Evaluating nitrogen isotopes as proxies for depositional redox conditions in shales: A case study from the Chang 7 Shale in the Ordos basin, North China.

<p>Bulk sedimentary nitrogen isotopes (δ<sup>15</sup>N) have been used as an accurate redox proxy in well-preserved sedimentary systems, however, fewer studies of N-isotope have been performed in lacustrine shales.  In this paper, we report the first δ<sup>15</sup>N data from the Chang 7 Shale from a core drilled in the Ordos Basin. Bulk δ<sup>15</sup>N values are significantly higher in Zone A (the Chang 7<sub>3</sub> and the lower part of the Chang 7<sub>2</sub> submembers, average = 9.4 ± 1.3‰) than in Zone B (the upper part of the Chang 7<sub>2</sub> and the Chang 7<sub>1</sub> submembers, average = 5.4 ± 1.5‰). Given the lithological characteristics and previous geochemical measurements, we suggest that sediments within Zone A of the Chang 7 Shale were mainly deposited under suboxic bottom water conditions, whereas Zone B sediments show evidence of deposition under oxic deep water regimes. Additionally, organic carbon isotopes (δ<sup>13</sup>C<sub>org</sub>) and total nitrogen (TN) values were measured to characterize any processes that might control alteration of the bulk δ<sup>15</sup>N signal, including changes in organic matter source and post-depositional processes. Our results show that there is no significant difference in the organic carbon isotopes (δ<sup>13</sup>C<sub>org</sub>) and total nitrogen (TN) values between the two zones. In conclusion, we suggest that the difference in δ<sup>15</sup>N values through the Chang 7 Shale primarily reflects differences in the depositional redox conditions and δ<sup>15</sup>N values of shale can provide important details regarding the depositional history of unconventional resource plays.</p>

Temperature exerts no influence on organic matter <i>δ</i>¹³C of surface soil along the 400 mm isopleth of mean annual precipitation in China

Soil organic carbon is the largest pool of carbon in the terrestrial ecosystem, and its isotopic composition is affected by a number of factors. However, the influence of environmental factors, especially temperature, on soil organic carbon isotope values (δ13CSOM) is poorly constrained. This impedes the application of the variability of organic carbon isotopes to reconstructions of paleoclimate, paleoecology, and global carbon cycling. Given the considerable temperature gradient along the 400 mm isohyet (isopleth of mean annual precipitation – MAP) in China, this isohyet provides ideal experimental sites for studying the influence of temperature on soil organic carbon isotopes. In this study, the effect of temperature on surface soil δ13C was assessed by a comprehensive investigation of 27 sites across a temperature gradient along the isohyet. Results demonstrate that temperature does not play a role in soil δ13C. This suggests that organic carbon isotopes in sediments cannot be used for paleotemperature reconstruction and that the effect of temperature on organic carbon isotopes can be neglected in the reconstruction of paleoclimate and paleovegetation. Multiple regressions with MAT (mean annual temperature), MAP, altitude, latitude, and longitude as independent variables and δ13CSOM as the dependent variable show that these five environmental factors together account for only 9 % of soil δ13C variance. However, one-way ANOVA analyses suggest that soil type and vegetation type are significant factors influencing soil δ13C. Multiple regressions, in which the five aforementioned environmental factors were taken as quantitative variables, and vegetation type, soil type based on the Chinese Soil Taxonomy, and World Reference Base (WRB) soil type were separately used as dummy variables, show that 36.2, 37.4, and 29.7 %, respectively, of the variability in soil δ13C are explained. Compared to the multiple regressions in which only quantitative environmental variables were introduced, the multiple regressions in which soil and vegetation were also introduced explain more of the isotopic variance, suggesting that soil type and vegetation type exert a significant influence on δ13CSOM.