Terrestrial Palaeoecology and Global Change

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Chapter 7. Climate change 229 Table 2. Stomatal lengths in the Triassic to Early Cretaceous bennettitalean leaves (Harris, 1932, 1969; Krassilov, 1967a, 1982; Doludenko & Orlovskaya, 1976; Barale, 1981; Watson & Sincock, 1992). Greenland Rhaetian Yorkshire Bajocian Taeniozamites Dictyozamites groenlandicus 22 hawellii 25 T.jourdeyi 30 Ptilophyllum pecten 37,5 Karatau Callovian Sphenozamites sphenozamioides 37,5 Anomozamites kornilovae 30 T.ajorpokensis 30 P.hirsutum 30 Nilssoniopteris aff vittata 65 France Kimmeridgian England Berriassian Otozamites reglei 36 Pseudocycas saportae 61 Mongolia Barremian Nilssoniopteris denticulata 21,3 Zamites pumilio 48 P.roemeri 63,75 Otozamites lacustris 28,4 Z.carpentieri 29 P.lesleyae 60 Neozamites verchojanicus 18 Anomozamites marginatus 40 P. pectinoides 45 N.boroldaica 50,6 Z.dubisensis 48 Otozamites titaniae 53,75 A.hartzi 25 Otozamites penna 32 N.karataviensis 28,5 Z.feneonis 52 Zamites carruthersii 36,25 Pterophyllum cf. angulatum 28,3 Primo Apti Nilssoniopte rhitidorachi N.robusta 30 Ptilophyllum 30 Zamiophyllu buchianum Zamites bor A.minor 25 O.leckenbyi 32 Z.distractus 52 Z.manoniae 38,75 Mean 24.0 Otozamites A.amdrupiana 30 O.thomasii 32,5 Mean 35.0 Z.dowellii 40 Dictyozamit grossinervis A.nitida 35 O.beanii 35 Mean 44.2 Z.wendyellisae 43,75 Pterophyllum sutschanens Pterophyllum hanesianum 30 O.gramineus 36 Z.corderi 56,25 Cycadolepis pterophylloi P.astartense 25-30 O.venosus 40 Z.nicolae 40 P.schenki 28 O.simpsonii 42 Z.tatianae 46,25 Mean 39.0 P.rosenkrantzi 20 O.marginatus 50 Z.notokenensis 56,25 P.pinnatifidum 40 Zamites gigas 48,7 Mean 44.9 P.kochi 20 Nilssoniopteris major 34,4 P.ptilum 20 N. pristis 37,5 P.zygotacticum 20 Anomozamites thomasii 35 P.xiphipterum 16 Pterophyllum cycadites 30 P.subaequale 30 P.fossum 40 Otozamites sp. 22 P.thomasii 47 Mean 27.0 Mean 37.3
230 Valentin A. Krassilov. Terrestrial Palaeoecology confirmed by the parallel variation of the cycadophyte index, a proxy of temperature changes over the Mesozoic (Krassilov, 1973b; Fig. 85). Dispersion of stomatal measurements has been calculated upon the assumption that a low level of atmospheric CO 2 would have caused a wider variation. The maximal dispersion was obtained for the Karatau assemblage but the next low in the mid-Neocomian coincides with a relatively narrow dispersion. Perhaps a more meaningful correlation will emerge with accumulation of data. A qualitative phytogeographic approach (Krassilov, 1997c) is based on a correlation of atmospheric/soil pCO 2 with plant tolerances in respect to other limiting factors. In particular, a water uptake by photosynthesis typically decreases with elevation of atmospheric pCO 2. In effect, the greenhouse conditions would have rendered plant species more tolerant of water deficit. Their geographic ranges would have been less dependent on precipitation. Since phytogeographic zonation is controlled both by temperature and precipitations, a reduced water uptake would make zonal boundaries more transparent, resulting in a mixing of zonal elements. An azonal plant distribution is thus a phytogeographic testimony of greenhouse climate. In the fossil plant record, the distinctness of the boundary between the mesotemperate deciduous and xerothermic evergreen zones varies with climate fluctuations. Smearing of the boundary is recorded for the relatively short “azonal” intervals in the Early Carboniferous (Visean), Early Triassic (Olenekian) and the Early-Middle Eocene, the supposed greenhouse episodes (VII.4). CO 2 trends and feedbacks. As discussed above, the atmospheric CO 2 levels depend not so much on the total amount of carbon released from the earth’ interior, as on its reallocation between the atmospheric, lithospheric, oceanic and biotic reservoirs. A directional change of atmospheric CO 2 concentration over times would have occurred with directional evolution of these reservoirs or any of them. Net CO 2 fluctuations result from a disbalance of multiple sinks and sources, their interaction being affected, among other variables, by the air temperatures and the SST. The time-average atmospheric CO 2 levels are sustained, with short-term fluctuations, by the negative feedbacks upon the temperature → CO 2 → temperature scheme. There is little doubt about the role of outgassing of the earth’s interior in the origin and regulation of atmosphere before the biospheric regulating mechanisms came to action and took over, of which the Proterozoic banded ore formations might have been an early evidence. Sea-level fluctuations, with an uncertain directional component, but probably of a deminishing amplitude since stabilization of continental crust over the Precambrian fold belts, administer a control over the sink rates of atmospheric CO 2 to deep-water circulation, terrestrial biomass and carbonate deposition. During the global regressions, both terrestrial biomass and chemical weathering increased with land area, consuming more CO 2 while, simultaneously, deep-water circulation was enhanced by shutting-down an influx of warm saline waters from epeiric seas. Transgressions inflicted the opposite trends (Fig. 94).
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Terrestrial Palaeoecology and Global Change

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