Rapid changes in ice core gas records Part 2: Understanding the rapid rise in atmospheric CO2 at the onset of the Bølling/Allerød

During the last glacial/interglacial transition the Earth's climate underwent rapid changes around 14.6 kyr ago. Temperature proxies from ice cores revealed the onset of the Bølling/Allerød (B/A) warm period in the north and the start of the Antarctic Cold Reversal in the south. Furthermore, the B/A is accompanied by a rapid sea level rise of about 20 m during meltwater pulse (MWP) 1A, whose exact timing is matter of current debate. In situ measured CO₂ in the EPICA Dome C (EDC) ice core also revealed a remarkable jump of 10±1 ppmv in 230 yr at the same time. Allowing for the age distribution of CO₂ in firn we here show, that atmospheric CO₂ rose by 20–35 ppmv in less than 200 yr, which is a factor of 2–3.5 larger than the CO₂ signal recorded in situ in EDC. Based on the estimated airborne fraction of 0.17 of CO₂ we infer that 125 Pg of carbon need to be released to the atmosphere to produce such a peak. Most of the carbon might have been activated as consequence of continental shelf flooding during MWP-1A. This impact of rapid sea level rise on atmospheric CO₂ distinguishes the B/A from other Dansgaard/Oeschger events of the last 60 kyr, potentially defining the point of no return during the last deglaciation

CH\u3csub\u3e4\u3c/sub\u3e and δ\u3csup\u3e18\u3c/sup\u3eO of O\u3csub\u3e2\u3c/sub\u3e records from Antarctic and Greenland ice: A clue for stratigraphic disturbance in the bottom part of the Greenland Ice Core Project and the Greenland Ice Sheet Project 2 ice cores

The suggestion of climatic instability during the last interglacial period (Eem), based on the bottom 10% of the Greenland Ice core Project (GRIP) isotopic profile, has been questioned because the bottom record from the neighboring Greenland Ice Sheet Project 2 (GISP2) core (28 km away) is strikingly different over the same interval and because records of the δ18O of atmospheric O2 from both cores showed unexpected rapid fluctuations. Here we present detailed methane records from the Vostok (Antarctica), GRIP, and GISP2 cores over the relevant intervals. The GRIP and GISP2 data show rapid and large changes in methane concentration, which are correlative with variations of the δ18O of the ice, while the Vostok record shows no such variations. This discrepancy reinforces the suggestion that the bottom sections of the Greenland records are disturbed. By combining the methane data with measurements of δ18O of O2 in the same samples, we attempt to constrain the nature of the stratigraphic disturbance and the age of the analyzed ice samples. Our results suggest that ice layers from part of the last interglacial period exist in the lower section of both ice cores and that some of the apparent climate instabilities in the GRIP core would be the result of a mixture of ice from the last interglacial with ice from the beginning of the last glaciation or from the penultimate glaciation

Carbon Isotope Constraints on the Deglacial CO2 Rise from Ice Cores

The stable carbon isotope ratio of atmospheric CO2 (d13Catm) is a key parameter in deciphering past carbon cycle changes. Here we present d13Catm data for the past 24,000 years derived from three independent records from two Antarctic ice cores. We conclude that a pronounced 0.3 per mil decrease in d13Catm during the early deglaciation can be best explained by upwelling of old, carbon-enriched waters in the Southern Ocean. Later in the deglaciation, regrowth of the terrestrial biosphere, changes in sea surface temperature, and ocean circulation governed the d13Catm evolution. During the Last Glacial Maximum, d13Catm and atmospheric CO2 concentration were essentially constant, which suggests that the carbon cycle was in dynamic equilibrium and that the net transfer of carbon to the deep ocean had occurred before then

Changes in the atmospheric CH4 gradient between Greenland and Antarctica during the Holocene

High-resolution records of atmospheric methane over the last 11,500 years have been obtained from two Antarctic ice cores (D47 and Byrd) and a Greenland core (Greenland Ice Core Project). These cores show similar trapping conditions for trace gases in the ice combined with a comparable sampling resolution; this together with a good relative chronology, provided by unequivocal CH4 features, allows a direct comparison of the synchronized Greenland and Antarctic records, and it reveals significant changes in the interpolar difference of CH4 mixing ratio with time. On the average, over the full Holocene records, we find an interpolar difference of 44±7 ppbv. A minimum difference of 33±7 ppbv is observed from 7 to 5 kyr B.P. whereas the maximum gradient (50±3 ppbv) took place from 5 to 2.5 kyr B.P. A gradient of 44±4 ppbv is observed during the early Holocene (11.5 to 9.5 kyr B.P). We use a three-box model to translate the measured differences into quantitative contributions of methane sources in the tropics and the middle to high latitudes of the northern hemisphere. The model results support the previous interpretation that past natural CH4 sources mainly lay in tropical regions, but it also suggests that boreal regions provided a significant contribution to the CH4 budget especially at the start of the Holocene. The growing extent of peat bogs in boreal regions would also have counterbalanced the drying of the tropics over the second half of the Holocene. Finally, our model results suggest a large source increase in tropical regions from the late Holocene to the last millennium, which may partly be caused by anthropogenic emissions

Atmospheric methane, record from greenland ice core over the last 1000 years

The atmospheric methane concentration in ancient times can be reconstructed by analysing air entrapped in bubbles of polar ice sheets. We present results from an ice core from Central Greenland (Eurocore) covering the last 1000 years. We observe variations of about 70 ppbv around the mean pre-industrial level, which is confirmed at about 700 ppbv on a global average. According to our data, the beginning of the anthropogenic methane increase can be set between 1750 and 1800. Changes in the oxidizing capacity of the atmosphere may contribute significantly to the pre-industrial methane concentration variations, but changes in methane emissions probably play a dominant role. Since methane release depends on a host of influences it is difficult to specify clearly the reasons for these emission changes. Methane concentrations correlate only partially with proxy-data of climatic factors which influence the wetland release (the main source in pre-industrial times). A good correlation between our data and a population record from China suggests that man may already have influenced the CH4-cycle significantly before industrialisation

A 60 yr record of atmospheric carbon monoxide reconstructed from Greenland firn air

We present the first reconstruction of the Northern Hemisphere (NH) high latitude atmospheric carbon monoxide (CO) mole fraction from Greenland firn air. Firn air samples were collected at three deep ice core sites in Greenland (NGRIP in 2001, Summit in 2006 and NEEM in 2008). CO records from the three sites agree well with each other as well as with recent atmospheric measurements, indicating that CO is well preserved in the firn at these sites. CO atmospheric history was reconstructed back to the year 1950 from the measurements using a combination of two forward models of gas transport in firn and an inverse model. The reconstructed history suggests that Arctic CO in 1950 was 140–150 nmol mol-1, which is higher than today's values. CO mole fractions rose by 10–15 nmol mol-1 from 1950 to the 1970s and peaked in the 1970s or early 1980s, followed by a ˜ 30 nmol mol-1 decline to today's levels. We compare the CO history with the atmospheric histories of methane, light hydrocarbons, molecular hydrogen, CO stable isotopes and hydroxyl radicals (OH), as well as with published CO emission inventories and results of a historical run from a chemistry-transport model. We find that the reconstructed Greenland CO history cannot be reconciled with available emission inventories unless unrealistically large changes in OH are assumed. We argue that the available CO emission inventories strongly underestimate historical NH emissions, and fail to capture the emission decline starting in the late 1970s, which was most likely due to reduced emissions from road transportation in North America and Europe

Российско-французский семинар 2015

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Summer Temperature Trend Over the Past Two Millennia Using Air Content in Himalayan Ice

Two Himalayan ice cores display a factor-two decreasing trend of air content over the past two millennia, in contrast to the relatively stable values in Greenland and Antarctica ice cores over the same period. Because the air content can be related with the relative frequency and intensity of melt phenomena, its variations along the Himalayan ice cores provide an indication of summer temperature trend. Our reconstruction point toward an unprecedented warming trend in the 20th century but does not depict the usual trends associated with Medieval Warm Period (MWP), or Little Ice Age (LIA)

High resolution measurements of carbon monoxide along a late Holocene Greenland ice core: evidence for in situ production

We present high-resolution measurements of carbon monoxide (CO) concentrations from a shallow ice core of the North Greenland Eemian Ice Drilling project (NEEM-2011-S1). An optical-feedback cavity-enhanced absorption spectrometer (OF-CEAS) coupled to a continuous melter system performed continuous, online analysis during a four-week measurement campaign. This analytical setup generated stable measurements of CO concentrations with an external precision of 7.8 ppbv (1σ), based on repeated analyses of equivalent ice core sections. However, this first application of this measurement technique suffered from a poorly constrained procedural blank of 48 ± 25 ppbv and poor accuracy because an absolute calibration was not possible. The NEEM-2011-S1 CO record spans 1800 yr and the long-term trends within the most recent section of this record (i.e., post 1700 AD) resemble the existing discrete CO measurements from the Eurocore ice core. However, the CO concentration is highly variable (75–1327 ppbv range) throughout the ice core with high frequency (annual scale), high amplitude spikes characterizing the record. These CO signals are too abrupt and rapid to reflect atmospheric variability and their prevalence largely prevents interpretation of the record in terms of atmospheric CO variation. The abrupt CO spikes are likely the result of in situ production occurring within the ice itself, although the unlikely possibility of CO production driven by non-photolytic, fast kinetic processes within the continuous melter system cannot be excluded. We observe that 68% of the CO spikes are observed in ice layers enriched with pyrogenic aerosols. Such aerosols, originating from boreal biomass burning emissions, contain organic compounds, which may be oxidized or photodissociated to produce CO within the ice. However, the NEEM-2011-S1 record displays an increase of ~0.05 ppbv yr⁻¹ in baseline CO level prior to 1700 AD (129 m depth) and the concentration remains elevated, even for ice layers depleted in dissolved organic carbon (DOC). Thus, the processes driving the likely in situ production of CO within the NEEM ice may involve multiple, complex chemical pathways not all related to past fire history and require further investigation

SUB-OCEAN: subsea dissolved methane measurements using an embedded laser spectrometer technology

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Environmental Science and Technology, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.est.7b06171.We present a novel instrument, the Sub-Ocean probe, allowing in situ and continuous measurements of dissolved methane in seawater. It relies on an optical feedback cavity enhanced absorption technique designed for trace gas measurements and coupled to a patent-pending sample extraction method. The considerable advantage of the instrument compared with existing ones lies in its fast response time of the order of 30 s, that makes this probe ideal for fast and continuous 3D-mapping of dissolved methane in water. It could work up to 40 MPa of external pressure and it provides a large dynamic range, from subnmol of CH4 per liter of seawater to mmol L-1. In this work, we present laboratory calibration of the instrument, intercomparison with standard method and field results on methane detection. The good agreement with the headspace equilibration technique followed by gas-chromatography analysis supports the utility and accuracy of the instrument. A continuous 620-m depth vertical profile in the Mediterranean Sea was obtained within only 10 min and it indicates background dissolved CH4 values between 1 and 2 nmol L-1 below the pycnocline, similar to previous observations conducted in different ocean settings. It also reveals a methane maximum at around 6 m of depth that may reflect local production from bacterial transformation of dissolved organic matter