Dependence of Ice-Core Relative Trace-Element Concentration on Acidification

To assess the role of methodological differences on measured trace-element concentrations in ice cores, we developed an experiment to test the effects of acidification strength and time on dust dissolution using snow samples collected in West Antarctica and Alaska. We leached Antarctic samples for 3 months at room temperature using nitric acid at concentrations of 0.1, 1.0 and 10.0% (v/v). At selected intervals (20 min, 24 hours, 5 days, 14 days, 28 days, 56 days, 91 days) we analyzed 23 trace elements using inductively coupled plasma mass spectrometry. Concentrations of lithogenic elements scaled with acid strength and increased by 100–1380% in 3 months. Incongruent elemental dissolution caused significant variability in calculated crustal enrichment factors through time (factor of 1.3 (Pb) to 8.0 (Cs)). Using snow samples collected in Alaska and acidified at 1% (v/v) for 383 days, we found that the increase in lithogenic element concentration with time depends strongly on initial concentration, and varies by element (e.g. Fe linear regression slope =1.66; r = 0.98). Our results demonstrate that relative trace-element concentrations measured in ice cores depend on the acidification method used

Centennial-scale variability of the Southern Hemisphere westerly wind belt in the eastern Pacific over the past two millennia

We present the first high-resolution (sub-annual) dust particle data set from West Antarctica, developed from the West Antarctic Ice Sheet (WAIS) Divide deep ice core (79.468° S, 112.086° W), and use it to reconstruct changes in atmospheric circulation over the past 2400 years. We find a background dust flux of ~4 mg m−2 year−1 and a mode particle size of 5–8 μm diameter. Through comparing the WAIS Divide record with other Antarctic ice core particle records, we observe that coastal and lower-elevation sites have higher dust fluxes and coarser particle size distributions (PSDs) than sites on the East Antarctic plateau, suggesting input from local dust sources at these lower-elevation sites. In order to explore the use of the WAIS Divide dust PSD as a proxy for past atmospheric circulation, we make quantitative comparisons between both mid-latitude zonal wind speed and West Antarctic meridional wind speed and the dust size record, finding significant positive interannual relationships. We find that the dust PSD is related to mid-latitude zonal wind speed via cyclonic activity in the Amundsen Sea region. Using our PSD record, and through comparison with spatially distributed climate reconstructions from the Southern Hemisphere (SH) middle and high latitudes, we infer that the SH westerlies occupied a more southerly position from circa 1050 to 1400 CE (Common Era), coinciding with the Medieval Climate Anomaly (MCA). Subsequently, at ca. 1430 CE, the wind belt shifted equatorward, where it remained until the mid-to-late twentieth century. We find covariability between reconstructions of El Niño–Southern Oscillation (ENSO) and the mid-latitude westerly winds in the eastern Pacific, suggesting that centennial-scale circulation changes in this region are strongly influenced by the tropical Pacific. Further, we observe increased coarse particle deposition over the past 50 years, consistent with observations that the SH westerlies have been shifting southward and intensifying in recent decades

The WAIS Divide deep ice core WD2014 chronology - Part 2: Annual-layer counting (0-31 ka BP)

International audienceWe present the WD2014 chronology for the upper part (0–2850 m; 31.2 ka BP) of the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core. The chronology is based on counting of annual layers observed in the chemical, dust and electrical conductivity records. These layers are caused by seasonal changes in the source, transport, and deposi-tion of aerosols. The measurements were interpreted manually and with the aid of two automated methods. We validated the chronology by comparing to two high-accuracy, absolutely dated chronologies. For the Holocene, the cos-mogenic isotope records of 10 Be from WAIS Divide and 14 C for IntCal13 demonstrated that WD2014 was consistently accurate to better than 0.5 % of the age. For the glacial period, comparisons to the Hulu Cave chronology demonstrated that WD2014 had an accuracy of better than 1 % of the age at three abrupt climate change events between 27 and 31 ka. WD2014 has consistently younger ages than Green-land ice core chronologies during most of the Holocene. For Published by Copernicus Publications on behalf of the European Geosciences Union. 770 M. Sigl et al.: The WAIS Divide deep ice core WD2014 chronology the Younger Dryas–Preboreal transition (11.595 ka; 24 years younger) and the Bølling–Allerød Warming (14.621 ka; 7 years younger), WD2014 ages are within the combined uncertainties of the timescales. Given its high accuracy, WD2014 can become a reference chronology for the Southern Hemisphere, with synchronization to other chronologies feasible using high-quality proxies of volcanism, solar activity , atmospheric mineral dust, and atmospheric methane concentrations

The WAIS Divide deep ice core WD2014 chronology - Part 2: Annual-layer counting (0-31 ka BP)

We present the WD2014 chronology for the upper part (0–2850 m; 31.2 ka BP) of the West Antarctic Ice Sheet (WAIS) Divide (WD) ice core. The chronology is based on counting of annual layers observed in the chemical, dust and electrical conductivity records. These layers are caused by seasonal changes in the source, transport, and deposition of aerosols. The measurements were interpreted manually and with the aid of two automated methods. We validated the chronology by comparing to two high-accuracy, absolutely dated chronologies. For the Holocene, the cosmogenic isotope records of ¹⁰Be from WAIS Divide and ¹⁴C for IntCal13 demonstrated that WD2014 was consistently accurate to better than 0.5% of the age. For the glacial period, comparisons to the Hulu Cave chronology demonstrated that WD2014 had an accuracy of better than 1% of the age at three abrupt climate change events between 27 and 31 ka. WD2014 has consistently younger ages than Greenland ice core chronologies during most of the Holocene. For the Younger Dryas–Preboreal transition (11.595 ka; 24 years younger) and the Bølling–Allerød Warming (14.621 ka; 7 years younger), WD2014 ages are within the combined uncertainties of the timescales. Given its high accuracy, WD2014 can become a reference chronology for the Southern Hemisphere, with synchronization to other chronologies feasible using high-quality proxies of volcanism, solar activity, atmospheric mineral dust, and atmospheric methane concentrations

Precise interpolar phasing of abrupt climate change during the last ice age

The last glacial period exhibited abrupt Dansgaard–Oeschger climatic oscillations, evidence of which is preserved in a variety of Northern Hemisphere palaeoclimate archives¹. Ice cores show that Antarctica cooled during the warm phases of the Greenland Dansgaard–Oeschger cycle and vice versa[superscript 2,3], suggesting an interhemispheric redistribution of heat through a mechanism called the bipolar seesaw[superscript 4–6]. Variations in the Atlantic meridional overturning circulation (AMOC) strength are thought to have been important, but much uncertainty remains regarding the dynamics and trigger of these abrupt events[superscript 7–9]. Key information is contained in the relative phasing of hemispheric climate variations, yet the large, poorly constrained difference between gas age and ice age and the relatively low resolution of methane records from Antarctic ice cores have so far precluded methane-based synchronization at the required sub-centennial precision[superscript 2,3,10]. Here we use a recently drilled high-accumulation Antarctic ice core to show that, on average, abrupt Greenland warming leads the corresponding Antarctic cooling onset by 218 ± 92 years (2σ) for Dansgaard–Oeschger events, including the Bølling event; Greenland cooling leads the corresponding onset of Antarctic warming by 208 ± 96 years. Our results demonstrate a north-to-south directionality of the abrupt climatic signal, which is propagated to the Southern Hemisphere high latitudes by oceanic rather than atmospheric processes. The similar interpolar phasing of warming and cooling transitions suggests that the transfer time of the climatic signal is independent of the AMOC background state. Our findings confirm a central role for ocean circulation in the bipolar seesaw and provide clear criteria for assessing hypotheses and model simulations of Dansgaard–Oeschger dynamics

Atmospheric Dust Deposition in West Antarctica Over the Past Two Millennia

I develop and interpret a late Holocene record of dust deposition from the West Antarctic Ice Sheet (WAIS) Divide deep ice core in order to reconstruct past changes in atmospheric circulation. The WAIS Divide core was collected from a high-accumulation (0.2 m weq yr-1) interior site (79.468° S, 112.086° W) with annually resolvable layering through the Holocene. My approach combines continuous and discrete physical and geochemical analyses of surface snow and ice samples from the upper 577 m (2400 years) of the core. Results from an experiment testing common glaciochemical methods show that acidification strength and time significantly increase trace elemental concentrations leached from impurities in snow/ice. Continuous measurements reveal elevated microparticle concentrations associated with acidity peaks for the Unknown (1258 C.E.), Kuwae (1458 C.E.) and Tambora (1815) volcanic eruptions. Ash particles from explosive tropical eruptions have particle size distributions (PSDs) 0.6-1.5 μm finer than the background atmospheric dust deposited at this site, and are deposited ~3-6 months earlier than sulfate aerosols. In contrast, particles from the Buckle Island, Antarctica (1839 C.E.) eruption produced a PSD \u3e 5 pm coarser-than-background. These observations may be used to infer the relative latitude and/or magnitude of unknown eruptions as measured in polar ice cores, and therefore their potential impact on global climate. Over the past two millennia, the background dust flux remained around ~4 mg m-2 y-1 with a modal particle diameter of 5-8 pm. West Antarctic dust flux and modal diameter are both higher than in central East Antarctica, but comparable to lower- elevation and coastal sites, suggesting a strong local dust emissions influence. The coarse particle percentage, a measure of the dust PSD, shows significant positive correlations with mid-latitude zonal wind speeds (r=0.4-0.5; p\u3c0.1). Through comparison with spatially distributed climate reconstructions from the Southern Hemisphere (SH) middle and high latitudes, I infer that the SH westerly wind belt occupied a more southerly position during the Medieval Climate Anomaly (~950-1350 C.E.), and shifted to a more northerly position at the onset of the Little Ice Age (~1430 C.E.) due to cooler surface temperatures and a contraction of the SH Hadley cell

Molecular phylogenetic analysis of a bacterial mat community, Le Grotte di Frasassi, Italy

The Frasassi Caves are a currently forming limestone karst system in which biogenic sulfuric acid may play a significant role. High concentrations of sulfide have been found in the Frasassi aquifer, and gypsum deposits point to the presence of sulfur in the cave. White filamentous microbial mats have been observed growing in shallow streams in Grotta Sulfurea, a cave at the level of the water table. A mat was sampled and used in a bacterial phylogenetic study, from which eleven 16S ribosomal RNA (rRNA) gene clones were sequenced. The majority of 16S clones were affiliated with the δ- proteobacteria subdivision of the Proteobacteria phylum, and many grouped with 16S sequences from organisms living in similar environments. This study aims to extend our knowledge of bacterial diversity within relatively simple geochemical environments, and improve our understanding of the biological role in limestone corrosion

Dominant Microbial Populations in Limestone-Corroding Stream Biofilms, Frasassi Cave System, Italy

Waters from an extensive sulfide-rich aquifer emerge in the Frasassi cave system, where they mix with oxygen-rich percolating water and cave air over a large surface area. The actively forming cave complex hosts a microbial community, including conspicuous white biofilms coating surfaces in cave streams, that is isolated from surface sources of C and N. Two distinct biofilm morphologies were observed in the streams over a 4-year period. Bacterial 16S rDNA libraries were constructed from samples of each biofilm type collected from Grotta Sulfurea in 2002. β-, γ-, δ-, and ɛ-proteobacteria in sulfur-cycling clades accounted for ≥75% of clones in both biofilms. Sulfate-reducing and sulfur-disproportionating δ-proteobacterial sequences in the clone libraries were abundant and diverse (34% of phylotypes). Biofilm samples of both types were later collected at the same location and at an additional sample site in Ramo Sulfureo and examined, using fluorescence in situ hybridization (FISH). The biomass of all six stream biofilms was dominated by filamentous γ-proteobacteria with Beggiatoa-like and/or Thiothrix-like cells containing abundant sulfur inclusions. The biomass of ɛ-proteobacteria detected using FISH was consistently small, ranging from 0 to less than 15% of the total biomass. Our results suggest that S cycling within the stream biofilms is an important feature of the cave biogeochemistry. Such cycling represents positive biological feedback to sulfuric acid speleogenesis and related processes that create subsurface porosity in carbonate rocks