A TEMPERATURE REGULATED ORGANIC CARBON CAPACITOR ON EARTH’S SURFACE

The early Paleogene was characterized by profound changes in Earth surface temperature and global carbon cycling. The latter is evidenced today through the sedimentary record, most notably by large amplitude variations in the d¹³C of carbon-bearing phases in lower Paleogene strata, and by widespread variations in carbonate preservation within lowere Paleogene deep-sea sediment. Critically, major changes in early Paleogene carbon cycling happened across a range of time scales. An increasingly clear series of short term (<200 kyr) warming events, commonly referred to as hyperthermals and including the PETM, were marked by rapid drops in d¹³C and seafloor carbonate dissolution. Less discussed are the surrounding, long-term changes. From about 62 to 57 Ma, a prominent rise in d¹³C and shaoling of the CCD occurred. This was followed by a large drop in d¹³C and deepening of the CCD from about 57 to 52 Ma, a second rise in d¹³C from about 52 to 48 Ma. The hyperthermals are widely considered to represent times of rapid global warming somehow associated with massive injections of carbon from some large ¹³C-depleted reservoir. The long0term changes in carbon cycling are best explained by the growth and decay of some large ¹³C-depleted reservoir. While the two reservoirs could be distinct, it seems more congruous to suggest they are the same, and Earth's surface has a large organic carbon capacitor, one missing in most geochemical models but crucial to understanding Earth systems during global warming. Possibilities include gas hydrates, peat, or permafrost. Crucially, throughout the early Paleogene, and at different time scales, warming generally precedes increases in carbon fluxes to the ocean and atmosphere. The fascination with profound changes in temperature and carbon cycling during the early Paleogene should not be focused on constraining climate sensitivity (how a given rise in CO_² increases Earth surface temperature) but rather on major carbon cycle feedbacks (how a given rise in temperature increases carbon fluxes from the shallow geosphere).