174-1 Environmental Change in the Early Jurassic: Unraveling the Causes and Consequences of Shallow Marine Biological Productivity Changes During the Toarcian Ocean Anoxic Event in North America

Session: Environmental Instability During Greenhouse Periods: Impact on Terrestrial and Marine Ecosystems

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Abstract:

We investigate changes in phosphorus (P) cycling across the Pliensbachian/Toarcian (Pl/To) boundary and Toarcian Oceanic Anoxic Event (T-OAE). We present sequential P extraction data, the oxygen isotopic composition of orthophosphate (δ¹⁸O_p) within the operationally defined authigenic P pool (P_auth), and sedimentary geochemistry of elements relevant to P cycling (sulfur, manganese, and iron) from the East Tributary section in Alberta, Canada. This sedimentary succession was deposited in a semi-restricted basin connected to the eastern Panthalassa Ocean. Throughout the studied interval, we infer that persistent eutrophication resulted in extensive P remineralization, as indicated by extremely low δ¹⁸O_p values (10.7 ± 0.5 ‰, 1 SD). Variable bottom-water oxygenation but persistent P remineralization and organic carbon burial indicate that the latter depended more on P remineralization than on oxygen limitation. In the late Pliensbachian, oxic to suboxic bottom waters, eutrophic surface waters, and sedimentary condensation promoted sedimentary P burial. Between the Pl/To boundary and the onset of the T-OAE negative δ¹³C_org excursion, sedimentary P burial was further aided by increased carbonate platform productivity. The falling limb of the T-OAE negative δ¹³C_org excursion coincided with a global transgression that resulted in the drowning of the adjacent carbonate platform and P release. No concomitant change in δ¹⁸O_p values indicates that the increase in bioavailable P did not reduce P recycling. P remineralization was maintained by enhanced eutrophication, which also enhanced bottom water euxinia. Less euxinic conditions during the rising limb of the negative δ¹³C_org excursion permitted P sedimentary burial as authigenic phases.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8587

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