Nitrogen geochemistry as a tracer of fluid flow in a hydrothermal vent complex in the Karoo Basin, South Africa

We have investigated the N geochemistry of minerals and rocks from contact metamorphic aureoles and hydrothermal vent complexes (HVC) in the Karoo Basin in South Africa. The HVC formed during phreatic eruptions associated with rapid devolatilization and pressure build-up in contact aureoles around early Jurassic sill intrusions. By combining outcrop data from a HVC and core data from contact aureoles, we investigate the relationship between light element release during metamorphism and vertical fluid migration. Sandstone and breccia from the HVC contain early-diagenetic ammonium -bearing feldspar (buddingtonite) and illite. Ammonium occupies up to 95% of the A site in feldspar, corresponding to concentrations up to 5.2 wt% N. Bulk-rock N isotope data for rocks from inside and outside the hydrothermal vent complex fall into two distinct groups. Background samples have δ¹⁵N_air between +1.5‰ and +4.9‰, whereas minerals from the vent complex have δ¹⁵N in the range +7.5 to +10.6‰. The N geochemistry of contact metamorphic shale from the lower stratigraphic units of the Karoo Basin shows that the vitrinite reflectance and δ¹⁵N values are positively correlated. Shale with reflectivity values >4%Ro are enriched in ¹⁵N, with δ¹⁵N values between +6‰ and +14‰, implying the release of isotopically light N into metamorphic fluids (probably as N₂). We suggest that the relatively high δ¹⁵N values of the early-diagenetic buddingtonite in the HVC reflect exchange of buddingtonite with N-bearing fluids ascending from greater depth after their release during contact metamorphism and dehydration. We present a qualitative model whereby hydrothermal vent complexes represent fluid flow structures after their formation, focusing N-bearing metamorphic fluids sourced in deeper levels of the basin. The release of organic N from sediments at depth in volcanic basins could play a role in the geochemical cycle of N, becoming particularly important during periods of intense volcanic activity.     

Complex trajectories of aquatic and terrestrial ecosystem shifts caused by multiple human-induced environmental stresses

Large shifts in the isotopic compositions of organic matter (OM) in lake sediments, over the last few hundred years, are commonly interpreted as representing changes in photosynthetic productivity corresponding to eutrophication or in the input of terrestrial OM due to human disturbances. Based on multiple-proxy data (C:N ratio, δ¹³C and δ¹⁵N of OM, δ¹³C of calcite, lithology and fossil pollen) from a 700-year sediment core at White Lake, New Jersey (USA), we propose a new explanation that relates these large shifts in OM δ¹³C and δ¹⁵N to human-induced changes in aquatic OM producers. Combined records of geochronology, fossil pollen and lithology from White Lake reveal that the upland forest was cleared by European settlers for farmland beginning around 1745 A.D. and has gradually reforested since 1930 after the abandonment of the farmlands. For the pre-agricultural period, OM had relatively constant but extremely low δ¹³C_VPDB (−35.8 to −34.5‰) and δ¹⁵N_Air (−3.5 to −2.5‰) and high atomic C:N ratios (13.7 to 16.7), indicating a stable anoxic lake environment with prominent microbial producers. Following the human disturbance (since 1745), high OM mass accumulation rates and abundances of the green alga Pediastrum indicate an increase in aquatic photosynthetic productivity due to enhanced nutrient input from disturbed uplands. However, carbonate δ¹³C remains constant or even decreases during this period, implying that increasing productivity did not elevate the δ¹³C of dissolved inorganic carbon and thus cannot explain the observed large increase in OM δ¹³C (7.4‰) and δ¹⁵N (5.8‰) over this period. Instead, δ¹³C, δ¹⁵N and C:N ratios of OM and differences in δ¹³C between calcite and OM suggest that the large increase in OM δ¹³C and δ¹⁵N can be attributed to a human-induced ecological shift in the predominant organic source from anaerobic bacteria to autotrophic phytoplankton. During the post-agricultural period, mass accumulation rates of OM, carbonate and silicate, and the δ¹³C of OM and calcite all decreased significantly, corresponding to stabilization of the uplands. However, over the last 70 years, an intensifying aquatic stress from the deposition of ¹⁵N-enriched industrial pollutants has resulted in a steady increase of 1.9‰ in δ¹⁵N. Proxy records for lake (δ¹³C and δ¹⁵N of OM) and upland conditions (pollen and silicates) at White Lake show complex trajectories of the aquatic and terrestrial ecosystems in response to past human disturbances.     

Cape cormorants decrease,move east and adapt foraging strategies following eastward displacement of their main prey

Numbers of Cape cormorants Phalacrocorax capensis breeding in South Africa decreased by nearly 50% from approximately 107 000 pairs in 1977–1981 to 57 000 pairs in 2010–2014. Although four colonies had >10 000 pairs in 1977–1981, there was just one such colony in 2010–2014. Almost all the decrease occurred after the early 1990s off north-west South Africa, between the Orange River estuary and Dassen Island. South of this, the number breeding in the two periods was stable, with some colonies being formed or growing rapidly in the 2000s. The proportion of South Africa’s Cape cormorants that bred south of Dassen Island increased from 35% in 1977–1981 to 66% in 2010–2014, with the opposite situation observed in the north-west. This matched a shift to the south and east in the distributions of two of the Cape cormorant’s main prey species, anchovy Engraulis encrasicolus and sardine Sardinops sagax. In 2014, an apparent scarcity of prey in the north-west resulted in Cape cormorants attempting to take bait from hooks of fishing lines over an extended period, a behaviour not previously recorded. The number of Cape cormorants breeding in the south may be constrained by the absence of large islands between Dyer Island in the west and Algoa Bay in the east. If so, it may be possible to bolster the southern population through the provision of appropriate breeding habitat, such as platforms, or restricting human disturbance at suitable mainland cliff breeding sites.     

On the Occurrence of Marine Fungi in the Diet of Littorina angulifera and Observations on the Behavior of the Periwinkle*

Abstract. The feeding and resting patterns of Littorina angulifera, the southern periwinkle, were observed in mangrove habitats of Belize (Central America). The snails feed predominantly on the surface of prop roots of Rhizophora mangle in a narrow zone at and above the mean high water mark. This area contains large numbers of hyphae and chlamydospores of an unidentified marine fungus (Deuteromycetes) and filaments of a chlorophyte (Chlorochytrium sp.). Both organisms are ingested by snails whose digestive tracts and fecal pellets contain ground-up cork cells, tricho-sclereids, tracheids, calcium oxalate crystals, fungal hyphae and chlamydospores, as well as undigested cyanobacteria. Most fungal particles pass through the gut unchanged. During dry periods, L. angulifera is in a dormant state, usually attached with dried mucus to leaves high in the tree, causing necrotic, crescent-shaped marks. The leaf tissue under the area of shell attachment becomes meristematic, separating dead tissues from healthy mesophyll. The snails detach during rainfall and move downward to the feeding sites on the prop roots.     

Nitrogen recycling in subducted oceanic lithosphere: The record in high- and ultrahigh-pressure metabasaltic rocks

This paper provides the first measurements of the nitrogen (N) concentrations and isotopic compositions of high- and ultrahigh-pressure mafic eclogites, aimed at characterizing the subduction input flux of N in deeply subducting altered oceanic crust (AOC). The samples that were studied are from the Raspas Complex (Ecuador), Lago di Cignana (Italy), the Zambezi Belt (Zambia) and Cabo Ortegal (Spain), together representing subduction to 50-90 km depths. The eclogites contain 2-20 ppm N with δ¹⁵N_air values ranging from −1 to +8‰. These values overlap those of altered oceanic crust, but are distinct from values for fresh MORB (for the latter, ∼1.1 ppm N and δ¹⁵N_air ∼ −4‰). Based on N data in combination with other trace element data, the eclogite suites can be subdivided into those that are indistinguishable from their likely protolith, AOC, with or without superimposed effects of devolatilization (Lago di Cignana, Cabo Ortegal), and those that have experienced metasomatic additions during subduction-zone metamorphism (Zambezi Belt, Raspas). For the former group, the lack of a detectable loss of N in the eclogites, compared to various altered MORB compositions, suggests the retention of N in deeply subducted oceanic crust. The metasomatic effects affecting the latter group can be best explained by mixing with a (meta)sedimentary component, resulting in correlated enrichments of N and other trace elements (in particular, Ba and Pb) thought to be mobilized during HP/UHP metamorphism. Serpentinized and high-pressure metamorphosed peridotites, associated with the eclogites at Raspas and Cabo Ortegal, contain 3-15 ppm N with δ¹⁵N_air values ranging from +3 to +6‰, significantly higher than the generally accepted values for the MORB mantle (δ¹⁵N_air ∼ −5‰). Based on their relatively high N contents and their homogeneous and positive δ¹⁵N values, admixing of sedimentary N is also indicated for the serpentinized peridotites.One possible pathway for the addition of sediment-derived N into eclogites and peridotites involves mixing with fluids along the slab-mantle wedge interface. Alternatively, sedimentary N could be incorporated into peridotites during serpentinization at bending-related faults at the outer rise and, during later deserpentinization, released into fluids that then infiltrate overlying rocks. Deep retention of N in subducting oceanic crust should be considered in any attempt to balance subduction inputs with outputs in the form of arc volcanic gases. If materials such as these eclogites and serpentinized peridotites are eventually subducted to beyond sub-arc depths into the deeper mantle, containing some fraction of their forearc-subarc N inventory (documented here), they could deliver isotopically heavy N into the mantle to potentially be sampled by plume-related magmas.     

Nitrogen concentration and δN of altered oceanic crust obtained on ODP Legs 129 and 185: Insights into alteration-related nitrogen enrichment and the nitrogen subduction budget

Knowledge of the subduction input flux of nitrogen (N) in altered oceanic crust (AOC) is critical in any attempt to mass-balance N across arc-trench systems on a global or individual-margin basis. We have employed sealed-tube, carrier-gas-based methods to examine the N concentrations and isotopic compositions of AOC. Analyses of 53 AOC samples recovered on DSDP/ODP legs from the North and South Pacific, the North Atlantic, and the Antarctic oceans (with larger numbers of samples from Site 801 outboard of the Mariana trench and Site 1149 outboard of the Izu trench), and 14 composites for the AOC sections at Site 801, give N concentrations of 1.3 to 18.2 ppm and δ¹⁵N_Air of −11.6‰ to +8.3‰, indicating significant N enrichment probably during the early stages of hydrothermal alteration of the oceanic basalts. The N-δ¹⁵N modeling for samples from Sites 801 and 1149 (n = 39) shows that the secondary N may come from (1) the sedimentary N in the intercalated sediments and possibly overlying sediments via fluid-sediment/rock interaction, and (2) degassed mantle N₂ in seawater via alteration-related abiotic reduction processes. For all Site 801 samples, weak correlation of N and K₂O contents indicates that the siting of N in potassic alteration phases strongly depends on N availability and is possibly influenced by highly heterogeneous temperature and redox conditions during hydrothermal alteration.The upper 470-m AOC recovered by ODP Legs 129 and 185 delivers approximately 8 × 10⁵ g/km N annually into the Mariana margin. If the remaining less-altered oceanic crust (assuming 6.5 km, mostly dikes and gabbros) has MORB-like N of 1.5 ppm, the entire oceanic crust transfers 5.1 × 10⁶ g/km N annually into that trench. This N input flux is twice as large as the annual N input of 2.5 × 10⁶ g/km in seafloor sediments subducting into the same margin, demonstrating that the N input in oceanic crust, and its isotopic consequences, must be considered in any assessment of convergent margin N flux.     

Fluid-induced breakdown of white mica controls nitrogen transfer during fluid–rock interaction in subduction zones

ABSTRACT

In order to determine the effects of fluid–rock interaction on nitrogen elemental and isotopic systematics in high-pressure metamorphic rocks, we investigated three different profiles representing three distinct scenarios of metasomatic overprinting. A profile from the Chinese Tianshan (ultra)high-pressure–low-temperature metamorphic belt represents a prograde, fluid-induced blueschist–eclogite transformation. This profile shows a systematic decrease in N concentrations from the host blueschist (~26 μg/g) via a blueschist–eclogite transition zone (19–23 μg/g) and an eclogitic selvage (12–16 μg/g) towards the former fluid pathway. Eclogites and blueschists show only a small variation in δ¹⁵N_air (+2.1 ± 0.3‰), but the systematic trend with distance is consistent with a batch devolatilization process. A second profile from the Tianshan represents a retrograde eclogite–blueschist transition. It shows increasing, but more scattered, N concentrations from the eclogite towards the blueschist and an unsystematic variation in δ¹⁵N values (δ¹⁵N = + 1.0 to +5.4‰). A third profile from the high-P/T metamorphic basement complex of the Southern Armorican Massif (Vendée, France) comprises a sequence from an eclogite lens via retrogressed eclogite and amphibolite into metasedimentary country rock gneisses. Metasedimentary gneisses have high N contents (14–52 μg/g) and positive δ¹⁵N values (+2.9 to +5.8‰), and N concentrations become lower away from the contact with 11–24 μg/g for the amphibolites, 10–14 μg/g for the retrogressed eclogite, and 2.1–3.6 μg/g for the pristine eclogite, which also has the lightest N isotopic compositions (δ¹⁵N = + 2.1 to +3.6‰).

Overall, geochemical correlations demonstrate that phengitic white mica is the major host of N in metamorphosed mafic rocks. During fluid-induced metamorphic overprint, both abundances and isotopic composition of N are controlled by the stability and presence of white mica. Phengite breakdown in high-P/T metamorphic rocks can liberate significant amounts of N into the fluid. Due to the sensitivity of the N isotope system to a sedimentary signature, it can be used to trace the extent of N transport during metasomatic processes. The Vendée profile demonstrates that this process occurs over several tens of metres and affects both N concentrations and N isotopic compositions.     

Testing for the occurrence of pilchard herpesvirus (PHV) in South African sardine Sardinops sagax

Catches of South African sardine Sardinops sagax have declined in recent years from about 200 000 t harvested annually during the period 2002–2006 to less than 100 000 t. Consequently, some companies are now importing sardine from sources elsewhere in the world to meet local demand for canned sardine and bait. This importation has the potential for the introduction of sardine pathogens, in particular the pilchard herpesvirus (PHV), which could have a negative impact on the currently small South African sardine population. The aims of the current study were to determine whether PHV is present in the local sardine population and to assess the extent to which sardine is being imported into the country and whether imported fish are from countries where the virus is known to be endemic. Fish sampled from South Africa’s western (n = 150), southern (n = 182) and eastern (n = 96) putative stocks of S. sagax were analysed for the presence of PHV using real-time quantitative polymerase chain reaction (qPCR). The origin and amount of potentially infected material imported into South Africa during the period 2010–2014 was also assessed. None of the South African sardine collected during this study tested positive for PHV, suggesting that active PHV was not prevalent in the local population of S. sagax at the time of this study. Between 56 000 and 71 000 t of frozen sardine was imported annually into South Africa from countries where S. sagax occurs, including some from areas (Australia and New Zealand) where sardine infection by PHV is known to be endemic. Hence, it is plausible that the PHV pathogen, capable of perpetuating infections in local sardine populations, could be imported into South Africa along with the importation of frozen sardine. Should local sardine be naïve to the virus, as suggested by this study, then the population is at risk of infection and precautions against such must be taken.