The Geologic 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 four eons of Earth's history, order them from oldest to most recent, and indicate in which one we live and since when, approximately.

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.

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.

Situate the Cretaceous period by answering the following questions:

1. a) To which geological era does it belong?
2. b) Between what approximate ages (in millions of years) does it span?
3. c) What event brought it to an end?

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.

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.

What event marks the beginning of the Phanerozoic eon and of the Cambrian period? Explain what the "Cambrian explosion" was and what change it represents in the fossil record.

The Mesozoic extends from $252$ million years ago to $66$ million years ago. Calculate its duration and what percentage of the Phanerozoic ($541$ million years) it represents, rounding the result.

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 end-Permian extinction was the greatest in history: the trilobites and the vast majority of marine species disappeared. Explain which era it brought to a close, which groups benefited from the ecological void that followed, and what relationship this crisis has with the beginning of the age of the dinosaurs.

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.

The official boundaries of the scale are fixed at specific profiles called stratotypes or "golden spikes" (GSSP). Explain what a golden spike is, why a physical place must be agreed upon for each boundary, and what characteristics that profile must have.

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."

In a continuous stratigraphic borehole across the boundary between two geological periods where a major mass extinction took place, the following geochemical and sedimentary anomalies have been recorded:

1. An abrupt and global negative $\delta^{13}\text{C}$ excursion of more than $-5\ ‰$.

2. A sharp drop in $\delta^{18}\text{O}$ in carbonates revealing thermal warming of the oceanic water column between $8\ \text{°C}$ and $10\ \text{°C}$.

3. A widespread euxinic horizon (black shales with pyrite framboids and enrichment in vanadium and uranium).

4. Total absence of iridium anomalies, impact spherules, or shocked quartz in the transitional clay.

1. a) Deduce to which of the two most studied extinctions (end-Permian or end-Cretaceous) these data correspond, justifying your choice and refuting the other using two arguments supported by the described evidence.
2. b) Reconstruct the biogeochemical cause-and-effect chain linking continental flood basalt volcanism (traps) with the drop in $\delta^{13}\text{C}$, the global greenhouse effect, the destabilisation of methane hydrates, and deep-marine anoxia.
3. c) Based on the reconstructed causal mechanism, critically assess why this event from the geological past represents a much closer warning analogue for the current anthropogenic climate crisis than the Cretaceous-Paleogene boundary event.

The International Commission on Stratigraphy (ICS) defines the boundaries between units of the Geologic Time Scale using Global Boundary Stratotype Sections and Points (GSSPs or "golden spikes").

Imagine you are part of the committee that must decide between two proposals to mark the basal boundary of a new geological age:

• Proposal A: A record in a stalagmite from a karst cave showing a $\delta^{18}\text{O}$ signal synchronous on a planetary scale (an abrupt global mega-drought) dated with very high radiometric precision using uranium-series dating ($^{230}\text{Th}/^{234}\text{U}$) at $4\,200$ years before present.

• Proposal B: A marine sedimentary section containing the local disappearance of a benthic foraminifer, with an approximate biostratigraphic correlation dating of $\pm 1.5$ Ma.

1. a) Justify which of the two proposals meets the formal geological requirements to be approved as a GSSP (resolution, global correlation, isochrony, and preservation), explaining the shortcomings of the discarded proposal.
2. b) In recent decades, the base of the Cambrian Period has been readjusted from about $570$ Ma to be fixed at $538.8 \pm 0.2$ Ma. Explain what advances in radiometric dating techniques (e.g., ID-TIMS on intercalated volcanic zircons) and in ichnostratigraphy (Treptichnus pedum) have allowed this date to be refined without altering the prior relative ordering of the layers.
3. c) Refute the claim of a detractor of geology: "The fact that the age of the Cambrian has changed several times proves that dating methods lack rigor and that the Geologic Time Scale is merely an arbitrary convention."

In March 2024, the International Union of Geological Sciences (IUGS) rejected by a large majority the proposal to formalize the "Anthropocene" as a new epoch of the Quaternary Period with a GSSP located in 1952 (marked by the peak of artificial radioisotopes from thermonuclear tests in the sediments of Crawford Lake, Canada).

1. a) Analyze the major stratigraphic objections that justified this decision: why does restricting the Anthropocene to the last 70 years ignore the deep diachrony of the global human footprint (beginnings of Neolithic agriculture, massive deforestation, industrial revolution) and clash with the stratigraphic principle of isochrony applicable to units of epoch rank?
2. b) Compare the potential preservation in the future lithological record (on a scale of tens of millions of years) of the radioactive pulse of $^{239}\text{Pu}$ and the so-called "technofossils" (plastics, concrete) against classic geological boundary markers such as iridium layers or global biological extinctions.
3. c) Defend why classifying the human impact as a "complex geological event" (analogous to the Great Oxidation Event or oceanic anoxic events) is a methodologically sounder solution than forcing its definition as a formal epoch with a single golden spike.

Order these milestones from oldest to most recent and express their age in millions of years: the formation of the Earth, the appearance of Homo sapiens, the Cambrian explosion, the extinction of the dinosaurs and the origin of life.

If we assign $1$ year to every $100$ million years, the Earth would be $46$ years old. Calculate at what "age" of this planet-person the dinosaurs appear ($230$ million years ago) and Homo sapiens ($0.3$ million years ago), and comment on what the analogy reveals.

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.

In popular science debates, the longevity of non-avian dinosaurs (~$160$ Ma) is often compared with that of our species (Homo sapiens, ~$0.3$ Ma) to evaluate "evolutionary success."

1. a) Identify and deconstruct the conceptual and taxonomic error of comparing the duration of an entire higher-ranking clade (Dinosauria, with hundreds of successive genera and lineages) with the longevity of a single biological species. Compare the current age of Homo sapiens with the average longevity of a mammal species in the Cenozoic fossil record (~$1$ to $2$ Ma) and assess whether our species has reached its expected evolutionary lifespan or not.
2. b) Explain two macroevolutionary mechanisms (adaptive plasticity, trophic diversification, and ecological redundancy) that allowed dinosaurs to recover and radiate after the Triassic-Jurassic mass extinction, and compare them with the intrinsic vulnerability of a current, ultra-homogenized biosphere that relies on a single dominant species.
3. c) Formulate two reasoned evolutionary hypotheses about the stratigraphic fate of humanity over the next 5 million years: one considering cladogenesis/speciation and another considering terminal biological extinction without descendants, detailing in each case the selection pressures and isolation conditions that would make it possible.