A multi-peak organic carbon isotope excursion in the late Aeronian (Llandovery, Silurian): Evidence from Arisaig, Nova Scotia, Canada

摘要

The late Aeronian is now recognized as one of several time intervals of the Silurian Period that is characterized by a positive carbon isotope excursion believed to be of global significance. There is also some recent evidence that this interval may include several distinct delta C-13 peaks within a time interval of less than 300 kyr. The late Aeronian to earliest Telychian succession in the Ross Brook Formation near Arisaig, Nova Scotia, Canada, preserves a thick succession of marine, clastic strata that spans the interval of this excursion, and it shows evidence of three or possibly four positive organic carbon isotope (delta C-13(org)) excursions ranging in magnitude from at least +0.7% to approximately +3.5% above local baseline values. These excursions occur within an interval beginning near the base of the Stimulograptus sedgwickii Zone and ending near the base of the Telychian, as defined by the last occurrence of the brachiopod Eocelia intermedia. Comparison with a recently published delta C-13(org) record from Bohemia suggests that the three or four peaks may be correlated between these two regions indicating that these multiple excursions may represent a regional or global environmental signal of cyclical palaeoenvironmental change. In the Arisaig region, these delta C-13(org) peaks all occur within an overall shallowing-upward cycle but otherwise our study provides no new insights into the nature of the specific palaeoenvironmental processes that may have driven these short-term fluctuations in delta C-13(org) values. If the level of last occurrence of Eocelia intermedia can be reliably regarded as indicating correlation with the GSSP for the Base of Telychian, and if our proposed correlation between the Arisaig and Bohemian successions is correct, then the biostratigraphic level of the GSSP correlates with a level near the base of the Bohemian Lituigraptus rastrum Zone, which correlates closely with the base of the Stimulograptus halli Zone in eastern Avalonia and Baltica. (C) 2014 Elsevier B.V. and Nanjing Institute of Geology and Palaeontology, CAS. All rights reserved.