Production, Dissolution, Sedimentation Of Biogenic Silica And Its Preservation Mechanisms In Prydz Bay Of The Southern Ocean

The Southern Ocean represents a crucial study area for better comprehension of the global biogeochemical cycles of silicon and carbon. Diatoms are a critical component of the biological pump of the Southern Ocean, exporting both biogenic silicon and organic carbon to the deep sea. For biogeochemical and paleoceanographic reasons, it is very important that more studies be devoted to the key processes of the Si cycle in the Southern Ocean, and it would be much more helpful to better understand the spatial and temporal changes in carbon cycling in the southern ocean. In addition to the Weddell Sea, Ross Sea, Prydz Bay is the largest embayment along the Antarctic margin in the Antarctic continent, and lies in the Indian Ocean sector of East Antarctica. However, compared with the related research carried out in the Weddell Sea, Ross Sea, the crucial processes of biogenic silica in Prydz Bay cognitive are still very scarce. To characterize the biogenic silica production, sinking and transform processes, and reveal the preservation mechanism in the up surface waters, and sediments, the production, dissolution, sedimentation of biogenic silica were discussed based on the insitu data during the austral summer from2002to2013and laboratory-based experiments data. Detailed results are shown below:1. Biogenic silica production and export from surface water in Prydz Bay.(1) The results obtained during the CHINARE cruise show the PBSi concentrations ranged from0.38to8.62μmol.dm-3in the surface waters of Prydz Bay in the austral summer of2012/2013, and higher concentrations located in the southern area of67°S, decreasing with depth in the upper water columns. Chla in surface water showed similar distribution patterns to PBSi. The annual difference in the average concentration of PBSi in surface water is obvious, but the distribution pattern is similar. (2) Biological factor is the dominant control factor of PBSi content distribution in the austral summer of Prydz Bay. Climate change will have a certain impact on the variation of the sea ice, while the phytoplankton will change with the variation of the sea ice in the population structure correspondingly, as a result, the PBSi content and distribution is also impacted.(3) The average value of Si:C ratio during the bloom in surface waters of Prydz Bay was0.21, which is similar to the average value of the Southern Ocean. The Si:C ratio decreased with depth in the upper200m reflect the decoupling of the export progresses of biogenic silica and organic carbon from surface layer to deep water. Higher rates of biogenic silica dissolution, and lower rates of organic matter remineralization in the upper200m lead to the lower export flux of biogenic silica than organic carbon, and the reduced Si:C ratio.(4) Using seasonal depletions of Si(OH)4observed in the surface mixed layer, we estimate the biogenic silica production rates of Prydz Bay, the average production rate is14.54mmol/m/d, which is similar with the production rate20.20mmol/m/d, obtained by using Si:C ratio and multiplying the14C primary production. Combine the results of the insitu incubation experiment with Si isotope dilution technique, we calculated that the surface layer to the subsurface layer transfer was89%for the biogenic silica stock, and only36%down to200m, which mean that large fraction of biogenic silica dissolved during the transport progress below the euphotic zone layer to200m.2. Transfer of biogenic silica through the deep sea in Prydz Bay(1) Diatom debris and aggregates were most prevalent of the settling particulate matter collected from Prydz Bay polynyas. While in the north open sea of Prydz Bay, Larvacean houses was also found in the samples during high flux summer period. Different types of aggregates in the settling particulate matter of the inner area and outer area of Prydz Bay, illustrate the different source of particulate matter and associated biogeochemical process in the upper water of the inner area and outer area of Prydz Bay.(2) Biogenic material was a significant component of the sinking particle and biogenic silica represented the main portion of the biogenic matter, in three sediment trap periods. The fluxes of biogenic silica and organic carbon changed seasonally and were mainly influenced by the growth of the phytoplankton in the upper waters. Peak fluxes were observed during the summer months of January-February in all three trap survey periods, and low fluxes were throughout the winter-spring months. The settling particulate matters exported Prydz Bay polynyas displayed higher biogenic silica fluxes than that in the north open sea of Prydz Bay during the same summer months in three trap periods.(3) The impact of climate change to biological productivity in the surface water of Prydz Bay, could be reflected in the sinking particle fluxes. ElNin~o event leaded to the bloom of phytoplankton reaching the maximum in December and until January began to weaken during the austral summer of2009/2010. While impacted by La Nina event, phytoplankton productivity in the same austral summer period of2010/2011was much lower than2009/2010. Therefore, interannual difference was obvious of the biogenic silica fluxes in the austral summer of2009/2010and2010/2011.(4) Sibio/Corg molar export ratios in the austral summer of2009/2010and2010/2011were2.5and1.8, much higher than the ratio in200m waters. The enhancement of the Sibio/Corg export ratios with increasing water depth indicated an elevated sub-euphotic rate of Corg remineralization versus biogenic silica preservation. When the phytoplankton exported from the euphotic zone aged and die, leads to faster C degradation, the senescent population was the main part of the sinking, so that there was a rapid increase in the Si:C ratios.(5) We are able to determine water column preservation efficiencies (=rain rate/production rate) for silica. Biogenic silica export at480m represents26%of the summer biogenic silica production in surface waters during2009/2010, as compared to a preservation efficiency value of24%during2009/2010. For silica the export from50m to200m represents36%, then the dissolution of biogenic silica during the transfer from200m to deep water, was much less than the upper water.3. Biogenic silica recycling at the sediment-water interface(1) Surface sediments from the Prdz Bay are characterized by very high BSiO2contents, with the range4.89-75.32%, and a regional pattern of contents show that maximum concentrations of BSiO2were found in the continental shelf, and the lowest contents occurred in the north area of67°S. The down core concentration of BSiO2displayed a decrease from surface layer to the deep depth in the south area of67°S, however, the profile of BSiO2in the sediment core of north area was increased with depth.(2) The Sibio/Corg ratios in sediments of Prydz Bay range from4.6to18.48were much higher than the ratios in the water column and the profile of biogenic silica and organic carbon display the opposite trend, indicating the differences in their experiences of physical, chemical and biological reactions in the sedimentary burial processes.(3) The pore water concentrations of DSi in the surface sediments display a larger range of variation, from118.15-552.00μmol/dm3,with the average contents352.43μmol/dm3. Sharp gradients appeared near the sediment-water interface in the profiles except the profile of Fram bank area, and are more pronounced in the station of continental shelf. The concentration of DSi increased from the value75.19μmol/dm3, the mean DSi concentration in the bottom water, to the value of352.43μmol/dm3at0cm of the sediments.(4) In contrast with the profile of BSiO2in the core sediment, with depth increased the concentration of DSi in pore water, exponentially approached to a stable asymptotic concentration of Cd. Profile distributions of DSi show that the main diagenetic reactions of biogenic silica were dissolution of silicate. High value of Cd occurred in67.5°S and68°S station of continental shelf were644.01、651.26μmol/dm3, and the lowestcontent472.84μmol/dm3was found in the Fram bank area of66.86°S.(5) Using pore water modeling benthic dissolution fluxes of silica were estimated, the fluxes fall within a range,0.30-1.15mmol/m2/d, and a regional pattern of fluxes show that the values found in the Fram bank area is obviously higher than that in the continental area and ice margin area. However, we found that fluxes estimated from in situ incubation results were very similar in values and regional pattern to those from modeling estimated, which fluctuated between0.45and0.63mmol/m2/d. All combine to suggest that most portion of diffusive fluxes are result from dissolution in the upper5cm of the sediment core.4. Dissolution and preservation of biogenic silica in sediments(1)Batch silica solubility was measured in laboratory. The results show that the apparent silica solubility in surface sediment of continental shelf and ice margin was1936μmol/dm3and1540μmol/dm3, respectively. The solubilities in the cores show the decreasing trends with depth, however, the decreasing degree in the core of continental shelf was lower than ice margin core. Compared to the asymptotic pore water concentrations of DSi, the Cd values are clearly lower than the laboratory-measured solubilities.(2) The batch experiment results show that the biogenic silica dissolution rate in the laboratory of continental shelf sediment is higher than the ice margin sediment, and the dissolution rate of the two areas is2.01μmol/h/g and1.09μmol/h/g in surface sediments, respectively. In the top5cm sediments, the dissolution rate reduced quickly, however, under the5cm the dissolution rate undergo little change. Generally in Prydz Bay biogenic silica dissolution rate in surface sediments is higher than that in deep sediments, and in surface sediments (0-4.5cm) biogenic silica represent faster dissolution rather than in deep sediments.(3)The burial fluxes of biogenic silica in sediments of Prydz Bay were range from0.09to2.06mol/m2/a, the latitudinal distribution of the burial fluxes display a higher value occurred in continental shelf area of67.5-68°S and the lower value was found in Fram bank area. Compared with the results estimated in other areas of the Southern ocean, the preservation efficiency in Prydz Bay, which ranges between18-94%with a mean of79%, was much higher. These show that preservation efficiency of biogenic silica in the sediment of the Southern Ocean was varied spatially.(4) The preservation efficiency of biogenic silica depends upon several processes, not only the production in euphotic zone, but also including the transport and burial processes. In Prydz Bay, Al play important influence on the preservation of biogenic silica, and this effect has also been confirmed in other area. In addition, sedimentation rate and the bioturbation intensity also have a substantial impact on the diagenesis process and ultimately burial of biogenic silica.5. The budget of silicon in Prydz Bay(1) We estimated a production of biogenic silica of3.64mol/m2/a in Prydz Bay. This estimate is similar to the average production of2.35mol/m2/a of the Southern Ocean, reported by Treguer in2013. There64%of this flux is directly recycled in the upper surface water, and the rest part is exported to the deep ocean, the export flux is1.31mol/m2/a also similar to the average value of the Southern Ocean1.48mol/m2/a.(2)The exported biogenic silica continues to dissolve as it sinks through the deep ocean, the rate of the surviving silica deposition on the seafloor is1.09mol/m2/a, where dissolution continues. The material that escapes dissolution through sediment-water interface buried in sediment is0.86mol/m2/a. The fraction of exported biogenic silica buried in Prydz Bay sediments is around66%, is much higher than the average preservation efficiency26%of the Southern Ocean.