141-7 Tracking ocean acidification in the Early Jurassic using ooid size to reconstruct carbonate saturation state

Session: New Advances in Geobiology

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Abstract:

Reconstructing environmental conditions in ancient oceans is complicated; this difficulty is exacerbated with increasing geological age. Paleo-ocean acidification – a rapid decrease in pH, carbonate saturation state (Ω), or a drop in the concentration of carbonate ([CO₃^2-]) or bicarbonate ions ([HCO₃^-]) – is particularly difficult to reconstruct, with only a few proxies available, many of which are still contentious. Here, we utilize machine learning and the Python program ‘segmenteverygrain’ to measure ooid size across a putative acidification event in the Early Jurassic; ooid sizes are then used to model ancient Ω using the Oomegasolver tool. Together, this method represents a new technique to quantify paleo-ocean acidification.

The Toarcian Oceanic Anoxic Event (T-OAE), also known as the Jenkyns Event, in the Early Jurassic was an interval of extreme climate change, characterized by global warming, decreased oxygen in the oceans, enhanced carbon burial (black shales), enhanced hydrological cycling, and potentially, ocean acidification. These rapid environmental changes resulted in extinctions, a severe reef collapse, and a shutdown of the carbonate factory in many locations. The Central High Atlas Mountains of Morocco preserve an expanded record of the T-OAE in a tropical mixed carbonate-siliciclastic ramp setting. We sampled oolites from the Toarcian representing grainstones from before, during, and after the T-OAE as defined by the negative carbon isotope excursion identified in previous studies. Samples were collected from two sections representing proximal and distal sites along the ramp. Thin section photomicrographs (plane-polarized light, 2.5x, 2.5 cm x 5 cm) of the ooid grainstones were analyzed using a modified version of ‘segmenteverygrain’ adapted to measure ooid diameters. These data were analyzed with the Oomegasolver tool, which applies the dynamic equilibrium ooid size model to calculate Ω. The resulting Ω estimates, combined with published pCO₂ estimates, were used to calculate other parameters of seawater carbonate chemistry using the open-source geochemical equilibrium software PHREEQC. Our preliminary data show an absence of carbonates at the onset of the T-OAE, followed by grainstones with diminutive ooid sizes, which likely reflect depressed Ω values during the event. Although ooids recover to their pre-event size, we do not see significantly larger ooids indicative of a “carbonate overshoot” following the event. This new technique shows great promise for reconstructing carbonate chemistry across ancient ocean acidification events.

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

doi: 10.1130/abs/2025AM-8125

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