Abstract
Roughly a third (~30 ppm) of the carbon dioxide (CO2) that entered the ocean during ice ages is attributed to biological mechanisms. A leading hypothesis for the biological drawdown of CO2 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 (N2) fixation can induce a low latitude drawdown of 7–16 ppm CO2. This mechanism involves a closer coupling between N2 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 δ15N deposited in glacial sediments. The positive response of marine N2 fixation to dusty ice age conditions, first proposed twenty years ago, therefore compliments high latitude changes to amplify CO2 drawdown.
Highlights
A third (~30 ppm) of the carbon dioxide (CO2) that entered the ocean during ice ages is attributed to biological mechanisms
N2 fixers are highly sensitive to the aeolian supply of Fe18,19, they represent up to half of primary production and C export in oligotrophic waters[20,21,22,23,24], they are physiologically adapted to P scarcity[25,26], produce organic matter that is enriched in C27–29, and previous modelling has demonstrated the potential of N2 fixation to draw CO2 into the ocean[30]
We undertook multi-millennial simulations using a global ocean biogeochemical model to explore the link between Fe fertilisation, N2 fixation and CO2 drawdown
Summary
We increased the supply of aeolian Fe to the ocean model from its modern[35] to glacial rate[5] (see Methods; Supplementary Fig. 2) under preindustrial physical conditions The glacial Fe supply allowed N2 fixers to inhabit the low NO3:PO4 waters at the boundary to upwelling zones where local Fe-N co-limitation prevails today[14,16] This shift in N2 fixation initiated strong biogeochemical feedbacks that encouraged PO4 utilisation, C export, suboxic expansion, denitrification, and a local NO3 supply via N2 fixation. Large regions of the tropical ocean were low in NO3:PO4, which enabled large gains in N2 fixation (88 Tg N yr−1) and CO2 drawdown (43 ppm) as Fe supply increased from 50 to 2500% of its modern rate.
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