The Core Idea
'COSI' — Carbon, Oxygen, Strontium, Iridium
Chemostratigraphy uses the chemical signals locked into sediments, especially marine carbonates. Because the oceans mix in about a thousand years — instant on geologic time scales — a change in seawater chemistry is recorded at essentially the same time worldwide. Remember COSI: Carbon (the carbon cycle), Oxygen (temperature and ice), Strontium (weathering), Iridium (impacts).
1
C — Carbon isotopes (δ¹³C)
Track the carbon cycle. A sudden NEGATIVE excursion means a flood of light carbon entered the ocean-atmosphere system (volcanism, methane release); a positive shift reflects increased burial of organic carbon.
The Paleocene–Eocene Thermal Maximum (PETM, about 56 million years ago) shows a sharp negative carbon isotope excursion used to correlate sections on every continent.
2
O — Oxygen isotopes (δ¹⁸O)
Record temperature and ice volume. Ice sheets preferentially lock up light ¹⁶O, leaving seawater — and the shells that grow in it — enriched in ¹⁸O. Higher δ¹⁸O in foraminifera means a colder climate and/or more ice.
3
Sr — Strontium isotopes (⁸⁷Sr/⁸⁶Sr)
Strontium stays in seawater for millions of years, far longer than the ocean takes to mix, so the ratio is the same everywhere at any given time. It rises with continental weathering and falls with hydrothermal input. Matching a sample's ratio to the global reference curve gives a numerical age for marine carbonates.
4
Ir — Iridium anomaly
Iridium is rare in Earth's crust but common in meteorites. A thin iridium-rich clay layer found worldwide at the Cretaceous–Paleogene (K–Pg) boundary, about 66 million years ago, supports the Chicxulub impact hypothesis.
🖥️ Applied Scenario
Two limestone sections — one in Egypt and one in Wyoming — contain almost no age-diagnostic fossils near the Paleocene–Eocene boundary. How can you show which layers formed at the same time?
STEP 1
Sample the carbonates — measure δ¹³C bed by bed up through both sections.
STEP 2
Look for the signature — both sections show the same abrupt negative carbon isotope excursion followed by a gradual recovery — the PETM fingerprint.
STEP 3
Tie the sections together — draw your correlation line at the onset of the excursion in each section — that level marks the same moment in time.
CONCLUDE
Even without fossils, the shared chemical signal proves the layers are time-equivalent, because the ocean-atmosphere carbon change happened globally at once.
📌 Exam Application
Expect questions linking each isotope system to what it records (δ¹³C = carbon cycle, δ¹⁸O = temperature/ice, ⁸⁷Sr/⁸⁶Sr = weathering and age, iridium = impact). Know the two headline examples: the PETM carbon excursion (~56 Ma) and the K–Pg iridium anomaly (~66 Ma).
⚠️ Most Common Chemostratigraphy Mistakes
Watch the direction of the oxygen signal: higher (more positive) δ¹⁸O in shells means COLDER or icier conditions, not warmer. Also remember that diagenesis — chemical alteration after burial — can overprint original signals, so samples must be screened.
✓ Quick Self-Test
1) Why can a chemical shift in seawater be used for global correlation? 2) What does a negative carbon isotope excursion indicate? 3) What does higher δ¹⁸O in foraminifera tell you? 4) Why is the strontium isotope ratio the same across all oceans at a given time? 5) What does the K–Pg iridium anomaly point to?