Intensified hydrological cycle, not sea level rise, caused PETM shelf deoxygenation

The Paleocene-Eocene Thermal Maximum (PETM) ~56 million years ago, provides one of the best geological analogues for investigating how marine oxygen levels respond to rapid global warming and massive perturbations to the global carbon cycle. Various studies on PETM shelf sections have documented the deposition of an extensive organic-rich sapropel horizon, which provides a unique geological archive into better understanding the crucial drivers and interactions responsible for PETM deoxygenation within shallow shelf settings. Changes in relative sea level and/or the hydrological cycle during the PETM have both been invoked as potential drivers behind this marine deoxygenation. However, there is currently a lack of high resolution dated PETM records which integrate and resolve the temporal relationship between these mechanisms and the onset of shelf deoxygenation. Therefore, we have investigated Kheu River; a key PETM section in the northern Caucasus which has previously produced several geochemical records indicating the prevalence of intermittent shallow marine anoxia and euxinia within the sapropel horizon. Our new datasets, together with published palaeoceanographic and palaeoclimatic proxy-based reconstructions from Kheu River, have been evaluated using a sequence stratigraphic framework and calibrated to an orbitally-tuned age model. Similar shallowing and deepening trends inferred from co-variations in geochemical, micropalaeontological, and sedimentological datasets suggest bottom water redox conditions at Kheu River were influenced by changes in relative sea level over ~10⁵-year timescales. Despite this however, we show the deposition of the sapropel horizon occurred more rapidly during the first ~26 kyr of the PETM carbon isotope excursion (CIE), consistent with an intensified hydrological cycle driver. This transient hydrological driven deoxygenation event is also coeval with an interval of maximum continental weathering, nutrient, and sediment influx at Kheu River, suggesting shallow shelf environments were sites of diverse and dynamic biogeochemical process interactions during the onset of the PETM CIE. These results underscore the complexity of shallow marine ecosystem responses to climate forcing. They also provide valuable insights into the drivers of marine deoxygenation in a rapidly warming world, which can help us better predict future deoxygenation patterns.