The Ordovician witnessed an explosion in marine biodiversity punctuated by the first of the ‘big-5’ Phanerozoic mass extinctions, the Late Ordovician mass extinction (LOME). The LOME consists of two discrete pulses occurring at the beginning and end of the Hirnantian. Lithologic and geochemical evidence suggests widespread marine anoxia triggered the second LOME pulse; however, most of these redox proxies record local bottom water or porewater conditions rather than global seawater conditions. To evaluate global redox trends, we utilize uranium (U) isotopes and trace element geochemistry of marine carbonates as a global marine redox proxy.
Bulk carbonate samples were collected from the upper Katian, Hirnantian, and through the lowermost Rhuddanian on Anticosti Island, Quebec with a focus on the second LOME pulse. We target this specific location because the U-isotope seawater redox proxy can be directly compared to the well documented, high-resolution record of glacio-eustasy, Hirnantian positive carbon isotope excursion (HICE), and faunal diversity trends previously reported from Anticosti Island which removes any potential temporal mismatches among proxy records.
Measured δ238U values range from -0.83‰ to 0.44‰ with values averaging ~ -0.25‰ in the late Katian through late Hirnantian indicating more oxic conditions, an abrupt (<20 ky) -0.35‰ negative shift lasting ~450 ky representing more reducing conditions, followed by an abrupt positive shift back to values averaging ~0.25 in the earliest Silurian (early Rhuddanian). Th/U values average ~0.8 during the late Katian to late Hirnantian and shift to an average of ~2.2 in the late Hirnantian before returning to an average of ~1. Total organic carbon (TOC) values are low, (average = 0.19%) for the entire section with a brief increase in the latest Hirnantian. Combined, these proxies indicate a global record of an abrupt latest Hirnantian oceanic anoxic event (HOAE) during the Late Ordovician icehouse. Simple box modeling of the event suggests that there was a ~7x increase in anoxic sediment deposition and U sequestration to generate the observed shift and the rapidity of the onset implies that Latest Ordovician oceans had significantly lower U concentrations than modern oceans.
The HOAE onset is coincident with the LOME onset but lasted ~200 ky longer, suggesting anoxic conditions did not affect the biologic recovery. The HOAE began during the deglacial highstand before peak glaciation and persisted through the peak glacial lowstand and subsequent deglacial rise, which indicates water depths and shoreline positions did not influence anoxic sediment accumulation. The largest HICE peak occurs during the HOAE but is not coincident with its onset and demise indicating the processes controlling δ13C trends were partially decoupled with anoxic sediment deposition.
We propose that the HOAE was the result of global cooling, more vigorous thermohaline circulation and upwelling, increased nutrient delivery and enhanced primary productivity which resulted in an expanded oxygen minimum zone and anoxic sediment deposition. This icehouse-related model contrasts with those proposed for other Phanerozoic OAEs, which developed under greenhouse climate conditions.
Earth and Planetary Sciences
Level of Degree
Department of Earth and Planetary Sciences
First Committee Member (Chair)
Dr. Maya Elrick
Second Committee Member
Dr. Yemane Asmerom
Third Committee Member
Dr. Viorel Atudorei
uranium, isotope, proxy, anoxia, LOME
Bartlett, Rickey; Maya Elrick; Yemane Asmerom; and Viorel Atudorei. "Abrupt Ocean Anoxia During the Late Ordovician Mass Extinction Detected Using Uranium Isotopes of Marine Carbonates." (2016). https://digitalrepository.unm.edu/eps_etds/138