Norian vegetation history and related environmental changes: New data from the Chinle Formation, Petrified Forest National Park (Arizona, SW USA)

Fossil plant assemblages including spores and pollen grains provide useful information on past ecosystems and the response of terrestrial biotas to various environmental perturbations. New quantitative palynological data from the Chinle Formation of the American Southwest suggest that a floral turnover occurred in the middle Norian (between 217 and 213 Ma). Analysis of plant communities reveals that this turnover was followed by a complete reorganization of the riparian vegetation, driven by changes in fluvial styles and the tectonic regime of the basin, as well as a gradual transition toward a more arid climate. Marked increases in Klausipollenites gouldii, Patina­ sporites spp. , and Froelichsporites traversei are probable indicators of environmental stress, such as increased aridity, perturbations of atmospheric pCO2, acid rain, and atmospheric aerosol accumulation due to volcanism in connection with the Pangean rifting and uplift of the Cordilleran arc. Comparison of the vegetation turnover with younger assemblages from the Chinle Formation in New Mexico revealed similar floral turnover patterns, suggesting two distinct drier periods as a result of multiple climatic oscillations. The climate-induced floral turnover may have contributed to the vertebrate faunal turnover as the loss of wetland habitat space and an increase in xerophytic plants may have dwindled the supply of palatable vegetation for herbivores. The onset of the floral turnover in Arizona roughly corresponds to the Manicouagan impact event, but a direct causal link is still speculative. INTRODUCTION The Norian Chinle Formation (e.g., Irmis et al., 2011; Ramezani et al., 2011, 2014) provides a vegetation record of an ancient terrestrial ecosystem composed of riverine, lowland, and upland plant communities (e.g., Ash, 1972, 1999, 2005, 2014). The flora of the Chinle Formation provides an excellent opportunity to trace vegetation dynamics during the Norian Stage, which was marked by a series of environmental perturbations, including climate change (e.g., Kutzbach and Gallimore, 1989), carbon cycle perturbation (e.g., Prochnow et al., 2006; Cleveland et al., 2008a, 2008b; Schaller et al., 2015), a probable impact event (Hodych and Dunning, 1992; Ramezani et al., 2005), and changes in the tectonic regime in the western part of the North American continent (e.g., Dubiel, 1989; Dubiel and Hasiotis, 2011; Atchley et al., 2013; Howell and Blakey, 2013; Riggs et al., 2013; Trendell et al., 2013a, 2013b; Nordt et al., 2015; Riggs et al., 2016). Pollen and spore records provide a window into regional plant succession and are a useful tool for detecting vegetation dynamics in relation to global climatic oscillations or environmental stress. The palynological record can provide information on regional-scale vegetation changes, in contrast to the macrofossil rec ord, which can be biased to local vegetation elements often in wet habitats (Demko et al., 1998). The palynology of the Chinle Formation is known from the earlier works of, for example, Gottesfeld (1972), Scott (1982), Fisher and Dunay (1984), Litwin (1986), Litwin et al. (1991), and more recently Reichgelt et al. (2013), Whiteside et al. (2015), and Lindström et al. (2016). The Chinle Formation was originally thought to encompass the Carnian and Norian Stages based on biostratigraphy (Litwin et al., 1991; Heckert et al., 2007), but paleomagnetic correlations (Muttoni et al., 2004; Olsen et al., 2011) and recent radiometric dating indicate a middle to late Norian age (Riggs et al., 2003; Irmis et al., 2011; Ramezani et al., 2011, 2014). This new age assignment makes the Chinle floras of special interest, because continental Norian palynofloras are relatively uncommon (e.g., Kürschner and Herngreen, 2010). The Chinle Formation was deposited at tropical latitudes and subject to a monsoonal climate (Parrish and Peterson, 1988; Dubiel et al., 1991). Sedimentological and paleontological evidence indicates climate change with increasing aridity and seasonality throughout its depositional period (Dubiel and Hasiotis, 2011; Atchley et al., 2013; Nordt et al., 2015). This climate change is expected to have caused stress in the terrestrial ecosystems, including plants and animals (e.g., Whiteside et al., 2015). In triguingly, previous paleontological studies showed a significant faunal and floral turnover in the middle of the Chinle Formation between 217.7 and 213.1 Ma (e.g., Litwin et al., 1991; Parker and Martz, 2011; Reichgelt et al., 2013; Rame zani et al., 2014), which might be related to climate change. In addition to the climate, other environmental perturbations, for example, volcanism (Atchley et al., 2013; Nordt et al., 2015) and pCO2 variations (Cleveland et al., 2008a, 2008b; Atchley et al., 2013; Nordt et al., 2015; Schaller et al., 2015; Whiteside et al., 2015), could have contributed to the causes of the biotic turnover. Furthermore, the age of the Manicouagan impact event at 215–214 Ma (Ramezani et al., 2005; Walkden et al., 2002; Jourdan et al., 2009) is close to the age range of the floral and faunal turnover in the Chinle strata (Reichgelt et al., 2013; Lindström et al., 2016). So far, no global extinction has been matched with the ejecta hori zon of the Manicouagan impact ( Olsen et al., 2011). However, the interval around 215 Ma was associated with regional GSA Bulletin; Month/Month 2016; v. 128; no. X/X; p. 1–21; https:// doi . org /10 . 1130 /B31673 . 1; 9 figures; 2 tables; Data Repository item 2017345 . ; published online XX Month 2016 . viktoria . baranyi@geo . uio.no For permission to copy, contact editing@geosociety.org © 2017 Geological Society of America