This is a review of our discussion several weeks ago about the Glaciation and Pleistocene periods. There were four major periods in the history of Earth during glaciation period and probably the most important of these is the second period that occurred some millions of years ago between 850 Ma to 635 Ma during the late Proterozoic Age. It was suggested that during this age Earth was covered completely in ice and then led to the Cambrian Explosion which has been responsible for diversification of multi-cellular life during this era.
The current period of constant glaciations in Pleistocene epoch from 1.8 million years to 10,000 years BP where the present continents were positioned and the plates on which these continents rested have not rotated more than 100 km since the start of this period. Repeated glacial cycles have described the climate of Pleistocene period pushed to 40th parallel in some places. 30% of the surface of the Earth has been covered by ice at its highest extent. Also, there was a zone of permafrost that extended from southern edge of glacial sheet of North America to Eurasia. The average annual temperatures of the ice and permafrost were -6˚C and 0˚C respectively.
One of the papers we discussed in class was “Global heat budget, plate tectonics and climate change” (Harris, 2002). This paper stimulated curiosity and discussion among the students in class. We arrived to the following conclusions about climate change on Earth from the paper and discussion.
- The Earth’s surface temperature has fluctuated since past 2000 Ma.
- Individual locations on Earth have undergone long-term change in temperature at different times and in different places.
- We discussed new evidences concerning the difference in heat absorption by land and water; such as, the transport of excess heat pole ward from the tropics and the change in distribution of land and sea resulting from plate tectonics. These evidences explain the major fluctuation in the geological record-setting temperatures measured during last 350 Ma. However, these evidences create confusion since they are not supported by sufficient background data.
- he paper also dealt with various controls which resulted in the climate change on the Earth’s surface.
- First control dealt with changes in the distribution of land and sea due to plate tectonics. This explains the major temperature fluctuation (>25˚C) around the globe in the last 350 Ma.
- Second order control was large scale changes in ocean currents and thermohaline circulation (15-25˚C).
- Third order control was Milankovitch orbital cycle producing variations in the air temperature by order of 10˚C.
- Fourth order control was massive volcanic eruptions and how changes in carbon dioxide production caused minor perturbations (<5˚C).
The take home message of our discussion was: the Earth's climate change was influenced by many factors; some of which act independently, while others acted interdependent with each other. The process of climate change was a gradual process which took many millions of years to see any significant change in climate. This change in temperature and climate, at different locations of the Earth, resulted in the geological distribution of various types of habitats on Earth.
The second paper we discussed in class was “Pleistocene isolation in the northwestern pacific marginal seas and limited dispersal in marine fish, Chelon haematocheilus (Temminck and Sclegel, 1885)” (Liu et al, 2007). This paper deals with three marginal seas: the Sea of Japan, East China Sea and South China Sea. During the period of Pleistocene glaciation when the whole earth was mostly covered by ice, the populations of living organisms were isolated in the seas.
We discussed the hypothesis that the rise of post glacial sea level resulted in the homogenization of the population by high disperse potential. To test this hypothesis, researchers used Chelon haematocheilus as the model organism. This fish belongs to the Mugilidae family, and is present in shallow coast water as well as freshwater regions in north Japan through the Korean Peninsula, and to the coast of China South. The early life history characteristics indicate that potential larval dispersal of C. haematocheilus is high. If C. haematocheilus larvae could travel on the currents, the connectivity should be high among populations within this region. The distribution and biological characteristics of C. haematocheilus make it a good subject to test the homogenization hypothesis.
Molecular analysis revealed three lineages which might have diverged in the three marginal seas during Pleistocene low sea level. Analysis of molecular variance and population statistics revealed significant differences in genetic structure among populations of the marginal seas. The outcome of the above analysis revealed that gene flow in C. haematocheilus is far more restricted spatially than predicted by the potential dispersal capabilities of this species. These results provide evidence for strong genetic divergence among these fishes in the marginal seas of the Northwestern Pacific, which coincides with expected pattern of vicariance due to sea level changes during the Pleistocene.
In order to support the above hypothesis further investigation should be done on other species of the same region.
Stuart A. Harris (2002) Global Heat Budget, Plate Tectonics and Climatic Change. Geografiska Annaler. Series A, Physical Geography, 84:1-9
JIN-XIAN LIU, TIAN-XIANG GAO, SHI-FANG WU, and, YA-PING ZHANG. (2007) Pleistocene isolation in the Northwestern Pacific marginal seas and limited dispersal in a marine fish, Chelon haematocheilus (Temminck & Schlegel, 1845). Molecular Ecology 16:275-288. DOI: 10.1111/j.1365-294X.2006.03140.x