Selenium isotope evidence for progressive oxidation of the Neoproterozoic biosphere

Neoproterozoic (1,000–542 Myr ago) Earth experienced profound environmental change, including ‘snowball’ glaciations, oxygenation and the appearance of animals. However, an integrated understanding of these events remains elusive, partly because proxies that track subtle oceanic or atmospheric redox trends are lacking. Here we utilize selenium (Se) isotopes as a tracer of Earth redox conditions. We find temporal trends towards lower δ82/76Se values in shales before and after all Neoproterozoic glaciations, which we interpret as incomplete reduction of Se oxyanions. Trends suggest that deep-ocean Se oxyanion concentrations increased because of progressive atmospheric and deep-ocean oxidation. Immediately after the Marinoan glaciation, higher δ82/76Se values superpose the general decline. This may indicate less oxic conditions with lower availability of oxyanions or increased bioproductivity along continental margins that captured heavy seawater δ82/76Se into buried organics. Overall, increased ocean oxidation and atmospheric O2 extended over at least 100 million years, setting the stage for early animal evolution

Reconstructing terrestrial nutrient cycling using stable nitrogen isotopes in wood

Although recent anthropogenic effects on the global nitrogen (N) cycle have been significant, the consequences of increased anthropogenic N on terrestrial ecosystems are unclear. Studies of the impact of increased reactive N on forest ecosystems—impacts on hydrologic and gaseous loss pathways, retention capacity, and even net primary productivity— have been particularly limited by a lack of long-term baseline biogeochemical data. Stable nitrogen isotope analysis (ratio of ¹⁵N to ¹⁴N, termed δ¹⁵N) of wood chronologies offers the potential to address changes in ecosystem N cycling on millennial timescales and across broad geographic regions. Currently, nearly 50 studies have been published utilizing wood δ¹⁵N records; however, there are significant differences in study design and data interpretation. Here, we identify four categories of wood δ¹⁵N studies, summarize the common themes and primary findings of each category, identify gaps in the spatial and temporal scope of current wood δ¹⁵N chronologies, and synthesize methodological frameworks for future research by presenting eight suggestions for common methodological approaches and enhanced integration across studies. Wood δ¹⁵N records have the potential to provide valuable information for interpreting modern biogeochemical cycling. This review serves to advance the utility of this technique for long-term biogeochemical reconstructions

Shoaling behaviour in a surface-dwelling and a cave-dwelling population of a barb Garra barreimiae (Cyprinidae, Teleostei).

Abstract We studied shoaling behaviour in a species of fish (Garra barreimiae) from Oman. We compared two populations (a surface-dwelling and a cave-dwelling population) with different theoretical costs and benefits of shoaling. We measured the tendency to associate with a shoal of conspecifics. The stimulus shoal was confined to (1) clear Plexiglas cylinders in light, (2) wire-mesh cylinders in light, or (3) wire-mesh cylinders in darkness. The surface form exhibited a strong preference for the stimulus shoal during the experiments in light, but also in darkness, when only non-visual cues from the shoal could be perceived. The cave form did not show a preference when solely visual cues were available (Plexiglas cylinder). When non-visual cues from the shoal could be perceived (wire-mesh), the cave form did show a preference to associate with the shoal, but the shoaling tendency was considerably weaker than in the surface form. The shoaling tendency has probably been genetically reduced in the cave form