Searching for ancient sulfate in South Africa
One of the best things of being a researcher is when you get the chance to broaden your horizons. This summer, geochemist Desiree Roerdink traveled to the Barberton Greenstone Belt in South Africa and dove back into the research field of sedimentology to find out how sulfate minerals formed on ancient shorelines, more than 3.2 billion years ago.
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“The Barberton Greenstone Belt in South Africa and Eswatini is a really special place for early Earth enthusiasts like me!”, says Desiree. “Here we find some of the world’s oldest rocks that are well enough preserved to extract information about the evolution of early life, what the earliest surface environments looked like, and how all this interacted with geological processes like volcanism and hot springs.”
Ever since her PhD research, Desiree has been working on chemical sedimentary deposits in the older parts of the Barberton Greenstone Belt, which formed by precipitation from seawater and hot spring fluids on the seafloor. “For many years, I have been intrigued by the presence of large-scale barite (barium-sulfate) deposits in this part of the rock record, as seawater at this time carried very little sulfate. The environment in which these deposits formed is an important puzzle to solve, as they host some of the oldest evidence for early microbial sulfate reduction.”
But barite is not the only sulfate mineral that is found in the early rock record. For this fieldwork, Desiree and her team decided to focus on small-scale sulfate nodules found in sandstones of the Moodies Group, with an age of 3.22 billion years the youngest of the units in the Barberton Greenstone Belt. “I have not really looked much at sandstones and conglomerates since my undergraduate courses, so that required quite some catching up on cross-bedding, current ripples and grain sizes”, Desiree laughs. “But you are never too old to learn!”
For one week, Desiree and the team consisting of MSc student Enza Magnier, soon-to-start PhD student Friedrich Hellmuth, as well as long-term collaborators Paul Mason (Utrecht University, The Netherlands) and Mark van Zuilen (Naturalis Biodiversity Center, The Netherlands), recorded types and occurrences of the sulfate nodules in three different locations in the greenstone belt. These field observations will provide a crucial framework when interpreting the geochemical data from drill core material from the same locations, collected in the Barberton Archean Surface Environments (ICDP BASE) scientific drilling project.
“Whereas we have learned much from the barite deposits about the marine sulfur cycle on the early Earth, the sulfate concretions in the Moodies Group sandstones provide an important counterpart to teach us about sulfur cycling on the earliest land”, Desiree explains. “All together will help us paint a better picture of what Earth looked like when life developed, from the earliest shorelines to the deep sea”.
The fieldwork of #TeamSulfateConcretions was funded by a grant from the Dr. Schürmann Foundation and the TMF Centre for Deep Sea Research.
