Oxygen and Nitrogen Under the Ice: Trace Elements and δ15N Evidence for Oxic Weathering and Oxygenated Waters During the Snowball Earth Marinoan Glaciation, Ghaub Formation, Namibia
We present the first nitrogen isotopic and trace element analyses from during a “Snowball Earth” glaciation, and suggest they indicate oxidative continental weathering and mildly oxygenated marine waters.
Specifically, we measured three sections of the Ghaub formation, which is the Marinoan glacial unit in northern Namibia. The Marinoan is the second of two Cryogenian Snowball Earth glaciations.
In all sections, we analyzed δ15N values and redox sensitive trace elements, including U, V, and Mo. All measured units are from a marginal marine environment. Two sections are synglacial; one composed of detritial siliciclastics and one of detrital carbonate, both with a minor clay fraction. The third section was deposited during the terminal deglaciation and is comformably overlain by the basal Ediacarab Keilberg cap dolostone.
Trace element data demonstrate two key observations. First, the presence of trace elements in a marine setting is consistent with delivery from the continents via oxic weathering. Active weathering occurred on the continents during Snowball Earth. Secondly, enrichments are consistent with mildly oxygenated subglacial ocean waters.
δ15N values from both synglacial sections range from +1 to +3‰, consistent with small but persistent volumes of oxygenated water. Most dissolved N was NH4+ from N-fixing (δ15N =0‰), with small reservoirs of NO3-. Denitrification of NO3- enriched the oceanic N isotopes a few per mil. An increase in oxygenated water during the deglaciation was accompanied by increased NO3- and denitrification, causing δ15N values to increase to near modern levels. Deglacial values are thus more enriched, between +3 to +5‰.
Presenter: Dr. Ben Johnson (Assistant Professor, Iowa State University)
Ben is a geologist and geochemist with broad interests that span Earth History and Earth System Science. He uses a variety of rock archives, including chemical sediments, glacial tills, and igneous rocks as records of the interactions between life, the oceans, the atmosphere, and the solid Earth over deep time. He uses field work, analytical and isotope geochemistry, and numerical modeling to investigate deep time questions about Earth history. He is also interested in astrobiology and planetary science!
