Message "Damn fingers"

One of the main principles of geological research - the principle of actualism, formulated by James Hatton back in 1795, says that the processes on Earth today and in the deep past took place the same way. By and large, this is true. For example, mountains grow along the boundaries of tectonic plates, rivers run down them down, destroying their shores, the material from the continents thus enters the oceans. All this happens today, and happened in the past. It is easy to find other examples. The volcanoes erupted on land and under water before and continue to do it today. Nevertheless, it is no secret for modern geologists that the speeds of such processes did not necessarily remain constant during the entire geological history. Another proof of this was found by a group of researchers from the Gonauk Institute of the Polish Academy of Sciences [1], analyzing the isotopic composition of the strontium in the Belemnites, in the common people called “damn fingers” for the shape of their sinks (Fig. 1), the paleontological remains of mollusks living in the Mesozoic era. The essence of the study [1] is as follows: strontium enters the sink of mollusks from sea water along with calcium. Measuring the isotopic composition of the strontium in these mollusks, we can directly say what was the isotopic composition of the strontium in sea water during their life.

Strontius enters oceans of three main tanks: (1) from continents that are characterized by various, but generally high values ​​of 87SR/86SR, (2) from underwater volcanoes, mainly medium-oceanic ridges with low values ​​of 87SR/86SR, (3) from previous marine carbonses, which have an isotopic composition Strontia is also varying, but located between high continental and low volcanic values ​​(Fig. 2). From the data on the isotopic composition of the strontium in the water of modern oceans it is known that it is the same everywhere, regardless of the geographical location. This happens due to the very rapid mixing of sea water and homogenization, in particular, the isotopic composition of the strontium around the world. The principle of actualism in this case is quite applicable to the study of the Paleokeans.

Perennial studies allowed to restore the isotopic composition of the strontium for the Paleokeans, showing distinct maximums and minimums associated with a change in the intake of the strontium from the continents during the maximum glaciation and the oceanic bottom due to increased volcanic activity (Fig. 3). Despite the fact that the reliability of the general variations of this curve is high, the accuracy and detail of the curve is quite different for different time intervals. In particular, the moment of decreasing the values ​​of 87SR/86SR in the middle of the Mesozoic era (in the middle yura) remained poorly studied. The work under consideration was devoted to the Beemnites selected from the Middle Juri deposits of Poland and Germany, where such deposits are well represented.

As a result, variations of isotopes of strontium in sea water of this time were established with very high accuracy. A sharp drop in 87SR/86SR below 0.7069 is apparently associated with an abnormally high ore of the ocean bottom, which caused an abnormally high volcanic activity. At least this is told to us "dramatic fingers."

Alexey Ivanov

1. Wierzbowski H., Anczkiewicz R., Ba-Zarnik J., Pawlak J. Strontium Isotope Varisic (Late Ba-Jocian-Callovian) Seawater: Implications for Earth's Tectonic Activity and Marine Environments. Chemical Geologi, in Press http://dx.doi.org/10.1016/j.chem-geo 2012.10.019
2. Depaolo DJ Correlating Rocks with Strontium Isotopes. GEOTIMES, 1987, Dec., p. 16-18.
3. Armstrong RL Glacial Erosion and the Variable Isotopic Composition of Strontium in Sea Water. Nature Physical Sciences, 1971, v. 230, p. 132-133.