Ocean, marine life, and anoxic events during warm ice-free epochs: what Devonian rocks of Northwest Territories can tell usOnline pre-recorded
The ongoing global warming, poleward migration of species, and retreat of ice is undeniable. These changes, easily observable on land surface, are coupled with ocean response to global warming - slowdown in thermohaline circulation and linked processes that will be highlighted in this presentation. The ultimate global warming called the greenhouse Earth-surface condition, when polar ice caps melt completely and ocean is predicted to undergo a profound change from bottom to top, is unexperienced by people as a biological species. Being the smartest terrestrial life form, we have technological capacity to prevent such future. However, for our learning about this ice-free potential future, sedimentary rocks provide indispensable information about protracted greenhouse states of Earth’s surface lasted for tens millions of years during deep geologic past. One such sedimentary archive is the strata of the Horn River Group in the Northwest Territories. Excellent oucrops of these basinal shales, cherts, and carbonates occur on cordilleran ranges in Sahtu Lands on both sides of central Mackenzie Valley. Representing the geologic age range of ~373-386 Myr (Givetian and Frasnian stages of Devonian System in Geologic Time Scale), these strata provide a rare-to-find, well-preserved archive of paleo-oceanographic signals imprinted in oxic and anoxic facies deposited in close proximity to each other. Four global anoxic events are recognized in these strata by using elemental and organic geochemistry, gamma spectrometry, and stable carbon isotope data. Age contraints are biostratigraphic, mostly provided by conodonts. The discovery of aryl isoprenoids (organic molecular compounds specific for green sulfur bacteria) at and between the levels of anoxic events contributes to the growing evidence of shallow, sunlit chemocline (the boundary separating oxygenated and oxygen-free waters) as a normal condition of then-time shelfal seas. This condition is not possible under present-day vigorous ocean circulation unless the sea is landlocked and semi-isolated from ocean circulation cells. Presented data collectively support a school of thought that discards sea-level changes as a control over these ancient oceanic anoxic events. Rather, the data suggests these events were pulsatory expansions of thick and laterally extensive oxygen minimum zones. This non-actualistic state of ocean was conductive for accumulation of enormous hydrocarbon reserves of the Western Canada Sedimentary Basin, including its northern continuation in N.W.T. On the other hand, there are indications that Devonian pulses of anoxia could promote precipitation of metalliferous (Ni-Mo-Zn-Pt-Pd-Au-Re...) horizons under conditions of extreme and protracted sediment starvation. Such horizons are known in the Middle Devonian of NE Yukon. This work is a contribution to the Geomapping for Energy and Minerals Program.