Journal of South American Earth Sciences 32 (2011) 96e109
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Early Cretaceous coniferous woods from a paleoerg (Paraná Basin, Brazil) Etiene Fabbrin Pires a, *, Margot Guerra-Sommer b, Claiton Marlon dos Santos Scherer b, Adriano Rodrigues dos Santos c, Edivane Cardoso c a
Ciências Biológicas, Campus de Porto Nacional, Universidade Federal do Tocantins, Rua 07 quadra 15, s/n, Bairro Jardim dos Ipês, 77500-000 Porto Nacional, TO, Brazil CNPq, Departamento de Paleontologia e Estratigrafia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul, 91501-970 Porto Alegre, RS, Brazil c Museu de Minerais e Rochas, Instituto de Geografia, Universidade Federal de Uberlândia, 38400-902 Uberlândia, MG, Brazil b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 13 May 2010 Accepted 3 April 2011
We report here the occurrence of an assemblage of silicified coniferous wood within aeolian sandstones of the Early Cretaceous Botucatu Formation, a paleoerg along the margins of the Paraná Basin (Brazil). The characteristics of this monotypic wood assemblage indicate the occurrence of some more humid periods during the prevailing arid climate of a semi-desert biome. The growing conditions were seasonal and stressed during the life cycle of the trees. Quantitative parameters controlling growth ring development might be closely related to local environmental conditions and not just only a single consequence of climate. The conifer assemblage, developed in an arid desert biome during the climax of a greenhouse phase with increasing atmospheric CO2 does not have modern analogues in present icehouse world. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Dendrology Early Cretaceous Equatorial belt Botucatu Formation e Paraná Basin Paleoerg
1. Introduction During the Cretaceous the configuration of the continents changed from the prevailing Mesozoic pattern of two supercontinents (Laurasia and Gondwana) straddling an equatorial ocean (Tethys), to one with several continents separated by oceans which extend into high latitudes. Different geological clues give evidence that this period can be characterized as an extended greenhouse interval with diverse biota scattered across a variegated mosaic of land, shallow sea and ocean. In the Early Cretaceous, the South AmericaeAfrica land mass was included in the TropicaleEquatorial Hot arid belt (Chumakov, 1995), or in the Arid climatic belt, according to the palaeoclimatic map of Scotese (2003), without evidence of sufficient moisture to have supported rainforests. Nevertheless, it was shown by Doyle et al. (1982), Frakes et al. (1992) and Vakhrameev (1991), that the conditions were not monotonously equable, and the Cretaceous climate was marked by distinct latitudinal provinces. A generalized temperature curve of Martin (1995) estimates for this interval a climatic coolewarm interval during the evolution of a greenhouse phase. The Southern
* Corresponding author. Tel.: þ55 63 3363 0527; fax: þ55 63 3363 0500. E-mail addresses:
[email protected] (E.F. Pires),
[email protected] (M. Guerra-Sommer),
[email protected] (C.M.S. Scherer),
[email protected]. br (A.R. Santos),
[email protected] (E. Cardoso). 0895-9811/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.jsames.2011.04.001
Hemisphere was hotter and more arid than the Northern Hemisphere that was characterized by a seasonally dry and monsoonal climate. From combined flora and lithological data, Rees et al. (2000) established a palaeoclimate model for the Jurassic, recognizing five main biomes, from Early to Late Jurassic, with seasonally dry biomes in low latitudes. Studies of Pires and Guerra-Sommer (unpublished data) have yielded important qualitative and quantitative information about periodicity of wood production during the early Early Cretaceous (Berriasian) in the equatorial belt. This wood association was developed during the deposition of a large shallow basin, the AfroBrazilian Depression (Missão Velha Formation, Araripe Basin). Despite warm temperatures at this low latitude (more or less 8 S) dendrological data indicate that the climate was characterized by cyclical alternation of dry and rainy periods influenced by cyclical precipitations. However the climatic seasonality did not have homogeneous rhythm that was probably originated by year to year variability characterized by frequent disturbances in rainfall. The patterns of growth rings were considered as consistent with a tropical savanna climate included in the SummereWet biome of Rees et al. (2000), during a greenhouse climate phase. During the TithonianeBerriasian the fragmentation of Gondwana was intensified. While rift basins were formed in the southern and northeastern portions of South America, a wide topographic area of a cratonic basin (Paraná Basin) accumulated a thick package of aeolian dune deposits (Scherer and Goldberg, 2007). The accumulation of the Botucatu Formation at the southern part of the basin
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Fig. 1. Location map showing fossiliferous locality, A. an overview, showing the fossiliferous locality localization in the Paraná Basin; B. detail view of the Fazenda Sobradinho outcrop.
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represents, according to Scherer and Lavina (2006), a period of climate stability associated with hyper arid conditions. Vertebrate ichnofossils are common in the northeastern and northwestern parts of the basin. The theropod tracks comprise an endemic fauna of bipedal dinosaur of larger and smaller types, theropodian and ornithopodian, along with theromorphoid and mammaloid forms (Leonardi et al., 2007). An Early Cretaceous age (sensu Gradstein et al., 2005) for the end of accumulation of the Botucatu Formation was based on radiometric dating (Ar/Ar) obtained in the basalts of overlying Serra Geral Formation (Onstott et al., 1993; Turner et al., 1994). The present investigation documents the record of an assemblage of large silicified woods with well preserved growth rings in an outcrop of the Botucatu Formation (a paleoerg) interbedded in basaltic layers, at the northeastern margin of the Paraná Basin (Minas Gerais State e Brazil). Taking into account the available data and its effect on the model for climatic evolution of the Botucatu Formation of Paraná Basin at the Early Cretaceous, the aim of the present contribution is to: 1. record the presence of a hypoautochthonous-autochthonous wood assemblage; 2. document dendrological analysis of growth rings, 3. offer palaeoclimatic inferences and improve previous palaeoclimatic interpretations for the studied interval. It is beyond the scope of this paper to discuss in detail the lithofacies data; nevertheless some of the geological factors that probably controlled the water availability are briefly discussed. 2. Palaeogeographic and geological setting The Jurassic was characterized by the fragmentation of Pangaea. In the Late Jurassic to Early Cretaceous interval, the separation between Gondwana and Laurasia was well in progress, and a wide ocean had already developed between those continents (Scotese, 2003). Moreover, the eustatic sea level was high during this time interval, flooding large areas of Laurasia and allowing the development of extensive epicontinental seas. In Gondwana, however, marine deposits apparently formed only at the west margin along a narrow strip of the retro arc region of the Andes, while the continental interior displayed exclusively continental sedimentation. The absence of epicontinental seas on inland Gondwana is a result of the continental palaeotopography. A wide plateau with an elevation in excess of 100 m developed due to the high thermal flux, and consequently high continental freeboard, analogous to that observed in Africa today (Worsley et al., 1984). The period between the end of the Jurassic and the beginning of the Cretaceous was characterized by the fragmentation of Gondwana with the development of continental rift systems leading to
the formation of the South Atlantic. Rift basins were formed in the southern and northeastern portions of South America and a wide intracratonic area known as Paraná Basin accumulated a thick package of aeolian dune deposits. The Paraná Basin (1,700,000 km2) (Fig. 1), located in southeast South America, has six depositional supersequences (Milani, 1997) originated by second order eustatic and tectonic events. These supersequences, from base to top are: 1) Rio Ivaí (OrdovicianeSilurian), 2) Paraná (Devonian), 3) Gondwana I (Carboniferous e Early Triassic), 4) Gondwana II (Late Triassic), 5) Gondwana III (Jurassic e Early Cretaceous), 6) Bauru (Late Cretaceous). The Gondwana III (Jurassic e Early Cretaceous) supersequence (Fig. 2) comprises the basal Botucatu Formation, object of the present study, and the overlying 300e1,700 m thick succession of volcanic rocks of the Serra Geral Formation. The Botucatu Formation consists of aeolian sandstones that crop out along the borders of the Paraná Basin. The Botucatu paleoerg (sand sea) stretched out to the southwest. In Uruguay, the aeolian package is called the Rivera Formation (Ferrando and Montana, 1988) whereas in Argentina (Chaco e Paraná Basin) it is known as the San Cristobal Formation (Padula and Migramm, 1969; Pezzi and Mozetic, 1989; Garrasino, 1995). At the western portion of the basin, the sandstones of Botucatu Formation are found in Paraguay, where they are called the Missiones Formation (Clerici et al., 1986). The northern and northeastern limits of the Botucatu outcrop area are located in Brazilian states of Mato Grosso, Goiás and Minas Gerais. The question about an erosive or depositional nature of these deposits is unclear (Bigarella, 1979). A transgression of the aeolian sediments of the Botucatu Formation directly over Precambrian rocks in the regions of Estrela do Sul and Monte Carmelo, Uberlândia and Tupaciguara (Minas Gerais) was observed by Hasui (1969). There is no evidence of a physical continuity between the “Botucatu Basin” and the rift basins in northeastern Brazil, which suggests that topographic highs physically separated the aeolian sandstones of the Botucatu Formation from the rift basins. The Botucatu Formation is defined at the base by a regional unconformity that can be traced across the entire basin (Milani, 1997). It is composed of dominantly aeolian deposits, represented by large scale sets of cross strata (1e30 m) interpreted as aeolian dune deposits (Almeida, 1954; Bigarella and Salamuni, 1961). The lowermost deposits immediately above the basal contact locally contain conglomerates and gravelly sandstones deposited by ephemeral streams, as well as coarse-grained sandstones interpreted as aeolian sand sheets deposits (Bigarella and Salamuni, 1961; Soares, 1975; Almeida and Melo, 1981; Scherer, 2002). The thickness of Botucatu Formation varies from 0 to 800 m, the greatest thickness occurring in the northwestern portion of Paraná
Fig. 2. Stratigraphic chart highlighting Cretaceous of the Paraná Basin (Modified from Milani, 1997).
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Basin (Milani, 1997). Lava flows covered the previously active aeolian dunes in the erg, thereby entirely preserving their morphologies (Scherer, 2002). The onset of sedimentation in the Botucatu Formation is mainly inferred by vertebrate ichnofossils (Bonaparte, 1996; Leonardi et al., 2007) suggesting an age of Late Jurassic e Early Cretaceous. Furthermore, Scherer (2000) based on close relation between the aeolian sandstones, lava flow deposits and the lack of regional unconformities, indicates for the onset of the aeolian sedimentation a later age. Radiometric dating (Ar/Ar) has previously been carried out in volcanic rock samples of the Serra Geral Formation, which overlays Botucatu Formation, in the Alto Piquiri (Onstott et al., 1993). The resulting ages span a range from 136.6 1.5 to 130.8 Ma. Additionally, Turner et al. (1994) obtained an age of 137 0.7 Ma for the oldest volcanic rocks, which suggests an earliest Cretaceous age for the termination of aeolian accumulation in the Botucatu Formation. Roisenberg (2000) and Scherer and Lavina (2006), based on many ages stipulated by radiometric dating (Turner et al., 1994; Renne et al., 1996; Seward and Kerrick, 1996) estimated an age of 138e128 Ma for the deposition of Serra Geral volcanic rocks in the Rio Grande do Sul and Paraná States. Holz et al. (2007) assert that a mean value of 132 Ma supplies a reliable chronological reference for the end of the Botucatu sedimentation. According to Holz et al. (2007), the 10 Ma spread of numerical ages may reflect the difference from the earliest to the latest flows in a thick basalt pile in the central part of the basin. This indicates that in different parts of the basin the onset of the lava flows may have happened in different times. Scherer and Goldberg (2007) interpreted the direction of the sediment transport in the aeolian sandstones of the Botucatu Formation on the basis of cross strata dip directions which allowed the reconstruction of regional wind patterns in middle e western Gondwana. Regionally, cross strata dip directions indicate variations of paleowind directions across the outcrop area of the Botucatu paleoerg. The northern portion of the paleoerg (palaeolatitute < 20 ) was characterized by paleowinds blowing from the north, whereas the southern portion was under influence of paleowind coming from the SW. 2.1. The wood-bearing site The wood-bearing Fazenda Sobradinho (18 460 56,500 S/ 48 160 0,2 E e altitude 657 m), previously registered by Suguio and Coimbra (1972) and Brito (1979), lies between the cities of Uberlândia and Araguari (Minas Gerais State), bordering the railroad (Fig. 1) with a total area of about 15,000 m2. The exposed sedimentary sequence of 5 m is inserted between a subjacent metamorphites of Araxá Group (Precambrian) and superjacent lava flow layer of the Serra Geral Group. These sequences demonstrate a prominent aeolian facies, composed of a succession of medium to coarse-grained sandstones with trough cross-bedding where silicified woods are the exclusive fossils. The area of outcrops makes a total of 1000 m2 throughout the reconnaissance area (Fig. 1), where silicified woods are mainly included in coarse-grained sandstones (Fig. 3) or occur as rolled fragments over the soil. The most common assemblage comprises horizontal compressed wood fragments included in coarse-grained sandstones presenting cross strata and in saprolite sandstone (Fig. 4A). Five (5) large specimens, segmented, but showing continuity for up to 5 m were recovered. Apart from the large specimens, seventeen small compressed fragments (length up to 30 cm) were also observed (Fig. 4BeD). Buried specimens of elongate, irregular (diameter: 30 cm), sub-horizontal, crosscutting coarse-grained strata were also
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Fig. 3. Silicified wood fragment included in cross-bedded, coarse-grained sandstone.
recovered. Flared stumps that were not compressed were found as rolled forms over the soil (Fig. 4B). 3. Dendroclimatological analysis 3.1. Coniferous woods Based on external morphology the fossils studied here were classified in trunks, flared basal stumps and roots. The most common wood fragments are horizontally and dispersed large trunks (5 m high), with compressed diameters (diameters up to 80 cm) showing transverse and longitudinal compression bands (Fig. 5A), and branch scars (Fig. 5B). Few specimens contain preserved pith, without preserved cortex. Flared stumps, not compressed, are also present, but uncommon, with diameter of 30 cm, showing at the basal portions the area of insertion of the inferred root system (Fig. 4B). Two fragments of elongate woods (1 m in length), not compressed, showing sub-horizontal disposition, with irregular arched morphology cut across the coarse-grained strata (Fig. 4CeE). By external morphology and orientation these woods seem to represent in situ preserved roots that would indicate the original forest level, but the obliteration of anatomical detail by quartz deposits precludes such determination. Nevertheless, their relationship with any vertical basal stump was not evidenced. The presence of both, basal flared stems and putative “in situ” roots points to a hypoautochthonousautochthonous deposition for this wood assemblage. 3.2. Wood anatomical pattern Anatomic details were observed from thin slides on transmitted light optical microscopy. Thin slides were prepared along transverse, radial longitudinal and tangential longitudinal sections. The samples are stored in the Laboratory of Palaeobotany, Departamento de Estratigrafia e Paleontologia, Instituto de Geociências, Universidade Federal do Rio Grande do Sul, Porto Alegre, Rio Grande do Sul. Given the lack of cell structure in most fossils, only 10 mature woods were selected for detailed analysis. The wood is mostly silicified and light brown (Figs. 3e6). Despite the extremely poor preservation of wood anatomy, it is evident that all the specimens
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Fig. 4. A. prostrate fossil wood compressed and segmented included in altered sandy soil; B. basal flared stump; C, D, E. putative in situ main axis of roots, C. included in sandy soil (alteration product of the sandstone); D. another view of C, showing the sandstone blocks surrounding the fossil wood; E. included in saprolite sandstone.
correspond to gymnosperm wood, probably assigned to the Coniferales. In the absence of well preserved diagnostic structures, the woods were classified in a xylotype based on Page’s method (Page, 1979, 1980, 1981). One single xylotype was documented in almost all the specimens, characterizing a monotypic plant association. The pinoid xylotype (sensu Wheeler and Lehman, 2005), represented in Figs. 7 and 8 was recognized by parameters such as: round tracheids in transverse section (Fig. 7AeC); latewood characterized by reduction of radial size, thickened walls are not observed (Fig. 7C); vertical secretory channels with walls not thickened, concentrated in the boundary of growth rings (Fig. 7A, B, E); horizontal (Fig. 7D, F) and vertical secretory ducts perceivable (Fig. 7E); radial pitting of tracheids of mixed type of arrangement (Fig. 8D); low to medium rays (Fig. 8A) composed of up to 12 cells, heterogeneous, composed of radial parenchyma and transversal tracheids (Fig. 8B, C); crossfield pitting mainly indistinct; when observable they are of pinoid type (Fig. 8E). The main xylotype characteristics are present in Paleopinuxylon josuei Mussa, described for this outcrop included in the Protopinaceae, according to the criteria of Vogellehner (1967)
and Kräusel (1949) (Mussa, 1974). The phylogenetic position of woods placed in that group is not yet understood, but has been considered by some authors as a basal and extinct branch of the Coniferales. 4. Analysis of growth rings 4.1. Methodology Growth rings were measured from transverse polished sectioned blocks of the silicified wood using binocular stereoscopic microscopy or were measured with paquimeter directly on the polished wood surface. Details of growth ring structure were obtained by the observation of thin slides on transmitted light optical microscopy. Statistical procedures used in the analyses of growth rings from the 10 fragments of fossil wood followed those of Fritts (1976): based on Douglass (1928), Creber (1977), Creber and Chaloner (1984), Parrish and Spicer (1988) and Denne (1989). These include the
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Fig. 5. Lateral views of fossil woods showing A. transverse and longitudinal compression bands, and B. branche scars.
variance of ring width, Mean Sensitivity (MS), Annual Sensitivity (SA) and growth ring classification scheme. Earlywood/latewood boundary has been determined by different methods. Creber and Chaloner (1984) determined the boundary by the cumulative sum deviation from the mean (CSDM) radial cell across a single growth ring. Research in extant plants uses the ratio of cell wall thickness to lumen diameter to establish growth ring boundary (Mork, 1928; Denne, 1989). These two techniques were applied and produced different results for percent latewood.
4.2. Results Statistical data where obtained from 119 growth rings in 10 samples. The Mean Sensitivity ranges from 0.288 to 0.862; the average of Mean Sensitivity is 0.569; the minimum annual sensitivity is 0 and the maximum annual sensitivity is 1.729 (Table 1). The maximum annual sensitivity exceeds 0.7 in almost all samples. The annual sensitivity of the specimens increases year after year in samples 1302-09, 1306-02, 1306-20, PB 4312, and PB
Fig. 6. Cross sections of fossil woods, A. central portion showing circular pith and fractures filled by quartz; B. growth rings and a false growth ring (arrow); C. boundary of growth ring with secretory channels; D. sequence of growth rings showing the width variability.
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Fig. 7. Thin-sections of fossil woods, A, B, C. transverse sections, A. growth ring boundary with secretor channels; B. detail of Fig. 7A; C. earlywoodelatewood transition showing reduction of radial size of tracheids; wall thickening not observed; D, E, F. radial sections; D. bisserial sub-opposite bordered pits and horizontal secretor ducts; E. secretor channel and unisserial bordered pits in tracheids wall; F. cross-field with horizontal ducts.
4317, decreases in samples 1306-19, PB 4311, PB 4313, and PB 4314, and remains stable in PB 1306-25 (Figs. 9 and 10). All the specimens have growth rings (Fig. 6B) with range from 0.09 to 1.22 cm wide (mean: 0.37 cm). The widths of the growth rings vary from year to year (Fig. 6D, Fig. 9); this feature is common in all specimens. The transition early-latewood is gradual (Fig. 6C), according to the criteria of Creber and Chaloner (1984). The earlywood is characterized by thin-walled cells, and latewood is composed of 2e4 narrow cells. Using the statistic patterns of Denne (1989) based in Mork (1928) it becomes clear that the growth ring was characterized only by the decrease of the cell lumen, without thickening of the cell walls, like demonstrated in graphs of Fig. 11.
False growth rings are abundant in almost all samples (Fig. 6B). They may be distinguished from seasonal growth rings because they have a gradual transition to thick-walled narrow cells then a gradual reversal to large thin-walled cells (Spicer, 2003). 5. Discussion Simulations of the Early Cretaceous climate indicate that arid desert biomes associated with hyper arid conditions have prevailed at the southern lower latitudes during the pre-rift phase of Pangaea. In this climate, characterized by high stability, the equatorial zone is markedly dryer than today, with large continental interiors. Much of this land mass would be remote from moisture sources.
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Fig. 8. Thin-sections of fossil woods, A, B. tangential sections; A. with uniseriate rays; B. a vertical secretor duct; C, D, E. radial sections; C. uniseriate ray, composed of parenchyma (arrow) and tracheid cells; D. mixed type of bordered pits on tracheid walls; E. cross-field with poorly preserved pitting (pinoid?).
Paleofloras composed of ferns, conifers, and cycads are registered in low and high latitudes in both hemispheres. Due to latitudinal spread, these plant assemblages were interpreted as representing warm temperate climates in the past (Smiley, 1967; Douglas and Williams, 1982; Jefferson, 1982; Frakes et al., 1992; Chumakov, 1995; Scotese, 2003).
Palaeontologic data for the Botucatu Formation, until now, were constrained to dinosaurs tracks (Bonaparte, 1996; Leonardi et al., 2007). According to Willis and McElwain (2002) the absence of fossil floras together with an abundance of wind-blown (aeolian) sediments supports the presence and the extension of this subtropical-desert biome.
Table 1 Results of growth ring analysis. Samples 1 1302-09 2 1306-02 3 1306-19 4 1306-20 5 1306-25 6 PB 4311 7 PB 4312 8 PB 4313 9 PB 4314 10 PB 4317 Total/mean
Number of growth rings
Minimum ring width (cm)
Maximum ring width (cm)
Mean ring width (cm)
Mean sensitivity MS
Minimum annual sensitivity
Maximum annual sensitivity
16 21 06 04 11 08 10 08 10 25 119
0.19 0.09 0.50 0.32 0.09 0.05 0.03 0.25 0.04 0.04
1.10 0.70 1.22 0.50 1.00 0.90 0.70 0.52 1.12 0.90
0.46 0.30 0.75 0.42 0.32 0.30 0.26 0.36 0.24 0.31 0.37
0.449 0.618 0.297 0.288 0.544 0.862 0.778 0.432 0.861 0.565 0.569
0.029 0.105 0.121 0.171 0 0.186 0.026 0.214 0 0
1.308 1.494 0.681 0.420 1.077 1.600 1.722 0.568 1.704 1.729
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Fig. 9. Growth ring width graphs for each specimen analyzed. x-axis represents the number of growth rings, which range from 4 to 22, and the y-axis represents the width of each growth ring in centimetres.
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Fig. 10. Annual sensitivity graphs for each specimen analyzed. x-axis represents the number of growth rings, and the y-axis represents the annual sensitivity.
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Fig. 11. A. Graph based on methodology of Creber and Chaloner (1984), showing the variation in cell radial diameter along selected growth ring (solid line) and cumulative sum of deviations from mean radial diameter (dashed line); vertical line separates earlywood (EW) from latewood (LW); B. graph based on methodology of Mork (1928) and Denne (1989), showing the variation in a single growth ring in relation to: i. cell radial diameter along selected growth rings (more thick line); ii. thickness of two adjacent tracheids walls (intermediary line); and iii. thickness of single tracheids walls (thin line); EW e earlywood, LW e latewood.
The presence of a dense association of hypoautochthonousautochthonous large conifer woods within a sedimentary sequence characterized by large scale cross strata, identified as a dry aeolian system developed within the Botucatu Formation, Paraná Basin, Brazil, contrasts with the climatic prediction of stratigraphic data (Scherer, 2000; Scherer and Lavina, 2006) and palaeowind patterns (Scherer and Goldberg, 2007), which indicate there is no evidence of moisture enough to have supported forests. Two specific features of the outcrop described here allowed us to infer autochthony conditions: 1) a restricted stratigraphic distribution, representing a microenvironment interlayered between metamorphic rocks of Araxá Group and basalt flows of Serra Geral Formation, and 2) the monotypic pattern of fossil woods. These features eliminate the possibility of taphonomic bias as the accumulation of specimens from the same taxon, transported from different localities. Paleoclimate inferences based on fossil woods analysis are established on different latitudes by several authors. Géerards et al. (2007) confirm Early Cretaceous palaeoclimate in tropical latitudes utilizing fossil wood analysis in Mons Basin (Belgium), characterizing the palaeoclimate by a succession of marked dry and wet seasons, under unstable palaeoenvironmental conditions. Evidence of seasonal climate in the southern mid latitude was also obtained with growth ring analyses (Jefferson, 1982; Falcon-Lang and Cantrill, 2000) in the Early Cretaceous (Alexander Island e Antarctic). These parameters indicate moderate to high seasonality. The unusual conditions which allow the generation of growth rings in plants close to the paleopoles and those from the tropics in Early Cretaceous, are generally interpreted in terms of a high carbon-dioxide greenhouse world (Chaloner and McElwain, 1997). On the other hand, Falcon-Lang (2005) recommends that the quantitative percentage latewood data and mean ring data no longer can be used as a palaeoclimatic indicator, based on the data processed from the International Tree Ring Data Bank that established a global climate analysis of growth rings in woods. According to the author, quantitative growth rings analysis of fossil woods may be used only in well-constrained palaeoecological studies where taxonomic and climatic sources of variability can be controlled, and additionally, as a qualitative tool in palaeoclimatic and palaeoecological analysis. Prior studies of Parrish and Falcon-Lang (2007) reported the widespread occurrence of silicified conifer roots, stumps and trunks within interdune deposits from southeast Utah (EUA), from the Navajo Sandstone Formation, which represents a large Early
Jurassic paleoerg. The authors report a (par)autochthonous assemblage associated with the deposits of spring-fed carbonate lakes. A few stumps preserved in growth position are rooted in aeolian sandstone immediately below the lake deposits, established on interdune soils in response to a rising water table. Allochthonous woods assemblages were associated with massive sandstone beds, that were interpreted as fluidized mass deposits. The assemblage is composed of pycnoxylic wood and represents a single morphotaxon which has similarities with Xenoxylon morrisonense Medlyn and Tidwell (1975), a Protopinaceae. The absence of growth rings indicates that there was little or no seasonal variation in temperature or water supply throughout the year. In this case study, the unexpected evidence supplied by large coniferous fossil woods with the presence of growth rings is striking. The low percentage of latewood without thickening of the cell walls, and the gradual transition of early-latewood allowed us to infer conditions interpreted to have manifested as accentuated growing periods. These parameters contrast with different palaeoclimatic models (i.e. Rees et al., 2000 e Fig. 12). The characterization of growth rings has led to identification of the type E proposed by Creber and Chaloner (1984). This kind of wood is recorded in middle latitudes at Early Cretaceous for both hemispheres, occurring with a higher frequency in the Southern Hemisphere. However, Brison et al. (2001) included this growth ring type in the “progressive woods” i.e. those with a progressive transition from earlywood to latewood. Nevertheless this development is also correlated with wood taxonomy, since some morphogenera (i.e. Cupressinoxylon and Protopodocarpoxylon) can only build this type of tree ring. Taking into account the abrupt decrease of the cell lumen, without thickening of the cell walls evidenced by the use of Mork method, the pattern of wood growth is unusual in extant floras developed under icehouse conditions. Fossil woods show uneven sequences of growth rings, indicating fluctuations in growing conditions over several growth periods. Reduction of water supply was materialized in thin growth rings in a single specimen. This clearly irregular disposition of tree rings can represent erratic variations in growth conditions. In this sense, false growth ring, a common characteristic in almost all the specimens, can reflect palaeoclimatic and palaeoecological constrains which can lead to an early near cessation of cambial growth and a renewal of growth within the same growing period. Therefore, severe droughts could be the controlling factor.
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a geographical position of the plant community on the paleoerg margin. The moisture in the northern part of the Paraná Basin in the studied time interval can be explained by monsoonal paleowind pattern that allowed the transport of humidity from Tethys Ocean to the interior of the Gondwana continent (Scherer and Goldberg, 2007). An important biological evidence is the presence of vertebrate ichnofossils in the northeastern and northwestern parts of the Paraná Basin, reported by Leonardi et al. (2007), which indicates that the region was not an extreme desert, and fresh water may have been present as streams and lakes, even ephemeral, given that these animals would require water. The limited sedimentological and stratigraphical data summarized here preclude accurate interpretations about the probable palaeoenvironment that allowed the development of the conifer trees in arid desert biome; nevertheless a working hypothesis for future studies is that of forested dune deposit by large conifer trees that could be buried by dune migration. 6. Conclusions
Fig. 12. Palaeoclimatic map from the JurassiceCretaceous transition (TithonianeBerriasian) with the localization of fossil wood assemblage of Botucatu formation (modified from Rees et al., 2000).
Different geological data indicate that the conifer assemblage was developed during the climax of a greenhouse phase (Fischer, 1984; Frakes et al., 1992; Berner, 2001), with increasing atmospheric CO2 (Tajika, 1999). Controlled experiments with vegetation in greenhouses developed by Ruddiman (2001) to test growth ring signals as tools for climate science show that plant growth is enhanced by higher levels of CO2. Thus CO2 fertilization may be a factor in faster tree growth in dry regions regardless of other climatic parameters. In extant floras, coniferous species (especially Pinaceae) are particularly drought tolerant, well adapted to growth in hot, dry semi-desert climates with greater resistance to water stress for survival to more xeric habitats (Atzmon et al., 2004; Richardson, 1998). Under nutrient poor, drought-stressed conditions, the efficiency of the vascular system of conifers, and their small leaves are advantageous, acting conservatively under potential damaging conditions (Kershaw and McGlione, 1995). Furthermore, Piñol and Sala (2000) showed for several extant Pinaceae in the Pacific USA that species extending into drier habitats compensated with less resistance to water cavitation via different adjustments of a physiological nature (stomata control) and structural nature, by the increased relative biomass allocation to sapwood. The particular endogenous and extrinsic factors that could be involved with the production of growth rings in the studied assemblage suggest that growth ring width is not related to latitude in a simple way, as it was previously supposed by Creber and Chaloner (1984). The palaeoenvironmental significance of the large conifer trees documented here is difficult to assess. Clearly a detailed study of the depositional cyclicity and facies analyses are required to confirm dendrological data, that point to wet episodes in a large paleoerg that supported localized plant communities in an otherwise generally arid climate zone. These wet deposits may reflect
1. The record of a coniferous wood association from aeolian sandstones at the northern portion of the Botucatu Formation (Paraná Basin, Brazil) at the Early Cretaceous is evidence of some moisture in the paleoenvironment. 2. The presence of basal flared stems and putative “in situ” roots points to a hypoautochthonous-autochthonous deposition for this wood assemblage. 3. The presence of growth rings as a common character indicates cyclical variation in tree growing conditions; nevertheless, a typical latewood zone characterized by wall thickenings and reduction of lumen size was not found. 4. The use of quantitative growth ring parameters as palaeoecological indicators at this exceptional fossil wood site allows for the inference of not only seasonality, but also inter-annual variability of growth. The growth conditions were seasonal, but most stressed and in some periods it presented an erratic full stoppage. Moreover; individual woods display growth rings of variable widths, and the common presence of false growth rings could be related to plant stress represented by droughts during the growing period. 5. Quantitative parameters controlling growth ring development were related with environment features, and not just a single consequence of climate. Thus growth rings were not determined by external factors alone; taxonomic and physiologic parameters were decisive as a response to environmental constrains. 6. Integration between dendrologic, palaeogeographic and sedimentological data allowed the detection of peculiar conditions within tropicaledesert climatic conditions in “greenhouse ecosystems” during the Cretaceous that have no modern analogues in the present “icehouse world”. Acknowledgements We acknowledge the contribution of the Museu de Minerais e Rochas, Instituto de Geografia, Universidade Federal de Uberlândia, MG, Brazil for field support and to Dr. Rita de Cassia Tardin Cassab, of the Laboratory of Paleontology (DNPM e Rio de Janeiro, RJ) for loaning some specimens. The research was financed from research projects and grants by CNPQ. We are grateful to two reviewers, anomymous review and Mariana Brea (Universidad Autónoma de Entre Ríos), and for Reinhardt A. Fuck, Regional Editor of Journal of South American Earth Sciences for their constructive comments
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