Rare-earth elements (REE) in limestones were measured by ICP-MS from the Yangdi section across the Frasnian-Famennian (F-F) transition in Guilin, Guangxi of South China. The average ΣREE contents of measured samples are 33.32 ppm. The average LREE/HREE ratio is 6.35. Data show LREE-enriched REE distribution pattern in the Yangdi section. Most of samples are characterized by positive Eu anomaly with average of 1.13, and there are two maxima of Eu anomaly just below the F-F boundary. Almost all analytical samples show obvious tetrad effects. Remarkable M-shaped tetrad effects are observed in these samples with positive Eu anomaly. The characters of REE and some relative elements (Fe, Mn, Al, etc.) indicate that hydrothermal activities occurred and resulted in seawater acidification in Upper and Lower Kellwasser horizons of the Late Devonian. The changes of marine environment caused by hydrothermal activities, such as seawater acidification, heavy metal pollution, and eutrophication, would destroy the neritic ecosystem and finally result in the F-F mass extinction. Devonian, mass extinction, REE, hydrothermal activities, tetrad effects, seawater acidification Citation:Zeng J W, Xu R, Gong X M. Hydrothermal activities and seawater acidification in the Late Devonian F-F transition: Evidence from geochemistry of rare earth elements.

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The abundance curves derived from area proportions of brachiopods and Girvanella in the thin sections from the Yangdi section, South China, exhibits complex relationships during the Late DevonianFrasnian-Famennian transition. The living activity of brachiopods such as grazing and borrowing did great damage to the growth of Girvanella. However, there was more to just a mere a survival competing relationship between them, Girvanella actually improved marine environments by oxygenating the dysoxic ambient sea-water through photosynthesis in the lower Kellwasser Horizon. Profited from this improvement brachiopods' abundance increased subsequently and suppressed Girvanella again. Nonetheless, without Girvanella's photosynthesis, brachiopods were wiped out by the farther anoxic environments in the upper Kellwasser Horizon. The complex relationships between Girvanella and brachiopods may be a key to unlocking the relationships between geomicrobes and metazoans in the geological overturn periods.brachiopods, Girvanella, anoxia, Late Devonian, F-F, mass extinction, South ChinaThe Late Devonian Frasnian-Famennian (F-F) transition event was one of the five greatest mass extinctions in Phanerozoic time [1][2][3][4] . It was characterized by the demises of more than 60% marine creatures, particularly tropical shallow-water metazoans, whereas high-latitude, deep-water and terrestrial ecosystems were only slightly affected [5][6][7][8][9] . Since the 1990s, the F-F event has been considered a typical stepwise deterioration process rather than a sudden crash of the ecosystem [10][11][12] , and was closely related to oceanic environmental changes. Evidences of sedimentary facies, geochemistry and paleontology have demonstrated hostile and capricious environments in the Late Devonian ocean including anoxia in shallow sea-water [13][14][15] , frequent eustasy [16][17][18][19][20] , climatic change from warm to cold [21][22][23] , and blooming of microorganisms induced by eutrophication [15,[24][25][26][27][28][29] .The F-F environmental changes inflicted heavy losses on metazoans.There was always a spreading of geomicrobes on the heels of the mass extinction of metazoans during Phanerozoic time, with world-wide occurrences of various kinds of microbial sediments [30,31] . Two flourishes of cyanobacteria coupled with the Permian-Triassic faunal crisis were reported and were considered as the result of both the depression of survival stress and nutrients input [32] . It seems that the only chance of the geomicrobes to re-dominate the biosphere is for the crashes of metazoans and changes of environments that are suitable for the geomicrobes. Girvanella are the remains of the calcified autotrophic filamentous cyanobacterial sheaths. They were very common during the Late Devonian, usually twisted into nodular masses or sometimes formed microbial mats. Girvanella were one of the most important non-skeletal reef-building geomicrobes. Was there any relationship between the blooming of Girvanella and the decline of metazoans? This paper wil...

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