Marine nitrogen fixers mediate a low latitude pathway for atmospheric CO 2 drawdown.

Roughly a third (~30 ppm) of the carbon dioxide (CO ₂ ) that entered the ocean during ice ages is attributed to biological mechanisms. A leading hypothesis for the biological drawdown of CO ₂ is iron (Fe) fertilisation of the high latitudes, but modelling efforts attribute at most 10 ppm to this mechanism, leaving ~20 ppm unexplained. We show that an Fe-induced stimulation of dinitrogen (N ₂ ) fixation can induce a low latitude drawdown of 7-16 ppm CO ₂ . This mechanism involves a closer coupling between N ₂ fixers and denitrifiers that alleviates widespread nitrate limitation. Consequently, phosphate utilisation and carbon export increase near upwelling zones, causing deoxygenation and deeper carbon injection. Furthermore, this low latitude mechanism reproduces the regional patterns of organic δ ¹⁵ N deposited in glacial sediments. The positive response of marine N ₂ fixation to dusty ice age conditions, first proposed twenty years ago, therefore compliments high latitude changes to amplify CO ₂ drawdown.