Contribution of terrestrially derived phytolith as a marine silicon sink

Silicon is important as a nutrient for phytoplankton (diatom, radiolarian, silicoflagellates and sponges) and for the phytolith production by terrestrial vegetation. Silicon also contributes in removing carbon dioxide from the atmosphere through silicate weathering.  Hence it is important to understand the behavior of the silicon cycle throughout earth history. Silica is the second most abundant element in the earth's crust and the concentration of silicic acid in the marine environment has not changed since the past 10,000 years. Phytolith plays an important role in the silicon cycle. While the phytoplankton in marine environment bioengineers silica within the water column, phytolith transports terrestrial biogenic silica into the marine environment and act as a silicon sink. Though astonishingly, very few researches have been carried out in the field of marine phytolith sink and also on the phytoliths in the marine environment.

For this study, we have chosen the world highest terrestrial sediment receiving submarine fan, the Bengal fan. The core sample was extracted at a water depth of 3520m at 85.960985 N, 9.99351 E. 24 phytolith types were identified and all the morphotypes were counted dividing into three size classes. These size classes were specific to considering morphotypes. Most related simple geometries were used to calculate the volume of phytolith cells and these volume data were used in calculating the total volume of phytolith in one gram of sediment by combining with an absolute abundance of phytolith data for each size class, which were later used to calculate the total weight of phytolith in one gram of marine sediment. According to the results in deep oceanic sediment at the core, the location contains ⁓0.15mg/g phytolith during the low phytolith flux periods (ex. Late Holocene) and ⁓2.678mg/g of phytolith during the high phytolith flux periods such as 25ka to 30ka B.P. and around the beginning of deglaciation. After removing 10% from the total weight as phytolith occluded carbon (PhytOC), phytolith derived biogenic silica content in sediment varies from ⁓0.135mg/g - ⁓2.41mg/g. Thus, phytolith in marine sediment contributes as a permanent silicon and carbon sink. By considering average marine sediment density as 1.7g/cm³, in a 1cm thick, one square km sediment layer contains ⁓2 to 40 metric tons of biogenic silica derived from phytolith, during low and high phytolith flux periods. This study serves as the pioneer of this field of study and further it is important to investigate the release of biogenic silica in to marine environment by phytolith and PhytOC content in different morphotypes and in different geological regions, for better understanding the contribution of phytolith to the biogenic silicon cycle in the marine environment.

Keywords: Marine phytolith, Deep oceanic sediment, Silicon cycle, Phytolith Flux, Silicon sink.

Acknowledgements

This work was funded by the National Natural Science Foundation of China (NSFC 41876062) and Key Special Project for Introduced Talents Team of Southern Marine Science and EngineeringGuangdong Laboratory (Guangzhou) (GML2019ZD0206).