Geochemistry Geophysics Geosystems最新文献

The Late Pleistocene stratigraphic sequence of Heinrich event layers 1 to 5a shows a high petrological diversity with systematically varying abundances of specific ice-rafted detritus (IRD) species. Our study core GeoB18530-1 from the southeastern Grand Banks Slope of Newfoundland resolves these in continuity and considerable detail. Using digital light microscopy and thin section analysis, the compositional mineralogy of coarse-grained IRD particles was photographically reproduced, petrographically examined, accurately described and named. From 8,243 identified particles, we established a new IRD classification scheme of 22 optically discernible prevalent IRD rock types and rock-forming minerals. The abundance of each IRD species was counted at 2.3 cm resolution over the lower core section featuring six 30–50 cm thick Heinrich layers and similarly wide interlayers. This high-resolution time series provides a complex geological record of the Laurentide Ice Sheet calving and episodic collapse during MIS 3 and MIS 2. The count statistics allow IRD species to be divided into three separate groups based on their equal, over-, or under-representation in Heinrich layers relative to their interlayers. Heinrich layers 1, 2, 4, and 5 were found to be petrologically similar, while Heinrich layers 3 and 5a have some deviating IRD constituents, conforming to previous findings. Ooid-bearing dolomite IRD dominates the mid-section of all Heinrich layers, whereas muscovite-biotite granite IRD is enhanced at their tops and bottoms. The characteristic IRD lithology of the interlayers changes gradually over the investigated Late Wisconsinan Stage (60–15 ka).

The boron isotope ratio (δ¹¹B) of planktonic foraminifera is a robust proxy for determining oceanic pH and inferring atmospheric pCO₂ during the Cenozoic. However, measuring δ¹¹B in these small and low [B] samples (3–16 ppm) is challenging, as a precision below 0.8‰ (2SD) is required, equivalent to ∼0.1 pH units, which reflects the scale of anthropogenic acidification or glacial/interglacial alternations. Moreover, uncertainties in paleoclimate reconstructions from biocarbonates are due to biomineralization processes, requiring species-specific calibrations of the pH-δ¹¹B relationship to account for the so-called “vital effect.” While such calibrations exist for many species, the influence of foraminiferal size on δ¹¹B remains poorly studied. We measured δ¹¹B of Globigerina bulloides (80–190 tests, 3–5 ppm [B]), for three different size fractions (250–315, 315–400, and >400 μm), collected from the upper part of a core from the Chilean Margin. We validated a novel analytical method combining microdistillation for B purification and micro-direct injection (μ-dDIHEN) to the Multi-Collector Inductively Coupled Plasma Mass Spectrometer for accurate and precise (0.1–0.5‰, 2SD) δ¹¹B analysis of the smallest samples (only 1–2 ng B in solution). This approach injects only 10 μL of solution, producing short injection peaks that minimize blank effects through transient signal processing, thereby improving paleo-pH reconstructions from small-sized foraminifera. Our results show no size-dependent variations in δ¹¹B of symbiont-barren G. bulloides, confirming that its microenvironment is primarily influenced by respiration and calcification. This contrasts with symbiont-bearing species, where δ¹¹B varies with symbiont density due to photosynthetic activity.

Ocean-wide “Heinrich event” layers marked by weakly magnetic dolomitic ice-rafted detritus (IRD) from the Hudson Bay region are a prominent signature of Laurentide Ice Sheet surges. Contrary to intuition, the Heinrich layers of the Ruddiman Belt IRD exhibit maxima of magnetic susceptibility. Building on IRD petrology and abundance in sediments from the southeast Grand Banks Slope of Newfoundland described in Part I of this study, this Part II presents bulk rock magnetic, major element and grain-size data as well as rock magnetic analyses of 22 classified IRD lithologies. By stratigraphically, computationally and statistically comparing magnetic with other bulk sediment properties and lithology-specific IRD counts, we analyze, how the various magnetic IRD species and the glaciomarine background sedimentation generate the observed magnetic susceptibility record. How to infer from microscopically identifiable, but subordinate coarse IRD to the apparently greater impact of the unclassifiable fine IRD fraction is critical for this quest. We show that multi-domain magnetite, originating from plutonic Canadian Shield rocks, especially muscovite-biotite granite, dominates and shapes bulk susceptibility records more than all other IRD lithologies. Inverse correlations of Ca/Sr and K/Fe ratios, proxies for dolomitic and granitic IRD, delineate that the magnetic enhancement by the granitic detritus is partly compensated by co-deposited weakly magnetic dolomitic detritus in the mid-phase of Heinrich events. Modeling bulk susceptibility from various non-magnetic records suggests that large parts of fine IRD did not settle directly from melting overpassing icebergs, but seem to have been redistributed and sorted by ocean currents and gravity flows.

Kimberlites are rare and perplexing igneous rocks that may represent the deepest-sourced melt type extracted from within the Earth's mantle, and their origin may be associated with Large Low Shear Velocity Provinces (LLSVPs) along the core-mantle boundary. The dynamics and stability of LLSVPs provide precious insights into the nature and pattern of mantle convection, as they may either be passive features, easily swept away by mantle convection, or may play an essential role in creating and stabilizing global mantle flow. Studying the connection between backtracked kimberlite locations and LLSVPs is one of the few ways to assess the stability of LLSVPs over time. Although some previous studies exist, the kinematic models describing past and present motions of lithospheric plates have only recently undergone significant improvements that allow researchers to model plate motions as far back as 1 Ga. Using such models, we show that most of the kimberlites with ages between 120 and 680 Ma occurring on the North American continent were located over LLSVPs when they were emplaced. More importantly, we show that quiet periods, during which few kimberlites were emplaced, can occur when the North American plate drifted between the Pacific LLSVP and the African LLSVP. This indicates that LLSVPs have remained stable for at least 680 Ma and therefore play an essential role in creating and stabilizing global mantle flow. The emplacement of kimberlites younger than 120 Ma, that is, when North America does not overlie an LLSVP, seems to have been facilitated by edge-driven convection.

New helium isotope data for basalt glasses from the Gulf of Tadjoura and Gulf of Aden reveal a mantle plume signal that has ³He/⁴He up to 17 R_A. In the Gulf of Aden, plume helium is detectable ∼380 km from the Afar triple junction, up to the Shukra El Sheik Fracture Zone along the West Sheba Ridge, but not beyond. The trace of the fracture zone onto the coastal margins also marks the eastward extent of volcanic terranes attributed to Afar plume influence on the ocean-continent transition. The lack of elevated ³He/⁴He beyond this point and the history of westward propagation of spreading in the Gulf of Aden toward the triple junction indicate that the Afar plume has a limited influence on modern-day crustal accretion along most of the Sheba Ridge. Eastward of the Afar plume influence, basalts show uniform ³He/⁴He = 8.08 ± 0.20 R_A (1σ, n = 22) along >1,100 km of the ridge axis. This is among the most uniform sections of the global mid-ocean ridge system for helium isotopes. The uniformity likely results from enhanced homogenization by small-scale convection in the upper mantle beneath ultra-slow spreading ridges. Regional He–Pb–Nd–Sr isotope variations follow a pattern similar to that for ocean island basalts, showing a decrease in ³He/⁴He as the proportion of recycled material increases in the mantle source region. Source contributions include the Afar mantle plume, shallow asthenosphere, Pan-African lithosphere, and a sporadic HIMU component that originates either from the continental lithosphere or from within the Afar plume.

Processes that form and deform any rock leave distinct imprints on its microstructure. The description and interpretation of these microstructural features aids in understanding rock's evolution. The anisotropy of magnetic susceptibility is widely regarded as a method for characterizing rock microstructures. Microstructures generally vary based on the activity of deformation mechanisms, which are influenced by factors such as temperature, rock composition, strain rate and the presence of fluid or melt. To assess the impact of deformation mechanisms on magnetic fabric development, this study compares the microstructural and magnetic record evolution in a high-temperature shear zone (SZ) in marble with previously published data from a low-temperature SZ in similar rock. In the high-temperature SZ, evidence of high-temperature grain boundary migration recrystallization is observed. This results in a weak but evolving calcite crystallographic preferred orientation, accompanied by a continual reorientation of a shape preferred orientation of constant intensity. The magnetic fabric aligns closely with these changes in orientation but shows limited correlation between strain and magnetic fabric strength. In contrast, the earlier-studied low-temperature, dominated by subgrain rotation recrystallization, exhibits a magnetic fabric-strain relationship governed by the representation and mutual orientation of newly formed subfabrics at the microscale. These findings highlight the critical role of deformation mechanisms in shaping magnetic fabric patterns and their relationship to strain suggesting that certain types of deformation mechanisms may not necessarily lead to a significant increase in the magnetic fabric strength in direct correlation with strain gradient.

Supracrustal greenstone belts of the Rae craton of north Baffin Island have been historically attributed to the Mary River Group (MRG), a key tectonostratigraphic unit of economic significance. Best-preserved exposures of supracrustal rocks occur within the Eqe Bay Greenstone Belt. New mapping and U-Pb zircon dating of samples from Eqe Bay, combined with previous geochronological work, reveal that greenstone belts hitherto known as MRG consist of two temporally and spatially distinct episodes of volcanism: a ca. 2863–2830 Ma Mesoarchean pulse, to which we assign the name “Tuktuliarvik Group”, and a ca. 2759–2718 Ma Neoarchean pulse, which retains the designation “MRG.” High-purity Fe-ores of the world-class Mary River deposit occur exclusively within the Neoarchean-age MRG. Detrital zircon U-Pb dates from quartzite and psammite (e.g., Pond Inlet) constrain a transition within the MRG, from magmatism to clastic sedimentation, at ca. 2720 Ma. An extensive tract of turbiditic siliciclastic rocks of the Eqe Bay Greenstone Belt, originally thought to represent the upper succession of the MRG, yields a Paleoproterozoic maximum depositional age. This suggests that turbiditic rocks instead form part of the younger, ca. <2.19–2.16 to >1.88 Ga Piling Group, and that the MRG lacks a turbiditic sequence typical of the upper stratigraphy of many greenstone belts. An updated chronostratigraphy of the greenstone belts of north Baffin Island has implications for stratigraphic links with Greenland and mainland Canada, Archean geodynamics within the northeastern Rae craton and the footprint of Precambrian basins during the amalgamation of the Nuna supercontinent.

The Zagros orogen records the closure of the Neotethys Ocean by northward subduction of the oceanic slab beneath the Iranian plate. Northwestern Iran has undergone episodic volcanism since the Eocene, due to a shift of tectonic regimes from subduction to collision and post-collisional extension, although the dynamics responsible for such volcanism remain highly controversial. We report new zircon U–Pb ages and geochemical data from the Takab-Shahindezh volcanic rocks in the NW-trending Zagros orogen, adjacent to the ENE-trending Mianeh-Ardabil fault system, to explore their origin and geodynamic implications. Crystallization ages range from 44 to 39 Ma for the Shahindezh lava to 19‒16 Ma for the Takab lava. Integrated zircon Hf–O isotopes, whole-rock geochemistry, and Sr-Nd isotope data suggest that both volcanic suites originated from the partial melting of a subduction-modified, sediment-enriched mantle lithosphere, triggered by asthenospheric upwelling during ongoing geodynamic reorganization. We present evidence that the Eocene Shahindezh volcanism developed in a late stage of the Neotethyan subduction. This was followed by the Early-Middle Miocene Takab volcanism, which originated after subduction ended and slab tearing occurred. Subsequently, slab failure, likely controlled by the Aras-Mianeh-Ardabil fault system, triggered widespread Late Miocene-Quaternary volcanism, including the Sahand and Sabalan volcanoes, across northwestern Iran.

The Paleogene sedimentary basins of southeastern Tibet record sediment dispersal patterns associated with crustal deformation during the early stages of the India–Asia collision. We present detrital zircon U–Pb geochronology, tourmaline geochemistry, and petrographic data from the Gonjo and Mangkang basins that suggest a reginal interconnected paleodrainage during the Paleocene–Eocene. Sediments were likely sourced from the distal Songpan–Ganzi and proximal northern Qiangtang terranes. Stratigraphic relationships and paleoelevation estimates support the interpretation of a paleodrainage system that may have extended toward the Lanping–Simao region and northern Vietnam. This system appears to have been disrupted in the late Eocene, possibly due to sinistral shearing along the Ailao Shan–Red River fault zone. These findings offer new insights into sediment routing and landscape evolution along the southeastern margin of the Tibetan Plateau during the early Cenozoic.

Satellite observations have enabled an important advance in the near-real-time quantification of the dynamic parameters of the volcanic plume spreading in the atmosphere. However, the link between these observations and the estimation of eruption source parameters, such as the mass eruption rate (MER), remains a scientific obstacle to be overcome. The previously developed methods to estimate the MER are less efficient for weak eruptions and/or occurring under strong wind conditions, which are the most frequent. Here, we update a 1-D volcanic column model for the estimation of the MER based on satellite measurements of wind-impacted plumes. The new model allows predicting the plume geometry as seen from space, and thus linking the source MER to the geometry far from the source. We find that the predictions mostly depend on the wind speed and the MER. We test the model using measurements made on GOES-16 images during the 2021 eruption of La Soufrière, St Vincent, and find a good agreement between our MER estimates and those found in the literature (with a mean $��\Delta �$MER of −0.23). We finally test our ability in estimating the MER in near real-time using the HOTVOLC system and Meteosat-SEVIRI images of 10 paroxysms from Mt Etna. The GIS-based tool integrated to HOTVOLC allows easier measurements of the plume growth and will provide a robust tool for a rapid interpretation of satellite data in terms of source conditions, which are necessary inputs for tephra dispersion models, such as those used by the Volcanic Ash Advisory Centers.