Combined Effects of Atmospheric and Seafloor Iron Fluxes to the Glacial Ocean

Abstract

Changes in the ocean iron cycle could help explain the low atmospheric CO2 during the Last Glacial Maximum (LGM). Previous modeling studies have mostly considered changes in aeolian iron fluxes, although it is known that sedimentary and hydrothermal fluxes are important iron sources for today's ocean. Here we explore effects of preindustrial‐to‐LGM changes in atmospheric dust, sedimentary, and hydrothermal fluxes on the ocean's iron and carbon cycles in a global coupled biogeochemical‐circulation model. Considering variable atmospheric iron solubility decreases LGM surface soluble iron fluxes compared with assuming constant solubility. This limits potential increases in productivity and export production due to surface iron fertilization, lowering atmospheric CO2 by only 4 ppm. The effect is countered by a decrease in sedimentary flux due to lower sea level, which increases CO2 by 15 ppm. Assuming a 10 times higher iron dust solubility in the Southern Ocean, combined with changes in sedimentary flux, we obtain an atmospheric CO2 reduction of 13 ppm. The high uncertainty in the iron fluxes does not allow us to determine the net direction and magnitude of variations in atmospheric CO2 due to changes in the iron cycle. Our model does not account for changes to iron‐binding ligand concentrations that could modify the results. We conclude that when evaluating glacial‐interglacial changes in the ocean iron cycle, not only surface but also seafloor fluxes must be taken into account.

Glacial CO<sub>2</sub> decrease and deep-water deoxygenation by iron fertilization from glaciogenic dust

“…Our results show that the glacial CO2 reduction due to dust-bone iron fertilization is mainly driven by glaciogenic dust. Simulated total CO2 reduction of 16.8 ppm induced by iron fertilization is in the range of previous studies using OGCM (8-25 ppm CO2 drawdown (Bopp et al, 2003;Parekh et al, 2006;Tagliabue et al, 2009;Oka et al, 2011;Lambert et al, 2015;Muglia et al, 2017). DFe supply from dust also contributes to glacial CO2 reduction through enhancing the efficiency of the biological pump ( Table 2).…”

“…On the other hand, the hydrothermal DFe flux is increased by lower sea level and bottom pressure (Middleton et al, 2016). Muglia et al (2017) Our model-proxy comparison shows the importance of the combination of more sluggish SO circulation and enhanced biological transport of organic matter for the greater accumulation of respired carbon and deoxygenation in the deep SO.…”

Aerosol-Climate Interactions During the Last Glacial Maximum

“…In general, the models indicate that LGM dust-induced iron fertilization could be responsible for increased export production and for the associated drawdown of almost 20 ppmv of atmospheric CO 2 , despite the large spread in estimates (Fig. 3b) and the difficulty in establishing a coherent comparison given the diversity on experimental designs [175][176][177][178][179][180][181][182]. Box model studies (not included in Fig.…”

“…See also pre-existing global compilations of changes in ocean productivity in references (165,174). b Model-based estimates of CO 2 drawdown induced by increased LGM dust deposition [175][176][177][178][179][180][181][182]. The semi-circle on the x-axis in b marks the average of the model ensemble.…”

“…In synthesis, marine sediment core data generally agree on the occurrence of dust-borne iron fertilization as a mechanism that enhanced the efficiency of the biological pump and resulted in increased export production, at least in the subantarctic Southern Ocean [167,189]. Ocean biogeochemistry models indicate a reduction of~20 ppmv CO 2 during the LGM in response to iron fertilization [182]. The quantification of this effect and its impact on the global carbon cycle remain however highly uncertain, most notably because of uncertainties in the representation of key process in ocean biogeochemistry models [157], in the quantification of dust [90] and other potential sources of iron to HNLC areas in particular [186], and because of uncertainty in quantifying the bioavailable fraction of iron [190].…”