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

Abstract

Abstract. Increased accumulation of respired carbon in the deep ocean associated with enhanced efficiency of the biological carbon pump is thought to be a key mechanism of glacial CO2 drawdown. Despite greater oxygen solubility due to sea surface cooling, recent quantitative and qualitative proxy data show glacial deep-water deoxygenation, reflecting increased accumulation of respired carbon. However, the mechanisms of deep-water deoxygenation and contribution from the biological pump to glacial CO2 drawdown have remained unclear. In this study, we report the significance of iron fertilization from glaciogenic dust for glacial CO2 decrease and deep-water deoxygenation using our numerical simulation, which successfully reproduces the magnitude and large-scale pattern of the observed oxygen changes from the present to Last Glacial Maximum. Sensitivity experiments reveal that physical changes (e.g., more sluggish ocean circulation) contribute to only half of all glacial deep deoxygenation, whereas the other half is driven by enhanced efficiency of the biological pump. We found that iron input from the glaciogenic dust with higher iron solubility is the most significant factor for enhancement of the biological pump and deep-water deoxygenation. Glacial deep-water deoxygenation expands the hypoxic waters in the deep Pacific and Indian Ocean. The simulated global volume of hypoxic waters is nearly double the present value, which suggest that the glacial deep-water is sever environment for the benthic animals. Our model underestimated the deoxygenation in the deep Southern Ocean due to enhanced ventilation. The model-proxy comparison of oxygen change suggest that the stratified Southern Ocean is required for reproducing oxygen decline in the deep Southern Ocean. Enhanced efficiency of biological pump contributes to decrease of glacial CO2 by more than 30 ppm, which is supported by the model-proxy agreement of oxygen change. Our findings confirm the significance of the biological pump in glacial CO2 drawdown and deoxygenation.

The Flux and Provenance of Dust Delivered to the SW Pacific During the Last Glacial Maximum

“…The speciation of Fe varies between products of different weathering regimes, leading to a difference in Fe solubility and bioavailability (Schroth et al, 2009). Glacial sources have been found to have a higher Fe solubility and bioavailability, so dust produced by glacial activity in New Zealand could alleviate Fe limitation more effectively than dust produced in arid regimes like Australia (Schroth et al, 2009; Shoenfelt et al, 2018; Yamamoto et al, 2019).…”

Abrupt Bølling‐Allerød Warming Simulated under Gradual Forcing of the Last Deglaciation

“…The coefficient of the horizontal isopycnal layer thickness diffusivity of the ocean model was changed to 7.0 × 10 6 cm 2 /s from 3.0 × 10 6 cm 2 /s, and the present model produces weak AMOC under the LGM because of enhanced dense Antarctic Bottom Water (AABW) formation (Kawamura et al, ). The MIROC has been used to investigate the climate of the LGM with radiative forcing and climate feedback (Yoshimori et al, ), the effect of ice sheets on the climate and the AMOC (Abe‐Ouchi et al, ; Kawamura et al, ; Sherriff‐Tadano et al, ), ocean biogeochemical cycles (Kobayashi et al, ; Kobayashi & Oka, ; Yamamoto et al, ), and mass balance of the Antarctic ice shelves (Kusahara et al, ; Obase et al, ).…”

Author response to Reviewer Comments #3

“…RC: p21, L. 10 -p22, L. 5: This section really has to be discussed in light of all the work that has been done on the impact of iron fertilisation in the Southern Ocean. Some work on the topic: Watson et al, 2000, Nature;Jaccard et al, 2013, Science;Yamamoto et al, 2019, Climate of the Past;…”

“…According to Kohfeld et al (2005), based on sediment data, the Southern Ocean biological productivity mechanism could account for no more than half of the glacial CO2 drawdown. Others emphasise that Southern Ocean biological export productivity fluxes may have been weaker in the LGM, in absolute terms, but that with weaker Southern Ocean upwelling, the iron-enhanced productivity contributed to a stronger biological pump of carbon and was a major contributor to the LGM CO2 drawdown (Jaccard et al, 2013;Martinez-Garcia et al, 2014;Yamamoto et al, 2019).…”

“…References added following this reviewers' comments: Watson et al, 2000Joos et al, 2004Tarnocai et al, 2009Ganopolski et al, 2010Brovkin et al, 2012Jaccard et al, 2013Schneider et al, 2013Menviel et al, 2015Yu et al, 2016Ganopolski and Brovkin, 2017Lindgren et al, 2018Mauritz et al, 2018Yamamoto et al, 2019Treat et al, 2019 Specific comments: RC: 1) Abstract: The first line does not make sense. Please reformulate.…”