The geochemistry of modern calcareous barnacle shells and applications for palaeoenvironmental studies

Thoracican barnacles of the Superorder Thoracicalcarea Gale, 2016 are sessile calcifiers which are ubiquitous in the intertidal zone and present from very shallow to the deepest marine environments; they also live as epiplankton on animals and detritus. The geochemical composition of their shell calcite has been shown to yield information about environmental conditions, but comprehensive analyses of barnacle shell geochemistry are so far lacking. Here, a dataset is reported for Mg/Ca, Sr/Ca, Mn/Ca, Fe/Ca, as well as carbon and oxygen isotope ratios for 42 species from the Balaniformes, Verruciformes, Scalpelliformes and Lepadiformes. Barnacles predominantly form low-Mg-calcite with very high Sr/Ca ratios averaging 4.2 mmol/mol. The Mn/Ca and Fe/Ca ratios in shell plates are variable and can exceed > 4mmol/mol in barnacles that are attached to manmade structures or live close to (anthropogenic) sources of Mn and Fe. No strong phylogenetic control on the average element/Ca ratios is observed in barnacles. The Balaniformes show a ca. 40 % enrichment of Mg in their scuta and terga as compared to other shell plates — a pattern which is not seen in other barnacles. The combination of low to medium Mg/Ca ratios and high Sr/Ca ratios is rare for marine biogenic calcite and Barnacles may thus become important for robustly reconstructing past seawater composition, if this signature is also present in fossil barnacle calcite and can be used alongside other fossil taxa with different Sr incorporation behaviour. Carbon and oxygen isotope data support the view that the oxygen isotope thermometer for barnacles is robust and that most barnacle species form their calcite in, or near, isotopic equilibrium with ambient water. The Lepadiformes, however, show a tendency for strong co-variation of δ13C with δ18O values and depletion in 13C and 18O which is attributed to isotopic disequilibrium during shell secretion. Strong systematic fluctuations in Mg/Ca ratios over length scales of ca. 5 to 15 µm are exhibited by the scalpelliform species Capitulum mitella, the only studied species which consistently forms high-Mg-calcite, and are tentatively linked to tidal control on the shell secretion pattern. Cathodoluminescence images for this species suggest that additionally a seasonal pattern of Mn distribution in its shell plates is recorded, pointing to a potential use for reconstruction of seasonal changes in terrestrial element supply

Permian strontium isotope stratigraphy

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Partial diagenetic overprint of Late Jurassic belemnites from New Zealand: implications for the preservation potential of δ7Li values in calcite fossils

The preservation potential and trends of alteration of many isotopic systems (e.g. Li, Mg, Ca) that are measured in fossil carbonates are little explored, yet extensive paleoenvironmental interpretations have been made on the basis of these records. Here we present a geochemical dataset for a Late Jurassic (∼153 Ma) belemnite (Belemnopsis sp.) from New Zealand that has been partially overprinted by alteration. We report the physical pathways and settings of alteration, the resulting elemental and isotopic trends including δ7Li values and Li/Ca ratios, and assess whether remnants of the primary shell composition have been preserved or can be extrapolated from the measured values. The δ18O and δ13C values as well as Sr/Ca and Mn/Ca ratios were analysed along two profiles. In addition, 6 samples were analysed for 87Sr/86Sr, Sr/Ca and Mn/Ca ratios. Five samples from the same specimen and 2 from the surrounding sediment were analysed for δ7Li values, Li/Ca, Sr/Ca and Mn/Ca ratios and are compared to results for 6 other Late Jurassic belemnite rostra (Belemnopsis sp. andHibolithes sp.) from the same region. The 87Sr/86Sr ratios are lower (less radiogenic) in the most altered part of the rostrum, whereas δ7Li values become more positive with progressive alteration. The direction and magnitude of the trends in the geochemical record indicate that one main phase of alteration that occurred in the Late Cretaceous caused most of the diagenetic signature in the calcite. Despite relatively deep burial, down to 4 km, and thus elevated temperatures, this diagenetic signature has subsequently been preserved even for the highly mobile element lithium, suggesting that primary lithium-isotope values can be maintained over geological timescales, at least in thick macrofossil shells. Our best δ7Li estimate for pristine Late Jurassic (∼155–148 Ma) belemnites is +27 ± 1‰, which points to a Late Jurassic seawater δ7Li of ∼29–32‰, compatible with the modern value of 31‰

Phosphorus in belemnites: extraction, quantification, and variability

Phosphorus is generally considered the ultimate limiting nutrient for marine primary productivity over geological timescales and plays a key role in modulating several biogeochemical cycles. Most established methods for investigating P cycling do not provide direct evidence for water-column P concentrations, but recent work on carbonate associated phosphorus (CAP) has shown there is a potential to record ancient dissolved P concentrations. We present a method to extend the application of carbonate associated P measurements to belemnites, and we quantify variability in P contents within and between belemnite specimens from two stratigraphic levels in Jurassic rocks from the Yorkshire coast, UK, as well as in modern analogues of belemnites. We show that there is little difference in P measurements between different preparative methods in uncontaminated belemnite calcite samples. In samples with higher levels of contaminant phases of P (P other than CAP), or a greater extent of diagenetic alteration, cleaning with non-oxidative methods and dissolution in weak acids (acetic) was found to minimise the impact of contamination on the measured P contents. P concentrations vary within and between specimens, but variations are not a result of taxonomic differences, and overall P measurements are reproducible between replicate samples, within individual belemnites and within stratigraphic levels. There were statistically significant differences in belemnite P concentrations between the stratigraphic levels studied here, indicating the potential for this technique to be used to measure changes in belemnite CAP through time

Onset of main Phanerozoic marine radiation sparked by emerging Mid Ordovician icehouse

The Great Ordovician Biodiversification Event (GOBE) was the most rapid and sustained increase in marine Phanerozoic biodiversity. What generated this biotic response across Palaeozoic seascapes is a matter of debate; several intrinsic and extrinsic drivers have been suggested. One is Ordovician climate, which in recent years has undergone a paradigm shift from a text-book example of an extended greenhouse to an interval with transient cooling intervals – at least during the Late Ordovician. Here, we show the first unambiguous evidence for a sudden Mid Ordovician icehouse, comparable in magnitude to the Quaternary glaciations. We further demonstrate the initiation of this icehouse to coincide with the onset of the GOBE. This finding is based on both abiotic and biotic proxies obtained from the most comprehensive geochemical and palaeobiological dataset yet collected through this interval. We argue that the icehouse conditions increased latitudinal and bathymetrical temperature and oxygen gradients initiating an Early Palaeozoic Great Ocean Conveyor Belt. This fuelled the GOBE, as upwelling zones created new ecospace for the primary producers. A subsequent rise in δ13C ratios known as the Middle Darriwilian Isotopic Carbon Excursion (MDICE) may reflect a global response to increased bioproductivity encouraged by the onset of the GOBE

The Li isotope composition of marine biogenic carbonates: Patterns and mechanisms

Little is known about the fractionation of Li isotopes during formation of biogenic carbonates, which form the most promising geological archives of past seawater composition. Here we investigated the Li isotope composition (δ7Li) and Li/Ca ratios of organisms that are abundant in the Phanerozoic record: mollusks (mostly bivalves), echinoderms, and brachiopods. The measured samples include (i) modern calcite and aragonite shells from various species and natural environments (13 mollusk samples, 5 brachiopods and 3 echinoderms), and (ii) shells from mollusks grown under controlled conditions at various temperatures. When possible, the mollusk shell ultrastructure was micro-sampled in order to assess intra-shell heterogeneity. In this paper, we systematically characterize the influence of mineralogy, temperature, and biological processes on the δ7Li and Li/Ca of these shells and compare with published data for other taxa (foraminifera and corals)