Table2_Combining Nitrogen Isotopes and Redox Proxies Strengthens Paleoenvironmental Interpretations: Examples From Neoproterozoic Snowball Earth Sediments.csv

The history of the nitrogen cycle on Earth is linked to the redox evolution of the surface environment. Many nitrogen cycle fluxes are microbially mediated, and the particular fluxes operating at any given time in an ecosystem depend on the presence, absence or abundance of oxygen. However, interpreting this relationship is complicated as several isotopic fractionations associated with N-cycling are not diagnostic of a particular redox state. Thus, linking nitrogen isotopic analyses with redox-sensitive proxies is essential when interpretating past environments. Specifically, we use concentrations of U, V and Mo, along with Fe-speciation, to augment and contextualize nitrogen isotopic measurements. As an example, we consider samples from the Neoproterozoic Cryogenian period to suggest that there was oxygenated water, with associated aerobic N cycle fluxes. This interpretation is based on positive δ15N values between 4 to 80/00, Fe-speciation data consistent with anoxic bottom water during the Snowball ocean and oxygenated after, and redox-sensitive trace metals indicative of oxic weathering and surface water. Typically, high δ15N values are interpreted to reflect enhanced denitrification. We propose potential causes including a post-Snowball freshwater melt lid that suppressed deep water ventilation and that denitrification occurred more rapidly at high temperatures after the Snowball. These interpretations are buttressed by combined N isotope and redox analyses. This approach is especially useful during times of dynamic redox in the ocean-atmosphere system to interpret biologic isotopic signals.

地球氮循环的演化历史与地表环境的氧化还原演化密切相关。诸多氮循环通量由微生物介导，生态系统中任一时刻运行的特定氮循环通量，取决于氧气的存在、缺失或丰度水平。然而，由于与氮循环相关的多种同位素分馏（isotopic fractionations）无法指示特定的氧化还原状态，因此解读这一关联颇具复杂性。因此，在重建古环境时，将氮同位素分析与氧化还原敏感代用指标（redox-sensitive proxies）相结合至关重要。具体而言，我们借助铀（U）、钒（V）、钼（Mo）的浓度数据，结合铁物种形态分析（Fe-speciation），对氮同位素测量结果进行补充并限定其环境背景。作为示例，我们对新元古代成冰纪（Neoproterozoic Cryogenian period）的样品开展分析，结果表明当时存在含氧水体，并伴随有氧氮循环通量活动。该解释基于三项依据：其一，δ¹⁵N值介于4‰至80‰之间的正值；其二，铁物种形态分析（Fe-speciation）数据显示，“雪球海洋（Snowball ocean）”时期底层水体处于缺氧状态，后期转为含氧环境；其三，氧化还原敏感痕量金属数据指示了氧化风化作用与表层水体的氧化条件。通常来说，高δ¹⁵N值被认为反映了反硝化作用（denitrification）的增强。我们提出了两种潜在成因：一是雪球事件结束后形成的淡水熔盖层抑制了深水通风；二是雪球事件结束后，高温环境下反硝化作用速率显著加快。上述解释得到了氮同位素与氧化还原联合分析结果的支撑。该研究方法在解读海-气系统氧化还原状态动态变化时期的生物同位素信号时，具备尤为重要的应用价值。