Surface Chlorophyll-A Fronts in the Yellow and Bohai Seas Based on Satellite Data

Chlorophyll fronts are important to monitor and map the oceanic front, especially in the season when sea surface temperature (SST) fronts weaken. In this study, surface chlorophyll-a (chl-a) fronts in the Yellow and Bohai seas were characterized for the first time using satellite data. Five distinct chl-a fronts (i.e., the Bohai Strait, Shandong Peninsula, Jiangsu, Liaodong Peninsula, and Korean Peninsula fronts) were observed in summer along the 40 m isobaths and faded in other seasons. Notably, these fronts coincided with SST fronts. Strong chl-a fronts emerged during summer due to chl-a blooms in eutrophic coastal waters paired with surface chl-a fading in strongly stratified offshore waters and coastal physical fronts. Although SST fronts were strong during winter, light limitation and strong vertical mixing in offshore waters led to low chl-a in both coastal and offshore waters, suppressing chl-a front formation. Both chl-a and SST fronts coincided with steep seabed slopes (slope ratio > 1), suggesting that seabed slope may be an indicator of oceanic front location.

Whole-rock and zircon evidence for evolution of the Late Jurassic high Sr/Y Zhoujiapuzi granite, Liaodong Peninsula, North China Craton

Middle-Late Jurassic high Sr/Y granitic intrusions are extensively exposed in the Liaodong Peninsula, in the eastern part of the North China Craton (NCC). However, the genesis of the high Sr/Y signature in these intrusions has not been studied in detail. In this study, we report results of zircon U-Pb dating, Hf isotopic analysis and zircon and whole-rock geochemical data for the Late Jurassic Zhoujiapuzi granite in the middle part of the Liaodong Peninsula. The Zhoujiapuzi granite is high-K (calc-alkaline) and peraluminous in nature, with high SiO2 (68.1–73.0 wt %) and Al2O3 (14.5–16.8 wt %), low in TFe2O3 (1.10–2.49 wt %) and MgO (0.10–0.44 wt %), and with high Sr/Y (19.9–102.0) and LaN/YbN (14.59–80.40). Morphological and chemical studies on zircon grains show that there are two stages of zircon growth, interpreted as magmatic evolution in two distinct stages. The early stage of zircons (ESZ) reflects a crystallization environment of low oxygen fugacity and high TZr-Ti (Ti-in-zircon thermometer values: 669–792 °C); the late stage of zircons (LSZ) formed with high oxygen fugacity and lower TZr-Ti (498–720 °C). LA-ICP-MS U-Pb zircon dating yielded the formation ages of the ESZ and LSZ of ~162 ± 1 Ma and ~158 ± 1 Ma, respectively, with similar εHf(t) values in the range of &minus26.3– −22.8. Interpretation of the elemental and isotopic data suggests that the Zhoujiapuzi granite was a I-type granite derived from partial melting of basement in the region: ~2.17 Ga Liaoji granites. The high Sr/Y signature is most likely inherited from these source rocks. Based on the geochemical features and regional geological data, we propose that the Liaodong Peninsula in the Late Jurassic was part of a mature continental arc, with extensive melting of thick crust above the Paleo-Pacific subduction zone.