Oak leaves as biosensors of late neogene and early pleistocene paleoatmospheric CO2 concentrations

Marine Micropaleontology

27 (1996) 299-312

Oak leaves as biosensors of late Neogene and early Pleistocene paleoatmospheric CO2 concentrations Wolfram M. Kiirschner a,*, Johan van der Burgh a, Henk Visscher a, David L. Dilcher b ” Laboratory of Palaeobotany and Palynology, Utrecht University, Budapestlaan 4, 3.584 CD Utrecht, The Netherlands ’ Florida Museum of Natural History, Universify of Florida, Gainesville, FL 32611, USA Received 1 October 1994; accepted

1 May 1995

Abstract Complementary to the interpretation of 613Cvalues of biogenic carbonate and sedimentary organic carbon in marine sediments, CO2 levels can be estimated by considering the inverse relationship between atmospheric CO* concentration and stomata1 parameters (frequency, size) on leaves of land plants. In woody plants, the significance of this (species-specific) physiological response to changing CO2 regimes is now repeatedly confirmed, both experimentally and from historical sequences of leaves collected since the onset of industrialization. A corollary of this relationship is that analysis of stomata1 parameters on fossil leaves has the potential of determining changes in paleoatmospheric CO2 levels at different time scales. Well-preserved cuticle remains of oak leaves from late Miocene, Pliocene and early Pleistocene sediments of the Lower Rhine Embayment (Germany, The Netherlands) give promise of extending the record of stomata1 frequency response to the last 10 Ma. During intervals with warm-temperate to subtropical climatic conditions, oak leaves are characterized by a high stomata1 resistance (or low conductance) to CO* diffusion and low stomata1 frequencies; during cooler intervals we observe an opposite picture. Comparison with historical relations between CO2 concentration and stomatal properties suggests that paleoatmospheric COz concentrations were not significantly higher than during the last 200 years and fluctuated several times between 280 and 370 ppmv in covariation with contrasting regional climatic conditions. On a global scale, intervals with reduced CO* levels match glacial pulses characterized by the occurrence of ice-rafted detritus in high-latitude oceanic sediments. paleoatmospheric

1. Introduction

The prediction of global warming in the range of 2.5”-4.5”C for the next century (Houghton et al., 1990) has stimulated the study of past climatic variability. Particularly the Pliocene epoch has attracted considerable attention as an ancient, but geologically most modern analogue of a possible future “greenhouse world” (e.g. Budyko et al., 1985; Dowsett et al., 1994; Chandler et al., 1994). The main mechanisms suggested as cause for the Pliocene warming are higher levels of * E-mail: [email protected]. 0377~8398/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDfO377-8398(95)00067-4

ocean heat transport (Rind and Chandler, 199 1; Dowsett and Poore, 1991; Dowsett et al., 1992, 1994; Raymo et al., 1992; Raymo, 1994; Chandler et al., 1994) and elevated paleoatmospheric CO? concentrations (e.g. Crowley, 1991). Changing CO2 regimes have been mainly inferred from proxy signals derived from geochemical observations and by forward modelling. However, resulting estimates of Pliocene CO2 concentrations are still controversial. On the basis of S13C values for carbonate of benthic and planktonic foraminifers, Shackleton (1985) and Berger and Spitzy ( 1988) estimated the general upper limit of post-Eocene CO2 concentrations

300

WM. Kiirschner et al. /Marine

at 2.5 times the present-day level. Constraints for a CO,-enriched paleoatmosphere during the Late Miocene and Pliocene were established by Popp et al. ( 1989)) Freeman and Hayes ( 1992)) Raymo and Rau ( 1992) and Raymo ( 1994). From 613C values for biogenie carbonates and coeval sedimentary organic matter or specific organic compounds (geoporphyrins) they concluded that CO, concentrations were closer to 360-400 ppmv, 2540% higher than pre-industrial values. Geochemical data from the terrestrial realm, pertinent to paleoatmospheric CO, levels, were collected by Cerling ( 199 1,1992). A Late Miocene and Pliocene pC0, similar to present-day values was deduced on the basis of S’“C values for paleosol carbonates and organic matter. Global Circulation Models for the Pliocene (Crowley, 1991) and for a future greenhouse world (Houghton et al., 1990) result in COZ levels twice as high as the pre-industrial value. Further theoretical approaches were performed by Berner et al. ( 1983), Lasaga et al. ( 1985), Berner ( 1991) and Francois and Walker ( 1992) by modelling the long-term global carbon budget on a time scale > lo6 years. Their simulated paleoatmospheric curves for the past 10 million years indicate CO, levels up to 1.5 times the modern COZ level. In addition to the geochemical approach and modelling exercises, the fossil record may provide significant information with respect to past changes in atmospheric CO* concentration. Because of their prominent role in photosynthetic CO, fixation, notably land plants should be considered as potential biosensors of Pliocene CO2 levels. Physiological responses of plants to changes in the supply of basic resources (solar radiation, CO*, water, nutrients) require morphological, anatomical and/or biochemical adjustments. If physiologically determined adjustments can still be discriminated in fossil plant remains, their temporal analysis may provide a “paleophysiological” methodology for quantifying past climatic change. A novel paleobotanical approach is now available by considering the inverse relationship between the stomata1 frequency on leaves of land plants and atmospheric CO1 concentration. Following the pioneer study by the British ecologist Woodward ( 1987)) the significance of this physiological response to changing CO* regimes is repeatedly confirmed, both experimentally and from historical sequences of leaves collected since the onset of industrialization. Conflict-

Micropaleontology

27 (1996) 299-312

ing results on the basis of historical observations (e.g. KSrner, 1988) need further investigation; an important factor is formed by differences in methodology (see discussion by Beerling and Chaloner, 1992). Controversial results from experimental (usually CO, doubling) studies are discussed by Kiirschner et al. (in prep. ) . Carbon dioxide diffuses from the atmosphere into plant leaves to the carboxylation site of rubilose biphosphate (RuP,) in the chloroplasts through pores (stomata). Since diffusion rates depend on concentration gradients (Fick’s first law), appropriate gradients are required to ensure sufficient diffusion of CO, from the ambient air to the chloroplast. The diffusion pathway in land plants can be divided into a gaseous and a liquid phase, and each portion of this pathway imposes a resistance to diffusion. The first part of the pathway is determined by three consecutive processes: diffusion through the boundary layer of the leaf, through the stomata, and through the intercellular spaces between the mesophyll cell walls of the leaf. In this gaseous phase, the stomata1 component shows by far the largest influence (Jones, 1992). As a result, adjustment of this parameter provides the plant with an effective means of controlling CO;? assimilation. The stomata1 resistance is determined morphologically by the stomata1 frequency, the size of stomata1 pore area, the length of the stomatal tube, and the diameter of the substomatal cavity. The increase of atmospheric CO, concentrations over the last 200 years from a pre-industrial value of about 270 ppmv to a present-day level of about 360 ppmv (Neftel et al., 1985; Friedli et al., 1986; Keeling, 1993) has resulted in a decrease of nearly 40% of the stomata1 density of several European temperate forest tree and shrub species (Woodward, 1987, 1993; Penuelas and Matamala, 1990). This acclimatization to the current ambient CO* level has been confirmed by physiological experiments in controlled environments (Woodward, 1987; Woodward and Bazzaz, 1988). Analysis of the stomata1 frequency on fossil leaf remains of extant species offers a possibility to extend the record into geological times. Variations in stomata1 density have been used successfully as a biosensor for CO* levels during the last Glacial-Interglacial cycle (Beerling, 1993; Beerling and Chaloner, 1994; Van de Water et al., 1994).

W.M. Kiirschner et al. /Marine Micropaleontology 27 (19%) 299-312

Through the study of cuticle remains of late Neogene oak leaves (Quercuspetrueu) it was demonstrated that the relationship between stomata1 frequency and CO2 concentration can well be used to quantify CO2 levels during the last 10 million years (Van der Burgh et al., 1993; Visscher, 1994). The data suggest that late Miocene and Pliocene CO:! concentrations fluctuated between 280 and 370 ppmv. Following this initial study, the present paper presents an extended documentation of the morphological and physiological properties of recent and fossil oak leaves (Quercus petrueu) . In order to utilize late Miocene, Pliocene and early Pleistocene oak leaves as biosensors of paleoatmospheric CO2 levels, the objectives of this study are ( 1) to calculate stomata1 density, stomata1 index and stomata1 resistance to CO2 diffusion in oak leaves collected over the past 190 years and to assess the historical evolution of these parameters as a response to increasing atmospheric CO* concentrations, and (2) to calibrate stomata1 parameters for fossil leaf remains from the Lower Rhine Embayment (Germany, The Netherlands) against this historical response.

301

2. Geological setting and sampling The recent leaf material of Quercus petruea (durmast oak) was collected in the field at sites near Gulpen in southern Limburg, The Netherlands. The material includes both sun- and shade-leaf morphotypes. Herbarium specimens from different sites in Europe spanning the last 190 years were provided by the Herbarium Division of Utrecht University, the National Herbarium (Leiden, The Netherlands) and the National Museum of Natural History (London, England). The fossil leaves originate from the Lower Rhine Embayment (Fig. 1) , a fault-controlled depocentre marking the current southeastern margin of the Neogene North Sea Basin exposed in Germany and adjacent parts of The Netherlands (see Zagwijn and Hager, 1987). Conventionally the leaf remains are identified as the fossil species Q. pseudocustuneu and Q. roburoides. According to Van der Burgh ( 1993)) however, these species are likely to represent fossil representatives of Q. petrueu. The material has been collected from clay intercalations in the late Miocene, Pliocene

The Netherlands

Belgium Bruxelles

I

TertiaryMuatemary

1//1

Pm-Tertiary

Fig. 1. Location of the Neogene Lower Rhine Embayment.

-

Faults

302

W.M. Kiirschner et al. /Marine Micropaleontology 27 (19%) 299-312

and early Pleistocene coastal plain sequences above the middle/late Miocene main seam group of the browncoal formation (the Frimmersdorf and Garzweiler seams of the Ville Formation). The litho- and biostratigraphic framework for the continental and marine Late Cenozoic sequences in the Lower Rhine Embayment was established by Zagwijn (1960), Von der Brelie ( 1968)) Zagwijn and Doppert ( 1978)) Doppert ( 1980), Sliggers and Van Leeuwen ( 1987), Zagwijn and Hager (1987) and R. Stritzke (unpubl. data), based on foraminiferal, molluscan and palynological studies. Correlation with marine standard stages is discussed by Zagwijn and Hager ( 1987). The sampled interval (compare Fig. 4, based on the local development in open-cast brown-coal mine Hambath; Wutzler, 1991; Abraham, 1993) starts with the Inden Formation, which is characterized by a succession of coarse and fine-grained, cross-bedded sands alternating with clay horizons in several channel fills. On the basis of palynological data the Inden Formation is locally classified as Linne B/C, an informal chronostratigraphic unit that may represent part of the marine Tortonian Stage. The Inden Formation is followed by the predominantly coarse fluvial elastics of the Main Gravel Formation. This formation corresponds to the upper part of the regional Susterian Stage, which approximates the Messinian of marine standard classification. The following Lower Pliocene Red Clay Formation is characterized by a lower and an upper clay member, separated by a sequence of cross-bedded sands with gravel beds and small, local clay intercalations. The age of the formation is Brunssumian, a correlative of the marine Zanclean Stage. The Red Clay Formation is overlain by the Reuver Formation. The lower part of this unit is generally sandy; the upper part consists of clay with a thin brown-coal layer at the top. The Reuver Clay Formation typifies the regional Reuverian Stage, that corresponds to the lower part of the marine Piacenzian. In regional chronostratigraphic considerations the top of the formation is generally regarded to represent the Pliocene/Pleistocene junction, characterized by the onset of the first distinctive cold phase in the Northern Hemisphere (2.3 Ma). It should be realized, however, that this boundary does not correspond to the Pliocene/Pleistocene boundary of conventional marine standard chronostratigraphy ( 1.8 Ma). For arguments in favour of a 2.3 Ma boundary, see Zagwijn ( 1992). If we accept this boundary,

the Pleistocene sequence would start with the Tegelen Formation, composed of basal gravel-sand deposits followed by clay-dominant elastics. The Tegelen Formation corresponds to the glacial Praetiglian and the interglacial Tiglian Stages. In terms of marine standard classification, both units would still represent the Late Pliocene Piacenzian Stage (compare Fig. 5). Most of the sampled material is derived from the brown-coal open-cast mine at Hambach near Cologne; the material represents the Linne B and C, Susterian, Brunssumian and Tiglian. Reuverian leaves originate from collections made in the type locality at Reuver, which were described by Laurent and Marty ( 1923). The age of this flora is early Reuverian (Zagwijn, 1960). In addition, leaf material was collected from the Reuver Formation in the Laumanns pit near Abel. In this location paleomagnetic investigations (Arias et al., 1984) demonstrated the presence of the Gauss-Matuyama polarity reversal in the middle part of the Reuver clay, indicating an age of 2.6 Ma (Hilgen, 1991a).

3. Analytical techniques For cuticle morphology of the field and herbarium material, three small pieces (0.5 cm’) were taken from the middle part of a leaf and processed for 8 hours in a 5% NaClO, solution at 40°C to remove the mesophyll layer (Dilcher, 1974). The remaining cuticle samples were carefully washed with water and subsequently coloured in a Cr,03 solution. From the fossil material, the adhering sediment was removed from the leaf remains with HF and HCl (Kerp, 1990). The cuticle samples were then bleached for some minutes, alternating cold HNO, and KOH treatments until they were suitable for microscopic examination. All cuticle samples were mounted in glycerine jelly. Stomata1 and epidetmal cell densities and length of the stomata1 pore were measured on stored video images with a bit-map editor or with an image analyzer (Quantimed 500 C, Leica Cambridge). The field study included 54 cuticular samples from 30 modern leaves, the herbarium study 78 samples from 33 herbarium sheets. Per cuticular sample lo-15 counts were made. With respect to the fossil material 30-90 counts were made per stratigraphical horizon, samples usually originating from three fossil leaves. During the course of the study it appeared to be necessary to use a standard

W.M. Kiirschner et al. /Marine Micropaleontology 27 (1996) 299-312

30.7

field area for counting and calculation of stomata1 parameters. We selected a field area of 0.035 mm*. Tables (in ASCII or Excel (Microsoft) format) with the raw data can be obtained on request from the Laboratory of Palaeobotany and Palynology, Utrecht University, The Netherlands. The stomata1 index was calculated using the following equation (Salisbury, 1927) :

in air, as the reconstructed absolute temperature values are too variable and are only available for the Miocene. All cuticle preparations are stored at the Laboratory of Palaeobotany and Palynology, Utrecht University. The Netherlands.

g=

4.1. Cuticularparameters

(4 (ed+sd)

(1)

x loo

where SZ [ %] represents the stomata1 index, sd the stomata1 density per unit leaf area and ed the epidermal cell density per unit leaf area. The average stomata1 resistance was estimated utilizing the following equation (Parlange and Waggoner, 1970):

In(4alb) 77-a

(2)

where a [m] represents the major axis radius of the stomata1 pore (half pore length), b [m] the minor axis radius of the stomata1 pore (half pore width), D [ m2/ coefficient of CO, in air Sl the diffusive (D= 1.47x10-” m2 s-’ at 20°C and 101.3 kPa, after Monteith and Unsworth, 1991), d [m] the length of the stomata1 tube plus the diameter of the substomatal cavity, n [ 1/m2] the stomata1 density, and R [s/m] the stomata1 resistance at 20°C and at 1 atm (0.01013 MPa) . Transverse sections of fresh leaves were prepared in order to determine the length of the diffusive pathway (thickness of the lower epidermis and cuticle plus diameter of the substomatal cavity). The stomata1 resistance of the field and herbarium leaf material was calculated for a standard stomata1 aperture width of 4 pm, mean pore length of 10 pm, and standard length of the diffusion pathway of 50 pm. The stomata1 resistance of the fossil leaf material was calculated from measured stomata1 densities, measured major axis radius of the stomata1 pore, and the standard length of the diffusion pathway and the stomata1 aperture width derived from the studies on fresh material. Regional changes in paleotemperature during the climatic fluctuations in the studied area were not considered for calculation of the diffusion coefficient of CO,

4. Results

The stomata1 density and calculated stomata1 resistance to CO2 diffusion of Q. petruea leaves collected in 1992, as well as herbarium leaf material collected over the last 190 years is depicted in Fig. 2. Recent oak leaves show a wide range of mean stomata1 densities from 360 up to 600 pores per mm’. This variability is mainly caused by changing epidermal cell expansion as a response to differences in sunlight intensity and air humidity. Mean stomata1 densities for distinctive sun and shade morphotypes are in the range of 330-450 and 520-600 pores per mm’, respectively. Superimposed on these variations, the stomata1 density decreases by about 40% over the past 190 years. Thercfore, the overall trend depicts a band with its width determined by the differences in the sun and shade lea! morphotypes. The observed decrease in stomata1 density corresponds to a 50% increase of the average stomata1 resistance to CO, diffusion, calculated from the cuticle morphology and leaf anatomy for sun and shade leaves. respectively (Fig. 2). In addition to the calculations, stomata1 resistance to CO* diffusion was also determined experimentally (Ktirschner et al ., in prep. ) for shade leaves at 350 ppmv CO2 in controlled environment chambers. In agreement with other physiological studies (e.g. Van Gardingen et al., 1989), our experiments indicate a good correspondence between values inferred from calculations on leaves and whole-plant experimental measurements (330 s/m and 360 s/m. respectively; Kiirschner et al., in prep.). The stomata1 index of the oak leaves is plotted in Fig. 3 versus the historical atmospheric CO* concentrations derived from ice core data (Neftel et al., 1985; Friedli et al., 1986) and Mauna Loa monitoring (Kecling et al., 1989; Keeling, 1993). The mean stomata1 index of recent oak leaves varies between 10% and 16% (sun morphotypes > 12.8%. shade morphotypes

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W.M. Kiirschner et al. /Marine Micropaleontology 27 (19%) 299-312

2001800

1840

1920 1880 Year A. D.

1960

-590 zoo0

Fig. 2. Historical evolution of the stomatal density of Quercuspetraea leaves collected over the last 190 years and calculated stomatal resistances to CO2 diffusion. For data points of the field study, mean stomataf densities between 520 and 600 pores per mm’correspond to sun morphotypes, densities between 360 and 450 mm’ correspond to shade morphotypes.

< 12.8%)) that of oak leaves grown under a pre-industrial atmospheric COZ level ( <280 ppmv) between 15% and 20%. The mean standard deviation for a single cuticular sample (10-15 counts) is f 1.76. The computed sigmoidal regression curve shows a linear response interval between 280 and 340 ppmv. Above 340 ppmv CO*, the stomata1 index curve approaches asymptotically its lower limit at about 11%. The stomata1 index, stomata1 density, pore length and calculated stomata1 resistance of the fossil oak leaves are summarized in Table 1. The stomata1 indices for late Neogene to early Pleistocene Q. petruea leaves are plotted in Fig. 4. Morphological and anatomical studies (Ktirschner, 1994; Ktirschner, in prep.) indicate that most of the leaves preserved in the fossil record represent sun morphotypes. Moreover, taphonomic studies (e.g. Spicer, 1975; Roth and Dilcher, 1978; Ferguson, 1985) in recent depositional environments have shown that selective transport and degradation processes favour the preservation of sun leaf morphotypes. Q. petruea from the Linne B and C, early and late Brunssumian as well as early Reuverian and Tiglian show mean values between 13% and 15% (Fig. 4). On the other hand, the mean values of about 19% for the sun leaves from the Susterian and middle Brunssumian and latest Reuverian

25 23 21 tyl

a

19

260

272

284

296

308

320

332 344

356

368

380

Atm. CO2 [ppmv] Fig. 3. Stomatal indices of Quercus petraea leaves versus historical atmospheric CO2 concentrations derived from ice core data (Neftel etaf., 1985;Friedlietrd., 1986) andMounaLoamonitoring (Keeling et al., 1989; Keeling, 1993); computed sigmoidal regression curve with 95% confidence interval. For data points of the field study, mean stomatal indices > 12.8% correspond to sun morphotypes, indices < 12.8% correspond to shade morphotypes.

W.M. Kiirschner el al. /Marine Micropaleontology 27 (1996) 299-312

30.5

Table I Stomata1 properties and estimated resistances to CO2 diffusion of fossil oak leaves Age

Tiglian L. Reuverian E. Reuverian L. Brunssummian

Location

Hambach 6bel Reuver Hambach

S.D. [n/mm’]

f St. Dev.

E.D. [n/mm’]

+ St. Dev.

S.I.

505 670 437 477

61 69 65 13

3029 2863 2749 2720

259 164 236 230

14.31 18.96 13.7 14.83 (11.5) 22.43 13.03 19.89 (16.4) 16.05 (11.5) 13.69 (9.8)

M. Brunssummian E. Brunssummian Susterian

Hambach Hambach Hambach

908 498 828

92 60 68

3148 3325 3338

259 257 205

Linne C

Hambach

510

81

2672

158

Linne B

Hambach

515

93

3219

307

[%I

f St. Dev.

S.L. [pm]

f St. Dev.

P.L.

* St.

R(Q)

[km]

Dev.

[s/ml

1.39 1.5 1.41

20.71 19.71 21.65 19.07

1.53 1.52 1.7 1.57

10.3x 9.67 I 1.37 9.66

I.39 0.74 I .27

226 182 239

I.2

3.56

18.77 19.11 19.09

I .78

9.97 9.05 I I .08

1.39

1.35 1.74

I .26

131 261 I29

19.51

1.47

9.67

0.96

240

18.56

2.1

9.48

I .23

242

1.48 (1.4) 1.91 1.39 1.29 (1.1) 2.14 (1.2) 1.71 (1.7)

I 09

Where S.D. represents the mean stomatal density, St. Dev. the averaged standart deviation of a sample, E.D. the mean epidennal cell density. S.I. the mean stomatal index, S.L. the mean stomatal apparatus length, P.L. the mean pore length; values in brackets are from Van der Burgh et al. (1993).

are significantly higher. The averaged standard deviation of the mean values is I 2.14. The observed fluctuations of the stomata1 index during the studied interval are accompanied by changes in the stomata1 density between about 505 and 900 pores per mm2 (Fig. 5)) which mainly effect the variations in the stomatal resistance to CO* diffusion. The values deduced from the leaf morphological data fluctuate between 130 and 260 s/m. The epidennal cell density with a mean of 30 10 + 265 cells per mm2 remains fairly constant in the fossil leaf assemblage. 4.2. Determination of paleoatmospheric CO, levels Comparison with historical relations between CO1 concentration and stomatal properties enables the interpretation of the latter in terms of paleoatmospheric CO2 levels (Fig. 6). The measured stomata1 properties of the fossil oak leaves generally indicate that the CO2 concentrations were not significantly higher than during the last 200 years. The relative changes so far observed suggest that concentrations fluctuated several times between 280 and 360 ppmv. The high stomatal indices as well as high stomatal resistances of oak leaves grown under a warm temperate to subtropical climate indicate CO:! levels similar to the present-day level ( -360 ppmv). General lower values of both parameters (Fig. 5) during cooler periods suggests

COz concentrations similar to the pre-industrial level. The CO2 curve so far reconstructed may indicate a pattern of CO2 oscillations with a frequency of approximately 2 Ma and an amplitude of 80 ppmv.

5. Discussion 5.1. Cuticle parameters The heterogeneity in stomata1 density found in the 1992 oak leaves (Fig. 3) as a response to different h-radiance levels and air humidity is well known from other studies under natural conditions and from controlled environment experiments (e.g. Wild and Wolf, 1980; Sol&rovB and PospiSilova, 1988; Kiirschner, 1994; Kiirschner et al., in prep.). These variations in stomata1 density for sun and shade leaves of Q. perraea are accompanied by changes in the stomata1 index. Our results seem to contradict earlier investigations of other species, which supposed that the stomata1 index is fairly constant except under extreme environmental stress (e.g. Salisbury, 1927; Sharma and Dunn, 1968,1969). However, other studies (Schiirmann, 1959; Schoch et al., 1980) have shown that stomata1 indices can be influenced by light intensity and air humidity. A further factor that may account for the variety in the herbarium study is that these leaves could well have

306

W.M. Kiirschner et al. /Marine Micropaleontology 27 (19%) 299-312

10m I

12

I

14

I

16

I

18

I

20

I

22

24

Fig. 4. Stomata1 indices for Quercus perraea (sun leaves) plotted alongside the generalized stratigraphic column (after Wutzler, 1991) of the Late Miocene to Early Pleistocene strata in the open-cast mine Hambach; Pt.= Praetiglian.

been collected from trees growing in dissimilar habitats. The effect of altitude on stomata1 parameters has been well documented (e.g., Kiirner et al., 1979; Woodward and Bazzaz, 1988; Beerling and Chaloner, 1992). However, except for a single site in Switzerland (520 m), our herbarium material originates from WestEuropean lowland localities not exceeding 200 m. It is unlikely, therefore that the herbarium data are signifi-

cantly influenced by altitudinal variation. The present distribution of Q. petruea in Europe extends from southern Scandinavia to the Mediterranean regions. Hence the species has a considerable tolerance for differences in temperature, precipitation and soil conditions. The species is intolerant to high groundwater levels. Despite the wide area1 extend of Q. petruea, so far we have no indications of geographically deter-

W.M. Kiirschner et al. /Marine Micropaleontology 27 (1996) 299-312

Continental Standard

Stages

Stages (N;~&&a$s,

Stomatal Resistance RstW2) Wml ) , , , , , , , , , , , , ( 130 150 170 190 210 230 250

307

Stomata1Density [n/mw

Q. petfaea r I I I I I 400slocm7008009001000

1

_______.____.

dessinian

__________ __._B Tortonian -9 I I

Linne B/C .lO

4

-

Fig. 5. Stomata1 density and calculated stomata1 resistance to CO2 diffusion. Approximate correlation of the continental stages with marine standard stages after Zagwijn ( 1986) and Zagwijn and Hager ( 1987). Chronology for the marine standard stages after Harland er al. ( 1990) and Hilgen ( 199la, b).

mined variation. Moreover, it seems unlikely that such variation would be detectable in tree leaves. According to Kiirschner et al. (in prep.) sun leaves of Q. petruea from the north of the Netherlands (Groningen) have mean stomata1 densities varying between 460 and 610 pores per mm2 (stomata1 index between 14.6 and 16.4%)) whereas material from northern Italy (Torino) shows a density variation between 550 and 670 pores per mm2 (stomata1 index between 12.6 and 15.2%). Considering the large degree of overlap of these variations with the ranges of stomata1 parameters for sun morphotypes obtained from our field study, it is unlikely that other variations induced by local environmental conditions can ever be confidently detected among leaves of Q. petraea. The measured mean stomata1 geometry (pore length and width) of Q. petraea leaves is in good agreement with that given for a closely related species Q. robur by Meidner and Mansfield ( 1968). The measured general decrease in stomata1 density and stomata1 index for

Q. petruea leaves in response to the rise in atmospheric COZ over the past two centuries is in good agreement with other studies (Woodward, 1987; Peiiuelas and Matamala, 1990). The rate of this decline remains constant despite exponential increase of the atmospheric CO* content since 1900. Woodward ( 1993) mentioned that an initial strong response of the stomata1 density relative to a small rise in atmospheric CO, occurs before the 20th century. Our results suggest that stomatal adjustment in Q. petraea approaches a threshold value at atmospheric CO, concentrations above 5 340 ppmv (Fig. 3). Moreover, in accordance to other studies on a variety of woody and herbaceous plants (e.g. Madsen, 1973; Oberbauer et al., 1985; Woodward and Bazzaz, 1988; Berrymanet al., 1994; Ferris andTaylor. 1994; Rey and Jarvis, 1994), physiological cxperiments with young oak plants (Kiirschner et al., in prep.) indicate that the stomata1 index and the stomata1 density respond only slightly to further doubled CO2 concentrations (700 ppmv) . Other studies (Thomas

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Micropaleontology

27 (1996) 299-312

Climatecurve

Messinian

. . . . . . . . . . . . .. 8 Tortonian

I I

‘. ’ .\

Linne B/C

Fig. 6. Paleoatmospheric CO2 curve compared to a regional climate curve based on palynological analysis(fromZagwijnandHager,1987) andoccurrences of ice rafted debris (IRD) in the North Atlantic (after Shackleton et al., 1984; Jansen and Sj@holm,1991; Larsen et al., 1994).

and Harvey, 1983; Mousseau and Enoch, 1989; RadogIOUand Jarvis, 1990; Evans et al., 1993) report for some species even no extra reduction of stomata1 density at this elevated CO* concentrations (700 ppmv). Such conditions result however in modifications of leaf anatomy, i.e. a thicker mesophyll and a thicker cuticle, a common effect confirmed by other studies (e.g. Rogers et al., 1983; Thomas and Hrirvey, 1983; Radoglou and Jarvis, 1990, 1992; Kiirschner et al., in prep.). In Q.petruea the increase in leaf thickness is mainly determined by an increased cell size in the palisade parenchyma (39 zL-4 pm under 350 ppmv and 65 f 4 pm under 700 ppmv) , whereas the spongy parenchyma is hardly affected (49 f 5 pm and 50&4 pm, respectively) and the lower epidermal cell layer remains unchanged ( 10 + 2 pm). Consequently, the length of the diffusion pathway in the gaseous phase (d in Eq. 2) is not significantly influenced by increased CO* levels (Kiirschner et al., in prep.). The stomata1 indices for fossil Q. perraea earlier published by Van der Burgh et al. (1993) are in good

agreement with our present data, albeit that the current values are slightly higher. This disparity can mainly be ascribed to the effects of improved analytical techniques (image analyzer). It is here considered that all fossil leaves of Q. petrueu analyzed for the present study may represent sun morphotypes. This assumption is based on a comparison of total leaf size and epidermal cell size with recent material. In addition, mesophyll anatomy can sometimes provide conclusive evidence of a two-layered palisade parenchyma. A taphonomic analysis suggests that sun morphotypes of Q.petrueu are selectively preserved in fossil leaf assemblages (Kiirschner, 1994). In the stomata1 analysis, the resulting predominance of sun morphotypes has the advantage that it stabilizes the influence of different light regimes and minimizes any effect of extra humidity and COP released by evapotranspiration and respiration of the understorey vegetation and the soil surface. The variation in both stomata1 indices and stomata1 densities for the fossil leaf remains agree well with the variations

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observed for sun leaves of the herbarium and field studies. Fossil oak leaves grown under warm climatic conditions show generally stomata1 properties comparable to that of their living representatives. As discussed above, leaves of e. petraeu have reached almost under the ambient atmospheric CO2 concentration of 360 ppmv their lower limit of stomata1 index and stomata1 density. Consequently, these parameters would fail to detect paleoatmospheric CO, levels that are higher than the present-day value. On the other hand, stomata1 dimensions, particularly the pore length remains fairly constant over the studied interval (Table 1). As a result, the calculated stomatal resistances do not exceed present-day values. First results from an anatomical study of pristine preserved, relatively uncompressed, fossil oak leaves (Ktirschner, 1994) show no significant increase in mesophyll thickness, which also suggests that CO:! levels were not significantly higher than 360 ppmv. However, final confirmation has to await evidence from long-ranging ( > 10 Ma) plant species showing a clear response of stomatal index or stomata1 density to elevated atmospheric CO* concentrations > 350 ppmv, both under experimental conditions and in the fossil record. 5.2. Paleoatmospheric CO, curve In general, the maximum paleoatmospheric CO* levels inferred from our paleophysiological study of fossil oak leaves are more or less similar to estimates based on geochemical signals from the marine record (Freeman and Hayes, 1992; Raymo and Rau, 1992). The present data are in harmony with scenarios with present-day, rather than doubled, COz levels as required by recent global circulation modelling for the middle Pliocene (Chandler et al., 1994). Whereas the marine studies only showed long term trends in the paleoatmospheric CO2 regime, our measurements reveal higher order oscillations in the CO2 concentration of a pre-Quatemary atmosphere. The declining trends in the COz content, so far observed, coincide several times with cooling events indicated in the regional climate curve of Zagwijn and Hager ( 1987). On a global scale, these shifts in the paleoatmospheric CO* content correlate with early pulses of substantial glaciation in the Northern Hemisphere during the Late Miocene and Pliocene, preceding the final

309

Pleistocene cooling optimum. Sedimentological evidence for ice rafted detritus (IRD) in deep-sea sediments of the North Atlantic (ODP Holes 642A and 644B, Jansen and Sjaholm, 1991; ODP Leg 152, Larsen et al., 1994; ODP Leg 151, Leg 15 1 Shipboard Scientific Party, 1995)) indicates high-northem-latitude circumarctic glaciation at about 7.5.5.4.5 and 2.8 Ma ago (Fig. 6) prior the final establishment of the North Atlantic ice sheet at around 2.3 Ma (Shackleton et al., 1984). In the Southern Hemisphere Late Neogene ice rafted debris from Antarctica occurred around 6.5,4.5 and 3.0 Ma (Ehrmann et al., 1992). This covariation with climatic changes strongly implies a causal relationship between fluctuations in global paleoatmospheric COz level and temperature regime during Late Neogene to Early Pleistocene times. It should be emphasized, however, that the observed covariation does not provide definitive evidence with respect to the question whether late Neogene climatic trends are primarily induced by CO* or by oceanic heat transport. 6. Concluding remarks Analysis of stomata1 parameters on late Miocene to early Pleistocene leaves from Germany and The Netherlands demonstrates that paleophysiological methodology enables determination of CO* levels during at least the last 10 million years. It should be realized, however, that the fossil leaf record is characterized by its generally discontinuous nature. The relatively low number of data-points available in any one terrestrial sequence will limit the extent to which high-resolution patterns of paleoatmospheric change can be reconstructed exclusively with the aid of stomata1 analysis. Yet, there is every indication that this novel method may promote many collections of fossil leaves to become invaluable paleoatmospheric databases that can provide the quantitative information required for testing and refining scenarios of paleoatmospheric change based on the interpretation of the geochemical and microfossil record in both the marine and terrestrial realm. Acknowledgements

We thank Friederike Wagner, Gerard Versteegh, Karin Zonneveld, Han van Konijnenburg-van Cittert

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(Laboratory of Palaeobotany and Palynology, Utrecht University) and Harald Strauss (Ruhr University Bochum) for fruitful discussions. We thank two anonymous reviewers for critical but constructive comments. Gerhard Belz (University Ttibingen) is acknowledged for donation of some samples, and R. Stritzke (Geological Survey NRW, Krefeld) for providing stratigraphic data. This research was supported by The Netherlands Geosciences Foundation (GOA) of The Netherlands Organization for Scientific Research (NWO). NSG publication no. 941005.

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