Increased continental weathering flux during orbital-scale sea-level highstands: Evidence from Nd and O isotope trends in Middle Pennsylvanian cyclic carbonates

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• 作者：Bethany P. Theiling^a ; ^b ; ^{btheilin@purdue.edu} ; Maya Elrick^a ; Yemane Asmerom^a
• 关键词：Cycles ; Orbital-scale ; Neodymium ; Oxygen ; Sea-level ; Glacio-eustasy ; Continental weathering ; 未18O ; 蔚Nd ; Glacial&ndash ; interglacial
• 刊名：Palaeogeography ; Palaeoclimatology ; Palaeoecology
• 出版年：2012
• 期刊代码：68_00310182
• 类别：ear
• 出版时间：15 July, 2012
• 卷：342-343
• 期：Complete
• 页码：17-26
• 文件大小：535 K

摘要

Despite the common occurrence of orbital-scale (10⁴-10⁵ yr) sedimentary cycles in a wide range of Proterozoic through Neogene marine and non-marine depositional systems, understanding the effects and phase relationships of orbital-scale climate drivers on time-equivalent marine and non-marine deposits is difficult mainly due to correlation limitations between the geographically isolated deposition settings. Results from this study assess the relationships between orbital-scale continental weathering flux and glacial-interglacial marine cycles using Nd isotopes (from whole rock limestones) and 未¹⁸O values (from conodont apatite) from Middle Pennsylvanian cyclic marine carbonates in the U.S. Southwest.

Conodont 未¹⁸O trends from 2 of 4 sampled glacial-interglacial carbonate cycles support previous interpretations that observed water-depth changes were controlled by glacio-eustasy (30-50 m magnitudes) combined with < 1掳 seawater temperature changes. Two additional sampled cycles show initially increasing, then decreasing 未¹⁸O trends. Based on these results, we suggest that 未¹⁸O better defines a eustatic sea-level curve, rather than a facies-derived curve.

蔚_Nd trends in 5 of 8 sampled cycles are higher during regressive intervals (early glacial phase) and lower during sea-level highstands (interglacial phase), supporting the hypothesis that increases in precipitation and/or air temperatures during interglacial intervals result in increased continental weathering rates and/or increased flux of weathered solutes to the Middle Pennsylvanian marine basin. This hypothesis is in contrast to traditional sequence stratigraphic interpretations (increased siliciclastic shedding into marine basins during falling sea level/lowstands) and suggests that climatically-controlled precipitation and/or air temperature fluctuations influenced continental weathering flux more than sea level-controlled shoreline or baseline position in this paleotropical location. These results highlight the use of combined 蔚_Nd and 未¹⁸O analyses as a tool for evaluating the response of marine and coeval non-marine systems to orbital-scale climate changes, particularly in deep-time depositional systems.