Reconstructing paleo-ocean silicon chemistry and ecology during Last Glacial Maximum, a biogeochemical cycle modeling approach

It has been established by a number of investigators that opal content and Si-C isotope studies in the marine sediments reveal information about paleooceanography and the impact on silicic acid utilization by marine autotrophes (diatoms, silicoflagellates) and heterotrophes (radiolarians) during the Last Glacial Maximum (LGM). Opal, as an amorphous form of SiO₂, formed by marine Si-secreting organisms, has been used as a proxy to indicate chemical ocean evolution, paleoproductivity and temperature variations in the paleoenvironment and regional ocean water biogeochemical studies, both on million- and thousand-year scales. Here, we are using a model of the global silicon biogeochemical cycle to understand and reconstruct evolutionary history of the paleobiogeochemical cycle and paleoenvironment since LGM. The model is process-driven, temperature-driven, and land-ocean-sediment coupled with specific marine Si-secreting organisms that represent different trophic levels and physiological mechanisms. Specifically, Si utilization by marine silicoflagellates and radiolarians are each about 5% of that of ubiquitous marine diatoms. Available marine reactive Si is controlled by variation of diatom bioproduction that represents 5% of the total marine primary productivity (Si/C Redfield ratio in the marine organic matter is ~0.13, which is an order of magnitude higher than ratio in land organic matter). River input of Si is controlled by chemical weathering of silicate rocks and biocyling of land plant phytoliths. Decreasing dissolved and particulate Si input from land and less favorable climatic condition into LGM diminished the primary production of marine diatoms. However, because radiolarians favor deep-water habitat, where a higher level of DSi is found and that is less affected by temperature changes, a peak of relative abundance is usually observed in sedimentary record during LGM. Given that opal formation fractionated seawater δ³⁰Si (1‰) and enriched seawater with heavier ³⁰Si, the sediment isotope records of δ³⁰Si variations have been found to support the suppressed diatom Si biological bump. Since the LGM, the temperature increased about 5°C and contributed to almost 50% of land bio-productivity rise. On the other hand, warming climate and enhanced hydrological cycle drove the chemical weathering of continental silicate rocks and soil phytoliths remineralization that increased riverine input of dissolved Si, that promoted the Si utilization of diatom in the surface waters. We present a history of species-specific Si utilization due to the environmental and hydrological changes on land and in the ocean. We also use and compare available data of opal sedimentation, Si and C isotope records, as well as temperature history to better quantify the biogeochemistry of the Si cycle during LGM and in later time. The results provide insight into paleoproductivity of Si-C cycles in the ocean and assist the Si model in its projections of future environmental changes.