Pulses of the pastBursts of carbon dioxide, released into the atmosphere and occurring on centennial time scales, were seen during the cold periods of the last glacial cycle but not in older or ...warmer conditions. Nehrbass-Ahles et al. present a record of atmospheric carbon dioxide concentrations retrieved from the European Project for Ice Coring in Antarctica Dome C ice core showing that these carbon dioxide jumps occurred during both cold and warm periods between 330,000 and 450,000 years ago. They relate these pulses to disruptions of the Atlantic meridional overturning circulation caused by freshwater discharge from ice sheets. Such rapid carbon dioxide increases could occur in the future if global warming also disrupts this ocean circulation pattern.Science, this issue p. 1000Pulse-like carbon dioxide release to the atmosphere on centennial time scales has only been identified for the most recent glacial and deglacial periods and is thought to be absent during warmer climate conditions. Here, we present a high-resolution carbon dioxide record from 330,000 to 450,000 years before present, revealing pronounced carbon dioxide jumps (CDJ) under cold and warm climate conditions. CDJ come in two varieties that we attribute to invigoration or weakening of the Atlantic meridional overturning circulation (AMOC) and associated northward and southward shifts of the intertropical convergence zone, respectively. We find that CDJ are pervasive features of the carbon cycle that can occur during interglacial climate conditions if land ice masses are sufficiently extended to be able to disturb the AMOC by freshwater input.
Ice core measurements of the concentration and stable isotopic composition of atmospheric nitrous oxide (N2O) 74,000–59,000 years ago constrain marine and terrestrial emissions. The data include two ...major Dansgaard‐Oeschger (D‐O) events and the N2O decrease during global cooling at the Marine Isotope Stage (MIS) 5a‐4 transition. The N2O increase associated with D‐O 19 (~73–71.5 ka) was driven by equal contributions from marine and terrestrial emissions. The N2O decrease during the transition into MIS 4 (~71.5–67.5 ka) was caused by gradual reductions of similar magnitude in both marine and terrestrial sources. A 50 ppb increase in N2O concentration at the end of MIS 4 was caused by gradual increases in marine and terrestrial emissions between ~64 and 61 ka, followed by an abrupt increase in marine emissions at the onset of D‐O 16/17 (59.5 ka). This suggests that the importance of marine versus terrestrial emissions in controlling millennial‐scale N2O fluctuations varied in time.
Plain Language Summary
Nitrous oxide is a powerful greenhouse gas that is produced naturally in soils and oceans. An important unresolved question is the extent to which anthropogenic warming will stimulate additional emissions from these sources, further adding to the warming. Past variations in the abundance of nitrous oxide have been observed using ice core reconstructions, but the reasons for the variations are not well understood. Nitrous oxide produced in soils is isotopically distinct from nitrous oxide produced in oceans. New measurements of the isotopes of atmospheric nitrous oxide provide constraints on how marine and terrestrial sources must have changed, driving fluctuations in nitrous oxide concentration during two intervals of rapid warming and a prolonged period of global cooling. The reconstructed changes in nitrous oxide sources provide insights into relationships between marine and terrestrial ecosystems and climate.
Key Points
Stable isotopes of nitrous oxide constrain marine versus terrestrial production 74,000–59,000 years ago
Marine and terrestrial sources varied similarly across Dansgaard‐Oeschger 19 and during the Marine Isotope Stage 5‐4 transition
Marine emissions dominated across Dansgaard‐Oeschger 16/17; thus, abrupt N2O increases were not all identical during the last glacial period
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
Reconstructions of past atmospheric methane concentrations are available from ice cores from both Greenland and Antarctica. The difference observed between the two polar methane concentration levels ...represents a valuable constraint on the geographical location of the methane sources. Here we present new high-resolution methane records from the North Greenland Ice Core Project (NGRIP) and the European Project for Ice Coring in Antarctica (EPICA) Dronning Maud Land (EDML) ice cores covering Termination 1, the Last Glacial Maximum, and parts of the last glacial back to 32 000 years before present. Due to the high resolution of the records, the synchronisation between the ice cores from NGRIP and EDML is considerably improved, and the interpolar concentration difference of methane is determined with unprecedented precision and temporal resolution. Relative to the mean methane concentration, we find a rather stable positive relative interpolar difference throughout the record with its minimum value of 3.7 ± 0.7 % between 21 900–21 200 years before present, which is higher than previously estimated in this interval close to the Last Glacial Maximum. This implies that Northern Hemisphere boreal wetland sources were never completely shut off during the peak glacial, as suggested from previous bipolar methane concentration records. Starting at 21 000 years before present, i.e. several millennia prior to the transition into the Holocene, the relative interpolar difference becomes even more positive and stays at a fairly stable level of 6.5 ± 0.8 % during Termination 1. We thus find that the boreal and tropical methane sources increased by approximately the same factor during Termination 1. We hypothesise that latitudinal shifts in the Intertropical Convergence Zone (ITCZ) and the monsoon system contribute, either by dislocation of the methane source regions or, in case of the ITCZ, also by changing the relative atmospheric volumes of the Northern and Southern Hemispheres, to the subtle variations in the relative interpolar concentration difference on glacial/interglacial as well as on millennial time scales.
During the last glacial cycle, Greenland temperature showed many rapid temperature variations, the so-called Dansgaard–Oeschger (DO) events. The past atmospheric methane concentration closely ...followed these temperature variations, which implies that the warmings recorded in Greenland were probably hemispheric in extent. Here we substantially extend and complete the North Greenland Ice Core Project (NGRIP) methane record from the Preboreal Holocene (PB) back to the end of the last interglacial period with a mean time resolution of 54 yr. We relate the amplitudes of the methane increases associated with DO events to the amplitudes of the local Greenland NGRIP temperature increases derived from stable nitrogen isotope (δ15N) measurements, which have been performed along the same ice core (Kindler et al., 2014). We find the ratio to oscillate between 5 parts per billion (ppb) per °C and 18 ppb °C−1 with the approximate frequency of the precessional cycle. A remarkably high ratio of 25.5 ppb °C−1 is reached during the transition from the Younger Dryas (YD) to the PB. Analysis of the timing of the fast methane and temperature increases reveals significant lags of the methane increases relative to NGRIP temperature for DO events 5, 9, 10, 11, 13, 15, 19, and 20. These events generally have small methane increase rates and we hypothesize that the lag is caused by pronounced northward displacement of the source regions from stadial to interstadial. We further show that the relative interpolar concentration difference (rIPD) of methane is about 4.5% for the stadials between DO events 18 and 20, which is in the same order as in the stadials before and after DO event 2 around the Last Glacial Maximum. The rIPD of methane remains relatively stable throughout the full last glacial, with a tendency for elevated values during interstadial compared to stadial periods.
An important share of paleoclimatic information is buried within the lowermost layers of deep ice cores. Because improving our records further back in time is one of the main challenges in the near ...future, it is essential to judge how deep these records remain unaltered, since the proximity of the bedrock is likely to interfere both with the recorded temporal sequence and the ice properties. In this paper, we present a multiparametric study (δD-δ18Oice, δ18Oatm, total air content, CO2, CH4, N2O, dust, high-resolution chemistry, ice texture) of the bottom 60 m of the EPICA (European Project for Ice Coring in Antarctica) Dome C ice core from central Antarctica. These bottom layers were subdivided into two distinct facies: the lower 12 m showing visible solid inclusions (basal dispersed ice facies) and the upper 48 m, which we will refer to as the "basal clean ice facies". Some of the data are consistent with a pristine paleoclimatic signal, others show clear anomalies. It is demonstrated that neither large-scale bottom refreezing of subglacial water, nor mixing (be it internal or with a local basal end term from a previous/initial ice sheet configuration) can explain the observed bottom-ice properties. We focus on the high-resolution chemical profiles and on the available remote sensing data on the subglacial topography of the site to propose a mechanism by which relative stretching of the bottom-ice sheet layers is made possible, due to the progressively confining effect of subglacial valley sides. This stress field change, combined with bottom-ice temperature close to the pressure melting point, induces accelerated migration recrystallization, which results in spatial chemical sorting of the impurities, depending on their state (dissolved vs. solid) and if they are involved or not in salt formation. This chemical sorting effect is responsible for the progressive build-up of the visible solid aggregates that therefore mainly originate "from within", and not from incorporation processes of debris from the ice sheet's substrate. We further discuss how the proposed mechanism is compatible with the other ice properties described. We conclude that the paleoclimatic signal is only marginally affected in terms of global ice properties at the bottom of EPICA Dome C, but that the timescale was considerably distorted by mechanical stretching of MIS20 due to the increasing influence of the subglacial topography, a process that might have started well above the bottom ice. A clear paleoclimatic signal can therefore not be inferred from the deeper part of the EPICA Dome C ice core. Our work suggests that the existence of a flat monotonic ice–bedrock interface, extending for several times the ice thickness, would be a crucial factor in choosing a future "oldest ice" drilling location in Antarctica.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
This work presents a new, field-deployable technique for continuous, high-resolution measurements of methane mixing ratios from ice cores. The technique is based on a continuous flow analysis system, ...where ice core samples cut along the long axis of an ice core are melted continuously. The past atmospheric air contained in the ice is separated from the melt water stream via a system for continuous gas extraction. The extracted gas is dehumidified and then analyzed by a Wavelength Scanned-Cavity Ring Down Spectrometer for methane mixing ratios. We assess the performance of the new measurement technique in terms of precision (±0.8 ppbv, 1σ), accuracy (±8 ppbv), temporal (ca. 100 s), and spatial resolution (ca. 5 cm). Using a firn air transport model, we compare the resolution of the measurement technique to the resolution of the atmospheric methane signal as preserved in ice cores in Greenland. We conclude that our measurement technique can resolve all climatically relevant variations as preserved in the ice down to an ice depth of at least 1980 m (66 000 yr before present) in the North Greenland Eemian Ice Drilling ice core. Furthermore, we describe the modifications, which are necessary to make a commercially available spectrometer suitable for continuous methane mixing ratio measurements from ice cores.
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IZUM, KILJ, NUK, PILJ, PNG, SAZU, UL, UM, UPUK
The climate of the last glacial Marine Isotopic Stage 3 (MIS3) period is characterized by strong millennial-scale variability with a succession of Dansgaard–Oeschger events first identified in ...Greenland ice cores and associated with variations of the Atlantic Meridional Overturning Circulation (AMOC). These abrupt events have a smooth and lagged counterpart in water stable isotopes from Antarctic ice cores. In this study we aim at depicting and understanding the circum-Antarctic expression of this millennial-scale variability. To illustrate the mechanisms potentially at work in the response of the southern high latitudes to an abrupt decrease of the AMOC, we first present results from experiments performed with the IPSL-CM4 atmosphere-ocean coupled model under glacial boundary conditions. When the AMOC is perturbed by imposing an additional freshwater flux in the North Atlantic, our model produces the classical bipolar seesaw mechanism generally invoked to explain the warming of the Southern Ocean/Antarctic region. However, this mechanism can be locally offset by faster atmospheric teleconnections originating from the tropics, even though the precise location of this fast response is not coherent among different climate models. Our model results are confronted with a synthesis of Antarctic records of ice core stable isotope and sea-salt sodium, including new data obtained on the TALDICE ice core. The IPSL-CM4 produces a dipole-like pattern around Antarctica, with warming in the Atlantic/Indian sectors contrasting with an unexpected cooling in the East-Pacific sector. The latter signal is not detected in our data synthesis. Both ice core data and simulations are consistent in depicting a more rapid response of the Atlantic sector compared to the Indian sector. This feature can be explained by the gradual impact of ocean transport on which faster atmospheric teleconnections are superimposed. Detailed investigations of the sequence of events between different proxies are conducted in three ice cores. Earlier shifts in deuterium excess and significant changes in sea-salt sodium fluxes in the most coastal sites (TALDICE and EDML) compared to EDC suggest reorganizations in local moisture sources, possibly linked with sea-ice cover. This study demonstrates the added value of circum-Antarctic ice core records to characterize the patterns and mechanisms of glacial climate variability.
► Review of glacial millennial-scale climate variability in Antarctic ice cores. ► Comparison between Antarctic ice core data and global coupled model simulations. ► Included new Talos Dome ice core climatic data. ► Regional differences of Southern Ocean and Antarctica climate are observed when North Atlantic Ocean circulation is weakened.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A new route for the chlorination of methane is described using ultrasound irradiation, which allows for an intrinsically safe process at ambient pressure and temperature. By tuning the gas feed ...composition methyl chloride yields of up to 19% have been obtained.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK, ZRSKP
Efforts to extract a Greenland ice core with a complete record of the Eemian interglacial (130,000 to 115,000 years ago) have until now been unsuccessful. The response of the Greenland ice sheet to ...the warmer-than-present climate of the Eemian has thus remained unclear. Here we present the new North Greenland Eemian Ice Drilling ('NEEM') ice core and show only a modest ice-sheet response to the strong warming in the early Eemian. We reconstructed the Eemian record from folded ice using globally homogeneous parameters known from dated Greenland and Antarctic ice-core records. On the basis of water stable isotopes, NEEM surface temperatures after the onset of the Eemian (126,000 years ago) peaked at 8 ± 4 degrees Celsius above the mean of the past millennium, followed by a gradual cooling that was probably driven by the decreasing summer insolation. Between 128,000 and 122,000 years ago, the thickness of the northwest Greenland ice sheet decreased by 400 ± 250 metres, reaching surface elevations 122,000 years ago of 130 ± 300 metres lower than the present. Extensive surface melt occurred at the NEEM site during the Eemian, a phenomenon witnessed when melt layers formed again at NEEM during the exceptional heat of July 2012. With additional warming, surface melt might become more common in the future.
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DOBA, IJS, IZUM, KILJ, KISLJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
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