[M-IS12] Paleoclimatology and paleoceanography

[MIS12-28] Orbital-Scale Variability in the Miocene Silicon Cycle

*Jumpei Yoshioka¹, Junichiro Kuroda¹, TAKAAKI ITAI² (1.Department of Ocean Floor Geoscience, Atmosphere and Ocean Research Institute, the University of Tokyo, 2.Department of Earth and Planetary Science, the University of Tokyo)

Keywords:Miocene, Si cycle, Ge/Si ratio, δ30Si, orbital-scale oscillations

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 Ge/Si ratios and δ³⁰Si oscillated between the glacial and interglacial periods (e.g., Mortlock et al., 1991; Sutton et al., 2018). However, the lack of reconstructions with high time resolution leaves it unknown whether such orbital-scale oscillations existed during the Miocene or older periods. It is one of the obstacles to revealing a long-term shift in the Si cycle.
In this study, we constructed a cyclostratigraphic high-resolution age model of the Miocene diatomaceous sediment exposed on Sado Island in Niigata Prefecture based on cyclic oscillations in the ratios of bSi to detritus. We reconstructed Ge/Si ratios and δ³⁰Si of diatom frustules, which were extracted from sediment samples taken at an interval of ca. 15 kyr. 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. High-resolution reconstruction of the Miocene Si cycle reveals Quaternary-like orbital-scale oscillations in the Ge/Si ratios and δ³⁰Si, which are characterized by lower values of both during colder periods with lower bSi contents. It indicates that mechanisms controlling the Si cycle are similar between the Quaternary and the Miocene.
Additionally, we evaluated factors controlling the Si cycle by 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 dSi utilization ratio, a decline in bSi preservation efficiency, and a decrease in riverine input.
This study provides very new information about the Miocene Si cycle. However, further accumulation of temporal and spatial data is needed to understand a long-term evolution in the Si cycle and its background mechanisms.