ALBERT

Description: Rocks and ore deposits in subduction zones can provide clues to material cycling and related subduction processes. Here we demonstrate, based upon a new data compilation from more than 240 late-Mesozoic fluorite deposits in southeast China that most of them can be divided into three fluorite deposit belts (FDB) with distinct ages and orientations. The two older inland FDB with a NE-SW orientation were formed at 165–150 Ma and 150–130 Ma, respectively, whereas the younger coastal FDB, roughly perpendicular to the others, was generated at 110–70 Ma. The temporal-spatial distributions of the FDB are compatible with the subduction of the (paleo-) Pacific plate. We propose that the genesis of the earlier two inland FDB was due to northeastward slab rollback during low-angle subduction, while formation of the coastal FDB resulted mainly from high-angle northwestward hot/warm subduction. Large amounts of F, mainly derived from the decomposition of phengite (±apatite) in the subducted slabs, could be transported into the overlying crust via F-rich fluids and alkalic magmas. Subsequently, extensive crustal magmatism and fluid activities associated with subduction further concentrate F from the magmas and/or wall-rocks and facilitate fluorite mineralization. This study provides new insights into the interaction between the subducting (paleo-) Pacific plate and overlying eastern Eurasian continent in the late Mesozoic. It also highlights the potential genetic association of F transport/deposition with plate subduction that could be a common phenomenon in ancient and modern subduction zones.

Description: Geobiology explores how Earth's system has changed over the course of geologic history and how living organisms on this planet are impacted by or are indeed causing these changes. For decades, geologists, paleontologists, and geochemists have generated data to investigate these topics. Foundational efforts in sedimentary geochemistry utilized spreadsheets for data storage and analysis, suitable for several thousand samples, but not practical or scalable for larger, more complex datasets. As results have accumulated, researchers have increasingly gravitated toward larger compilations and statistical tools. New data frameworks have become necessary to handle larger sample sets and encourage more sophisticated or even standardized statistical analyses. In this paper, we describe the Sedimentary Geochemistry and Paleoenvironments Project (SGP; Figure 1), which is an open, community-oriented, database-driven research consortium. The goals of SGP are to (1) create a relational database tailored to the needs of the deep-time (millions to billions of years) sedimentary geochemical research community, including assembling and curating published and associated unpublished data; (2) create a website where data can be retrieved in a flexible way; and (3) build a collaborative consortium where researchers are incentivized to contribute data by giving them priority access and the opportunity to work on exciting questions in group papers. Finally, and more idealistically, the goal was to establish a culture of modern data management and data analysis in sedimentary geochemistry. Relative to many other fields, the main emphasis in our field has been on instrument measurement of sedimentary geochemical data rather than data analysis (compared with fields like ecology, for instance, where the post-experiment ANOVA (analysis of variance) is customary). Thus, the longer-term goal was to build a collaborative environment where geobiologists and geologists can work and learn together to assess changes in geochemical signatures through Earth history. With respect to the data product, SGP is focused on assembling a well-vetted and comprehensive dataset that is tractable to multivariate statistical analyses accounting for multiple geological and methodological biases. Phase 1 of the project, which focused on the Neoproterozoic and Paleozoic, has been completed. Future phases will capture a broader range of geologic time, data types, and geography. The database contains tens of thousands of unpublished data points provided by consortium members, as well as detailed metadata that go beyond what is contained in papers. In many cases, these represent measurements that are tangential to a given published study but still of high utility to database studies; these allow the community to address questions that would be impossible to answer solely with the published data. For instance, in order to use a proxy such as Mo/TOC (total organic carbon) ratios in mudrocks deposited under a euxinic water column, the full suite of trace metal, iron speciation, and total organic carbon data is needed. Likewise, geospatial information is required to account for sampling biases, and many statistical learning approaches cannot accept, or have difficulty with, incomplete geological predictor variables. Ultimately, it is this complete data matrix that will allow for SGP’s most insightful analyses.

Description: Several recent studies have shown that garnet U-Pb dating may potentially provide robust constraints on the ages of carbonatite, alkaline magmatism and skarn ore formation. Here we present textural, trace element, and laser ablation inductively coupled plasma mass spectrometry U-Pb isotope data of garnet from the Yinan Au-Cu skarn deposit in Luxi District, eastern North China Craton, to place tight constraints on the timing of mineralization. Three hydrothermal andradite-rich garnet samples (JL-8, THZ-38 and TJ-13) collected respectively from Jinlong, Tonghanzhuang and Gongquan ore segments of the Yinan Au-Cu skarn deposit, contain 2.31–10.36 ppm U and are characterized by flat time-resolved depth profiles for U, indicating that this element is hosted in crystallographic lattice in the garnets. The positive correlation between U and LREEs concentration suggests that the incorporation of U into the garnet is largely controlled by substitution mechanisms. The three garnet samples yield weighted mean 206Pb/238U dates of 127 ± 3 Ma, 127 ± 5 Ma and 126 ± 7 Ma (2σ), respectively. These ages are reproducible within analytical uncertainties and are consistent with a zircon U-Pb age of 129 ± 2 Ma of the ore-related diorite intrusion, and thus, are interpreted as the age of Au-Cu mineralization at Yinan. The results presented here, when combined with existing isotopic age data, demonstrate that Au-Cu skarn mineralization throughout the southeastern Luxi District occurred, coevally, in the Early Cretaceous and is genetically associated with the Tongjing and Jingchang granitic-dioritic complexes. The Au-Cu mineralization and associated magmatism coincide in time with the iron skarn mineralization and associated magmatism (134–126 Ma) across the Luxi District, strongly suggesting a causal link between the district-wide skarn mineralization and pervasive intermediate to felsic magmatism; both are products of destruction of lithospheric mantle beneath the North China Craton. This study confirms that garnet U-Pb dating can be a robust geochronometer for garnet-bearing hydrothermal deposits and can elucidate associated geological processes.

Description: To expand the newly developed ARM glasses as reference materials for in situ microanalysis of isotope ratios and iron oxidation state by a variety of techniques such as SIMS, LA-MC-ICP-MS and EPMA, we report Li-B-Si-O-Mg-Sr-Nd-Hf-Pb isotope data and Fe2+/ΣFe ratios for these glasses. The data were mainly obtained by TIMS, MC-ICP-MS, IR-MS and wet-chemistry colorimetric techniques. The quality of these data was cross-checked by comparing different techniques or by comparing the results from different laboratories using the same technique. All three glasses appear to be homogeneous with respect to the investigated isotope ratios (except for B in ARM-3) and Fe2+/ΣFe ratios at the scale of sampling volume and level of the analytical precision of each technique. The homogeneity of Li-B-O-Nd-Pb isotope ratios at the microscale (30–120 μm) was estimated using LA-MC-ICP-MS and SIMS techniques. We also present new EPMA major element data obtained using three different instruments for the glasses. The determination of reference values for the major elements and their uncertainties at the 95% confidence level closely followed ISO guidelines and the Certification Protocol of the International Association of Geoanalysts. The ARM glasses may be particularly useful as reference materials for in situ isotope ratio analysis.

Description: The observational records have shown a strengthening of the Pacific Walker circulation (PWC) since 1979. However, whether the observed change is forced by external forcing or internal variability remains inconclusive, a solid answer to more societal relevantly question of how the PWC will change in the near future is still a challenge. Here we perform a quantitative estimation on the contributions of external forcing and internal variability to the recent observed PWC strengthening using large ensemble simulations from six state-of-the-art Earth system models. We find the phase transition of the Interdecadal Pacific Oscillation (IPO), which is an internal variability mode related to the Pacific, accounts for approximately 63% (~51–72%) of the observed PWC strengthening. Models with sufficient ensemble members can reasonably capture the observed PWC and IPO changes. We further constrain the projection of PWC change by using climate models’ credit in reproducing the historical phase of IPO. The result shows a high probability of a weakened PWC in the near future.

Description: Extratropical SST variability associated with ocean dynamics has well-documented effects on the atmospheric boundary layer, but it is still unclear to what degree and through which mechanisms it influences the free troposphere. The mesoscale features of this SST variability (eddies, fronts) are not captured using typical climate model resolutions, making their large-scale effects difficult to study. We use the Norwegian Earth System Model version 2 (NorESM2) with two ocean setups - standard 1° resolution (MM) and eddy-permitting 0.125° resolution (MX) - both coupled to a 1° atmosphere. With a higher resolution ocean, MX is overall warmer and has different large-scale SST patterns compared to MM. MX exhibits stronger surface turbulent heat fluxes from the ocean to the atmosphere in regions where it is warmer than MM (notably, the western boundary currents, eastern tropical Pacific, and South Atlantic), indicating that SST differences drive the heat flux anomalies. In the extratropical free atmosphere, MX exhibits more energetic storm tracks and a poleward shift of the eddy-driven jets and storm tracks in all sectors and seasons except the North Atlantic during winter. These analyses, together with atmosphere-only experiments forced by SSTs from MX and MM, suggest that the large-scale SST patterns and fine-scale ocean features in an eddy-permitting ocean play a role in setting the jets and storm tracks. Future work will use SST pacemaker experiments to isolate and investigate the underlying coupled mechanisms for how mesoscale ocean variability affects the extratropical atmospheric circulation.

Description: Rossby wave propagation is one of the most important mechanisms for teleconnections between regions. This propagation relies on the meridional gradient of vorticity, which gets progressively weaker at higher latitudes. Despite this, we show that Rossby waves do have a role to play in high latitude variability, albeit a weaker one than at lower latitudes. In this presentation we outline two teleconnection pathways: 1) how waves propagating from lower latitudes behave as they reach the polar regions; 2) how waves can be triggered at high latitudes by anomalous surface heating and then propagate equatorward.

Description: The Southern Ocean sea-ice budget, which consists of dynamic and thermodynamic processes, synthesizes many important aspects of air-sea-ice interaction. However, many Earth System Models struggle to reproduce realistic sea-ice budget partly because of inadequate understanding of required model parameter adjustments. In this study, we investigate the sensitivity of sea-ice concentration and volume budgets to 18 sea-ice model parameters in the Southern Ocean. The analysis is based on a total of 449 model runs and two sensitivity methods. We found that the simulated sea-ice budgets are sensitive to sea-ice strength, snow thermal conductivity, the number of sea-ice thickness categories, lateral melting, and air-ice drag and ice-ocean drag. Optimized ice-ocean and air-ice drag coefficients reduce the model error in terms of sea-ice concentration budget about 10%. Ten combinations of sea-ice model parameter values yielding to improved sea-ice extent, and sea-ice volume and concentration budgets were identified.

Description: Utilizing the Community Atmospheric Model version 4, the role of Arctic sea ice concentration (SIC) in triggering the Ural Blocking (UB) event formation on sub-seasonal timescale is investigated. Taking eight zonal flows as the reference states, the optimal Arctic SIC perturbations that trigger zonal flows into UB events on sub-seasonal timescale are obtained with the conditional nonlinear optimal perturbation approach. The numerical results show that the Arctic SIC decline in the Greenland, Barents and Okhotsk Seas can trigger zonal flows into UB events in four pentads. Further diagnosis shows that SIC decline in these regions locally warms the low troposphere via diabatic processes in the first pentad. Then, dynamic processes, such as temperature advection, modulate the temperature in the middle troposphere and weaken the meridional temperature gradient between the Arctic and mid-latitudes upstream of the Ural sector. The weakened meridional temperature gradient further decelerates the westerly near the Ural sector and triggers UB formation in four pentads. This investigation highlights the role of Arctic SIC in the Greenland, Barents and Okhotsk Seas in triggering the formation of UB events on sub-seasonal timescale. The spatial characteristics of the optimal SIC perturbations may provide scientific support for SIC target observations in capturing the precursor of UB formation and improving UB prediction skills on sub-seasonal timescales.