Drier Winters Drove Cenozoic Open Habitat Expansion in North America

Abstract

The shift from denser forests to open, grass‐dominated vegetation in west‐central North America between 26 and 15 million years ago is a major ecological transition with no clear driving force. This open habitat transition (OHT) is considered by some to be evidence for drier summers, more seasonal precipitation, or a cooler climate, but others have proposed that wetter conditions and/or warming initiated the OHT. Here, we use published (n = 2,065) and new (n = 173) oxygen isotope measurements (δ18O) in authigenic clays and soil carbonates to test the hypothesis that the OHT is linked to increasing wintertime aridity. Oxygen isotope ratios in meteoric water (δ18Op) vary seasonally, and clays and carbonates often form at different times of the year. Therefore, a change in precipitation seasonality can be recorded differently in each mineral. We find that oxygen isotope ratios of clay minerals increase across the OHT while carbonate oxygen isotope ratios show no change or decrease. This result cannot be explained solely by changes in global temperature or a shift to drier summers. Instead, it is consistent with a decrease in winter precipitation that increases annual mean δ18Op (and clay δ18O) but has a smaller or negligible effect on soil carbonates that primarily form in warmer months. We suggest that forest communities in west‐central North America were adapted to a wet‐winter precipitation regime for most of the Cenozoic, and they subsequently struggled to meet water demands when winters became drier, resulting in the observed open habitat expansion.

Tectonic extension and paleoelevation influence mammalian diversity dynamics in the Basin and Range Province of western North America

“…Continuous Cenozoic regional climate data are not currently available for North America, although recent climate model results for key intervals over the Neogene ( 19 ) may be used in future evaluation of spatial and temporal diversity patterns. Likewise, an increasing number of studies in the western US will help address the spatial and temporal gaps in existing compilations of regional vegetation ( 1 , 61 ). Data from phytolith and paleobotanical studies will be valuable to distinguish the distinct role of grassland expansion that has been demonstrated in other systems ( 23 ), and we expect to influence mammalian diversity and turnover in the late Miocene.…”

Frontiers of Carbonate Clumped Isotope Thermometry

“…Increasingly sophisticated assessment of depositional environment, diagenesis, proxy system, and hydroclimate effects (e.g., Kelson et al 2020, Ingalls et al 2020, Song et al 2022 is key to these and future applications. There is opportunity to combine clumped isotope thermometry with carbonate geochronology and submillennial sampling (e.g., Huth et al 2020), and to collect complementary observations [e.g., carbonate 17 O (Beverly et al 2021) or clay δ 18 O analyses (Kukla et al 2022)], all of which could be included in proxy-system models. In the next decade, carbonate clumped isotope thermometry could significantly improve atmospheric pCO 2 reconstruction from soils over the Phanerozoic and reconstruct past Earth surface environments to inform predictions of future environmental change.…”

Toward a Cenozoic history of atmospheric CO 2

“…2D). Therefore, the rise of C 4 plants to their dominance in many tropical and subtropical ecosystems was likely driven (and is maintained today) by other factors, such as fire, seasonality of rainfall, and herbivory (i.e., grazing that keeps landscapes open) ( 97 , 98 ). The temporal evolution of these factors warrants further study as we move toward a future where CO 2 may rise above the 550-ppm threshold that was key to the origin, taxonomic diversification, and spread of C 4 plants.…”