Chemical Geology最新文献

Microfossils identification, especially in very old rocks, is extremely challenging because morphological and chemical signatures of microfossils are subtle and inevitably altered with aging. Chert nodules from the Doushantuo Formation in the Yangtze Gorges area of South China have captured significant interest due to their remarkable preservation of Ediacaran microfossils. To understand the taphonomic processes leading to an exceptional preservation of microbes in these rocks, we studied the morphological and chemical features of microfossils within the lower Doushantuo Formation chert nodules down to atomic scales via advanced microscopy techniques. Our results align with previous studies, confirming that most microfossils there are preserved by silicification. Further analysis of a representative filamentous microfossil, Polytrichoides lineatus, reveals that both the cell wall (or extracellular sheath) and cytoplasm are preserved by loose aggregates of subrounded or irregular quartz grains, along with patchy organic residuals, which are distinctive from the closely packed and angular-shaped quartz grains in the non-fossil matrix of the chert nodule. The cell wall or extracellular sheath likely provided narrower spaces and more nucleation sites for silicification, resulting in smaller quartz grains (i.e., 115 ± 42 nm) compared to those formed in the cytoplasm (i.e., 1150 ± 258 nm). The permeation and precipitation of quartz grains within the cell wall or extracellular sheath could contribute to an exceptional preservation of subcellular structures. This study offers valuable insights into the preservation of microbes in the Doushantuo Formation chert nodules and even older siliceous sedimentary rocks.

Rare earth element and yttrium (REY) compositions of bioapatite (e.g., fish tooth and bone) can serve as potential paleoceanographic indicators. However, the REY enrichment of bioapatite and REY transfer from FeMn micronodule to bioapatite remain unclear owing to a lack of comparative study on these processes under variable redox conditions (oxic vs. suboxic), which hampers the utility of these indicators. To address these uncertainties, we conducted in situ geochemical analyses of fish teeth and FeMn micronodules from two REY-rich sediment cores (GC01 and GC02) collected from the Clarion–Clipperton fracture zone in the central equatorial Pacific. We found that the Ce/Ce* ratios of fish teeth from GC01 (sediments ΣREY = 723 ± 274 ppm) and GC02 (sediments ΣREY = 506 ± 65 ppm) gradually increased with depth under oxic conditions, with calculated oxic pore water-derived REY increasing from ∼0–3% on the surface to ∼11–24% at 200 cmbsf. In deep sediment columns (>200 cmbsf), the suboxic pore water contributed a small amount of REY (∼4% to ∼13%) to fish teeth, as evidenced by sharp increases in Ce/Ce* ratios of fish teeth and decreases in Ce/Ce* ratios and increases in Y_N/Ho_N ratios of micronodules. Therefore, the REY-patterns of fish teeth in core-deep samples were overprinted by oxic–suboxic pore waters may be unreliable archives of ancient bottom seawater.

Several missions have reported complex alteration mineralogies on early Mars, which preserve environmental records of multiple water-rock-atmosphere interactions. The MSL and M2020 missions in Gale and Jezero have identified Fe and Mn-bearing secondary phases. These elements are used as tracers for the redox conditions on both Earth and Mars. However, to fully understand the short-lived and local-scale processes observed on Mars, it is necessary to go beyond thermodynamic models and experiments. Enhancing our ability to interpret the redox and hydrological conditions from the observed phase assemblage requires understanding the evolution of Fe and Mn during weathering. This study reports the results of kinetic alteration experiments and geochemical models conducted under Mars-like conditions. We tested variable pO₂, pCO₂, temperatures, and starting solutions. The results suggest that Fe is more mobile on Mars than on Earth, with a pseudo-equilibrium concentration that is kinetically controlled by dissolution and oxidation rates. Despite some initially modeled siderite precipitation, no siderite precipitation was observed in the altered powder. Solutions with higher acid concentrations were primarily controlled by dissolution kinetics, with both Fe and Mn being mobile, even when a minor amount of P, Fe, and S bearing secondary phases are formed. Based on our experimental results, we updated the model and conducted two large-scale sensitivity tests on our kinetic simulation. We confirmed that our experiments were too high in pO₂ for siderite to form; however, we found that over a range of clearly oxidizing conditions from an equilibrium standpoint, Fe and Mn are mostly mobile, and siderite precipitation can occur. We were able to determine the pO₂, pCO₂ and the temporal space where Fe-oxide or siderite predominate or coexist, constraining the meaning of reducing or oxidizing conditions. Moreover, we also observed that siderite formation would require a much longer water residence time than Fe-oxide to precipitate, interpreted as higher weathering rates, or later evaporation required to effectively precipitate under any conditions. On ancient Mars, both Fe and Mn would be relatively mobile and prone to be leached from their host rock. Observing siderite or oxide would not primarily be a redox marker but would be a clue to a different hydrological regime. At the planetary scale, it would be challenging to form authigenic siderite during alteration. Although siderite would not indicate the presence of a particularly reducing atmosphere and that Mn-oxides are mainly pH controlled and do not require terrestrial-level amounts of oxygen, a collocated precipitation of siderite and Mn-oxides could also provide valuable information to constrain the redox environment of the ancient Mars.

The subduction of the Paleo-Pacific plate and the formation of a big mantle wedge have influenced the tectonic-magmatic evolution of eastern Asia. However, the timing and mechanism of big mantle wedge formation remain obscure. Here we report molybdenum (Mo) isotope compositions of Mesozoic-Cenozoic mafic rocks in eastern China, which provide constraints on the geodynamics of Paleo-Pacific subduction. The >125 Ma rocks have high δ⁹⁸Mo values of −0.27‰ to 0.17‰ and arc-like features, whereas the <125 Ma rocks have low δ⁹⁸Mo values of −0.67‰ to −0.04‰ and ocean-island basalt (OIB)-like features. The mantle sources of these two types of rocks contain subducted Paleo-Pacific slab components at sub-arc and mantle transition zone depths, respectively. Therefore, the Paleo-Pacific subduction involves shallow subduction yielding a small mantle wedge in the early stage and deep subduction yielding a big mantle wedge in the late stage. The dramatic change in Mo isotopes reveals that the big mantle wedge beneath eastern Asia was initiated at ∼125 Ma.

Permafrost degradation has led to increased riverine ion concentrations and export to the sea. This study uses major ion data collected in summer in 2012 and 2013 and during spring flood in 2015 to investigate the spatio-temporal variability in chemical weathering patterns and the associated CO₂ consumptions in one of the major Arctic Rivers – the Lena River and its tributaries. The catchment shows strong spatial variations in major ion concentrations in the main river and tributaries. The weathering flux represented by TIS (total inorganic solids) is calculated to be 112 Tg/yr, which is almost double that calculated in an earlier study 20 years ago for the same region. The CO₂ consumption is estimated to be 4.9 Tg C/yr, which is approximately equally shared between weathering of carbonates and silicates, despite two thirds of TIS derived from carbonates and the rest of TIS by silicates and evaporites. Our results suggest an enhanced role for silicate weathering in elemental export and CO₂ drawdown due to the ongoing transition from a near surface-dominated system towards a deep groundwater dominated system caused by permafrost degradation in the Arctic region under a warmer climate. Such an enhanced weathering pattern is also expected in other Arctic rivers; hence, a re-evaluation of the weathering budgets is clearly needed. Our findings improve our understanding of the response of the weathering regime in large Arctic river catchments to ongoing climate-driven permafrost loss, which also sheds lights into the role of land-sea element fluxes in sustaining primary production and carbon cycling on the Arctic shelf seas.

Long-chain alkenones (LCAs) are important tools for paleotemperature reconstructions in lacustrine and marine environments. However, some neutral lipid compounds that co-elute with LCAs often occur in natural sediment samples, which seriously interferes with the identification and quantification of the LCAs. In this study, we report a new flash column chromatography based on silver(I)-dimercaptotriazine (Ag-DMT) functionalized silica material (≡Si-DMT-Ag) for purifying and isolating the individual LCAs from complex lake and marine sediment samples. Compared to silver(I)-mercaptopropyl (Ag-MP) functionalized (≡Si-(CH₂)₃-S-Ag) and silver nitrate (SiO₂ + AgNO₃) impregnated silica materials used by previous studies, the Ag-DMT stationary phase displays the best efficiency in removing co-eluting compounds (including wax ester, hopanoid, hopanoic acid, sterane, alkanoic acid, and some unknown compounds) from the LCAs in our study lake and marine sediment samples. The Ag-DMT material also shows its high retention capacity for the LCAs, probably due to a strong interaction of more positively charged silver sites on Ag-DMT with double bonds. Employing the Ag-DMT chromatography, we develop an optimal solvent elution scheme to efficiently isolate the individual LCAs with the same chain length but different numbers of unsaturation for their isotopic analysis in the future. Our study provides a highly effective method for eliminating co-eluting compounds and isolating individual LCAs for paleoclimate studies.

Stable silicon (Si) isotopes in fluvial sediments can provide insights into understanding silicate weathering processes on the Earth's surface. However, a lack of comprehensive studies has hindered full understanding of the factors influencing Si isotope fractionation during continental weathering. In this study, through the analysis of Si isotopes in fine-grained sediments from 13 rivers surrounding the South China Sea, significant variation of Si isotopes in bulk detrital sediments (<63 μm) was observed, with δ³⁰Si values ranging from −0.17‰ to −1.09‰. At basin scale, the δ³⁰Si values are influenced by multiple controlling factors such as climatic conditions, lithology, and tectonic settings, which have a close relationship with the content of clay minerals. The characteristics of weathering types and intensities are ultimately reflected in the weathering products, specifically clay minerals. Compiling data across multiple grain sizes from major rivers globally, robust correlations based on clay mineral classification between δ³⁰Si and Al/Si ratio have been observed, which are unaffected by regional and grain-size variations. As the dominant clay mineral group transitions from illite/chlorite to smectite and kaolinite, the degree of Si isotope fractionation increases progressively. This sequence indicates a shift from stronger physical erosion to more intensive chemical weathering, suggesting a transition in the weathering regime from weathering-limited to transport-limited. This study reveals the intrinsic link between Si isotopic compositions and clay mineral assemblages, providing implications for similar stable isotope research and offering a potential indicator for understanding continental weathering processes and their contributions to the global carbon cycle.

Cordilleran Granitic batholiths or Andean/Cordilleran batholiths serve as plutonic expressions of continental arcs, offering valuable insights into the processes that operate in large silicic magmatic systems at subduction environments. This work presents a comprehensive geochronological study of the Carboniferous Tabaquito batholith in the Frontal Cordillera of western Argentina, which is the best exponent of the Carboniferous arc-related magmatism in this region. Using new data obtained through LA-MC-ICP-MS and comparing it with previously published data (LA-MC-ICP-MS and SHRIMP techniques), we identify five distinct magmatic events in the Tabaquito batholith: 325 ± 2 Ma; 332 ± 2 Ma; 337 ± 2 Ma; 346 ± 1 Ma, and 362 ± 2 Ma. The 325 Ma event is considered as the best estimate of the crystallization age during the emplacement. The 332 Ma, 337 Ma, 346 Ma and 362 Ma events suggest the presence of zircon antecrysts, implying a complex magmatic system that worked for a protracted time lapse of ca. 40 Myr, or ca. 15 Myr if we consider the ages of 346 and 362 Ma as xenocrysts from a source like the Potrerillos pluton (346 ± 3 Ma to 356 ± 3 Ma), which was interpreted in previous studies as possible source of the parental magmas of the Tabaquito batholith. Furthermore, it has been also identified inherited zircons with ages older than ca. 370 Ma that are consistent with the detrital zircon pattern obtained for the country rock. Inherited zircon ages from the referred country rock yield a maximum deposition age estimated at 387 Ma that is consistent with the fossil ammonoids record, indicating that the granitic rocks were emplaced in Devonian rocks.

Igneous rocks in central Iran preserve proof of several episodes of Late Mesozoic–Cenozoic Neotethyan subduction, and record temporal and spatial variations from subduction to collisional geodynamic regimes that remain a subject of debate. We revisit magmatic rocks from the central Urumieh–Dokhtar magmatic arc (UDMA; 32° 30′ N to 36° 00′ N) and report new major and trace element analyses, whole-rock Sr–Nd–Pb-isotopic data and zircon U–Pb–Hf ages on plutonic rocks from the Kajan region. The data reveal magmatic pulses between 32 and 22 Ma that are represented by igneous rocks with geochemical and isotopic features resulting from melting of a metasomatized, enriched mantle. Modeling indicates the samples represent a mixture of 97% batch melt (from a source with 98.5% mantle melt +1.5% terrigenous sediment) with 3% upper continental crust. The oldest yielded age (i.e., 32 Ma) rules out a lull in magmatism in the central UDMA between 37 and 26 Ma and speaks against distinct flare-up magmatic episodes at ∼54–37 Ma and ∼ 20–5 Ma in the frontal arc. We hypothesize that magmatism records down-going slab tearing into two pieces beneath the central to SE UDMA, which then changes the geometry and buoyancy of subducting Neotethyan lithosphere. The down-going slab remained involved in central UDMA (i.e., rollback) during Early Miocene whilst arc retreat beneath the SE UDMA proceeded with detachment of subducted oceanic lithosphere (i.e., break-off). We hypothesize a diachronous collision in the UDMA, first in the southeastern segments, and subsequently in the central UDMA. These findings bear important implications for the geodynamic evolution of the Zagros orogen and run counter to a model suggesting that the collision initiated in the northwest to central and propagated progressively to the southeast along the Zagros suture zone.

Boron isotopes ($\delta$¹¹B) in coral skeletons of Porites have been widely applied to reconstruct past seawater pH (pH_SW) on decadal to centennial timescales. However, due to biological regulation within corals, an additional transfer function is required to estimate ambient seawater chemistry during the skeleton growth under the calcification site fluid pH. Temperature may also interfere with coral calcification fluid pH (pH_CF) due to changes in kinetics of coral aragonite precipitation, or buffering capacity in coral calcification fluid. To decipher how coral Porites adjusts pH_CF in response to pH_SW from complex environmental controls, long-term records from sites with least fluctuations in environmental conditions other than pH_SW are essential. Here we present a 37-year record of coral $\delta$¹¹B and B/Ca ratios derived from a coral core collected from southern Maldives, the tropical Indian Ocean. Our results show no clear seasonality in the coral $\delta$¹¹B and B/Ca ratios between monsoons, but a long-term decline in coral pH_CF is evident across the entire record. When applying different existing transfer functions, we also observe discrepancies among the calculated pH_CF values, model results and short-term instrumental data. Calculated calcification fluid dissolved inorganic carbon concentration ([DIC]_CF) values are relatively low compared to literature, suggesting that coral calcification fluid carbonate chemistry may be under different levels of control, even within the same coral taxa. Thus, coral records from a wider geographic range are required to better quantify coral response to ocean acidification, and our results can serve as a baseline for future comparisons.