Ocean Circulation in the Toarcian (Early Jurassic): A Key Control on Deoxygenation and Carbon Burial on the European Shelf

Abstract

The Toarcian Oceanic Anoxic Event (T‐OAE, ∼183 Myr) was a long‐lasting episode of ocean deoxygenation during the Early Jurassic. The event is related to a period of global warming and characterized by major perturbations to the hydrological and carbon cycles with high rates of organic matter burial in shelf seas. Ocean circulation during the Toarcian and its influence on marine biogeochemical cycles are still not fully understood. Here we assess the spatial extent of anoxia in the NW Tethys Ocean during the T‐OAE, the relationship with ocean circulation and the impact on organic carbon burial, using new and existing sedimentary records from the European Epicontinental Shelf in combination with general circulation model results. We demonstrate that bottom waters on the southwestern part of the shelf were mainly oxic during the T‐OAE, while those in the northeastern basins were mostly anoxic or even sulfidic. Results for two ocean‐atmosphere models (Fast Ocean‐Atmosphere Model and Massachusetts Institute of Technology general circulation model) suggest the presence of a strong clockwise gyre over the European Epicontinental Shelf, which brought oxygenated equatorial waters from the Tethys Ocean to the southern shelf. The northward limb of the gyre was significantly weakened due to the rough bathymetry of the northern shelf, making this relative small region highly sensitive to local ocean stratification. These sluggish ocean dynamics promoted bottom water anoxia and enhanced burial of organic carbon in the northeastern basins, which accounted for 3–5% of the total carbon extracted from the ocean‐atmosphere system as recorded by the positive carbon isotope shift.

Enhanced Organic Carbon Burial in Sediments of Oxygen Minimum Zones Upon Ocean Deoxygenation

“…In all our scenarios with increased phosphorus and iron inputs, existing OMZs expanded and between 0.3 and 1.3% of the seafloor was covered by anoxic bottom waters after 50 kyrs. This compares well to the known areas with anoxic bottom waters during the Toarcian-OAE and the Cretaceous OAE 2, during which at least ∼0.3% (Dickson et al, 2017;Ruvalcaba Baroni et al, 2018) and ∼5% (Owens et al, 2013(Owens et al, , 2016Dickson et al, 2016) of the global ocean sea floor was overlain by waters depleted in oxygen, respectively. High atmospheric pCO 2 and high rates of nutrient input, sustained over a longer period of time, similar to our scenario for the modern ocean, have been deduced for these past OAEs (e.g., Schlanger and Jenkyns, 1976;Poulsen et al, 2001;Cohen et al, 2004Cohen et al, , 2007Kemp et al, 2005;Barclay et al, 2010;Goddéris et al, 2012;van Bentum et al, 2012;Poulton et al, 2015).…”

“…However, geological evidence and model results suggest that during T-OAE protracted anoxia/euxinia was geographically widespread (Gill et al, 2011;Them et al, 2018), but likely mainly confined to relatively shallow areas (e.g., van de Schootbrugge et al, 2005;Dickson et al, 2017). These areas account for at least ∼0.3% of the seafloor (Ruvalcaba Baroni et al, 2018) and provide evidence for high rates of organic carbon burial. Trace metal and thallium isotope analysis further suggest that bottom water deoxygenation did not remain constant over the event (e.g., Montero-Serrano et al, 2015;Them et al, 2018).…”

“…This difference in vertical extent and intensity of the anoxia is likely due to the differences in continental configuration and ocean circulation, allowing parts of the ocean to be more prone to anoxia in the past than today (Donnadieu et al, 2006;Goddéris et al, 2012). During the Toarcian, this holds specifically for the northern European Epicontinental Shelf, which was subject to a flow pattern that promoted the development of anoxia (Ruvalcaba Baroni et al, 2018). During OAE 2, this was the case in the proto-North Atlantic (e.g., Sinninghe Damsté and Köster, 1998;Kuypers et al, 2002;Owens et al, 2013).…”

“…Sedimentary rock records indicate that large amounts of organic-rich sediments were deposited in low oxygen areas in the ocean during the T-OAE and OAE 2 (Jenkyns, 1985(Jenkyns, , 2010. This is thought to have led to sequestration of atmospheric CO 2 and may have contributed to the termination of these past events (Sinninghe Damsté and Köster, 1998;van Bentum et al, 2012;Xu et al, 2017;Ruvalcaba Baroni et al, 2018).…”

“…Trace metal and thallium isotope analysis further suggest that bottom water deoxygenation did not remain constant over the event (e.g., Montero-Serrano et al, 2015;Them et al, 2018). Organic carbon burial during T-OAE is thought to have contributed to at least 8 to 14% of the drawdown of atmospheric CO 2 (Xu et al, 2017;Ruvalcaba Baroni et al, 2018). Both OAE 2 and T-OAE share similar biogeochemical characteristics, such as prolonged periods of regional photic zone euxinia, major losses in marine biodiversity, large perturbations in most elemental cycles and high rates of organic carbon burial (e.g., Schlanger and Jenkyns, 1976;Jenkyns, 1985Jenkyns, , 2010McArthur et al, 2008;Dickson et al, 2017).…”