2:52 PM - 3:15 PM
[MIS14-25] The Miocene Silicon Cycle and Siliceous Sediments in the Japan Sea
★Invited Papers
Keywords:silicon (Si) cycle, Miocene, the Japan Sea, siliceous sediment, biogenic silica (bSi)
The global silicon (Si) cycle is one of the important factors in understanding climate systems in the geological timescale since the Si cycle is deeply related to the carbon cycle through silicate weathering and biological pumps. There are two developing proxies for reconstruction of the past Si cycle: germanium/silicon (Ge/Si) ratios and Si stable isotope (δ30Si) of biogenic silica (bSi). These proxies are usually applied for the Quaternary sediments, and it is known that the proxies oscillated between the glacial and interglacial periods. On the other hand, scarce proxy data from the Miocene sediments make it impossible to reveal the details of the Miocene Si cycle.
In this study, we focused on the Miocene siliceous sediments in the Japan Sea. They sometimes show orbital-scale oscillations in the sediment compositions, especially the ratios of bSi to detritus. Since bSi is the largest output in the Si cycle, the deposition of the siliceous sediments was possibly subject to the strong influence of the Si cycle. Taking advantage of such properties of the siliceous sediments, we constructed a cyclostratigraphic high-resolution age model based on cyclic oscillations in the ratios of bSi to detritus and reconstructed Ge/Si ratios and δ30Si of diatom frustules separated from the sediment samples. Ge concentrations, Si concentrations, and δ30Si were measured by using isotope dilution-hydride generation-inductively coupled plasma-mass spectrometry (ID-HG-ICP-MS), ICP-atomic emission spectrometry (ICP-AES), multicollector-ICP-MS (MC-ICP-MS), respectively.
The reconstructed Ge/Si ratios and δ30Si show orbital-scale oscillations, whose characteristic is similar to the Quaternary Si cycle, with lower values during colder periods and lower bSi contents. It indicates that the mechanism controlling the Miocene Si cycle is conformable to that of the Quaternary. Otherwise, the absolute values and the amplitudes of the Miocene Ge/Si ratios and δ30Si differ from those in the Quaternary. Then, we evaluated factors controlling the Si cycle using a box model. We examined the individual effects of five factors on Ge/Si ratios and δ30Si of bSi: dissolved silica (dSi) utilization ratio in the surface layer, bSi preservation efficiency into the sediment, riverine input, hydrothermal input, and sill depth of the Japan Sea. The box model calculation reveals that lower Ge/Si ratios and δ30Si during colder periods can be explained by a combination of a decrease in the dSi utilization ratio, a decline in the bSi preservation efficiency, and a decrease in the riverine input.
The newly obtained data in this study have a higher time resolution than the previous reconstructions of the Miocene Si cycle and provide a lot of new information. However, further accumulation of temporal and spatial data is needed to understand a long-term evolution in the Si cycle and evaluate its actual influence on the global climatic evolution.
In this study, we focused on the Miocene siliceous sediments in the Japan Sea. They sometimes show orbital-scale oscillations in the sediment compositions, especially the ratios of bSi to detritus. Since bSi is the largest output in the Si cycle, the deposition of the siliceous sediments was possibly subject to the strong influence of the Si cycle. Taking advantage of such properties of the siliceous sediments, we constructed a cyclostratigraphic high-resolution age model based on cyclic oscillations in the ratios of bSi to detritus and reconstructed Ge/Si ratios and δ30Si of diatom frustules separated from the sediment samples. Ge concentrations, Si concentrations, and δ30Si were measured by using isotope dilution-hydride generation-inductively coupled plasma-mass spectrometry (ID-HG-ICP-MS), ICP-atomic emission spectrometry (ICP-AES), multicollector-ICP-MS (MC-ICP-MS), respectively.
The reconstructed Ge/Si ratios and δ30Si show orbital-scale oscillations, whose characteristic is similar to the Quaternary Si cycle, with lower values during colder periods and lower bSi contents. It indicates that the mechanism controlling the Miocene Si cycle is conformable to that of the Quaternary. Otherwise, the absolute values and the amplitudes of the Miocene Ge/Si ratios and δ30Si differ from those in the Quaternary. Then, we evaluated factors controlling the Si cycle using a box model. We examined the individual effects of five factors on Ge/Si ratios and δ30Si of bSi: dissolved silica (dSi) utilization ratio in the surface layer, bSi preservation efficiency into the sediment, riverine input, hydrothermal input, and sill depth of the Japan Sea. The box model calculation reveals that lower Ge/Si ratios and δ30Si during colder periods can be explained by a combination of a decrease in the dSi utilization ratio, a decline in the bSi preservation efficiency, and a decrease in the riverine input.
The newly obtained data in this study have a higher time resolution than the previous reconstructions of the Miocene Si cycle and provide a lot of new information. However, further accumulation of temporal and spatial data is needed to understand a long-term evolution in the Si cycle and evaluate its actual influence on the global climatic evolution.