Fe is the most abundant variable-valence element in igneous rocks, and is also an important mineralizing element, mainly in the solid (mineral) and liquid (fluid) phases in Fe²⁺ or Fe³⁺ valence state, and participates in magmatic processes and various mineralization throughout. With the development of test analytical techniques (e.g. MC-ICPMS), the analysis of non-traditional stable isotope compositions such as Fe has become possible and has been successfully applied to the study of important geological processes such as magma source tracing, tracing of crystallization evolutionary processes and mineralization analysis in the last decade or so. Based on the analysis of the fractionation effect of Fe isotopes during magmatism, this paper summarized the latest results of Fe isotope composition studies in tracing the action of seafloor basaltic magmas (MORB, OIB, IAB and BABB, etc.) and discussed the main problems in the application of Fe isotope composition in tracing the action of seafloor magmas. The results of the comprehensive analysis show that the Fe isotope fractionation effect in igneous rocks is influenced not only by the processes of partial melting of magma source material, magma diffusion, fluid exsolution and crystallization differentiation, but also by the assimilation of surrounding rock material and seafloor alteration. Since Fe isotope analysis techniques (methods) have yet to be further refined, and the available data are limited and need to be screened for artifacts, caution is still needed when using Fe isotope compositions to analyze or recover magmatic sources and processes. It is urgent to establish a complete and reliable Fe isotope tracing system, which requires the recent work to select as many suitable samples as possible representing different tectonic environments and different rock types, to obtain (accumulate) more fine analytical data of original (unmodified or altered) samples, and to pay attention to the combination or mutual corroboration of multiple data in the process of using Fe isotope tracing for seafloor magmatism.

The Atlantic meridional overturning circulation (AMOC) is an important component of the climate system, of which change in the strength can affect meridional heat distribution between the northern and southern hemispheres. Proxy records show that changes in Atlantic Ocean circulation during the Late Pleistocene is associated with precessional cycle, but its physical mechanism remains unclear. Here we use a fully coupled climate model to investigate dynamics associated with AMOC changes in precessional band under glacial-interglacial climate conditions. Our results show that increase in boreal summer insolation can effectively weaken the AMOC during warm interglacial periods, while this weakening effect is reduced under glacial maximum. We further demonstrate that during the warm interglacial period increase in boreal summer insolation leads to sea surface warming and subpolar rainfall increase in North Atlantic, which jointly reduces sea surface density and hence the strength of deep water formation. During the glacial maximum period, climate responses to precessional change is of anti-phase impacts on the AMOC. At the low latitudes, a low pressure anomaly triggered by subtropical warming weakens atmospheric moisture export from the subtropical Atlantic to Pacific, increasing in net precipitation and hence freshening tropical sea surface in the North Atlantic. At the high latitudes, the warming-induced sea ice retreat promotes ocean heat loss via the enlarged ice-free area, and hence tends to strengthen the vertical mixing. The combined effects of low- and high-latitude responses finally leads to a trivial weakening of the AMOC. Overall, our results provide a systematic understanding of governing mechanism for precessionally-induced AMOC change under glacial-interglacial climatic backgrounds, shedding light on our interpretation of precessional periodicity in reconstructed ocean circulation changes during the Pleistocene.

This study utilizes a new functional analysis tool, multiscale window transform (MWT), to decompose the ocean circulation system in the Bay of Bengal (BOB) into three scale windows, namely, the background flow window (>96 days), the mesoscale window (24–96 days) and the high-frequency window (<24 days), and then uses the canonical energy transfer theory to investigate the intrinsic nonlinear multiscale interactions among these windows, on the basis of an eddy-resolving model simulation. It is found that multiscale interactions are strongest along the northwestern boundary and east of Sri Lanka. With intense barotropic and baroclinic instabilities, the canonical transfers of kinetic energy (KE) and available potential energy (APE) are mainly forward in these two regions. Mesoscale eddy kinetic energy (EKE) reservoir is mainly filled by the barotropic energy pathway with the kinetic energy of the background flow transferring to EKE, and secondarily from the baroclinic energy pathway with APE of the background flow transferring to the mesoscale APE and further converting to EKE. The gained EKE is found to further cascade to high-frequency motions, acting as an important dissipation mechanism of the mesoscale eddies in these regions. In contrast, the central BOB is mainly characterized by inverse KE cascades, where EKE and high-frequency kinetic energy (HKE) are gained via the baroclinic energy pathway, and then feed the background flow through inverse cascade processes. The northwest of Sumatra is also an area with strong mesoscale and high-frequency variability. Both barotropic and baroclinic energy pathways are the sources for EKE and HKE reservoirs in this region, with the baroclinic energy pathway playing the dominant role.

Previous studies have shown that the decadal modulation of the Kuroshio extension (KE) system is controlled by the Pacific decadal oscillation-associated forcing from downstream. However, recent observation reveals that this mechanism ceases to function after August 2017. Meanwhile, a large meander is under development in the KE’s upstream, i.e., south of Japan. Using the self-organizing map (SOM), we investigate the characteristic spatial and temporal patterns of the Kuroshio south of Japan and the KE and their causal relations, based on the 26-year (1993−2018) satellite altimetry data of sea level anomaly (SLA). The typical spatial patterns are well extracted, and their temporal trajectories indicate that the KE tends to be stable (unstable) when the upstream Kuroshio takes a large meander (an offshore nonlarge meander) path. To further unravel the underlying cause-and-effect relation between the two systems, we apply the information flow-based causality analysis to the typical regions of SLA and its associated temporal modes identified with the SOM. It is found that during the large meander event, the Kuroshio south of Japan and the KE are mutually causal, but have different hotspots. The information flows from the former to the latter mainly occur in the southeastern area off the Kii Peninsula and the time-mean ridge and trough of the KE jet, while those from the latter to the former are mainly concentrated in the time-mean ridge and trough of the KE jet, and the recirculation gyre of the Kuroshio. These results indicate that the Kuroshio large meander is an important factor influencing the KE’s stability, while the KE affects its upstream Kuroshio via modulating the associated recirculation gyres. In contrast, when the offshore nonlarge meander path is taken, a one-way causality is identified from the Kuroshio to the KE, mainly occurring over the Izu-Ogasawara Ridge and in the recirculation gyres. This may be attributed to the constantly downstream transport of negative SLAs into the KE’s recirculation gyre, which leads to an unstable KE.