Detrital Sources Research Articles

Organic carbon source variability in Arctic bivalves as deduced from the compound specific carbon isotopic composition of amino acids

In this study we used compound-specific carbon isotope analysis of amino acids (δ13CAA) to determine organic carbon sources utilized by the dominant benthic bivalve species collected along a latitudinal gradient in the northern Bering and Chukchi Seas, specifically at productivity hotspots identified within the Distributed Biological Observatory (DBO) program, and over Hanna Shoal in the northern Chukchi Sea. The recent shift to earlier sea-ice melt is one of the climate change consequences influencing Pacific Arctic ecosystems, which we integrate within our observations. Our goals included investigating the utilization of organic matter resources by several dominant Arctic bivalves and their trophic elasticity to changes in primary productivity patterns following changes in the onset of the annual productive season. Based upon δ13CAA patterns observed, these species utilized different carbon sources along the latitudinal gradient, including a strong input of bacterially reworked material and microalgae, mainly in particulate organic matter mixtures. Species type and the sampling location both played roles in δ13CAA variability, suggesting the influence of local production and decomposition processes. Macoma calcarea and Ennucula tenuis were shown to utilize organic matter of different quality, suggesting they may switch their feeding preferences to more detrital sources on a seasonal basis, but this was also affected by geographical location. These observations may have important implications for the benthic populations as microbial reworking of organic material is expected to increase with climate warming and likely shifts in food web structure.

Tectonic evolution of an Early Cryogenian late- magmatic basin in central Madagascar

Central and southern Madagascar comprise a number of distinctive Archaean crustal blocks (the Antongil-Masora and Antananarivo domains) overlain by Proterozoic supracrustal sequences, preserved in the East African Orogen. Here, we present U–Pb and Lu–Hf isotopic data for two supracrustal units from detrital and metamorphic zircon grains. The lower sequence is comprised of quartzite and calc-silicate units with a major Palaeoproterozoic detrital zircon source and a minor Archaean contribution with a maximum depositional age of ca. 1780 Ma. This sequence reflects a stable shelf sedimentation within the Antananarivo Domain and is correlated with the Itremo Group. U–Pb and Hf data are equivocal in determining the direct sources for the Archaean and early Palaeoproterozoic detrital zircon grains. However, the abundant ca. 2.3–1.8 Ga detrital grains are correlative with the Congo-Tanzania-Bangweulu Block and as these are close to the inferred age of the Itremo Basin, these are interpreted to be single cycle detritus. This implies that the Congo-Tanzania-Bangweulu craton was close to central Madagascar at ca. 1.8–1.6 Ga and the lower sequence would correspond to an originally contiguous late Palaeoproterozoic to early Mesoproterozoic sedimentary basin across central Madagascar. The upper metasedimentary unit has contrasting detrital sources and is represented mostly by biotite-plagioclase paragneiss, with an inferred psammitic protolith interleaved with volcanic/subvolcanic andesitic/rhyolitic dikes. The predominant Tonian-aged population (ca. 860-710 Ma) are igneous zircon grains with εHf(t) values varying from −15.1 to −29.2 and TDM Hf model ages between ca. 3.4 and 2.6 Ga. These grains were derived from the ca. 850-750 Ma Imorona-Itsindro magmatic suite. Their Neoarchaean-Palaeoproterozoic cores are interpreted as xenocrysts, reinforcing that the Imorona-Itsindro magmatism has a prominent continental reworking component. The probable tectonic setting for this Early Cryogenian sedimentary basin would represent a transition from an intra-arc to an intracontinental setting related to an outboard subduction, partially jammed at ca. 710 Ma due to the subduction of a ridge-transform system. The analogue would be the western US, where the Basin and Range region corresponds to a wide rift associated with a major mantle thermal anomaly. The absence of geological units and structures between ca. 720 and 635 Ma in central Madagascar corroborate with this model for a transition to a transform continental setting. The pre-Gondwana amalgamation convergence in the Ediacaran-Cambrian, that deformed and metamorphosed all units in central Madagascar units, is accounted for by ca. 550 Ma metamorphic rims on zircon grains from the quartzites in the Itremo Group.

Sediment provenance and tidal current-driven recycling of Yellow River detritus in the Bohai Sea, China

A detailed sediment-provenance study is presented on the modern Bohai Sea, China, the innermost gulf of the Yellow Sea, using framework petrography, heavy-mineral and grain-size analyses. A series of subaqueous river deltas, characteristic of multisource, is distinguished along the coastal zone. First-cycle fluviodeltaic sediments are attributed principally to three large source areas: the Loess Plateau, the Yanshan highlands, and the Horqin sandland. Coastal sands along eastern Liaodong Bay are produced by weathering and physical erosion of near-source Mesozoic granitoid rocks during Quaternary lowstands. Apart from subaqueous deltas and coastal sands, sediment provenance is only attributed to two end-member mixing with various proportions between recycled Holocene Yellow River sediments and ancient fluvial sediments of the Liaohe river system, even though detrital sources are multiple along the coast. Relative contributions are quantified on a ternary diagram of heavy-mineral compositions, using the proportion of relative distances between a given mixed area and the two end-member deltas. The eastern Bohai flood-tidal delta, characterized by abundant garnet and ilmenite, is regarded as a large repository of ancient Liaohe sediments, which had been intermittently accumulated during Quaternary lowstands and reworked during multiple transgressions. As a secondary sediment source detached from the present-day Liaohe River delta, it is remobilized and involved in the modern sedimentation. A complete tidal current-dominated depositional pattern is proposed throughout the Bohai Sea to explain sediment accumulation and recycling. We suggest that dominant ebb tidal currents, prevailing in the southern Bohai Strait, are a primary driver carrying fine-grained Yellow River sediments eastward to the Yellow Sea. In contrast, the recycling of Yellow River sediments is driven by flood tidal currents, prevailing in the northern Bohai Strait. The internal recycling process in the Bohai Sea builds the sand sheet and its terminal lobe, i.e., the eastern Bohai mud belt, whereas the external recycling process that involves the depocenters of Yellow River sediments in the outside North Yellow Sea builds the sand ridges and its terminal lobe, i.e., the quartz-rich area. This study improves our understanding on the Yellow River dispersal system. It also provides a typical example that demonstrates how ancient alluvial sediments accumulated during eustatic lowstands are reactivated and join the modern sedimentation processes.

Contrasting fatty acids with other indicators to assess nutritional status of suspended particulate organic matter in a turbid estuary

Achieving a precise nutritional characterization of suspended particulate organic matter (SPOM) is key for understanding the efficiency of bottom-up trophic fluxes in estuaries. Assessing the nutritional status of SPOM in turbid estuaries is challenging due to the complex mixing and degradation dynamics of the autotrophic, heterotrophic and detrital sources configuring SPOM. Using a recently described index based on fatty acids (FA), SPOM nutritional value was quantitatively characterized in the Guadalquivir River Estuary (GRE). Over a two-year study, this FA-based nutritional index (FANI) discerned changes in the nutritional value of SPOM to a greater extent than other nutritional indices. Spatiotemporal results of FANI in the GRE indicated SPOM nutritional quality was 2.2-fold higher in the 20–35 salinity range respecting to the 0–5 salinity range and that SPOM in summer was 1.5-fold nutritionally richer than in winter. A mixing model using bacterial FA estimated a maximum 40–60% range of variation for a complementary contribution of autotrophs and heterotrophs to microbial biomass and was in agreement with results of carbon-based estimates of phytoplankton and total microbes. In the spatiotemporal scale, and referred to particulate organic carbon (POC), phytoplankton represented from 9.3% (winter) to 31.0% (summer), and from 15.0% in lower salinity (0–20) to 24.5% in higher salinity reaches (20–35). Phytoplankton was positively correlated with SPOM nutritional value. Specific FA markers indicated a low contribution of land plants to POC. The lipid-based approach here described can improve understanding of the nutritional status of basal food resources in turbid estuaries.

Provenance of Neogene sandstones in western Taiwan traced with garnet geochemistry and zircon geochronology

AbstractIn this study, we use provenance analysis to shed light on the Neogene evolution of river drainage and sediment dispersal in eastern Asia as a consequence of the Himalayan orogeny and topographic rise of the Tibetan Plateau. As an attempt to envisage how sediment‐routing systems operated in the past, and to identify the ultimate source of siliciclastic detritus originally generated in mainland China and now incorporated in, and recycled from, the western Taiwan accretionary prism, we have used Raman spectroscopy of detrital garnet in modern river sands and upper Neogene sandstones, complemented by detrital‐zircon geochronology and by petrographic and heavy‐mineral analysis. Garnet grains are relatively abundant and largely represented by almandine with various percentages of pyrope molecules in Yangtze sand as in western Taiwan sandstones and modern river sand derived from them. Instead, they are rare and largely represented by grossular or andradite in the sand of the Pearl River and of coastal rivers of SE China facing the Taiwan Strait. This finding supports the result of detrital geochronology, which documents a sharp difference between U‐Pb age spectra of Pearl River and coastal SE China zircons (characterised by a Paleozoic‐Mesozoic triple peak) and Yangtze and western Taiwan zircons (characterised by prominent Neoproterozoic, Paleoproterozoic, and latest Archean clusters). The Pearl River and minor rivers facing the Taiwan Strait are thus ruled out as ultimate sources of upper Neogene sandstones exposed today along the western front of the Taiwan fold‐thrust belt. In the envisaged scenario, sand mostly supplied by the paleo‐Yangtze River was entrained for ~ca.1,000 km southward by longshore currents and deposited on the Chinese passive margin before being accreted along the front of the Taiwan orogen since ~6 Ma. Littoral sand drift represents a major factor of long‐distance sediment transport in modern shelves and needs to be taken into full account in source‐to‐sink studies and paleogeographic reconstructions.

Ultrahigh-temperature granulites from the southern margin of the Madurai Block, India: Petrology, metamorphic phase equilibria and zircon-monazite U–Pb geochronology

The Madurai Block is the largest crustal block in the Southern Granulite Terrane of India, and is composed of a dominantly Neoarchean northern segment, Paleoproterozoic central segment, and dominantly Neoproterozoic southern segment. Here we investigate garnet – orthopyroxene – spinel – cordierite – plagioclase – K-feldspar – quartz bearing granulites from the southern margin of the Madurai Block. We present results from petrology, mineral chemistry, pseudosection modeling, geothermobarometry, and U–Pb geochronology of zircon and monazite. The prograde minerals assemblages comprise Grt + Pl + Qtz + Mt. + Ilm + Sill which define pressure-temperature (P-T) conditions of 7.2–7.8 kbar and 900–980 °C. The peak minerals assemblages of Grt + Pl + Opx + Crd + Qtz + Mt. + Ilm yield P-T conditions of 5.8–6.5 kbar and 980–1030 °C. Subsequent cooling and decompression are inferred by the break down of garnet at 2–4 kbar and < 850 °C. Detrital zircon grains in the garnet-orthopyroxene-cordierite- bearing granulite samples show similar age distributions and can be divided into magmatic cores (1047–600 Ma with age peaks at 869 Ma and 609 Ma) and metamorphic overgrowths (623–511 Ma), representing the early Neoproterozoic detrital source and Late Neoproterozoic - Cambrian ultrahigh-temperature metamorphism. Monazite shows a prominent age peak at 543 Ma with a few early Neoproterozoic detrital grains that show Pb loss associated with the Cambrian metamorphic event. The Cambrian ultrahigh-temperature metamorphism as recorded in our samples is similar to the reports from several other localities in the Southern Granulite Terrane and coincides with the tectonics associated with the final assembly and the Gondwana supercontinent. The common overprinting of the Cambrian UHT event at the junction of assembly of the East Gondwana fragments suggest mantle upwelling beneath this wide region, and possible heat input through underplated magmas.

A combined geochemical, Nd, and stable Ca isotopic investigation of provenance, paleo-depositional setting and sub-basin connectivity of the Proterozoic Vindhyan Basin, India

Geochemical and isotopic investigation of Proterozoic clastic and chemical sedimentary rocks provide insights into surface conditions of the early Earth. The mid-Proterozoic Vindhyan Basin of central India, comprising ~5 km thick, mostly undeformed and unmetamorphosed clastic and chemical sediments, is one such archive which is exposed in two sub-basins, the Son Valley Vindhyan sub-basin in the east (SVV) and Chambal Valley Vindhyan sub-basin in the west (CVV). Geochemical and Nd isotopic compositions of siliciclastic sedimentary rocks from the CVV sub-basin are reported in order to understand the provenance of the Vindhyan sediments in this sub-basin and the results are compared with the provenance of the sediments exposed in the SVV sub-basin to evaluate whether the two sub-basins had shared a common provenance. Elemental ratios, such as Th/Co, La/Sc suggest that the clastic sedimentary rocks from the CVV have dominantly upper crustal silicic/felsic provenances and are broadly similar to those of equivalent stratigraphic horizons from the SVV. Initial Nd isotopic composition (εNd(t)) of the siliciclastic rocks from the Lower Vindhyans of the CVV, calculated at their respective times of deposition, suggest sediment derivation from the Banded Gneissic Complex, Berach Granitoids, 1.82 Ga continental arc granitoids of the Aravalli Range and the Hindoli Volcanics. In contrast, the εNd(t) values of the Upper Vindhyan siliciclastics suggest that the Delhi Belt granitoids were the primary source of detritus during the deposition of these sediments. Paleo-depositional environment and connectivity between the SVV and CVV sub-basins are evaluated using geochemical, radiogenic Nd, and stable Ca isotopic compositions of carbonates from both sub-basins. Non-redox sensitive REY anomalies and Y/Ho ratios of carbonates from the two sub-basins suggest deposition in an epeiric sea with limited connection to the open ocean. Neodymium isotopic compositions of the carbonate units suggest that the two sub-basins were connected during the deposition of both Lower and Upper Vindhyan sediments. Carbonates from both sub-basins record a significant shift towards more radiogenic Nd isotopic compositions around 1.6 Ga, possibly as a result of an Andean-type arc magmatism in near vicinity of the Son Valley sub-basin. The δ44/40Ca compositions of the SVV carbonates are typically lower than the CVV carbonates. This difference is likely due to greater freshwater input in the SVV sub-basin which is consistent with the existence of paleo-gradients in the Vindhyan Basin.

High-Resolution Environmental Magnetism Using the Quantum Diamond Microscope (QDM): Application to a Tropical Speleothem

Quantum diamond microscope (QDM) magnetic field imaging is a recently developed technique capable of mapping magnetic field sources in geologic samples at 1 micrometer resolution. Applying QDM imaging to speleothems can provide high‐resolution time series of detrital input into the cave environment, which, in turn, can yield useful paleoenvironmental information. Here we map the magnetic field over a speleothem from midwest Brazil over a 174 year timespan with annual to sub-annual resolution and perform backfield remanence acquisition experiments to quantify changes in the magnetic grain population through time. We find that magnetic particles occur in highly enriched layers of 10–100 µm thickness that sample the same detrital source population. Combined with petrographic observations and electron microprobe mapping of Mg and Ca, we conclude that detrital enrichment in our sample is caused by drier conditions leading to slow or halted speleothem growth. This interpretation is compatible with oxygen isotopic data and implies that speleothem magnetism can be used to infer the past occurrence of drought and potentially quantify their duration. Future high-resolution magnetic imaging of speleothems may provide additional insight into the mechanism of detrital enrichment and establish their role as a proxy for local moisture and infiltration.

Source of Detritus in Subducted Turbidites, Tectonic Mélange, Port Macquarie Block, Southern New England Orogen, Australia—A Geochemical Perspective

A tectonic mélange containing blueschists and eclogites, Middle Ordovician mid-ocean ridge basalt, cherts, and clastic sediments occurs at Port Macquarie in the southern New England Orogen, Australia. The clastics are quartz-poor and are dominated by mafic volcanic and fragmented plagioclase clasts; felsic volcanic clasts are less common. They exhibit erosional bases, graded bedding, load structures, and lithologies ranging from laminated mudstones to pebbly sandstones. Based on these features, they are interpreted as turbidites. During subduction, these turbidites were deformed and metamorphosed under prehnite–pumpellyite and lower greenschist facies conditions. Geochemically, they have a calc-alkaline, intra-oceanic arc signature; show no recycling; and have been derived from a provenance dominated by mafic volcanic rocks of basaltic-andesite composition. Further, chemical index of alteration (44–69) and Index of Compositional Variability (0.8–2.4) data reveal they show little weathering and are immature. The lack of weathering of rocks in a location where tropical climatic conditions existed is attributed to extreme erosion associated with a dynamic setting resulting in rapid transportation of the sediments to the fore arc basin and subsequently to the trench. As a consequence, little time was available for weathering to take place. The detritus in the turbidites is thought to have been derived from Late Ordovician volcanics in the Macquarie Arc and fore arc basin sequences of the Murrawong Formation. The cherts with which they are associated record both a continental and oceanic arc geochemical signature.

GEOCHEMISTRY OF MAIGANGA COAL IN GOMBE STATE, UPPER BENUE TROUGH, NORTHEASTERN NIGERIA

Major and trace element composition of the Maiganga coal in the Gombe sandstone of the Upper Benue trough was carried out with the aim of determining their source, paleoweathering/climatic conditions, tectonic setting and depositional environment. Nine (9) of the coal samples and two (2) of the non-coal samples were selected for analysis of major and trace elements. The combined Inductively Coupled Plasma- Mass Spectrometry (ICP-MS) and Inductively Coupled Plasma Atomic Emission Spectrometry (ICP-AES)were used to analyze the samples. The thickness of the Maiganga Coal seams vary from 0.5 – 4.2 m with a lateral extent of 750 m. Analysis of the results through bivariate plots, pearson correlation and cluster analysis shows that there is a strong correlation between Na2O, K2O and TiO2 with SiO2 and Al2O3 and ash yield indicate a common detrital source while others with weak or negative correlations, are believed to be autothochnous. Similarly, trace elements such as Cu, Pb, U, Th, Sb, Bi, V, Cr, W, Sn, Sr, and Sc showed strong correlations with Al2O3 and with each other, indicating that they were derived syngenetically or epigenetically from the same detrital input to the precursor swamp. Those elements that showed negative or weak correlations like Mo, Ni, As, Cd, Co, Cu, Zn, Be, Zr and Tl may have been authigenic in origin.The trace element concentrations of the Maiganga coal falls within the range for world coals and the exceptionally low concentrations of Pb, Be, As, and U makes it less of a threat to the environment and health thereby improving its quality. The discriminant plot on the basis of oxide of Al2O3 and TiO2 revealed a provenance of intermediate composition with little input from mafic source which is also supported by the plots of Co/Th against La/Sc and Cr/Th against Sc/Th.Weathering processes at the source as determined by CIA and PIA values both suggest a dominantly high weathering in a humid environment. Low U/Th, Ni/Co, Th/U, Sr/Ba ratios and Ni/Co vs V/Cr plot suggest that the coals were deposited under oxic and suboxic conditions reductive littoral to brackish swamp environment during deposition. In addition, low Mo, U and {(CaO+MgO+Fe2O3)/(SiO2+Al2O3)} ratio indicates a deltaic depositional environment for the Maiganga coal.They were formed in a passive- margin tectonic setting.

Elemental and isotopic compositions of trench-slope black shales, Bohemian Massif, with implications for oceanic and atmospheric oxygenation in early Cambrian

This study examines a lower Cambrian pyrite-bearing black shale–graywacke succession in the Czech Republic interpreted as infill of a deep-marine, extensional trench-slope basin on top of an accretionary wedge that developed during subduction of an oceanic plate beneath the northern margin of Gondwana. The new U–Pb detrital zircon geochronology of the graywacke constrains the maximum age of deposition to 533 +5/−6 Ma, whereas the Re–Os dating of pyrite yielded a younger age of 507 ± 16 Ma. Distribution of major and trace elements and calculated enrichment factors (EF) indicate that this succession was deposited under changing redox conditions over a short time span and that the basin was presumably controlled by tectonic subsidence and varying supply of terrigenous arc-derived material. This depositional setting is reflected by largely variable EFs and δ98Mo and δ53Cr values. Euxinic conditions detected at the base of the sampled black shale interval are documented by the highest values of EFs of redox-sensitive metals (e.g., Mo, U, V, Ni, Co, As) and also the Corg/P (>1000) and DOPT (>0.7) values. Black shales show lower δ56Fe values due to the excess of authigenic pyrite-Fe with a mean δ56Fe value of −0.02‰ over detrital Fe in graywacke with a mean δ56Fe value of +0.25‰. However, the Fe isotopic signatures of the black shales are not consistent with iron shuttling, mixing of authigenic and detrital sources, or hydrothermal metal enrichment. Instead, they most likely resulted from partial oxidation of pyrite through the syndepositional oxidizing hydrothermal fluids (Si–Ba enrichment), which resulted in precipitation of isotopically heavy Fe-oxyhydroxides. We propose that our maximal recorded δ98Mo value (+ 0.98‰) might represent the best estimate for the ancient local seawater Mo composition at around 533 Ma and argues against deep-ocean oxygenation in the early Cambrian. On the other hand, the Se/Co ratios of synsedimentary pyrite indicate a mean atmosphere O2 value of ~27%.

Detrital Subsidies in the Diet of Mytilus edulis; Macroalgal Detritus Likely Supplements Essential Fatty Acids

Detritus is a frequent, poorly defined, component of bivalve growth and carrying capacity models. The purpose of this study was to determine the proportional contributions of detrital material derived from primary producers (phytoplankton, macroalgae, Spartina alterniflora, and terrestrial leaf litter) to particulate organic matter (POM) and blue mussel’s (Mytilus edulis) diet within a temperate bay (Saco Bay, ME, United States). We assessed which detrital sources, if any, warranted incorporation into modeling efforts. Stable isotopes (δ13C and δ15N) and fatty acid biomarkers (FA) of mussels, size fractionated (&amp;lt;100 μm) POM, and primary producer endmembers (phytoplankton, Saccharina latissima, Ascophyllum nodosum, Chondrus crispus, Spartina alterniflora and leaf litter) collected between 2016 and 2017 were used to estimate endmember contributions to POM and mussel diets. Based on FAs dinoflagellates were the most abundant phytoplankton in Saco Bay, even during the fall diatom bloom. Diatoms within the bay were primarily centric, but pennate diatoms were at times present in the water column (e.g., in September). Following abundances of dinoflagellates, and centric and pennate diatoms, 22:6ω3 (DHA) was the most abundant essential FA (8.6 ± 0.1% total FAs), followed by 20:5ω3 (EPA: 7.0 ± 0.1%) and 20:4ω6 (ARA: 0.3 ± 0.1%). On average, phytoplankton derived organic matter contributed 22.1 ± 0.3% of the total POM in the bay. The concentration of non-fresh phytoplankton organic matter, or remaining organic matter (REMORG), was positively correlated with all endmember biomarkers. However, the proportion (%) of vascular plant, macroalgal, and detrital FAs was negatively correlated with the concentration of REMORG. This finding suggests in periods of low productivity, vascular plant and macroalgal detritus are proportionally more important contributors to POM. Mussels were broad spectrum omnivores, consuming phytoplankton, zooplankton, and detrital material. Detrital contributions to mussel diets were important (minimum of 16% of diet). Although small, macroalgae’s dietary contribution (8%) to M. edulis may be important. Macroalgal detritus contained essential FAs (20:5ω3 and 20:4ω6) that could supplement mussel diets, as M. edulis in Saco Bay were likely limited by 20:5ω3. Consideration of how macroalgal detritus affects the availability of essential FAs in POM may be useful to incorporate into aquaculture site selection.

Neodymium isotope composition of Palaeoproterozoic Birimian shales from the Wa‐Lawra Belt, north‐west Ghana: Constraints on provenance

An Nd‐isotopic study was carried out on Palaeoproterozoic Birimian shales from the Wa‐Lawra volcanic belt, located in the southeastern margin of the West African Craton. The objective of this study was to assess the nature and average crustal residence ages of the source areas, and deduce the provenance of these shales. Nd‐model ages range from 2.12 to 2.58 Ga, which are within the range of Nd‐model ages reported for the Birimian volcanic rocks, suggesting that the Birimian volcanic rocks are the main sources of detritus for the Birimian shales. The εNd values of the Birimian shales, calculated at an age of 2.1 Ga, which represents the time of the deposition of the Birimian shales, ranges from −1.65 to +3.01. The range of observed εNd values (i.e., −1.65 to +3.01) suggest that the Birimian shales were predominantly derived from the juvenile Palaeoproterozoic crust, with minor contribution of older crustal material. εNd(T) versus Th/Sc, and ƒSm/Nd versus εNd(T) systematics indicate that the provenance can be explained by mixing between Palaeoproterozoic volcanic arc rocks and variable amounts of Archean continental crustal rocks. Provenance modelling shows that the Birimian shales received at least 80% detritus from a juvenile Birimian volcanic arc. The mean Archean component is 8%, suggesting that the continental crust of the Wa‐Lawra Belt is clearly dominated by material derived directly from the mantle.

Regional geochemical variations in a metamorphosed black shale: a reconnaissance study of the Silurian Smalls Falls Formation, Maine, USA

A reconnaissance geochemical study of 21 samples of sulphidic black phyllite and schist from the Silurian Smalls Falls Formation in Maine was undertaken in order to evaluate compositional changes during regional metamorphism. These samples represent variably metamorphosed black shale. Analyzed samples come from the chlorite zone in northern Maine and the biotite, garnet, and staurolite-andalusite zones in west-central Maine. Strata of the Smalls Falls Formation are distinctive in containing abundant pyrite and/or pyrrhotite (total S = 1.2–9.7 wt%), but only minor organic matter or graphite (TOC = 0.43–1.85 wt%); TOC/S ratios are uniformly low (average = 0.37 ± 0.22). Median enrichment factors were calculated for each element by normalizing the concentration to Ti in each sample to the Ti-normalized median composition of global black shale. In the chlorite zone, moderate to large decreases in enrichment factors (-23.1 to -49.8%) are evident for V, Cr, Cu, Ni, Zn, Pb, Sb, and U, attributed here to various factors during sedimentation plus variable element mobility during diagenesis. With increasing metamorphic grade (biotite through staurolite-andalusite zones), systematic small to extreme decreases (-14.5 to -99.0%) were found for Ba, Sb, Au, and U, together with less-systematic moderate to large decreases (-35.4 to -61.1%) for V and As. Molybdenum shows an extreme decrease (-94.7%) from the garnet to staurolite-andalusite zones. Excluding Ba, these results are interpreted to mainly reflect mobility of trace elements during pyrite recrystallization, and during the metamorphic transformations of organic matter to graphite and of pyrite to pyrrhotite. Moderate to large increases for Rb (+28.1 to +61.5%) and Th (+39.1 to +47.3%) from the biotite to staurolite-andalusite zones likely record the introduction of alkalis and mass loss, respectively, during metamorphism. Three samples from one site in the garnet zone differ in having anomalously high Fe/Al and low La/Yb ratios, attributed here to epigenetic formation of pyrite and related leaching of light rare earth elements during syn-metamorphic, channelized fluid flow.Geologic and geochemical data indicate that strata of the Smalls Falls Formation were deposited during an interval of anoxia on the northwestern flank of the Central Maine Basin, for which detrital sources included an evolved continental arc. Onset of anoxia coincided with deposition of the Mayflower Hill Formation of the Vassalboro Group, on the basin’s southeastern flank, related to emergence of the Brunswick subduction complex. We suggest that this emergence played a role in promoting both lateral and vertical circulation changes, nutrient loading, and deoxygenation through subsequent basin closure that culminated with Acadian deformation and metamorphism. Based on the relatively high contents of total sulphur present in our Smalls Falls samples, sediments in the Black Sea represent the only known plausible candidate among those in modern suboxic to euxinic basins.

Seasonal patterns of fine root dynamics and their contribution to net primary production in hinoki cypress (Chamaecyparis obtusa) and konara oak (Quercus serrata) forests

Key messageFine root and litterfall are major contributor of NPP and fine root production may reflect forest productivity in a warm-temperate forest in Japan.Forest ecosystems play an important role as the major carbon sink on land, with fine root dynamics and litterfall representing major carbon fluxes. The objectives of this research were to estimate NPP including annual fine root production values, to investigate fine root dynamics and the relationships between above– and belowground organs in konara oak (Quercus serrata) and hinoki cypress (Chamaecyparis obtusa) forests. Litterfall was collected seasonally for 1 year from June 2013. The ingrowth core method and the sequential soil core method were applied with a root litterbag experiment to estimate fine root (< 2 mm) production (FRP), mortality (FRM), and decomposition (FRD) for 1 year (from 2013 to 2014), using the continuous inflow estimate method and the simplified decision matrix. The total NPP ranged from 8.2 to 13.9 (t ha− 1 yr− 1), and the sum of aboveground litterfall and FRP accounted for 60% of the total NPP on average, confirming the significance of above- and belowground litter for the forest NPP as a source of detritus for the decomposer system. In hinoki cypress stand, fine root biomass peaked in the end of winter while fine root necromass showed the highest peak in late summer. In konara oak stand, only very fine root (< 0.05 mm) biomass and necromass demonstrated significant seasonal patterns. The seasonal patterns of fine root production did not differ between forest types and root diameter classes. We found a possible relationship between above- and belowground production and fine root production tended to be high in productive forests. This study improves our understanding of different patterns of carbon dynamics between temperate broadleaved and coniferous forest ecosystems.

Detrital food web contributes to aquatic ecosystem productivity and rapid salmon growth in a managed floodplain.

Similar to many large river valleys globally, the Sacramento River Valley has been extensively drained and leveed, hydrologically divorcing river channels from most floodplains. Today, the former floodplain is extensively managed for agriculture. Lack of access to inundated floodplains is recognized as a significant contributing factor in the decline of native Chinook Salmon (Oncorhynchus tshawytscha). We observed differences in salmon growth rate, invertebrate density, and carbon source in food webs from three aquatic habitat types—leveed river channels, perennial drainage canals in the floodplain, and agricultural floodplain wetlands. Over 23 days (17 February to 11 March, 2016) food web structure and juvenile Chinook Salmon growth rates were studied within the three aquatic habitat types. Zooplankton densities on the floodplain wetland were 53x more abundant, on average, than in the river. Juvenile Chinook Salmon raised on the floodplain wetland grew at 0.92 mm/day, 5x faster than fish raised in the adjacent river habitat (0.18 mm/day). Two aquatic-ecosystem modeling methods were used to partition the sources of carbon (detrital or photosynthetic) within the different habitats. Both modeling approaches found that carbon in the floodplain wetland food web was sourced primarily from detrital sources through heterotrophic pathways, while carbon in the river was primarily photosynthetic and sourced from in situ autotrophic production. Hydrologic conditions typifying the ephemerally inundated floodplain—shallower depths, warmer water, longer water residence times and predominantly detrital carbon sources compared to deeper, colder, swifter water and a predominantly algal-based carbon source in the adjacent river channel—appear to facilitate the dramatically higher rates of food web production observed in the floodplain. These results suggest that hydrologic patterns associated with seasonal flooding facilitate river food webs to access floodplain carbon sources that contribute to highly productive heterotrophic energy pathways important to the production of fisheries resources.

Middle Tonian calc-alkaline picrites, basalts, and basaltic andesites from the Jiangnan Orogen: Evidence for rear-arc magmatism

The chemical composition of a basalt is a critical indicator of mantle thermal condition and tectonic setting. The Yangtze Block, which is a part of the Rodinia supercontinent, formed during the Neoproterozoic era presents an enigmatic thermal state of the mantle and tectonic evolution. We compiled a large database of petrogeochemical data on the Middle Tonian volcanic rocks covering the entire Jiangnan Orogen with a view to evaluate the evolution of the magma compute the composition of the primary magma, estimate the mantle potential temperature (Tp), and distinguish the tectonic setting. Calc-alkaline picrites, basalts, and basaltic andesites (type A) from ca. 845–825 Ma and tholeiitic basalts (type B) from ca. 860–830 Ma were both identified as interbedded layers accompanied with the Middle Tonian sedimentary sequences (the Fanjingshan Group and its equivalents) and geographically intersecting with each other along the Jiangnan Orogen. Geochemical and isotopic features of type A volcanic rocks indicate that they originated from an enriched mantle source. Both slab-derived fluids and fractionation of olivine and clinopyroxene played important roles during magma generation and evolution. The calculated composition of the primary magma is picritic and the estimated average Tp is 1404 °C. This temperature is consistent with the melting of a mantle wedge due to a rapid slab roll-back process, but it is lower by 166 °C than that produced by a hot mantle plume. Considering the occurrence of the continental arc basalts (CAB)-like type A volcanic rocks and the back-arc basin basalts (BABB)-like type B basalts along with contemporaneous sedimentary sequences with the bidirectional detritus sources in the Jiangnan Orogen, we hereby speculate a rear-arc setting for type A volcanic rocks and a back-arc setting for Type B basalts.

Preliminary U-Pb Detrital Zircon Ages from Tufiti di Tusa Formation (Lucanian Apennines, Southern Italy): Evidence of Rupelian Volcaniclastic Supply

U-Pb spot ages have been determined on detrital zircons from two samples of volcaniclastic arenites belonging to the Tufiti di Tusa Formation (TTF) outcropping in the Lucanian Apennines (Southern Italy). Many petrographic and geochemical studies have been performed on these sandstones with the aim of defining their detritus source. A new and precise evaluation of the mineralogical composition of metamorphic lithic fragments, together with U-Pb detrital zircon ages, helps to clarify the deposition age of these syn-sedimentary volcaniclastic sandstones and constrains their source areas. Volcaniclastic arenites consist of andesitic fragments and single minerals of plagioclases, clinopyroxenes, and hornblendes, while the metamorphic lithics are mainly fragments of blue amphibole-bearing micaschists, serpentinites, ophicalcites, phyllites, and medium-grade micaschists. Phaneritic plutonic fragments consist of quartz, feldspar, and micas. Carbonate components include biomicritic and biosparitic fragments. Eighty age data collected from 56 zircons reveal a wide age spectrum, ranging from Neoarchean to Rupelian (from 2712 ± 25 to 30 ± 1 Millions of years (Ma)). The age data show that in volcaniclastic sandstones there is evidence of ancient crystalline basements involved in Cadomian and Variscan orogenesis (ages from 2712 ± 25 Ma to ≈260 Ma), whereas the measured ages of 157 Ma testify the events of Pangea fragmentation and the ages between 78 and 67 Ma are related to subduction metamorphism connected to the Alpine orogenesis. Fifty percent of the estimated detrital zircon ages show a mean concordant age of 33 ± 1 Ma, they have been measured on idiomorphic crystals with undisturbed magmatic oscillatory zoning. These data reveal the true sedimentation age of Tufiti di Tusa sandstones at least at the sampled levels, coeval with that recorded in other Rupelian volcaniclastic successions outcropping in the Northern Apennines and in the Western Alps (e.g., Val d’Aveto-Petrignacola Formation, Ranzano Formation and Taveyannaz Sandstone). These data represent preliminary suggestions of the sedimentation age of the Tufiti di Tusa Formation and Rupelian paleogeography in the Western Mediterranean area. During Priabonian-Rupelian times, the volcanic calc-alkaline detritus linked to a widespread syn-sedimentary igneous activity on the hinterland terranes of the foredeep basins in the Apennine-Maghrebian orogen extended from north to south in the Western-Central Mediterranean area.

Major and trace element characterization of Oceanic Anoxic Event 1d (OAE 1d): Insight from the Umbria-Marche Basin, central Italy

The Pialli Level in the Umbria-Marche Basin (central Italy) correlates with the lowermost part of the positive carbon isotopic excursion characterizing the late Albian–early Cenomanian Oceanic Anoxic Event 1d (OAE 1d). High-resolution litho-, bio- and chemostratigraphic data from the Monte Petrano and Le Brecce sections allow for a bed-by-bed comparison of the two successions, and discriminate local from basin-scale signals. We present new X-ray fluorescence, ICP-MS and TOC data for both the limestones and the Pialli Level shales integrated with available carbonate carbon isotopes and nannofossil temperature and nutrient indices. Data indicate a homogenous background sedimentation dominated by pelagic carbonates, biogenic silica with little contribution by clays. The limited variation in lithogenic elements points to an essentially homogeneous detrital source area with limited fluvial terrigenous input. Higher Mn concentrations coupled with low enrichments in redox-sensitive elements, such as U, Fe, S, Re, Mo, Ag, suggest that the Pialli Level shales represent temporary suboxia without reaching anoxia. Furthermore, P, authigenic Ba, Cd and Ni enrichments, together with nannofossil nutrient index indicate generally low primary productivity conditions along the entire succession, with only minor increases for some of the black-to-dark grey shales of the Pialli Level. The nannofossil temperature index highlights a warm climate during the OAE 1d, with the warmest conditions experienced during the deposition of the Pialli Level shales. During the late Albian, the warm and humid climate was interrupted by brief episodes of relatively warmer and less saline surface waters ensuring slower rates of bottom water renewal and producing temporary suboxic conditions. Such paleoceanographic dynamics would be the continuation of episodic warmer and humid pulses characterizing the late Albian interval in the Umbria-Marche Basin. As such, the Pialli Level can be considered the result of a last episode closing a cycle before the establishment of a steadier climate during the early Cenomanian.

Revealing exhumation of the central Alps during the Early Oligocene by detrital zircon U\u2013Pb age and fission-track double dating in the Taveyannaz Formation

The late Eocene-to-early Oligocene Taveyannaz Formation is a turbidite series deposited in the Northern Alpine Foreland Basin (close to the Alpine orogenic front). Double dating of zircons with the fission-track and the U–Pb methods is applied on samples from the Taveyannaz Formation to reconstruct the exhumation history of the Central-Western Alps and to understand the syn-collisional magmatism along the Periadriatic lineament. Three samples from this unit show similar detrital zircon fission-track age populations that center at: 33–40 Ma (20%); 69–92 Ma (30–40%); and 138–239 Ma (40–50%). The youngest population contains both syn-volcanic and basement grains. Combined with zircon U–Pb data, it suggests that the basement rocks of Apulian-affinity nappes (Margna Sesia, Austroalpine) were the major sources of detritus, together with the Ivrea Zone and recycled Prealpine flysch, that contributed debris to the Northern Alpine Foreland Basin. Furthermore, the rocks of the Sesia–Lanzo Zone or of equivalent units exposed at that time presumably provided the youngest basement zircon fission-track ages to the basin. The Biella volcanic suite was the source of volcanogenic zircons. Oligocene sediment pathways from source to sink crossed further crystalline basement units and sedimentary covers before entering the basin from the southeast. The lag times of the youngest basement age populations (volcanic zircons excluded) are about 11 Myr. This constrains average moderate-to-high exhumation rate of 0.5–0.6 km/Myr in the pro-side of the orogenic wedge of the Central Alps during the late Eocene to early Oligocene.