The silicon isotopes of Permian biogenic cherts from the South China unveils marine silicon cycle response to the termination process of late Paleozoic ice age

Abstract

There is a global chert accumulation event during the Permian period - Permian Chert Event (PCE, ∼280 Ma to 251 Ma), indicating large perturbations in the marine silicon cycle. The Permian also witnessed termination of Earth's penultimate icehouse (late Paleozoic ice age - LPIA, ∼340 Ma to ∼260 Ma), final assembly of the Pangea supercontinent and significant changes in biosphere. However, few studies have been focused on using these PCE cherts to understand the marine silicon cycle during this major environmental and climatic transition period. Here, we study the petrology, mineralogy and geochemistry of widespread cherts formed during the PCE period on the northeastern margin of the South China, which was located near the equator in the eastern Paleo-Tethys Ocean and isolated from the assembling Pangea supercontinent. Our studied period of the cherts is from ∼280 to ∼253 Ma and overlaps with the demise of LPIA. The petrologic, mineralogical and geochemical (silicon isotopes, oxygen isotopes, major and trace elements) results support that these studied Permian cherts are biogenic in origin. New and compiled silicon isotope data of biogenic cherts from South China generally show lowest δ³⁰Si values (−0.4 ‰ to 1.6 ‰, with an average of 0.5 ‰) during the coldest stage from 273 to 262 Ma in the middle Permian, highest δ³⁰Si values (0.4 ‰ to 2.5 ‰, with an average of 1.4 ‰) during the warm stage from ∼280 to 273 Ma of the early Permian and intermediate δ³⁰Si values (0.1 ‰ to 1.8 ‰, with an average of 0.9 ‰) during the warmest stage from ∼262 to 252 Ma in the late Permian. Comparing our results with previous records of tectonics, volcanic, marine lithium and strontium isotopes, we demonstrate the major controlling role of marine siliceous productivity on the silicon isotopes of these biogenic cherts. Our results of generally higher δ³⁰Si values in warm periods in contrast with lower values in cold periods indicate decreased oceanic dissolved silicon concentration and enhanced marine siliceous productivity in warming oceans, probably due to more nutrients supplied by enhanced costal upwelling during warm non-glacial periods. However, the silicon isotopes of the late Permian cherts from ∼262 to 252 Ma are probably also affected by continental weathering inputs. Our study provides a typical example of interpreting changes of silicon isotopes in biogenic cherts through major climatic transition period in terms of changes of oceanic siliceous biological productivity and provide insights into Permian marine silicon cycle during the termination process of late Paleozoic ice age.