Abstract
Estimates of the permafrost-climate feedback vary in magnitude and sign, partly because permafrost carbon stability in warmer-than-present conditions is not well constrained. Here we use a Plio-Pleistocene lacustrine reconstruction of mean annual air temperature (MAAT) from the Tibetan Plateau, the largest alpine permafrost region on the Earth, to constrain past and future changes in permafrost carbon storage. Clumped isotope-temperatures (Δ47-T) indicate warmer MAAT (~1.2 °C) prior to 2.7 Ma, and support a permafrost-free environment on the northern Tibetan Plateau in a warmer-than-present climate. Δ47-T indicate ~8.1 °C cooling from 2.7 Ma, coincident with Northern Hemisphere glacial intensification. Combined with climate models and global permafrost distribution, these results indicate, under conditions similar to mid-Pliocene Warm period (3.3–3.0 Ma), ~60% of alpine permafrost containing ~85 petagrams of carbon may be vulnerable to thawing compared to ~20% of circumarctic permafrost. This estimate highlights ~25% of permafrost carbon and the permafrost-climate feedback could originate in alpine areas.
Highlights
Estimates of the permafrost-climate feedback vary in magnitude and sign, partly because permafrost carbon stability in warmer-than-present conditions is not well constrained
Carbonate stable isotope (δ18Oc and δ13Cc), carbonate content (CaCO3), total organic carbon (TOC), total nitrogen (TN), carbon to nitrogen ratios (C/N), carbon isotope ratios in organic matter (δ13Corg), and grain size have been widely used for paleoenvironmental reconstruction as these proxies provide qualitative constraints on temperature, evaporation-precipitation balance, primary productivity, erosion and other climate conditions that might indirectly relate to temperature[18–20]
Given that the persistence of permafrost depends on temperature-ecosystem interactions[22,23], reliable reconstructions of mean annual air temperatures (MAAT) from Plio-Pleistocene sediments would enable us to assess the presence of permafrost during the mid-Pliocene Warm Period (mPWP) warmth, and projects the modern permafrost regions that might be vulnerable to future global warming
Summary
Stratigraphy and age model of the Kunlun Pass Section. The KP site is located on the northern Tibetan Plateau (35°39′N, 94°03′E, elevation ~4.7 km) south of the Eastern Kunlun Mountains that separate the modern cold steppe environments (Supplementary Fig. 1a) from the hyper-arid desert climate of the Qaidam Basin to the north[25] (Fig. 1b). Given that the high covariance of δ18Oc and δ13Cc throughout the section, the increased mean grain size of the sediments, and published data indicating regional aridification during the PlioPleistocene climate transition[28], we infer that cooling at the KP site at 2.7 Ma, rather than opening of the hydrological system (e.g., lake level rise overtopping the topographic basin), provides a better explanation of the observations. Changes in the proportion of aquatic and submerged plants with different δ13Corg values could play an important role in controlling variations in the bulk δ13Corg of lake sediments In concert, these qualitative proxies support regional cooling at 2.7 Ma coincident with the intensification of Northern Hemisphere glaciation that in turn, played a fundamental role in changing paleoenvironments at the KP site, including by influencing evaporation rates, impacting lake area and productivity, and sedimentation. The decrease in lake water δ18Ow value at the KP site at 2.7–2.6 Ma may be associated with reduced evaporation in this region
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