Diagenetic evaluation of a Pennsylvanian carbonate succession (Bird Spring Formation, Arrow Canyon, Nevada, U.S.A.) 鈥?1: Brachiopod and whole rock comparison

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• 作者：Uwe Brand^a ; ^{ubrand@brocku.ca} ; Ganqing Jiang^b ; Karem Azmy^c ; James Bishop^d ; Isabel P. Montañ ; ez^e
• 关键词：Pennsylvanian ; Bird Spring Formation ; Diagenetic evaluation ; Brachiopods whole rock ; Trace chemistry ; Carbon oxygen and strontium isotopes
• 刊名：Chemical Geology
• 出版年：2012
• 期刊代码：12_00092541
• 类别：ear
• 出版时间：20 May, 2012
• 卷：308-309
• 期：Complete
• 页码：26-39
• 文件大小：3031 K

摘要

Diagenetic evaluation of the Pennsylvanian Bird Spring Formation, Arrow Canyon, Nevada (U.S.A.) involved the detailed petrographic and geochemical examination of its brachiopod and whole rock components. 未¹³C, 未¹⁸O and ⁸⁷Sr/⁸⁶Sr of whole rock (WR) statistically (p = 0.0001) do not agree with isotope values of coeval brachiopod low-Mg calcite (LMC). More importantly, a detailed investigation of the best preserved, fine-grained whole rock material demonstrates that it does not universally preserve its carbon isotope values, in contrast with hypothetical predictions. Furthermore, lumped trace chemistry or stable isotope tests do not unequivocally identify brachiopod or whole rock (fine- to coarse-grained) samples that carry an 鈥榦riginal鈥?seawater signal. Instead, diagenetic evaluation, involving as many screening tests as possible, on a horizon-by-horizon level to compensate for spatial and temporal variations, is necessary to identify the sample(s) that carries an original seawater signature. Many brachiopods from the Bird Spring Formation are blue- to non-luminescent and carry microstructures and trace element contents similar to those of their modern tropical counterparts. Concurrence of chemical results between well preserved brachiopods and whole rock (if originally low-Mg calcite) is achieved in 22%of coeval samples for 鈭?sup>13C_bLMC-WR (with natural limits of 卤 0.25鈥?, in 20%of samples for 鈭?sup>18O_bLMC-WR (with natural limits of 卤 0.25鈥?, and in 9%of samples for 鈭?sup>87Sr_bLMC-WR (with global limits of 卤 0.000025). After an adjustment for mineralogical fractionation, if whole rock is considered originally aragonite, the concurrence of 鈭?sup>13C_bLMC-WR drops to 0%, and for 鈭?sup>18O_bLMC-WR it drops to 6%. Furthermore, comparison of stable isotopes of our well-preserved brachiopods with those of preserved whole rock from the literature, suggests that they are significantly different (p = 0.0001), with results of the latter dataset being more negative by about 鈭?#xA0;0.77 (鈭?#xA0;2.57鈥?with mineralogical adjustment) for 未¹³C (PDB) and 鈭?#xA0;1.31 (鈭?#xA0;2.11鈥? for 未¹⁸O (PDB). Thus, our integrated screening and comparative tests indicate that: (1) an overwhelming number of the whole rock samples from the Bird Spring Formation carry a diagenetic instead of a primary signal for both carbon and oxygen isotopes, and (2) many of the biogenic low-Mg calcite brachiopods in this succession carry an original chemical signal suitable for the reconstruction of high-resolution seawater isotope curves representative of the Pennsylvanian North American epeiric sea, and potentially of the global oceans.