Permo-Pennsylvanian Palaeotemperatures from Fe-Oxide and Phyllosilicate Δ18o Values ⁎ Neil J
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Earth and Planetary Science Letters 253 (2007) 159–171 www.elsevier.com/locate/epsl Permo-Pennsylvanian palaeotemperatures from Fe-Oxide and phyllosilicate δ18O values ⁎ Neil J. Tabor Department of Geological Sciences, Southern Methodist University, Dallas, TX, 75275-0395, United States Received 13 February 2006; received in revised form 9 October 2006; accepted 11 October 2006 Available online 21 November 2006 Editor: H. Elderfield Abstract The oxygen isotope composition of fossil roots that have been permineralized by hematite are presented from eight different stratigraphic levels spanning the Upper Pennsylvanian and Lower Permian strata of north-central Texas. Hematite δ18O values range from −0.4% to 3.7%. The most negative δ18O values occur in the upper Pennsylvanian strata, and there is a progressive trend toward more positive δ18O values upward through the lower Permian strata. This stratigraphic pattern is similar in magnitude and style to δ18O values reported for penecontemporaneous authigenic palaeosol phyllosilicates and calcites, suggesting that all three minerals record similar paragenetic histories that are probably attributed to temporal palaeoenvironmental changes across the Late Pennsylvanian and Early Permian landscapes. Palaeotemperature estimates based on paired δ18O values between penecontemporaneous hematite and phyllosilicate samples suggest these minerals co-precipitated at relatively low temperatures that are consistent with a supergene origin in a low-latitude soil-forming environment. Hematite–phyllosilicate δ18O pairs indicate (1) relatively low soil temperatures (∼24±3 °C) during deposition of the upper Pennsylvanian strata followed by (2) a considerable rise in soil temperatures (∼35–37±3 °C) during deposition of the lowermost Permian strata. Significantly, δD and δ18O values of contemporaneous phyllosilicates provide single mineral palaeotemperature estimates that are analytically indistinguishable from temperature estimates based on hematite– phyllosilicate oxygen isotope pairs. The results between the two temperature-proxy methods suggest that the inferred large temperature change across the Upper Pennsylvanian–Lower Permian boundary might be taken seriously. If real, such a significant climate change would have undoubtedly had far-reaching ecological effects within this region of Pangaea. Notably, there are important lithological and palaeobotanical changes, such as disappearance of coal and coal swamp floras, across the Upper Pennsylvanian–Early Permian boundary of north-central Texas that may be consistent with major climatic change toward warmer conditions. © 2006 Elsevier B.V. All rights reserved. Keywords: Hematite; Phyllosilicate; Oxygen isotope equilibrium; Palaeotemperatures; Permian 1. Introduction mineralogical component of ancient sedimentary strata [1]. Many recent studies have demonstrated that the Fe(III) oxyhydroxides commonly form in near- isotopic composition of Fe(III) oxyhydroxides, as well surface sedimentary environments and are an abundant as other minerals that typically form in low temperature environments such as calcite and phyllosilicate, may ⁎ Tel.: +1 214 768 4175. provide important palaeoenvironmental information E-mail address: [email protected]. related to the conditions of mineral formation in the 0012-821X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2006.10.024 160 N.J. Tabor / Earth and Planetary Science Letters 253 (2007) 159–171 Fig. 1. Pangaean Continental Reconstruction and Permo-Pennsylvanian physiography (∼300 Ma BP). The palaeogeographic position of the north- central Texas study area is marked by the square symbol along western Pangaea, south of the equator. Modified from [47]. (B) Stratigraphic and Geological map of the field area in north central Texas. The stratigraphic column (left) shows epoch and stage boundaries according to conventionalisms for West Texas, whereas the thin dashed lines upon the geological map (right) show stage boundaries according to the International Geological Timescale [48] based upon conodont biostratigraphic zonation in north-central Texas strata (B.Wardlaw, pers. comm., 2005). Bold letters A, B, C, D, E, F, G, H mark approximate locations from which Fe-oxides were sampled in this study. See text for discussion. N.J. Tabor / Earth and Planetary Science Letters 253 (2007) 159–171 161 early burial environment. In particular, these minerals 2. Geologic background can provide information about the δ18O values of local meteoric water and temperature of crystallization 2.1. Lithostratigraphy [1–20]. In this regard, pedogenic Fe(III) oxides and phyllo- The dominantly terrestrial Upper Pennsylvanian and silicates have the potential to provide isotopic records of Lower Permian succession of north-central Texas was palaeoclimate, and palaeoclimatic change, through a deposited in three broad depositional belts (lower and succession of sedimentary rocks. To date, however, upper coastal plain, and piedmont facies) distributed there has been no systematic stratigraphic analysis of across the low-sloping eastern shelf of the Midland basin. oxygen isotope compositions of co-existing Fe-oxides The succession comprises ∼1100mofupwardfining and phyllosilicates preserved in sedimentary strata fluvio-alluvial cycles [21] and contains abundant, well- [17,20]. developed palaeosols [17,22]. The study area remained in This study presents mineralogical, chemical and the western equatorial region of Pangaea, within 5° of the oxygen isotope compositions of Fe(III) oxides from equator, throughout Permo-Pennsylvanian time [M. Upper Pennsylvanian through Early Permian sedimen- Steiner, unpublished data; 23]. Fe (III) oxides occur as tary strata of the eastern Midland basin, north-central permineralized plant fossils of in-situ root systems within Texas, U.S.A. These new Fe-oxide data are compared mudstones and claystones of overbank-floodplain depos- with previously published oxygen isotope values of its (Fig. 2). Reflected light microscopy of these samples penecontemporaneously formed pedogenic phyllosili- show preservation of original cellular root morphology of cates of the eastern Midland basin to investigate the secondary xylem (Fig. 2). This sort of preservation (1) isotopic models of mineral formation in the early suggests these roots were likely permineralized soon after burial environment, (2) the feasibility of mineral-water death, in the very early burial environment, prior to any oxygen isotope fractionation equations, and (3) Permo- significant humification of the woody root material [24]. Pennsylvanian palaeoclimate over western equatorial Regionally extensive fluvial sandstone sets [21] are Pangaea (Fig. 1A). intercalated with marine limestone marker beds throughout Fig. 2. (A) Photographic image of permineralized fossil root structures (Sample E). Scale is marked in millimeters. (B) Reflected light image of polished transverse section from sample E showing permineralized celluar structure of the root system. Scale bar is 1 mm. (C) Magnified reflected light image of permineralized cells of the secondary xylem in (B). Dark areas define cell walls, whereas light areas define cellular lumen. Field of view is 1 mm. (D) Close up of C. Light circular area is ∼0.1 mm across. See Text for discussion. 162 N.J. Tabor / Earth and Planetary Science Letters 253 (2007) 159–171 the succession and define a high-resolution stratigraphic Table 1 framework for correlation throughout the study region. Name, unit, and position of the Fe-oxide samples, and mineralogy as determined by X-ray diffraction Upper Pennsylvanian (and/or Carboniferous) and Lower Permian epoch and stage boundaries have been identified Sample name Stratigraphic Stratigraphic position Mineralogy on the basis of conodont and fusilinid biostratigraphy of unit (m)1 intercalated marine rocks in the north-central Texas strata H Clear Fork Gp. 850 Hematite (B. Wardlaw, pers. comm., 2005; Fig. 1B). The Pennsyl- G Clear Fork Gp. 805 Hematite – F Clear Fork Gp. 760 Hematite vanian Permian boundary [301± 2 Ma; 25] occurs within E Clear Fork Gp. 710 Hematite the upper Markley Formation of the Bowie Group D Nocona Fm. 315 Hematite (Fig. 1B). C Markley Fm. 230 Hematite, Goethite, 2.2. Previous palaeoclimate inferences Kaolinite B Markley Fm. 205 Hematite A Markley Fm. 180 Hematite Palaeosols are an important stratigraphic component of the Upper Pennsylvanian and Lower Permian succession of north-central Texas [22,26]. Tabor and Montañez floras across the Permo-Pennsylvanian landscape [26]. [17,22] defined eight pedotypes [27] in Permo-Pennsyl- Nevertheless, temperature estimates based upon phyllo- vanian strata of north-central Texas based on palaeosol silicate δ18OandδD values are not without uncertainties. macro- and micromorphological characteristics. Those Necessary assumptions that must be made for these studies interpreted the stratigraphic distribution of pedo- palaeotemperature estimates from phyllosilicate include types to record a relatively rapid transition from humid (1) chemical equilibrium with meteoric water during conditions typical of the Late Pennsylvanian to signifi- Permo-Pennsylvanian pedogenesis, (2) knowledge of the cantly drier conditions in the earliest Permian. A climate phyllosilicate-water oxygen isotope fractionation factors trend toward increasingly drier conditions is recorded and (3) little or no post-pedogenic alteration that would throughout the Lower Permian by changes in the strati- have resulted in isotopic change