5:15 PM - 7:15 PM
[MIS17-P08] Climatic change and distribution of continents in the Proterozoic
Keywords:Climate change, Oxygen isotopes, Supercontinent
The origin of life, emergence of eukaryotes, and appearance of multicellular animals are considered the three major evolutions of life. It remains controversial whether such macroevolution is the result of a series of microevolutions or whether a driving force is required to cause the major evolution. However, it is also difficult how to answer this question. We aim to clarify this question by deciphering the two evolutions of life and the surface environment estimated from geological samples.
The Proterozoic Era spans a period from 2.5 billion to 540 million years ago. It has been suggested that Snowball Earth events occurred at the beginning and end of the Era and that the oxidation of the atmosphere and oceans progressed during these periods. Previous studies have suggested that eukaryotes appeared after the Great Oxidation Event (GOE) in the Paleoproterozoic based on the fossil record (1.9 Ga) and molecular clocks (2.2 Ga), and that the atmospheric oxygen content was stable at a level slightly higher than the Pasteur point required for aerobic respiration through the Proterozoic. However, recent studies have suggested that the atmospheric oxygen concentration fluctuated greatly throughout the Proterozoic Era and was below the Pasteur point, which has become a major topic of discussion. On the other hand, glacial deposits have been reported from several Proterozoic strata, but it was later shown that many of them were formed in the Neoproterozoic. No reliable glacial deposits have been found except for the early and late Proterozoic Snowball Earth and around 1.9 to 1.8 Ga glaciation. Therefore, the Proterozoic is considered to have been a warm and relatively stable climatic period.
Methods to estimate the past climate of the Precambrian period, when fossil records are scarce, include the occurrence of glacial deposits, oxygen isotope ratios of carbonate rocks/minerals, phosphate minerals, and quartz, clumped isotope thermometers of carbonate minerals, and Si isotopes of chert. However, conventional proxies of seawater temperatures have estimated that the Proterozoic seawater temperature ranges between 20 °C and 100 °C, and the extremely high temperature and very wide temperature ranges are unreliable. The high estimated temperature is generally considered to be caused by secular change of the oxygen isotopes of seawater and the effects of secondary alteration. However, this extremely wide temperature range cannot be explained by the former alone.
It is widely considered that two supercontinents, Columbia and Rodinia, existed in the Proterozoic era. In general, when supercontinents exist, it is easier to reconstruct the paleogeography and paleolatitude of each continent than when continents are dispersed.
We constructed a new database of oxygen isotope values of carbonate rocks and investigated the relationship between the estimated temperature of Proterozoic seawater and paleogeography, based on the paleolatitude and distribution of continents. We used recent reconstruction of the secular change of oxygen isotope values of seawater through the time to estimate the paleo-temperature. The calculation shows that the seawater temperature in the Proterozoic ranged from 0 to 20 °C, which is not consistent with the high-temperature environment suggested by previous studies, but suggests that it was equivalent to the modern environment. The calculation also shows that the seawater temperature dropped to -10 to 10 °C at approximately 1.6–1.5 Ga and 0.8–0.6 Ga. The latter is consistent with the cold period of the Neoproterozoic, but the former is not supported by the geological record, since no glacial deposits with 1.6–1.5 Ga ages have been found at the moment. In addition, the calculation shows that the estimated ocean temperatures were very high around 2.3 and 1.9 Ga. It is considered that the high estimated temperatures are arbitrary and may be caused by a decrease in oxygen isotope values of seawater in the regions due to the inflow of large amounts of meltwater with low oxygen isotopes associated with the Paleoproterozoic Snowball Earth and the 1.9 Ga ice period.
In terms of the relationship between paleolatitude and estimated ocean temperatures, there were some contradictions, but there was a general tendency for high-latitude data to be colder at the same ages. Therefore, some of the large temperature ranges can be explained by the paleogeography.
The Proterozoic Era spans a period from 2.5 billion to 540 million years ago. It has been suggested that Snowball Earth events occurred at the beginning and end of the Era and that the oxidation of the atmosphere and oceans progressed during these periods. Previous studies have suggested that eukaryotes appeared after the Great Oxidation Event (GOE) in the Paleoproterozoic based on the fossil record (1.9 Ga) and molecular clocks (2.2 Ga), and that the atmospheric oxygen content was stable at a level slightly higher than the Pasteur point required for aerobic respiration through the Proterozoic. However, recent studies have suggested that the atmospheric oxygen concentration fluctuated greatly throughout the Proterozoic Era and was below the Pasteur point, which has become a major topic of discussion. On the other hand, glacial deposits have been reported from several Proterozoic strata, but it was later shown that many of them were formed in the Neoproterozoic. No reliable glacial deposits have been found except for the early and late Proterozoic Snowball Earth and around 1.9 to 1.8 Ga glaciation. Therefore, the Proterozoic is considered to have been a warm and relatively stable climatic period.
Methods to estimate the past climate of the Precambrian period, when fossil records are scarce, include the occurrence of glacial deposits, oxygen isotope ratios of carbonate rocks/minerals, phosphate minerals, and quartz, clumped isotope thermometers of carbonate minerals, and Si isotopes of chert. However, conventional proxies of seawater temperatures have estimated that the Proterozoic seawater temperature ranges between 20 °C and 100 °C, and the extremely high temperature and very wide temperature ranges are unreliable. The high estimated temperature is generally considered to be caused by secular change of the oxygen isotopes of seawater and the effects of secondary alteration. However, this extremely wide temperature range cannot be explained by the former alone.
It is widely considered that two supercontinents, Columbia and Rodinia, existed in the Proterozoic era. In general, when supercontinents exist, it is easier to reconstruct the paleogeography and paleolatitude of each continent than when continents are dispersed.
We constructed a new database of oxygen isotope values of carbonate rocks and investigated the relationship between the estimated temperature of Proterozoic seawater and paleogeography, based on the paleolatitude and distribution of continents. We used recent reconstruction of the secular change of oxygen isotope values of seawater through the time to estimate the paleo-temperature. The calculation shows that the seawater temperature in the Proterozoic ranged from 0 to 20 °C, which is not consistent with the high-temperature environment suggested by previous studies, but suggests that it was equivalent to the modern environment. The calculation also shows that the seawater temperature dropped to -10 to 10 °C at approximately 1.6–1.5 Ga and 0.8–0.6 Ga. The latter is consistent with the cold period of the Neoproterozoic, but the former is not supported by the geological record, since no glacial deposits with 1.6–1.5 Ga ages have been found at the moment. In addition, the calculation shows that the estimated ocean temperatures were very high around 2.3 and 1.9 Ga. It is considered that the high estimated temperatures are arbitrary and may be caused by a decrease in oxygen isotope values of seawater in the regions due to the inflow of large amounts of meltwater with low oxygen isotopes associated with the Paleoproterozoic Snowball Earth and the 1.9 Ga ice period.
In terms of the relationship between paleolatitude and estimated ocean temperatures, there were some contradictions, but there was a general tendency for high-latitude data to be colder at the same ages. Therefore, some of the large temperature ranges can be explained by the paleogeography.