The content of organic carbon, nitrogen and phosphorus in deep-water biocenoses of coralline algae of the South China sea

The study on the influence of the depth of habitation on the content of organic carbon (Corg), organic and mineral forms of nitrogen (N) and phosphorus (P) in the biocenosis of coralline algae, was carried out in spring in the South China Sea at depths of 5 to 150 m. It was shown that with an increase in depth to 150 m, the content of Corg in the tissues decreased from 72 to 42 g/m2. The content of N also decreased with depth, but to a lesser extent — from 15 to 10 g/m2, while the content of P increased linearly from 0.75 to 4.29 g/m2. The atomic ratio C/N/P in the biocenosis at depths of 5–10 m does not vary significantly and is on the average 187:31:1, which is comparable to the mean values obtained for mass species of reef-building corals. Unlike organic carbon and nitrogen, the phosphorus content in the algal biocenosis increased with depth more than 5 times. Correspondingly, the C/N/P ratio becomes 24:5:1 at a depth of 150 m. Such depth-dependent increase in the proportion of phosphorus in the biocenosis, with a simultaneous decline in the contents of organic carbon and, to a lesser extent, nitrogen, can be explained by release of phosphorus during destruction of the organic matter in the biocoenosis of coralline algae with the formation of insoluble calcium phosphates.

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Structure and Composition of Micro-Manganese Nodules in Deep-Sea Carbonate from the Zhaoshu Plateau, North of the South China Sea

Minerals ◽

10.3390/min10111016 ◽

2020 ◽

Vol 10 (11) ◽

pp. 1016

Author(s):

Hengchao Xu ◽

Xiaotong Peng ◽

Kaiwen Ta ◽

Taoran Song ◽

Mengran Du ◽

...

Keyword(s):

South China Sea ◽

South China ◽

Deep Sea ◽

Carbonate Rocks ◽

The South China Sea ◽

Coralline Algae ◽

Manganese Nodules ◽

The South ◽

Raman Spectrometry ◽

China Sea

The occurrence of deep-sea ferromanganese nodules and crusts on the seafloor is widespread, providing an important resource for numerous metals such as Ni, Co, and Cu. Although they have been intensively studied in the past, the formation of micro-manganese nodules within carbonate rocks has received less attention, despite the considerable amounts of manganese released from the dissolution of the calcareous framework. The micro-petrographic and geochemical characteristics of reef carbonate rocks recovered from the Zhaoshu plateau in the Xisha uplift, north of the South China Sea, were studied using optical microscopy, scanning electron microscopy, confocal Raman spectrometry, and an electron probe micro-analyzer. The carbonate rocks are composed of biogenic debris, including frameworks of coralline algae and chambers of foraminifer, both of which are suffering strong micritization. Within the calcite micrite, numerous micro-manganese nodules were identified with laminated patterns. Mineral and elemental evidence showed that the Mn oxides in the carbonates are mixed with 10 Å vernadite, 7 Å vernadite and todorokite, both of which are closely associated with the carbonate matrix. The micro-nodules were found to have high Mn/Fe ratios, enriched in Ni and Cu and depleted in Co. We infer that theses nodules are mixed type with early diagenetic growth under oxic–suboxic conditions. The re-distribution of manganite within the rocks is likely influenced by micritization of the calcareous framework. We deduce that microbial-associated reduction of manganite induces the formation of diagenetic todorokite similar to nodules buried in marine sediments.

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