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 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 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.

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 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.

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.

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.