1748-9326/6/3/034007

Abstract

The 13C concentration in atmospheric CO 2 has been declining over the past 150 years as large quantities of 13C-depleted CO 2 from fossil fuel burning are added to the atmosphere. Deforestation and other land use changes have also contributed to the trend. Looking at the 13C variations in the atmosphere and in annual growth rings of trees allows us to estimate CO 2 uptake by land plants and the ocean, and assess the response of plants to climate. Here I show that the effects of the declining 13C trend in atmospheric CO 2 are recorded in the isotopic composition of paper used in the printing industry, which provides a well-organized archive and integrated material derived from trees' cellulose. 13C analyses of paper from two European and two American publications showed, on average, a − 1.65 ± 1.00‰ trend between 1880 and 2000, compared with − 1.45 and − 1.57‰ for air and tree-ring analyses, respectively. The greater decrease in plant-derived 13C in the paper we tested than in the air is consistent with predicted global-scale increases in plant intrinsic water-use efficiency over the 20th century. Distinct deviations from the atmospheric trend were observed in both European and American publications immediately following the World War II period.

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1. Introduction

The preferential uptake of 12CO 2 over 13CO 2 in plant photosynthesis (Francey and Farquhar 1982, Farquhar et al 1982) has two important implications. First, the extent of this biologically dominated discrimination is sensitive to environmental conditions, making 13C content in wood and other plant materials a useful indicator of climate change and plant response to it (Feng 1999, Leavitt et al 2003, Peñuelas et al 2008). Second, removal of 13C-depleted CO 2 from the atmosphere via land photosynthesis results in 13C accumulation in the remaining atmospheric CO 2 , the extent of which reflects the strength of the land sink (Ciais et al 1995, Tans et al 1993), while CO 2 exchange with the ocean, in contrast, is dominated by physical processes that have relatively little effect on 13C. In both cases we must also account for the effects of anthropogenic activities over the industrial period. Fossil fuel emissions and biomass burning involve the combustion of photosynthetically derived material. In all cases, 13C-depleted CO 2 is returned to the atmosphere diluting its 13C content. Continuous records of the atmospheric 13C trend over the last 1000 years exist mainly based on air trapped in firn and ice (e.g. Francey et al 1999). While extensive, high-precision monitoring of atmospheric 13C only started in 1990s (www.esrl.noaa.gov/gmd/ccgg/globalview/co2c13/co2c13_intro.html), there have been several attempts to detect the fossil fuel signal on century time-scales via the 13C/12C ratio in organic material, primarily using tree rings (McCarroll and Loader 2004, Feng and Epstein 1995, Waterhouse et al 2004). Paper used in the printing industry also provides a well-preserved,-organized and -dated archive of partially purified cellulose from plants. The pulping industry made major technological advancements in the early 19th century, and since then, paper is made almost exclusively from cellulose extracted from wood (Britt 2011). The sources of wood for the paper industry in the developed world are largely confined to major forestry centers in the northern temperate and boreal forests. Further, pulp production is well tuned to paper consumption, with short time-gaps between wood harvesting and dated publications that use the paper products. Paper integrates materials from many trees and the pulp contains a record of the entire age of the trees (about 40 years at the time of harvest). On the one hand, this may limit the identification of specific tree species and the ability to obtain isotopic records at high spatial and temporal resolution. But on the other hand, it has the advantage of providing large-scale, running mean values of the isotopic composition of the wood, often the ultimate goal even in the high resolution tree-ring studies. Using paper may also involve several complications. For example, manipulations of paper quality may involve various sources of pulp, paper treatment to improve quality may affect it isotopic compositions, and the increasing extent of recycling can also result in the loss of time-resolved signal. The main question addressed here is can we see the temporal signal in atmospheric 13C, in spite of the range of the potential pitfalls. The surprisingly optimistic results obtained in our trial could motivate further development of this attractive but neglected tool.

2. Materials and methods

Paper was sampled from one American and two European periodicals (Science magazine, Nature magazine, and the Journal of the Chemical Society of London, respectively), and one daily newspaper (the Boston Globe) for which I had accessible archives spanning at least one hundred years. I used samples from publications dated between 1880 and 2000. Samples from the periodicals were obtained from my local campus libraries and from the Boston Globe's archives in Boston, Massachusetts, USA. Preliminarily, attempts to trace the sources of the papers at the 100-year range were generally difficult. For Nature, during the past few decades, the paper used for printing could generally be traced to Finnish pulp. Material for the Boston Globe, as for most daily newspapers, came from regional wood, in this case, from the northeastern US and Canada (personal communications with paper manufacturers). In all cases, 20 mm × 5 mm non-printed samples were clipped from the bottom of a page of a single, arbitrary selected, volume/issue from each year for which the publication was available. (For the Journal of the Chemical Society, volumes from only every 2–3 years were sampled.) About 0.2 mg sub-samples were combusted, quantitatively converted to CO 2 , purified and analyzed on an isotope ratio mass spectrometer for determination of the 13C/12C ratio, as described before (Klein et al 2005). Twelve arbitrarily selected sub-samples from one publication (Journal of the Chemical Society), spread over the entire record length, were used for chemical purification of α-cellulose (Klein et al 2005), which was subsequently analyzed for 13C/12C ratios, as above. Isotopic ratios were reported in the conventional delta notation, where δ13C = R sample /R standard − 1 and the standard is PDB carbonate. Trends of atmospheric 13C in atmospheric CO 2 over the past 120 years were obtained from the literature on air samples (Francey et al 1999) and tree-rings samples (Feng and Epstein 1995, Feng 1999). The isotopic data for each publication was filtered by detrending the data and excluding data points that were different by more than ± 2σ from the mean. For the Journal of the Chemical Society, samples of paper dated before 1900 were off by about + 2‰ from subsequent samples. These samples may not have been derived from wood—early paper was sometimes made from cloth—and were not included in the data analysis. Because photosynthesis results in plant organic matter depleted by 16–18‰ relative to the δ13C value of atmospheric CO 2 (Farquhar et al 1982), trends in both atmospheric and plant samples were normalized for comparison. The atmospheric trend was normalized by setting the best-fit line to zero in 1882, and all other data sets were normalized to best fit the starting point of this record (see table 1). Table 1. Summary of the atmospheric and paper 13C values associated with the results presented in figure 1. The starting point values used to adjust all data to zero in 1880 are indicated as the normalizing factor. The number of samples used and the outliers (greater than ± SD from the detrended fit line) excluded from the data analysis is also indicated. 13Δ indicates discrimination during photosynthetic CO 2 uptake [(13Δ = (δ13C air − δ13C plant )/(1 + δ13C plant /1000))]. Source Normalizing factor (‰) 1882–2000 change (‰) Number of samples/outliers Atmospheric trend (Francey et al 1999) + 6.57 − 1.45 — (Feng 1999) + 6.56 − 1.57 — European paper Nature + 21.85 − 3.05 97/19 Royal Chem. Soc + 22.64 − 1.71 32/11 American paper Boston Globe + 24.24 − 0.91 83/18 Science + 23.49 − 0.94 68/21 Mean (paper) + 23.05 − 1.65 13Δ 16.87 + 0.17

3. Results and discussion

Acknowledgment

The assistance of Mrs Manuela Negreanu in the paper collection and isotopic analysis is greatly appreciated.