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Proc Natl Acad Sci U S A. Sep 17, 2002; 99(19): 12011–12014.
Published online Aug 29, 2002. doi:  10.1073/pnas.182420699
PMCID: PMC129389
Geology

Rapid atmospheric CO2 changes associated with the 8,200-years-B.P. cooling event

Abstract

By applying the inverse relation between numbers of leaf stomata and atmospheric CO2 concentration, stomatal frequency analysis of fossil birch leaves from lake deposits in Denmark reveals a century-scale CO2 change during the prominent Holocene cooling event that occurred in the North Atlantic region between 8,400 and 8,100 years B.P. In contrast to conventional CO2 reconstructions based on ice cores from Antarctica, quantification of the stomatal frequency signal corroborates a distinctive temperature–CO2 correlation. Results indicate a global CO2 decline of ≈25 ppm by volume over ≈300 years. This reduction is in harmony with observed and modeled lowering of North Atlantic sea-surface temperatures associated with a short-term weakening of thermohaline circulation.

Estimates of Holocene atmospheric CO2 concentrations strongly rely on CO2 measured in air extracted from Antarctic ice cores. Records for the past millennium indicate significantly reduced CO2 levels from A.D. 1550 to 1800, which are temporally related to the historical Little Ice Age climate deterioration (1, 2). By contrast, Antarctic ice-core data do not clearly support a temperature–CO2 correlation during the eight earlier short-term cooling pulses that punctuated Holocene climatic conditions in the North Atlantic region with a periodicity of ≈1,500 years (3). Only the so-called 8.2-ka-B.P. cooling event has been associated with a long-term, modestly declining CO2 trend recognized in the Taylor Dome record (2). This most prominent century-scale instability of Holocene climate is reflected as a negative δ18O excursion in Greenland ice cores (4, 5) as well as in a variety of marine and terrestrial proxy records indicative of pronounced modifications of regional temperature and/or precipitation regimes in the North Atlantic region (3, 4, 6–11). The instability is related to a brief episode of massive release of meltwater associated with the final demise of the Laurentian ice sheet (12).

The conventional iced-based concept of relatively stabilized CO2 concentrations during the greater part of the Holocene is challenged increasingly by stomatal frequency analysis of fossil leaves (13–15). Species of C3 plants are often characterized by a plastic phenotype capable of consistent adjustment of numbers of leaf stomata in response to changes in ambient CO2 concentration (16–18). Identification of a CO2-sensitive gene involved in stomatal development in Arabidopsis thaliana demonstrates the genetic control of the response (19). As a corollary of this responsiveness, stomatal frequency analysis of fossil leaves enables the detection and quantification of atmospheric CO2 changes at different time scales (14, 17–25). High-resolution analysis of tree leaves buried in Lateglacial and Holocene peat and lake deposits suggests temporal correlation between global CO2 dynamics and Northern Hemisphere temperature. In Europe and North America, reconstructed CO2 fluctuations (14, 15, 20) appear to parallel well documented δ18O and chrinomid-based temperature changes associated with the Younger Dryas stadial and the Preboreal Oscillation, the first of the Holocene climatic instabilities occurring at ≈11,000 calendar years B.P. (26). Both Younger Dryas and Preboreal Oscillation are not apparent in the CO2 record of Antarctic ice (2, 27), which may be due to generally low temporal resolution of Lateglacial and early Holocene CO2 data from Antarctica.

To corroborate the concept of a coupling between recurrent Holocene cooling pulses and CO2 fluctuations, we document stomatal frequency data that constrain timing and magnitude of CO2 shifts associated with the prominent 8.2-ka-B.P. cooling event. We analyzed leaves of European tree birches (Betula pubescens and Betula pendula) preserved in cores of gyttja deposits from Lake Lille Gribsø, North of Copenhagen, Denmark (55°58′43′′N; 12°18′52′′E). Well preserved leaf remains occur continually through an interval corresponding to the period between ≈8,700 and ≈6,800 calendar years B.P. Temporal control is provided by a series of six accelerator mass spectrometry 14C chronologies measured on single leaves (Table (Table1).1).

Table 1.
Leaf-bearing and accelerator mass spectrometry 14C-dated horizons in Lake Lille Gribsø, Denmark

Stomatal frequency can be expressed in terms of stomatal density and stomatal index (SI): SI = (stomatal density/[stomatal density + epidermal cell density]) × 100. In contrast to stomatal density, SI reflects stomatal frequency independently of variation in epidermal cell size related to light intensity, temperature, or nutrient and water availability. In angiosperm leaves, therefore, mean SI is the more sensitive parameter for detecting stomatal frequency responses to changing CO2 levels (28, 29). Effects of variation within and between leaves can be eliminated by using large data sets (17, 18, 28). Leaves of B. pendula and B. pubescens display essentially similar SI patterns and can be treated therefore as a single category in stomatal frequency analysis (18). There is sufficient evidence from field studies and experiments that CO2-induced trends in mean SI for Betula leaves are not disturbed significantly by environmental factors other than CO2 (30, 31).

Standardized, computer-aided determination of stomatal parameters on leaf cuticles was performed on a Leica Quantimet 500C/500+ image-analysis system. Measured parameters include stomatal density and epidermal cell density (including guard cells). Counting areas are restricted to stomata-bearing alveoles. Calculated SIs are mean values for up to seven leaves per data point. Seven digital images (field area, 0.035 mm2) per leaf were analyzed (standard deviations are constant after seven counts). We used the rate of historical CO2 responsiveness of the European tree birches (Fig. (Fig.1)1) to quantify early Holocene SI-based CO2 levels. Our approach of inferring an unknown value of a quantitative variable (CO2/x0) from the quantitative response of a single taxon (SIbirch/y) meets the basic requirements for a classical linear regression, which allows a better performance at the extremes and with slight extrapolation (32–34).

Fig 1.
Historical response of SI for European tree birches (B. pendula and B. pubescens) to global atmospheric CO2 increase from 287 to 356 ppmv. The training set includes mean SI values for herbarium material (•) collected in The Netherlands and Denmark ...

The reconstructed CO2 record shows a fluctuating pattern (Fig. (Fig.2).2). Inferred CO2 minima with averages of ≈275 ppm by volume (ppmv) occur at ≈8,680 years B.P. and between ≈8,430 and ≈8,040 years B.P.; prominent maxima with values of 300–325 ppmv occur at ≈8,640 years B.P. and between ≈7,920 and ≈7,270 years B.P. The series of low CO2 concentrations around 8,300 years B.P. follow a declining trend of ≈25 ppmv within a time interval of <100 years. This interval ends with a return to levels >300 ppmv after ≈300 years. Timing and duration of the century-scale CO2 excursion are in harmony with the proxy records for the 8.2-ka-B.P. cooling event embracing a period of 200–300 years between ≈8,400 and ≈8,100 years B.P. (3–12). The SI signals thus indicate a temporal association between the 8.2-ka-B.P. cooling and atmospheric CO2 concentration. The exact phase relationship between changes in temperature and CO2 cannot be determined yet.

Fig 2.
Reconstructed CO2 concentrations for the time interval between ≈8,700 and ≈6,800 calendar years B.P. based on CO2 extracted from air in Antarctic ice of Taylor Dome (left curve; ref. 2; raw data available via www.ngdc.noaa.gov/paleo/taylor/taylor.html ...

Our CO2 reconstructions reflect rapid changes with a significantly greater magnitude than the smooth and modest atmospheric CO2 decline to values ≈260 ppmv inferred from the low-resolution Taylor Dome ice-core record (Fig. (Fig.2).2). The data also confirm the regular occurrence of early Holocene atmospheric CO2 concentrations well above 300 ppmv, unknown from Antarctic ice cores but common in leaf-based time series (9, 14). These apparent controversies between leaf-based and ice-based CO2 data have not been resolved yet (ref. 35; see text at www.sciencemag.org/cgi/content/full/286/5446/1815a). It should be noted that early Holocene records from Greenland ice cores have repeatedly indicated rapidly fluctuating CO2 levels including values >300 ppmv (36, 37). At present, the Antarctic record is usually considered to be reliable, so that discrepancies are ascribed to CO2 enrichment within the Greenland ice (38, 39). However, there is evidence that in polar ice also postdepositional CO2 depletion could occur, but underlying chemical processes of this potential source of error have not yet been investigated in detail (38, 39).

The documented coupling between CO2 fluctuations and the 8.2-ka-B.P. cooling implies a distinctive involvement of the oceans, where short-term perturbations of sea-surface temperature and/or salinity allow rapid CO2 transfer between the atmosphere and surface waters. Holocene cooling events are generally related to a reduction in the thermohaline circulation, resulting in sea-surface temperature lowering in large parts of the North Atlantic (3, 6). This forcing mechanism is evident particularly for the 8.2-ka-B.P. cooling event, where weakening of the thermohaline circulation was triggered by catastrophic release of Laurentide meltwater (12). By applying a freshwater perturbation over 20 years, a global atmosphere-sea-ice-ocean model simulation of the 8.2-ka-B.P. event produces a 320-year lasting weakening of the North Atlantic thermohaline circulation and 1–5°C surface cooling over the adjacent continents (40).

The reconstructed atmospheric CO2 reduction of ≈25 ppmv indicates a temporarily enhanced North Atlantic sink for CO2 at the time of the 8.2-ka-B.P. cooling event. While regional palynological data support temperature changes (11), vegetation reconstructions do not provide evidence of an extended terrestrial sink. The occurrence of global CO2 fluctuations substantiates the interpretation of δ18O records in ice cores from Antarctica and Greenland in terms of globally parallel climate changes on the Northern and Southern Hemispheres (41). The CO2 fluctuations falsify conclusively the concept of an approximate antiphase relation between short-term temperature changes on the two hemispheres that would cause buffering of North Atlantic CO2 drawdown by the effects of synchronous sea-surface temperature increase in the Southern Ocean (42). It thus may be concluded that leaf-based CO2 data support a much more dynamic evolution of the Holocene CO2 regime than previously thought. In effect, there seems to be every indication that the occurrence of Holocene CO2 fluctuations is more consistent with current observations and models of past global temperature changes than the common notion of a relatively stable CO2 regime until the onset of the Industrial Revolution.

Acknowledgments

We thank Frans Bunnik, David Dilcher, Morten Fischer-Mortensen, Lenny Kouwenberg, Wolfram Kürschner, Jette Raal-Hanssen, David Robinson, Bas van Geel, Tom van Hoof, and two anonymous reviewers for constructive comments and stimulating discussions on the topic. This work was supported by the Danish Research Councils and the Council for Earth and Life Sciences of the Netherlands Organization for Scientific Research. This paper is Netherlands Research School of Sedimentary Geology publication no. 20020602.

Abbreviations

  • ka, thousand years
  • ppmv, ppm by volume

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