450 research outputs found
Radiation damage allows identification of truly inherited zircon
Many studies have reported U-Pb dates of zircon that are older than the igneous rocks that contain them, and they are therefore thought to be inherited from older rock complexes. Their presence has profound geodynamic implications and has been used to hypothesize about concealed micro-continents, continental crust beneath ocean islands, and recycling of continental material in the mantle beneath mid-ocean ridges. Here, we combine single zircon U-Pb dates and structural radiation damage determined by Raman spectroscopy from a Pliocene mid-ocean ridge gabbro and from Cenozoic igneous rocks to test whether radiation damage allows distinction between contamination and truly inherited zircon. We find that Precambrian zircon found in the Pliocene sample has accumulated substantially more radiation damage than could be explained if they had truly been inherited. In the Cenozoic samples, however, we find that the radiation damage of old grains corresponds with that of young magmatic zircon, suggesting they are genuinely inherited.publishedVersio
Volcanic evolution of an ultraslow-spreading ridge
Nearly 30% of ocean crust forms at mid-ocean ridges where the spreading rate is less than 20âmm per year. According to the seafloor spreading paradigm, oceanic crust forms along a narrow axial zone and is transported away from the rift valley. However, because quantitative age data of volcanic eruptions are lacking, constructing geological models for the evolution of ultraslow-spreading crust remains a challenge. In this contribution, we use sediment thicknesses acquired from ~4000âkm of sub-bottom profiler data combined with 14C ages from sediment cores to determine the age of the ocean floor of the oblique ultraslow-spreading Mohns Ridge to reveal a systematic pattern of young volcanism outside axial volcanic ridges. Here, we present an age map of the upper lava flows within the rift valley of a mid-ocean ridge and find that nearly half of the rift valley floor has been rejuvenated by volcanic activity during the last 25âKyr.publishedVersio
A Highly Depleted and Subduction-Modified Mantle Beneath the Slow-Spreading Mohns Ridge
The Mohns Ridge is a very slow-spreading ridge that, together with the Knipovich Ridge, marks the boundary between the North American and Eurasian plates in the Norwegian-Greenland Sea. In this study, we report the major and trace element composition of spatially associated basalts and peridotites from a gabbro-peridotite complex âŒ20 km west of the Mohns Ridge rift flank. Formation of the âŒ4â5 Myr crustal section involved accretion of normal mid-ocean ridge basalts with Na-content suggesting derivation from a depleted mantle source. This is consistent with the degree of partial melting estimated for clinopyroxene poor harzburgites using the Cr-number of spinel (14%â18%) and rare earth element modeling of orthopyroxene (16%â24%) and reconstructed whole-rock composition (14%â20%). If all the melting took place beneath the paleo-Mohns Ridge, a crustal thickness of âŒ7â8 km is expected, which is nearly double the observed thickness. Orthopyroxene trace elements are not consistent with typical fractional melting expected for mid-ocean ridges but rather resemble that seen in supra-subduction zone peridotites. The geochemistry of both the basalts and the peridotites suggests that a water-rich slab flux in the past has influenced the mantle source. In turn, this caused hydrous melting which increased the depletion of the pyroxene components, leading to a highly depleted mantle that is now underlying much of the Arctic Mid-Ocean Ridges and represents the source for the spreading related magmatism.publishedVersio
Correlating microbial community profiles with geochemical data in highly stratified sediments from the Arctic Mid-Ocean Ridge
Microbial communities and their associated metabolic activity in
marine sediments have a profound impact on global biogeochemical
cycles. Their composition and structure are attributed to geochemical
and physical factors, but finding direct correlations has remained a
challenge. Here we show a significant statistical relationship between
variation in geochemical composition and prokaryotic community
structure within deep-sea sediments. We obtained comprehensive
geochemical data from two gravity cores near the hydrothermal
vent field Lokiâs Castle at the Arctic Mid-Ocean Ridge, in the Norwegian-
Greenland Sea. Geochemical properties in the rift valley
sediments exhibited strong centimeter-scale stratigraphic variability.
Microbial populations were profiled by pyrosequencing from
15 sediment horizons (59,364 16S rRNA gene tags), quantitatively
assessed by qPCR, and phylogenetically analyzed. Although the
same taxa were generally present in all samples, their relative
abundances varied substantially among horizons and fluctuated
between Bacteria- and Archaea-dominated communities. By independently
summarizing covariance structures of the relative
abundance data and geochemical data, using principal components
analysis, we found a significant correlation between changes in
geochemical composition and changes in community structure.
Differences in organic carbon and mineralogy shaped the relative
abundance of microbial taxa. We used correlations to build hypotheses
about energy metabolisms, particularly of the Deep Sea Archaeal
Group, specific Deltaproteobacteria, and sediment lineages
of potentially anaerobic Marine Group I Archaea. We demonstrate
that total prokaryotic community structure can be directly correlated
to geochemistry within these sediments, thus enhancing our
understanding of biogeochemical cycling and our ability to predict
metabolisms of uncultured microbes in deep-sea sediments
Trace Element and Sulfur Isotope Signatures of Volcanogenic Massive Sulfide (VMS) Mineralization: A Case Study from the Sunnhordland Area in SW Norway
The Sunnhordland area in SW Norway hosts more than 100 known mineral occurrences,
mostly of volcanogenic massive sulfide (VMS) and orogeny Au types. The VMS mineralization
is hosted by plutonic, volcanic and sedimentary lithologies of the Lower Ordovician ophiolitic
complexes. This study presents new trace element and ÎŽ
34S data from VMS deposits hosted by
gabbro and basalt of the Lykling Ophiolite Complex and organic-rich sediments of the LangevÄg
Group. The AlsvÄgen gabbro-hosted VMS mineralization exhibits a significant Cu content (1.2 to
>10 wt.%), with chalcopyrite and cubanite being the main Cu-bearing minerals. The enrichment of
pyrite in Co, Se, and Te and the high Se/As and Se/Tl ratios indicate elevated formation temperatures,
while the high Se/S ratio indicates a contribution of magmatic volatiles. The ÎŽ
34S values of the sulfide
phases also support a substantial influx of magmatic sulfur. Chalcopyrite from the AlsvÄgen VMS
mineralization shows significant enrichment in Se, Ag, Zn, Cd and In, while pyrrhotite concentrates
Ni and Co. The LindĂžya basalt-hosted VMS mineralization consists mainly of pyrite and pyrrhotite.
Pyrite is enriched in As, Mn, Pb, Sb, V, and Tl. The ÎŽ
34S values of sulfides and the Se/S ratio in pyrite
suggest that sulfur was predominantly sourced from the host basalt. The LitlabĂž sediment-hosted
VMS mineralization is also dominated by pyrite and pyrrhotite. Pyrite is enriched in As, Mn, Pb, Sb,
V and Tl. The ÎŽ
34S values, which range from â19.7 to â15.7 â° VCDT, point to the bacterial reduction
of marine sulfate as the main source of sulfur. Trace element characteristics of pyrite, especially the
Tl, Sb, Se, As, Co and Ni concentrations, together with their mutual ratios, provide a solid basis for
distinguishing gabbro-hosted VMS mineralization from basalt- and sediment-hosted types of VMS
mineralization in the study area. The distinctive trace element features of pyrite, in conjunction with
its sulfur isotope signature, have been identified as a robust tool for the discrimination of gabbro-,
basalt- and sediment-hosted VMS mineralization
The Methylococcus capsulatus (Bath) Secreted Protein, MopE*, Binds Both Reduced and Oxidized Copper
Under copper limiting growth conditions the methanotrophic bacterium Methylococcus capsulatus (Bath) secrets essentially only one protein, MopE*, to the medium. MopE* is a copper-binding protein whose structure has been determined by X-ray crystallography. The structure of MopE* revealed a unique high affinity copper binding site consisting of two histidine imidazoles and one kynurenine, the latter an oxidation product of Trp130. In this study, we demonstrate that the copper ion coordinated by this strong binding site is in the Cu(I) state when MopE* is isolated from the growth medium of M. capsulatus. The conclusion is based on X-ray Near Edge Absorption spectroscopy (XANES), and Electron Paramagnetic Resonance (EPR) studies. EPR analyses demonstrated that MopE*, in addition to the strong copper-binding site, also binds Cu(II) at two weaker binding sites. Both Cu(II) binding sites have properties typical of non-blue type II Cu (II) centres, and the strongest of the two Cu(II) sites is characterised by a relative high hyperfine coupling of copper (
Mapping Microbial Abundance and Prevalence to Changing Oxygen Concentration in Deep-Sea Sediments Using Machine Learning and Differential Abundance
Oxygen constitutes one of the strongest factors explaining microbial taxonomic variability in deep-sea sediments. However, deep-sea microbiome studies often lack the spatial resolution to study the oxygen gradient and transition zone beyond the oxic-anoxic dichotomy, thus leaving important questions regarding the microbial response to changing conditions unanswered. Here, we use machine learning and differential abundance analysis on 184 samples from 11 sediment cores retrieved along the Arctic Mid-Ocean Ridge to study how changing oxygen concentrations (1) are predicted by the relative abundance of higher taxa and (2) influence the distribution of individual Operational Taxonomic Units. We find that some of the most abundant classes of microorganisms can be used to classify samples according to oxygen concentration. At the level of Operational Taxonomic Units, however, representatives of common classes are not differentially abundant from high-oxic to low-oxic conditions. This weakened response to changing oxygen concentration suggests that the abundance and prevalence of highly abundant OTUs may be better explained by other variables than oxygen. Our results suggest that a relatively homogeneous microbiome is recruited to the benthos, and that the microbiome then becomes more heterogeneous as oxygen drops below 25 ÎŒM. Our analytical approach takes into account the oft-ignored compositional nature of relative abundance data, and provides a framework for extracting biologically meaningful associations from datasets spanning multiple sedimentary cores.publishedVersio
- âŠ