Constraints on Earth System Functioning at the Paleocene‐Eocene Thermal Maximum From the Marine Silicon Cycle

Guillaume Fontorbe,Katharine R Hendry,Christina L De La Rocha,Daniel J Conley,Patrick J Frings

doi:10.1029/2020pa003873

Abstract

AbstractThe Paleocene‐Eocene Thermal Maximum (PETM, ca. 56 Ma) is marked by a negative carbon isotope excursion (CIE) and increased global temperatures. The CIE is thought to result from the release of 13C‐depleted carbon, although the source(s) of carbon and triggers for its release, its rate of release, and the mechanisms by which the Earth system recovered are all debated. Many of the proposed mechanisms for the onset and recovery phases of the PETM make testable predictions about the marine silica cycle, making silicon isotope records a promising tool to address open questions about the PETM. We analyzed silicon isotope ratios (δ30Si) in radiolarian tests and sponge spicules from the Western North Atlantic (ODP Site 1051) across the PETM. Radiolarian δ30Si decreases by 0.6‰ from a background of 1‰ coeval with the CIE, while sponge δ30Si remains consistent at 0.2‰. Using a box model to test the Si cycle response to various scenarios, we find the data are best explained by a weak silicate weathering feedback, implying the recovery was mostly driven by nondiatom organic carbon burial, the other major long‐term carbon sink. We find no resolvable evidence for a volcanic trigger for carbon release, or for a change in regional oceanography. Better understanding of radiolarian Si isotope fractionation and more Si isotope records spanning the PETM are needed to confirm the global validity of these conclusions, but they highlight how the coupling between the silica and carbon cycles can be exploited to yield insight into the functioning of the Earth system.

Highlights

The early Paleogene (65 to 50 Ma) is characterized by atmospheric and ocean temperatures up to 15 °C warmer than the preindustrial period (e.g., Cramwinckel et al, 2018; Evans et al, 2018), ice‐free poles, and atmospheric CO2 concentrations around 1,000 ppmv (Anagnostou et al, 2016)
Using a box model to test the Si cycle response to various scenarios, we find the data are best explained by a weak silicate weathering feedback, implying the recovery was mostly driven by nondiatom organic carbon burial, the other major long‐term carbon sink
At the Paleocene‐Eocene Thermal Maximum (PETM) onset, δ30Sirad decreases from about +1.2‰ to +0.5‰, after which they are constant with time (+0.65 ± 0.17‰, n = 24)

Summary

Introduction

The early Paleogene (65 to 50 Ma) is characterized by atmospheric and ocean temperatures up to 15 °C warmer than the preindustrial period (e.g., Cramwinckel et al, 2018; Evans et al, 2018), ice‐free poles, and atmospheric CO2 concentrations (pCO2) around 1,000 ppmv (Anagnostou et al, 2016). The PETM is characterized by a negative carbon isotope excursion (CIE) of 2–6‰ in both marine and terrestrial archives (McInerney & Wing, 2011). The existence of this globally manifest CIE requires a significant amount—thousands of Gt—of 13C‐depleted carbon to have been added to the exogenic carbon pool. An internally consistent understanding of the PETM requires that the amount and δ13C of carbon is compatible with terrestrial and marine observations of the CIE, as well as the magnitudes and rates of warming, changes in ocean pH, and any carbonate compensation depth (CCD) change. Additional unknowns include the rate and duration of carbon addition and the site(s) of its release

Methods

Results

Discussion

Conclusion