Iron Fertilization in the Subantarctic South Pacific?

The scarcity of iron limits marine export production and carbon uptake in about 25% of the global ocean where the concentration of major nutrients is high, yet phytoplankton growth is low. Of these, the Southern Ocean is the region where variations in iron availability can have the largest effect on Earth’s carbon cycle through its fertilizing effect on marine ecosystems, both in the modern and in the past.

Recent work in the Subantarctic South Atlantic (Martínez-Garcia et al., 2009, 2014, Anderson et al., 2014) suggests that dust-driven iron fertilization lowered atmospheric CO₂ from about 225 ppm to 185 ppm in the latter half of each glacial cycle of the late Pleistocene, with the increase in Subantarctic productivity consuming a greater fraction of the surface nutrients and thus driving more storage of carbon in the ocean interior. The opposite effect, reduced iron fertilization, has been hypothesized to drive the 15-20 ppm increase during Heinrich Events in the last glacial cycle (Martínez -Garcia et al, 2014).

The vast majority of the information we have so far is from observations in the Subantarctic Atlantic and therefore our current estimates of the role of the Southern Ocean in lowering CO₂ rely critically on the assumed extrapolation of the results from the Atlantic Sector to the entire Southern Ocean.

However, the Pacific Sector of the Southern Ocean not only accounts for the largest surface area of the Subantarctic Southern Ocean, but the deep Pacific Ocean also can be inferred - based on its volume – to have stored the largest fraction of carbon that was extracted from the atmosphere and from the terrestrial biosphere during glacial periods.

Here we report first results from a set of cores from the Subantarctic Pacific (PS75, Lamy et al 2014), including a high-resolution sediment core (PS75/056-1) from the flank of the East Pacific Rise that allows to resolve millennial year variability over the past glacial cycle. We test how tightly dust and biological productivity are coupled over glacial/interglacial and millennial timescales in the Subantarctic Pacific. The results will allow us to determine whether or not the Subantarctic Pacific experienced increased nutrient utilization due to iron fertilization during glacial periods, in phase with iron fertilization in the South Atlantic.