Research Paper GEOSPHERE Pollen evidence of floristic turnover forced by cool aridity during the Oligocene in Colorado GEOSPHERE; v. 15, no. 1 Estella B. Leopold and Stephanie Zaborac-Reed University of Washington, Department of Biology, Box 351800, Seattle, Washington 98195, USA https://doi.org/10.1130/GES01689.1 7 figures; 6 plates; 11 tables; ABSTRACT Manchester, 1997). In Colorado, the EOT is associated with a period of cooling 1 set of supplemental files and severe aridity lasting most of the Oligocene. One of our strongest contri- New pollen data from four Oligocene floras in volcanic landscapes of Colo­ butions is the addition of the pollen record from four Oligocene floras, which CORRESPONDENCE: eleopold@ uw.edu rado record important climatic shifts that reshaped the local flora and promoted had not been reported previously. This pollen record is an important aspect the development of sub­arid vegetation types. We combined new pollen data of this paper for assessing the total flora. The emphasis of this study is on the CITATION: Leopold, E.B., and Zaborac-Reed, S., 2019, Pollen evidence of floristic turnover forced by with previous megafossil evidence to assess vegetation changes during the evolutionary and climatic significance of the floristic changes during the EOT. cool aridity during the Oligocene in Colorado: Geo- Eocene–Oligocene Transition (EOT). Pollen data are the basis for updating Five Colorado floras of late Eocene through Oligocene age lie within the sphere, v. 15, no. 1, p. 254–294, https:// doi .org /10 the list of flora identified at Creede. Local extinctions in response to lower sum­ geographic mix of caldera and volcanic settings in the central Colorado vol- .1130 /GES01689.1. mer rainfall abruptly removed many of the exotic woody taxa of eastern North canic area and in the San Juan volcanic field in southwestern Colorado (Fig. 1). American and Asian affinity. This loss was followed by the appearance of xeric The floras are, in ascending order: the Florissant (34.1 Ma), Antero (33.8 Ma), Science Editor: Shanaka de Silva Associate Editor: Rhawn Denniston shrubland taxa of the Ponderosa pine­fir woodland and sagebrush flora that Pitch-Pinnacle (between 32.9 and 29 Ma), Platoro (28.0 Ma), and Creede characterize the Colorado area today. Tell­tale genera appear and suggest an (26.9 Ma) assemblages (Fig. 2; Table 1). The purpose of this report is to sup- Received 6 March 2018 understory of shrubs such as Artemisia, Elaeagnus, Ribes, Ephedra, Jamesia, plement previous work on plant megafossils of the Oligocene in Colorado with Revision received 22 August 2018 and Shepherdia. Poaceae are also present. Significantly, herbaceous taxa of our new work on fossil pollen. We characterize the pollen and megafossil floras Accepted 9 November 2018 the Asteraceae, Rosaceae, Cucurbitaceae, Euphorbiaceae, and Caryophyllaceae from just before and after the EOT and interpret the climatic and floristic shifts Published online 10 January 2019 make their first appearances in the fossil record of Colorado here. that they demonstrate with emphasis on the following questions: The new Oligocene pollen data record a significant drop in summer rainfall and a climatic cooling at 33.8 Ma of several degrees that relates to the Oi­1 (1) What was the scale of the climatic shift based on the floras, and how glaciation in Antarctica. The particular taxa that continued after the EOT were does it relate to possible uplift of the region? a basis for estimating changes in soil moisture during this time. The condi­ (2) What was the evolutionary significance of the floristic changes during tions in Colorado are reminiscent of Wolfe’s “terminal Eocene event.” This re­ the Colorado EOT? markable shift precipitated the development of a local pollen and megafossil (3) What estimates of paleoelevation may be based on these floras and the flora more “modern” in aspect (e.g., a larger proportion of extant local genera local geology of the sites? are present). The impressive floristic turnover was probably a response to the (4) How do climate tools such as coexistence approach, detrended corre- increasingly continental climate that embraced the area during the Oligocene. spondence analysis, the Sørensen index, and nearest living relatives analysis assist the interpretation of the floras in terms of inferred mean OLD G 1. INTRODUCTION annual temperature (MAT) and precipitation (PPT)? (5) How do the inferred climate shifts compare to conditions in other parts The global climate change at the Eocene–Oligocene transition (EOT) of the world during the EOT? (33.4 Ma; Prothero and Berggren, 1992) is the largest abrupt cooling of the OPEN ACCESS Cenozoic (Zachos et al., 2001). During the EOT, changes in circulation result- 2. BACKGROUND ing from the development of deep-water passages separating Antarctica from South America (beginning ca. 33 Ma) (Graham, 2011; Lawver et al., 2011) may Our fossil records span the latest Eocene Florissant flora (34.1 Ma) and have contributed to the major worldwide cooling and associated aridity re- four progressively younger Oligocene floras (33.8 Ma–26.9 Ma; Table 1). This lated to Antarctic glaciation Oi-1 (Cather et al., 2008). This post-Eocene cool- interval is closely synchronous with large-scale explosive volcanism in the ing and aridity resulted in local and regional extinctions of plant, vertebrate, southern Rocky Mountains (mainly 37–27 Ma), the development of the Oi-1 This paper is published under the terms of the and invertebrate taxa (Prothero, 1985a, 1985b; Hutchison, 1992; Legendre and glaciation and the “Ice House,” and the period of great aeolian activity along CC-BY-NC license. Hartenberger, 1992; Prothero and Berggren, 1992; Wolfe, 1992; Meyer and the Arizona–New Mexico border (Zachos et al., 2001; Cather et al., 2008). © 2019 The Authors GEOSPHERE | Volume 15 | Number 1 Leopold and Zaborac-Reed | Pollen evidence of floristic turnover during the Colorado Oligocene Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/1/254/4619230/254.pdf 254 by guest on 03 October 2021 Research Paper 2.1. Regional Geological Setting of the Five Paleogene Floras in Colorado The Florissant fossil beds formed after a comparatively small volcanic event that dammed a lake at the end of the Eocene. In contrast, the Oligocene floras that immediately followed in southwestern Colorado developed in local volcanic sediments. The San Juan volcanic field produced 28 large-volume ignimbrite eruptions between 36.9 Ma and 26.9 Ma (Lipman and McIntosh, 2008; Lipman et al., 2015). Ponding of distal ash from these and the formation of lakes within source calderas are the main sites for the floras discussed here. The Florissant Formation formed when a lahar blocked the Florissant Valley in the Thirtynine Mile volcanic area (36.9 Ma to 34.3 Ma), leading to the devel- opment of Lake Florissant (McIntosh and Chapin, 2004; Fig. 1). Distal ash asso- ciated with Mount Aetna nearby accumulated in another intermontane lake to form the Oligocene Antero Formation (33.8 Ma). In addition, lake deposits of the Pitch-Pinnacle flora accumulated within and near the Marshall caldera at ca. 33.8 Ma (Lipman et al., 2013, 2015). Shortly after this, a dune field of aeolian dust and sand in the Chuska Mountains of northwestern New Mexico Figure 1. Reference map of the Colorado and New Mexico Oligocene calderas, with the five and Arizona (Fig. 1) formed at ca. 33.5 Ma and lasted until 28–26 Ma (Cather flora sites indicated (after Chapin, 2012). The floras studied are: (1) Florissant;( 2) Antero; et al., 2008, 2012). Called the Chuska erg, or sand sea, these deposits (as thick (3) Pitch­Pinnacle; (4) Platoro, and (5) Creede. The La Garita caldera is represented by the as 535 m) resulted from large accumulations of airborne and fluvial material in outline surrounding the group of San Juan volcanic field calderas clustered around Creede. the central and southern Colorado Plateau (McIntosh et al., 1992). Figure 2. Map of Creede area, including lo­ cations of pollen collections (D numbers) and megafossil collections (circles). Based on The National Map (http:// nationalmap .gov) shaded relief map and the 7.5 min series Creede quadrangle map, U.S. Geo­ logical Survey, 2013. GEOSPHERE | Volume 15 | Number 1 Leopold and Zaborac-Reed | Pollen evidence of floristic turnover during the Colorado Oligocene Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/15/1/254/4619230/254.pdf 255 by guest on 03 October 2021 Research Paper TABLE 1. IE ROCY MONTAIN PALEOGENE LORAS O COLORADO IN STRATIGRAPHIC ORDER lora Present elevation lora source and No. of taa for Ages of associated formations or volcaniclastics km)Vegetation type stratigraphic source this report Oligocene Creede 2.7–3.3 Open coniferous forest, mid-montane and Aelrod 1987 49 26.87 0.03 Ma (40Ar/39Ar using l-rich dacite youngest montane olfe and Schorn 1989, 1990) from Snowshoe Mountain Tuff; Lipman, 2007) eposures* Ecotone between mied conifer forest upland Lanphere 2000 and pinyon uniper woodlands and scrub Leopold and Zaborac-Reed 2014 Aelrod This report Platoro 3.1 Open coniferous forest, montaneChase et al. 1998 36 27.98 0.11 Ma (40Ar/39Ar using biotite from Meyer 1986 Chiuito Peak Tuff; Lipman and Bachmann, Lipman 2000 2015 Lipman and McIntosh 2008 This report pollen Pitch Pinnacle 3 Conifer mied forest Gregory and McIntosh 1996 28 Between 32.9 oldest and 29 youngest Ma Lipman et al. 2015 (40Ar/39Ar using sanidines from Pitch-Pinnacle This report pollen ormation;