Relationships of interannual variability in SST and phytoplankton blooms with giant jellyfish (Nemopilema nomurai) outbreaks in the Yellow Sea and East China Sea

Giant jellyfish (Nemopilema nomurai) outbreaks in relation to satellite sea surface temperature (SST) and chlorophyll-a concentrations (Chl-a) were investigated in the Yellow Sea and East China Sea (YECS) from 1998 to 2010. Temperature, eutrophication, and match–mismatch hypotheses were examined to explain long-term increases and recent reductions of N. nomurai outbreaks. We focused on the timing of SST reaching 15 °C, a critical temperature enabling polyps to induce strobilation and enabling released ephyra to grow. We analyzed the relationship of the timing with interannual variability of SST, Chl-a, and the timing of phytoplankton blooms. Different environmental characteristics among pre-jellyfish years (1998–2001), jellyfish years (2002–2007, 2009), and non-jellyfish years (2008, 2010) were assessed on this basis. The SST during late spring and early summer increased significantly from 1985 to 2007. This indicated that high SST is beneficial to the long-term increases in jellyfish outbreaks. SST was significantly lower in non-jellyfish years than in jellyfish years, suggesting that low SST might reduce the proliferation of N. nomurai. We identified three (winter, spring, and summer) major phytoplankton bloom regions and one summer decline region. Both Chl-a during non-blooming periods and the peak increased significantly from 1998 to 2010 in most of the YECS. This result indicates that eutrophication is beneficial to the long-term increases in jellyfish outbreaks. Timing of phytoplankton blooms varied interannually and spatially, and their match and mismatch to the timing of SST reaching 15 °C did not correspond to long-term increases in N. nomurai outbreaks and the recent absence.     

Late Cenozoic volcanic activity in the Chugoku area, southwest Japan arc during back-arc basin opening and reinitiation of subduction

Abstract Temporal–spatial variations in Late Cenozoic volcanic activity in the Chugoku area, southwest Japan, have been examined based on 108 newly obtained K–Ar ages. Lava samples were collected from eight Quaternary volcanic provinces (Daisen, Hiruzen, Yokota, Daikonjima, Sambe, Ooe–Takayama, Abu and Oki) and a Tertiary volcanic cluster (Kibi Province) to cover almost all geological units in the province. Including published age data, a total of 442 Cenozoic radiometric ages are now available. Across‐arc volcanic activity in an area approximately 500 km long and 150 km wide can be examined over 26 million years. The period corresponds to syn‐ and post‐back‐arc basin opening stages of the island arc. Volcanic activity began in the central part of the rear‐arc ca 26 Ma. This was followed by arc‐wide expansion at 20 Ma by eruption at two rear‐arc centers located at the eastern and western ends. Expansion to the fore‐arc occurred between 20 and 12 Ma. This Tertiary volcanic arc was maintained until 4 Ma with predominant alkali basalt centers. The foremost‐arc zone activity ceased at 4 Ma, followed by quiescence over the whole arc between 4 and 3 Ma. Volcanic activity resumed at 3 Ma, covering the entire rear‐arc area, and continued until the present to form a Quaternary volcanic arc. Adakitic dacite first occurred at 1.7 Ma in the middle of the arc, and spread out in the center part of the Quaternary volcanic arc. Alkali basalt activities ceased in the area where adakite volcanism occurred. Fore‐arc expansion of the volcanic arc could be related to the upwelling and expansion of the asthenosphere, which caused opening of the Japan Sea. Narrowing of the volcanic zone could have been caused by progressive Philippine Sea Plate subduction. Deeper penetration could have caused melting of the slab and resulted in adakites. Volcanic history in the Late Cenozoic was probably controlled by the history of evolution of the upper mantle structure, coinciding with back‐arc basin opening and subsequent reinitiation of subduction.