How have the continents grown through time? What where the ancient atmosphere, oceans, and biosphere like, and how did they evolve? Are there connections between the solid Earth and its surface, including the biosphere? What analogs can we study on Earth, past and present, to inform us in searching for life and planetary habitability elsewhere in the Solar System? These are some of the major scientific questions that isotope geochemistry can contribute to. Some of my research pursuits involve the study of astrobiology, a broad interdisciplinary approach to addressing the origin and evolution of life on Earth and elsewhere. Others fall more into the category of “the solid Earth”, specifically orogenic systems, past and present. Still others focus on relatively young sedimentary systems.
Focus: Astrobiology and the Early Earth
Early Archean jaspers are Fe-rich cherts that contain a mix of hematite, +/- magnetite, +/- siderite. They share many features with banded iron formations (BIFs), although units defined as jaspers tend to be much smaller and less laterally extensive than BIFs, certainly the very large Meso to Neoarchean and Early Paleoproterozoic BIFs.
the preservation of Archean jasper in the rock record can give insights into the redox history of ancient seawater and the evolution of early microorganisms. New research has suggested that there may have been transient increases in oxygen as far back as 3.0 Ga. Our most recent research has focused on sequences older than 3.0 Ga, in order to constrain the earliest increases in seawater oxygenation. The sequences we have focused on are the 3.46 Ga Marble Bar chert (western Australia) and the 3.26 Ga Manzamanyama jasperlite from South Africa. To understand the redox of ancient seawater, we use Fe (isotopes) and U (concentrations), which are both redox sensitive elements. Unique to our research, we combine these data with U-Th-Pb geochronology, which allows us to assess the fidelity of the redox proxies.
Jasper from the 3.46 Ga Marble Bar chert has the highest δ56Fe values ever measured on terrestrial samples, implying that the hematite formed by a very small amount of partial oxidation. Currently, we are working on the slightly older jaspers in the Dresser Formation, which has a larger range of Fe isotope compositions that record mixing between restricted and open marine environments. These jaspers samples also have low U concentrations, and U-Th-Pb geochronology confirmed that no U has been lost, confirming the low U contents of these rocks since deposition. Combined with the very positive δ56Fe values, both data sets suggest that oxygen contents of 3.46 Ga seawater were low, implying that oxygenic photosynthesis was not operating prior to 3.5 Ga.
Jasper samples from the 3.26 Ga Manzaminyama jasperlite, in contrast, have lower δ56Fe values and higher U contents than the samples from Marble Bar, suggesting O2 contents in 3.26 Ga seawater may have been higher than at 3.46 Ga. Unique to the Manzaminyama samples, is the preservation of deep and shallow precipitated samples. Here, the shallow water (low-Fe) samples have lower δ56Fe values and higher U contents than deeper (high-Fe) samples. U-Th-Pb geochronology confirms that the differences seen in U contents from deep to shallow water samples are primary and not a result of later alteration. The lower δ56Fe values and higher U contents in the shallow water (low-Fe) samples suggest that shallow waters at this time had elevated O2 contents, likely due to oxygen produced thought oxygenic photosynthesis. This suggests that cyanobacteria had likely evolved prior to 3.26 Ga, which is 200 million years than previously thought.
The work in the Pilbara is in collaboration with Prof. Martin Van Kranendonk and Dr. Tara Djokic at the University of New South Wales, Australia, as well as Prof. Xinyuan Zheng at the University of Minnesota, and Dr. Brian Beard at UW-Madison.
A second line of early Earth research is focussed Mesoarchean BIFs in South Africa. Oxygenic photosynthesizers have played a critical role in shaping the evolution of Earth’s surface over the last two billion years, but molecular phylogeny, and geologic evidence, tells us oxygenic photosynthesis did not dominate the biosphere early in Earth history. The Mesoarchean (3.2 to 2.8 b.y. ago) is an era that likely recorded the development of nascent metabolic systems and redox cycles that preceded the “Great Oxidation Event” (GOE) at ~2.3 Ga. Iron reduction, Mn cycling, and methane cycling are all deeply rooted microbial metabolisms—suggesting an early appearance in the evolution of life. Pinpointing the earliest occurrences of these metabolisms provide a valuable anchor point that links the tree of life to Earth’s tangible history. Due to the deeply rooted nature, and the easy availability of their electron donors, these metabolisms could also be important in astrobiology’s search for extraterrestrial life.
Current research is focused on the microbial metabolism of Fe(III) reduction—trying to identify whether there was a benthic iron shuttle operating in the 3 billion year old Witwatersrand—Pongola basin. Favoring the lighter isotopes, the metabolism of microbial Fe(III) reduction fractionates carbon and iron, producing light isotopic signatures that can be preserved in minerals such as magnetite and siderite. Preliminary work on iron isotopes of microdrilled pyrite, whole rocks, and magnetite separates show a trend of increasingly light iron isotope signatures moving from the proximal to distal marine settings—in tandem with whole-rock iron enrichment in the distal basin. In Severmann et al. (2008)’s study on the black sea, a similar trend was observed. Measuring the iron isotopes and iron concentrations of sediments across the basin, it was found that there was an inverse correlation between δ56Fe and the Fe/Al ratio of whole rock samples. This was interpreted to indicate a benthic iron shuttle, whereby reactive Fe microbial flux is preferentially removed from the shelf and sequestered in the deeper depofacies. Our data suggests that a benthic iron shuttle operated 3 billion years ago—making this the oldest proposed benthic iron shuttle.
The work on the Mesoarchean sequences is in collaboration with Profs. Nic Beukes and Bertus Smith and the University of Johannesburg, as well as Dr. Aaron Satkoski at UT-Austin and Dr. Brian Beard and UW-Madison.