Paradox of the peak-PCIM (Paleocene Carbon Isotope Maxima; ~57.8Ma) and Abrupt Global Warming

Abstract:

The Paleocene Carbon Isotope Maxima (PCIM; ~57.8Ma) represents a major transition in global δ¹³C during the late Paleocene, when the long-term positive trend in δ¹³C reversed from positive to negative. The peak-PCIM (~57.7Ma) has been tightly resolved in new high-resolution, astronomically-tuned benthic isotope records from IODP Sites 1209 (Pacific) and 1262 (Atlantic), which show the final phase of δ¹³C enrichment as abrupt (~1‰ in <100kyrs) and coinciding with a 0.5‰ decline in benthic δ¹⁸O indicative of 2-3ºC of bottom water warming, effectively marking the onset of a gradual 8Myr long warming trend [Littler et al., EPSL 2014]. Typically, during the Paleogene on orbital time scales, warming is observed during intervals of δ¹³C depletion, consistent with release of isotopically light carbon to the atmosphere. This event, which has the appearance of a bifurcation in the long-term coupling of climate system and the carbon cycle, poses an interesting paradox as any rapid carbon release to the atmosphere should, in theory, create a negative excursion because all of the major carbon sources are isotopically light, whether volcanic outgassing, weathering/oxidation of organic carbon, or methane release [Dunkley-Jones et al., Phil. Trans. R. Soc. A 2010]. If global, there are several testable mechanisms that may explain the shift including increase in burial flux of light carbon, a reduction in heavy carbon burial flux, or a large-scale circulation change perhaps associated with the transition of a major oceanic gateway. Using trace metal (B/Ca and Mg/Ca) and stable isotope (δ¹¹B, δ¹⁸O, and δ¹³C) geochemistry, here we establish the nature of the peak-PCIM at sites from 3 different ocean basins (IODP Sites 690, 1209, and 1262) and begin to test several of the possible mechanisms for change. Mg/Ca in mixed-layer planktonic foraminifera show 2-3°C of sea surface warming coinciding with, and abrupt as, the benthic carbon isotope enrichment at all sites. Bottom water ∆[CO₃^2-], as indicated by B/Ca in benthic foraminifera, abruptly increases by 30-40µmol/kg_sw. While this may indicate a change in bottom water circulation, surface B-based proxies also respond with a positive shift during the peak-PCIM indicating a slight increase in surface pH and highlighting the global nature of the event.