Exercises: The geological time scale

Order these units of the geological time scale from longest to shortest duration: epoch, eon, period, and era. Give a real example of each.

Name the three eras of the Phanerozoic eon, list them in order from oldest to newest, and cite a characteristic group of living organisms for each.

Place each of these events into the corresponding era and, if possible, period: the extinction of non-avian dinosaurs, the appearance of the genus Homo, and the "Cambrian explosion" of animals.

The Phanerozoic eon began $541$ million years ago, and Earth formed about $4\,600$ million years ago. Calculate what percentage of Earth's history the Phanerozoic occupies and explain why the previous eons (the "Precambrian") are much less well understood.

Explain why the boundaries between eras on the geological time scale coincide with mass extinctions, and use the Mesozoic-Cenozoic boundary as an example, stating what caused it.

The diagram summarises the major intervals of geological time (eons and eras) with their boundary ages. Name the interval marked with "?", state between which ages it extends, and cite an important biological event that occurred during it.

Imagine that the history of the Earth ($4\,600$ Ma) is compressed into a one-calendar-year timeline ending at midnight on December 31.

1. a) Calculate at what approximate date and time you would place three fundamental milestones: the Great Oxidation Event (~$2\,400$ Ma), the appearance of the first macroscopic fossils with shells or skeletons in the Cambrian explosion (~$541$ Ma), and the origin of the genus Homo (~$2.5$ Ma). Show the proportional calculations carried out.
2. b) A popular science documentary states: "The history of the Earth is fundamentally the history of land animals and plants, which have populated the planet since its origins, culminating in the arrival of human beings." Based on the temporal results obtained in the previous section, refute this statement point by point and argue why the geological time scale invalidates any anthropocentric or zoocentric reading of Earth's time.

A geology team analyzes two continuous stratigraphic columns with a thickness of 150 meters: Column A corresponds to the Jurassic period ($201$-$145$ Ma) and Column B belongs to an Archean sedimentary basin ($3\,200$-$3\,000$ Ma).

1. a) Justify why in Column A it is feasible to establish a high-resolution chronology by subdividing strata into biozones spanning barely hundreds of thousands of years, whereas in Column B this method is completely inapplicable. What biological and taphonomic conditions exclusive to the Phanerozoic explain this?
2. b) Design a geochronological strategy to date and correlate the strata of Column B. Specify which isotopic or geochemical methods you would use and what obstacles arising from the rock cycle (metamorphism, diagenesis, tectonics) you must anticipate due to the antiquity of the materials.
3. c) Refute the following conclusion drawn by a student: "Since the Archean lasted more than three times longer than the entire Phanerozoic, its stratigraphic record should be much broader and easier to decipher than that of the Jurassic."

The background extinction rate throughout the Phanerozoic is estimated to have been between 0.1 and 1 extinction per million species-years (E/MSY). During the great end-Permian crisis (~$252$ Ma), around $95\ \%$ of marine species became extinct within an interval of about $60\,000$ years. Currently, estimates place the extinction rate caused by human activity between 100 and $1\,000$ E/MSY.

1. a) Based on these data, compare the average extinction rate in the Permian with the current rate. Explain why the contemporary crisis is classified as a potential "sixth extinction" even though the absolute number of extinct species documented by the IUCN in recent centuries is much lower than those of the five great geological extinctions.
2. b) Analyze the taphonomic biases of the marine fossil record that complicate direct comparison between historical diversity curves (such as Sepkoski's) and modern terrestrial biodiversity censuses.
3. c) Predict what characteristic biofacies and lithofacies a palaeontologist would find 50 million years from now in sediments corresponding to our era if the current pace of anthropogenic transformation continues.