1,721,299 research outputs found
In situ Nd isotopic analysis of geological materials by laser ablation MC-ICP-MS
No full textNeodymium (Nd) isotopes have widespread applications in the earth sciences, both in geochronology and in provenance/tracer studies. The isotopic analyses are conventionally done by chemical separation of pure Nd from the sample matrix. This process is time consuming and loses potentially vast amounts of information held at small spatial scales. Here, we describe a laser ablation protocol for the measurement of Nd isotopes in geological materials with relatively high Nd concentrations (100 ppm; e.g., apatite, titanite, and ferromanganese nodules). The procedure allows, for the first time, the exploitation of information held in Nd isotopes at scales of less than 0.1 mm. The principal analytical issues to be overcome are the interferences from isobaric Sm isotopes, the correction for which must be very accurate in order to maintain a high accuracy for the Nd isotopic ratios. We show that careful attention to mass discrimination effects along with an iterative correction procedure allows a correction that results in an accuracy on the 143Nd/144Nd isotope ratio of the order of 0.5 epsilon units (0.005%) for Sm/Nd ratios as high as 1.2. The total analysis time is short (2–5 min), enabling a large number of analyses (>30) in each analytical session. Spatial resolution is governed by the Nd concentration and the required accuracy and for apatites and titanites it is typically 90 µm. Analytical precision is degraded by around a factor of 2–3 relative to the best TIMS and solution MC-ICP-MS data. Crucially, however, the rapid sample throughput and the extremely high spatial resolution by far outweigh this reduction in precision. A high resolution study of an Atlantic ferromanganese crust illustrates the usefulness of the described approach.Neodymium (Nd) isotopes have widespread applications in the earth sciences, both in geochronology and in provenance/tracer studies. The isotopic analyses are conventionally done by chemical separation of pure Nd from the sample matrix. This process is time consuming and loses potentially vast amounts of information held at small spatial scales. Here, we describe a laser ablation protocol for the measurement of Nd isotopes in geological materials with relatively high Nd concentrations (100 ppm; e.g., apatite, titanite, and ferromanganese nodules). The procedure allows, for the first time, the exploitation of information held in Nd isotopes at scales of less than 0.1 mm. The principal analytical issues to be overcome are the interferences from isobaric Sm isotopes, the correction for which must be very accurate in order to maintain a high accuracy for the Nd isotopic ratios. We show that careful attention to mass discrimination effects along with an iterative correction procedure allows a correction that results in an accuracy on the 143Nd/144Nd isotope ratio of the order of 0.5 epsilon units (0.005%) for Sm/Nd ratios as high as 1.2. The total analysis time is short (2–5 min), enabling a large number of analyses (>30) in each analytical session. Spatial resolution is governed by the Nd concentration and the required accuracy and for apatites and titanites it is typically 90 µm. Analytical precision is degraded by around a factor of 2–3 relative to the best TIMS and solution MC-ICP-MS data. Crucially, however, the rapid sample throughput and the extremely high spatial resolution by far outweigh this reduction in precision. A high resolution study of an Atlantic ferromanganese crust illustrates the usefulness of the described approach
Geochemical proxy systems in marine CaCO3 biominerals record both environmental changes and biomineralisation processes
Full text linkThe isotopic and elemental composition of calcium carbonate formed by marine organisms underpins a substantial portion of our knowledge of past climates. These geochemical ‘proxy’ systems have revolutionised our understanding of palaeoenvironmental change, but remain largely rooted in empiricism because of poorly understood biological ‘vital effects’. Here, we outline how this is both a problem and an opportunity—while some proxies have their basis in biological processes, this is the root cause of uncertainty in others. Moreover, integrating geochemistry into biomineralisation models provides additional constraint on cellular mechanisms; geochemical data have untapped potential in the field of biomineralisation and could be used to simultaneously understand the proxies in question and to determine why biomineralising organisms are sensitive to environmental change
Intercomparison of satellite-derived SST with logger data in the Caribbean—Implications for coral reef monitoring
Full text linkSince the early 1980s measurements of Sea Surface Temperature (SST) derived from satellite-borne instruments have provided a wide range of global gridded products documenting changes in SST. However, there are many sources of uncertainty in these records and significant differences exist among them. One use of these products is identification of coral bleaching events, and the predictions of the impact of future warming on coral reefs. This relies on an understanding of how temperatures near reefs as recorded by SST products differ from the in-situ SST experienced by the corals. This difference is a combination of real spatio-temporal variations, inadequate in product resolution and errors in the products. This paper investigates the relationship between the local temperature measured in-situ by loggers at coral sites in the western tropical Atlantic and two high resolution satellite SST products. Using differences among ESA SST CCI v2.1 (CCI analysis SST), NOAA CoralTemp SST products and in-situ logger data from coral reefs, an assessment of the satellite products with focus on coral reef monitoring is carried out. Discrepancies between the two products can be large, especially in coastal areas and for the warmest and coldest months when there is a particular risk of bleaching. By comparison to the stable CCI analysis SST product, CoralTemp was found to overestimate the rise in SST by as much as 0.20°C per decade. In almost all cases SSTs from CCI analysis SST were more consistent with temperatures measured near the corals than those from CoralTemp
Future climate forcing potentially without precedent in the last 420 million years
No full textThe evolution of Earth’s climate on geological timescales is largely driven by variations in the magnitude of total solar irradiance (TSI) and changes in the greenhouse gas content of the atmosphere. Here we show that the slow ∼50 Wm−2 increase in TSI over the last ∼420 million years (an increase of ∼9 Wm−2 of radiative forcing) was almost completely negated by a long-term decline in atmospheric CO2. This was likely due to the silicate weathering-negative feedback and the expansion of land plants that together ensured Earth’s long-term habitability. Humanity’s fossil-fuel use, if unabated, risks taking us, by the middle of the twenty-first century, to values of CO2 not seen since the early Eocene (50 million years ago). If CO2 continues to rise further into the twenty-third century, then the associated large increase in radiative forcing, and how the Earth system would respond, would likely be without geological precedent in the last half a billion years
An improved boron isotope pH proxy calibration for the deep-sea coral Desmophyllum dianthus through sub-sampling of fibrous aragonite
Full text linkThe isotopic composition of boron (?11B) in marine carbonates is well established as a proxy for past ocean pH, however, its robust application to palaeo-environments relies on the generation of species-specific calibrations. Existing calibrations utilising the deep-sea coral (DSC) Desmophyllum dianthus highlight the potential application of this pervasive species to pH reconstructions of intermediate depth waters. Nevertheless, considerable uncertainty remains regarding the estimation of seawater pH from these bulk skeletal ?11B measurements, likely resulting from microstructural heterogeneities in ?11B of D. dianthus. To circumvent this problem, thus improving the reliability of the D. dianthus ?11B-pH calibration, we present a new ?11B calibration of micro-sampled fibrous aragonite from this species.Modern coral specimens recovered from the Atlantic, Pacific, and Southern Oceans, micro-sampled using microdrilling, micromilling, and laser cutting extraction, were analysed for trace element (B/Ca, Mg/Ca, Sr/Ca, and U/Ca) and boron isotopic composition. We find the best calibration against the ?11B of borate in local ambient seawater (a function of pH and taken from hydrographic data sets; pH range 7.57 to 8.05) utilises ?11B measurements of fibres with likely slow growth rates and minimal contamination from adjacent microstructures (identified by low Mg/Ca) for each coral specimen. This new calibration exhibits a stronger, and better-defined dependence on ambient seawater pH compared to bulk coral ?11B; ?11Bfibre = (0.93 ± 0.17) × ?11Bborate + (12.02 ± 2.63). We suggest that the majority of the variability in measured ?11B between replicate bands of fibrous aragonite from a D. dianthus specimen can be explained by small incorporation of non-fibrous aragonite and surface impurities during microsampling and growth rate effects. This study confirms the utility of D. dianthus as an archive of precise palaeo-pH (± 0.07 pH units), provided that suitable sampling strategies are applied
Constraining uncertainty in boron isotope systematics using a Bayesian inversion engine reveals contrasting parameter sensitivities
No full textPhysical parameters within boron isotope systematics form a complex interplay that determine the boron isotopic composition of rocks, minerals, and fluids, but to date, providing constraints on uncertainty within boron equilibrium isotope modelling remains elusive. This underlying uncertainty limits the potency of boron isotopes as a tool for detecting fluid-rock exchange. A new equilibrium boron mineral-fluid fractionation modelling approach, named EquiB, coupled with a Bayesian inversion engine is presented, providing robust and reproducible constraints on the uncertainty of physical parameters encoded into a boron isotopic composition of a rock in equilibrium with a fluid. We demonstrate the validity of our approach by applying the model to several basalt-fluid and peridotite-fluid exchange scenarios. The model output generates multi-dimensional posterior probability distributions that show temperature is the greatest control on mineral-fluid fractionation in all applied scenarios. At high temperatures (defined as >50 °C) pH-dependent fractionation is negligible, but at low temperatures (defined as <50 °C) pH-dependent fractionation is a control on boron isotopic compositions. At geologically reasonable conditions other parameters such as salinity, fluid density, and pressure have little effect on the extent of boron mineral-fluid fractionation. Model outputs agree with experimentally derived fractionation factors at typical hydrothermal conditions but diverge at low temperatures. This approach provides robust constraints of parameter uncertainty, enabling meaningful interpretation of boron isotope analyses and the ability to fingerprint isotopic compositions with greater confidence.</p
Reconstructing ocean pH with boron isotopes in foraminifera
No full textIn order to better understand the effect of CO2 on the Earth system in the future, geologists may look to CO2-induced environmental change in Earth's past. Here we describe how CO2 can be reconstructed using the boron isotopic composition (?11B) of marine calcium carbonate. We review the chemical principles that underlie the proxy, summarize the available calibration data, and detail how boron isotopes can be used to estimate ocean pH and ultimately atmospheric CO2 in the past. ?11B in a variety of marine carbonates shows a coherent relationship with seawater pH, in broad agreement with simple models for this proxy. Offsets between measured and predicted ?11B may in part be explained by physiological influences, though the exact mechanisms of boron incorporation into carbonate remain unknown. Despite these uncertainties, we demonstrate that ?11B may provide crucial constraints on past ocean acidification and atmospheric CO2
Placing our current 'hyperthermal' in the context of rapid climate change in our geological past
Full text link'. . . there are known knowns. These are things we know that we know. There are known unknowns. That is to say, there are things that we know we don't know. But there are also unknown unknowns. There are things we don't know we don't know.' Donald Rumsfeld 12th February 2002.</p
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