Carbonate geochemistry via ICP-MS of the Ireviken Biogeochemical Event from the Altajme core, Gotland, Sweden

The global carbon cycle is a useful tool for looking at what changes the Earth and oceans have undergone throughout history. These changes are preserved in the rock record by marine sediments. Carbon has two stable isotopes, ¹²C and ¹³C, and we measure the ratio (¹³C/¹²C) of these two carbon isotopes to give us the carbon isotope record, ô¹³C, of the oceans. Changes in the carbon cycle appear as “excursions” in the ô¹³C record where ô ¹³C values rapidly increase or decrease over a short interval of time. This study investigates one such event, the Ireviken Biogeochemical Event, which occurred during the Silurian, ~431 million years ago. This event includes a positive carbon isotope excursion and a global extinction of many marine species including conodonts, corals, graptolites, and brachiopods.

Anoxic conditions in the ocean (low oxygen levels) are believed to have been a major role in this event. We are able to look at certain elements that respond to changes in ocean oxygen levels and see how they behaved during the event. These redox-sensitive elements can help indicate if the ocean was experiencing anoxia. This study documents many of these redox sensitive elements and finds they respond as expected for anoxic conditions.

Additionally, this study separated out the carbonate vs. clastic lithology in the marine rock record. Carbonate material forms in the ocean and is therefore very useful for understanding ocean conditions at the time of formation. Analyzing solely the carbonate component revealed more evidence of anoxia from redox-sensitive elements. A cooling event was also identified from the carbonate component analysis and named the Altajme Cooling Event.