Abstract
Marked by the expansion of ice sheets in the high latitudes, the intensification of Northern Hemisphere glaciation across the Plio/Pleistocene transition at ~ 2.7 Ma represents a critical interval of late Neogene climate evolution. To date, the characteristics of climate change in North America during that time and its imprint on vegetation has remained poorly constrained because of the lack of continuous, highly resolved terrestrial records. We here assess the vegetation dynamics in northwestern North America during the late Pliocene and early Pleistocene (c. 2.8–2.4 Ma) based on a pollen record from a lacustrine sequence from paleo-Lake Idaho, western Snake River Plain (USA) that has been retrieved within the framework of an International Continental Drilling Program (ICDP) coring campaign. Our data indicate a sensitive response of forest ecosystems to glacial/interglacial variability paced by orbital obliquity across the study interval, and also highlight a distinct expansion of steppic elements that likely occurs during the first strong glacial of the Pleistocene, i.e. Marine Isotope Stage 100. The pollen data document a major forest biome change at ~ 2.6 Ma that is marked by the replacement of conifer-dominated forests by open mixed forests. Quantitative pollen-based climate estimates suggest that this forest reorganisation was associated with an increase in precipitation from the late Pliocene to the early Pleistocene. We attribute this shift to an enhanced moisture transport from the subarctic Pacific Ocean to North America, confirming the hypothesis that ocean-circulation changes were instrumental in the intensification of Northern Hemisphere glaciation.
Highlights
The ‘mid’-Pliocene to early Pleistocene time period (~ 3.6– 2.0 Ma) comprises the full range of climatic boundary conditions as realised in the context of present-day and near-future climate change (e.g. Robinson et al 2008)
The glaciation connected to Marine Isotope Stage (MIS) G6 (~ 2.7 Ma) is often considered to mark the onset of intensification of Northern Hemisphere glaciation (iNHG) because it resulted in the first occurrence of ice-rafting debris (IRD) in the North Atlantic Ocean (e.g. Shackleton et al 1984; Bailey et al 2013)
Three pollen assemblage zones (PAZ) were defined, with PAZ A comprising the late Pliocene part of the record (733– 624 m; c. 2.8–2.7 Ma), PAZ B spanning across the Plio/
Summary
The ‘mid’-Pliocene to early Pleistocene time period (~ 3.6– 2.0 Ma) comprises the full range of climatic boundary conditions as realised in the context of present-day and near-future climate change (e.g. Robinson et al 2008). Atmospheric carbon-dioxide concentrations (pCO2) varied between preindustrial levels and anthropogenically perturbed values as they may be reached in few decades (Pachauri et al 2014; Martínez-Botí et al 2015), whereas the palaeogeography was very similar to that of today. Together, these facts make the ‘mid’-Pliocene to early Pleistocene well suited to examine the response of the Earth’s climate to increasing greenhousegas forcing as a consequence of anthropogenic CO2 emissions (Robinson et al 2008; Haywood et al 2016). Marine Isotope Stage 100, ~ 2.5 Ma ago, was likely the first glacial during which the Laurentide Ice Sheet advanced into the midlatitudes (Balco and Rovey, 2010); together with the subsequent, strongly pronounced glacials MIS 98 and 96 (e.g. Lisiecki and Raymo 2005; Bolton et al 2010; BrighamGrette et al 2013), it represents the first of a series of early Pleistocene glaciations, resulting in the glacial/ interglacial cycles that have dominated the Earth’s climate throughout the Quaternary (Lisiecki and Raymo 2005; Bartoli et al 2006; De Schepper et al 2014)
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