1:45 PM - 2:00 PM
[BCG06-07] Oceanographic structure and evolution of sponge in Cryogenian interglacial: biomarker and carbon isotopic records
Keywords:Proterozoic, Cryogenian, carbon isotopes, sponges
The Cryogenian period was an age of ice, when the Earth was frozen twice, and it is also an age of the animal evolution during its interglacial interval. When the global freeze disappeared, the Earth experienced a strong warming and weathering due to the sustained high CO2 levels. An enhanced primary production in the eutrophic ocean produced a large amount of organic matter that cannot be fully remineralized and had to accumulate or remain in the stratified ocean. Decomposition of the organic matter that accumulated in the anaerobic parts of the ocean took over millions of years, and eventually the ocean becoming oxygen-rich. This series of scenarios is well recorded in the isotopic stratigraphy of the Ediacaran period after the Marinoan glaciation, and the evolution of life appeared in the fossil record. On the other hand, research on the Cryogenian interglacial period after the Starchian glacial period has been relatively sparse, but it has become clear that this period has some similarities with the Ediacaran interglacial. For example, the carbon isotope stratigraphy can be summarized as following scenarios: 1) negative value of cap carbonate, 2) increasing trend up to around +8‰, 3) first negative anomaly value, 4) long-term high value (up to +10‰), 5) a significant decrease to -10‰, and 6) a slow recovery. This trend of change has much in common with the carbon isotope stratigraphy of the Cryogenian interglacial. Here, we introduce examples of research on the period in China and Australia, and explain the ocean structure and ecosystem during the Cryogenian interglacial.
The cryogenic interglacial in southern China is composed of the Fulu and Datangpo Formations. Both are mainly composed of sandstone and shale, but iron ore layers are recognized as chemical carbonate rocks in the Fulu Formation and Mn carbonate in the Datangpo Formation. There is a Mn deposit in the Datangpo Formation in western Hunan Province, and well-preserved biomarkers were obtained from the organic-rich calcareous shale. The biomarker composition shows that green sulfur bacteria, purple sulfur bacteria, and methane bacteria are predominant, and green algae and cyanobacteria are extremely rare. In addition, no marker indicating sponges was detected. It is thought that anoxygenic photosynthesis, methane production, sulfate reduction, and anaerobic methane decomposition probably occurred in the stratified ocean area. This corresponds to Stage 2 or 4 in terms of carbon isotope stratigraphy.
A thick sequence of carbonate and clastic rocks developed during this period in the Flinders Ranges of South Australia. The uppermost Trezona Formation contains fossil particles that were believed to be sponges. The Trezona fossil layers recorded a low carbon isotope ratio down to -8‰, called Trezona Anomaly, which has characteristics similar to the Ediacaran Shrum Anomaly. This corresponds to Stage 5.
Considering that the record of biomarkers from the Cryogenian period shows that sponges appeared in the late interglacial period, it is highly likely that sponges evolved during this period, regardless of the authenticity of the Trezona sponge fossil. The period would be above the Datangpo Mn deposits and after the Trezona fossils. In the fossil layers of the Trezona Formation, the carbon isotope ratio gradually returns to a positive direction. Porifera probably removed the organic matter that had accumulated in the ocean, and when it was almost gone, the ocean returned to an oxidative state.
The cryogenic interglacial in southern China is composed of the Fulu and Datangpo Formations. Both are mainly composed of sandstone and shale, but iron ore layers are recognized as chemical carbonate rocks in the Fulu Formation and Mn carbonate in the Datangpo Formation. There is a Mn deposit in the Datangpo Formation in western Hunan Province, and well-preserved biomarkers were obtained from the organic-rich calcareous shale. The biomarker composition shows that green sulfur bacteria, purple sulfur bacteria, and methane bacteria are predominant, and green algae and cyanobacteria are extremely rare. In addition, no marker indicating sponges was detected. It is thought that anoxygenic photosynthesis, methane production, sulfate reduction, and anaerobic methane decomposition probably occurred in the stratified ocean area. This corresponds to Stage 2 or 4 in terms of carbon isotope stratigraphy.
A thick sequence of carbonate and clastic rocks developed during this period in the Flinders Ranges of South Australia. The uppermost Trezona Formation contains fossil particles that were believed to be sponges. The Trezona fossil layers recorded a low carbon isotope ratio down to -8‰, called Trezona Anomaly, which has characteristics similar to the Ediacaran Shrum Anomaly. This corresponds to Stage 5.
Considering that the record of biomarkers from the Cryogenian period shows that sponges appeared in the late interglacial period, it is highly likely that sponges evolved during this period, regardless of the authenticity of the Trezona sponge fossil. The period would be above the Datangpo Mn deposits and after the Trezona fossils. In the fossil layers of the Trezona Formation, the carbon isotope ratio gradually returns to a positive direction. Porifera probably removed the organic matter that had accumulated in the ocean, and when it was almost gone, the ocean returned to an oxidative state.