The rock record preserves evidence of past climate and sea-level change: rock types act as climate indicators (coal for warm wet swamps, evaporites for hot arid conditions, tillite for cold glacial conditions, reef limestone for warm shallow seas); rising sea level (transgression) gives a fining-upward, deepening sequence and falling sea level (regression) a coarsening-upward sequence; these changes are driven by ice ages, plate movement and changes in the volume of the ocean basins.

What this dot point is asking

Eduqas wants you to use the rock record as an archive of past climate and sea-level change. You need to read rock types as climate indicators (coal, evaporites, tillite and reef limestone), to interpret transgression and regression from a sequence (rising sea level gives a deepening, fining-upward sequence; falling sea level gives a coarsening-upward one), and to know the causes of sea-level change (ice ages, plate movement and changes in the volume of the ocean basins). This is an AO2 and AO3 interpretation skill, so it carries higher marks.

The answer

Rocks as climate indicators

Certain rocks can only form under particular climates, so finding them tells you what the climate was like when they formed:

• Coal forms from plant material in waterlogged swamps. It needs a warm, wet climate with lush plant growth and ground wet enough to slow decay. Coal therefore signals warm, wet conditions.
• Evaporites (rock salt, gypsum) form when water evaporates faster than it is replaced, leaving the dissolved salts behind. This needs a hot, arid climate in a restricted basin. Evaporites signal hot, dry conditions.
• Tillite is lithified glacial till: poorly sorted, unstratified, often with striated (scratched) pebbles. It is dropped directly by ice, so it signals a cold, glacial climate.
• Reef limestone is built by corals and other shelly organisms that need warm, shallow, clear sea. It signals a warm, shallow marine setting.

Because these conditions are so different, a change in rock type up a sequence can record a change in climate over time.

Transgression and regression

When sea level rises or falls relative to the land, the shoreline moves and the environments shift, leaving a tell-tale vertical sequence:

• Transgression is the sea advancing over the land as sea level rises. Each point ends up in progressively deeper water, so the sequence deepens upward and the grain size fines upward (coarse beach sands at the base, finer offshore muds above).
• Regression is the sea retreating as sea level falls. Each point ends up in progressively shallower water, so the sequence shallows upward and the grain size coarsens upward (offshore muds at the base, beach sands above).

So a fining-upward, deepening sequence equals a transgression; a coarsening-upward, shallowing sequence equals a regression.

What changes sea level

Sea level changes for several reasons:

• Ice ages. When large ice sheets grow, water is locked up on land as ice, so sea level falls worldwide. When the ice melts, sea level rises. These are global (eustatic) changes.
• Plate movement. Tectonic uplift or subsidence raises or lowers the land relative to the sea (a local change), and continents drifting into warm or cold latitudes change the regional climate.
• The volume of the ocean basins. Fast sea-floor spreading builds large, hot mid-ocean ridges that take up space in the ocean basins, pushing sea level up; slow spreading lets it fall.

Distinguishing a global change in sea level (from ice volume or ocean-basin volume) from a local change (from tectonic uplift) is a higher-mark skill: a global change should appear in rock sequences worldwide, a local one only in one region.

Examples in context

Example 1. Coal Measures of Britain. The thick coal seams of northern England formed in vast tropical swamps when Britain lay near the equator, recording a warm, wet climate hundreds of millions of years ago.

Example 2. Ice-age sea level. During the last ice age, so much water was locked in ice sheets that sea level was about 120 metres lower than today, and Britain was joined to mainland Europe by dry land.

Try this

Q1. Name the rock type that indicates a hot, arid climate. [1 mark]

• Cue. Evaporites (for example rock salt or gypsum).

Q2. A sequence coarsens upward from offshore mud to beach sand. Is this a transgression or a regression? Explain. [2 marks]

• Cue. A regression: the sequence shallows upward, recording the sea retreating as sea level falls.

Q3. State one cause of a worldwide fall in sea level. [1 mark]

• Cue. The growth of large ice sheets in an ice age (water is locked up on land as ice).