Air was sampled from the porous firn layer at the NEEM site in Northern Greenland. We use an ensemble of ten reference tracers of known atmospheric history to characterise the transport properties of the site. By analysing uncertainties in both data and the reference gas atmospheric histories, we can objectively assign weights to each of the gases used for the depth-diffusivity reconstruction. We define an objective root mean square criterion that is minimised in the model tuning procedure. Each tracer constrains the firn profile differently through its unique atmospheric history and free air diffusivity, making our multiple-tracer characterisation method a clear improvement over the commonly used single-tracer tuning. Six firn air transport models are tuned to the NEEM site; all models successfully reproduce the data within a 1s Gaussian distribution. A comparison between two replicate boreholes drilled 64 m apart shows differences in measured mixing ratio profiles that exceed the experimental error. We find evidence that diffusivity does not vanish completely in the lock-in zone, as is commonly assumed. The ice age- gas age difference (?age) at the firn-ice transition is calculated to be 182+3-9 yr. We further present the first intercomparison study of firn air models, where we introduce diagnostic scenarios designed to probe specific aspects of the model physics. Our results show that there are major differences in the way the models handle advective transport. Furthermore, diffusive fractionation of isotopes in the firn is poorly constrained by the models, which has consequences for attempts to reconstruct the isotopic composition of trace gases back in time using firn air and ice core records

An understanding of the mechanisms that control CO 2 change during glacial-interglacial cycles remains elusive. Here we help to constrain changing sources with a high-precision, high-resolution deglacial record of the stable isotopic composition of carbon in CO 2 (δ 13 C-CO 2 ) in air extracted from ice samples from Taylor Glacier, Antarctica. During the initial rise in atmospheric CO 2 from 17.6 to 15.5 ka, these data demarcate a decrease in δ 13 C-CO 2 , likely due to a weakened oceanic biological pump. From 15.5 to 11.5 ka, the continued atmospheric CO 2 rise of 40 ppm is associated with small changes in δ 13 C-CO 2 , consistent with a nearly equal contribution from a further weakening of the biological pump and rising ocean temperature. These two trends, related to marine sources, are punctuated at 16.3 and 12.9 ka with abrupt, century-scale perturbations in δ 13 C-CO 2 that suggest rapid oxidation of organic land carbon or enhanced air-sea gas exchange in the Southern Ocean. Additional century-scale increases in atmospheric CO 2 coincident with increases in atmospheric CH 4 and Northern Hemisphere temperature at the onset of the Bølling (14.6-14.3 ka) and Holocene (11.6-11.4 ka) intervals are associated with small changes in δ 13 C-CO 2 , suggesting a combination of sources that included rising surface ocean temperature.ice cores | paleoclimate | carbon cycle | atmospheric CO 2 | last deglaciation O ver thirty years ago ice cores provided the first clear evidence that atmospheric CO 2 increased by about 75 ppm as Earth transitioned from a glacial to an interglacial state (1, 2). After decades of research, the underlying mechanisms that drive glacial-interglacial CO 2 cycles are still unclear. A tentative consensus has formed that the deglaciation is characterized by a net transfer of carbon from the ocean to the atmosphere and terrestrial biosphere, through a combination of changes in ocean temperature, nutrient utilization, circulation, and alkalinity. Partitioning these changes in terms of magnitude and timing is challenging. Estimates of the glacial-interglacial carbon cycle budget are highly uncertain, ranging from 20-30 ppm for the effect of rising ocean temperature, 5-55 ppm for ocean circulation changes, and 5-30 ppm for decreasing iron fertilization (3, 4), with feedbacks from CaCO 3 compensation accounting for up to 30 ppm (5, 6).A precise history of the stable isotopic composition of atmospheric carbon dioxide (δ 13 C-CO 2 ) can constrain key processes controlling atmospheric CO 2 (7,8). A low-resolution record from the Taylor Dome ice core (9) identified a decrease in δ 13 C-CO 2 at the onset of the deglacial CO 2 rise that was followed by increases in both CO 2 and δ 13 C-CO 2 (Fig. 1). A higher-resolution record from the European Project for Ice Coring in Antarctica Dome C (EDC) ice core (10) provided additional support for the rapid δ 13 C-CO 2 decrease associated with the initial CO 2 rise, and box modeling indicated that this decrease was consistent with changes in marine productivity. The r...

Atmospheric methane (CH 4) is a potent greenhouse gas, and its mole fraction has more than doubled since the preindustrial era 1. Fossil fuel extraction and use are among the largest anthropogenic sources of CH 4 emissions, but the precise magnitude of these contributions is a subject of debate 2,3. Carbon-14 in CH 4 (14 CH 4) can be used to distinguish between fossil (14 C-free) CH 4 emissions and contemporaneous biogenic sources; however, poorly constrained direct 14 CH 4 emissions from nuclear reactors have complicated this approach since the middle of the 20th century 4,5. Moreover, the partitioning of total fossil CH 4 emissions (presently 172 to 195 teragrams CH 4 per year) 2,3 between anthropogenic and natural geological sources (such as seeps and mud volcanoes) is under debate; emission inventories suggest that the latter account for about 40 to 60 teragrams CH 4 per year 6,7. Geological emissions were less than 15.4 teragrams CH 4 per year at the end of the Pleistocene, about 11,600 years ago 8 , but that period is an imperfect analogue for present-day emissions owing to the large terrestrial ice sheet cover, lower sea level and extensive permafrost. Here we use preindustrial-era ice core 14 CH 4 measurements to show that natural geological CH 4 emissions to the atmosphere were about 1.6 teragrams CH 4 per year, with a maximum of 5.4 teragrams CH 4 per year (95 per cent confidence limit)-an order of magnitude lower than the currently used estimates. This result indicates that anthropogenic fossil CH 4 emissions are underestimated by about 38 to 58 teragrams CH 4 per year, or about 25 to 40 per cent of recent estimates. Our record highlights the human impact on the atmosphere and climate, provides a firm target for inventories of the global CH 4 budget, and will help to inform strategies for targeted emission reductions 9,10. 14 CH 4 emissions from nuclear power plants 4,5. By contrast, palaeoatmospheric 14 CH 4 measurements from ice cores offer a direct constraint on natural geological CH 4 emissions without these complications. Whereas geological CH 4 emissions have the potential to change on tectonic-and glacial-cycle timescales 14 , they have very probably been constant over the past few centuries. The preindustrial-era emission estimates can therefore be applied to the modern CH 4 budget with confidence. Ice core 14 CH 4 analysis is challenging owing to both the very large sample requirement (~1,

We report atmospheric methane carbon isotope ratios (delta13CH4) from the Western Greenland ice margin spanning the Younger Dryas-to-Preboreal (YD-PB) transition. Over the recorded approximately 800 years, delta13CH4 was around -46 per mil (per thousand); that is, approximately 1 per thousand higher than in the modern atmosphere and approximately 5.5 per thousand higher than would be expected from budgets without 13C-rich anthropogenic emissions. This requires higher natural 13C-rich emissions or stronger sink fractionation than conventionally assumed. Constant delta13CH4 during the rise in methane concentration at the YD-PB transition is consistent with additional emissions from tropical wetlands, or aerobic plant CH4 production, or with a multisource scenario. A marine clathrate source is unlikely.