Climates through Late Triassic EarlyJurassic

the Mesozoic The Late Triassic and Early Jurassic were times of heat and aridity. They also were times of marked seasonality; that is, well-defined seasons. Paleontologists studying Late Triassic paleoclimates have recognized three broad climate regimes: an equatorial, year-round dry belt; narrow belts north and south of the dry belt of strongly seasonal rainfall; and middle and upper latitude humid belts. These are based upon the distribution of Upper Triassic deposits of sedimentary rocks indicative of aridity. Such rocks include sand dune deposits (the remains of ancient deserts) and evaporites, rocks that are made up of minerals formed during desiccation. Triassic evaporites and sand dune deposits do not today form continuous belts around the globe;

the continents had to be rotated back into the positions they had during the Late Paleozoic and Early Mesozoic in order to obtain the banding pattern.

Other climatic indicators that are keys to Late Triassic-Early Jurassic paleoclimates have been found. First, continental rocks from this time interval are very commonly red beds, rocks of an orange/red color due to an abundance of iron oxide. Such rocks today generally form in climates that are relatively warm. Secondly, a variety of fossilized soils have been found with caliche (calcium carbonate nodules) in them. Such nodules commonly form today in soils that are located in arid climates. Finally, evidence of warm, dry climatic conditions has been obtained from stable isotopes, isotopes that do not spontaneously decay. It has been shown that the amount of a stable isotope of oxygen, 180, varies with temperature and salinity. Therefore, by measuring how much 180 is present, one can obtain a direct measure of ancient temperatures and salinities. Obviously, isotopes are an extremely important tool for learning about ancient climates as well as a variety of other subjects, including (as we shall see) warm-bloodedness in dinosaurs (see Box 2.1 and Chapter 15). The Late Triassic-Early Jurassic interval, then, seems to have been a time that was generally warmer and perhaps drier than the present, with strong seasonality.

0 0

Post a comment