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3.3.7. Sedimentary Rocks

Many of the techniques used for palaeoclimatic reconstruction discussed in the preceding sections have only a limited time scale open to their period of study. Most ice cores are restricted to the last million years, whilst tree ring analysis can only provide proxy climate information for at best the last 10,000 years. Ocean sediments provide some of the longest proxy records available, and offer a window on palaeoclimates dating back to the age of the dinosaurs, 100 million years ago. Most older sediments, however, will have been subducted beneath overriding tectonic plates as the continents continue to drift about the Earth. To reconstruct climates older than this, therefore, one needs to look elsewhere for the evidence.

Sediments laid down on the ocean floor become progressively buried by subsequent debris transported from continental interiors. Deeply buried sediments are subjected to considerable pressures from the overlying layers, and after tens to hundreds of millions of years, the sediments are gradually lithified, forming sedimentary rocks. If, through tectonic movements, these sedimentary rocks are uplifted and exposed, scientists may study them, as they do other forms of evidence, to reconstruct past climates.

Numerous techniques of analysing sedimentary rocks are used for palaeoclimate reconstruction. Principally, rock type provides valuable insights into past climates, for rock composition reveals evidence of the climate at the time of sediment deposition. However, depositional climatic regimes vary not only due to actual climatic changes but also due to continental movements. The Carboniferous limestones and coals (evidence of warm, humid climates) of Northern England (300Ma), for example, were laid down at a time when Britain was located near the equator, whilst large scale glaciation was occurring in the high latitudes of the Southern Hemisphere (section 5.2.2.2).

The study of rock type is geologically known as facies analysis. Facies analysis investigates how the rock type changes over time, and therefore provides a potential tool for investigating past climatic change. A sedimentary formation consisting of a shale layer (fine-grained mudstone) interbedded between two sandstone layers (coarse-grained), for example, provides evidence of a changing sea level, potentially linked to climatic change (caused either by epeirogeny (section 2.6.2) or ice formation (section 2.5.2.4)). Sandstones are deposited in coastal zones where the water is shallow, whilst mudstones (shales) are deposited in deeper water of the continental shelf region. A change in the rock type in the vertical cross section must therefore reflect a change in sea level and associated coastline movements.

Other principal marker rock types include evaporites (lithified salt deposits and evidence of dry arid climates), coals (lithified organic matter and evidence of warm, humid climates), phosphates and cherts (lithified siliceous and phosphate material and evidence of ocean upwelling due to active surface trade winds) and reef limestone (lithified coral reef and evidence of warm surface ocean conditions).

As well as facies analysis, other techniques, including analysis of sedimentation rates, sediment grain morphology and chemical composition provide information on the climatic conditions prevailing at the time of parent rock weathering. In addition, some of the methods used to reconstruct past climate discussed in earlier sections may be equally applied to sedimentary rocks. For example, the type and distribution of marine and continental fossils within fossil-bearing rocks (principally limestones and mudstones, but occasionally sandstones) are valuable palaeoclimate indicators. Microfossil type, abundance and morphology may also be studied, and palaeotemperatures derived from their oxygen isotope analysis (section 3.3.4.1).