Representing high-latitude deep carbon in the pre-industrial state of the ORCHIDEE-MICT land surface model (r8704)

Field measurements, after extrapolation, suggest that deep Yedoma deposits (ice-rich, organic-rich permafrost, formed during the late Pleistocene) and peatlands (formed mostly during the Holocene) account for about 600 Pg C of soil carbon storage. Incorporating this old, deep, cold carbon into land surface models (LSMs) is crucial for accurately quantifying soil carbon responses to future warming. However, it remains underrepresented or absent in current LSMs, which typically include a passive soil carbon pool (a conceptual soil carbon pool with the longest turnover time) to represent all "old carbon"and lack the vertical accumulation processes that deposited deep carbon in the layers of peatlands and Yedoma deposits. In this study, we propose a new, more realistic protocol for simulating deep and cold carbon accumulation in the northern high latitudes (30-90° N), using the ORCHIDEE-MICT (ORganizing Carbon and Hydrology in Dynamic EcosystEms-aMeliorated Interactions between Carbon and Temperature) model. This is achieved by (1) integrating deep carbon from Yedoma deposits whose formation is calculated using Last Glacial Maximum climate conditions and (2) prescribing the inception time and location of northern peatlands during the Holocene using spatially explicit data on peat age. Our results show an additional 157 Pg C in present-day Yedoma deposits, as well as a shallower peat carbon depth (by 1-5 m) and a smaller passive soil carbon pool (by 35 Pg C, 43 %) in northern peatlands, compared to the old protocol that ignored Yedoma deposits and applied a uniform, long-duration (13 500 years) peat carbon accumulation across all peatlands. As a result, the total organic carbon stock across the Northern Hemisphere (30-90° N) simulated by the new protocol is 2028 Pg C, which is 226 Pg C higher than the previous estimate. Despite the significant challenge of simulating deep carbon with ORCHIDEE-MICT, the improvements in the representation of carbon accumulation from this study provide a model version to predict deep carbon evolution during the last glacial-deglacial transition and its response to future warming. The methodology implemented for deep carbon initialization in permafrost and cold regions in ORCHIDEE-MICT is also applicable to other LSMs.