The impact of neogene grassland expansion and aridification on the isotopic composition of continental precipitation

Abstract

The late Cenozoic was a time of global cooling, increased aridity, and expansion of grasslands. In the last two decades numerous records of oxygen isotopes have been collected to assess plant ecological changes, understand terrestrial paleoclimate, and to determine the surface history of mountain belts. The δ18 O values of these records, in general, increase from the mid-Miocene to the Recent. We suggest that these records record an increase in aridity and expansion of grasslands in midlatitude continental regions. We use a nondimensional isotopic vapor transport model coupled with a soil water isotope model to evaluate the role of vapor recycling and transpiration by different plant functional types. This analysis shows that increased vapor recycling associated with grassland expansion along with biomechanistic changes in transpiration by grasses themselves conspires to lower the horizontal gradient in the δ 18O of atmospheric vapor as an air mass moves into continental interiors. The resulting signal at a given inland site is an increase in δ 18O of precipitation with the expansion of grasslands and increasing aridity, matching the general observed trend in terrestrial Cenozoic δ 18O records. There are limits to the isotopic effect that are induced by vapor recycling, which we refer to here as a "hydrostat." In the modern climate, this hydrostatic limit occurs at approximately the boundary between forest and grassland ecosystems.

Role of the westerlies in Central Asia climate over the Cenozoic

“…5b). As a result of our modifications to the calculation of δ e , the δ 18 O value above the "isotopic hydrostat" is invariant with changing E/T , in contrast to Chamberlain et al (2014).…”

“…Instead, a high degree of moisture recycling flattens this δ 18 O gradient (Gat and Matsui, 1991;Mix et al, 2013). To examine the role of moisture recycling in detail, we employ a modified version of the isotopic reactive transport model of Chamberlain et al (2014) and Winnick et al (2014) for atmospheric moisture transport over terrestrial environments.…”

“…For a full derivation and model details, seeHendricks et al (2000),Chamberlain et al (2014), andWinnick et al (2014). To simplify further, N d is approximated as the ratio of E T to runoff (i.e., the ratio of recycled moisture to moisture lost).…”

The Sensitivity of Terrestrial δ18O Gradients to Hydroclimate Evolution

“…Specifically, δ 18 O records are most sensitive to regional hydroclimate changes around a wet initial state. Chamberlain et al () recognized that isotope records are only sensitive to hydroclimate within some discrete range of variability. Their study introduced the term “hydrostat” that refers to the point along an aridification trend—increasing ET / P in their paper—where isotope records are no longer sensitive to further drying out.…”

Understanding Neogene Oxygen Isotopes in the Southern Great Plains Using Isotope‐Enabled General Circulation Model Simulations

“…Compared with the proxy values, the model was unable to capture the drying trend in western California and Nevada, possibly because the model failed to capture the dryness in those regions (Figure S1). The decrease in the δ 18 O of the leeward side of the Cascades was attributed to the uplift of the mountains (Chamberlain et al, ), but the model results suggest that global cooling can also explain the decrease. I speculate that the increase in δ 18 O from mid‐Miocene to late Pleistocene in the western United States is due to the increased dryness, whereas the decrease in the leeward side of the Cascades and the SPG region is due to the decreased temperature, possibly because of the strengthened stationary eddies with global cooling (Wills & Schneider, ), making the longitudinal contrast larger.…”