Late Pleistocene Carbon Cycle Revisited by Considering Solid Earth Processes

Peter Köhler,Guy Munhoven

doi:10.1029/2020pa004020

Abstract

AbstractThe importance of volcanic CO2 release, continental weathering, and coral reef growth on the global carbon cycle has been highlighted by several different studies. Based on these independent approaches, we here revisit the last 800 kyr with the box model BICYCLE, which has been extended to be able to address these solid Earth contributions to the carbon cycle. We show that the volcanic outgassing of CO2 as a function of sea level change from mid‐ocean ridges and hot spot island volcanoes cannot be the generic process that leads during phases of falling obliquity to a sea level‐CO2 decoupling as has been suggested before. The combined contribution from continental and marine volcanism, if both lagging sea level change by 4 kyr, might have added up to 13 ppm to the glacial/interglacial CO2 rise. Coral reef growth as suggested by an independent model is during glacial terminations about an order of magnitude too high to be reconciled with meaningful carbon cycle dynamics. Global riverine input of bicarbonate caused by silicate and carbonate weathering is suggested to have been stable over Termination I. However, if weathering fluxes are changed by up to 50% in sensitivity experiments, the corresponding bicarbonate input might contribute less than 20 ppm to the deglacial atmospheric CO2 rise. The overall agreement of results with the new process‐based sediment module and the previously applied time‐delayed response function to mimic carbonate compensation gives confidence in the results obtained in previous applications of the BICYCLE model without solid Earth processes.

Highlights

Atmospheric CO2 is a global integrating variable that captures changes in various different parts of the global carbon cycle
We find the following dynamics during glacial terminations, in line with what would be expected from theory (e.g., Sarmiento & Gruber, 2006): After large amounts of CO2 have been shifted by various processes from the ocean to atmosphere and terrestrial biosphere, oceanic pH is rising, leading to a rise in CO23− concentration, and to a deepening of calcite saturation horizon (CSH) by more than 1 km allowing a larger amount of CaCO3 to accumulate above the CSH, leading thereafter to an oceanic loss of CO23−
The model has been extended by a process-based model of early diagenesis in the marine sediments and without further tuning of already implemented processes we here investigated the consequences of volcanic CO2 release, silicate and carbonate weathering, and coral reef growth

Summary

Introduction

Atmospheric CO2 is a global integrating variable that captures changes in various different parts of the global carbon cycle. On glacial/interglacial (G/IG) timescales of the late Pleistocene, the period for which reliable reconstruction of CO2 from ice cores exist (Bereiter et al, 2015), most hypotheses trying to explain parts of the observed CO2 changes deal with marine processes (e.g., Ganopolski & Brovkin, 2017; Khatiwala et al, 2019), since small changes in the ocean that contains nearly 2 orders of magnitude more carbon than the atmosphere are able to explain rather larger changes in the atmospheric CO2 content. We do not investigate in that direction any further but note that others have done so (e.g., Ganopolski & Brovkin, 2017)

Results

Discussion

Conclusion