# Carbon isotope budget indicates biological disequilibrium dominated ocean carbon storage at the Last Glacial Maximum

## TL;DR

This study uses carbon isotope data to show that biological processes and sea ice expansion helped store more carbon in the ocean during the Last Glacial Maximum.

## Contribution

The study introduces a new mass balance approach using carbon isotopes to quantify biological disequilibrium in ocean carbon storage during the Last Glacial Maximum.

## Key findings

- Biologically sequestered carbon in the ocean explains a 75 ± 40 ppmv atmospheric CO2 change.
- Carbon isotopic analysis suggests similar regenerated carbon inventories at the LGM and during the Holocene.
- The inferred carbon storage change is attributed to sea ice expansion or changes in ocean circulation.

## Abstract

Understanding the causes of the  ~90 ppmv atmospheric CO2 swings between glacial and interglacial climates is an important open challenge in paleoclimate research. Although the regularity of the glacial-interglacial cycles hints at a single driving mechanism, Earth System models require many independent physical and biological processes to explain the full observed CO2 signal. Here we show that biologically sequestered carbon in the ocean can explain an atmospheric CO2 change of 75 ± 40 ppmv, based on a mass balance calculation using published carbon isotopic measurements. An analysis of the carbon isotopic signatures of different water masses indicates similar regenerated carbon inventories at the Last Glacial Maximum and during the Holocene, requiring that the change in carbon storage was dominated by disequilibrium. We attribute the inferred change in carbon disequilibrium to expansion of sea-ice or change in the overturning circulation.

Atmospheric CO2 was  ~ 90 ppmv lower at the Last Glacial Maximum (LGM) than during the Holocene. Based on a carbon isotope budget, this study infers that at the LGM biological production pumped carbon into the deep ocean and sea ice kept it there.