Plate Tectonics and Global Glaciation Tectonic plate motions move the continents and determine the form of the ocean basins. Paleoclimatologists have suggested that land must be located over the polar regions in order to provide a site for ice accumulation and corresponding positive feedback mechanism (e.g., increase in albedo or uptake in atmospheric CO 2 ) to promote global glaciation. 620 Ma 240 Ma 330 Ma 30 Ma

620 Ma 240 Ma 330 Ma 30 Ma Inclination of the earth’s magnetic field varies with latitude. It is parallel to the surface over the equator and vertical over the magnetic poles. Paleomagnetic inclination can be used to infer paleo-latitude of igneous rocks and to reconstruct tectonic plate configurations.

Movement of tectonic plates (including relative movement of continents can explain the apparent wandering of the paleomagnetic pole over geologic time. Without invoking plate motions, paleomagnetic evidence preserved in the rock record would require unique magnetic poles for different aged rock and locations. Apparent motion of the magnetic pole can be used to reconstruct the relative positions of tectonic plates to each other.

Over earth’s climatic history there are at least four protracted cold periods (Ice Age) preserved in the rock/sediment record. Two of these ice ages occurred during the Precambrian while the more recent occurred during the Late Paleozoic and Cenozoic Eras. Proterozoic tillite and striated bedrock Late Paleozoic Dwyka tillite, South Africa To explain these protracted periods of global glaciation, paleoclimatologists require causal mechanisms that extend over time periods of millions of years.

Lithified till (tillite) deposited during the Late Paleozoic Era (Permo-Carboniferous Period) is preserved in South Africa. Similar Late Paleozoic-aged tillites are preserved in southern South America, India, Australia and Antarctica. Dwyka Tillite, South Africa

The modern distribution of late Paleozoic glacial deposits can be understood within a context of plate tectonics. Plate configurations during the late Paleozoic placed the present day land masses of southern South America, Africa, India, Australia and Antarctica over the South Pole, where continental ice sheets could develop and expand.

Paleontologic and geologic evidence support the paleomagnetic reconstructions of the plate configurations during the late Paleozoic and early Mesozoic Eras. Following the break-up of the super-continent during the Mesozoic, faunal assemblages underwent divergent evolution and each continent had its own unique fauna. Closer faunal relationships existed between continental land masses that remained contiguous for longer time periods, such as Africa-South America and North America-Eurasia.

About 240 M.Y. ago the super-continent Pangaea began to break-up. Note the latitudinal distribution of the continental land masses and configuration of the ocean basins. An equatorial current existed along the Tethyan Seaway. Increased sea floor spreading added CO 2 to the atmosphere directly from volcanism, and indirectly by causing the ocean floor to be more buoyant, which would cause a rise in eustatic sea level. Reduction in land area reduced chemical weathering (uptake of acidified rain) and the uptake of atmospheric CO 2. Increasing sea water temperatures furhter adds CO 2 to the atmosphere, as the solubility of CO 2 gas is reduced. These positive feedback mechanisms enhanced global warming during the Mesozoic Era. Tethyan Seaway

Throughout the Mesozoic Era the continental land masses were distributed near the equator and global climatic conditions were much warmer than during the Cenozoic. Note that Australia and Antarctica remained isolated from the other continental land masses over much of Mesozoic and Cenozoic. What impact do you think this had on the evolution of Australian fauna versus the other continents? Also note how the continental and ocean basin configurations changed during the Mesozoic.

By the early Cenozoic (50 M.Y. ago), large continental land masses of North America and Eurasia had migrated to higher latitudes in the northern hemisphere. The Antarctic continent was situated over the South Pole, the Indian sub-continent was impinging upon Asia, and ocean basin configurations were changing, as the Tethyan Seaway closed and the Antarctic Ocean began to form.

During the Cenozoic Era several important tectonic events occurred that had a major impact on the onset of the Cenozoic cooling: 1. Following the break-up of Pangaea North America and Eurasia migrated to high latitudes, 2. Antarctica was situated over the South Pole, 3. The Circum-Antarctic Current formed with opening of Drakes Passage, 4. Collision of the Indian Sub-continent with the Eurasian continent (uplift of the Himalayas and Tibetan Plateau), 5. Miocene marine regression and isolation of the Mediterranean Sea, and 6. Closure of Isthmus of Panama and creation of the Gulf Stream and existing thermo-haline circulation pattern.

Onset of Cenozoic Cooling Cenozoic surface water temperature reconstruction (using planktonic foraminifera) of the Antarctic Ocean. The relative rapid adjustment of circulation systems that followed tectonic events is the likely cause of the observed, uneven, stepped history of Cenozoic cooling shown on the upper left. Onset of Cenozoic cooling is likely related to the poleward migration of North America and Eurasia. Antarctica becomes isolated over the South Pole by ~50 Ma BP as Australia moved north. The southern ocean develops and initiation of Antarctic Circumpolar Current (ACC). Think about feedback mechanisms (e.g., albedo, CO 2 uptake). 90 Ma 30 Ma

Onset of Cenozoic Cooling Cenozoic surface water temperature reconstruction (using planktonic foraminifera) of the Antarctic Ocean. The relative rapid adjustment of circulation systems that followed tectonic events is the likely cause of the observed, uneven, stepped history of Cenozoic cooling shown on the upper left. India collides with the Eurasian continent ~35 Ma BP causing uplift of Himalayas and Tibetan Plateau. Drakes Passage opens ~30 Ma BP further enhancing the Antarctic Circumpolar Current (ACC). The ACC restricts warm tropical air from reaching Antarctica. Antarctic ice begins to develop (think about feedback mechanisms again with ice development, ocean cooling). 90 Ma 30 Ma Drakes Passage

Onset of Cenozoic Cooling Cenozoic surface water temperature reconstruction (using planktonic foraminifera) of the Antarctic Ocean. The relative rapid adjustment of circulation systems that followed tectonic events is the likely cause of the observed, uneven, stepped history of Cenozoic cooling shown on the upper left. By the Miocene the Antarctic ice sheet is well-developed (ice-rafted sediment observed at sea level). Marine regression (40-50 m) in late Miocene caused isolation of the Mediterranean Sea and Messinian salinity crisis. Water spilling into basin evaporated and major salt deposition occurred. Global ocean salinity fell by 6%. What effect would this have on sea ice production. 30 Ma Present

Onset of Cenozoic Cooling Cenozoic surface water temperature reconstruction (using planktonic foraminifera) of the Antarctic Ocean. The relative rapid adjustment of circulation systems that followed tectonic events is the likely cause of the observed, uneven, stepped history of Cenozoic cooling shown on the upper left. By about 3 Ma BP the northern hemisphere ice sheets began to develop (evidence ice-rafted debris in deep sea deposits). This coincident with the timing of the closure of the Isthmus of Panama and development of the Gulf Stream and north Atlantic thermo-haline circulation,as well as major mountain building in the northern hemisphere. How do you think these events are related? 30 Ma Present

Proterozoic Ice Age Enigmas 1. global glaciation (evidence of continental ice sheet deposits wide- spread). 2. paleomagnetic data is not consistent with geological data (i.e., Proterozoic glacial deposits deposited at low latitude locations and near sea level). 3. Fe-rich rock mixed with glacial sediment (no oxygen in oceans during ice sheet development, then rapid oxygenation). 4. Rapid accumulation of warm water carbonates, immediately following deglaciation (cap carbonates overlie glacial deposits). 5. Carbon isotope ( 13 C/ 12 C ratios) suggest prolonged drop in biological activity during glaciation. Hoffman and Schrag, 1999

Modern distribution of Proterozoic glacial deposits and carbon isotope data.