Isotopic analyses of Ordovician–Silurian siliceous skeletons indicate silica‐depleted Paleozoic oceans

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.

显生宙标志着从完全通过非生物途径的海洋二氧化硅沉积到一个以生物源二氧化硅沉积为主的系统的重大转变。几十年来，主流观点预测这种从非生物到生物的转变以海水中溶解二氧化硅浓度的显著下降为标志；然而，由于海绵和放射虫相对于硅藻的丰度和摄取亲和力被认为较低，海洋溶解二氧化硅被认为一直保持在高于现代的值，直到硅藻在新生代辐射。对现代海洋二氧化硅生物矿化生物的研究表明，由于这些生物之间硅同位素分馏的差异，海绵骨针以及硅藻或放射虫产生的浮游二氧化硅生物矿物的硅同位素比值（δ30Si）可以作为过去海水溶解二氧化硅浓度的定量指标。我们对来自加拿大育空地区黑尔山组奥陶纪 - 志留纪燧石矿床的海绵骨针和放射虫进行了446次离子微探针分析，测定其δ30Si和δ18O。这些同位素数据显示，生活在海洋斜坡和盆地环境中的海绵相对于同时期的放射虫表现出较小的硅同位素分馏。通过构建海洋二氧化硅循环中的主要通量和储库以及二氧化硅生物矿化生理的数学模型，我们发现古生代早期海水中溶解二氧化硅的浓度小于约150μM——这个值明显低于先前的估计。我们认为古生代早期海洋二氧化硅循环的拓扑结构比先前假设的更接近现代海洋。