Did the Boreal Realm extend into the equatorial region? New paleomagnetic evidence from the Tuva–Mongol and Amuria blocks

Abstract

Fossil organisms of the Boreal Realm in the Tuva–Mongol Block (TMB) and Amuria Block (AMB) are commonly thought to have lived at mid to high latitudes during the Carboniferous Period; however, reliable paleomagnetic constraints have not been available previously to test this hypothesis. Here we report newly obtained combined paleomagnetic and geochronological results from the Upper Mississippian Gunbayan Formation in the AMB and the Lower Pennsylvanian Altan-Ovoo Formation in the TMB. Zircon U–Pb dating of tuff beds from the Gunbayan and Altan-Ovoo formations yield ages of 331.0 ± 2.7 and 315.5 ± 2.4 Ma, respectively. A total of 263 paleomagnetic specimens underwent stepwise thermal demagnetization. After removing the viscous remanent magnetizations of the recent geomagnetic field, stable high-temperature components (HTCs), carried by magnetite, were successfully acquired from 240 specimens. The HTCs of the Gunbayan Formation passed a fold test and a reversal test, and those of the Altan-Ovoo Formation passed a fold test, suggesting that they represent primary remanence magnetizations. We used the elongation/inclination method to test and correct for all the HTC directions of the clastic rocks of the studied formations. Their corresponding paleomagnetic poles are 46.0°N/320.0°E (A=952.0°) at ca. 330 Ma for the AMB and 43.5°N/355.9°E (A=952.9°) at ca. 315 Ma for the TMB. The updated paleomagnetic database indicates with certainty that both the TMB and the AMB were located in northern hemisphere low-latitude regions and close to the equator at ca. 330 Ma, suggesting that the Boreal Realm might have extended into the equatorial region at the onset of the large ice age of the Carboniferous. The paleobiogeographic characteristics and distribution of climate-sensitive lithologies indicate that this realm occupied a wide northern temperate belt during 330–315 Ma.