A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions

The chromium isotope system (53Cr&thinsp;∕&thinsp;52Cr, expressed as δ53Cr relative to NIST SRM 979) in marine biogenic and non-biogenic carbonates is currently being evaluated as a proxy for the redox state of the ocean. Previous work has concentrated on using corals and foraminifera for this purpose, but investigations focusing on the behavior of Cr in bivalves as potential archives are lacking. Due to their often good preservation, fossil marine biogenic carbonates have the potential to serve as useful archives for the reconstruction of past ocean redox fluctuations and eventually link those to climatic changes throughout Earth's history. Here, we present an evaluation of the Cr isotope system in shells of some modern bivalves. Shell species from Lucidinadae, Cardiidae, Glycimerididae and Pectenidae, collected systematically from one Mediterranean location (Playa Poniente, Benidorm, Spain) over a 3-year period reveal δ53Cr values ranging from 0.15&thinsp;‰ to 0.65&thinsp;‰, values that are systematically below the local seawater δ53Cr value of 0.83±0.05&thinsp;‰. This attests to a significant reduction of dissolved seawater chromium in the process leading to calcification and thus for control of Cr isotope fractionation during biological routes. A similar, constant offset in δ53Cr values relative to surface seawater is observed in shells from Mytilius edulis from an arctic location (Godhavn, Disko Bay, Greenland). Chromium concentrations in the studied shells are significantly controlled by organic matter and typically range from 0.020 to 0.100&thinsp;ppm, with some higher concentrations of up to 0.163&thinsp;ppm recorded in Pectenidae. We also observe subtle, species-dependent differences in average Cr isotope signatures in the samples from Playa Poniente, particularly of Lucidinadae and Cardiidae, with considerably depressed and elevated δ53Cr values, respectively, relative to the other species investigated. Intra-species heterogeneities, both in Cr concentrations and δ53Cr values, are favorably seen to result from vital effects during shell calcification rather than from heterogeneous seawater composition. This is because we observe that the surface seawater composition in the particular Playa Poniente location remained constant during the month of July of the 3 years we collected bivalve samples. Intra-shell heterogeneities – associated with growth zones reflecting one to several years of growth, both in δ53Cr and Cr concentrations – are observed in a sample of Placuna placenta and Mimachlamys townsendi. We suspect that these variations are, at least partially, related to seasonal changes in δ53Cr of surface seawaters. Recognizing the importance of organic substances in the bivalve shells, we propose a model whereby reduction of Cr(VI) originally contained in the seawater as chromate ion and transported to the calcifying space, to Cr(III), is effectively adsorbed onto organic macromolecules which eventually get included in the growing shell carbonates. This study, with its definition of statistically sound offsets in δ53Cr values of certain bivalve species from ambient seawater, forms a base for future investigations aimed at using fossil shells as archives for the reconstruction of paleo-seawater redox fluctuations.</p

Heterogeneity and incorporation of chromium isotopes in recent marine molluscs (Mytilus)

The mollusc genus Mytilus is abundant in various modern marine environments and is an important substrate for palaeo-proxy work. The redox-sensitive chromium (Cr) isotope system is emerging as a proxy for changes in the oxidation state of the Earth's atmosphere and oceans. However, potential isotopic offsets between ambient sea water and modern biogenic carbonates have yet to be constrained. We measured Cr concentrations ([Cr]) and isotope variations (δ53 Cr) in recent mollusc shells (Mytilus) from open and restricted marine environments and compared these to ambient sea water δ53 Cr values. We found a large range in mollusc [Cr] (12-309 ppb) and δ53 Cr values (-0.30 to +1.25‰) and in the offset between δ53 Cr values of mollusc shells and ambient sea water ( Δ53CrseawaterbulkMytilus , -0.17 to -0.91‰). Step digestions of cultivated Mytilus edulis specimens indicate that Cr is mainly concentrated in organic components of the shell (periostracum: 407 ppb, n = 2), whereas the mollusc carbonate minerals contain ≤3 ppb Cr. Analyses of individual Cr-hosting phases (i.e., carbonate minerals and organic matrix) did not reveal significant differences in δ53 Cr values, and thus, we suggest that Cr isotope fractionation may likely take place prior to rather than during biomineralisation of Mytilus shells. Heterogeneity of δ53 Cr values in mollusc shells depends on sea water chemistry (e.g., salinity, food availability, faeces). The main control for δ53 Cr values incorporated into shells, however, is likely vital effects (in particular shell valve closure time) since Cr can be partially or quantitatively reduced in sea water trapped between closed shell valves. The δ53 Cr values recorded in Mytilus shells may thus be de-coupled from the redox conditions of ambient sea water, introducing additional heterogeneity that needs to be better constrained before using δ53 Cr values in mollusc shells for palaeo-reconstructions.Sylvie Bruggmann, Robert M. Klaebe, Cora Paulukat, Robert Fre

Facies-dependent δ 13 C variation and diagenetic overprinting at the onset of the Sturtian glaciation in North-East Greenland

Time-significant surfaces are used to construct a stratigraphic test of the intrabasinal reproducibility of a 16‰ Neoproterozoic carbon-isotope (δ 13 C) anomaly in the NE Greenland Caledonides. The δ 13 C excursion from +6‰ to -10‰ occurs in the carbonate-dominated Andrée Land Group and lies below glacial diamictites of the Tillite Group commonly correlated as Sturtian in age (~720 Ma) and has been widely interpreted to record a global isotopic event reflecting a perturbation in the carbon cycle preceding the initiation of the snowball ice age. δ 13 C stratigraphic patterns were determined in two strike sections with relatively shallow platform deposits at Kap Weber and slope deposits on Ella Ø preserved directly below the first evidence for glaciation. The top and bottom of the δ 13 C profile was bounded by contiguous chronostratigraphic surfaces including an unconformity (sequence boundary) at the base recognisable in both sections and a subaerial exposure surface with a basinal correlative conformity at the top. The δ 13 C profile at Kap Weber shows δ 13 C values of +6‰ that drop to -8‰ in fine-grained slope deposits before returning to +6‰ in platform carbonates at the base of the glacial diamictite. The slope section on Ella Ø shows similar values for platformal carbonates of +6‰ with a similar drop to -10‰ in slope deposits, but this section lacks the return to platform deposits evident at Kap Weber beneath the diamictite as well as a return to positive δ 13 C values. The absence of the platformal sediments and positive δ 13 C values on Ella Ø cannot be attributed to erosional truncation because the contact in this section shows a conformable, interbedded transition into the glacial deposits indicating that a continuous record is preserved. This disparity in δ 13 C values below the base of the Tillite Group suggests that the most negative δ 13 C values recorded in slope mudstone facies (-10‰) occur near synchronously with values of +6‰ on the platform. Elemental mapping of the mudstone facies indicates that carbonate is largely pore-filling and authigenic, representing a secondary phase that is unlikely to record a seawater value. Elevated δ 13 C values in shallow inner-ramp carbonate intervals may record locally modified seawater enriched in 13 C by photosynthesis and evaporation. The excursion in δ 13 C values in the upper Andrée Land Group is therefore interpreted to result from a shift in facies from platform carbonates to carbonate cemented and diagenetically overprinted mudstone and does not record the secular change in seawater δ 13 C used for correlation or interpretation of biogeochemical events preceding the snowball ice age

The impact of libraries as creative spaces

Ready access to information through digital media has challenged the perceived societal roles of public libraries. Since the mid 1990s, libraries have reoriented themselves towards public participation beyond lending and reading. Libraries now offer an increasing range of community-focused creative activities.Library spaces are transforming. In addition to housing archival and loan materials, desks and reading spaces, libraries are becoming even more exible and activity-oriented. Given these transformations, understanding and demonstrating the new contributions public libraries make to their communities is critical.In 2015, the State Library of Queensland (SLQ) commissioned researchers at the Digital Media Research Centre at Queensland University of Technology (QUT) to explore the impact of libraries as creative spaces.The objectives of the Libraries as Creative Spaces project were to:• Investigate the community impact of creative spaces in public libraries• Provide clear evidence of this impact• Articulate the opportunities to further embed creative spaces in public libraries or community spaces.The methods used included:• A contextual review of thinking regarding libraries as creative spaces• A literature review on the evaluation of creative activity in libraries and creative spaces more generally• Interviews and observational eld research at selected Queensland libraries• The development and refinement of an evidence-based Creative Spaces Impact Framework• The development of an associated rich media package (including videos, photos and digital stories) showcasing library-based creative activity across Queensland in 2015.The contextual review highlights the attention being paid to makerspaces as a site of creative activity in public libraries. However, the review also points to the many other ways public libraries are facilitating creative activity. It also demonstrates that while libraries have been formally evaluated for more than a hundred years, only recently has there been a concerted e ort to develop frameworks that can qualitatively assess the social impact on individuals and their communities. Moreover, few of these frameworks directly assess libraries as creative spaces.Following the contextual review, an extended literature review was undertaken for two reasons. Firstly, to develop our understanding of how creative spaces are evaluated (outside the library context). And secondly, to develop our understanding of how libraries are evaluated in more general terms. From this initial evidence base, a preliminary Creative Spaces Impact Framework was developed.The framework was used to interrogate creative activity at ve public library sites across Queensland – Ayr, Cleveland, Helensvale, Roma, and Victoria Point. The framework was revised in conjunction with the eldwork and further feedback from a range of stakeholders. The Creative Spaces Impact Framework allows for the evaluation of libraries as creative spaces through the creative activities they enable. These activities may be formally organised by the library, informally enacted by library clients, or involve a combination of both approaches. The contents of the framework represent a range of impact potentials, some of which will be relevant to certain library creative space activities more than others, if at all

The Iso2k database: a global compilation of paleo-δ18O and δ2H records to aid understanding of common era climate

Reconstructions of global hydroclimate during the Common Era (CE; the past ~2,000 years) are important for providing context for current and future global environmental change. Stable isotope ratios in water are quantitative indicators of hydroclimate on regional to global scales, and these signals are encoded in a wide range of natural geologic archives. Here we present the Iso2k database, a global compilation of previously published datasets from a variety of natural archives that record the stable oxygen (δ18O) or hydrogen (δ²H) isotopic composition of environmental waters, which reflect hydroclimate changes over the CE. The Iso2k database contains 756 isotope records from the terrestrial and marine realms, including: glacier and ground ice (205); speleothems (68); corals, sclerosponges, and mollusks (145); wood (81); lake sediments and other terrestrial sediments (e.g., loess) (158); and marine sediments (99). Individual datasets have temporal resolutions ranging from sub-annual to centennial, and include chronological data where available. A fundamental feature of the database is its comprehensive metadata, which will assist both experts and non-experts in the interpretation of each record and in data synthesis. Key metadata fields have standardized vocabularies to facilitate comparisons across diverse archives and with climate model simulated fields. This is the first global-scale collection of water isotope proxy records from multiple types of geological and biological archives. It is suitable for evaluating hydroclimate processes through time and space using large-scale synthesis, model-data intercomparison and (paleo)data assimilation. The Iso2k database is available for download at: https://doi.org/10.25921/57j8-vs18 (Konecky and McKay, 2020) and is also accessible via through the NOAA/WDS Paleo Data landing page: https://www.ncdc.noaa.gov/paleo/study/29593

Sedimentology and stable isotope geochemistry of the Trezona Formation; A local or global biogeochemical event?

This item is only available electronically.High magnitude δ13C shifts (>12‰) restricted to Neoproterozoic carbonate successions are widely interpreted to reflect a vastly different regime of carbon cycling that changed into the Phanerozoic. Despite isotopic values in the Neoproterozoic being anomalous, they are considered to reflect the δ13C composition of sea water because the values appear to be reproducible, change systematically, and occur in a similar stratigraphic interval relative to overlying glacial intervals within successions in different basins. The relation to a primary marine origin of the isotopic values in carbonates is key to these isotopic excursions providing constraints on the global carbon mass balance during the Neoproterozoic, that are central to present models of the ancient carbon cycle. The Trezona Formation in the Central Flinders Ranges in South Australia records a large (~9‰) pre-Marinoan glacial δ13C excursion widely correlated to basins globally and termed the ‘Trezona Anomaly’. This study examines the depositional setting of the Trezona Formation using outcrop exposures, petrographic studies, and stable isotope geochemical data and investigates the origin of δ13C values with respect to lithological and diagenetic controls. δ13C and δ18O data was collected using Isotope Ratio Mass Spectrometry. Field observations reported here are inconsistent with an open marine or tidal origin for the Trezona Formation. Sequence boundaries in the form of paleosols and fluvial deposits at the basal and upper contacts respectively indicate that it represents its own discrete depositional cycle. This is contrary to previous interpretations that the Trezona Formation records a broad shallowing upwards trend of widespread marine shales of the underlying Enorama Formation into the overlying glacial sediments of the Elatina Formation. Evidence of frequent desiccation throughout the basal half of the Formation and the limited spatial distribution of the Trezona Formation is suggestive of a consistently shallow, restricted marine or periodically lacustrine depositional setting. Covariant and diverging relationships between δ13C and δ18O values in stratigraphic profiles suggest a lithological relation to isotope values. Furthermore, petrographic data suggests that intervals of the Trezona Formation housing strongly negative δ13C values (<-5‰) may have undergone digenetic recrystalisation. A diagenetic origin for these values makes them typical of meteorically altered successions in the Phanerozoic, and removes the need for currently popular global biogeochemical models calling for dramatic differences in Precambrian carbon cycling. These observations also imply that the Trezona Formation is not a record homogeneous, open marine δ13C values and is therefore inappropriate as a correlation of chemostratigraphic events. Rather, it likely records the common alteration of coastal or lacustrine carbonates responding to exposure and alteration during sea-level fall coinciding with the Marinoan glaciationThesis (B.Sc.(Hons)) -- University of Adelaide, School of Earth and Environmental Sciences, 201