A groundbreaking study published in Nature Geoscience has revealed the crucial role of the Southern Ocean in the end of the last Ice Age, approximately 12,000 years ago. This period marked a significant shift in global temperatures and the emergence of early Holocene human settlements. Led by Dr. Huang Huang from the Laoshan Laboratory in Qingdao, with the expertise of Dr. Marcus Gutjahr, a geochemist at GEOMAR, the research team delved into the spatial evolution of Antarctic Bottom Water (AABW) over the past 32,000 years.
Dr. Huang explained the motivation behind their study: "We aimed to unravel how the influence of Antarctic Bottom Water, the coldest and densest water mass globally, evolved during the last deglaciation and its impact on the global carbon cycle."
To achieve this, the researchers analyzed nine sediment cores from the Atlantic and Indian sectors of the Southern Ocean, collected from depths ranging from 2,200 to 5,000 meters and widely spaced locations. By examining the isotopic composition of neodymium, a trace metal incorporated into sediments from surrounding seawater, they reconstructed the extent of AABW over tens of thousands of years.
Dr. Marcus Gutjahr elaborated on the significance of neodymium: "Dissolved neodymium and its isotopic signature in seawater provide invaluable insights into the origin of deep-water masses. Our previous studies revealed that the neodymium signature in the deep South Atlantic only attained its modern composition around 12,000 years ago. Sediments from the last Ice Age exhibited values not found in the Southern Ocean today, initially leading us to question our methods or the integrity of the sediment core. However, the real enigma was: What could have generated such an exotic isotopic signature? Such a signature can only arise when deep water remains nearly motionless for extended periods, allowing benthic fluxes from the seafloor to dominate the isotopic imprint in marine sediments."
The study identified two distinct phases of AABW expansion, coinciding with known warming events in Antarctica. As the planet warmed and ice sheets melted between approximately 18,000 and 10,000 years ago, the volume of AABW expanded, releasing stored carbon into the atmosphere.
"The expansion of AABW is linked to several processes," Gutjahr explained. "Warming around Antarctica reduced sea-ice cover, leading to increased meltwater entering the Southern Ocean. The AABW formed during this transitional climate period had a lower density due to reduced salinity, allowing it to spread further through the Southern Ocean and destabilize existing water-mass structures, enhancing exchanges between deep and surface waters."
Contrary to previous assumptions, the study suggests that changes in the North Atlantic, including the formation of North Atlantic Deep Water (NADW), had more limited impacts on deep-water circulation in the South Atlantic. Instead, the displacement of a glacial, carbon-rich deep-water mass by newly formed AABW is believed to have played a central role in the rise of atmospheric CO2 at the end of the last Ice Age.
"Comparisons with the past are inherently imperfect," Gutjahr acknowledged. "Ultimately, it boils down to the energy within the system. Understanding how the ocean responded to warming in the past can enhance our grasp of current Antarctic ice shelf melting."
The Southern Ocean's sheer size makes it a significant regulator of Earth's climate. Over the past five decades, waters deeper than approximately 1,000 meters around Antarctica have warmed significantly faster than most other parts of the global ocean. To comprehend how these changes impact the ocean's capacity to absorb and release carbon dioxide, long-term monitoring of physical and biogeochemical processes and their integration into climate models are essential.
"I aim to comprehend the modern ocean to interpret signals from the past," Gutjahr emphasized. "By tracing how Antarctic Bottom Water has changed over the last few thousand years, we can more accurately assess the potential rate of Antarctic Ice Sheet mass loss in the future."
Palaeoclimate data derived from sediment cores is invaluable for this endeavor, providing insights into past climates warmer than today and improving projections of future climate change.
