Earth’s youngest banded iron formation implies ferruginous conditions in the Early Cambrian ocean

摘要

>It has been proposed that anoxic and iron-rich (ferruginous) marine conditions were common through most of Earth history. >This view represents a major shift in our understanding of the evolution of marine chemistry. >However, >thus far, >evidence for ferruginous conditions comes predominantly from Fe-speciation data. >Given debate over these records, >new evidence for Fe-rich marine conditions is a requisite if we are to shift our view regarding evolution of the marine redox landscape. >Here we present strong evidence for ferruginous conditions by describing a suite of Fe-rich chemical sedimentary rocks—banded iron formation (BIF)—-deposited during the Early Cambrian in western China. >Specifically, >we provide new U-Pb geochronological data that confirm a depositional age of ca. >527 Ma for this unit, >as well as rare earth element (REE) data are consistent with anoxic deposition. >Similar to many Algoma-type Precambrian iron formations, >these Early Cambrian sediments precipitated in a back-arc rift basin setting, >where hydrothermally sourced iron drove the deposition of a BIF-like protolith, >the youngest ever reported of regional extent without direct links to volcanogenic massive sulphide (VMS) deposits. >Their presence indicates that marine environments were still characterized by chemical- and redox-stratification, >thus supporting the view that—despite a dearth of modern marine analogues—ferruginous conditions continued to locally be a feature of early Phanerozoic seawater.