Glacial and Holocene Middle East Stalagmite Proxies Interpreted Using GCM Simulations and Karst Environment Numerical Modeling

Abstract:

We present multi-proxy records combined with modeling to reconstruct the Middle East climate response to two distinct past climate forcings: abrupt climate events of the early glacial period, and precessional change over the Holocene. We utilize both global circulation model simulations as well as catchment-scale numerical modeling in our interpretations.

We first investigate the Middle East climate response to abrupt climate events. Here we use the fully coupled climate model HadCM3, which shows good qualitative agreement with the observed atmospheric flow in the region. During stadial events, stalagmite δ¹⁸O values increase by 2-3‰ in both NW and NE Iran. In the model experiment, dry conditions are found across the region in response to stadial event forcing, with a smaller fractional loss of moisture on the upstream west-to-east trajectory. We therefore interpret the observed stalagmite δ¹⁸O increases at both Iranian sites to be caused by overall drier conditions upstream. A simple vapor transport model shows quantitative agreement between the δ¹⁸O increase in the stalagmite archive and the rainfall δ¹⁸O increase calculated from the HadCM3 output. Strong evidence of drying is further provided by observed trace element-to-calcium ratio increases, which numerical modeling (open CaveCalc software) shows can be explained by drier conditions along the water flow path through karst conduits (Carolin et al., 2019).

We also present an Iranian stalagmite record which spans the full Holocene to investigate the region’s response to orbital-scale seasonality changes. CCSM3-isoCAM3 model experiments over the land area just west of Iran (Levant, Syria, Iraq) find increased winter and summer precipitation under extreme maximum seasonality conditions (similar to early Holocene-type conditions) (Orland et al., 2019). Our stalagmite δ¹⁸O record is dominated by a gradual 2‰ increase from 7 to 2 ka, a clear overlap with decreasing local summer and increasing local winter insolation strength. The model experiments indeed show lighter δ¹⁸O in both summer and winter rainfall during early Holocene-type conditions. Our interpretation of an early-to-late Holocene trend from wet to dry conditions is further supported by several other stalagmite environmental proxies (Andrews et al., 2020).