The role of gastropod shell composition and microstructure in resisting dissolution caused by ocean acidification.

Organisms, such as molluscs, that produce their hard parts from calcium carbonate are expected to show increased difficulties growing and maintaining their skeletons under ocean acidification (OA). Any loss of shell integrity increases vulnerability, as shells provide protection against predation, desiccation, and disease. Not all species show the same responses to OA, which may be due to the composition and microstructural arrangement of their shells. We explore the role of shell composition and microstructure in resisting dissolution caused by decreases in seawater pH using a combination of microCT scans, XRD analysis, and SEM imaging. Two gastropods with different shell compositions and microstructure, Tegula funebralis and Nucella ostrina, were exposed to simulated ocean acidification conditions for six months. Both species showed signs of dissolution on the exterior of their shells, but changes in density were significantly more pronounced in T. funebralis. XRD analysis indicated that the exterior layer of both shell types was made of calcite. T. funebralis may be more prone to dissolution because their outer fibrous calcite layer has more crystal edges and faces exposed, potentially increasing the surface area on which dissolution can occur. These results support a previous study where T. funebralis showed significant decreases in both shell growth and strength, but N. ostrina only showed slight reductions in shell strength, and unaffected growth. We suggest that microstructural arrangement of shell layers in molluscs, more so than their composition alone, is critical for determining the vulnerability of mollusc shells to OA.

Characterization and implications of solids associated with hydraulic fracturing flowback and produced water from the Duvernay Formation, Alberta, Canada.

Public concern is heightened around flowback and produced water (FPW) generated by the hydraulic fracturing process. FPW is a complex mix of organic and inorganic solutes derived from both the injected hydraulic fracturing fluid and interactions with the subsurface lithology. Few studies to date have systematically investigated the composition of FPW or its individual components. Here, we provide the first systematic characterization of the composition of the solids associated with FPW by analyzing samples from three wells drilled into the Duvernay Formation in Alberta, Canada. The FPW initially returned to the surface with high total dissolved solids (greater than 170 000 mg L-1) and enriched with Fe(ii), silica, sulfate, barium, and strontium. The solids form two distinct phases once the FPW reached the surface: (1) silica-enriched Fe(iii) oxyhydroxides, and (2) a barite-celestine solid solution. We hypothesize that the precipitation of the amorphous silica-enriched Fe(iii) oxyhydroxide is a two-step process, where first the silica precipitates as a function of the cooling of the FPW from elevated subsurface temperatures to ambient surface temperatures. Next, the silica acts as a template for the precipitation of Fe(iii) oxyhydroxide as the diffusion of oxygen into the subsurface causes oxidation of aqueous Fe(ii). The barite-celestine solid solution precipitates solely as a function of cooling. Elevated dissolved Fe concentrations in FPW and modeled saturation indices from five North American shale plays (Marcellus, Fayetteville, Barnett, Bakken, and Denver-Julesburg) indicate that solids similar to those found in Duvernay FPW, specifically Fe(iii) oxyhydroxides, barite and quartz, are likely to occur. With the solids known to carry a significant portion of FPW's toxicity and organic contaminant load, the development of new treatment technologies, such as the oxidation of the Fe(ii) in FPW, may increase FPW reuse and reduce the environmental risk posed by FPW.

Determination of the Settling Rate of Clay/Cyanobacterial Floccules.

The mechanisms underpinning the deposition of fine-grained, organic-rich sediments are still largely debated. Specifically, the impact of the interaction of clay particles with reactive, planktonic cyanobacterial cells to the sedimentary record is under studied. This interaction is a potentially major contributor to shale depositional models. Within a lab setting, the flocculation and sedimentation rates of these materials can be examined and measured in a controlled environment. Here, we detail a protocol for measuring the sedimentation rate of cyanobacterial/clay mixtures. This methodology is demonstrated through the description of two sample experiments: the first uses kaolin (a dehydrated form of kaolinite) and Synechococcus sp. PCC 7002 (a marine coccoid cyanobacteria), and the second uses kaolin and Synechocystis sp. PCC 6803 (a freshwater coccoid cyanobacteria). Cyanobacterial cultures are mixed with varying amounts of clay within a specially designed tank apparatus optimized to allow continuous, real-time video and photographic recording. The sampling procedures are detailed as well as a post-collection protocol for precise measurement of chlorophyll a from which the concentration of cyanobacterial cells remaining in suspension can be determined. Through experimental replication, a profile is constructed that displays sedimentation rate.

Response to comment on "Bilaterian burrows and grazing behavior at >585 million years ago".

Gaucher et al. suggest that their field observations and petrographic analysis of one thin section do not support an Ediacaran age for the trace fossils-bearing strata of the Tacuarí Formation. We have strengthened our conclusion of an Ediacaran age for the Tacuarí Formation based on reassessment of new and previously presented field and petrographic evidence.

Bilaterian burrows and grazing behavior at >585 million years ago.

Based on molecular clocks and biomarker studies, it is possible that bilaterian life emerged early in the Ediacaran, but at present, no fossils or trace fossils from this time have been reported. Here we report the discovery of the oldest bilaterian burrows in shallow-water glaciomarine sediments from the Tacuarí Formation, Uruguay. Uranium-lead dating of zircons in cross-cutting granite dykes constrains the age of these burrows to be at least 585 million years old. Their features indicate infaunal grazing activity by early eumetazoans. Active backfill within the burrow, an ability to wander upward and downward to exploit shallowly situated sedimentary laminae, and sinuous meandering suggest advanced behavioral adaptations. These findings unite the paleontological and molecular data pertaining to the evolution of bilaterians, and link bilaterian origins to the environmental changes that took place during the Neoproterozoic glaciations.