What would happen in the near future with continued global warming? What environmental conditions would the life on Earth most likely confront? The episodes of climate changes in the Earth’s deep past, similar to the current global warming, may provide valuable clues to these questions.
A recent study led by scientists from China, U.S., and New Zealand reveals that an abrupt warming linked to massive carbon emission during an icehouse climate state caused approximately 20% of anoxic areal extent of the seafloor, and significant biodiversity drop. The finding was published by the Proceedings of the National Academy of Sciences of the United States of America on May 2, 2022.
As is known, we are currently living under the Cenozoic icehouse climate that has started since 34 Myr ago. However, global temperature rises rapidly under this icehouse over a couple of centuries, in tandem with accelerated ablation of polar glaciers, rising sea-level, and aggravating marine de-oxygenation, and undoubtedly leading to a significant drop in biodiversity. Where would the global warming lead us to in the future? There are substantial uncertainties regarding the modeling results based on current observations, which drive the current focus on understanding past episodes of carbon emission and ocean deoxygenation, particularly under an icehouse climate state.
The Late Paleozoic Ice Age (LPIA, between 360 and 280 million years ago) is the longest-lived and the only icehouse that recorded the transition from icehouse to greenhouse climate states since the occurrence of advanced plants and terrestrial ecosystem. The LPIA is also the only geological period that is featured by low atmospheric CO2 and high O2 concentrations, highly comparable to those of the modern day. It is, therefore, critical to study carbon emissions and their consequences during the LPIA for better understanding the processes and feedbacks of the icehouse Earth system, and thus for more precisely predicting the future environmental and biodiversity changes.
A international research team led by Dr. CHEN Jitao from the Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences (NIGPAS), Dr. WANG Xiangdong from the Nanjing University, and Dr. Isabel P. Montanez from the University of California, Davis studied the Carboniferous strata from the southern China for over 10 years, with respect to sedimentology, stratigraphy, paleontology, and geochemistry.
The Naqing and Narao sections from the Luodian region, Guizhou Province crop out continuous Carboniferous carbonate successions that register geochemical signal of sea water.
Scientists collected samples by every 20 cm for over 40-m-thick strata from the two sections and carried out carbon and uranium isotopes to explore the global carbon cycling and marine anoxia. "We utilized global carbon cycle model (LOSCAR) and paleo carbon dioxide concentrations to simulate a total amount of 9,000 Gt C emitted over 300 kyr, causing an increase in sea-surface temperature by ~4℃", CHEN says.
"We also modelled an increase in areal extent of the anoxic seafloor from 4% to 22%, causing a dramatic decrease in biodiversity" CHEN Added, "and finally, we performed climate model simulations using the fully coupled Community Earth System model (CESM) to explore the potential mechanisms for the marine anoxia, which are linked to enhanced thermocline stratification and increased nutrient fluxes during the warming."
The study further finds that warming-induced marine anoxia may be more pronounced in a glaciated than in an unglaciated period.
Reference: Chen, J.T., Montanez, I.P., Zhang, S., Isson, T.T., Macarewich, S.I., Planavsky, N.J., Zhang, F., Rauzi, S., Daviau, K., Yao, L., Qi, Y.P., Wang, Y., Fan, J.X., Poulsen, C.J. Anbar, A.D., Shen, S.Z., Wang, X.D., 2022. Marine anoxia linked to abrupt global warming during Earth’s penultimate icehouse, PNAS, https://doi.org/10.1073/pnas.2115231119.
Fig. 1. An abrupt global warming occurred at ~304 Ma under the Carboniferous icehouse climate state, consistent with doubling of atmospheric carbon dioxide, significant negative excursion in carbon isotopes, sea-surface temperature increase, and drop in biodiversity.
Fig. 2. Carbon and uranium isotopes recorded in the Naqing section, Guizhou Province, southern China, showing remarkable negative excursions across the Kasimovian-Gzhelian boundary. Modeling results suggest a distinct perturbation in global carbon cycle and an increase in marine anoxia.
Fig. 3. Comparison between the KGB warming event during the late Carboniferous icehouse and greenhouse C-perturbation events over the last 300 Myr, indicating that the KGB warming had a more extreme impact on the marine redox landscape relative to its rate of C injection and SST increase than the other events.
LIU Yun, Propagandist
Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences
Nanjing, Jiangsu 210008, China