No glacial fertilization effect in the Antarctic Ocean

In the 200-meter-long deep-sea core U1537, the climate history of the last 1.5 million years was recorded in detail. "We have a similar resolution as in the ice cores, but we can look further back and thus already 'predict' what it will look like at Oldest Ice, at least in the dust cycles," reports Dr Thomas Ronge from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), who was involved in both the JOIDES Resolution expedition and the Nature Communications study. "The core was also unofficially christened Ice Core of the Seas," says the AWI geologist with a wink, referring to the Beyond EPICA - Oldest Ice project, in which AWI researchers and international partners are conducting deep ice core drilling on the Antarctic ice sheet. By analysing the current sediment core, the time span mapped to reconstruct the dust input can be nearly doubled, as Antarctic ice cores only cover the last 800,000 years. Current records from the deep ocean show that dust deposition during the ice ages was actually five to 15 times higher. This is also reflected in the ice cores.

However, the researchers found no evidence of a fertilization effect from dust in the Antarctic Ocean during the ice ages. Rather, the production of algae, for example, and thus carbon CO₂ sequestration, was high only during warm periods when dust input into the Scotia Sea was low. This means that during cold periods, other processes prevented the CO₂ captured in the ocean from escaping into the atmosphere and triggering warming. The main factors here are much more extensive sea ice cover, more intense stratification in the ocean, and reduced dynamics of the current systems, which contributed to a reduction in the CO₂ content of the atmosphere during cold periods.

The opposing trends in dust deposition and oceanic productivity during the ice ages and interglacial periods of the Pleistocene are accompanied by long-term, gradual changes in the climate system in the southern polar region. Bioproductivity was particularly high during the interglacial periods of the last 400,000 years, but during the mid-Pleistocene transition 1.2 million to 700,000 years ago, it differed little from that during cold periods. As the transition progressed, the dust input covered larger and larger areas in the Southern Hemisphere. Abrupt changes continued to occur 900,000 years ago, indicating greater glaciation of Antarctica.

"There is indeed evidence of a fertilization effect during the ice ages in cores outside the Antarctic zone," Weber concludes. "However, our study shows that atmospheric CO₂ fluctuations do not depend solely on iron fertilization from dust deposition. In the Antarctic Ocean, it is rather a complex interplay of a westerly wind system, productivity, and feedback with sea ice. This relationship has been consistent over the last 1.5 million years."