Antarctic Climate History Unveiled: Sunlight & Sediments Tell the Story

The remnants of ice attached to the coast offer astounding insights into the climate history of past millennia. An international research team led by the CNR Institute of Polar Sciences (Italy) and involving the University of Bonn has applied a new method of analyzing sediment drill cores to show the climate history of the past 3,700 years in Antarctica. Surprisingly, it is connected to the natural fluctuations in solar activity. The study has now been published in the journal Nature Communications. Ice forms wherever there is water and it gets cold enough. It can float freely in the sea as drift ice or can form pack ice when the wind and ocean currents connect ice floes together. For "fast ice," however, freedom is a thing of the past: It is no longer able to move, as it is firmly attached to the coast or the shallows. If this phenomenon were compared to a cocktail, the drift ice would be the ice cubes floating in the glass. The fast ice, however, would stick to the edge of the glass—for instance, if the cocktail mixer had frozen the glass beforehand to keep the drink cold longer. An international team of researchers focused on precisely this fast ice—not in a cocktail glass but instead along the coast. Even in freezing Antarctica, the ice is subjected to constant change. If it gets warmer, it breaks up, melts, and gets thinner. If there is a cold period, it spreads. Fast ice influences biogeochemical cycles on the coast, governs the life of many Antarctic species—including penguins—and, in some regions of the continent, even serves as a natural landing platform for aviation. The study developed a new method of tracking the thawing and freezing of the fast ice over thousands of years. These findings are crucial for understanding the natural factors that influence all forms of frozen water in Antarctica. The knowledge of this phenomenon helps researchers to distinguish natural fluctuations from human-induced climate change. "Fast ice is one of the 'missing pieces' in the puzzle of Antarctica," says Dr. Tommaso Tesi, the main author of the study by the Institute of Polar Sciences in Bologna. "We discovered that the time at which the fast ice breaks up is linked to long-term solar cycles." Dr. Michael Weber from the Institute of Geosciences at the University of Bonn, who provided the method, adds, "This offers a fundamentally new insight into how distant solar variability can bring about major changes in the Antarctic atmosphere and in the ocean." New technique decodes ice history with high precision Direct satellite observations of the fast ice in Antarctica only extend over the last few decades. This was not sufficient for the researchers, as they wanted to obtain data about the fast ice from past millennia to learn more about its long-term natural cycles. As a result, the research team focused on automated counting techniques for sediment cores taken from the Edisto Inlet on the northern coast of Victoria Land (Ross Sea). What sets these sediment cores apart is that they feature thin horizontal layers (laminae). The cores contain alternating dark and light layers, which depend on the conditions of the fast ice. Dark layers indicate the initial breaking up of the fast ice in early summer. Then high concentrations of diatoms, which live in the sea ice, are seen. Light layers, however, indicate prolonged ice-free conditions with open water, characterized by the presence of the diatom Corethron pennatum. Using the dark and light layers in the sediment drill cores, the researchers created a continuous record of the variability of fast ice over the last 3,700 years with the aid of further data on the tiny organisms and automated image analyses of these layers. The evaluation of this data showed that the breaking up of the fast ice does not follow a simple annual cycle, but instead shows a more complex pattern over longer timescales. Solar cycles as drivers The analysis revealed persistent cyclical patterns in the breaking up of the fast ice that occur about every 90 years and 240 years and correspond to the known Gleissberg and Suess-de Vries solar cycles. Such cycles occur due to fluctuations in solar activity. The study describes a "cascade of events" that links solar activity to the stability of coastal ice. Changes in solar activity disrupt the zonal winds over the Southern Ocean. These altered winds drive the retreat of free-floating sea ice (regional pack ice) along the coast of Victoria Land. Satellite data confirm a strong link: If the protective pack ice retreats early, the mainland ice in the bay is exposed to local winds, waves and warming—causing the ice to break up. Climate model simulations using exaggerated solar influences confirm this. They show that the increased solar radiation is a main driver for the warming of the sea surface, which reduces the insulating sea ice and increases heat exchange between the ocean and atmosphere. "Our approach offers a practicable way to expand the records about mainland ice far beyond the limits of the measuring instruments," said Dr. Tesi.