Hubertus Fischer’s research while affiliated with University of Bern and other places


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Publications (220)


The location of the SIR drilling site
SIR is shown as a red dot and other existing and proposed ice core sites around WAIS as blue dots. The purple colour shows exposed rock outcrops. a, Overview of West Antarctica. b, Detail of the SIR region. Contours are from the Cryosat2 elevation model⁴². Maps in a and b were generated using QGIS with the Quantarctica mapping environment⁴³, under a Creative Commons licence CC BY 4.0.
Sea salt sodium (ssNa) concentration at SIR comparing the Holocene and LIG
a, Holocene (10–0 ka, 100 year averages). b, LIG (126–117 ka, 100 year averages). The red line in a and b is the average value for 7–0 ka. c, Data from a published spatial survey (blue circles) of sea salt concentrations on the Ronne Ice Shelf³⁷. The solid black line is the best fit³⁸ and the green circle is the present concentration at SIR (top 25 m of core) which fits closely onto the line. The vertical position of the black arrow is the average ssNa concentrations from 10–8 ka, when the Ronne Ice Shelf was extended. The red arrows labelled LIG indicate average ssNa values from 125–121 ka (lower arrow) and 120–118 ka (upper arrow).
δ¹⁸O for the SIR ice and East Antarctic cores in the LIG and Holocene
a, Reference data for the LIG for EPICA Dome C (EDC)⁶ (red, δD) and EPICA Dronning Maud Land (EDML)⁴⁴ (blue, δ¹⁸O), scaled 8:1, with the horizontal line being an indicative level for the late Holocene. b, As a but for the last 8 kyr. c, SIR data for the LIG. d, SIR data for the last 8 kyr. The dashed line in c and d is the average of 2–0 ka. All three records are synchronized to the AICC2012 age model. The y-scaling is the same for all three cores and both time periods.
A model reconstruction of the Antarctic Ice Sheet at the LIG
The model is consistent with both our data showing the presence of ice shelves (absence of exposed ocean or sea ice) and data indicating the existence of seaways. a–c, Antarctica (a); close-up of region around SIR (b); close-up of region in Ross Ice Shelf (c). In a–c, blue is grounded ice, pink is ice shelf. b and c show water column thickness contours, with units of metres, for the ocean beneath the ice shelves. d, Time series of modelled magnitude (blue) and rate (red) of sea-level-equivalent mass loss for the shown scenario. Model output maps were plotted using Generic Mapping Tools v.6.
The Ronne Ice Shelf survived the last interglacial
  • Article
  • Full-text available

January 2025

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78 Reads

Nature

Eric W. Wolff

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The fate of the West Antarctic Ice Sheet (WAIS)¹ is the largest cause of uncertainty in long-term sea-level projections. In the last interglacial (LIG) around 125,000 years ago, data suggest that sea level was several metres higher than today2, 3–4, and required a significant contribution from Antarctic ice loss, with WAIS usually implicated. Antarctica and the Southern Ocean were warmer than today5, 6, 7–8, by amounts comparable to those expected by 2100 under moderate to high future warming scenarios. However, direct evidence about the size of WAIS in the LIG is sparse. Here we use sea salt data from an ice core from Skytrain Ice Rise, adjacent to WAIS, to show that, during most of the LIG, the Ronne Ice Shelf was still in place, and close to its current extent. Water isotope data are consistent with a retreat of WAIS⁹, but seem inconsistent with more dramatic model realizations¹⁰ in which both WAIS and the large Antarctic ice shelves were lost. This new constraint calls for a reappraisal of other elements of the LIG sea-level budget. It also weakens the observational basis that motivated model simulations projecting the highest end of projections for future rates of sea-level rise to 2300 and beyond.

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Location of ice cores and marine sediment cores used in the manuscript
The white circles indicate marine sediment (Ocean Drilling Program (ODP), Polarstern (PS)) and ice core positions and the black arrows illustrate the typical mean 5 day air mass back-trajectory to EDML for all trajectories (dashed line) and trajectories connected to snow fall at EDML (solid line, arrows adapted from ref. ⁴⁶). Dashed lines illustrate the positions of the Polar Front (PF), Subantarctic Front (SAF) and the Subtropical Front (STF). Map adapted from the original version provided by Bethan Davies (www.AntarcticGlaciers.org).
Measured EDML aerosol and temperature tracer data
a, The total Na⁺ (Na⁺tot) and ssNa. b, The total Ca²⁺ (Ca²⁺tot) and nssCa (total Ca²⁺ data are nearly indistinguishable from the nssCa data). c, The surface temperature (Tsurf) derived from δ¹⁸O (ref. ³⁶) (note that the surface temperature is significantly colder than the condensation temperature at the top of the inversion layer depicted in Fig. 4). d, The Δ³³SO4²⁻ (with excursions indicative for mass-independent fractionation of volcanic sulfate in the stratosphere indicated by ‘v’). e, The δ³⁴SO4²⁻ (with samples indicative of volcanic horizons of stratospheric or tropospheric input indicated by ‘v’). f, The total measured SO4²⁻. The red dots in f refer to values of the discrete samples taken for SO4²⁻ isotope analysis and the black line shows the high-resolution SO4²⁻ data from continuous melt analysis. All values are provided on the extended AICC2023 age scale⁴⁷. The orange bar indicates the LIG and the blue bar indicates the PGM with termination II (TII) in between.
Contributions from different sources to the total SO4²⁻ budget at EDML
Top: the relative contributions of sea-salt (light blue), mineral dust (brown), biogenic (green) and volcanic (purple) emissions to total SO4²⁻. a, Sea-salt SO4²⁻ (SO4,sea). b, Biogenic SO4²⁻ (SO4,bio). c, Terrestrial SO4²⁻ (SO4,dust). d, Volcanic SO4²⁻ (SO4,volc). e, Total measured SO4²⁻ (SO4,tot). The filled symbols and their error bars (1 s.d.) were derived using Monte Carlo error propagation of analytical uncertainties for fixed (nssSO4²⁻/nssCa)dust = 0.7 (Supplementary Information). The open circles represent alternative decompositions for (nssSO4²⁻/nssCa)dust = 1.0 and (nssSO4²⁻/nssCa)dust = 0.18. Note that these are not discernible from the standard run for biogenic SO4²⁻. The magenta peak in volcanic SO4²⁻ indicates one sample, where a negative volcanic δ³⁴SO4²⁻ signature needs to be assumed to close the budget. The light-green dot in biogenic SO4²⁻ (with its volcanic contribution as indicated in pink) represents an outlier that would require a much lighter isotopic signature of biogenic SO4²⁻ to obtain values that are similar to the neighbouring samples. As we have no objective reason to reject this outlier, we included this sample in our dataset, but refrain from interpreting this value as being indicative of an even higher biogenic SO4²⁻ emission at that time. All values are provided on the extended AICC2023 age scale⁴⁷, the orange bar indicates the LIG and the blue bar indicates the PGM with termination II (TII) in between.
Modelled atmospheric concentration of biogenic SO4²⁻
a, Tsource (grey circles). b, The atmospheric biogenic SO4²⁻aerosol concentration at the source (SO4,bio source). c, The atmospheric biogenic SO4²⁻ aerosol concentration on the ice sheet (SO4,bio site). d, The ice core concentration of biogenic SO4²⁻ (SO4,bio ice). The filled symbols and their error bars (1 s.d.) in b and c were derived using Monte Carlo error propagation of the transport model for our standard decomposition (nssSO4²⁻/nssCa)dust = 0.7. The orange-/red-coloured dot represents an outlier and suggests that the source decomposition is biased for this data point. As we have no objective reason to reject this outlier, we included this sample in our dataset, but refrain from interpreting this value as being indicative of an even higher biogenic SO4²⁻ emission at that time. e, The condensation temperature at the top of the inversion layer over the ice sheet, Tsite (black circles). All values are provided on the extended AICC2023 age scale⁴⁷, the orange bar indicates the LIG and the blue bar indicates the PGM with termination II (TII) in between.
Bioproductivity in the Atlantic Sector-Southern Ocean north and south of the mAPF
a, The alkenone concentration (C) at ODP site ODP1090 north of the mAPF⁴⁸. b, The deep ocean oxygenation tracer authigenic uranium at ODP1094 south of the mAPF⁴³. c, Biogenic Ba flux at ODP1094 (ref. ⁸). d, Atmospheric biogenic SO4²⁻ aerosol concentration at the source. The orange-coloured dot in d represents an outlier and suggests that the source decomposition is biased for this data point. We refrain from interpreting this value as being indicative of an even higher biogenic SO4²⁻emission at that time. e–g, Biogenic Ba flux south of the mAPF at the Antarctic continental margin at PS1575 (e), PS1648 (f) and PS1821 (g), respectively (see Fig. 1 for their location)⁴⁹. Note that all records are provided on their individual age scale (AICC2013 in case of EDML), which can largely differ from each other. The orange bar indicates the LIG and the blue bar indicates the PGM with termination II (TII) in between.
Limited decrease of Southern Ocean sulfur productivity across the penultimate termination
Nature Geoscience

Productivity in the Pleistocene glacial Southern Ocean was probably enhanced owing to iron fertilization by aeolian dust. Marine sediments indicate such an increase north of the modern Antarctic Polar Front but reduced biogenic activity south of it. However, quantitative estimates for the integrated net effect are difficult to obtain. Here we use the SO4²⁻ isotopic composition and other geochemical ice core records from the Atlantic sector of the Southern Ocean to reconstruct net changes in integrated biogenic sulfur productivity in the surface ocean over the penultimate glacial termination. We show that biogenic SO4²⁻ aerosol contributes 58% and 85% to the sulfate budget in Dronning Maud Land during glacial and interglacial times, respectively, and that biogenic sulfate is derived predominately from the seasonal sea ice zone. Using our quantitative reconstruction of biogenic aerosol production in the Southern Ocean source region, we show that the average biogenic sulfate production integrated over the Atlantic sector was 16% higher in the penultimate glacial 137,000–153,000 years ago compared with the later Last Interglacial 120,000–125,000 years ago. An intermittent decrease in productivity observed during early peak interglacial warming suggests that a reduction in the seasonal sea ice zone may disrupt Southern Ocean ecosystems.


δ¹³C–CH4 for HE5 and DO12, HE4 and DO8 and HE1
a, WDC CFA CH4 data (blue)39,53. b, δ¹³C–CH4 data from ref. ²¹ (green), ref. ⁴¹ (yellow) and this study (red diamonds, black line) each plotted with 1σ measurement uncertainties. c,d, Relative changes in pyrogenic (red) and microbial (green) emissions from the one-box model used to interpret the δ¹³C–CH4 data. Shaded bands show Monte-Carlo-derived 95% confidence intervals related to analytical, firn diffusive, geologic emission and source signature uncertainties (Methods). e, δ¹⁸O–CaCO3 records from speleothems in Asian Monsoon (ASM) (brown; HE4 and HE5, ref. ⁵⁴; HE1, ref. ⁵⁵) and South American Monsoon (SAM) regions (green; HE4, ref. ¹⁸; HE1, ref. ⁴⁴). DO events and the Bölling Alleröd (BA) warming are labelled and HEs are indicated by vertical dashed lines. Note that the approximately 50 ppb CH4 pulses discussed here are assumed synchronous with HEs¹.
Source Data
δD–CH4 across HE4 and DO8
a, WDC CFA CH4 data in blue1,9, TALDICE CH4 data in magenta⁵³ using the WDC-synchronized gas age scale from refs. 56,57. b, δ¹³C–CH4 data from this study (red diamonds, black line) and 1σ measurement uncertainties. c,d, Relative changes in modelled geologic (light blue) and microbial (green) emissions assuming constant pyrogenic emissions. Shaded bands show Monte-Carlo-derived 95% confidence intervals related to analytical, firn diffusive and source signature uncertainties (Methods). e, Calculated theoretical δD–CH4 using one-box model output for assumptions of constant geologic (red) and constant pyrogenic (dark blue) emissions. Measured δD–CH4 and 1σ measurement uncertainties from WDC ice (this study, light green) and TALDICE (this study and ref. ⁵⁸, magenta) plotted on the WDC-synchronized gas age scale from refs. 21,57. Shaded bands in c–e show Monte-Carlo-derived 95% confidence intervals related to analytical, firn diffusive and geologic emission and source signature uncertainties (Methods). DO interstadials are labelled and the onset of HE4 is indicated by a vertical dashed line.
Source Data
Sensitivity of analyses to variability in microbial δ¹³C–CH4 across the HE4, DO8 interval
a, CFA CH4 data9,58 (blue). b, One-box-modelled microbial δ¹³C–CH4 assuming constant pyrogenic and geologic δ¹³C–CH4 and emissions. c, Changes in tropical microbial CH4 emissions derived from C3-dominant (purple) or C4-dominant (orange) precursor biomass that are required to explain the observed atmospheric CH4 and δ¹³C–CH4 variability, assuming constant pyrogenic and geologic δ¹³C–CH4 and emissions (b). DO interstadials are labelled and the onset of HE4 is indicated by a vertical dashed line.
Source Data
Modelled CO2 across HE5, HE4 and HE1
a, WDC CFA CH4 data (blue)9,53. b, Atmospheric CO2 data and 1σ measurement uncertainties (WDC, black)7,49,53,59, baseline carbon cycle box model output with stable terrestrial-atmosphere carbon flux (red) (model output from ref. ⁴⁷), and the error envelope of the model output (95% confidence) including CO2 fluxes related to pyrogenic emissions calculated using our CH4 pyrogenic emissions (yellow shading). The error envelope was determined using a Monte Carlo error propagation related to uncertainties in pyrogenic CO2 emissions and post-fire regrowth times (Methods). Data are plotted relative to CO2 at the start of each interval. c, Same as b but for relative changes in δ¹³C–CO2. Ice core δ¹³C–CO2 data (plotted with 1σ measurement uncertainties) were measured using ice from Taylor Glacier (TG) and were shifted 150 years younger to match the WDC gas age scale⁷. d, Histogram showing the magnitude of CO2 increase across each HE. Baseline model change (red)⁴⁹, WDC measured change (black) and Monte-Carlo-derived probability density function of the change of the baseline + pyrogenic results, which incorporates all uncertainties described in b (yellow).
Source Data
Abrupt changes in biomass burning during the last glacial period

January 2025

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204 Reads

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1 Citation

Nature

Understanding the causes of past atmospheric methane (CH4) variability is important for characterizing the relationship between CH4, global climate and terrestrial biogeochemical cycling. Ice core records of atmospheric CH4 contain rapid variations linked to abrupt climate changes of the last glacial period known as Dansgaard–Oeschger (DO) events and Heinrich events (HE)1,2. The drivers of these CH4 variations remain unknown but can be constrained with ice core measurements of the stable isotopic composition of atmospheric CH4, which is sensitive to the strength of different isotopically distinguishable emission categories (microbial, pyrogenic and geologic)3, 4–5. Here we present multi-decadal-scale measurements of δ¹³C–CH4 and δD–CH4 from the WAIS Divide and Talos Dome ice cores and identify abrupt 1‰ enrichments in δ¹³C–CH4 synchronous with HE CH4 pulses and 0.5‰ δ¹³C–CH4 enrichments synchronous with DO CH4 increases. δD–CH4 varied little across the abrupt CH4 changes. Using box models to interpret these isotopic shifts⁶ and assuming a constant δ¹³C–CH4 of microbial emissions, we propose that abrupt shifts in tropical rainfall associated with HEs and DO events enhanced ¹³C-enriched pyrogenic CH4 emissions, and by extension global wildfire extent, by 90–150%. Carbon cycle box modelling experiments⁷ suggest that the resulting released terrestrial carbon could have caused from one-third to all of the abrupt CO2 increases associated with HEs. These findings suggest that fire regimes and the terrestrial carbon cycle varied contemporaneously and substantially with past abrupt climate changes of the last glacial period.




Improvement of the temporal resolution of the ice-core atmospheric CO2 record since the composite CO2 record from ref. ⁴⁴ over the last 500 ka
a, At the top, high-resolution records are represented in blue¹, red (this study), brown¹², green⁸, dark green⁴⁵, grey⁴⁶ and light-green dots⁴⁷, plotted on their own published age scales. They are superimposed on the composite CO2 record⁴⁴ (black line). At the bottom, temporal resolutions of all records computed as the running mean of 20 data points (colour codes similar to top panel). Data are not corrected for potential offsets due to the use of different experimental set-ups. Shaded red bands indicate periods where the ice-core CO2 record is associated with a temporal resolution better than 500 years. b, At the top, zoom on our EDC CO2 record between 260 and 190 ka (red dots, black line) and CH4 record from this study (light green) and previous studies48,49 (dark green) plotted on the Antarctic Ice Core Chronology 2023 (AICC2023) timescale²². Vertical black dashed lines indicate the newly identified CDJs (Methods). In the middle, CO2 rates of change from the detrended CO2 record for the smoothing spline (1 kyr cut-off period)¹. At the bottom, residual CO2 amplitude of the detrended CO2 record during the events occurring at a rate higher than 1.5 ppm per century. Vertical back dashed lines indicate the timing of the identified CDJs. A centennial-scale CO2 release is identified when the rate is higher than 1.5 ppm per century and the amplitude is higher than 5 ppm (dashed horizontal red lines)¹. MIS: Marine Isotope Stage.
Climatic and obliquity states for centennial-scale releases of atmospheric CO2 during the three periods associated with high-resolution ice-core CO2 records
a, EDC deuterium record (δD)⁵⁰ on the AICC2023 ice age scale²² (blue line). b, Atmospheric CO2 high-resolution records on the AICC2023 gas age scale²² for the time interval 500–67 ka, on the WD201451,52 timescale for the 67–12-ka interval and on several original timescales for the 12–0 ka interval (this study; refs. 1,8,12,44–47; red line). c, Obliquity²⁴ (black). Horizontal dashed lines indicate the average value for each considered proxy record over the last 500 ka. Yellow, orange and white dots indicate the value of the record during the occurrence of CDJ−, CDJ+ and undetermined CDJ, respectively1,8,17. The CDJ− identified during HS 5 (~49 ka) and simulated with LOVECLIM is identified with black stars on the three represented climate variables.
Simulated carbon cycle response to an AMOC shutdown
a, Evolution of atmospheric CO2 for the eight simulations as a function of time. The dashed grey line corresponds to a 1.5 ppm per century CO2 increase, which is also the threshold used to identify CDJ events in the ice-core CO2 record. Bold lines are smoothing spline filters. b, ΔCO2 between HighObl–LowObl (black line), HighObl_SHW–LowObl_SHW (red line), HighObl_NoVeg–LowObl_NoVeg (green line) and HighObl_NoVeg_SHW–LowObl_NoVeg_SHW (blue line). SHW, Southern Hemisphere Westerlies; Veg, vegetation. A positive value means that the CO2 anomaly due to the perturbation is higher under a high-obliquity phase than under a low-obliquity phase. Grey dots with dashed line correspond to the atmospheric CO2 record from the West Antarctic Ice Sheet (WAIS) ice core during HS 5, the HS that is simulated in the present study. c, Paired coloured bars correspond to the atmospheric CO2 offset values 400 years after the freshwater perturbation for simulations using the obliquity value at 49 ka (left, 24.3°) and a prescribed low-obliquity value (right, 22.1°). Words in italic refer to simulation names. d, Same simulation results as in c but with carbon stock in GtC. Negative carbon anomaly represents a carbon release from that reservoir into the atmosphere. Plain coloured bars represent the oceanic carbon stock, and coloured bars with white dots represent the terrestrial carbon stock. Colours refer to the same simulation as in c. The year 400 snapshot represents the period with the largest atmospheric CO2 difference between the high- and low-obliquity simulations and corresponds to a time period where most of the CO2 response to the freshwater forcing has occurred (Supplementary Fig. 10).
Simulated impacts of the freshwater forcing and the obliquity state on the climatic conditions and terrestrial carbon stock
a, Air temperature anomalies resulting from (top) a freshwater forcing under a high-obliquity phase vs no freshwater forcing under a high-obliquity phase (HighObl–HighOblCTR), (middle) a low-obliquity phase without a freshwater forcing vs a high-obliquity phase without a freshwater forcing (LowOblCTR–HighOblCTR) and (bottom) the impact of the freshwater forcing in a high- vs low-obliquity phase (HighObl–HighOblCTR)–(LowObl–LowOblCTR) on air temperatures. b, Same as a for precipitation anomalies. c, Same as a for the terrestrial carbon anomalies. Basemap outlines from ETOPO5 Global relief model (https://www.ncei.noaa.gov/products/etopo-global-relief-model). CTR, control; terr., terrestrial. Figure/panel created with Ferret.
Centennial-scale variations in the carbon cycle enhanced by high obliquity

October 2024

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363 Reads

Centennial-scale increases of atmospheric carbon dioxide, known as carbon dioxide jumps, are identified during deglacial, glacial and interglacial periods and linked to the Northern Hemisphere abrupt climate variations. However, the limited number of identified carbon dioxide jumps prevents investigating the role of orbital background conditions on the different components of the global carbon cycle that may lead to such rapid atmospheric carbon dioxide releases. Here we present a high-resolution carbon dioxide record measured on an Antarctic ice core between 260,000 and 190,000 years ago, which reveals seven additional carbon dioxide Jumps. Eighteen of the 22 jumps identified over the past 500,000 years occurred under a context of high obliquity. Simulations performed with an Earth system model of intermediate complexity point towards both the Southern Ocean and the continental biosphere as the two main carbon sources during carbon dioxide jumps connected to Heinrich ice rafting events. Notably, the continental biosphere appears as the obliquity-dependent carbon dioxide source for these abrupt events. We demonstrate that the orbital-scale external forcing directly impacts past abrupt atmospheric carbon dioxide changes.


Fig. 1. Paleoclimate records of polar climate, greenhouse gas variability, and East Asian monsoon intensity reflecting meridional displacement of the Intertropical Convergence Zone. (A) Averaged δ 18 O from Greenland Ice Core Project (GRIP) and Greenland Ice Sheet 2 (GISP2) (black) (32, 33) on the GICC05 timescale (34) multiplied by 1.0063 after 31 ka (35), (B) WAIS Divide (WD) CH 4 (gray) (15), (C) Chinese speleothem δ 18 O (blue) (11), (D) Antarctic six-core average d ln (dark green) (36), (E) Antarctic six-core average δ 18 O ice (light green) (36), (F) WD CO 2 (orange dots) with binomial smoothed average (dark orange line), and pooled 1σ standard uncertainty (orange shading) (this study; 18, 21). Gray bars show the timing of Heinrich Stadials (HS) 1 to 5 (SI Appendix, Table S1). Dashed lines represent timing of decadal-to centennial-scale rises and decreases in WD CO 2 and CH 4 , respectively.
Fig. 2. Centennial-scale CO 2 and CH 4 variability during Heinrich Stadials 1 to 5. Upper panel: WD CH 4 (15), WD CO 2 from this study (closed circles), and previous publications (open circles) (18, 21) and replicated from previous publications (closed black diamonds). Light orange shading represents pooled 1σ standard uncertainty of individual CO 2 measurements. Shaded vertical bars highlight centennial-scale jumps in WD CO 2 and CH 4 . Green lines indicate breakfit determinations (see text). Lower panel: WD CO 2 as in Upper panel sections A-E, with the magnitude and duration of abrupt CO 2 transitions shown in green.
Fig. 3. Atmospheric δ 13 C-CO 2 , CO 2 , and Southern Ocean marine sediment proxies during HS-4 and HS-1. Left panel: Taylor Glacier δ 13 C-CO 2 (light blue) (20), WD CO 2 (orange) (this study; 21), South Atlantic sediment proxies for deep-water oxygenation (blue) (43), and deep-water temperature (44). Right panel: δ 13 C-CO 2 (light blue) (29), WD CO 2 (orange) (this study; 21), simulated atmospheric CO 2 (pink) (26;LH1-SO-SHW simulation), and difference in Southern Ocean and atmosphere 14 C ages from shallow (yellow), intermediate (green), and deep (dark green) corals (27, 45) including 2σ U-Th dating uncertainties. Respectively, 160 and 150 y were subtracted from the Taylor Glacier chronology to align Taylor Glacier and WD CO 2 peaks at HS4 and HS1.
Fig. 4. Centennial-scale Antarctic temperature response during Heinrich Stadials. (A) Stacked average change in WD CH 4 (gray) (15), WD CO 2 (orange) (this study), Antarctic δ 18 O (yellow) (36), Antarctic d ln (green) (36), vapor source temperature (blue) (67), and site temperature (purple) (67) during HSs 1 to 5. Time "0" represents the mid-point of the associated WD CH 4 jump in HSs 1 to 5 (see text). (B) SST anomaly forcing based on Ferreira et al. (64) in response to a poleward displacement of the SH westerlies. The SST response from Ferreira et al. (64) was multiplied by two to better match Antarctic water isotopologs. Spatial pattern of the differences in (C) modeled surface temperature, (D) δ 18 O (D), and (E) d ln simulated in iCAM5 before and after imposing SST temperature forcing as shown in (B). Location, abbreviation, and HSs proxy value of each ice core are indicated on each map by the colored dots. (F) Scatter plots showing the magnitude of change in WD CO 2 concentrations and HSs proxies associated with HSs 1 to 5 versus background values for CO 2, d ln , and δ 18 O).
Southern Ocean drives multidecadal atmospheric CO2 rise during Heinrich Stadials

May 2024

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314 Reads

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2 Citations

Proceedings of the National Academy of Sciences

The last glacial period was punctuated by cold intervals in the North Atlantic region that culminated in extensive iceberg discharge events. These cold intervals, known as Heinrich Stadials, are associated with abrupt climate shifts worldwide. Here, we present CO 2 measurements from the West Antarctic Ice Sheet Divide ice core across Heinrich Stadials 2 to 5 at decadal-scale resolution. Our results reveal multi-decadal-scale jumps in atmospheric CO 2 concentrations within each Heinrich Stadial. The largest magnitude of change (14.0 ± 0.8 ppm within 55 ± 10 y) occurred during Heinrich Stadial 4. Abrupt rises in atmospheric CO 2 are concurrent with jumps in atmospheric CH 4 and abrupt changes in the water isotopologs in multiple Antarctic ice cores, the latter of which suggest rapid warming of both Antarctica and Southern Ocean vapor source regions. The synchroneity of these rapid shifts points to wind-driven upwelling of relatively warm, carbon-rich waters in the Southern Ocean, likely linked to a poleward intensification of the Southern Hemisphere westerly winds. Using an isotope-enabled atmospheric circulation model, we show that observed changes in Antarctic water isotopologs can be explained by abrupt and widespread Southern Ocean warming. Our work presents evidence for a multi-decadal- to century-scale response of the Southern Ocean to changes in atmospheric circulation, demonstrating the potential for dynamic changes in Southern Ocean biogeochemistry and circulation on human timescales. Furthermore, it suggests that anthropogenic CO 2 uptake in the Southern Ocean may weaken with poleward strengthening westerlies today and into the future.


Impact of subsurface crevassing on the depth–age relationship of high-Alpine ice cores extracted at Col du Dôme between 1994 and 2012

May 2024

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46 Reads

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1 Citation

Three seasonally resolved ice core records covering the 20th century were extracted in 1994, 2004, and 2012 at a nearly identical location from the Col du Dôme (4250 m above sea level, m a.s.l.; Mont Blanc, French Alps) drill site. Here, we complete and combine chemical records of major ions and radiometric measurements of 3H and 210Pb obtained from these three cores with a 3D ice flow model of the Col du Dôme glacier to investigate in detail the origin of discontinuities observed in the depth–age relation of the ice cores drilled in 2004 and 2012. Taking advantage of the granitic bedrock at Col du Dôme, which makes the ice core 210Pb records sensitive to the presence of upstream crevasses, and the fact that the depth–age disturbances are observed at depths for which absolute time markers are available, we draw an overall picture of a dynamic crevasse formation. This can explain the non-disturbed depth–age relation of the ice core drilled in 1994 and the perturbations observed in those drilled in 2004 and 2012. Since crevasses are common at high-Alpine glacier sites, our study points to the important need for rigorous investigations of the depth–age scale and glaciological conditions upstream of drill sites before interpreting high-alpine ice core records in terms of atmospheric changes.



of all discrete-sample CH4 data and corrections used in this study
a–c, NEEM (a), GISP2 (b) and WD (c) CH4 data plotted on their original gas age scale chronologies, coloured by measurement campaign. The data are blank, solubility, gravity and analytical campaign offset-corrected. The Greenland data (NEEM and GISP2) are also corrected for CH4xs. Colours and symbols correspond to the year samples were measured (see legend). The vertical dashed line at the HS1–BA [CH4] rise, which should be synchronous in each core, demonstrates the small disagreements between the three independently developed gas age scale chronologies that we correct for in our synchronization (Methods).
Source data
The CH4 rIPD records derived using GISP2 and NEEM
a,e, Tie points (black triangles), CH4xs-corrected (blue) and -uncorrected (red) discrete Greenland [CH4] records (a: GISP2, e: NEEM), discrete WD [CH4] record (green), and WD continuous flow analysis (CFA) [CH4] record (black)⁶¹. The WD CFA record is shown for visual aid when discrete data are not available. b,f, The [Ca²⁺] record for GISP2 (b)⁶² and NEEM (f)⁶³ plotted on each core’s WD-synchronized gas age scales (orange). c,g, IPD measurements corrected (blue) and uncorrected (red) for CH4xs derived using GISP2 (c) and NEEM (g). d,h, rIPD measurements corrected (blue) and uncorrected (red) for CH4xs and their 95% confidence intervals derived using GISP2 (d) and NEEM (h). In the NEEM-derived rIPD, abrupt transitions in [CH4] where the rIPD is not well defined are not shown in g and h. We suspect that the abrupt oscillations in the rIPD from 22 to 21 ka are an artefact of our analysis and are caused by a brief lack of available tie points and low data resolution. The interval-averaged rIPD estimates from previous studies are plotted in black¹², green⁹, purple²⁹ and magenta³¹. Heinrich stadial45,64 and DO interstadial¹⁷ climate intervals are shaded in light blue and grey, respectively.
Source data
Four-box-model results for the NEEM- and GISP2-derived rIPDs
a, NEEM (orange) and GISP2 (light blue) rIPDs (mean of Monte Carlo analysis), each plotted with 95% confidence intervals, as well as insolation curves for 60° N in June (solid black curve) and 30° S in December (dashed black curve)⁴⁹. b, The CH4xs-corrected [CH4] records for NEEM (orange), GISP2 (light blue) and WD (green). c, Speleothem-derived δ¹⁸O records for Botuverà Cave in Brazil (27° 13′ S; orange)⁴² and a composite of Chinese records (32° 30′ N; purple)⁴⁰. d, Global mean sea level (GMSL) data (squares) and model results (curve)⁵⁸, each plotted with 1σ uncertainty. e, CH4 rIPD box-model results: source strengths for the northern extratropical (N; blue), total tropical (T; green) and southern extratropical (S; yellow) boxes and their 95% confidence interval uncertainties. In the NEEM-derived rIPD, abrupt transitions in [CH4] where the rIPD and box-modelled emissions are untrustworthy are not shown (a and e). Heinrich stadial45,64 and DO interstadial¹⁷ climate intervals are shaded in light blue and grey, respectively.
Source data
Atmospheric methane variability through the Last Glacial Maximum and deglaciation mainly controlled by tropical sources

November 2023

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6 Citations

Constraining the causes of past atmospheric methane variability is important for understanding links between methane and climate. Abrupt methane changes during the last deglaciation have been intensely studied for this purpose, but the relative importance of high-latitude and tropical sources remains poorly constrained. The methane interpolar concentration difference reflects past geographic emission variability, but existing records suffered from subtle but considerable methane production during analysis. Here, we report an ice-core-derived interpolar difference record covering the Last Glacial Maximum and deglaciation, with substantially improved temporal resolution, chronology and a critical correction for methane production in samples from Greenland. Using box models to infer latitudinal source changes, we show that tropical sources dominated abrupt methane variability of the deglaciation, highlighting their sensitivity to abrupt climate change and rapidly shifting tropical rainfall patterns. Northern extratropical emissions began increasing ~16,000 years ago, probably through wetland expansion and/or permafrost degradation induced by high-latitude warming, and contributed at most 25 Tg yr⁻¹ (45% of the total emission increase) to the abrupt methane rise that coincided with rapid northern warming at the onset of the Bølling–Allerød interval. These constraints on deglacial climate–methane cycle interactions can improve the understanding of possible present and future feedbacks.


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Citations (72)


... An ice core was drilled to bedrock at SIR (651 m depth) 34 , and was subsequently dated, giving an age model tied to those of other Antarctic ice cores 35 . Although there is evidence of flow disturbance in the lowest 50 m of the ice (beyond 108 ka), ice from 117-126 ka (617-627 m) in the LIG is present and is in stratigraphic order 35 (Methods). ...

Reference:

The Ronne Ice Shelf survived the last interglacial
The ST22 chronology for the Skytrain Ice Rise ice core – Part 2: An age model to the last interglacial and disturbed deep stratigraphy

... To Figure 4 we have also added summer values of water soluble HULIS from Col du Dôme icecore data [68], normalized to its 1986 value and anchored to that date. Clearly, this is a World War II peak, but the reason is not known why the peak value is shifted somewhat to the earlier war years, perhaps an artifact of ice-core dynamics [69]. This spike in temperature during World War II as reflected by the Gottschalk curve could be at least partly influenced by HULIS aerosols produced by massive coal and biomass burning during WW2. ...

Impact of subsurface crevassing on the depth–age relationship of high-Alpine ice cores extracted at Col du Dôme between 1994 and 2012

... Two-box model equilibrium for (a) δ 13 C-CH 4 and (b) δD-CH 4 for scenarios with varying latitudinal source distribution (indicated by the x-axis: rIPD), but the same isotopic source signature. Varying latitudinal source distribution is displayed on the x-axis as the rIPD, which is the percent difference in NH and SH atmospheric CH 4 mixing ratio 11,25,70,95 . The range of observed rIPD values indicated by ice core studies are shaded in blue. ...

Atmospheric methane variability through the Last Glacial Maximum and deglaciation mainly controlled by tropical sources

... In Table 2, we list the averages of L1 and L2 for a rise of 10 %-90 % and decay of 90 %-10 %, respectively. L1 and/or L2 are often defined as the depth resolution of a CFA system Erhardt et al., 2023;Grie-man et al., 2022). This definition gives a depth resolution of 35-40 mm for the δ 18 O, Na, and rBC data over the depth interval between 6.17 and 112.87 m. ...

High-resolution aerosol data from the top 3.8 kyr of the East Greenland Ice coring Project (EGRIP) ice core

... Figure 4. Comparison of the diet of the Ifri Oussaïd bear with isotopic data from several European Holocene brown bears (DÖPPES et al., 2008;ERSMARK et al., 2019;GARCÍA-VÁZQUEZ et al., 2018). A) Greenland Ice-Core climate curve (RASMUSSEN et al., 2023). B) Radiocarbon dates of brown bear remains (CalBP) in relation to their δ 15 N values. ...

Ice-core data used for the construction of the Greenland Ice-Core Chronology 2005 and 2021 (GICC05 and GICC21)

... The EGRIP CFA data were measured by the Bernese setup and the data set is available at PANGAEA with the measurements described by Erhardt et al. (2023). In addition to the CFA system already successfully deployed for the NEEM ice core, the setup was extended by an inductively coupled plasma time-of-flight mass spectrometer (icpTOF, TOFWERK, Thun, Switzerland). ...

High resolution aerosol data from the top 3.8 ka of the EGRIP ice core

... Production of trace gases can occur both in the firn and in deeper ice below bubble lock-in presumably from ice impurities (Butler et al., 1999;Fain et al., 2014;Mühl et al., 2023). If the excess gas amount is comparable to the mean atmospheric levels, a correlation between the gas and the ice impurities would start to emerge. ...

Methane, ethane, and propane production in Greenland ice core samples and a first isotopic characterization of excess methane

... These processes have also been linked to the kinetics of global atmosphere-ocean disequilibria and corresponding fluxes of chlorofluorocarbons, CO 2 , and O 2 (Keeling et al., 1998;Takahashi et al., 1997;Wang et al., 2021). A recent study suggested that changes in the magnitude of air-sea disequilibria in the LGM ocean could have an appreciable effect on paleoatmospheric inert gas ratios, raising the possibility that previous ice-core based MOT estimates-which have not accounted for changes in air-sea disequilibrium-could be biased (Pöppelmeier et al., 2023). Thus, for example, a reduction in air-sea disequilibrium in the LGM ocean could increase the ocean inventory of Xe and thereby reduce atmospheric Xe/N 2 independent of ocean cooling, leading to a cold bias in ice-core estimates of MOT. ...

The Effect of Past Saturation Changes on Noble Gas Reconstructions of Mean Ocean Temperature

... Ice core data imply emissions increased as much as 6 to 21 Tg/year, causing methane growth rates as high as 6 ppb/yr (Rhodes et al., 2015). It is likely methane's rapid growth was primarily driven by rising tropical emissions, responding to abrupt changes in tropical rainfall (Bock et al., 2017;Möller et al., 2013;Riddell-Young et al., 2023;Schaefer et al., 2006). ...

Tropical sources dominated methane changes of the last glacial maximum and deglaciation

... The air is liberated by sublimating the ice under vacuum through irradiation with a near-infrared laser, while avoiding melting by controlled water vapor trapping within the extraction vessel. [1] The QCLAS instrument is based on mid-infrared direct absorption spectroscopy. The dual-laser concept allows us to simultaneously quantify the CO 2 concentration and its stable carbon isotopic ratio (δ 13 C-CO 2 ) as well as CH 4 and N 2 O concentrations with precisions of 0.4 ppm in CO 2 , 3 ppb in CH 4 , 1 ppb in N 2 O and 0.04‰ in δ 13 C-CO 2 . ...

Laser-induced sublimation extraction for centimeter-resolution multi-species greenhouse gas analysis on ice cores