A mechanistic analysis of early Eocene latitudinal gradients of isotopes in precipitation

Abstract

We use a one‐dimensional reactive transport model of isotopes in precipitation (δ18O) to investigate the physical mechanisms controlling global meridional isotope profiles under early Eocene hothouse conditions. Simulations of early Eocene precipitation isotopes display reduced meridional gradients relative to the modern climate with the largest increases in δ18O occurring at high latitudes, matching proxy data. These reduced gradients are controlled primarily by polar amplification that increases high‐latitude length scales of specific humidity and match our compilation of proxy‐based reconstructions. Comparing Eocene general circulation model simulations run with pCO2 of 2240 and 4480 ppm, we find that meridional isotopic profiles are insensitive to the associated 5°C change in global temperatures due to the relative lack of polar amplification. Finally, we hypothesize that observed negative δD anomalies in precipitation during peak warming of early Eocene hyperthermal events are the result of a theorized reduction in the strength of midlatitude transient eddies.

Late‐Pleistocene precipitation δ18O interpolated across the global landmass

“…[Winnick et al, 2015]. Global hot (Eocene, [Winnick et al, 2015]) and cold (late-Pleistocene, this study) house latitude-d 18 O gradients suggest that detectable impacts of modern climate warming driven by anthopogenic greenhouse gas emissions on precipitation d 18 O are likely to be maximized at the high latitudes rather than the equator.…”

“…Conversely, only two‐thirds of late‐Pleistocene δ 18 O records within 35° of the equator are 18 O‐depleted relative to the late‐Holocene (51 of 76 subtropical and tropical sites), while the remaining sites have positive Δ 18 O late‐Pleistocene values (Figure a). Negative Δ 18 O late‐Pleistocene values linked to the cool late‐Pleistocene climate corroborate precipitation δ 18 O estimates for the Eocene climate that simulate small modern‐Eocene δ 18 O changes across the low latitudes juxtaposed by higher magnitude, positive (δ 18 O Eocene > δ 18 O Modern ) shifts closer to the poles [ Winnick et al ., ]. Global hot (Eocene, [ Winnick et al ., ]) and cold (late‐Pleistocene, this study) house latitude‐δ 18 O gradients suggest that detectable impacts of modern climate warming driven by anthopogenic greenhouse gas emissions on precipitation δ 18 O are likely to be maximized at the high latitudes rather than the equator.…”

“…Some of the possible processes impacting the global latitude-D 18 O late-Pleistocene relationship include deglacial changes to global circulation patterns, sea surface temperatures, and moisture sources [e.g., Rozanski, 1985;Hoffmann and Heimann, 1997;Sultan et al, 1997;Aggarwal et al, 2004;Cruz et al, 2006;Tierney et al, 2008]. [Winnick et al, 2015]. Global hot (Eocene, [Winnick et al, 2015]) and cold (late-Pleistocene, this study) house latitude-d 18 O gradients suggest that detectable impacts of modern climate warming driven by anthopogenic greenhouse gas emissions on precipitation d 18 O are likely to be maximized at the high latitudes rather than the equator.…”

The Sensitivity of Terrestrial δ18O Gradients to Hydroclimate Evolution

“…We make three assumptions that allow us to link climate to hydroclimate and isotopic evolution: (1) The δ 18 O of the source moisture follows an empirical δ 18 O‐temperature relationship of 0.55‰/K (Kohn & Welker, ), (2) the moisture source region has a constant relative humidity (80%), so the initial precipitable water tracks the Clausius‐Clapeyron relationship (Held & Soden, ), and (3) potential evapotranspiration increases at a rate of 1.6%/°C, which is a characteristic scaling for the midlatitudes (Ibarra et al, ). The first assumption is purely illustrative because similar scaling relationships are sometimes used to reconstruct temperature, but we note that any empirical δ 18 O‐temperature link will not necessarily hold through space and time (e.g., Winnick et al, ).…”

“…At any given site, this implies a positive correlation between δ 18 O and temperature. We can decompose the total change in δ 18 O to three individual contributions: (1) the temperature effect on the δ 18 O of source moisture, although this is illustrative here and not expected to operate in all climate states (Winnick et al, ); (2) the orographic effect due to the temperature dependence of fractionation; and (3) the hydroclimate effect due to atmospheric moisture content increasing faster than precipitation rates. Both wet and dry warming simulations exhibit δ 18 O sensitivity to the temperature effect and the orographic effect, but only the wet simulation is sensitive to hydroclimate (Figures a and b).…”