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GEOLOGY | PALAEOCLIMATES

Image by Nicholas Ceglia

How do we know about the different climates that Have existed in the past? 

Scientists collect climate data from remote sensing platforms in near space (satellites) and automated/manual terrestrial instruments 24 hours a day, 365 days a year. The longevity of the instrumental record enables us to analyse and model weather and climates, but without a greater.

 

However, the instrument record of climate is relatively short-lived; for example the Radcliffe Observatory in Oxford has records dating from 1767, with continuous rainfall and temperature readings from 1813.  The UK Meterological Office (Met Office) records date from 1910. The satellite record is a mere baby in comparison, with little over 20 years of data on record. The data collected via satellite and terrestrial instrumentation  too short to accurately describe climate with any certainty - in fact to we need to data collection over hundreds to thousands of years for certainty.

To extend the 'instrumental records', palaeoclimatologists look for clues in Earth’s wealth of natural environmental records. If you know how to read it, the record of climate is stored in the fabric of the environment around us. Clues are written in sediments at the bottom of the oceans, locked away in coral reefs, frozen in the ice of glaciers and ice caps, and preserved in the growth rings of trees. The physical world provides geoscientists with sources of temperature and precipitation data stretching back over millennia. The records drawn from a range of natural records can be scientifically combined to retconstruct past global climates.

Image: This U shaped valley was carved out by a glacier when a much colder climate prevailed over this landscape.

rocks

The characteristics of sedimentary rocks depend on the environment in which the sediments that form them were deposited. 

  • Some sands and gravels are deposited by retreating glaciers and they become a distinctive sediment known as till and boulder clay.   Where till is found, we know that there must once have been glaciers and therefore a cold climate.

  • Rocks that form in a hot desert environment are often coloured red by iron deposits. Geological evidence of a once hot, arid climate.

  • In regions with high temperatures, water can evaporate quickly leaving behind a layer of salt on the ground that becomes preserved in the rocks and is another indicator of a hot climate.

 

 

Image:  This image was taken at an OGG field meeting at Westbury Garden Cliff, Gloucestershire. These Mercia Mudstone Group rocks are a mixture of aeolian dust, lake and sheetflood sediments deposited in an arid or semi-arid environment (evaporite minerals gypsum and halite confirm this environment). Hardly the temperate environment of western England of today.

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FOSSILS

Different species of plants and animals need specific conditions to survive. Some can be very sensitive and do not adapt easily to change, eg. large coral reefs develop in shallow tropical waters: within a particular temperature range, water chemistry, specific depths and receiving just the right amount of light. Changes to the marine environment, even by a relatively small amount, means they cannot thrive and in some cases not survive.

So, if you come across a site where there are fossil corals in the rock you can make some informed assumptions about the environment where the rock was formed.   So, if You're at Rock Edge Quarry or Workhouse Pit in the Oxford suburb of Headington,  and you're looking in wonder at the Thecasmilea, Isastrea and Thamnastrea coral fossils preserved beautifully in the Upper Jurassic limestones there, you can make a scientifically-informed assumption about the environment in which the limestones were deposited.

 

IMAGE: An OGG visit to Rock Edge Quarry, Headington, Oxford to look at the patch reefs preserved in the rock record.

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LANDSCAPE

Today’s landscape gives clues to the climate of the past. For example, glaciers leave tell-tale signs of their activity. As glaciers move slowly down river valleys, they can carve out deep, U-shaped valleys such as the Norwegian fjords. Glaciers also pick up pieces of rock and gravel and, as they move forwards, this debris scratches grooves called ‘striations’ into the rocks on the valley floor.

Although in the glaciers and ice sheets of the past stadia didn't reach down across Oxfordshire, the ice limit did come close to the northern edge of the county.  As such the land here was subjected to freezing temperatures and permafrost would have covered the landscape, rendering the permeable rocks underlying the surface impermeable. Meltwaters flowing across the landscape would have carved out large valleys into the frozen rocks.  Following climate change and the melting of the ice, the rocks became permeable once again and the valleys that evolved in the permafrost became abandoned by flowing water and dry.

 

 

 

 

Image: The Manger, a dry valley at the foot of White Horse Hill, Uffington, Oxfordshire.

CLIMATE PROXIES

Learn more of climate proxies and how geoscientists use them by reading         Noam Vogt-Vincent's Climate Proxies page.

 

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