Decoding the Past – Reading Nature’s Clues with Science Part 1 Lecture 4 - ATM 103 Review of Previous Lecture • Earth’s climate can change if any of the following change: • Energy/radiation from the Sun • Composition of the atmosphere • Refectivity of the planet (albedo) • Earth system is all connected. • Matter and energy are transported through cycles, processes, and events through Earth system reservoirs. • Our resource consumption and waste disposal choices are currently exceeding the Earth’s ability to replenish them by 70%. • Current Lecture: How do we know about Earth’s past climate? How do we know about climate in the past? • Historical records of temperature • We have a variety of historical records of temperature and precipitation for some regions. • Evidence of prolonged periods when climate was diferent in some places • “Little Ice Age” • “Medieval Warm Epoch” The Hunters in the Snow by Pieter Brueghel the Elder, 1565 Historical Evidence of Climate in the Past 2000 years • 1960s–1970s - Historical evidence (temperature records) of warm conditions from 1000–1200 AD (“Medieval Warm Epoch”) followed by cooling trend (“Little Ice Age”) • Medieval Warm Epoch led to Viking expansion and settlements in Greenland. • Little Ice Age resulted in cooler temperatures in Europe. • But probably not representative of changes in Earth’s global climate. • Historical temperature records Black line – land and ocean surface temperatures o Land temperatures from over 4000 land stations o Ocean surface temperature measurements from ships and buoys since 1853 • Brown line – land temperatures only (mainly Europe and North America) • Orange line – 4 European land stations only warmer than normal colder than normal Land and ocean surface Although it seems like a long time, 150–300 years worth of temperature records is not long enough to study Earth’s past climate. But, can we go back in time further? We need to look for climate proxies. • Climate proxy – a preserved physical record of past climate • Allows scientists to reconstruct Earth’s climate before direct temperature records were available • Examples include tree rings, ice cores, corals, and lake and ocean sediments (next lecture) Extending the historical record back to 700 AD with proxies… • General agreement in trends • Relatively cool period before ~1900 • Relatively warm period around 1000 AD • Lots of diferences among proxy records • Last 100 years has been period of rapid warming warmer than normal colder than normal How about even further back in time (pre-historic times)? • We must decode climate clues left by nature • Geologic evidence – reading the rocks for information about past climates • Fossil evidence – ancient life forms tell us about past climates • Geomorphological evidence – reading the surface of the land for clues to past climates • Chemical and physical evidence – the chemical fngerprints of past climates • Before we can understand this climate evidence, we need to understand • Some basics about Earth’s history • How we can date the evidence • How we determine climate from proxies Geologic and Geographic Features  Climate Clifs of D�over, England, made of chalk produced by phytoplankton at the ocean surface Yeager Rock, a 440-ton “glacial erratic” on the Waterville Plateau, Washington, ice-rafted to its present location 13,000 years ago inside the 4,000-foot-thick Cordilleran Glacier. Glacial till made of sand, gravel, and rock carried by ice at West Tarbert, UK Long Island formed mainly by glacial moraines left from the Ice Ages U-shaped valley cut by glacial action, Yosemite, CA This 6-foot long, 52-million-year-old palm frond was found near Fossil Butte National Monument (Wyoming) and suggests a subtropical climate. Petrifed Forest National Park provides evidence of a thick forest of tall trees in what is now Arizona 200 million years ago. Geologic Time Scale • Geologists have pieced together the history of the Earth. • Earth is about 4.6 billion years old, based on radiometric dating. • The history of the Earth in distant past is divided into units based on major events. Dating Techniques • Relative Dating • Based on interpreting time sequence of geologic strata to determine their age relative to one another Youngest • Principle of Superposition - the oldest strata are on the bottom • Principle of Horizontality sediments are originally deposited in horizontal layers • Principle of Lateral Continuity sedimentary layers extend laterally (across the surface) • By analogy, determining whether person A is older or younger than person B doesn’t tell you the age of either person. We need a way to determine the actual age. horizontal Oldest Example of Lateral Continuity – Colorado River Same layers seen in adjacent geologic features Fossil Dating New York State Fossil Eurypterus Remipes This “sea scorpion” was a predator in Silurian Seas (415–430 million years ago) covering North America. • Fossils - preserved remains or traces of animals, plants, and other organisms from the remote past • Fossils occur in rock formations, particularly in sedimentary strata • Mineral rich water flls the buried body of the organism, replacing the tissue with minerals • Principle of Faunal Succession – fossil organisms follow one another in a defnite order, so we can determine time periods through fossils • Index fossils are particularly useful in dating Index Fossils through Time Using Index Fossils to Date By identifying a known-age index fossil in a rock, we can identify the rock’s approximate age. Amplexograptus, an index fossil, from the Ordovician Radiometric Dating • Allows us to determine the age of rocks, geological features, and even biologic materials, due to the radioactive decay of certain isotopes of elements • Radioactivity – radiation emitted from an unstable atom • First discovered by Antoine Henri Becquerel (1896) • Becquerel, Marie Curie, and Pierre Curie received the Nobel Prize for this discovery in 1903 • Radiometric dating frst developed by Bertram Boltwood in 1907 for uranium, applied to dating the age of rocks Antoine Henri Becquerel (1852-1908) Marie Curie (18671934) Bertram Boltwood (1870-1927) Pierre Curie (1859-1906) Matter • All matter is composed of atoms of individual elements. • Atoms are composed of neutrons, protons, and electrons. • Protons have a positive electric charge. • Electrons have a negative electric charge. • Neutrons are a proton and electron combined, and have no electric charge. • If an atom does not have a charge, the number of protons and electrons is the same. • Atomic number is the number of protons. • Atomic mass number is the number of protons + neutrons. The classical orbit representation of the ground state of a Helium atom shows the nucleus of two protons and two neutrons orbited by two electrons. Elements • Matter is composed of elements, which exist alone or in combination with other elements in the form of molecules. • An element is a chemical substance with a specifc set of properties dependent on the details of its atomic structure. Oxygen (O), the most abundant element on Earth Silicon (Si), the second most abundant element on Earth Carbon (C), an essential element for life on Earth Nitrogen (N), the most abundant element in the atmosphere - 78% of air is N Elements Elements are distinguished by their atomic number (the number of protons). Periodic table of elements Isotopes • Elements can have multiple isotopes. • An isotope is an element with same number of protons, but a diferent number of neutrons. • Because the mass of the isotope is diferent, the physical, chemical, and biological behavior of isotopes is slightly diferent • 91 naturally occurring elements have isotopes. • Some isotopes are stable, meaning that they do not break apart by radioactive decay. • Others are unstable, meaning they break apart through radioactive decay. Hydrogen isotopes: protium (H-1), deuterium (H-2), Radioactive decay • Stable isotopes typically have the same number of protons and neutrons. • When the number of additional neutrons of an isotope is too large, or the nucleus itself is too large (atomic numbers >83), the nucleus becomes unstable and the atom is radioactive. • Unstable isotopes (radioisotopes) lose energy by emission of particles of ionizing radiation by two main processes: • Alpha decay – nucleus emits an alpha particle (2 protons + 2 neutrons) • Beta decay – nucleus emits an electron and other particles, increasing number of protons by 1 • Through this decay process, unstable atoms can change from one element to another element. Radioactive decay of an unstable isotope of lead (Pb-212), to Bismuth (Bi-212), to Thalium (Tl-208) or Polonium (Po-212), and a diferent isotope of lead (Pb-208). D�ecay occurs through emission of alpha (⍺) and Radioisotopes • Radioisotopes decay over time (milliseconds to hundreds of millions of years) to another isotope of an element. • Radioisotopes continue to decay into other daughter radioisotopes until they become a stable isotope that is not radioactive. • The half-life of a parent radioisotope is the amount of time needed for ½ of its atoms to transform to a daughter radioisotope • If the daughter is not radioactive, it will not decay further. • If the daughter is radioactive, it will decay into another daughter radioisotope. Radioactive decay of uranium-235 into daughter radioisotopes. Radiometric Dating The age of a sample can be determined from the relative abundance of the parent radioactive isotope to its daughter isotope. If there are equal amounts of the parent and daughter, then the age of the sample is one half-life. If there is 25% of the parent and 75% of the daughter, then the age of the sample is two half-lives. etc … etc … Radiometric Dating Click on the image below to watch a YouTube video. https://www.youtube.com/watch?v=phZeE7Att_s Geologically-Useful Radioisotopes The radioactive decay of certain elements are useful for determining the age of rocks. • All uranium isotopes – a trace element in Earth’s crust • U (uranium)  238 Pb (lead) 206 • Half-life: 4.5 billion years • U 235 Pb 207 • Half-life of 703 million years • U 234 Th (thorium) 230 • Half-life of 80,000 years Pitchblende is one of the many naturally occurring minerals that contains uranium. • 3 naturally occurring potassium (K) isotopes - 7th most Radiometric abundant element in Earth’s dating allows us crust to get actual 40 40 • K (potassium)  Ar (argon) ages of rocks. • Half life of 1.3 billion years Geologic Time Scale Radiometric dating has allowed us to determine ages for the geologic time scale. Rocks and Climate • Radiometric dating reveals the age of rocks. • Index fossils of known age can also be used to identify the age of rocks. • What can we learn about Earth’s prehistoric climate from rocks? • Geologic evidence • Geomorphological evidence • Fossil evidence Reading the Rocks Sedimentary rocks can record evidence of ancient environments • Ripple marks indicate past shallow seas. Dinosaurs left their footprints on the beaches of shallow seas Uplifted strata at D�inosaur Ridge, CO (Cretaceous) Ripples on beach in Wales, 2001 Reading the Rocks • Desiccation cracks indicate past arid conditions. This strata experienced a period of drought Rocks from NW Scotland, showing ancient mud cracks (Pre-Cambrian) Thick layer of mud develops deep cracks during dry periods Reading the Rocks • Burrow marks indicate specifc types of life. The environmen t could support life that could burrow Trace left by a burrowing organism from the late Precambrian (Ediacaran) Creatures that disturb the soil (Meysman, Middelburg and Heip, 200 Reading the Rocks Coal • Coal forms in rock strata over hundreds of millions of years. • Dead tropical or temperate forest plants accumulate in swampy areas, are buried, heated, and compressed into coal seams. • Coal provides evidence for ancient tropical or temperate forests. Reading the Rocks Oil shale • Forms from marine clay deposits rich in organic remains from thriving marine plankton • Presence indicates an ancient ecosystem with high productivity in a region isolated from rapid ocean circulation Reading the Rocks Limestone • Makes up ~10% of the volume of all sedimentary rocks on Earth • Composed of calcite and aragonite, which are forms of calcium carbonate (CaCO3) • Many deposits consist of skeletal fragments of marine organisms (coral or foraminifera) • Also deposited by chemical precipitation (cave formations) • Presence indicates a past warm, tropical environment with “plenty” of atmospheric CO2. The Burren landscape in Western Ireland (Aran Islands and much of County Clare) is composed of thick limestone deposits from the middle Carboniferous (~350 millions years ago). Reading the Rocks Chalk deposits • Composed of calcium carbonate (CaCO3) • Remains of marine phytoplankton called coccolithophores • Presence indicates “plenty” of atmospheric CO2 • During the Cretaceous, atmospheric CO2 was much greater (>1000 parts per million) than today (~400 parts per million). • Increase Earth’s albedo through release of aerosols at the ocean surface, promoting cloud formation, and an increase in ocean refectivity The Clifs of D�over, on the coast of England, are composed of ~110 m (350 ft) of chalk deposits from the Cretaceous Phytoplankton bloom in the Barents Sea increases the refectivity of the ocean Reading the Rocks Evaporites • Composed of calcite (CaCO3), halite (salt), gypsum, and other minerals • Form when water evaporates and leaves dissolved salts as mineral deposits • Presence indicates past hot, dry conditions that led to the evaporation of lakes and seas • e.g., Great Salt Lake, Dead Sea, and Bonneville Salt Flats Great Salt Lake, Utah Reading the Rocks Glaciers and Ice Sheets • Form characteristic features in the landscape due to the erosion by the glacier or ice sheet of the surface below it • Moraines - glacially accumulated debris • Till - thick deposits of rocks, sand, silt, and clay • Tillite - rock composed of metamorphosed glacial till • Dropstones/Glacial Erratics – large boulders dropped by glaciers Moraine Glacial Erratic Till U-shaped valley Tillite Reading the Rocks Glaciers and Ice Sheets • The past 2 million+ years of Earth’s history have featured repeated periods of glacial advance and retreat • Glacial periods - when the landscape is dominated by ice, temperatures are cooler, and sea level is lower (water trapped in ice) • Interglacial periods – when glaciers have retreated, temperatures are warmer, and sea level is higher Ice extent during last glacial period (left) compared to the present (right) Fossils as Environmental Indicators The fossil record provides a record of ancient environments • Fossil clam shells in limestone rock • Past presence of shallow sea • Fossil palm fronds Modern brachiopods (Phillipines) • Past presence of a temperate to tropical climate • Fossil corals • Type of coral sensitive to water temperature Shale slab containing brachiopods from the Pennsylvanian, Kansas. Review: Decoding the Past Part I • Direct temperature measurements on Earth only go back a few hundred years at most • Nature has left clues of our past climate that we can study by studying climate proxies, which allow us to study Earth’s climate much further back in time. • We can use multiple techniques to study Earth’s history • Relative dating • Fossil dating • Radiometric dating • Earth is about 4.6 billion years old • We can link types of rocks and fossils with the climate they existed in, and how climate may have changed in the past. 
Purchase answer to see full attachment