Rivers are the dominant source of many elements and isotopes to the ocean. But this input from the continents is not balanced by the loss of the elements and isotopes through hydrothermal and ...sedimentary exchange with the oceanic crust, or by temporal changes in the marine inventory for elements that are demonstrably not in steady state. To resolve the problem of the observed imbalance in marine geochemical budgets, attention has been focused on uncertainties in the hydrothermal and sedimentary fluxes. In recent Earth history, temporally dynamic chemical weathering fluxes from the continents are an inevitable consequence of periodic glaciations. Chemical weathering rates on modern Earth are likely to remain far from equilibrium owing to the physical production of finely ground material at glacial terminations that acts as a fertile substrate for chemical weathering. Here we explore the implications of temporal changes in the riverine chemical weathering flux for oceanic geochemical budgets. We contend that the riverine flux obtained from observations of modern rivers is broadly accurate, but not representative of timescales appropriate for elements with oceanic residence longer than Quaternary glacial-interglacial cycles. We suggest that the pulse of rapid chemical weathering initiated at the last deglaciation has not yet decayed away and that weathering rates remain about two to three times the average for an entire late Quaternary glacial cycle. Taking into account the effect of the suggested non-steady-state process on the silicate weathering flux helps to reconcile the modelled marine strontium isotope budget with available data. Overall, we conclude that consideration of the temporal variability in riverine fluxes largely ameliorates long-standing problems with chemical and isotopic mass balances in the ocean.
Proxies for past seawater chemistry, such as Mg/Ca and Sr/Ca ratios, provide a record of the dynamic exchanges of elements between the solid Earth, the atmosphere, and the hydrosphere and the ...evolving influence of life. We estimated past oceanic Mg/Ca and Sr/Ca ratios from suites of 1.6- to 170-million-year-old calcium carbonate veins that had precipitated from seawater-derived fluids in ocean ridge flank basalts. Our data indicate that before the Neogene, oceanic Mg/Ca and Sr/Ca ratios were lower than in the modern ocean. Decreased ocean spreading since the Cretaceous and the resulting slow reduction in ocean crustal hydrothermal exchange throughout the early Tertiary may explain the recent rise in these ratios.
Underground railway stations are known to have elevated particulate matter (PM) loads compared to ambient air. As these particles are derived from metal-rich sources and transition metals may pose a ...risk to health by virtue of their ability to catalyze generation of reactive oxygen species (ROS), their potential enrichment in underground environments is a source of concern. Compared to coarse (PM10) and fine (PM2.5) particulate fractions of underground railway airborne PM, little is known about the chemistry of the ultrafine (PM0.1) fraction that may contribute significantly to particulate number and surface area concentrations. This study uses inductively coupled plasma mass spectrometry and ion chromatography to compare the elemental composition of size-fractionated underground PM with woodstove, roadwear generator, and road tunnel PM. Underground PM is notably rich in Fe, accounting for greater than 40% by mass of each fraction, and several other transition metals (Cu, Cr, Mn, and Zn) compared to PM from other sources. Importantly, ultrafine underground PM shows similar metal-rich concentrations as the coarse and fine fractions. Scanning electron microscopy revealed that a component of the coarse fraction of underground PM has a morphology indicative of generation by abrasion, absent for fine and ultrafine particulates, which may be derived from high-temperature processes. Furthermore, underground PM generated ROS in a concentration- and size-dependent manner. This study suggests that the potential health effects of exposure to the ultrafine fraction of underground PM warrant further investigation as a consequence of its greater surface area/volume ratio and high metal content.
ODP/IODP Hole 1256D penetrates an in situ section of ocean crust formed at the East Pacific Rise, through lavas and sheeted dikes and ∼100 m into plutonic rocks. We use mineralogy, oxygen isotopes, ...and fluid inclusions to understand hydrothermal processes. The lavas are slightly altered at low temperatures (<150°C) to phyllosilicates and iron oxyhydroxides, with a stepwise increase in grade downward to greenschist minerals in the upper dikes. This resulted from generally upwelling hydrothermal fluids in the dikes mixing with cooler seawater solutions in the lavas, also producing minor metal sulfide mineralization in the upper dikes. Alteration grade increases downward in the dikes, with increasing recrystallization to amphibole and loss of metals at higher temperatures (>350°C up to ∼600°C). Intrusion of gabbro bodies into the lower dikes resulted in contact metamorphism to granoblastic hornfels at 850°C–900°C, representing a thermal boundary layer between the axial melt lens and the overlying hydrothermal system. Downward penetration of hydrothermal fluids led to rehydration of granoblastic dikes and plutonic rocks at ∼800°C down to <300°C. Fluid inclusion and oxygen isotope data show that vein quartz formed at ∼300°C to >450°C from hydrothermal fluids that were affected by supercritical phase separation. Fluids had variable salinities and were enriched in 18O (+0.4‰ to +3.5‰) relative to seawater, similar to seafloor vent fluids. Dike margins are brecciated and mineralized, suggesting hydrothermal activity coeval with magmatism. Anhydrite formed mainly in the upper dikes when partly reacted seawater fluids were heated as they penetrated deeper into the system. Low‐temperature alteration of the volcanic section continued as cold seawater penetrated along fluid pathways, forming minor iron oxyhydroxides in the rocks. Hydrothermal processes at Site 1256 fit with current models whereby greenschist alteration of dikes at low water/rock ratios is overprinted by fracture‐controlled alteration and mineralization by upwelling hydrothermal fluids, a conductive boundary layer above gabbroic intrusions, leaching of metals from dikes and gabbros in the deep “root zone,” and stepped thermal and alteration gradients in the basement. The Site 1256 section, however, is intact and retains recharge effects (anhydrite), allowing an integrated view of processes in the subsurface.
Although metabasaltic rocks have been suggested to be important source rocks for orogenic gold deposits, the mobility of Au and related elements (As, Sb, Se, and Hg) from these rocks during ...alteration and metamorphism is poorly constrained. We investigate the effects of increasing metamorphic grade on the concentrations of Au and related elements in a suite of metabasaltic rocks from the Otago and Alpine Schists, New Zealand. The metabasaltic rocks in the Otago and Alpine Schists are of MORB and WPB affinity and are interpreted to be fragments accreted from subducting oceanic crust. Gold concentrations are systematically lower in the higher metamorphic grade rocks. Average Au concentrations vary little between sub-greenschist (0.9 ± 0.5 ppb) and upper greenschist facies (1.0 ± 0.5 ppb), but decrease significantly in amphibolite facies samples (0.21 ± 0.07 ppb). The amount of Au depleted from metabasaltic rocks during metamorphism is on a similar scale to that removed from metasedimentary rocks in Otago. Arsenic concentrations increase with metamorphic grade with the metabasaltic rocks acting as a sink rather than a source of this element. The concentrations of Sb and Hg decrease between sub-greenschist and amphibolite facies but concentration in amphibolite facies rocks are similar to those in unaltered MORB protoliths and therefore unaltered oceanic crust cannot be a net source of Sb and Hg in a metamorphic environment. The concentrations of Au, As, Sb, and Hg in oceanic basalts that have become integrated into the metamorphic environment may be heavily influenced by the degree of seafloor alteration that occurred prior to metamorphism. We suggest that metasedimentary rocks are much more suitable source rocks for fluids and metals in orogenic gold deposits than metabasaltic rocks as they show mobility during metamorphism of all elements commonly enriched in this style of deposit.
Fluids play a key role in modifying the chemical and physical properties of fault zones, which may prime them for repeated rupture by the generation of high pore fluid pressures and precipitation of ...commonly weak, secondary minerals. Fluid flow paths, sources and fluxes, and the permeability evolution of fault zones throughout their seismic cycles remain poorly constrained, despite their importance to understanding fault zone behaviour. Here we use geochemical tracers of fluid–rock exchange to determine budgets for meteoric, metamorphic and mantle fluids on a major compressional tectonic plate boundary.
The Alpine Fault marks the transpressional Pacific–Australian plate boundary through South Island, New Zealand and appears to fail in regular (329±68 yrs) large earthquakes (Mw∼8) with the most recent event in 1717 AD. Significant convergent motion has formed the Southern Alps and elevated geothermal gradients in the hangingwall, which drive crustal fluid flow. Along the Alpine Fault the Alpine Schist of the Pacific Plate is thrust over radiogenic metasedimentary rocks on the Australian plate. The absence of highly radiogenic (87Sr/86Sr > 0.7200) strontium isotope ratios of hangingwall hot springs and hydrothermal minerals formed at a range of depths in the Alpine Fault damage zone indicates that the fluid flow is restricted to the hangingwall by a cross-fault fluid flow barrier throughout the seismogenic crust. Helium isotope ratios measured in hot springs near to the Alpine Fault (0.15–0.81 RA) indicate the fault is a crustal-scale feature that acts as a conduit for fluids from the mantle. Rock-exchanged oxygen, but meteoric water-like hydrogen isotope signatures of hydrothermal veins indicate that partially rock-exchanged meteoric fluids dominate down to the top of the brittle to ductile transition zone at ∼6 km. Geochemical tracer transport modelling suggests only ∼0.02 to 0.05% of total rainfall west of the Main Divide penetrates to depth, yet this recharge flux is sufficient to overwhelm other fluid contributions. Calculated mantle fluid fluxes of CO2 and H2O (0.2 and 3 to 13 mol/m2/yr respectively) and metamorphic H2O fluxes (4 to 750 mol/m2/yr) are considerably lower than the focused meteoric water discharge flux up the Alpine Fault (4 × 103 to 7 × 104 mol/m2/yr), driven by the >3000 m hydrologic head of the Southern Alps. Meteoric waters are primarily responsible for modifying fault zone permeability during fluid–rock interactions and may facilitate the generation of high pore fluid pressures that could assist episodic earthquake rupture.
•Helium isotopes indicate the Alpine Fault is a conduit for fluids from the mantle.•The Alpine Fault is a barrier to cross-fault flow throughout the seismogenic crust.•This barrier and the Southern Alps high topography focuses meteoric fluid discharge.•Meteoric fluids dominate with minor volumes of metamorphic and mantle fluids.•Focused fluid flow through the Alpine Fault zone may promote fault weakening.
The Oman Drilling Project (OmanDP), performed under the
International Continental Scientific Drilling Program (ICDP), is an
international scientific research project that undertook drilling at a ...range of sites in the Semail ophiolite (Oman) to collect core samples spanning the stratigraphy of the ophiolite, from the upper oceanic crust down to the basal thrust. The cores were logged to International Ocean Discovery Program (IODP) standards aboard the D/V Chikyu. During ChikyuOman2018 Leg 3 (July–August 2018), participants described cores from the crust–mantle transition (CM) sites. The main rock types recovered at these sites were
gabbros, dunites and harzburgites, rocks typically forming the base of the
oceanic crust and the shallow mantle beneath present-day spreading centres.
In addition to the core description, selected samples were analysed by X-ray fluorescence spectrometry (XRF) for their chemical compositions, including major, minor and some trace elements. To complement these standard procedures, we developed new
approaches to measure ultra-trace element concentrations using a procedure
adapted from previous works to prepare fine-grained pressed powder pellets
coupled with laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis using instrumentation aboard the D/V Chikyu. First, three (ultra)mafic reference materials were investigated to test and validate our procedure (BHVO-2, BIR-1a and JP-1), and then the procedure was applied to a selection of gabbro and dunite samples from the CM cores to
explore the limitations of the method in its current stage of development.
The obtained results are in good agreement with preferred values for the
reference materials and with subsequent solution replicate analyses of the same samples performed in shore-based laboratories following Leg 3 for the
CM samples. We describe this procedure for the determination of 37 minor and (ultra-)trace elements (transition elements and Ga, Li and Large-Ion Lithophile Elements (LILE), Rare Earth Elements (REE), High-Field-Strength Elements (HFSE), U, Th, and Pb) in mafic and ultramafic rocks. The presented method has the major advantage that it allows the determination at sea of the
(ultra-)trace element concentrations in a “dry”, safe way, without using
acid reagents. Our new approach could be extended for other elements of
interest and/or be improved to be adapted to other rock materials during
future ocean drilling operations aboard the D/V Chikyu and other platforms.
International CO
2
emissions reduction commitments are insufficient to avert damaging global warming and imperil a sustainable future. Climate engineering approaches are increasingly proposed as ...near-term intervention strategies, but deployment of these controversial techniques will require careful engagement with and the support of the public. New quantitative measurements of public perceptions for six climate engineering approaches show that the public of the United Kingdom (UK), United States (US), Australia (AU) and New Zealand (NZ) continue to have little knowledge of climate engineering. All approaches are regarded unfavourably, albeit less so for carbon dioxide removal (CDR) than solar radiation management (SRM). Knowledge and perceptions are remarkably similar between countries although UK and US respondents are more favourable towards SRM and UK respondents are more favourable towards CDR. Stratospheric aerosol injection is the most negatively perceived approach. Support for small-scale trials is also higher for CDR approaches than SRM. Statistical analyses yield mixed relationships between perceptions of climate engineering and age, political affiliation and pro-ecological views. Thus far, attempts to engage the public with climate engineering have seen little change over time and consequently, there is growing urgency to facilitate careful citizen deliberation using objective and instructive information about climate engineering.
Few data exist that provide insight into processes
affecting the long-term carbon cycle at shallow forearc depths. To better
understand the mobilization of C in sediments and crust of the
subducting ...slab, we investigated carbonate materials that originate from the
subduction channel at the Mariana forearc (< 20 km) and were
recovered during International Ocean Discovery Program Expedition 366.
Calcium carbonates occur as vein precipitates within metavolcanic and
metasedimentary clasts. The clasts represent portions of the subducting
lithosphere, including ocean island basalt, that were altered at lower
blueschist facies conditions and were subsequently transported to the
forearc seafloor by serpentinite mud volcanism. Euhedral aragonite and
calcite and the lack of deformation within the veins suggest carbonate
formation in a stress-free environment after peak metamorphism affected
their hosts. Intergrowth with barite and marked negative Ce anomalies in
carbonate attest the precipitation within a generally oxic environment, that
is an environment not controlled by serpentinization. Strontium and O
isotopic compositions in carbonate (87Sr∕86Sr = 0.7052 to
0.7054, δ18OVSMOW = 20 to 24 ‰)
imply precipitation from slab-derived fluids at temperatures between
∼130 and 300 ∘C. These temperature estimates are
consistent with the presence of blueschist facies phases such as lawsonite
coexisting with the carbonates in some veins. Incorporated C is inorganic
(δ13CVPDB = −1 ‰ to +4 ‰) and likely
derived from the decarbonation of calcareous sediment and/or oceanic crust.
These findings provide evidence for the mobilization of C in the downgoing
slab at depths of < 20 km. Our study shows for the first time in
detail that a portion of this C forms carbonate precipitates in the
subduction channel of an active convergent margin. This process may be an
important asset in understanding the deep carbon cycle since it highlights
that some C is lost from the subducting lithosphere before reaching greater
depths.
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