Stable isotopes and predation marks shed new light on ammonoid habitat depth preferences

Ammonoids are extinct cephalopods with external shells which predominated in many late Paleozoic and Mesozoic marine ecosystems. Stable isotope data from ammonoid shells constitute primary tools for understanding their palaeohabitats. However, in most sedimentary successions globally the aragonitic shells of ammonoids are dissolved during fossilisation process and therefore not available for geochemical studies. We overcome this taphonomic bias by analysing the better preservable calcitic elements of the ammonoid jaws (aptychi). We study moulds and aptychi of two successive members, temporal subspecies in our interpretation, of a scaphitid evolutionary lineage from a Late Cretaceous chalk succession in Poland. In order to reconstruct their habitat depth preferences, we apply the powerful combination of stable isotope data from aptychi and co-occurring benthic and planktic foraminifera with an analysis of predation marks preserved on scaphitid specimens. On this basis we conclude that the populations of the older subspecies led a nektic, and those of the younger subspecies, a nektobenthic lifestyle. The shift in habitat depth preferences took place probably as a response of local populations to the shallowing of the sea. Previous studies largely assumed stable depth preferences for ammonoid species, genera and even higher clades. Our study casts doubts over such generalizations by pointing out that ammonoids could have been more flexible in their depth-related behaviour than anticipated.

Soil water sources in permafrost active layer of Three-River Headwater Region, China

Water in permafrost soil is an important factor affecting the ecology of cold environments, climate change, hydrological cycle, engineering, and construction. To explore the variations in soil water in the active layer due to permafrost degradation, the soil water sources in the Three-River Headwater Region were quantified based on the stable isotope data (δ2H and δ18O) of 1140 samples. The results showed that the evaporation equation was δ2H = 7.46 δ18O - 0.37 for entire soil water. The stable isotope data exhibited a spatial pattern, which varied over the soil profile under the influence of altitude, soil moisture, soil temperature, vegetation, precipitation infiltration, soil water movement, ground ice, and evaporation. Based on the stable isotope tracer model, precipitation and ground ice accounted for approximately 88 % and 12 % of soil water, respectively. High precipitation contributed to the soil water in the 3900–4100 m, 4300–4500 m, and 4700–4900 m zones, whereas ground ice contributed to the soil water in the 4500–4700 m and 4900–5100 m zones. Precipitation contributed approximately 84 % and 80 % to the soil water in grasslands and meadows, respectively, whereas ground ice contributed approximately 16 % and 20 %, respectively. Precipitation; evapotranspiration; physical and chemical properties of soil; and the distribution of ground ice, vegetation, and permafrost degradation were the major factors affecting the soil water sources in the active layer. Therefore, establishing an observation network and developing technologies for ecosystem restoration and conservation is critical to effectively mitigate ecological problems caused by future permafrost degradation in the study region.

Ore–Bearing Fluids of the Blagodatnoye Gold Deposit (Yenisei Ridge, Russia): Results of Fluid Inclusion and Isotopic Analyses

The Blagodatnoye deposit with 340 t gold reserves is one of the most productive mines in Russia. Modern methods of studying fluid inclusions were used to determine the properties of fluids that formed this deposit. A comprehensive study revealed that the Blagodatnoye gold deposit was formed between 120 and 350 °C and at 0.2–2.6 kbar, and from fluids with salinities ranging from 0.5 to 30 wt.% (NaCl–eq.). These fluids are: 1—water–carbon dioxide; 2—carbon dioxide–hydrocarbon; 3—highly saline aqueous. According to Raman spectroscopy and gas chromatography–mass spectrometry, ore–forming fluids contained H2O, CO2, hydrocarbons and oxygenated organic compounds, sulfonated, nitrogenated and halogenated compounds. Early oxidized water–carbon dioxide fluids formed barren associations of the deposit. Later reduced carbon dioxide–hydrocarbon fluids had a key role in the formation of gold-bearing quartz veins. The stable isotope data (δ34S = 0.8 to 21.3‰, δ13С = –2.8 to –20.9‰, 3He/4He = 0.14 ± 0.3*10–6) suggest the ore-forming fluids have a crustal source.

Diagnosing underdetermination in stable isotope mixing models

Stable isotope mixing models (SIMMs) provide a powerful methodology for quantifying relative contributions of several sources to a mixture. They are widely used in the fields of ecology, geology, and archaeology. Although SIMMs have been rapidly evolved in the Bayesian framework, the underdetermination of mixing space remains problematic, i.e., the estimated relative contributions are incompletely identifiable. Here we propose a statistical method to quantitatively diagnose underdetermination in Bayesian SIMMs, and demonstrate the applications of our method (named β-dependent SIMM) using two motivated examples. Using a simulation example, we showed that the proposed method can rigorously quantify the expected underdetermination (i.e., intervals of β-dependent posterior) of relative contributions. Moreover, the application to the published field data highlighted two problematic aspects of the underdetermination: 1) ordinary SIMMs was difficult to quantify underdetermination of each source, and 2) the marginal posterior median was not necessarily consistent with the joint posterior peak in the case of underdetermination. Our study theoretically and numerically confirmed that β-dependent SIMMs provide a useful diagnostic tool for the underdetermined mixing problem. In addition to ordinary SIMMs, we recommend reporting the results of β-dependent SIMMs to obtain a biologically feasible and sound interpretation from stable isotope data.

Variability in Neogloboquadrina pachyderma stable isotope ratios from isothermal conditions: implications for individual foraminifera analysis

Individual foraminifera analysis (IFA) holds promise to reconstruct seasonal to interannual oceanographic variability. Even though planktonic foraminifera are reliable recorders of environmental conditions on a population level, whether they also are on the level of individuals is unknown. Yet, one of the main assumptions underlying IFA is that each specimen records ocean conditions with negligible noise. Here we test this assumption using stable isotope data measured on groups of four shells of Neogloboquadrina pachyderma from a 16–19 days resolution sediment trap time series from the subpolar North Atlantic. We find a within-sample variability of 0.11 and 0.10 ‰ for δ18O and δ13C respectively that show no seasonal pattern and exceed water column variability in spring when conditions are homogeneous down to 100s of metres. We assess the possible effect of life cycle characteristics and delay due to settling on foraminifera δ18O variability with simulations using temperature and δ18Oseawater as input. These simulations indicate that the observed δ18O variability can partially be explained by environmental variability. Individual N. pachyderma are thus imperfect recorders of temperature and δ18Oseawater. We estimate the excess noise on δ18O to be 0.11 ± 0.06 ‰. The origin and nature of the recording imprecision require further work, but our analyses highlight the need to take such excess noise into account when interpreting the geochemical variability among individual foraminifera.