A New Approach to Characterizing Deposit Type Using Mineral Inclusion Assemblages in Gold Particles

Mineral inclusions within native gold are features of lode gold occurrences that are preserved in detrital particles. Inclusion assemblages in populations of gold particles in placers from specific localities are revealed through inspection of polished sections, and assimilation of robust data sets permits reconstruction of the lode source mineralogy. Inclusion assemblages differ considerably according to the source deposit type, and various approaches have been employed to graphically represent inclusion mineralogy. We present a simple method for depicting and comparing inclusion assemblages using a single standardized radar diagram template that illustrates the proportions of 11 metal and 5 nonmetal (and metalloid) elements in each inclusion assemblage. The Canadian Cordillera hosts many different gold-bearing deposit types and is an ideal terrane in which to develop a globally applicable methodology. Although placer gold is widespread, the location and nature of source mineralization is commonly unclear. This study is based on the inclusion suites recorded in 37 sample sets of gold particles from both placer and lode localities. Radar diagrams describing inclusion assemblages show clear generic differences according to deposit type. Diagnostic signatures have been established and act as templates against which samples of unknown origin may be compared. This approach permits differentiation between populations of gold particles formed in different magmatic systems (low-sulfidation epithermal, calc-alkalic porphyry, and alkalic porphyry), which may all be distinguished from gold formed in orogenic (amagmatic) mineralization. Metallic element signatures are most useful in differentiating gold from different magmatic hydrothermal systems, whereas nonmetallic elements allow for classification of orogenic gold subtypes. Comparisons of mineral inclusion signatures from gold in the Canadian Cordillera with samples from similar geologic settings worldwide suggest that this approach to gold fingerprinting is globally applicable. Therefore, the geochemical signatures of inclusion assemblages provide a robust indication of deposit type and may be applied in exploration to illuminate regional metallogeny in areas where relationships between placer deposits and their source(s) may be unclear.

Diatom Dominance Enhances Resistance of Phytoplanktonic POM to Mesopelagic Microbial Decomposition

Particulate organic matter (POM) lability is one of the key factors determining the residence time of organic carbon (OC) in the marine system. Phytoplankton community composition can influence the rate at which heterotrophic microorganisms decompose phytoplankton detrital particles and thus, it controls the fraction of OC that reaches the ocean depths, where it can be sequestered for climate-relevant spans of time. Here, we compared the degradation dynamics of POM from phytoplankton assemblages of contrasting diatom dominance in the presence of mesopelagic prokaryotic communities during a 19-day degradation experiment. We found that diatom-derived POM exhibited an exponential decay rate approximately three times lower than that derived from a community dominated by flagellated phytoplankton (mainly coccolithophores and nanoflagellates). Additionally, dissolved organic matter (DOM) released during the degradation of diatom particles accumulated over the experiment, whereas only residual increases in DOM were detected during the degradation of non-diatom materials. These results suggest that diatom-dominance enhances the efficiencies of the biological carbon pump and microbial carbon pump through the relatively reduced labilities of diatom particles and of the dissolved materials that arise from their microbial processing.

Zooplankton trophic dynamics drive carbon export efficiency

The flux of detrital particles produced by plankton is an important component of the biological carbon pump. We investigate how food web structure and organisms’ size regulate particulate carbon export efficiency (the fraction of primary production that is exported via detrital particles at a given depth). We use the Nutrient-Unicellular-Multicellular (NUM) mechanistic size-spectrum model of the planktonic community (unicellular pro-tists and copepods), embedded within a 3D model representation of the global ocean circulation. The ecosystem model generates emergent food webs and size distributions of all organisms and detrital particles. Model outputs are compared to field data. We find that strong predation by copepods increases export efficiency, while protist predation reduces it. We find no clear relation between primary production and export efficiency. Temperature indirectly drives carbon export efficiency by affecting the biomass of copepods. High temperatures, combined with nutrient limitation, result in low growth efficiency, smaller trophic transfer to higher trophic levels, and decreased carbon export efficiency. Even though copepods consume a large fraction of the detritus produced, they do not markedly attenuate the particle flux. Our simulations illustrate the complex relation between the planktonic food web and export efficiency, and highlights the central role of zooplankton and their size structure.Plain Language SummaryPlankton are small organisms that live in the ocean. Plankton remove CO2 from the atmosphere by doing photosynthesis and sinking to the deep ocean, where the CO2 is sequestered. Photosynthesis can be measured by satellites, and therefore, knowing the fraction of photosynthesis that sinks to the deep ocean could allow making more accurate predictions of the concentration of CO2 in the atmosphere. This fraction of photosynthesis that is exported is termed “carbon export efficiency”. However, the drivers that define this carbon export efficiency are not well understood. To explore these drivers, we used computer simulations that include many planktonic organisms in a 3D model of the oceans. The model generates a detailed representation of the body sizes of plankton and of particle sizes, which is one of the main features defining sinking rates of particles in nature. We find that export efficiency is high when large zooplankton consume large amounts of prey. Temperature decreases export efficiency by reducing how efficient large plankton grow. Finally, we do not find a clear relation between photosynthesis and export efficiency, which has been much discussed in the literature. This provides mechanistic explanations to previous field observations and generates new hypotheses to be tested.Key Points:We used a 3D size-spectrum model of the planktonic community to understand the drivers of particulate carbon export efficiencyWe find that high temperature decreases growth efficiency, trophic transfer efficiency and associated carbon export efficiency.Systems that are top-down controlled by zooplankton can have high export efficiencies depending on the size of the dominant zooplankton.

Improving the Retrieval of Carbon-Based Phytoplankton Biomass from Satellite Ocean Colour Observations

Phytoplankton is at the base of the marine food web and plays a fundamental role in the global carbon cycle. Ongoing climate change significantly impacts phytoplankton distribution in the ocean. Monitoring phytoplankton is crucial for a full understanding of changes in the marine ecosystem. To observe phytoplankton from space, chlorophyll-a concentration (Chl) has been widely used as a proxy of algal biomass, although it can be impacted by physiology. Therefore, there has been an increasing focus towards estimating phytoplankton biomass in units of carbon (Cphyto). Here, we developed an algorithm to quantify Cphyto from space-based observations that accounts for the spatio-temporal variations of the backscattering coefficient associated with the fraction of detrital particles that do not covary with Chl. The main findings are: (i) a spatial and temporal variation of the detritus component must be accounted for in the Cphyto algorithm; (ii) the refined Cphyto algorithm performs better (relative bias of 23.7%) than any previously existing model; and (iii) our algorithm shows the lowest error in Cphyto across areas where picophytoplankton dominates (relative bias of 14%). In other areas, it is currently not possible to accurately assess the performance of the refined algorithm due to the paucity of in situ carbon data associated with nano- and micro-phytoplankton size classes.

Resilience of plankton trophic structure to an eddy-stimulated diatom bloom in the North Pacific Subtropical Gyre

We investigated the response of an open-ocean plankton food web to a major ecosystem perturbation event, the Hawaiian lee cyclonic eddy Opal, using compound-specific isotopic analyses of amino acids (CSIA-AA) of individual zooplankton taxa. We hypothesized that the massive diatom bloom that characterized Opal would lead to a shorter food chain. Using CSIA-AA, we differentiated trophic position (TP) changes that arose from altered transfers through protistan microzooplankton, versus metazoan carnivory, and assessed the variability at the base of the food web. Contrary to expectation, zooplankton TPs were higher in the eddy than in ambient control waters (up to 0.8 trophic level), particularly for suspension feeders close to the food-web base. Most of the effect was due to increased trophic transfers through protistan consumers, indicating a general shift up, not down, of grazing and remineralization in the microbial food web. Eucalanus sp., which was 15-fold more abundant inside compared to outside of the eddy, was the only taxon observed to be a true herbivore (TP = 2.0), consistent with a high phenylalanine (Phe) δ15N value indicating feeding on nitrate-fueled diatoms in the lower euphotic zone. Oncaea sp., an aggregate-associated copepod, had the largest (1.5) TP difference, and lowest Phe δ15N, suggesting that detrital particles were local hot spots of enhanced microbial activity. Rapid growth rates and trophic flexibility of protistan microzooplankton apparently allow the microbial community to reorganize to bloom perturbations, as microzooplankton remain the primary phytoplankton grazers—despite the dominance of large diatoms—and are heavily preyed on by the mesozooplankton.

Finding Coastal Megaclast Deposits: A Virtual Perspective

Coastal megaclast deposits are dominated by detrital particles larger than 1 m in size. These attract significant attention of modern researchers because of the needs of sedimentary rock nomenclature development and interpretation of storm and tsunami signatures on seashores. If so, finding localities that exhibit coastal megaclast deposits is an important task. Field studies do not offer a quick solution, and, thus, remote sensing tools have to be addressed. The application of the Google Earth Engine has permitted to find four new localities, namely Hondarribia in northern Spain (Biscay Bay), the Ponza Island in Italy (Tyrrhenian Sea), the Wetar Island in eastern Indonesia (Banda Sea), and the Humboldt o Coredo Bay at the Colombia/Panama border (eastern Pacific). In these localities, coastal megaclast deposits consisting of blocks (1–10 m in size) and some megablocks (>10 m in size) are delineated and preliminary described in regard to the dominant size of particles, package density, mode of occurrence, etc. The limitations of such virtual surveys of coastal megaclast deposits are linked to an insufficiently high resolution of satellite images, as well as ‘masking’ effects of vegetation cover and cliff shadows. However, these limitations do not diminish the importance of the Google Earth Engine for finding these deposits. Consideration of some tourism-related information, including photos captured by tourists and bouldering catalogues, facilitates search for promising areas for subsequent virtual surveying of megaclast distribution. It is also established that the Google Earth Engine permits quantitative analysis of composition of coastal megaclast deposits in some areas, as well as to register decade-long dynamics or stability of these deposits, which is important to interpret their origin. The current opportunities for automatic detection of coastal megaclast deposits seem to be restricted.

ANTHROPOGENIC COMPONENT OF ATMOSPHERIC AEROSOL SUBSTANCE OF ZAPORIZHZHIA CITY

The article is devoted to the study of the influence of technogenic processes on the environment, in particular the determination of the contribution of a substance formed in the course of economic activity to the total composition of the solid component of atmospheric aerosol. The actual material was obtained on the basis of the system monitoring of surface air flows within such a powerful industrial center as Zaporizhzhia city. Samples of suspended atmospheric matter accumulated on the filter textile were taken monthly for two years. Parallel continuous observations of hydrometeorological factors (speed, direction, duration of winds, rainfall) provided an integrated approach to the development and synthesis of research results. Field observations and laboratory studies determined a number of patterns in the distribution of the sedimentation substance in the air and the interrelations of the anthropogenic and natural constituents of the substance. The publication describes the features of the chemical composition and morphology of detrital particles, the entry of which into the air caused by technogenic processes, as well as the condensation component of the aeolian suspension, the origin of which is associated with high-temperature processes. The changes in the ratio of the natural / anthropogenic components in the material composition of the aerosol under the influence of external factors (natural and anthropogenic) were investigated. These changes also affect the redistribution of quantitative ratios between the condensed and dispersed components of the aeolian suspension. At the same time, the detrital component can reach the aleuritic dimension, the iron content exceeds 90 %, and the proportion of impurities of a number of ferrous and heavy metals, in particular titanium, manganese, nickel, copper, lead, zinc can reach 40% or more. Heavy metals are also found in the form of separate detrital and condensation formations, where their content varies in the range of 50 – 70 %. Analysis of the condensation component in the aerosol samples for the period of the research allowed us to give a general characteristic of it and to carry out a preliminary classification by chemical composition. The possibility of determining the focus of atmospheric pollution in terms of the shape, chemical composition and size of aerosols, seasonal factors and characteristics of atmospheric flows were also outlined.

Detrital Sr–Nd isotopes, sediment provenances and depositional processes in the Laxmi Basin of the Arabian Sea during the last 800 ka

Abstract87Sr/86Sr ratios and εNd values of detrital particles at International Ocean Discovery Program (IODP) Site U1456 in the Laxmi Basin of the Arabian Sea were measured to trace changes in sediment provenance over glacial–interglacial cycles. Based on the correlation of planktonic foraminiferal (Globigerinoides ruber) δ18O fluctuations with the LR04 stack of benthic foraminifera δ18O values, combined with shipboard biostratigraphic and palaeomagnetic data, the studied interval spans ∼1.2 Ma. Over the past 800 ka, 87Sr/86Sr values ranged from 0.711 to 0.726 while εNd values ranged between −12.5 and −7.3 in the detrital particles. By comparing 87Sr/86Sr ratios and εNd values of the possible sources of river sediments with our data, we found that sediments in the Laxmi Basin were influenced to various degrees by proportions of at least three sediment sources (i.e. Tapi River, Narmada River and Indus River). The Indus River might be a more important contributor to glacial sediments. Although 87Sr/86Sr ratios and εNd values varied quasi-cyclically, this pattern did not correspond precisely to the glacial–interglacial cycles. In particular, low-magnetic-susceptibility (low-MS) intervals coinciding with pelagic carbonates were characterized by low 87Sr/86Sr ratios and high εNd values, whereas high-MS intervals matching turbidite deposits showed high 87Sr/86Sr ratios and low εNd values. Thus, this study reveals that differences in the depositional processes between glacial and interglacial periods, governed by changes in sea level and monsoon activity, are an important factor in deciding 87Sr/86Sr ratios and εNd values of the detrital fraction in the Indus Fan of the Arabian Sea.