1,721,168 research outputs found
Imaging element distributions within small marine calcifiers: a NanoSIMS perspective
No full textClimate change is one of the major challenges of our time. Human induced increase in atmospheric carbon dioxide will, without the effort to decrease carbon emissions world-wide, likely lead to a global mean surface temperature rise exceeding 1.5 °C by the end of the 21st century compared to pre-industrial times. In order to mitigate climate change, we rely on model simulations of the Earth System, which are based on observations of past climate. Indirect observations can be made by making use of proxies: chemical or isotopic signatures of old materials we can measure today, of which a relationship with environmental parameters has been established. Many proxies are based on inorganic chemical signatures of fossil shells of marine calcifiers, such as foraminifera and coccolithophores. However, all estimates based on proxies are burdened with uncertainties, not in the least because interactions between multiple environmental factors and biological processes, the so-called vital effects, influence the proxy signal. Understanding of the fundamental controls of biomineralization and thereby the underlying mechanisms of proxies, thus the how and why proxies actually work, is presently of large interest in the scientific community. In this thesis, nano-scale secondary ion mass spectrometry (NanoSIMS) is the analytical technique of choice to investigate the submicron-scale distribution of minor and trace elements in small marine calcifiers. NanoSIMS provides several important advantages over bulk analytical techniques: (1) high spatial resolution, which is sufficient to resolve small samples, such as coccoliths, in considerable detail; (2) high mass resolution, which is sufficient to gain data largely free of isobaric interferences; and (3) the capability to collect data as images, which enables one to selectively choose uncontaminated sample areas for investigation. As this technique is expensive in both time and money, it is used in this thesis not to establish analytical routines to construct proxy records, but rather to investigate, with a sub-micrometer resolution, the distribution of minor and trace elements in marine carbonates to enhance our understanding of paleoceanographic proxies based on coccoliths and foraminiferal calcite. Overall, this thesis emphasizes the importance of micro-scale measurements for gaining insights into the mechanisms involved in marine calcification and the concomitant incorporation of minor and trace elements into marine carbonates. Getting a handle on the underlying mechanisms of calcification improves the reliability of paleoceanographic proxies and will thus potentially help to advance our understanding of the climate system as a whole. Not only were pilot studies conducted whether additional, not yet investigated elements such as Na in coccoliths, or Cl in foraminifera, may potentially serve as new paleo proxies, this thesis also highlights the necessity in scrutinizing analytical procedures with highly-sensitive instruments such as the NanoSIMS
Chlorine isotopic compositions of deep saline fluids in Ibusuki coastal geothermal region, Japan: using B–Cl isotopes to interpret fluid sources
Full text linkWe report chlorine stable isotopic compositions (δ37Cl, expressed in ‰ relative to the standard mean ocean chloride) as well as δ2H and δ18O values of deep saline fluids taken at eight drill-holes reaching from 73 to 780 m below sea level in the Ibusuki coastal geothermal region, Japan. Analytical results show that the δ37Cl values narrowly range between −0.26 and +0.21 ‰ with an analytical precision of ±0.06 ‰. Except for one sample, the samples examined are negative in δ37Cl value with varying Cl/B molar ratios from 117 to 1265. A correlation study between the Cl/B molar ratio and the δ37Cl/δ11B ratio indicates a hyperbola-type mixing of at least two Cl sources in the Ibusuki region. One of them depletes in 37Cl with a higher value of Cl/B molar ratio; and the other one enriches in 37Cl with a lower Cl/B molar ratio. The former is chemically identical to that of the deep brine, which is altered seawater through the seawater–hot rock interaction. The latter is chemically similar to gas condensate derived from the high-temperature (890 °C) vent of an island-arc volcano near the Ibusuki region
Attributing seasonal pH variability in surface ocean waters to governing factors
No full textOn-going ocean acidification and increasing availability of high-frequency pH data have stimulated interest to understand seasonal pH dynamics in surface waters. Here we show that it is possible to accurately reproduce observed pH values by combining seasonal changes in temperature (T), dissolved inorganic carbon (DIC), and total alkalinity (TA) from three time series stations with novel pH sensitivity factors. Moreover, we quantify the separate contributions of T, DIC, and TA changes to winter-to-summertime differences in pH, which are in the ranges of −0.0334 to −0.1237, 0.0178 to 0.1169, and −0.0063 to 0.0234, respectively. The effects of DIC and temperature are therefore largely compensatory, and are slightly tempered by changes in TA. Whereas temperature principally drives pH seasonality in low-latitude to midlatitude systems, winter-to-summer DIC changes are most important at high latitudes. This work highlights the potential of pH sensitivity factors as a tool for quantifying the driving mechanisms behind pH changes
Recent seasonal hypoxia on the Western Black Sea shelf recorded in adjacent slope sediments
No full textPhosphorus burial in sediments of the sulfidic deep Black Sea: Key roles for adsorption by calcium carbonate and apatite authigenesis
Full text linkAbstract Sedimentary burial of the essential nutrient phosphorus (P) under anoxic and sulfidic conditions is incompletely understood. Here, we use chemical and micro-scale spectroscopic methods to characterize sedimentary P burial along a water column redox transect (six stations, 78–2107 m water depth) in the Black Sea from the shelf with its oxygenated waters to the anoxic and sulfidic deep basin. Organic P is an important P pool under all redox regimes, accounting for up to 60% of P burial. We find a general down-core increase in the relative importance of organic P, especially on the shelf where P bound to iron (Fe) and manganese (Mn) (oxyhydr)oxides is abundant in the uppermost sediment but rapidly declines in concentration with sediment depth. Our chemical and spectroscopic data indicate that the carbonate-rich sediments (Unit I, ∼3000 years, ∼0–30 cm depth) of the sulfidic deep Black Sea contain three major P pools: calcium phosphate (apatite), organic P and P that is strongly associated with CaCO3 and possibly clay surfaces. Apatite concentrations increase from 5% to 25% of total P in the uppermost centimeters of the deep basin sediments, highlighting the importance of apatite formation for long-term P burial. Iron(II)-associated P (ludlamite) was detected with X-ray absorption spectroscopy but was shown to be a minor P pool (∼5%), indicating that lateral Fe–P transport from the shelf (“shuttling”) likely occurs but does not impact the P burial budget of the deep Black Sea. The CaCO3–P pool was relatively constant throughout the Unit I sediment interval and accounted for up to 55% of total P. Our results highlight that carbonate-bound P can be an important sink for P in CaCO3-rich sediments of anoxic, sulfidic basins and should also be considered as a potential P sink (and P source in case of CaCO3 dissolution) when reconstructing past ocean P dynamics from geological records
Generalised expressions for the response of pH to changes in ocean chemistry
No full textThe extent to which oceans are capable of buffering chemical changes resulting from the uptake of carbon dioxide (CO2) or other acidifying processes can be quantified using buffer factors. Here, we present general expressions describing the sensitivity of pH and concentrations of CO2 and other acid–base species to a change in ocean chemistry. These expressions can include as many acid–base systems as desirable, making them suitable for application to, e.g., upwelling regions or nutrient-rich coastal waters. We show that these expressions are fully consistent with previously derived expressions for the Revelle factor and other buffer factors, which only included the carbonate and borate acid–base systems, and provide more accurate values. We apply our general expressions to contemporary global ocean surface water and possible changes therein by the end of the 21st century. These results show that most sensitivities describing a change in pH are of greater magnitude in a warmer, high-CO2 ocean, indicating a decreased seawater buffering capacity. This trend is driven by the increase in CO2 and slightly moderated by the warming. Respiration-derived carbon dioxide may amplify or attenuate ocean acidification due to rising atmospheric CO2, depending on their relative importance. Our work highlights that, to gain further insight into current and future pH dynamics, it is crucial to properly quantify the various concurrently acting buffering mechanisms
Diatom chemistry as a potential paleoproxy for sea surface conditions
No full textDiatoms are responsible for over 40% of the total primary production and yet they are not a leading paleo-proxy. Instead, over the past few decades, foraminiferal-based proxies have dominated climate reconstruction research. Developments in analytical techniques have made it possible to look at chemical characteristics of samples on nanometer scale and may thus improve our understanding of the nature of diatoms. In this study we aim to identify changes in elemental ratios related to environmental changes, in both the diatom frustule itself and the occluded organic matter. Here we present the first results from Nano-SIMS (Secondary Ion Mass Spectrometer) analysis on Mediterranean diatoms obtained during the 2016 NESSC cruise
Composition and variability in the export of biogenic silica in the Changjiang River and the effect of Three Gorges Reservoir
Full text linkSilicon (Si) plays an essential role in biogeochemical processes, but is still poorly characterized in the river system. This study addressed the biogenic silica (BSi) composition, origin and variation in the Changjiang River, and estimated the impacts of natural processes and human activities on the river Si cycling. Our results indicate that phytoliths comprised 14%-64% of BSi, while diatoms accounted for 34%-85% of BSi. The Changjiang River transported 620Ggyr-1 of BSi and 2100Ggyr-1 of dissolved silicate (DSi) loadings, respectively; 55% of the BSi and 51% of the DSi fluxes are transported during the high discharge period from June to September. The Changjiang River carried phytolith BSi mostly comes from the middle and lower reaches area. The ratio of BSi/(BSi+DSi) has decreased from 0.47 before 1980 to 0.19 in 2013-2014 due to the direct retention of BSi. The BSi sedimentation in the Three Gorges Reservoir would cause a decrease of total reactive silica, but contribute to approximately 4%-16% of the DSi loading at the Jiangyin station due to its dissolution. This study demonstrates that phytoliths represent a significant contribution to the biogeochemical cycle of silica in coastal waters, and in-stream process exerts a great influence on the river Si loading and cycling
The role of calcification in carbonate compensation
Full text linkThe long-term recovery of the oceans from present and past acidification is possible due to neutralization by the dissolution of biogenic CaCO3 in bottom sediments, that is, carbonate compensation. However, such chemical compensation is unable to account for all features of past acidification events, such as the enhanced accumulation of CaCO3 at deeper depths after acidification. This overdeepening of CaCO3 accumulation led to the idea that an increased supply of alkalinity to the oceans, via amplified weathering of continental rocks, must accompany chemical compensation. Here we discuss an alternative: that changes to calcification, a biological process dependent on environmental conditions, can enhance and modify chemical compensation and account for overdeepening. Using a simplified ocean box model with both constant and variable calcification, we show that even modest drops in calcification can lead to appreciable long-term alkalinity build-up in the oceans and, thus, create overdeepening; we term this latter effect biological compensation. The chemical and biological manifestations of compensation differ in terms of controls, timing and effects, which we illustrate with model results. To better predict oceanic evolution during the Anthropocene and improve the interpretation of the palaeoceanographic record, it is necessary to better understand biological compensation
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