Letters to Nature

Nature 406, 981-985 (31 August 2000) | doi:10.1038/35023143; Received 15 July 1999; Accepted 15 June 2000

Diagenetic origin of quartz silt in mudstones and implications for silica cycling

Jürgen Schieber1, Dave Krinsley1,2 & Lee Riciputi3

  1. Department of Geology, University of Texas at Arlington, Arlington, Texas 76019 , USA
  2. Department of Geological Sciences, University of Oregon, Eugene, Oregon 97403 , USA
  3. Chemical and Analytical Sciences Division, Oak Ridge National Laboratory, PO Box 2008, Oak Ridge , Tennessee 37831-6365, USA

Correspondence to: Jürgen Schieber1 Correspondence and requests for materials should be addressed to J.S. (e-mail: Email: schieber@uta.edu).

Mudstone—the most abundant sedimentary rock type1, composed primarily of clay- or silt-sized particles—contains most of the quartz found in sedimentary rocks2. These quartz grains, which are chemically and mechanically resistant and therefore preserve their characteristics well, have long been considered to be derived from the continental crust1. Here we analyse quartz silt from black shales in the eastern USA, dating back to the Late Devonian period (about 370 million years ago), using backscattered electron and cathodoluminescence imaging and measure oxygen isotopes with an ion probe. Our results indicate that up to 100% of the quartz silt in our samples does not originate from the continental crust. Instead, it appears to have precipitated early in diagenesis in algal cysts and other pore spaces3, with silica derived from the dissolution of opaline skeletons of planktonic organisms, such as radiolaria and diatoms. Transformation of early diatoms into in situ quartz silt might explain the time gap between the earliest fossil occurrences of diatoms about 120 Myr ago4 and molecular evidence for a much earlier appearance between 266 or even 500 Myr ago5, 6. Moreover, if many other mudstone successions show similarly high proportions of in situ precipitated—rather than detrital—quartz silt, the sedimentary record in mudstones may have been misinterpreted in the past, with consequences for our estimates of palaeoproductivity as well as our perceptions of the dynamics and magnitude of global biogeochemical cycling of silica.