database: a global resource to document oxygen and carbon isotope records from speleothems

Stable isotope records from speleothems provide information on past climate changes, most particularly information that can be used to reconstruct past changes in precipitation and atmospheric circulation. These records are increasingly being used to provide “out-of-sample” evaluations of isotope-enabled climate models. SISAL (Speleothem Isotope Synthesis and Analysis) is an international working group of the Past Global Changes (PAGES) project. The working group aims to provide a comprehensive compilation of speleothem isotope records for climate reconstruction and model evaluation. The SISAL database contains data for individual speleothems, grouped by cave system. Stable isotopes of oxygen and carbon (δ 18O, δ 13C) measurements are referenced by distance from the top or bottom of the speleothem. Additional tables provide information on dating, including information on the dates used to construct the original age model and sufficient information to assess the quality of each data set and to erect a standardized chronology across different speleothems. The metadata table provides location information, information on the full range of measurements carried out on each speleothem and information on the cave system that is relevant to the interpretation of the records, as well as citations for both publications and archived data. The compiled data are available at https://doi.org/10.17864/1947.147

The growth mechanism of the aragonitic stalagmite laminae from Yunnan Xianren Cave, SW China revealed by cave monitoring

Stalagmites from Xianren Cave in Yunnan Province, SW China have visible bands of alternating compact and porous sub-layers, which have recently been demonstrated to be annual: the compact sub-layer consists of elongated columnar aragonites with a general longitudinal orientation (parallel to the vertical growth axis), while the porous sub-layer is composed mainly of needle aragonites forming radiating masses. In order to understand the growth mechanism of this type of stalagmite laminae, factors that could determine the growth rate and crystal structures of modern carbonates, such as cave-air temperature, CO2 concentration, drip rate and the hydrochemical parameters of drip water, have been monitored systematically. In this study, we selected five active drip sites to monitor for two years at a monthly frequency. Based on the monitoring results and the analyses of stalagmite laminae, we conclude that the compact sub-layer forms in winter, when the drip rate is moderate and the degassing is slow, continuous and prolonged. The porous sub-layer forms in early spring, when the drip rate is lower and Mg/Ca is higher than that during the formation of the compact sub-layer. We also found exceptionally some calcite laminae. Furthermore, we discovered modern calcites precipitating at all study sites in the first hydrologic year when the rainfall was very abundant. Therefore, it is suggested that the occurrence of localized calcites in an aragonitic stalagmite indicates an interval of higher rainfall

Stalagmite based high resolution precipitation variability for past four centuries in the Indian Central Himalaya: Chulerasim cave re-visited and data re-interpretation

Although the variations in δO and δC and the U/Th dating in the speleothems are considered as key proxies, improved dating with better quality resolution as well as composition of stalagmites and growth rate along with the cave monitoring are equally important for understanding the high resolution precipitation variability in the past. With a total of six dates on a 11.5 cm long stalagmite, we re-interpret the decadal to century scale climatic changes with multi-year droughts from the Indian Central Himalaya between ca. 1622 and 1950 AD. The sample is composed of aragonite (both compact sub-layers and porous sub-layers). Although, the age model of this young speleothem may be within age uncertainty owing to the high Th/ Th isotope ratios, yet the distinction of this study lies in recording various historical drought events which are otherwise never reported from the Himalayan foothills. Additionally, the sample consists of reasonable amount of U (>2 ppm), thus the age correction requirement may be minimum. The higher growth rate and comparatively lower values of δO and δC are observed during the Little Ice Age (LIA) until ca. 1820 AD, indicating its being wet in the Himalayan foothills in contrast to the Peninsular India and other regions which are solely influenced by the Indian Summer Monsoon (ISM). This is mainly because the monsoon trough moves from the plains to the Himalayan foothills during break-monsoon conditions and provides more orographic precipitation in form of the Westerlies in the south facing Himalayan slopes. The post-LIA period from ca. 1820 AD onwards is interpreted as comparatively drier than the LIA

Early anthropogenic impacts on the Indian summer monsoon induced by land-use and land-cover changes

The impacts of anthropogenic forcing on Indian summer monsoon (ISM) rainfall are obscure, partly due to limited availability of highly resolved hydroclimate proxy records as well as the highly regionalized nature of precipitation. Here, we report an annually-resolved speleothem oxygen isotope record from Xianren Cave, southwestern China, which represents rainfall change over the broad ISM region. We find that the region has endured at least six decadal-scale weak monsoon events in the past three hundred years. One of them, lasting from the early to mid 19th century, shares the similar gradual, persistent trend as the most recent decline in ISM rainfall and both have a magnitude substantially larger than the others dominated by natural variability. This early weak monsoon event occurred during a historical time of intensive deforestation in the region. We conclude that the ISM trend could have been altered by the changes in land-use and land-cover since the early 19th century.Ministry of Education (MOE)National Research Foundation (NRF)Published versionWe are in debt to Dr. Ming Tan who initiated the research in Xianren Cave with a grant from the National Natural Science Foundation of China (NSFC-41272204). This work has been possible thanks to the support by the Earth Observatory of Singapore via its funding from the National Research Foundation Singapore (NRF), the Singapore Ministry of Education (MOE) under the Research Centres of Excellence initiative. X. W. acknowledges the financial support from an MOE Tier 2 grant (MOE2019-T2-1-174) and an NRF grant (NRF2017NRF-NSFC001-047). U-Th dating at the High-Precision Mass Spectrometry and Environment Change Laboratory was supported by grants from the Science Vanguard Research Program of the Ministry of Science and Technology, Taiwan, ROC (109-2123-M-002-001 to C.-C.S.), the Higher Education Sprout Project of the Ministry of Education, Taiwan, ROC (109L901001 to C.-C.S.), and the National Taiwan University (110L8907 to C.-C.S.). K. L. acknowledges the financial support (SKLLQG1815) from State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, China

Stalagmite evidence for Early Holocene multidecadal hydroclimate variability in Ethiopia

A multiproxy oxygen and carbon isotope (δ13 C and δ18 O), growth rate, and trace element stalagmite paleoenvironmental record is presented for the Early Holocene from Ethiopia. The annually laminated stalagmite grew from 10.6 to 10.4 ka and from 9.7 to 9.0 ka with a short hiatus at ∼9.25 ka. Statistically significant and coherent spectral frequencies in δ13 C and δ18 O are observed at 15–25 and 19–23 years, respectively. The observed ∼1‰ amplitude variability in stalagmite δ18 O is likely forced by nonequilibrium deposition, due to kinetic effects during the progressive degassing of CO 2 from the water film during stalagmite formation. These frequencies are similar to the periodicity reported for other Holocene stalagmite records from Ethiopia, suggesting that multidecadal variability in stalagmite δ18 O is typical. Several processes can lead to this multidecadal variability and operate in different directions. A hydroclimate forcing is likely the primary control on the extent of the partial evaporation of soil and shallow epikarst water and associated isotopic fractionation. The resulting oxygen isotope composition of percolation water is subsequently modulated by karst hydrology. Further isotopic fractionation is possible in-cave during nonequilibrium stalagmite deposition. Combined with possible recharge biases in drip-water δ18 O, these processes can generate multide- cadal δ18 O variability

Variability of PDO identified by a last 300-year stalagmite delta O-18 record in Southwest China

Stalagmite oxygen isotope (delta O-18) record has been widely used in characterizing Asian Monsoon variations. In southwest (SW) China, although the stalagmite delta O-18 was usually suggested as a precipitation proxy, it remains a subject of debate. Here we present two replicated, high-precise-dated stalagmite delta O-18 records for the interval 1710-2003 AD from Yunnan province, SW China. The stalagmite delta O-18 shows no correlation with the local monsoon precipitation, but a significantly positive correlation with the Pacific Decadal Oscillation (PDO) index. It demonstrates that the stalagmite delta O-18 in SW China is most likely influenced by the atmospheric circulation associated with the PDO through the upstream depletion mechanism. During the warm (cold) PDO period, the deficit rainfall over India would make the delta O-18 of water vapor transporting to SW China less (more) negative due to the weaker Rayleigh distillation process, which would lead to the less (more) negative stalagmite delta O-18. Consequently, the stalagmite delta O-18 in SW China is suitable to reconstruct the PDO variations. It is important to note that the other factors, such as El Nino-Southern Oscillation (ENSO) or sea surface temperature (SST) also possibly affect the stalagmite delta O-18 in SW China, and the relationship between them requires further studies. (C) 2021 Elsevier Ltd. All rights reserved