Deglacial Hydroclimate of Midcontinental North America

Abstract

During the last deglaciation temperatures over midcontinental North America warmed dramatically through the Bølling-Allerød, underwent a cool period associated with the Younger-Dryas and then reverted to warmer, near modern temperatures during the early Holocene. However, paleo proxy records of the hydroclimate of this period have presented divergent evidence. We reconstruct summer relative humidity (RH) across the last deglacial period using a mechanistic model of cellulose and leaf water δ18O and δD combined with a pollen-based temperature proxy to interpret stable isotopes of sub-fossil wood. Midcontinental RH was similar to modern conditions during the Last Glacial Maximum, progressively increased during the Bølling-Allerød, peaked during the Younger-Dryas, and declined sharply during the early Holocene. This RH record suggests deglacial summers were cooler and characterized by greater advection of moisture-laden air-masses from the Gulf of Mexico and subsequent entrainment over the mid-continent by a high-pressure system over the Laurentide ice sheet. These patterns help explain the formation of dark-colored cumulic horizons in many Great Plains paleosol sequences and the development of no-analog vegetation types common to the Midwest during the last deglacial period. Likewise, reduced early Holocene RH and precipitation correspond with a diminished glacial high-pressure system during the latter stages of ice-sheet collapse.

Late Pleistocene proboscidean population dynamics in the North American Midcontinent

“…1). This was a period of substantial warming with higher levels of effective precipitation (Voelker et al 2015). Although Mammuthus retained a presence in forested areas of the Midwest, they were no longer the dominant proboscidean on the landscape (Fig.…”

“…We used 12.9AE0.125 ka as the age of the start of the YD, as defined globally in the NGRIP (Rasmussen et al 2006;Steffensen et al 2008) and supported by dendroclimatological work in the Midwest (Voelker et al 2015). In order to estimate the age of extinction, we must account for sampling and preservation biases at a regional scale.…”

“…1). This was a period of substantial warming with higher levels of effective precipitation (Voelker et al 2015). These conditions promoted the growth of a non-analogue vegetative community in the southern Great Lakes that was dominated by spruce (Picea) and black ash (Fraxinus nigra).…”

“…In Hypothesis 1 assessed whether the extinction of mastodons occurred after the beginning of the YD (H1), as one might expect if shifts in climate played a role in extinction, or if extinction occurred prior to the YD (H1A). We used 12.9AE0.125 ka as the age of the start of the YD, as defined globally in the NGRIP (Rasmussen et al 2006;Steffensen et al 2008) and supported by dendroclimatological work in the Midwest (Voelker et al 2015). Although this boundary is precisely dated in ice-core records, we use the more generous error in the regional dendroclimatological record due to unknown lag effects.…”

“…Although often characterized as an abrupt return to glacial conditions, the YD in the Midwest manifests as a slightly cooler, but still humid period (Gonzales & Grimm 2009;Voelker et al 2015). Although often characterized as an abrupt return to glacial conditions, the YD in the Midwest manifests as a slightly cooler, but still humid period (Gonzales & Grimm 2009;Voelker et al 2015).…”