FEEDBACKS AMONG CARBON CYCLE, SEA LEVEL, AND CLIMATE OVER THE PAST 66 MILLION YEARS

We reconstruct climate and related feedbacks with sea level and tectonics over the past 66 million years using carbon isotopes from mean deep water (δ¹³C_MDW), the large mobile carbon reservoir. Periodic δ¹³C variations are dominated by long eccentricity (405 kyr) and short obliquity (41 kyr) astronomical beats, with recurring long obliquity (1.2 Myr), eccentricity (2.4 Myr and quasi 100 kyr), and precessional (19/23 kyr) cycles. On these Milankovitch scales, high δ¹³C values are associated with cooler intervals and low sea level during glacials, likely through increased ocean circulation, upwelling, and export productivity. In contrast, on long time scales (>1.2 Myr), high δ¹³C values are associated with high sea-level, warmer climates, and increased organic carbon burial on flooded continents and with Large Igneous Province (LIP) volcanism due to increased nutrient supply (particularly Fe). Our analysis of recent CO₂ compilations shows that: 1) high atmospheric CO₂ was associated with high sea level and high burial of organic carbon on continental margins; and 2) decreasing CO₂ largely attributed to temperature-weathering feedback was associated with increased ice volume, falling sea level, and decreased organic carbon burial. Differences between mean deep water δ¹³C and phosphate-free surface water δ¹³C (δ¹³C_PFSW-_MDW), a potential proxy for ocean phosphate concentration, collapsed at the end-Cretaceous mass extinction in response to decreased export productivity, whereas it peaked in the Late Paleocene (ca. 63-56) in response to volcanic input. δ¹³C_PFSW-_MDW stabilized in the Early Eocene Hothouse (ca. 55-48 Ma), then decreased in the Middle Eocene to Early Miocene (ca. 40-20 Ma) with the advent of the Icehouse world, suggesting oligotrophic oceans. δ¹³C_PFSW-_MDW increased with Miocene LIP volcanism, but was subsequently fell until 7 Ma, when global δ¹³C decreased due to the expansion of C4 grasslands and marine diatoms. We conclude that global δ¹³C variations were controlled primarily by sea level on the >1.2 Myr scale and Milankovitch changes on the 1.2 Myr, 405 kyr, quasi-100 kyr, and 41 kyr scales.