Analytical improvements and assessment of long-term performance of the oxidation-denitrifier method.

The analysis of the nitrogen (N) isotopic composition of organic matter bound to fossil biomineral structures (BB-δ15 N) using the oxidation-denitrifier (O-D) method provides a novel tool to study past changes in N cycling processes. METHODS: We report a set of methodological improvements to the O-D method, including (a) a method for sealing the reaction vials in which the oxidation of organic N to NO3 - takes place, (b) a recipe for bypassing the pH adjustment step before the bacterial conversion of NO3 - to N2 O, and (c) a method for storing recrystallized dipotassium peroxodisulfate (K2 S2 O8 ) under Ar atmosphere. RESULTS: The new sealing method eliminates the occasional contamination and vial breakage that occurred previously while increasing sample throughput. The protocol for bypassing pH adjustment does not affect BB-δ15 N, and it significantly reduces the processing time. Storage of K2 S2 O8 reagent under Ar atmosphere produces stable oxidation blanks over more than 3.5 years. We report analytical blanks, accuracy, and precision for this methodology from eight users over the course of ~3.5 years of analyses at the Max Planck Institute for Chemistry. Our method produces analytical blanks characterized by low N content (0.30 ± 0.13 nmol N, 1σ, n = 195) and stable δ15 N (-2.20 ± 3.13‰, n = 195). The analysis of reference amino acid standards USGS 40 and USGS 65 indicates an overall accuracy of -0.23 ± 0.35‰ (1σ, n = 891). The analysis of in-house fossil standards gives similar analytical precision (1σ) across a range of BB-δ15 N values and biominerals: zooxanthellate coral standard PO-1 (6.08 ± 0.21‰, n = 267), azooxanthellate coral standard LO-1 (10.20 ± 0.28‰, n = 258), foraminifera standard MF-1 (5.92 ± 0.28‰, n = 243), and tooth enamel AG-Lox (4.06 ± 0.49‰, n = 78). CONCLUSIONS: The methodological improvements significantly increase sample throughput without compromising analytical precision or accuracy down to 1 nmol of N.

Enameloid-bound δ15 N reveals large trophic separation among Late Cretaceous sharks in the northern Gulf of Mexico.

The nitrogen isotopic composition (15 N/14 N ratio, or δ15 N) of enameloid-bound organic matter (δ15 NEB ) in shark teeth was recently developed to investigate the biogeochemistry and trophic structures (i.e., food webs) of the ancient ocean. Using δ15 NEB , we present the first nitrogen isotopic evidence for trophic differences between shark taxa from a single fossil locality. We analyze the teeth of four taxa (Meristodonoides, Ptychodus, Scapanorhynchus, and Squalicorax) from the Late Cretaceous (83-84 Ma) Trussells Creek site in Alabama, USA, and compare the N isotopic findings with predictions from tooth morphology, the traditional method for inferring shark paleo-diets. Our δ15 NEB data indicate two distinct trophic groups, with averages separated by 6.1 ± 2.1‰. The lower group consists of Meristodonoides and Ptychodus, and the higher group consists of Scapanorhynchus and Squalicorax (i.e., lamniforms). This δ15 NEB difference indicates a 1.5 ± 0.5 trophic-level separation between the two groups, a finding that is in line with paleontological predictions of a higher trophic level for these lamniforms over Meristodonoides and Ptychodus. However, the δ15 NEB of Meristodonoides is lower than suggested by tooth morphology, although consistent with mechanical tests suggesting that higher trophic-level bony fishes were not a major component of their diet. Further, δ15 NEB indicates that the two sampled lamniform taxa fed at similar trophic levels despite their different inferred tooth functions. These two findings suggest that tooth morphology alone may not always be a sufficient indicator of dietary niche. The large trophic separation revealed by the δ15 NEB offset leaves open the possibility that higher trophic-level lamniforms, such as those measured here, preyed upon smaller, lower trophic-level sharks like Meristodonoides.

Dietary and homeostatic controls of Zn isotopes in rats: a controlled feeding experiment and modeling approach.

The stable isotope composition of zinc (δ66Zn), which is an essential trace metal for many biological processes in vertebrates, is increasingly used in ecological, archeological, and paleontological studies to assess diet and trophic level discrimination among vertebrates. However, the limited understanding of dietary controls and isotopic fractionation processes on Zn isotope variability in animal tissues and biofluids limits precise dietary reconstructions. The current study systematically investigates the dietary effects on Zn isotope composition in consumers using a combined controlled feeding experiment and box-modeling approach. For this purpose, 21 rats were fed one of seven distinct animal- and plant-based diets and a total of 148 samples including soft and hard tissue, biofluid, and excreta samples of these individuals were measured for δ66Zn. Relatively constant Zn isotope fractionation is observed across the different dietary groups for each tissue type, implying that diet is the main factor controlling consumer tissue δ66Zn values, independent of diet composition. Furthermore, a systematic δ66Zn diet-enamel fractionation is reported for the first time, enabling diet reconstruction based on δ66Zn values from tooth enamel. In addition, we investigated the dynamics of Zn isotope variability in the body using a box-modeling approach, providing a model of Zn isotope homeostasis and inferring residence times, while also further supporting the hypothesis that δ66Zn values of vertebrate tissues are primarily determined by that of the diet. Altogether this provides a solid foundation for refined (paleo)dietary reconstruction using Zn isotopes of vertebrate tissues.

Severe cold-water bleaching of a deep-water reef underscores future challenges for Mesophotic Coral Ecosystems.

Elevated sea surface temperatures are causing an increase in coral bleaching events worldwide, and represent an existential threat to coral reefs. Early studies of Mesophotic Coral Ecosystems (MCEs) highlighted their potential as thermal refuges for shallow-water coral species in the face of predicted 21st century warming. However, recent genetic evidence implies that limited ecological connectivity between shallow- and deep-water coral communities inhibits their effectiveness as refugia; instead MCEs host distinct endemic communities that are ecologically significant in and of themselves. In either scenario, understanding the response of MCEs to climate change is critical given their ecological significance and widespread global distribution. Such an understanding has so far eluded the community, however, because of the challenges associated with long-term field monitoring, the stochastic nature of climatic events that drive bleaching, and the paucity of deep-water observations. Here we document the first observed cold-water bleaching of a mesophotic coral reef at Clipperton Atoll, a remote Eastern Tropical Pacific (ETP) atoll with high coral cover and a well-developed MCE. The severe bleaching (>70 % partially or fully bleached coral cover at 32 m depth) was driven by an anomalously shallow thermocline, and highlights a significant and previously-unreported challenge for MCEs. Prompted by these observations, we compiled published cold-water bleaching events for the ETP, and demonstrate that the timing of past cold-water bleaching events in the ETP coincides with decadal oscillations in mean zonal wind strength and thermocline depth. The latter observation suggests any future intensification of easterly winds in the Pacific could be a significant concern for its MCEs. Our observations, in combination with recent reports of warm-water bleaching of Red Sea and Indian Ocean MCEs, highlight that 21st century MCEs in the Eastern Pacific face a two-pronged challenge: warm-water bleaching from above, and cold-water bleaching from below.