Marine nitrogen fixers mediate a low latitude pathway for atmospheric CO2 drawdown

Published: 10 October 2019 by, Pearse J. Buchanan, Zanna Chase, Richard J. Matear, Steven J. Phipps & Nathaniel L. Bindoff

https://www.nature.com/articles/s41467-019-12549-z

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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.

Introduction

As much as 30 ppm of the total glacial-interglacial difference in atmospheric CO2 is attributed to marine biological mechanisms

1. The most prominent biological mechanism is the fertilisation of Fe-limited high latitude regions, namely the Southern Ocean

2 and subarctic Pacific

3, with dust-borne Fe under dusty glacial conditions

4,5. Today, phytoplankton that inhabit these high latitude regions are unable to consume all available macronutrients, which allows CO2 to escape to the atmosphere as deep waters mix into surface layers. Iron fertilisation of the high latitude glacial ocean therefore stands as a leading hypothesis to explain a more efficient biological carbon (C) pump and the associated drawdown of atmospheric CO2. Yet, modelling focussed on the high latitudes has sequestered less than 10 ppm of atmospheric CO2 via Fe fertilisation

5,6,7 and indicates that additional biological mechanisms are required.

There are good reasons to accommodate the lower latitudes in our search for additional mechanisms. First, the region is enormous. Surface waters between 40°S and 40°N represent over two thirds of CO2 outgassing to the atmosphere

8 and more than half of global C export

9,10. Second, unconsumed phosphate (PO4) at concentrations in excess of 0.1 to 0.2 mmol m−3 exists in surface waters across the tropics, which is evidence for unrealised biological CO2 fixation. Third, tropical oceans produce organic matter that is enriched in C because tropical phytoplankton are adapted to fix more C per unit phosphorus (P) under P scarcity

11. Fourth, oxygen-deficient waters in the tropical Pacific, Indian and Atlantic allow organic matter to sink deeper into the ocean interior

10,12,13. If these mechanisms are combined, the co-occurrence of more complete PO4 utilisation and the production of C-enriched organic matter near to oxygen-deficient zones would constitute an effective pathway of CO2 drawdown.

Enabling greater PO4 utilisation and CO2 drawdown in the lower latitudes, however, requires simultaneously relieving Fe limitation in upwelling zones

14, nitrate (NO3) limitation in the tropics

14,15 and their co-limitation at the boundary of both regimes

16. An aeolian Fe-induced stimulation of dinitrogen (N2) fixation is therefore an obvious candidate to alleviate low latitude nutrient limitation. Originally proposed by Falkowski

17, this mechanism is now supported by many independent lines of evidence. N2 fixers are highly sensitive to the aeolian supply of Fe

18,19, 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 C

27,28,29, and previous modelling has demonstrated the potential of N2 fixation to draw CO2 into the ocean

30. Dinitrogen fixation is also inextricably linked to suboxic zones (dissolved oxygen (O2) < 10 mmol m−3) where denitrification strips NO3 from the waters that upwell at the equator, creating a potential niche for N2 fixers across the wide expanse of the lower latitudes. The strength of N2 fixation, which strengthens PO4 utilisation, whole community C:P ratios and C export

20, is thus tied to the strength of denitrification, which in turn strengthens N2 fixation.

In this study, we use an ocean model to demonstrate that aeolian Fe supply to the tropical oceans under glacial conditions

31,32 relieves low latitude nutrient limitation

14,15,16 by stimulating N2 fixation, which in turn drives PO4 consumption, suboxic zone expansion, the acceleration of the nitrogen (N) cycle and a more efficient C export to the interior ocean. Furthermore, we estimate the contribution of this mechanism to CO2 drawdown and reveal evidence of its existence within glacial-interglacial sedimentary records of N isotopes (δ15Norg).

Results

A low latitude pathway

Inspired by these insights, we undertook multi-millennial simulations using a global ocean biogeochemical model to explore the link between Fe fertilisation, N2 fixation and CO2 drawdown. The ocean biogeochemical model is part of the Commonwealth Scientific and Industrial Research Organisation (CSIRO) Mark 3L—Carbon of the Ocean, Atmosphere and Land (Mk3L-COAL)

33. The model is designed for long-term, global oceanographic studies. It resolves multi-millennial timescales and so produces equilibrium circulation states under a given set of atmospheric conditions. It is equipped with prognostic C, PO4, NO3, 15NO3, and Fe cycles

34 (see Methods), and includes a dynamic ecosystem component where phytoplankton alter their nutrient requirements, stoichiometry and remineralisation rates according to their environment

33 (Supplementary Fig. 1). 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 (Mk3Lmild state in 

Table 1; Supplementary Note 1; Supplementary Figs. 3 and 4; Supplementary Table 1) with an atmospheric CO2 held at 280 ppm, and assessed changes to elemental cycling. To isolate the response of the lower latitudes, we nudged subsurface Fe concentrations to 0.6 μmol m−3 on a yearly timescale, which ensured that Fe was near non-limiting in regions of strong mixing, like the Southern Ocean and subarctic Pacific.

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