Natural ocean iron fertilization and climate variability over geological periods

Marine primary producers are largely dependent on and shape the Earth's climate, although their relationship with climate varies over space and time. The growth of phytoplankton and associated marine primary productivity in most of the modern global ocean is limited by the supply of nutrients, including the micronutrient iron. The addition of iron via episodic and frequent events drives the biological carbon pump and promotes the sequestration of atmospheric carbon dioxide (CO2) into the ocean. However, the dependence between iron and marine primary producers adaptively changes over different geological periods due to the variation in global climate and environment. In this review, we examined the role and importance of iron in modulating marine primary production during some specific geological periods, that is, the Great Oxidation Event (GOE) during the Huronian glaciation, the Snowball Earth Event during the Cryogenian, the glacial-interglacial cycles during the Pleistocene, and the period from the last glacial maximum to the late Holocene. Only the change trend of iron bioavailability and climate in the glacial-interglacial cycles is consistent with the Iron Hypothesis. During the GOE and the Snowball Earth periods, although the bioavailability of iron in the ocean and the climate changed dramatically, the changing trend of many factors contradicted the Iron Hypothesis. By detangling the relationship among marine primary productivity, iron availability and oceanic environments in different geological periods, this review can offer some new insights for evaluating the impact of ocean iron fertilization on removing CO2 from the atmosphere and regulating the climate. Ocean iron fertilization regulates climate by promoting phytoplankton and biological carbon pumps, but the process is affected by many other factors. The relationship between natural iron fertilization and climate change in geological time cannot prove that iron fertilization is the main factor leading to the ice age. In the glacial-interglacial cycle in the Pleistocene, there were strong correlations among marine primary productivity, temperature, and iron bioavailability in the ocean. In addition to biologically mediated carbon sequestration, ocean circulation and physical dissolution pumps also played important roles in regulating the climate over many geological periods, which is even considered to surpass the contribution of biologically mediated processes to global climate change.image