Stratigraphy: Sediments and Layers

Stratigraphy

• Stratigraphy involves layers of evidence.
• Lecture covers stratigraphy, what it is and why it matters
• It also discusses stratigraphy and the Quaternary period.

Sediments and Stratigraphy

• Sediment refers to any mineral or organic matter deposited by air, water, or ice.
• Sediment accumulates in places like the bottom of lakes, oceans, on land, and within bogs.
• Strata are horizontal layers of sediment (singular: stratum).
• Stratigraphy is formed by strata.

Stratigraphy Importance

• In 1027, Avicenna noted mountains are built up in layers over time, with the oldest layers at the bottom.

The Law of Superposition

• Nicholas Steno in 1669 introduced the Law of Superposition
• "In a sequence of strata, any stratum is younger than the sequence of strata on which it rests, and is older than the strata that rest upon it”.
• In the 17th century, people divided geological time into pre-Flood and post-Flood periods based on fossils
• “Pre-Flood" rocks did not contain fossils.

Stratigraphy and Age

• Stratigraphy is chronologically ordered, meaning the lowest sediment accumulated first.
• The deeper in the sequence, the older the sediment.
• The "time-depth relationship" refers to this concept.

Relevance of Studying Stratigraphy

• Stratigraphic changes provide insights into environmental changes.
• Changes in material, color, or compaction indicate events

Proxy Records

• Changes in sediment type can reveal changes in the environment.
• Cores from NW European bogs and lakes show evidence of a sudden "cold snap" at the end of the Last Glacial period, also known as the Younger Dryas Stadial.

The Quaternary Period

• Geologists divide Earth's time into four slices
• The Quaternary is the top slice, representing the current period
• It is separated from the slice below (the Tertiary) by a geological marker.
• The marker is a reversal of the Earth's magnetic field known as the Guass-Matuyama Reversal.
• Major cooling events occurred between 2.8 to 2.4 million years ago.
• The events were possibly due to the closure of the Panama Isthmus.
• The base of the Gelasian is astronomically tuned to 2.588 million years ago.
• The Quaternary's start is now placed at 2.6 million years ago (formerly 1.8 million years ago).

Quaternary Controversy

• There was a proposal to drop the Quaternary as a geological period by the Subcommission on Quaternary Stratigraphy
• It was proposed because there was no global, stratigraphic boundary to define it
• It was met with uproar from Quaternary Scientists and the International Union for Quaternary Science (INQUA) objected to proposed changes in 2006
• Recognition of Quaternary was ratified in 2009

Climate Cycles of the Quaternary

• Subdivisions within the Quaternary are based onclimato-stratigraphy.
• Climato-stratigraphy refers to units formed under different climatic regimes
• The Quaternary's climate has alternated between glacials (ice ages) and interglacials (warm periods).
• The current interglacial period is the Holocene.
• It will eventually transition into a future glacial period.

Glacial-Interglacial Cycles

• How many Glacial and Interglacial periods occurred are not known for sure.
• Each Glacial tends to wipe out the evidence of the preceding one
• Glaciers form during the Ice Age
• Glaciers expand and remodel the landscape
• At the start of an interglacial, the glacier melts, leaving moraines, eskers, and similar landforms to mark its former presence.

Lost Evidence and Glaciation

• At the start of the next glaciation, a new glacier forms.
• It scours the landscape, destroying traces of the former glaciation.
• As a result, tracing glaciation on land is problematic.
• By the early 20th century, four glacial cycles were recognized from land-based studies; now, six are recognized within Europe.
• Many land-based glacial studies are from the Northern Hemisphere.
• Ireland was almost entirely glaciated during the last Glacial period.
• However, there aren't human remains found in Ireland that date to before the start of the Holocene.
• In southern Britain, fairly extensive evidence exists for early hominids dating back to 800,000 years ago.
• During the Last Glacial period, Homo neanderthalensis and Homo sapiens (anatomically modern humans) occupied parts of Britain

Pleistocene Ireland

• Two organic horizons exist within geological section at Knocknacran, Co. Monaghan
• Pollen evidence suggests these formed during warmer intervals.

Evidence from the Sea

• Kilometers-thick marine mud has been deposited throughout numerous Glacial/Interglacial cycles.
• Cores can date back 2.5 million years i.e. they span the Quaternary.
• Marine cores suggest that there were between 30-50 Glacial-Interglacial cycles during the Quaternary

Ice Evidence

• During Glacials, northern and southern latitudes experienced huge ice sheets
• The ice-free tropics were much cooler than today
• Global sea-levels were reduced, as much water was locked up in the ice sheets
• Ice cores from Greenland extend back 120,000 years and from the Antarctic to 740,000 years
• These provide a shorter record than marine cores, but dating accuracy is much better

Identifying Glacials and Interglacials

• Oxygen isotope studies can tell when it got warmer/colder.
• "Oxygen Isotope Stages" is the name (aka Marine Isotope Stages)
• The terms “ky” or “ka" = 1,000 years and "Ma" = million years

Nomenclature of Previous Periods

• Glacial nomenclature is not internationally accepted.
• The Last Glacial is known as Midlandian, Devensian, Würmian, Weichselian, Wisconsinan.
• Difficulty in agreement as unable to align cycles across the globe.
• Oxygen isotopes stages now commonly used.
• all of the Glacials and Interglacials before the Holocene are known as the Pleistocene
• The Quaternary is the Pleistocene + Holocene.

Last Ice Age

• The last Ice Age began around 115,000 years ago
• It consisted of periods of very cold conditions (STADIALS) and warmer conditions (INTERSTADIALS)
• There is agreed terminology for these events, which is based on ice core data such as Greenland Stadial 1, 2, etc. (GS-1, GS-2...) and Greenland Interstadial 1, 2 etc. (GI-1, GI-2...)

The Eemian Interglacial and Long EPICA Ice Core

• The Eemian Interglacial occurred 127,000-115,000 years ago
• The Eemian Interglacial was warmer than the Holocene (proxy records)
• Lions, hippos, and elephants roamed Britain at that time
• It was longer than the Holocene so far (dating methods)
• The base of the Greenland ice cap today has where the was Borel forest grew
• The long EPICA (Antarctic) ice core indicates eight Glacial-Interglacial cycles during the last 740,000 years
• Interglacials lasted between 10,000 to 30,000 years versus Glacials that lasted between 70,000 to 90,000 years
• The earlier four Interglacials were less warm than the four most recent ones
• The four most recent Interglacials have been shorter than the previous four

Rapidity of Change

• Research from the ice cores find climate has shifted from one state to another in less than 100 years
• It changed possibly as quickly as 20 years, therefore less than a human lifetime
• Milankovitch Theory may explain some of these changes, but not all

Stratigraphy Summary

• Stratigraphy provides a record of past environmental change over time.
• Stratigraphy has changes to physical conditions, organisms, and chemical compositions.
• Stratigraphy is interpreted "upwards,” from the oldest layers to the most recent.
• On land, later events can destroy earlier strata.
• 50 Glacial-Interglacial cycles occurred during the Quaternary.
• Stratigraphy of ice cores shows major and rapid climate swings during and since last Ice Age.