Since the seminal work of John H. Martin, who first discover the widespread iron limitation in the polar oceans, the effect of iron on marine ecosystems structure, productivity and organic carbon sequestration has been extensively studied by the scientific community. Paleoceanographic studies provide a unique opportunity to directly test the influence of continued large-scale iron fertilization on marine phytoplankton productivity, and to estimate its potential effect on atmospheric CO2 levels. Using a combination of geochemical proxies we have shown that the increase in the deposition of iron rich dust in the Subantarctic during ice ages is systematically associated with periods of high marine productivity over the past million years, as expected from Martin’s iron fertilization hypothesis (Martinez-Garcia et al 2009; Lamy et al. 2014). The extension of our reconstructions back to 4 million years has provided new evidence of a tight connection between high dust/iron input to the Southern Ocean and the development of stronger glaciations, indicating that iron fertilization may have also played an important role in long-term climate evolution (Martinez-Garcia et al 2011).
Iron-rich dust streaming from Patagonian deserts fertilizes nutrient-poor southern oceans. NASA/GODDARD SPACE FLIGHT CENTER, WILLIAM M. PUTMAN AND ARLINDO M. DA SILVA
Recently, we have added one of the last remaining pieces to the iron fertilization puzzle. Using a newly developed geochemical technique based on the analysis of the nitrogen isotopic composition of foraminifera-bound organic matter we have demonstrated that peak glacial times and millennial cold events are characterized not only by increases in dust deposition and productivity, but also by an increase in the degree of nitrate consumption by marine phytoplankton, a combination that is uniquely consistent with Subantarctic iron fertilization (Martinez-Garcia et al, 2014). Thus, our research suggests that this process played a fundamental role in controlling changes in atmospheric CO2 levels, and therefore in modulating global climate evolution, in a variety of timescales from millennia to millions of years.
Iron Fertilization during Ice ages
Southern Ocean overturning and CO2
Changes in Southern Ocean overturning and deep ocean ventilation are key to explain the full magnitude of the glacial interglacial atmospheric CO2 change. An increase in Southern Ocean vertical stratification during ice ages is thought to have reduced CO2 outgassing from the deep ocean to the atmosphere contributing to explain the glacial atmospheric CO2 decrease. Our work has offered strong support for this view by providing empirical evidence of reduced nutrient supply to the Antarctic surface (Studer et al. 2015; Wang et al. 2017), decreased deep ocean oxygenation (Jaccard et al. 2016), and increased residence time for Southern Ocean surface waters (Hasenfratz et al. 2019) during late Pleistocene ice ages. In fact, our data suggests that the combination of these processes and iron fertilization largely determined the temporal structure of the glacial-interglacial atmospheric CO2 record over the past 800,000 years (Jaccard et al. 2013), including interglacial CO2 trends. During the Holocene, increased nutrient supply to the Southern Ocean would have weakened the ocean’s biological pump that stores CO2 in the ocean interior, possibly explaining the pre-industrial 20 ppm atmospheric CO2 rise observed over the past 6000 years (Studer et al. 2018). During the last interglacial (∼127,000 years ago) a slowdown in AABW formation likely driven coastal freshening due to mass loss from the Antarctic ice sheet, possibly caused the atmospheric CO2 minimum observed at that time (Hayes et al. 2014).
Southern Ocean Icebergs. Photograph taken during expedition ANT-XXVI/2 to the Pacific sector of the Southern Ocean
Quaternary Ice Ages
One of the main research in interests of the the laboratory is the study of past changes in climate and ocean biogeochemistry over the Glacial/Interglacial cycles of the Pleistocene. In particular, over the past years we have focussed on: the role of the Southern Ocean in controlling the concentration of carbon dioxide in the atmosphere, the processes that control changes in marine N2 fixation, and the causes of the Middle Pleistocene Climatic Transition. Below, we briefly describe some of the results of our research on these topics.
N2 fixation during ice ages
Modern studies have yielded diverse views of the controls on N2 fixation. For example, changes in temperature and iron bioavailability are considered to play an important role on the distribution of N2 fixation in the modern ocean. Therefore, ice-age cooling might be expected to reduce N2 fixation, while an increase in the supply of iron-bearing dust has been suggested to cause glacial enhancements in N2 fixation. However, our biogeochemical reconstructions using foraminifera bound N isotopes suggest that the supply of waters with excess phosphorus is the master variable that controls N2 fixation during the glacial interglacial cycles of the Pleistocene. In the North Atlantic, the supply of excess phosphorus appears to be driven by changes in equatorial Atlantic upwelling and Antarctic Intermediate Water circulation (Straub et al. 2013). In contrast, in the South China sea it appears to be related to sea level-driven variations in shallow sediment denitrification associated with the cyclic drowning and emergence of the continental shelves (Ren et al. 2017). Our data also indicates a strong coupling between N2 fixation and denitrification, i.e. the dominant N sources and sinks in the ocean, suggesting a stable oceanic fixed N reservoir over glacial cycles.
Inferred glacial/interglacial N cycle changes along the SCS margin (Ren et al. 2017)
The Middle Pleistocene Climatic Transition
The Mid-Pleistocene Transition (MPT) marked a major shift in the response of Earth’s climate system to orbital forcing. During the Early Pleistocene, glacial–interglacial (G-IG) climate cycles were paced by ∼40,000 y obliquity cycles, whereas G-IG cycles after the MPT gradually intensified over multiple obliquity cycles (i.e., 80- to 120-ky periodicity), and acquired a distinctively asymmetric character. These changes gave rise to longer and colder late Pleistocene ice ages. The MPT occurred in the absence of any significant change in the pacing or amplitude of orbital forcing, indicating that it arose from an internal change in the response of the climate system. It has been suggested that a decrease in radiative climate forcing exerted by a CO2 decline may have caused the MPT. Our work suggest that this decrease in atmospheric CO2 levels may have been triggered by a combination of increased iron fertilization (Martínez-García et al. 2011) and a decrease in the residence time of Antarctic surface waters (Hasenfratz et al. 2019). Our work suggests that the MPT was initiated by a change in ice sheet dynamics, but that longer and deeper post-MPT ice ages were sustained by carbon cycle feedbacks related to dust fertilization of the Subantarctic ocean and changes in the Antarctic ocean (Chalk et al. 2017).
Reconstructed decrease in the water supply to the Antarctic surface after the MPT (Hasenfratz et al. 2019).