1 - 1 SRSC Introduction

Question 1

How does chronostratigraphy differ from lithostratigraphy? (2)

Answer 1

Two beds may correlate lithostratigraphically yet not be contemporaneous: they are diachronous. This is seen, for example, in the Kimmeridge Clays around the North Sea.

Question 2

Summarise chronostratigraphic methods. (2.1)

Answer 2

Biostratigraphic zones: FAD, LAD, assemblage, acme. Well preserved, widespread and rapidly evolving species. Viosca Knoll dates, Aptian 8.8Ma 8 ammonite zones in Tethys.

Chemostratigraphy: correlation of isotope ratios C12/C13 (12 preferentially incorporated into organic matter), Sr87/Sr86 (Sr87 from terrestrial sources: Himalayan oregony), O16/O18 (16 preferentially evaporated so reflects glaciation).

Magnetostratigraphy: chrons may be normal or reversed. Magnetic minerals and clays. Past shape and size of ocean basins.

Astrochronology: Fourier analysis, Milankovitch cycles: precession (19, 23ka), obliquity (41ka), ellipsis (95, 123, 413ka). Methane hydrate release event, high resolution Cretaceous clays.

Radiometric: decay of isotopes, 14C, K40-Ar40, Ar40-Ar39, Rb87-Sr87, Uranium-Lead. Igneous, volcanic, organic and metamorphic but not sedimentary. Black Sea.

Question 3

How has chronostratigraphy been used to study the Palaeocene/Eocene methane hydrate release event? (2.2)

Answer 3

Approx 55Ma, 13C -3ppt equivalent to 120-200gT carbon into atmosphere. Contemporary with loss of 35-50% benthic foraminifers and evidence for shift to warmer wetter climate.

Fourier analysis recognised magnetic cycles of two periodicities, identified as 19 and 23ka processions. Count the number of cycles in each stage. Estimated initial event few thousand years, but 120ka before carbon returns to steady state (6 cycles occur).

Question 4

How have Ryan and Pitman (1998) identified and quantified the flooding of the Black Sea? (3.1)

Answer 4

Boreholes: homogenous olive-green mud containing marine mollusc fossils overlies sandstones, sand dunes, subaerially desiccated mudstones and gravels containing abraded freshwater fossils.

Palaeocoastline 160-170m below present.14C dating: 7600BP. Rapid rise (15cm per day) and transgression (1 mile per day).

Question 5

How can we detect changes to sea-level? (3.2)

Answer 5

1. d18O: higher means more glaciation and so lower eustatic sea-level. Use benthic foraminifers as also affected by temp, and less fluctuation on floor.
   - - d18O curve last 20Ma from DSDP
   - - Rangitikei: shelf siltstones and shore sandstones 2-2.2Ma.
2. Paleocoastlines and hypsometric charts, though modern relief only accurate for recent periods.
3. Tide gauges. But only last 150 years and only RSL (e.g. since 1950 Alaska -40m, New York +20m).

Question 6

Why does sea-level change? (3.3)

Answer 6

Changes in volume of seawater:

• Glacio-eustasty (60-80m, reduce land 20%)
• Thermal expansion (10 degrees = 10m)

Changes in volume of basins holding water:

• Mid-ocean ridges (Cretaceous)
• Continental collision decreases land area

Some of this occurs on a local scale:

• Subsidence (e.g. glacial loading) and uplift (e.g. isostatic rebound)
• Sediment filling a basin