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Earth's History and Changing Climate

The Theory of Earth - James Hutton (1785)

  • James Hutton, in his "Theory of the Earth" (1785), proposed the idea that "The present is the key to the past."
  • This suggests that the processes observable on Earth today have also operated throughout its history.
  • Catastrophism (OLD)
    • From 1600 to 1700.
    • Idea: Earth is young (only a few thousand years old) and the features of the earth were created by great catastrophes
    • Based On: Belief…NOT scientific data
    • Example: Grand canyon formed in a matter of months from a huge flood that happened in the past – floods like this no longer occur
  • The Theory of Uniformitarianism (NEW)
    • late 1700’s
    • Idea: Earth is very old and the same processes that operate today have operated throughout Earth’s long history. “The present is the key to the past.”
    • Based On: Scientific data and thinking (Reasoning!)
    • Example: Grand Canyon was carved slowly by the river eroding its bed over 10’s of thousands of years (slow processes if given enough time can cause major changes to Earth’s surface

Earth’s History - Big Idea #1

  • Processes observed on Earth now also operated in the past: Uniformitarianism
    • Sedimentation - process by which solid particles settle out of a liquid or gas and accumulate on a surface
      • Force of gravity exceeds the upward force from the fluid (liquid or gas)
      • Commonly occurs in bodies of water such as rivers, lakes, and oceans,
        • Eroding particles settle to the bottom to form sediment layers.
        • Over time, layers can become compacted and cemented together to form sedimentary rocks.
    • Plate tectonic movement - motion of the large, rigid plates that make up the Earth's lithosphere (the outermost layer of the Earth), which float on the semi-fluid asthenosphere beneath them.
      • Movement driven by the convective currents in the Earth's mantle.
    • Volcanic activity - processes associated with the eruption of molten rock (magma), gases, and other materials from a volcano onto the Earth's surface or into the atmosphere.
      • Intruding magma
      • Gas emissions
      • Lava flows
      • Ash eruptions
      • Pyroclastic flows
      • Volcanic Debris
    • Biological Evolution - the process by which living organisms change over successive generations through the gradual accumulation of heritable variations. Subject to natural selection, whereby individuals with advantageous traits are more likely to survive and reproduce, passing on those traits to their offspring. Over time, this process can lead to the emergence of new species and the diversification of life on Earth.

Earth’s History - Big Idea #2

  • Most Earth processes are slow but continuous (change is nearly undetectable over short spans of time)

Earth’s History - Big Idea #3

  • The rock record provides evidence of geological events and past life forms.
    • Rock record - the geological history of Earth as preserved in layers of sedimentary rocks, igneous rocks, and metamorphic rocks. Crucial for understanding the history and processes that have shaped our planet over geological time scales.

Earth’s History - Big Idea #4

  • Earth is very old (4.6 billion years) and has changed over geologic time
    • Relative Dating - method of placing geological events in their order of occurrence (you know what is older and what is younger but not how old anything is)
      • Tells you the timing of an event compared to other events in a sequence but not exactly when it happened
    • Absolute Dating - method that uses radiometric techniques to determine the number of years since a geologic event happened (e.g., 10,000 yrs ago)
      • Gives an age in years for a rock or fossil.

Earth’s History - The Stories In Rock Layers

  • Outcrop - exposed bedrock on the surface of the Earth.
  • Strata - layers of rock or sediment that have been deposited over time. These layers often vary in composition, density, and characteristics, and they provide valuable information about the history of the Earth's surface.

Relative Dating Laws - Superposition

  • Law of Superposition – unless sedimentary rock layers have been disturbed (deformed = folded, faulted or overturned) the oldest rock layer is on the bottom (because it was deposited first).

Relative Dating Laws - Original Horizontality

  • Law of Original Horizontality – No matter the present angle or orientation of sedimentary rock layers, it is almost certain that the layers were initially deposited flat on the ocean floor.

Relative Dating Laws - Cross-Cutting Relationships

  • Law of Cross-Cutting Relationships – faults and igneous intrusions are younger than the layers they cut across.

Relative Dating Laws - Inclusions

  • Law of Inclusions – Inclusions (fragments of one kind of rock enclosed within another rock) are older than the rock they are in.

Relative Dating Laws - Unconformities

  • Law of Unconformities – Unconformities are surfaces that represent a gap in the geologic record. Sediment was not deposited or weathering and erosion have removed rock layers.
    • Nonconformity - occurs when younger sedimentary rocks overlay older, eroded igneous or metamorphic rocks. The younger sedimentary rocks were deposited on top of pre-existing, much older rocks that were subjected to erosion and weathering before the deposition of the younger rocks.
      • Significant gap in the geologic record, often spanning millions or even hundreds of millions of years.
    • Angular Unconformity - Occurs when younger sedimentary rocks are deposited on tilted or folded layers of older sedimentary rocks. The angular discordance between the older and younger rock layers indicates deformation and uplift followed by erosion before the deposition of the younger rocks.
    • Disconformity - Occurs when there is a gap in the geologic record between parallel layers of sedimentary rocks. The erosion or nondeposition event may be less obvious compared to an angular unconformity or nonconformity, as the layers above and below the unconformity are parallel to each other.

Earth’s History - Correlating Rock Layers

  • Rock Correlation - a method used by geologists to establish the relationship between rock layers (strata) at different locations. It involves identifying and matching specific rock layers or sequences of rock layers in different outcrops or drill cores based on their lithology, fossil content, or other characteristics.
    • Stratigraphic Relationships: Geologists also consider the relative position of rock layers within the stratigraphic sequence. Layers that are in the same relative position within the sequence and share similar lithological and fossil characteristics are likely to be correlated.

Earth’s History - Fossils

  • Fossils - any preserved evidence of past life, typically found within sedimentary rocks, that provides information about ancient organisms and environments.
    • Can be the remains of organisms
      • Bones
      • Shells
      • Teeth
      • Soft tissues and other body structures (rare → require conditions for preservation)
    • Can be traces of organisms
      • Footprints
      • Tracks
      • Burrows
      • Imprints of soft tissue
  • Fossils - any remnant which provides evidence of past life, typically found within sedimentary rocks. Provide data of both organisms and their environment: location, ecosystem, climate.
    • Most fossils are marine invertebrates (many had shells)
    • Organisms with hard parts and underwent quick burial
    • Complete organisms are extremely rare
      • Bits and pieces of body parts are common
  • Steps To Become A Fossil
    1. Organism dies
    2. Organism is buried in sediment or preserved (frozen, amber, etc)
    3. Sediments continue to pile up until the layer the organism is in becomes sedimentary rock.
    4. The body of the organism decays away over time (if not preserved in some way) and minerals are deposited within the space its body occupied.
    5. Tectonic forces or weathering/erosion bring the fossil to the surface to be found!

Earth’s History - Types of Fossils (Summary)

  • Body Fossils
    • Definition: These fossils preserve parts of an organism's actual body. They can include bones, teeth, shells, feathers, and skin impressions. Provide direct evidence of the organism's physical structure.
    • Examples: Mold and Casts, carbon films, petrified, preserved (frozen, amber, desiccated, bog bodies)
  • Trace Fossils
    • Definition: Indirect evidence of ancient life. They include footprints, tracks, burrows, nests, and coprolites (fossilized feces). Trace fossils reveal behavior, movement, and ecological interactions of ancient organisms.
    • Examples: Coprolites, gastroliths, tracks, trails, burrows, chemical signatures

Earth’s History - Types of Trace Fossils

  • Trace Fossils: Indirect evidence of ancient life. They include footprints, tracks, burrows, nests, and coprolites (fossilized feces). Trace fossils reveal behavior, movement, and ecological interactions of ancient organisms.
    • Coprolites - Fossilized or preserved remains of the contents of the intestine and the excrement of organisms: fossilized feces
      • Indicate the former presence of organisms in the area where found
      • Aid in the interpretation of the organism’s diet
      • Good indicator of the ecosystem around that organism
    • Gastroliths - fossilized gizzard stones and are usually only applicable in the fossil study of reptiles.
      • Help to grind food to pieces in reptiles gizzard
      • Have rounded edges and smooth polished surfaces
      • Help identify feeding habits and areas where ancient reptiles lived
    • Tracks - markings of movement that vertebrates leave
      • Formed when an organism moves over the surface of soft sediment and leaves an impression of its movement behind.
      • Tell more about the organism's behavior rather than the organism itself.
    • Trails - markings commonly characterized as impressions left by snails or worms crawling, jelly-fish dragging its tentacles, or the markings left by the movements of crustaceans or sea urchins
      • Often made on the soft sediment beneath the water surface.
    • Burrows - fossilized holes or tunnels excavated into the ground or seafloor - by animals to create a space suitable for habitation.

Earth’s History - Types of Body Fossils (Molds and Casts)

  • Molds and Casts: When an organism's remains decay and leave behind an impression in sediment, it forms a mold. If the mold later gets filled with minerals, it creates a cast, replicating the original shape of the organism.
    1. Organism dies
    2. Buried in sediments
    3. Sediments compact into rock (usually sandstone which is porous)
    4. Carbonated ground water permeates and dissolve the original tissue leaving a open hole (mold).
    5. Sediments and minerals fill mold creating a 3D representation of the organism (cast).

Earth’s History - Types of Body Fossils (Petrification)

  • Petrified Fossils: Occur when organic material is replaced with minerals, turning it into stone. Petrified wood is a common example, where the original wood structure is replaced by minerals such as quartz or calcite. Often evidence of a volcanic eruption in an area.
    • Reproduces the original tissue in every detail. This kind of fossilization occurs in both hard and soft tissues.

Earth’s History - Types of Body Fossils (Carbon Films)

  • Carbonized Fossils (Carbon Films): Formed when the organic material in an organism is compressed and heated, leaving behind a thin residue of carbon.
    • Decomposition of organic matter under anaerobic water or sediment, the hydrogen, oxygen, and nitrogen are driven off, leaving the carbon residue.
    • Delicate structures like leaves or feathers in detail.
    • Most compressions are found around coal seams.
    • Reveal evidence of the vast swamps containing luxuriant forests in particular areas, such as today's China, India, Australia, Africa, North America and parts of Europe.

Earth’s History - Types of Body Fossils (Preservations)

  • Preservations: When resin from trees hardens over time, it can trap small organisms like insects, spiders, or even small vertebrates. These organisms become encased in amber, preserving them and their original structures with remarkable detail.
    • Freezing: In environments where temperatures are consistently below freezing, organisms can become frozen and preserved in ice. This is common in polar regions, where mammoths and other animals have been found preserved in permafrost.
    • Tar Seeps: Similar to amber, tar seeps can trap and preserve organisms such as plants, insects, and small animals in sticky tar-like substances. Over time, the tar (oil that bubbles to the surface) hardens and preserves the trapped organisms.
    • Amber: Certain organisms, such as insects and small vertebrates, can become trapped and preserved in tree resin that hardens into amber over time. Amber fossils often preserve intricate details of the organism's soft tissues, providing valuable insights into ancient ecosystems.
    • Bog Bodies: In certain bog environments with low oxygen levels and acidic conditions, bodies can become naturally mummified. Bog bodies are often remarkably well-preserved, with intact skin, hair, and internal organs. Examples include the Tollund Man and Grauballe Man from Denmark.
    • Desiccation Fossils: In arid or desert environments, organisms can become desiccated, or dried out, and preserved in a mummified state. This can occur naturally through dehydration or burial in dry sand or sediments.

Earth’s History - Types of Body Fossils (Preservations) Molecular Fossils

  • Molecular Fossils - Often referred to as biomarkers or biosignatures and represent products of cellular biosynthesis that are incorporated into sediments and eventually into a rock. Many of these chemicals become altered in known ways and can be stable for billions of years.
    • Remnants of once-living organisms that have undergone chemical transformations over geological time scales.
    • Provide valuable information about ancient organisms, their metabolic processes, and even the environments they inhabited.
    • Can indicate the presence of specific types of bacteria or algae, the abundance of oxygen in ancient oceans or atmosphere, or the occurrence of past climatic events.
    • Common types of molecular fossils include sterols, hopanes, and alkanes, among others
    • Sterols are a type of lipid found in the cell membranes of eukaryotic organisms like plants and animals, while hopanes are derived from the degradation of bacterial hopanoids.
    • Alkanes are simple hydrocarbon molecules that can originate from a variety of biological and non-biological sources.

Earth’s History - The Fossil Record

  • The fossil record - The collection of all known fossils, which are the preserved remains or traces of organisms that lived in the past.
    • Provides a window into the history of life on Earth, documenting the diversity, evolution, and extinction of organisms over geological time scales.
    • Spans billions of years and encompasses a wide range of organisms, from microscopic bacteria to complex vertebrates.
  • By studying fossils, paleontologists can reconstruct past environments, track the evolution of species, and identify patterns of biodiversity and extinction events.
  • The fossil record is a crucial source of evidence for understanding the processes that have shaped life on Earth:
    • Evolutionary change
    • Adaptation to changing environments
    • Impact of mass extinctions

Earth’s History - Index Fossils

  • Index Fossils - Fossils of organisms that lived during specific time periods and have a wide geographic distribution. These fossils are particularly useful in the field of stratigraphy, which is the study of rock layers (strata) and their relative ages.
    1. Widespread around the Earth
    2. Easily identifiable
    3. From species that lived for a short span of geologic time
    4. Abundant (easy to find)

Earth’s History - The Principle of Fossil Succession

  • The Principle of Fossil Succession - fossil organisms succeed one another in a definite and determinable order, and therefore, any time period can be recognized by its fossil content.
    • Based on the observation that different types of organisms have lived on Earth at different times throughout its history.
      • The fossils of these organisms are preserved in distinct layers of sedimentary rock.
      • Different fossil species always appear and disappear in the same order, and that once a fossil species goes extinct, it disappears and cannot reappear in younger rocks.
  • By examining the types of fossils present in different rock layers, geologists can infer the relative ages of the rocks and establish a chronological sequence of events.
    • Especially powerful when index fossils are present in strata.
  • Fossil Assemblages - a collection of fossils found within a particular geological deposit or stratum. These fossils can include the remains of plants, animals, and other organisms that lived during a specific period of Earth's history.
    • Provide valuable information about the evolution of life on Earth.
    • Help reconstruct past ecosystems to understand how organisms interacted and what their environment was like.
  • The presence of certain index fossils in assemblages can help geologists establish the relative ages of rock layers and correlate them across different regions.

Fossil Succession and Evolution

  • Biological Evolution - the process by which living organisms change over successive generations through the gradual accumulation of heritable variations.
  • Transitional Fossils: Intermediate forms that exhibit traits of both ancestral and descendant groups, providing evidence for evolutionary transitions between different species or lineages. Help bridge the gaps in the evolutionary history of organisms and support the concept of gradual change over time.
  • Biogeography: The distribution of fossils in different geographic regions. Provides insights into the evolutionary history of organisms and the processes that have influenced their dispersal and diversification. Patterns of fossil succession observed in different parts of the world contribute to our understanding of how organisms have migrated, adapted to new environments, and evolved in response to changing ecological conditions.

Earth’s History - Bias In The Fossil Record

  • The fossil record, while invaluable for understanding the history of life on Earth, is inherently biased in several ways, leading to an incomplete representation of past life forms. Some of the biases in the fossil record include:
    1. Preservation bias
    2. Taphonomic bias
    3. Temporal bias
    4. Sampling bias
  • Preservation (Taxonomic) Bias: The fossilization process is highly selective, with only a small fraction of organisms becoming fossilized under specific conditions. Organisms with hard parts, such as shells, bones, and teeth, have a higher chance of being preserved as fossils compared to those with soft tissues. As a result, the fossil record primarily consists of organisms with hard parts, while soft-bodied organisms are underrepresented.
  • Taphonomic Bias: Taphonomy refers to the processes that affect organic remains from the time of death to their discovery as fossils. Various factors such as decay, scavenging, transportation, and burial can affect the preservation and distribution of fossils.
    • Example: Organisms living in environments where rapid burial occurs frequently, such as marine sediments or volcanic ash, are more likely to be preserved as fossils than those living in terrestrial environments.
  • Temporal Bias: The likelihood of fossilization varies over time, with certain geological periods being better represented in the fossil record than others. Factors such as environmental conditions, sedimentary deposition rates, and tectonic activity influence the preservation of fossils. Additionally, younger rocks are often more accessible and better studied than older rocks, leading to a bias towards more recent fossils.
  • Sampling Bias: The recovery of fossils is influenced by factors such as geographic location, accessibility of outcrops, and human activity. Fossil discoveries tend to cluster around areas with active geological research and accessible rock exposures, leading to sampling biases in certain regions or geological formations.

Earth’s History - Absolute Dating

  • Absolute Dating - a method used to determine the specific age (exact or a defined range) of a fossil, artifact, or geological feature in numerical terms.
    1. Radiometric dating
    2. Luminescence dating
    3. Dendrochronology
    4. Varve Chronology

Absolute Dating Techniques - Radiometric Dating

  • In order to understand the first absolute dating technique, radiometric dating, it is important to review a few terms: isotope, radiation and radioactive decay…
  • All matter is made of atoms, but not all atoms of the same element are identical!
    • Isotope - atoms of an element which have different numbers of neutrons in their nuclei.
      • Written as an elements name followed by a dash and the atomic mass.
      • While the # of protons determines the element's identity, the # of neutrons affects its stability.
  • The most abundant (commonly found in the universe) isotopes are STABLE while many less commonly found elements are UNSTABLE and undergo radioactive decay…
    • Radioactive decay - Spontaneous process by which an unstable isotope, one which has excess internal energy in the nucleus due to an imbalance of protons and neutrons, releases radiation (the emission or transmission of energy in the form of waves or particles through space or a medium) to become more stable. In doing so, it transforms into a different element or more stable nuclear arrangement.
  • Review: X = parent nucleus
  • A = the atomic mass (#N + #P)
  • Z = the atomic number (#P)
  • Y = daughter nucleus

Types of Radioactive Decay - A Closer Look

  • Alpha Decay - an atomic nucleus emits an alpha particle, resulting in the transformation of the original nucleus into a different nuclide. An alpha particle is a helium-4 nucleus consisting of two protons and two neutrons.
    • Occurs primarily in heavy, unstable isotopes with atomic numbers greater than 82 (lead)
    • Alpha particles are highly energetic and can interact strongly with matter (ionize) causing damage to biological tissue.
  • Beta Decay - characterized by the emission of energetic beta particles, which are electrons or positrons resulting in the transformation of the original nucleus. The mass number of the daughter nucleus remains the same as that of the parent nucleus.
    • Beta-Minus (\beta−) Decay: A neutron in the nucleus is converted into a proton, and an electron (beta particle) and an antineutrino are emitted
    • Beta-Plus (\beta+) Decay: A proton in the nucleus is converted into a neutron, and a positron (positive beta particle) and a neutrino are emitted.
  • Gamma Decay - An unstable atomic nucleus releases gamma rays, resulting in the transformation of the original nucleus. The emission of gamma rays occurs following alpha or beta decay or as a result of nuclear reactions.
    • Unlike alpha and beta particles, which change the composition of the nucleus, gamma decay does not change the atomic or mass number of the nucleus.
    • A process by which the nucleus transitions to a lower-energy state, releasing excess energy in the form of gamma rays

Absolute Dating Techniques - Radiometric Dating

  • Radiometric Dating - A method of absolute dating which relies on the principles of radioactive decay to determine the age of rocks, minerals, and fossils. Measures the abundance of radioactive isotopes and their decay products in a sample to calculate the time since the material was last heated or crystallized.
    • Isotopes of different elements decay at predictable rates.
    • Half-life - the time it takes for half of the atoms in a sample of the substance to undergo radioactive decay. In other words, it is the time it takes for the quantity of the radioactive substance to decrease by half. Parent Atoms Daughter Products

Radioactive Carbon Dating

  • Radioactive Carbon Dating
  • C-14, along with other carbon isotopes (C-12, C-13), reacts with oxygen to form CO2. Plants absorb all three isotopes of carbon as CO2 during photosynthesis. Animals absorb C-14 when they consume plants. Concentration of C-14 decreases as it decays to N-14.
  • half-life = 5730 years

Absolute Dating Techniques - Luminescence Dating

  • Luminescence - the emission of light by a substance when it is stimulated by energy sources such as heat or light.

Absolute Dating Techniques - Luminescence Dating

  • Optically Stimulated Luminescence (OSL) Dating: commonly used to determine the age of sediments and geological deposits, as well as archaeological artifacts buried in sedimentary contexts. It relies on the accumulation of trapped electrons within mineral grains like quartz or feldspar over time due to exposure to ionizing radiation from natural sources such as cosmic rays and radioactive isotopes in the surrounding environment. By exposing the mineral grains to light (optical stimulation) in the laboratory, the trapped electrons are released, resulting in luminescence emission proportional to the time since burial. This luminescence signal can be measured and used to calculate the age of the sample.
  • Thermoluminescence (TL) Dating: TL dating is similar to OSL dating but is typically used for materials such as pottery, ceramics, and burnt stones in archaeological contexts. Like OSL dating, TL dating relies on the accumulation of trapped electrons over time due to exposure to ionizing radiation. However, instead of optical stimulation, TL dating involves heating the sample (thermally stimulating) in the laboratory. This heating releases the trapped electrons, resulting in luminescence emission that can be measured and used to determine the age of the sample.

Absolute Dating Techniques - Dendrochronology

  • Dendrochronology - A dating technique that uses the analysis of tree rings to determine the age of wooden objects or structures. A powerful tool for reconstructing past climate and environmental conditions, enhancing our understanding of Earth's climatic history and informing projections of future climate change.

Absolute Dating Techniques - Dendrochronology

  • Each year, trees in temperate climates undergo a cycle of growth and dormancy → form one growth ring per year.
    • During the growing season, trees forms a new layer of wood called earlywood = lighter in color and less dense.
    • Growth slows towards the end of the season, a darker and denser layer called latewood is formed. Together, these layers form a distinct annual growth rings.
  • Tree Core - a cylindrical section of wood extracted from the trunk of a tree for dendrochronological analysis.
    • Obtained using a specialized tool called an increment borer
    • Provide valuable information about the tree's growth patterns, environmental conditions, and historical events.
  • By examining the patterns of wide and narrow rings, as well as other characteristics such as ring width and density, sequences of rings that match across different samples can be identified. Then, the pattern in the sample can be matched with patterns in other samples to create a chronological sequence extending back in time. This larger sequence of rings is known as a master chronology.
  • Cross-Dating: To ensure accuracy, dendrochronologists cross-date samples by comparing their ring patterns with the master chronology. If a sample's ring pattern matches the master chronology and aligns with historical records or other dating methods, it can be accurately dated.

Dendrochronology → Dendroclimatology

  • Dendroclimatology - subfield of dendrochronology, focuses on using tree-ring data to reconstruct past climate conditions.
    1. Ring Width: Generally, wider rings indicate favorable growth conditions, such as ample moisture and mild temperatures, while narrower rings suggest periods of stress, such as drought or cold temperatures.
    2. Ring Density: The density of tree rings can reflect variations in the availability of water and nutrients. Higher density rings typically form during periods of slower growth, often associated with drier or colder conditions.
    3. Isotopic Composition: The isotopic composition of tree rings, particularly stable isotopes of oxygen and carbon, can provide information about past temperature and precipitation patterns.
  • By analyzing tree ring patterns from multiple trees in a region, past climate variations can be reconstructed, extending back hundreds or even thousands of years. These reconstructions help scientists understand natural climate variability, identify long-term climate trends, and provide context for recent climate changes attributed to human activities.

Absolute Dating Techniques - Varve Chronology

  • Varve Chronology - method used in geology and paleoclimatology to establish precise and detailed chronological sequences of sedimentary deposits, particularly in glacial and lake environments. Varves are pairs of sedimentary layers, usually consisting of a light-colored layer (spring/summer) and a dark-colored layer (fall / winter), which form annually in certain depositional settings.
  • Varve chronology involves:
    • Core Sampling: Sediment cores are extracted from the depositional environment using techniques such as drilling or coring.
    • Visual Examination: The sediment cores are visually examined to identify the alternating light-dark layers characteristic of varves.
    1. Microscopic Analysis: Conducted to confirm the annual nature of the varves and to identify changes in sediment composition or biological remains.
    2. Counting and Dating: Done either directly if an unbroken sequence is present, or by correlating the sequence with other dating methods (e.g., radiocarbon dating, tephrochronology) if gaps exist.

Absolute Dating Techniques - Varve Chronology

  • Varve chronologies can provide high-resolution records of past environmental changes, including:
    • Seasonal and annual climate variability.
    • Glacial fluctuations and meltwater dynamics.
    • Environmental responses to volcanic eruptions, earthquakes, and other events.
  • Varve chronologies have been instrumental in reconstructing past climate variations and understanding the timing and magnitude of environmental changes over various timescales, from decades to millennia.
  • Varve chronology can serve as a form of absolute dating when certain conditions are met, particularly in settings where varves are formed annually and continuously:
    • Annual Deposition: In environments such as glacial or proglacial lakes, where varves are formed annually due to seasonal changes in sediment input, each varve pair represents one year of deposition. This annual deposition allows for precise dating of sediment layers.
    • Varve Counting: Each varve pair represents one year, so by counting the varves from the top of the sediment sequence to a known depth, researchers can determine the absolute age of the sediment layers. Requires careful examination of the sediment cores often using specialized equipment such as microscopes.

The Geologic Timescale

  • The geologic time scale - a chronological framework that divides Earth's history into distinct intervals based on major geological and biological events. It provides a way to organize and understand the vast expanse of time over which Earth has existed. The geologic time scale is divided into several hierarchical units, including eons, eras, periods, epochs, and ages.
  • Eons > Eras > Periods > Epochs > Ages