A new study reveals that shifts in Antarctic Bottom Water circulation around 18,000 to 10,000 years ago drove the release of stored carbon into the atmosphere, contributing to the warming at the end of the last Ice Age. Researchers analyzed sediment cores to trace these changes, highlighting the Southern Ocean's role in the global carbon cycle. The findings suggest implications for modern Antarctic ice melt and climate projections.
Around 12,000 years ago, as the last Ice Age ended and the Holocene began, global temperatures rose, marking a major climate transition. A study published in Nature Geoscience, led by Dr. Huang Huang of the Laoshan Laboratory in Qingdao and including Dr. Marcus Gutjahr from GEOMAR, reconstructed the extent of Antarctic Bottom Water (AABW) over the past 32,000 years.
The team examined nine sediment cores from the Atlantic and Indian sectors of the Southern Ocean, at depths between 2,200 and 5,000 meters. They analyzed the isotopic composition of neodymium in the sediments to track changes in deep-water masses. "We wanted to understand how the influence of Antarctic Bottom Water, the coldest and densest water mass in the global ocean, changed during the last deglaciation, and what role it played in the global carbon cycle," says Huang, who completed his PhD at GEOMAR in 2019.
During the Ice Age, the deep Southern Ocean contained carbon-rich waters originating from the Pacific, a precursor to today's Circumpolar Deep Water, which isolated carbon and kept atmospheric CO2 low. Between 18,000 and 10,000 years ago, AABW expanded in two phases, coinciding with Antarctic warming events. This expansion, driven by reduced sea-ice cover and meltwater lowering water density, increased vertical mixing and brought stored carbon to the surface, releasing it into the atmosphere.
"The expansion of the AABW is linked to several processes," explains Gutjahr. "Warming around Antarctica reduced sea-ice cover, resulting in more meltwater entering the Southern Ocean. The Antarctic Bottom Water formed during this transitional climate period had a lower density due to reduced salinity. This late-glacial AABW was able to spread further through the Southern Ocean, destabilizing the existing water-mass structure and enhancing exchanges between deep and surface waters."
The study challenges prior assumptions that North Atlantic changes primarily drove Southern Ocean shifts, emphasizing Antarctic influences on CO2 rise. Today, the Southern Ocean has warmed faster than most oceans below 1,000 meters over the past 50 years. "Comparisons with the past are always imperfect," says Gutjahr, "but ultimately it comes down to how much energy is in the system. If we understand how the ocean responded to warming in the past, we can better grasp what is happening today as Antarctic ice shelves continue to melt." Such paleoclimate insights from sediment cores aid in modeling future ice loss and carbon dynamics.