Đang chuẩn bị nút TẢI XUỐNG, xin hãy chờ
Tải xuống
CHAPTER FIVE Climate Change at the Neoproterozoic-Cambrian Transition Varangerian and lower Sinian glacial deposits are found in Argentina, Uruguay, Mato Grosso (Brazil), Namibia, Laurentia, and probably southern Brazil, which were all situated close together during Neoproterozoic-Cambrian times. | 05-C1099 8 10 00 2 05 PM Page 90 CHAPTER FIVE Toni T. Eerola Climate Change at the Neoproterozoic-Cambrian Transition Varangerian and lower Sinian glacial deposits are found in Argentina Uruguay Mato Grosso Brazil Namibia Laurentia and probably southern Brazil which were all situated close together during Neoproterozoic-Cambrian times. According to continental paleoreconstructions glacial deposits of these regions together with those of Scotland Scandinavia Greenland Russia Antarctica and Australia formed the Varangerian-Sinian Glacial Zone of the supercontinent Rodinia. Tectonic activity associated with the amalgamation of Rodinia and Gondwana was probably related to the origin of these deposits as in the case of mountain glaciers that formed in uplifted areas of fragmenting or colliding parts of this supercontinent. In such circumstances the Pan-African and Brasiliano orogenies and the site of opening of the Iapetus Ocean would have been in key positions. However some paleomagnetic reconstructions locate these regions near the South Pole where glaciers could have formed even in the absence of tectonic events. In this case the change to warm climate and the evolutionary explosion of the Cambrian could have been due to rapid shift of continents to equatorial latitudes although these changes might also have been triggered by supercontinent breakup. These events are reflected in the isotopic records of strontium and carbon which provide some of the best available indicators of the climatic and environmental changes that occurred during the Neoproterozoic-Cambrian transition. They also appear to reveal the occurrence of a discrete cold period in the Cambrian the disputed lower Sinian glaciation. INTRODUCTION The Neoproterozoic-Cambrian transition was characterized by ophiolite formation Yakubchuk et al. 1994 the formation and breakup of supercontinents e.g. Bond et al. 1984 the Cambrian evolutionary explosion Moores 1993 Knoll 1994 and intense climatic changes among .
