7.2 - Evidence for Climate Variations (Ancient) Flashcards
How do pollen grains contribute to our understanding of ancient variations in global temperature?
Pollen grains are easily distributed by the wind. They are also resistant to decay and persist in sediment for a long time. Pollen blown from surrounding environments into rivers could have fossilised in sedimentary rocks, and analysis of this provides information of the types of vegetation growing in the area, thus allowing for an idea of the climate at that time. The composition of pollen also tends to change with changing temperatures, so analysis of this could provide information about variations in global temperatures.
Explain how scientists can interpret past environments from the study of sedimentary rocks
Sedimentary rocks contain structures that reflect the climates in which they form. A common structure in sedimentary rocks is cross-bedding, which forms in dunes and is produced by flowing air or water. Sand dunes on land lead to large cross beds with characteristic fine sand compositions. They are often iron-stained, reflecting the arid environments in which they form. Cross-beds are also common in river-deposited sandstones. Large sets of cross beds reflect the movement of high volumes of water, and the resulting rocks, as part of a sequence, can be used to interpret the environment and climate in which they formed. Rocks with striations could be a result of glacial ice moving over the rock surface, reflecting a time when the climate was cold and glaciers were present. Dropstones also indicate glacial environments.
Igneous and metamorphic rocks also provide evidence of variations in temperature. The presence of igneous rocks indicates volcanic activity. Metamorphic rocks vary based on degree of metamorphism. For example, ecologites tend to form under intense heat and pressure, while hornfels form under high heat and low pressure such as that of contact metamorphism.
How do fossils provide evidence for ancient climates?
One of the ways that fossils can provide evidence for ancient climates is through our understanding of the native climates of many of today’s species. For instance, coral reefs only form in warm tropical waters, so if we find fossilised coral, we can deduce that the climate in this place at the time the fossil was formed must have been a warm, tropical climate. We can also use our understanding of plate tectonics - for example, the presence of dinosaur and plant fossils in Antarctica indicates that the climate of Antarctica was at that point much warmer. Through our knowledge of plate tectonics, we know that while Antarctica wasn’t directly over the south pole at this time, it was still far enough south that the climate should have been too cold for dinosaurs and these plants. Thus, we can deduce that the global climate at this time must have been much warmer in order for these species to exist so close to the pole.
Explain how deep sea sediments can indicate climate variation
Much of the sediment on the deep ocean floor is the accumulated remains of calcium carbonate from marine organisms. The calcium carbonate contains two forms of oxygen isotopes. Analysis of the ratio of O16 and O18 isotopes in carbonate skeletons can be used to determine past temperatures. O16, because it is lighter, evaporates more easily. During glacial periods, O16 becomes locked up in the glacial ice, and lower amounts in the ocean indicate low temperatures. Meanwhile, O18 remains in ocean waters causing the skeletons to become enriched in O18 too.
During interglacial periods when temperatures are warmer, the glaciers and ice melts, releasing O16 into the oceans and as a result the O16:O18 ratio returns to normal and this is again reflected in the chemistry of the skeletons. These skeletons break down to format the sediment on the ocean floor.
