Geochemistry Geophysics Geosystems最新文献

Dating young lava flows is essential for understanding volcano's eruption frequency, yet challenging due to methodological limitations of commonly used dating techniques. Ruapehu (Aotearoa New Zealand) produced many lava flows during the Holocene, but constraints on the timing of these eruptions are scarce. Here, we use paleomagnetic dating to deliver new eruption ages of 18 lava flows with uncertainties ranging between 500 and 2,700 years (at the 95% confidence level). Comparison between lava flows' paleomagnetic directions and a local paleosecular variation record indicates that the large lava flow field located on the Whakapapa area was emplaced during at least three distinct eruptive episodes between 10600 and 7400 BP. Two of these episodes closely followed a large collapse event that affected Ruapehu's northern area and generated large volumes of lava between 10600 and 8800 BP, with the third episode producing less voluminous lava flows between 8100 and 7400 BP. Following a smaller collapse of the southeastern sector of the edifice at ca. 5300 BP, several low-volume lava flows were emplaced during at least two distinct eruptive episodes prior to ca. 1000 BP, which supplied the Whangaehu valley with lava. The youngest age inferred from our data represents the youngest eruption age provided for a lava flow outside Ruapehu's summit region. This research provides greater detail to the Holocene effusive chronology at Ruapehu, shedding light on partial cone reconstructions after edifice collapses during the Holocene, and the time relationships between trends observed in its effusive and explosive activity.

Bamboo corals are promising archives of paleoceanographic conditions. Existing calibrations for element-to-calcium ratio (El/Ca) proxies of bamboo corals, however, are not necessarily calibrated to contemporaneous environmental parameters, thus weakening the reliability of the proxies. Here, we aim at calibrating the proxies by comparing El/Ca in the outermost surface of the calcareous skeletons of live-collected bamboo corals from the South China Sea (SCS) with modern environmental records. Statistical analysis suggests that Mg/Ca and Ba/Ca can be expressed as a function of in situ seawater temperature and silicate concentration, respectively, that is, Mg/Ca (mmol/mol) = 2.17 ± 0.51 * T (°C) + 74.90 ± 2.66 and Ba/Ca (μmol/mol) = 0.070 ± 0.020 * Silicate (μmol/kg) + 7.27 ± 2.42. The slope of the Mg/Ca-T equation from this study is slightly different from that in a previous study on bamboo corals, likely due to taxonomic and/or geographic differences of the corals and/or differences in sampling strategy and pre-treatment method. Intra- and inter-coral variations have small effects on Mg/Ca, yielding an uncertainty of 2.04 mmol/mol in Mg/Ca (95% confidence interval), equivalent to 0.94°C in estimated temperature. The slope of the Ba/Ca-silicate equation is the same as that in a previous study, suggesting little effect of geographic difference on Ba/Ca. Intra- and inter-coral variations in Ba/Ca are larger than those in Mg/Ca, possibly reflecting incorporation of multiple Ba-rich particulate phases and/or highly variable nutrient concentrations in the micro-environment near corals. These new calibrations allow reconstructions of paleo-temperature and nutrient concentration in the SCS on decadal and longer timescales.

Upper mantle deformation is mainly controlled by the mechanical behavior of olivine. Crystallographic preferred orientations (CPOs) develop in olivine due to crystal-plastic deformation during mantle flow, where the a-axes of olivine polycrystalline aggregates are aligned with the flow direction. Therefore, the observed CPO in olivine-rich rocks is used as an indicator of the mantle flow direction. Experimental data show that olivine rheology is strongly controlled by the microstructure. While the influence of plastic deformation is in general well characterized, the role of dynamic recrystallization during deformation is not totally understood, limiting our ability to interpret the deformation history of naturally deformed rocks. This contribution presents microdynamic numerical simulations of olivine polycrystalline aggregates with different iron content (i.e., fayalite content) with the aim of exploring the CPO and grain size response to dynamic recrystallization. We use a full-field approach with an explicit simulation of viscoplastic deformation and dynamic recrystallization processes under simple shear boundary conditions up to high strain. The simulations show that the CPOs are similar and practically reach the same maximum regardless of the iron content. CPOs are characterized by a single cluster of a-axis and two-clusters of b-axis, reveling a joint activity of the easy glide [100](010) and the moderate strength [100](010) slip systems. High-strain domains of our models are consistent with experimental results, showing an A-type fabric with double maxima, and where the CPO is aligned with the shear direction. The model provides a deeper understanding of the dynamic recrystallization influence on olivine CPOs resulting from plastic deformation.

A large mantle helium anomaly and separate domains of high heat flow are the predominant manifestations of bimodal magmatic activity in the Milford valley. The mantle helium anomaly (1.9–2.6 R/Ra) covers 270 km² and is subdivided into two separated domains: a cold shallow groundwater regime and high temperature hydrothermal activity. The zone of anomalous heat flow covers >100 km² and is also subdivided into two adjacent domains, comprising hydrothermal activity at Roosevelt Hot Springs (RHS) (3–7 W/m²) and conductive heat flow (100–180 mW/m²). While the transfer of heat and mantle helium at RHS are coupled, heat and helium transfer are decoupled in the adjacent cold groundwater regime to the west. Both the mantle helium and geothermal anomalies are attributed to recent mafic-felsic magmatic intrusions of >400 km³, however, the absence of volcanic eruptions <500,000 years indicates magmas stall before rising to shallow crustal level <10 km depth. Deep level magmatism produces a felsic composition melt, which is inferred to be responsible for the widespread and near uniform range of diluted mantle helium values. A thick and impermeable mass of crystalline granitic basement rock at the mid-crustal level divides the ascent of mantle helium into separate flow paths. It may also impede the rise of buoyant magma trapping thermal energy that facilitates partial melting, slow cooling, and development of a thick thermal aureole. Partitioning of convective and conductive thermal regimes and independent flow paths supplying deeply derived helium characterize the development of a large long-lived magma-related geothermal system.

The Caribbean-South America subduction zone is a flat subduction zone, with Laramide-style thick-skinned uplifts occurring in the Merida Andes, Sierra de Perija Range, and Santa Marta Massif. Geodetic measurements and historical seismicity show this region is storing strain energy and is capable of a mega-thrust earthquake (M ≥ 8.0). Previous seismic investigations of the lithosphere and upper mantle in this area are either very large scale, very local, or only peripheral to this area; therefore, details of the Caribbean plate subduction geometry beneath the Maracaibo block remain unclear. In this study, we used a new data set acquired by the Caribbean-Merida Andes seismic experiment (CARMA), which comprised 65 temporary broadband stations and 44 permanent stations from the Colombian and Venezuelan national seismic networks. We jointly inverted ambient noise Rayleigh wave Z/H ratios, phase velocities in the 8–30 s band and ballistic Rayleigh wave phase velocities in 30–80 s band to construct a 3-D S-wave velocity model in the area between 75°–65°W and 5°–12°N. The 3-D model reveals a general increase in crust thickness from the trench to the southeast. An anomalous area is the Lake Maracaibo, which is underlaid by the thinnest crystalline crust in the region. This observation may indicate that the Maracaibo block is experiencing a contortion deformation within the crust. We also identified a high velocity anomaly above the subducting Caribbean slab, likely representing a detached piece of eclogitized Caribbean large igneous province from the base of the Maracaibo block. Additionally, our V_s model clearly indicates a slab tear within the subducted Caribbean slab, approximately beneath the Oca-Ancon Fault.

Knowledge of Late Miocene-Early Pliocene environmental change is critical for understanding the interactions among global cooling, regional tectonic activity and Asian climatic evolution. However, their relationships remain unclear, partly due to the scarcity of quantitative reconstructions of temperature and hydroclimatic conditions, which limits our understanding of the effect of topography on montane climatic evolution. Here, we quantitatively reconstructed the Late Miocene-Early Pliocene temperature, hydroclimate, and pH of the Xiaoshuizi peneplain, northeastern Tibetan Plateau (NETP), based on glycerol dialkyl glycerol tetraethers. Our results indicate that the Xiaoshuizi temperature was relatively high with large fluctuations during 6.2–4.6 Ma. The temperature then gradually decreased until 4.0 Ma, when this trend was interrupted by an intensive warming event. Additionally, the combined R_i/b and pH proxies revealed that the Xiaoshuizi region experienced a hydroclimatic transition from relatively wet to dry conditions after 4.0 Ma. Our integrated results show that the Xiaoshuizi climate became warm and dry during 4.0–3.6 Ma, in contrast to the global cooling trend and the occurrence of a humid climate on the eastern Chinese Loess Plateau and the North China Plain. We ascribe this anomalous change to a warm and dry climate during 4.0–3.6 Ma to a topographic Foehn effect triggered by surface uplift of the Maxian Mountains associated with extensive tectonic uplift of the NETP during the Middle Pliocene.

Deep fracture-hosted fluids of Precambrian bedrock cratons are relatively stagnant over long time spans compared to near-surface systems. However, episodic events, such as fracture reactivations, transgressions, and deglaciations, may introduce dilute water, replacing, and mixing with the deep continental brines, thereby sparking microbial activity. Secondary minerals that line bedrock fractures serve as important geochemical archives for such episodic events. Here we explore the fracture mineral record of Archean rocks of Western Greenland by analyzing samples from deep boreholes with the aim to trace and characterize episodic paleofluid flow and paleomicrobial activity. A sequence of hydrothermal to low temperature fluid flow events is demonstrated. For the youngest generation, microscale S-isotope analysis of pyrite reveals substantial ³⁴S-depletion (minimum δ³⁴S:−58‰V-CDT) compared to fracture-hosted barite (δ³⁴S:13‰ ± 2‰) and gypsum (δ³⁴S:2.6‰–10.6‰). This suggests the formation of pyrite following S isotope fractionation during microbial sulfate reduction. This metabolism is further indicated by several methyl-branched fatty acids preserved in calcite. A general discrepancy between calcite and groundwater δ¹⁸O-values suggests that calcite formed from water different from the presently residing glacial meltwater-influenced groundwater mix. High spatial resolution U-Pb carbonate geochronology of the youngest generation of calcite yielded ages for two samples: 64 ± 3, 75 ± 7 Ma (2σ). These ages overlap with tectonic events related to early stages, or prestages, of the opening of the Atlantic and Labrador Seas. This suggests that deep fracture networks in Western Greenland were colonized by microorganisms, such as sulfate reducers, in the course of this extensional event.

Anthropogenic activities increase the level of dissolved heavy metals in some tropical near-shore environments threatening reef ecosystems. The skeleton of stony corals like Porites species potentially provides a high-resolution geochemical archive for past heavy metal concentrations, with potentially century long records revealing baseline values before large-scale human disturbance. However, few data exist for heavy metal partitioning into coral skeleton aragonite. To address this, culturing experiments exposing Porites lobata and Porites lichen to a mixture of dissolved Cr, Mn, Ni, Cu, Zn, Ag, Cd, Sn, Hg, and Pb over a wide concentration range have been performed. Water samples were taken frequently to monitor changes in the heavy metal concentration. Laser ablation ICP-MS measurements of the coral aragonite revealed metal concentrations that were positively correlated with Cr, Mn, Ni, Zn, Ag, Cd, and Pb concentrations in seawater. The D_TE values for most metals appear dependent on the seawater metal content, approximating a power law, and therefore stabilize at higher seawater metal/Ca ratios. The partitioning of Pb into the coral skeleton is a notable exception, with D_Pb being stable around 2 to 1 across a large range of “natural” to highly polluted seawater Pb concentrations. This and the general agreement with partition coefficients estimated by previous work suggests that the reconstruction of the heavy metal concentration in seawater for ecosystem monitoring is possible. However, the high variability within and between coral colonies requires further study and suggests that multiple records from multiple coral colonies should be combined to obtain robust reconstructions.

Seawater transported into the South Atlantic from the Indian Ocean via “Agulhas leakage” modulates global ocean circulation and has been linked to glacial-interglacial climate cycles. However, constraining past Agulhas leakage has been a challenge. We sampled a transect of the Cape Basin in winter 2017 that intersected a mature Agulhas eddy and found that the ¹⁵N/¹⁴N ratio (δ¹⁵N) of mixed-layer nitrate, zooplankton, and foraminifera (tissue and shells) was 2‰–3‰ lower in the eddy than in the background Atlantic even though the δ¹⁵N of the underlying thermocline nitrate was indistinguishable between the two settings. We suggest that the δ¹⁵N of foraminifera and other zooplankton in the eddy reflects the original Agulhas Current thermocline nitrate, which is ∼2‰ lower than that of the South Atlantic due to N₂ fixation that occurs in the Indian Ocean. Foraminifera δ¹⁵N may have been lowered further during eddy migration by in situ N₂ fixation and/or recycling of low-δ¹⁵N ammonium. The absence of low-δ¹⁵N Agulhas nitrate in the eddy thermocline can be explained by partial assimilation of the nitrate as it was mixed into the euphotic zone during and after eddy formation, raising its δ¹⁵N. The low δ¹⁵N of eddy foraminifera, apparent even after several months of eddy migration across the Cape Basin, suggests that fossil foraminifer-bound δ¹⁵N from the region could record variations in past Agulhas leakage.