We thank E. R. Schroeter for scientific advice. S. M. Webb and C. Roach assisted during the SRS-XRF analysis. T. Wigren produced the artistic reconstructions. Financial support was provided by a Grant for Distinguished Young Researchers (642-2014-3773, Swedish Research Council) to J.L., as well as a National Science Foundation INSPIRE grant (EAR-1344198) to M.H.S. and W.Z., and donations from F. M. and S. P. Orr, and V. and G. Mullis. The Paul Scherrer Institute, Switzerland, provided beamtime at the TOMCAT beamline of the Swiss Light Source. SRS-XRF data were collected at the Stanford Synchrotron Radiation Lightsource using beamline 10-2. NanoSIMS measurements were obtained at the Chemical Imaging Infrastructure, Chalmers University of Technology and University of Gothenburg, which is supported by the Knut and Alice Wallenberg Foundation. Part of this work was performed at the Analytical Instrumentation Facility (AIF) of North Carolina State University. The AIF is supported by the State of North Carolina and the National Science Foundation (ECCS-1542015) and is a member of the North Carolina Research Triangle Nanotechnology Network, a site within the National Nanotechnology Coordinated Infrastructure.

Reviewer information

Nature thanks A. Houssaye, B. Kessler, S. Kiel and R. N. S. Sodhi for their contribution to the peer review of this work.