Carbon and nitrogen isotopes in grasslands of northern China

<p>Carbon (C) and nitrogen (N) isotopes are important traits to characterize terrestrial ecosystems. Studying the relationships between carbon and nitrogen isotopes of soils and plants in different grassland types and under different environmental conditions is of great importance to the reconstruction of past climate. In this study, we selected three different grassland ecosystems (temperate meadow steppe, temperate typical steppe and temperate desert steppe) in northern China, collected meteorological data and plant and soil samples, determined the basic physical and chemical properties, C and N isotopes to explore the patterns and controlling factors of C and N isotopes in plants and soils of grasslands in northern China. The results showed that there were significant differences in soil δ<sup>13</sup>C and δ<sup>15</sup>N between different grassland types in the northern temperate zone. The soil δ<sup>13</sup>C and δ<sup>15</sup>N of different depths of the northern temperate grassland soil increased with the increase of soil depth. The surface soil δ<sup>13</sup>C of temperate meadow steppe and temperate desert steppe had a good correlation with plant sample δ<sup>13</sup>C. The surface soil δ<sup>15</sup>N temperate typical steppe and temperate desert steppe had a good correlation with plant sample δ<sup>15</sup>N. Mean annual temperature (MAT) and mean annual precipitation (MAP) had a complicated relationship with carbon and nitrogen isotopes of surface soil and plant sample in northern temperate grassland. The surface soil δ<sup>13</sup>C and δ<sup>15</sup>N and the plant sample δ<sup>15</sup>N can be used as indicators of the change of MAT.</p><p><em>Keywords:</em> carbon isotopes, nitrogen isotopes, grasslands, climate, soil depth</p>

Why Tolbachik Diamonds Cannot be Natural

Taking into account recent publications, we provide additional comprehensive evidence that type Ib cuboctahedral diamonds and some other microcrystalline diamonds from Kamchatka volcanic rocks and alluvial placers cannot be natural and undoubtedly represent synthetic materials, which appear in the natural rocks by anthropogenic contamination. The major arguments provided in favor of the natural origin of those diamonds can be easily disproved. They include the coexistence of diamond and deltalumite from Koryaksky volcano; coexistence with super-reduced corundum and moissanite, Mn-Ni silicide inclusions, F-Cl enrichment and F/Cl ratios, and carbon and nitrogen isotopes in Tolbachik diamonds, as well as microtwinning, Mn-Ni silicides, and other inclusions in microcrystalline diamond aggregates from other Kamchatka placers. We emphasize the importance of careful comparison of unusual minerals found in nature, which include type Ib cuboctahedral diamonds and super-reduced phase assemblages resembling industrial slags, with synthetic analogs. The cavitation model proposed for the origin of Tolbachik diamonds is also unreliable since cavitation has only been shown to cause the formation of nanosized diamonds only.