The continental Si cycle and its impact on the ocean Si isotope budget

Summary, in English

The silicon isotope composition of biogenic silica (δ³⁰Si_BSi) in the ocean is a function of the δ³⁰Si of the available dissolved Si (DSi; H₂SiO₄), the degree of utilisation of the available DSi, and, for some organisms, the concentration of DSi. This makes δ³⁰Si_BSi in sediment archives a promising proxy for past DSi concentrations and utilisation. At steady-state, mean δ³⁰Si_BSi must equal a weighted average of the inputs, the majority of which are of continental origin. Variation in the functioning of the continental Si cycle on timescales similar to the residence time of DSi in the ocean (~10 ka) may therefore contribute to downcore variability in δ³⁰Si_BSi on millennial or longer timescales. The direction and magnitude of change in published δ³⁰Si_BSi records over the last few glacial cycles is consistent among ocean basins and between groups of silicifiers. They document glacial values that are typically 0.5 to 1.0‰ lower than interglacial values and together hint at coherent and predictable glacial-interglacial variability in whole-ocean δ³⁰Si driven by a change in mean δ³⁰Si of the inputs. In this contribution, we review the modern inputs of DSi to the ocean and the controls on their isotopic composition, and assess the evidence for their variability on millennial-plus timescales.Today, 9.55 × 10¹² mol yr^-1 DSi enters the ocean, of which roughly 64% and 25% are direct riverine inputs of DSi, and DSi from dissolution of aeolian and riverborne sediment, respectively. The remainder derives from alteration or weathering of the ocean crust. Each input has a characteristic δ³⁰Si, with our current best estimate for a weighted mean being 0.74‰, although much work remains to be done to characterise the individual fluxes. Many aspects of the continental Si cycle may have differed during glacial periods that together can cumulatively substantially lower the mean δ³⁰Si of DSi entering the ocean. These changes relate to i) a cooler, drier glacial climate, ii) lowered sea level and the exposure of continental shelves, iii) the presence of large continental ice-sheets, and iv) altered vegetation zonation. Using a simple box-model with a Monte-Carlo approach to parameterisation, we find that a transition from a hypothesised glacial continental Si cycle to the modern Si cycle can drive an increase in whole ocean δ³⁰Si of comparable rate and magnitude to that recorded in δ³⁰Si_BSi. This implies that we may need to revisit our understanding of aspects of the Si cycle in the glacial ocean. Although we focus on the transition from the last glacial, our synthesis suggests that the continental Si cycle should be seen as a potential contributory factor to any variability observed in ocean δ³⁰Si_BSi on millennial or longer timescales.

• Author: Patrick J. Frings; Wim Clymans; Guillaume Fontorbe; Christina L. De La Rocha; Daniel J. Conley
• Department/s: Quaternary Sciences
• Publishing year: 2016-05-01
• Language: English
• Pages: 12-36
• Publication/Series: Chemical Geology
• Volume: 425
• Links: http://dx.doi.org/10.1016/j.chemgeo.2016.01.020
• Document type: Journal article
• Publisher: Elsevier
• Topic: Geochemistry
• Keywords: Biogenic silica; Biogeochemical cycling; Global silicon cycle; LGM; Palaeoceanography; Silicon isotopes
• Status: Published
• ISBN/ISSN/Other: ISSN: 0009-2541