Led by geoscientists from the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research (AWI), the study provides the southernmost directly assessable climate and environmental data from the Cretaceous and poses new challenges for climate modellers around the globe. The study has now been published in the journal Nature.
The mid-Cretaceous time interval, from ca. 115 million to 80 million years ago, is not only considered the age of the dinosaurs, but was also the warmest period in the past 140 million years. Sea surface temperatures in the tropics at this time were likely as high as some 35 degrees Celsius, and the sea level would have been 170 metres higher than today. Yet we still know very little about environmental conditions in the Cretaceous south of the polar circle, since there are virtually no reliable climate archives that extend that far back in time. The new sediment core offers the team of experts the first chance to reconstruct the West Antarctic climate during the warmest interval of the Cretaceous, thanks to the unique evidence it contains.
In the sediment core, which the team collected with the University of Bremen’s seafloor drill rig MARUM-MeBo70 near the Pine Island Glacier on an RV Polarstern expedition, they found pristinely preserved forest soil from the Cretaceous. “During the initial shipboard assessments, the unusual colouration of the sediment layer quickly caught our attention; it clearly differed from the layers above it. Moreover, the first analyses indicated that, at a depth of 27 to 30 metres below the ocean floor, we had found a layer originally formed on land, not in the ocean,” reported first author Dr Johann Klages, a geologist at the AWI in Bremerhaven.
The sedimentologist and palaeoclimatologist Professor Werner Ehrmann, head of the Institute for Geophysics and Geology at Leipzig University, was involved in planning and preparing the Polarstern expedition and was also on board the research vessel in February and March 2017. Ehrmann assisted with evaluating the sediment samples in the laboratory. “We were very surprised indeed when we discovered the first root tubes while opening and cleaning the cores. We had never seen this before in Antarctic sediment cores, and we weren’t expecting it either. My analyses prove a dominance of the clay mineral kaolinite for the sediments in which we find the root tubes. We also find this mineral around Leipzig in connection with the local lignite seams. It is indicative of boggy conditions. Together with the clay mineral smectite, it suggests a warm and humid climate,” he explained.
Yet it didn’t become clear just how unique the climate archive truly was until the sediment core was subjected to X-ray computed tomography (CT) scans. The CT images revealed a dense network of roots that spread through the entire soil layer of fine-grained clay and silt, and which was so well-preserved that the researchers could make out individual cell structures. In addition, the soil sample contains countless traces of pollen and spores from various vascular plants, including the first remnants of flowering plants ever found at these high Antarctic latitudes.
The sediment core analysed in the study was collected in the Amundsen Sea during the German polar research vessel Polarstern’s expedition PS104 (6 February–19 March 2017). This was accomplished with the seafloor drill rig MARUM-MeBo70, which was developed at MARUM in Bremen and used for the first time in the Antarctic. The X-ray computed tomography (CT) scans were conducted at the Klinikum-Mitte hospital in Bremen. The research project and Polarstern expedition PS104 as a whole were jointly financed by the AWI, MARUM, the British Antarctic Survey and the UK division of the International Ocean Discovery Program (UK-IODP).
The AWI press release contains further information.
Original publication in Nature:
“Temperate rainforests near the South Pole during peak Cretaceous warmth”, Nature, doi.org/10.1038/s41586-020-2148-5