Silicon isotope-based optimization of biogenic silica measurements and its application in marine sediments

Die Li^1,3, Quanchao Cui^1,3，Haiyang Liu^1,4，Tzu-Hao Huang⁵，Wei-li Hong^5,6， Yuanyuan Xiao^2,4，Weidong Sun^1,3,4，Xiaole Sun^1,3*
1 Key Laboratory of Ocean Observation and Forecasting, Center of Deep Sea Research, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, PR China
2 CAS Key Laboratory of Marine Geology and Environment, Institute of Oceanology, Chinese Academy of Sciences, Qingdao 266071, PR China
3 University of Chinese Academy of Sciences, Beijing 100049, PR China
4 Ocean Sciences and Interdisciplinary Frontiers, Laoshan Laboratory, Qingdao 266237, PR China
5 Department of Geological Sciences, Stockholm University, 10691, Sweden
6 Baltic Sea Centre, Stockholm University, 10691, Sweden

*Corresponding author xiaole.sun@qdio.ac.cn

Keywords: Stable Si isotopes; Biogenic silica; Silicon phases; Marine sediment; Pore water
Sedimentary biogenic silica (BSi) has been well known as an important achieve of marine productivity. However, biogeochemical processes in marine sediments can lead to large alterations of different silicon (Si) phases, which challenges the application of BSi in tracking primary production. This study proposes to use stable Si isotopes to 1) improve the traditional wet-chemical sequential leaching method for determination of BSi contents and 2) optimize the recovery efficiency of the brucite co‐precipitation method (MAGIC) that used to purify dissolved Si (DSi)in porewater. We have applied the method to four sediment cores with distinct BSi contents and porewater with different matrix. Our results show that 1) for sediment samples with BSi content below 5%, the mineral phase reaches a linear dissolution within 1 hour, while it takes at least 3 hours for those with BSi content above 5%, 2) silicon isotope values of the first-hour Na₂CO₃ leachates of sediment samples with BSi content of > 10% could reflect the Si isotope composition of BSi. Meanwhile, we also suggest a pretreatment of hydrogen peroxide to remove organic matter and facilitate BSi dissolution during alkaline digestion. For MAGIC, when the ratio of magnesium to silicon in porewater samples is below 100, DSi recovery efficiency cannot be guaranteed to be better than 95%, possibly leading to biased Si isotope values due to the isotope fractionation during chromatographic purification. Our findings indicate that complete dissolution of BSi depends on various factors, including the concentration of the alkaline solution, organic matter content, the specific types of BSi, and alterations via early diagenetic processes. Proper BSi leaching and porewater methods should be carefully assessed based on the characterizations of sediment cores and marine environment that the cores are taken.

 

stable Si isotopes, biogenic silica, silicon phases, marine sediment, pore water