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1. Evaluating the timing and structure of the 4.2 ka event in the Indian summer monsoon domain from an annually resolved speleothem record from Northeast India

   https://doi.org/10.5194/cp-14-1869-2018  

   Kathayat, Gayatri ; Cheng, Hai ; Sinha, Ashish ; Berkelhammer, Max ; Zhang, Haiwei ; Duan, Pengzhen ; Li, Hanying ; Li, Xianglei ; Ning, Youfeng ; Edwards, R. Lawrence ( January 2018 , Climate of the Past)

   Abstract. A large array of proxy recordssuggests that the “4.2ka event” marks an approximately300-year long period (∼3.9 to 4.2ka) ofmajor climate change across the globe. However, the climatic manifestation ofthis event, including its onset, duration, and termination, remains lessclear in the Indian summer monsoon (ISM) domain. Here, we present new oxygenisotope (δ¹⁸O) data from a pair of speleothems (ML.1 and ML.2)from Mawmluh Cave, Meghalaya, India, that provide a high-resolution record ofISM variability during a period (∼3.78 and 4.44ka) that fullyencompasses the 4.2ka event. The sub-annually to annually resolved ML.1δ¹⁸O record is constrained by 18 ²³⁰Th dates with anaverage dating error of ±13 years (2σ) and a resolution of ∼40 years, which allows us to characterize the ISM variability withunprecedented detail. The inferred pattern of ISM variability during theperiod contemporaneous with the 4.2ka event shares broad similarities andkey differences with the previous reconstructions of ISM from the MawmluhCave and other proxy records from the region. Our data suggest that the ISMintensity, in the context of the length of our record, abruptly decreased at∼4.0ka ($\sim \pm \mathrm{13}$ years), marking the onset of a multi-centennialperiod of relatively reduced ISM, which was punctuated by at least twomulti-decadal droughts between ∼3.9 and 4.0ka. The latter stands outin contrast with some previous proxy reconstructions of the ISM, in which the4.2ka event has been depicted as a singular multi-centennial drought.

    

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2. A stalagmite test of North Atlantic SST and Iberian hydroclimate linkages over the last two glacial cycles

   https://doi.org/10.5194/cp-14-1893-2018  

   Denniston, Rhawn F. ; Houts, Amanda N. ; Asmerom, Yemane ; Wanamaker Jr., Alan D. ; Haws, Jonathan A. ; Polyak, Victor J. ; Thatcher, Diana L. ; Altan-Ochir, Setsen ; Borowske, Alyssa C. ; Breitenbach, Sebastian F. ; et al ( January 2018 , Climate of the Past)

   Abstract. Close coupling of Iberian hydroclimate and North Atlantic seasurface temperature (SST) during recent glacial periods has been identifiedthrough the analysis of marine sediment and pollen grains co-deposited on thePortuguese continental margin. While offering precisely correlatable records,these time series have lacked a directly dated, site-specific record ofcontinental Iberian climate spanning multiple glacial cycles as a point ofcomparison. Here we present a high-resolution, multi-proxy (growth dynamicsandδ¹³C, δ¹⁸O, and δ²³⁴Uvalues) composite stalagmite record of hydroclimate from two caves in westernPortugal across the majority of the last two glacial cycles (∼220ka).At orbital and millennial scales, stalagmite-based proxies for hydroclimateproxies covaried with SST, with elevated δ¹³C,δ¹⁸O, and δ²³⁴U values and/or growth hiatusesindicating reduced effective moisture coincident with periods of lowered SSTduring major ice-rafted debris events, in agreement with changes inpalynological reconstructions of continental climate. While in many cases thePortuguese stalagmite record can be scaled to SST, in some intervals themagnitudes of stalagmite isotopic shifts, and possibly hydroclimate, appearto have been somewhat decoupled from SST.

    

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3. Supercooled liquid fogs over the central Greenland Ice Sheet

   https://doi.org/10.5194/acp-19-7467-2019  

   Cox, Christopher J. ; Noone, David C. ; Berkelhammer, Max ; Shupe, Matthew D. ; Neff, William D. ; Miller, Nathaniel B. ; Walden, Von P. ; Steffen, Konrad ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. Radiation fogs at Summit Station, Greenland (72.58^∘ N,38.48^∘ W; 3210 m a.s.l.), are frequently reported by observers. Thefogs are often accompanied by fogbows, indicating the particles are composedof liquid; and because of the low temperatures at Summit, this liquid issupercooled. Here we analyze the formation of these fogs as well as theirphysical and radiative properties. In situ observations of particle size anddroplet number concentration were made using scattering spectrometers near 2 and 10 m height from 2012 to 2014. These data are complemented bycolocated observations of meteorology, turbulent and radiative fluxes, andremote sensing. We find that liquid fogs occur in all seasons with thehighest frequency in September and a minimum in April. Due to thecharacteristics of the boundary-layer meteorology, the fogs are elevated,forming between 2 and 10 m, and the particles then fall toward the surface.The diameter of mature particles is typically 20–25 µm in summer.Number concentrations are higher at warmer temperatures and, thus, higher insummer compared to winter. The fogs form at temperatures as warm as −5 ^∘C, while the coldest form at temperatures approaching −40 ^∘C. Facilitated by the elevated condensation, in winter two-thirds offogs occurred within a relatively warm layer above the surface when thenear-surface air was below −40 ^∘C, as cold as −57 ^∘C,which is too cold to support liquid water. This implies that fog particlessettling through this layer of cold air freeze in the air column beforecontacting the surface, thereby accumulating at the surface as ice withoutriming. Liquid fogs observed under otherwise clear skies annually imparted1.5 W m⁻² of cloud radiative forcing (CRF). While this is a smallcontribution to the surface radiation climatology, individual events areinfluential. The mean CRF during liquid fog events was 26 W m⁻², andwas sometimes much higher. An extreme case study was observed toradiatively force 5 ^∘C of surface warming during the coldest partof the day, effectively damping the diurnal cycle. At lower elevations ofthe ice sheet where melting is more common, such damping could signal a rolefor fogs in preconditioning the surface for melting later in the day.

    

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4. Spatial and temporal variability of snowfall over Greenland from CloudSat observations

   https://doi.org/10.5194/acp-19-8101-2019  

   Bennartz, Ralf ; Fell, Frank ; Pettersen, Claire ; Shupe, Matthew D. ; Schuettemeyer, Dirk ( January 2019 , Atmospheric Chemistry and Physics)

   Abstract. We use the CloudSat 2006–2016 data record to estimate snowfall over theGreenland Ice Sheet (GrIS). We first evaluate CloudSat snowfall retrievalswith respect to remaining ground-clutter issues. Comparing CloudSatobservations to the GrIS topography (obtained from airborne altimetrymeasurements during IceBridge) we find that at the edges of the GrISspurious high-snowfall retrievals caused by ground clutter occasionallyaffect the operational snowfall product. After correcting for this effect,the height of the lowest valid CloudSat observation is about 1200 mabove the local topography as defined by IceBridge. We then use ground-based millimeter wavelength cloud radar (MMCR) observations obtained from the Integrated Characterization of Energy, Clouds, Atmospheric state, and Precipitation at Summit, Greenland (ICECAPS) experiment to devise a simple,empirical correction to account for precipitation processes occurringbetween the height of the observed CloudSat reflectivities and the snowfallnear the surface. Using the height-corrected, clutter-cleared CloudSatreflectivities we next evaluate various Z–S relationships in terms ofsnowfall accumulation at Summit through comparison with weekly stake fieldobservations of snow accumulation available since 2007. Using a set of threeZ–S relationships that best agree with the observed accumulation at Summit,we then calculate the annual cycle snowfall over the entire GrIS as well asover different drainage areas and compare the derived mean values and annualcycles of snowfall to ERA-Interim reanalysis. We find the annual meansnowfall over the GrIS inferred from CloudSat to be 34±7.5 cm yr⁻¹liquid equivalent (where the uncertainty is determined by the range invalues between the three different Z–S relationships used). In comparison,the ERA-Interim reanalysis product only yields 30 cm yr⁻¹ liquid equivalentsnowfall, where the majority of the underestimation in the reanalysisappears to occur in the summer months over the higher GrIS and appears to berelated to shallow precipitation events. Comparing all available estimatesof snowfall accumulation at Summit Station, we find the annually averagedliquid equivalent snowfall from the stake field to be between 20 and 24 cm yr⁻¹, depending on the assumed snowpack density and from CloudSat 23±4.5 cm yr⁻¹. The annual cycle at Summit is generally similar betweenall data sources, with the exception of ERA-Interim reanalysis, which showsthe aforementioned underestimation during summer months.

    

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5. The Sediment Green‐Blue Color Ratio as a Proxy for Biogenic Silica Productivity Along the Chilean Margin

   https://doi.org/10.1029/2022GC010350  

   Li, Chen ; Clementi, Vincent J. ; Bova, Samantha C. ; Rosenthal, Yair ; Childress, Laurel B. ; Wright, James D. ; Jian, Zhimin ; Expedition 379T Scientists ( September 2022 , Geochemistry, Geophysics, Geosystems)

   Sediment cores recently collected from the Chilean Margin during D/VJOIDES ResolutionExpedition 379T (JR100) document variability in shipboard‐generated records of the green/blue (G/B) ratio. These changes show a strong coherence with benthic foraminiferal δ¹⁸O, Antarctic ice core records, and sediment lithology (e.g., higher diatom abundances in greener sediment intervals), suggesting a climate‐related control on the G/B. Here, we test the utility of G/B as a proxy for diatom productivity at Sites J1002 and J1007 by calibrating G/B to measured biogenic opal. Strong exponential correlations between measured opal% and the G/B were found at both sites. We use the empirical regressions to generate high‐resolution records of opal contents (opal%) on the Chilean Margin. Higher productivity tends to result in more reducing sedimentary conditions. Redox‐sensitive sedimentary U/Th generally co‐varies with the reconstructed opal% at both sites, supporting the association between sediment color, sedimentary U/Th, and productivity. Lastly, we calculated opal mass accumulation rate (MAR) at Site J1007 over the last ∼150,000 years. The G/B‐derived opal MAR record from Site J1007 largely tracks existing records derived from traditional wet‐alkaline digestion from the south and eastern equatorial Pacific (EEP) Ocean, with a common opal flux peak at ∼50 ka suggesting that increased diatom productivity in the EEP was likely driven by enhanced nutrient supply from the Southern Ocean rather than dust inputs as previously suggested. Collectively, our results identify the G/B ratio as a useful tool with the potential to generate reliable, high‐resolution paleoceanographic records that circumvent the traditionally laborious methodology.

    

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