Massive Antarctic Algal Blooms Significantly Reduced Atmospheric CO2 14,000 Years Ago, Study Finds

Massive algal blooms in the Southern Ocean approximately 14,000 years ago played a crucial role in regulating global carbon dioxide levels, according to new research published in Nature Geoscience. The study, which utilized ancient DNA analysis from seabed sediments, identified the genus Phaeocystis as the primary driver of significant carbon sequestration during this period.

The research focused on the Antarctic Cold Reversal (ACR), a climatic event that occurred between roughly 14,700 and 12,700 years ago. This era saw a temporary cooling trend following the last Ice Age, with Antarctic ice core records indicating a temperature drop of 1.5-2°C. The specific climatic conditions during the ACR, including prolonged winter sea ice followed by vigorous spring melt, created an ideal environment for Phaeocystis blooms. These algae absorbed substantial amounts of atmospheric CO2, thereby moderating the increase of this critical greenhouse gas.

A team from the Alfred Wegener Institute (AWI) conducted the analysis of sedimentary ancient DNA (sedaDNA), establishing a direct link between the ACR's environmental conditions and the extensive algal activity. This historical role of Phaeocystis in stabilizing atmospheric CO2 offers valuable insights into the sensitivity of marine ecosystems to climate shifts.

The findings also resonate with more recent observations. Studies analyzing 25 years of satellite data and ocean samples have indicated a decline in diatoms, another vital phytoplankton group for carbon storage, with a concurrent rise in smaller phytoplankton like haptophytes and cryptophytes. This shift, observed after 2016 and associated with sea ice loss, highlights how changes in sea ice extent can significantly alter phytoplankton communities.

The Southern Ocean is a critical conduit for atmospheric CO2 entering the ocean, absorbing over 40% of anthropogenic CO2. The potential weakening of this biological carbon pump due to changes in phytoplankton populations could have far-reaching consequences for the global climate system. The historical evidence from Phaeocystis blooms provides a vital perspective on current climate dynamics and the ocean's capacity to act as a carbon sink.