The rhythmic expression of mid-Cretaceous Oceanic Anoxic Event 2 southwest of Australia (IODP U1513, U1516)
Résumé
The widespread deposition of organic-rich black shales during the mid-Cretaceous hothouse at ~94 Ma
marked a climatic extreme that is particularly well studied in the Northern Hemisphere. The expression of
Oceanic Anoxic Event 2 (OAE 2) in the NH was characterised by low oceanic oxygen concentrations, likely
caused by the input of nutrients through volcanism and/or weathering in combination with a peculiar geography
in which the proto-North Atlantic was semi-restricted (Jenkyns, 2010; Trabucho Alexandre et al.,
2010). The extent of water column anoxia outside the North Atlantic and Tethyan domains remains poorly
resolved, as few Southern Hemisphere records have been recovered that span OAE 2, and only a portion of
those Indian and Pacific Ocean localities experienced anoxia and organic matter deposition (Dickson et al.,
2017; Hasegawa et al., 2013).
Here we present new results from IODP Expedition 369 offshore southwestern Australia. Sedimentary records
across the Cenomanian-Turonian transition from Sites U1513 and U1516 in the Mentelle Basin (Indian Ocean)
display rhythmic lithologic banding patterns. The OAE 2 interval is marked by a dramatic drop in carbonate
content and the occurrence of several thin organic-rich black bands. The spacing of dark bands within a
rhythmic sequence suggests a potential orbital control on organic matter deposition at our study sites. Time
series analyses of high-resolution (cm-scale) elemental data from XRF-core scanning reveal the imprint of
periodicities that can be confidently linked to Earth’s orbital parameters. The new OAE 2 records from Sites
U1516 and U1513 allow us to i) evaluate existing time scales over the Cenomanian-Turonian transition, and
ii) investigate the mechanisms leading to a recurrent lack of oxygen in the Indian Ocean.
Climatic mechanisms translating changes in insolation to variations in organic matter deposition may have
included variations in nutrient input from nearby continents and shifts in water column structure affecting
local to regional stratification versus deep water formation and advection. Investigating ventilation of the
deep sea during the OAE2 interval is of heightened relevance as current global warming is leading to a
worldwide expansion of oxygen minimum zones (Pörtner et al., 2019).