Anomalously Low δ18O Values of High-Latitude Permo-Triassic Paleosol Siderite: Implications for the Hydrologic Cycle during Warm Climate Intervals
Abstract The most severe extinction in Earth history occurred during a time of extreme climate change, caused in part by a massive release of carbon into the atmosphere. Isotopic measurements of siderite occurring in paleosols during intervals of global warming suggest high-latitude depletions in δ 18 O of precipitation, often attributed to an amplified hydrologic cycle. Here, Late Permian and Early Triassic paleosol siderite from Alaska, Antarctica, eastern Australia, Siberia, and South Africa indicate similar or greater meridional gradients in siderite δ 18 O compared to other past warm intervals. An isotope-tracer-enabled version of the Global Environmental and Ecological Simulation of Interactive Systems (GENESIS) general circulation model (GCM) was used to compare to siderite δ 18 O data. The model, when deriving siderite δ 18 O at a specified paleoatmospheric CO 2 concentration of 12.7 × the preindustrial atmospheric level (PAL), matches a small subset of relatively less depleted high-latitude siderites but does not produce the conditions necessary to explain the most depleted siderite δ 18 O values. Siderite has been thought to record mean annual precipitation δ 18 O, though this study suggests that many may not. A seasonal bias, where siderite growth occurs in the summertime in wetlands that receive most of their recharge from melting winter precipitation, may be responsible. GENESIS indicates soil moisture recharge during the spring ahead of the rainy season for high-latitude Permo-Triassic (PT) siderite localities that reach below freezing winter temperatures. Drainage from high altitude regions throughout the growing season may also be responsible. Biological cloud feedbacks and monsoon-related amount effects are not likely the cause for low siderite δ 18 O because the enrichment of water vapor δ 18 O associated with warming is too significant.