4:15 PM - 4:30 PM
[MIS12-29] Theoretical constraints on ocean anoxic conditions with a coupled model of ocean biogeochemical cycle and microbial ecosystem
Keywords:Ocean Anoxic Events, 1-D ocean microbial ecosystem model
During the Phanerozoic (~541–0 Ma), the redox state of the ocean has fluctuated both gradually and transiently. During the Cambrian, the ocean may have been depleted in oxygen. Around 400 million years ago, the ocean had been oxygenated following an increase in the atmospheric oxygen level (pO2) associated with the emergence of vascular plants (Dahl et al., 2010). Since then, ocean anoxic events (OAEs), events of a temporal development of oxygen-poor water masses in the ocean, occurred repeatedly (e.g., Reershemius and Planavsky, 2021). OAEs would be caused by oceanic eutrophication owing to climate warmings associated with massive volcanisms. The enhanced chemical weathering of continents and the nutrient supply to the ocean would have increased the oceanic primary productivity, increasing the consumption of oxygen in the ocean by the organic matter decomposition (Ozaki et al., 2011). Records of carbon isotope excursions during the periods of the OAEs also indicate the fluctuations in the activity of marine ecosystems (e.g., Cramer and Jarvis, 2020). For example, during many OAEs, such as Permian Triassic boundary OAE and Cretaceous OAEs, the biomarker of green sulfur bacteria is found in coastal sediments, which indicates that water masses with high hydrogen-sulfide content have reached the photic zone, which is called the photic zone euxinia (PZE) (e.g., Grice et al., 2005; Jenkyns, 2010). However, the responses of the biogeochemical cycle in the surface ocean to changes in global-mean temperature and pO2 over the Phanerozoic time and differences of the conditions for the photic zone euxinic the responses in the coastal and open ocean regions remain ambiguous. In this study, we developed a coupled model of ocean biogeochemical and microbial ecosystem which considers biogeochemical reactions from oxic to anoxic marine environmental conditions. We systematically investigated the responses of the marine environment to changes in atmospheric pO2 and global average surface temperature.
We show that when atmospheric pO2 decreases and/or global average temperature increases, it leads to ocean deoxygenation and the development of euxinic water masses. In the pelagic surface ocean, an anoxic water mass occurred but PZE did not occur. On the other hand, in the upwelling region, PZE occurred easily: under the present condition, PZE occurs with warming of ~5 ℃. Following the changes in the nutrient concentrations and the redox state of the ocean, the activities of the primary producer also change. Starting from the present condition where algae are the dominant primary producers, it shifts to nitrogen-fixing (diazotrophic) cyanobacteria following a decrease in atmospheric pO2 and global warming. This is consistent with observations that the biomarker of cyanobacteria increases during many OAEs (e.g., Cao et al., 2009; Kuypers et al., 2009). In the upwelling region, green sulfur bacteria become active in the lower part of the photic zone when PZE occurs, which is consistent with the detections of the biomarker of green sulfur bacteria in coastal sediments (e.g., Grice et al., 2005; Jenkyns, 2010).
By comparing our results and the temperature and atmospheric pO2 variations during the Phanerozoic estimated in the previous study (Lenton et al., 2018), we show that during the early Paleozoic when the atmospheric pO2 was low, euxinic water masses can develop in the ocean, while the ocean would have been oxygenated after about 400 million years ago, following the increase in the atmospheric pO2. This is consistent with the records of molybdenum isotope (Dahl et al., 2010). Since then, PZE may occur only transiently following the global warming associated with massive volcanisms.
We show that when atmospheric pO2 decreases and/or global average temperature increases, it leads to ocean deoxygenation and the development of euxinic water masses. In the pelagic surface ocean, an anoxic water mass occurred but PZE did not occur. On the other hand, in the upwelling region, PZE occurred easily: under the present condition, PZE occurs with warming of ~5 ℃. Following the changes in the nutrient concentrations and the redox state of the ocean, the activities of the primary producer also change. Starting from the present condition where algae are the dominant primary producers, it shifts to nitrogen-fixing (diazotrophic) cyanobacteria following a decrease in atmospheric pO2 and global warming. This is consistent with observations that the biomarker of cyanobacteria increases during many OAEs (e.g., Cao et al., 2009; Kuypers et al., 2009). In the upwelling region, green sulfur bacteria become active in the lower part of the photic zone when PZE occurs, which is consistent with the detections of the biomarker of green sulfur bacteria in coastal sediments (e.g., Grice et al., 2005; Jenkyns, 2010).
By comparing our results and the temperature and atmospheric pO2 variations during the Phanerozoic estimated in the previous study (Lenton et al., 2018), we show that during the early Paleozoic when the atmospheric pO2 was low, euxinic water masses can develop in the ocean, while the ocean would have been oxygenated after about 400 million years ago, following the increase in the atmospheric pO2. This is consistent with the records of molybdenum isotope (Dahl et al., 2010). Since then, PZE may occur only transiently following the global warming associated with massive volcanisms.