The Phanerozoic Eon marked a major transition from marine silica deposition exclusively via abiotic pathways to a system dominated by biogenic silica sedimentation. For decades, prevailing ideas predicted this abiotic-to-biogenic transition were marked by a significant decrease in the concentration of dissolved silica in seawater; however, due to the lower perceived abundance and uptake affinity of sponges and radiolarians relative to diatoms, marine dissolved silica is thought to have remained elevated above modern values until the Cenozoic radiation of diatoms. Studies of modern marine silica biomineralizers demonstrated that the Si isotope ratios (δ³⁰Si) of sponge spicules and planktonic silica biominerals produced by diatoms or radiolarians can be applied as quantitative proxies for past seawater dissolved silica concentrations due to differences in Si isotope fractionations among these organisms. We undertook 446 ion microprobe analyses of δ³⁰Si and δ¹⁸O of sponge spicules and radiolarians from Ordovician–Silurian chert deposits of the Mount Hare Formation in Yukon, Canada. These isotopic data showed that sponges living in marine slope and basinal environments displayed small Si isotope fractionations relative to coeval radiolarians. By constructing a mathematical model of the major fluxes and reservoirs in the marine silica cycle and the physiology of silica biomineralization, we found that the concentration of dissolved silica in seawater was less than ~150 μM during early Paleozoic time—a value that is significantly lower than previous estimates. We posit that the topology of the early Paleozoic marine silica cycle resembled that of modern oceans much more closely than previously assumed.

中文翻译：

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奥陶纪-志留纪硅质骨骼的同位素分析表明，古生代海洋贫硅

显生宙 Eon 标志着从完全通过非生物途径的海洋二氧化硅沉积到以生物二氧化硅沉积为主的系统的重大转变。几十年来，普遍的观点预测这种非生物向生物转变的特点是海水中溶解二氧化硅的浓度显着降低。然而，由于海绵和放射虫相对于硅藻的感知丰度和吸收亲和力较低，人们认为海洋溶解二氧化硅一直高于现代值，直到硅藻的新生代辐射。现代海洋二氧化硅生物矿化剂的研究表明，Si 同位素比 (δ ³⁰由硅藻或放射虫产生的海绵针状体和浮游二氧化硅生物矿物的 Si) 可用作过去海水溶解二氧化硅浓度的定量代理，因为这些生物之间 Si 同位素分馏的差异。我们对δ ³⁰ Si和δ ¹⁸进行了446离子探针分析来自加拿大育空地区野兔组奥陶纪-志留纪燧石矿床的海绵针状体和放射虫的 O。这些同位素数据表明，生活在海洋斜坡和盆地环境中的海绵相对于同时代的放射虫显示出较小的 Si 同位素分馏。通过构建海洋二氧化硅循环中的主要通量和储集层以及二氧化硅生物矿化生理学的数学模型，我们发现早古生代海水中溶解二氧化硅的浓度小于~150 μM——该值明显低于比之前的估计。我们假设早期古生代海洋二氧化硅循环的拓扑结构比以前假设的更接近现代海洋的拓扑结构。