Revised records of atmospheric trace gases CO<sub>2</sub>, CH<sub>4</sub>, N<sub>2</sub>O, and <i>δ</i><sup>13</sup>C-CO<sub>2</sub> over the last 2000 years from Law Dome, Antarctica

Abstract

Abstract. Ice core records of the major atmospheric greenhouse gases (CO2, CH4, N2O) and their isotopologues covering recent centuries provide evidence of biogeochemical variations during the Late Holocene and pre-industrial periods and over the transition to the industrial period. These records come from a number of ice core and firn air sites and have been measured in several laboratories around the world and show common features but also unresolved differences. Here we present revised records, including new measurements, performed at the CSIRO Ice Core Extraction LABoratory (ICELAB) on air samples from ice obtained at the high-accumulation site of Law Dome (East Antarctica). We are motivated by the increasing use of the records by the scientific community and by recent data-handling developments at CSIRO ICELAB. A number of cores and firn air samples have been collected at Law Dome to provide high-resolution records overlapping recent, direct atmospheric observations. The records have been updated through a dynamic link to the calibration scales used in the Global Atmospheric Sampling LABoratory (GASLAB) at CSIRO, which are periodically revised with information from the latest calibration experiments. The gas-age scales have been revised based on new ice-age scales and the information derived from a new version of the CSIRO firn diffusion model. Additionally, the records have been revised with new, rule-based selection criteria and updated corrections for biases associated with the extraction procedure and the effects of gravity and diffusion in the firn. All measurements carried out in ICELAB–GASLAB over the last 25 years are now managed through a database (the ICElab dataBASE or ICEBASE), which provides consistent data management, automatic corrections and selection of measurements, and a web-based user interface for data extraction. We present the new records, discuss their strengths and limitations, and summarise their main features. The records reveal changes in the carbon cycle and atmospheric chemistry over the last 2 millennia, including the major changes of the anthropogenic era and the smaller, mainly natural variations beforehand. They provide the historical data to calibrate and test the next inter-comparison of models used to predict future climate change (Coupled Model Inter-comparison Project – phase 6, CMIP6). The datasets described in this paper, including spline fits, are available at https://doi.org/10.25919/5bfe29ff807fb (Rubino et al., 2019).

Advancing Scientific Understanding of the Global Methane Budget in Support of the Paris Agreement

“…It is expressed in units of nanomoles per mole, which is abbreviated for simplicity as parts per billion (ppb). In the lower atmosphere, globally averaged mole fractions approached 1,860 ppb in 2018, up from 1,775 ppb in 2006 (National Oceanic and Atmospheric Administration, NOAA/ESRL, www.esrl.noaa.gov/gmd/ccgg/trends_ch4/) and approximately 710 ppb in preindustrial 1750 (Etheridge et al, 1998;Rubino et al, 2019) (Figure 1). Atmospheric CH 4 mole fractions have exhibited large fluctuations year to year, and these are observed in the high-frequency measurement record that began in 1983.…”

Very old firn air linked to strong density layering at Styx Glacier, coastal Victoria Land, East Antarctica

“…In the firn layer, air moves through the open pores and is occluded into the adjacent ice at total porosity of ∼ 0.1 (Schaller et al, 2017). Firn air moves downward with the adjacent ice (advection), but is furthermore mixed by diffusion and affected by thermal and gravitational fractionation (Craig et al, 1988;Johnsen et al, 2000;Severinghaus et al, 2001;Goujon et al, 2003).…”

“…First, we can calculate the z COD from firn densification models. Typically, the close-off occurs when the density of ice reaches about 830 kg m −3 (Blunier and Schwander, 2000), equivalent to a critical porosity of around 0.1 (Schaller et al, 2017). Also, if temperature is known, the average density at close-off can be estimated from empirical relations (Martinerie et al, 1992).…”

“…Firn air models help calculate the firn air age using some parameters such as temperature and accumulation rate. However, several studies found that layering also affects the movement of firn air (e.g., Mitchell et al, 2015;Schaller et al, 2017). This implies that physical properties of the ice may affect the age of the firn air as well.…”

“…With regard to the lock-in and close-off processes, recent studies have focused on snow layers and microstructure of the firn (Hörhold et al, 2011;Gregory et al, 2014;Mitchell et al, 2015;Schaller et al, 2017). Density variability on scales of millimeters to tens of centimeters is observed at all polar sites.…”