Abstract:
Evidence for ocean deoxygenation during past intervals of global warming can help to set observations of expanding oxygen minimum zones in the moderns oceans into a longer term geological context. The molybdenum (Mo) isotope composition of the hydrogenous Mo in marine sediments that accumulated in euxinic settings can preserve the seawater Mo-isotope composition, which in turn reflects the balance between oxic, anoxic, and euxinic sinks in the global ocean. As such, it can be used to determine the extent of de-oxygenated waters in past oceans when the local depositional environment is well characterised. We present new Mo isotope data of samples from Arctic Ocean IODP Site 302 and from continental shelf sites on the northern Tethyan margin (Guru-Fatima and Kheu River) that accumulated during the Paleocene-Eocene Thermal Maximum (PETM) approximately 56 Ma ago. The PETM is characterised by a global C-isotope excursion (CIE) caused by the introduction of a large amount of isotopically depleted carbon into the earth-ocean-atmosphere system, which in turn caused global temperatures to rise by 5-8 degrees C. Mo isotope data from IODP Site 302 broadly mirror the organic carbon isotope excursion recorded in the same sample suite, with near uniform values of 2.1 ppm during the peak of the CIE and lower values during the late Paleocene and during the PETM recovery interval. Samples that accumulated during the peak of the CIE were deposited under locally euxinic conditions, demonstrated by trace element data measured in the same samples and also by comparison to published organic geochemical data. Their Mo-isotope values consequently record the seawater value at this time, which was only approximately 0.2 ppm lower than modern seawater ( approximately 2.3 ppm). This finding suggests that (1) once euxinia in the Arctic Ocean is accounted for, global ocean anoxia was not widespread during the PETM and (2) the Arctic ocean was unrestricted during the PETM. In contrast, preliminary Mo isotope data from the northern Tethys Ocean suggest a slight expansion of ocean anoxia at the onset of the CIE, an interval that is missing within the Arctic core.