Nianqiao Fang

Changes in ocean ventilation have been pivotal in regulating carbon sequestration and release on centennial to millennial timescales. However, paleoceanographic reconstructions documenting changes in deep-ocean ventilation using 14C dating, may bear multidimensional explanations, obfuscating the roles of ocean ventilation played on climate evolution. Here, we show that previously inferred poorly ventilated conditions in the North Atlantic were linked to enhanced pre-aged organic carbon (OC) input during Heinrich Stadial 1 (HS1). The 14C age of sedimentary OC was approximately 13,345 ± 692 years older than the coeval foraminifera in the central North Atlantic during HS1, which is coupled to a ventilation age of 5,169 ± 660 years. Old OC was mainly of terrigenous origin and exported to the North Atlantic by ice-rafting. Remineralization of old terrigenous OC in the ocean may have contributed to, at least in part, the anomalously old ventilation ages reported for the high-latitude North Atlantic during HS1.

Natural thermoluminescence (TL) from the core of MD81349 marine calcareous biological ooze samples in the Ninetyeast Ridge of the equatorial northeast Indian Ocean and from the core of IODP306-U1312B in the high latitudes of the North Atlantic Ocean was studied. The spurious TL intensity of 395 °C at its peak is dose independent when the heating rate is 6 °C/s in a nitrogen atmosphere. TL signals have exhibited a significant correlation with the marine isotope stages (MIS) in the two oceans since the mid-Pleistocene era. High TL intensity corresponds to a negative δ¹⁸O in the interglacial stages, and low TL intensity corresponds to a positive δ¹⁸O in the glacial stages. The TL of both cores from the two oceans reveal that global climate has experienced eight cold and warm cycles since the mid-Pleistocene era. In this study, a single-frequency spectrum analysis of the MD81349 and U1312B TL cores in the last 300 ka is performed. Near the equator of the northeast Indian Ocean, the short cycles of 38 ka and 5 ka are more significant, while the cycle of 8 ka is more significant in the North Atlantic Ocean. In addition, a correlation analysis shows that the TL has a significant positive correlation with the trace element ¹³⁵Ba and a significant negative correlation with ⁴⁷Ti. The impurity ions (e.g., Ba²⁺, Mn²⁺ and Ti²⁺) doped in carbonate act as activators and suppressants, respectively. A time series of the TL of the calcareous biological ooze tests provide an important record of climate change. The source of the TL signal is also discussed.

Centimeter-scale magmatic enclaves are abundant in peralkaline felsic volcanic rocks in the Sanshui Basin. Their lithology is mainly syenite and syenitic porphyry, and they mainly comprise alkali feldspar and amphibole, which is similar to the mineral assemblage of the host trachyte and comendite. The SiO₂ content in the syenitic enclaves is ~63 wt%, which is similar to that of the host trachyte but lower than that of the comendite. Thermobarometric calculations showed that the syenitic enclaves crystallized at similar temperature and pressure conditions as their host trachyte. The results of mass-balance modeling and MCS modeling indicate that the syenitic enclaves likely experienced an approximately 74% fractional crystallization from the basaltic parental magma. Combined with the similar mineral assemblages and geochemical characteristics of the host trachyte, we think that the enclaves resulted from the in situ crystallization of trachytic magma in the shallow crust and that they had a cogenetic origin with their host volcanic rocks, which means that they were likely to derived from the identical magma chamber which was formed from different batches of magma mixing/mingling. The recharge and mixing of basaltic magma triggered the eruption of trachytic magma eruption. The syenitic crust may have been disaggregated by the ascending trachytic magma and brought to the surface as syenitic enclaves. The syenitic enclaves in volcanic rocks provide unique information on the magmatism of the shallow crust as evidence of magma mixing/mingling.

The Sanshui Basin is located at the northern continental margin of the South China Sea and characterized by a continental rift basin. The bimodal volcanic rocks in Sanshui Basin record the early Cenozoic magmatic activity in the South China Block, but the magmatic evolution that produced the bimodal volcanic rocks is poorly understood. Clinopyroxenes in bimodal volcanic rocks in the Sanshui Basin provide an opportunity to investigate magma during magma ascent. In this work, we classified nine types of clinopyroxene phenocrysts according to composition and texture in cogenetic basalt-trachyandesite-comenditic trachyte, while the composition of unzoned clinopyroxene have an evolution sequence of diopside-hedenbergite-aegirine along with an increase in trace element contents with a decrease of Mg^#, indicating that the genesis of clinopyroxene was dominated by fractional crystallization in a closed magma system. However, the clinopyroxenes with reversed zoning and multiple zoning record the process of magma mixing and recharge indicating an open magma system. While fractional crystallization is the dominant process, magma mixing, recharge, and crystal settling were also found to influence magma evolution. Thermobarometric calculations showed that clinopyroxene crystallized a several structural levels in the crust during magma ascent. In this study, we established a magma plumbing system that provides new constraints for the magma evolution in the Sanshui Basin.

Southeastern Guangxi is located in the southwestern South China Block and to the northwest of the South China Sea (SCS), with abundant records of the Cretaceous magmatism. A detailed study of igneous rocks will contribute to a better understanding of the late Mesozoic tectonic environment. Zircon U–Pb dating yields ages of 93.37 ± 0.43 Ma for Yulin andesites and 107.6 ± 1.2 Ma for Luchuan granites. Yulin andesites are hornblende andesites, of which w(MgO) is between 7.72% and 8.42%, and Mg# is between 66.7 and 68.0, belonging to high magnesian andesites (HMAs) from peridotite sources. Luchuan granites are medium- to fine-grained monzogranites. Monzogranites and clastoporphyritic lava are high-K calc-alkaline series and metaluminum to weakly peraluminous series, which belong to the I-type granites. Those are enriched in Rb, Th, K and LREEs and depleted in Nb, Ta, P and Eu, showing the geochemical characteristics related to subduction. Unlike the contemporary “bimodal igneous rock assemblages” in Zhejiang and Fujian, the intermediate-acid magmatites in the southeastern Guangxi imply the compressive tectonic environment. The assemblage of HMAs and adakitic rocks indicates that the southwestern South China Block was under the Neo-Tethyan subduction during Cretaceous, and slab melting contributed to the magma in this area.

Many siliceous rocks representing different geologic environments are exposed in the Palaeo-Tethys orogenic belt of western Yunnan, China. In this paper, silicon and oxygen isotopes and major and trace elements including rare earth elements (REEs) of siliceous rocks collected from the region are analysed. According to geochemical discriminant proxies, such as Al/(Al + Fe + Mn), REE patterns, the Al-Fe-Mn and δ³⁰Si-Al₂O₃ diagrams, most siliceous rocks in this study are found to have derived from marine biogenic silica, and a few samples were affected by hydrothermal activity. The redox-sensitive trace elements, such as V, U, Ni and Mo, of the siliceous rocks suggest that the water body of the Palaeo-Tethys oceanic basin in western Yunnan experienced great changes between oxic and anoxic environments during its evolution. The major element discriminant diagrams and the δCe, La_N/Ce_N and δ³⁰Si values indicate that the basin of the Palaeo-Tethys was limited and difficult to match with an open ocean basin. The Palaeo-Tethys oceanic basin may have been delimited by several islands with poor circulation of seawater, which is called an archipelagic ocean. The δ³⁰Si values of siliceous rocks deposited in the archipelagic ocean range from 0.3‰ to 0.8‰, which is narrower than the range of radiolarian siliceous rocks reported previously. Unlike typical shallow sea, hemipelagic and pelagic environments, the silicon isotope characteristics of siliceous rocks in the archipelagic ocean do not show a clear trend from shallow sea to deep sea.