Abstract The mean circulation pattern and its mechanism over Caiwei Guyot (1,308–5,600 m) in the Northwest Pacific were studied utilizing 3 years of in situ data. A deep anticyclonic cap was found to enclose the entire guyot from its bottom up to a depth of 728 m, which is composed of a stable but highly asymmetric anticyclonic circulation at the foot and a bottom‐trapped anticyclonic circulation over the summit. On the slope, the circulation is complex with a dominant anticyclonic circulation near the bottom and a weak cyclonic circulation at ∼2,200 m. The anticyclonic cap intensity over the summit is significantly modulated by the time‐varying impinging flow. An intensified cold ring above the summit edge was observed at Caiwei Guyot, which differs from the cold domes observed over traditional conic seamounts. Further analysis suggests that the impinging flow is primarily responsible for the cap formation, and the M 2 tide‐seamount interaction plays a secondary role. The anticyclonic cap may play a role in the local geological distribution.
Abstract Observations of currents and temperatures from four moorings deployed around the deep slope (∼2500 m) of Caiwei Guyot in the Pacific Prime Crust Zone were utilized to investigate topographically trapped waves at low-latitude seamounts. Contrasting with commonly reported persistent diurnal seamount-trapped wave cases at middle and high latitudes, the subinertial variability in deep currents and temperatures at the slope of Caiwei Guyot was primarily characterized by two distinct lower-frequency bands (i.e., 13–24 and 3.3–4.7 days). These subinertial variabilities are interpreted as intermittent seamount-trapped waves and topographic Rossby waves (TRWs). During certain time periods, the observations include key signatures of seamount-trapped waves, such as near-opposite phases of azimuthal velocity (and temperature) on opposite flanks of the seamount, and patterns of temporal current rotation consistent with counterrotating cells of horizontal current propagating counterclockwise around the seamount. After comparing these observations to idealized seamount-trapped wave solutions, we conclude that the 13–24-day (3.3–4.7-day) energy is mainly due to radial–vertical mode 5 (3) for azimuthal wavenumber 1 (3). Sometimes the subinertial energy remained pronounced at only one flank of the seamount, primarily explained as TRWs with 192–379-m vertical trapping scale and 14–28-km wavelength. Upper-layer mesoscale perturbations might provide energy for deep seamount-trapped waves and TRWs. This study highlights the role of topographically trapped waves in modulating the deep circulation at low-latitude seamounts.
Abstract Magmatism of various magnitudes or intensities was widely recognized worldwide in accompanying with the end-Permian mass extinction (EPME) event across the Permian/Triassic boundary (PTB). Meanwhile, hydrocarbon source rocks were pervasively occurring in later-Permian marine carbonate successions. The EPME-related magmatism and later-Permian source rocks were associated spatially and temporally. However, the features of this magmatism and its effects on underlying source rocks were not elucidated. The current study investigated episodes and magnitudes of the magmatism across the PTB from typical South-China profile (i.e., the Pingdingshan section) using conodont-based geochemical proxies at a high-resolution scale (~50 kyr). Integrated trace elemental (Mn, Sr, Rb, and Th) and stable/radioactive isotopic (δ 18 O, δ 13 C, and 87 Sr/ 86 Sr) results revealed that conodonts provided an ideal proxy for chemostratigraphic signatures of ancient seawater, largely because it was more resistant to diagenetic alterations or thermal recrystallization. The conodont-based high-resolution 87 Sr/ 86 Sr values from studied interval (250.50 Ma to 252.00 Ma) showed three decreasing cycles upwardly against a long-term increasing background across the PTB, reflecting three episodes of magmatism. By contrary, the δ 18 O of same resolution and from same interval displayed no similar trend. This inconsistency was probably because that the δ 18 O composition of carbonates from studied section was limitedly altered due to long distance from magmatism center and/or buffering from thick water column. The micrite-based high-resolution δ 13 C exhibited an evolving pattern consistent with long-term background, revealing that the δ 13 C signatures of multiple stages of magmatism during this short-term interval were not inherited by micrites. The episodes of magmatism across the PTB can be correlated to underlying Chihsian source rocks from studied section according to the clustering of oxygen and strontium isotopic compositions of two sets of strata that were spatially and temporally related. The EPME-related magmatism across the PTB exerted great influences on formation of underlying marine source rocks by bring massive heat and pervasive oceanic anoxia.
As the secondary fault of the Niushoushan-Luoshan Fault Zone, the Liumugao Fault records the northeastward expansion of the Tibetan Plateau. Based on WorldDEM data, 11 rivers (R1-R11) that flow across the Liumugao Fault and their drainage basins were extracted using ArcGIS technology and a MATLAB script. The longitudinal profiles, Hack profiles, and stream length-gradient (SL) indexes of these rivers, as well as the hypsometric integral (HI) of their drainage basins, were extracted and calculated and combined with field observations to quantitatively analyze the activity of Liumugao Fault. The results show that: (1) the longitudinal profiles of the 11 rivers are steep, the Hack profiles of most rivers drop sharply at the main fault, and the average HI value of the drainage basins is ∼0.4. These phenomena indicate that the current activity of the fault is strong. (2) The average HI values of the drainage basins in the northern, middle, and southern segments of Liumugao Fault are 0.32, 0.37 and 0.45, respectively. Accordingly, the average normalized stream-gradient (SL/K) values of the three segments are 3.72, 4.64 and 7.16, respectively. These data show that the activity of the Liumugao Fault gradually increases from north to south.
Near-bottom observation data from the manned deep submersible Jiaolong with high-precision underwater positioning data from Weijia Guyot, Magellan Seamounts in the Western Pacific Ocean are reported. Three substrate types were identified: Sediment, ferromanganese crust, and ferromanganese crust with a thin cover of sediment. The ferromanganese crusts show clear zoning and their continuity is usually disturbed by sediments on areas of the mountainside with relatively gentle slope gradients. The identified substrate spatial distributions correspond to acoustic backscatter intensity data, with regions of high intensity always including crust development and regions of low intensity always having sediment. Therefore, acoustic backscatter intensity surveying appears useful in the delineation and evaluation of crust resources, although further more work is needed to develop a practicable methodology.
Ferromanganese nodules are huge metal resources and windows into Earth processes, being widely distributed in vast deep-sea basins covered by sediments. Co-rich ferromanganese nodules are typical hydrogenetic deposits that can effectively scavenge and enrich multiple trace elements in seawater. However, uncertainty still exists regarding the enrichment process of hydrogenetic nodules and their interactions with the distribution of trace elements in seawater. Herein, we analyzed up to 73 elements in Co-rich ferromanganese nodules from the western Pacific and found that high-field strength and redox-sensitive elements are selectively distributed between the Fe and Mn hydroxide phases. These elements are highly enriched in ferromanganese nodules over seawater and upper continental crustal values. The enormous amounts of ferromanganese deposits make them the major, even exclusive, budget for Te, Mn, Co, Ce, Pb, Bi, Pt, Ru, Rh, Ni, and Mo. The distributions of trace elements in seawater are both the cause and result of scavenging by ferromanganese deposits and of biogeochemical cycling. In particular, ferromanganese deposition is responsible for the distributions of scavenged-type elements such as Mn, Co, Ce, Pb, Bi, and Te in seawater. Based on the distributions of elements in seawater and at the water–sediment interface, we propose a new two-stage model for nodule metallogenesis. Stage I is the initial enrichment of trace elements by the sinking of ferromanganese hydroxide colloids, which regulate the distributions of scavenged-type elements in the water column. Stage II is the top-down migration of trace elements dominated by bioparticle cycling, which promotes the re-enrichment of trace elements by ferromanganese hydroxides at the water–sediment interface.
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