Assessing Eocene hydrological pattern by reconstructing seasonally enhanced fluvial discharge into marginal seas

Shifts in the hydrological cycle, such as the location and magnitude of seasonal precipitation, are changing as a result of current climate change. The Eocene, as the warmest epoch during the Cenozoic, is increasingly used as a test of our understanding of how broad-scale features of the climate system respond to greenhouse gas forcing. Here we demonstrate an attempt to reconstruct hydrology of northwest Europe by assessing the riverine input into the Anglo-Paris Basin (an adjacent basin to the Paleo North Sea) during the Lutetian. Marginal seas, like the Anglo-Paris Basin, are well-placed to track seasonal atmospheric variations as well as changes in fluvial discharge.

For our reconstruction we measured stable oxygen (δ18O) and clumped isotopes (∆47) in combination with spatially-resolved Ba/Ca ratios in pristinely preserved bivalve shells (Venericor planicosta). The combined isotope systems were used for the calculation of temperature as well as to identify salinity changes. Barium was simultaneously employed as proxy for terrigenous input. To further evaluate the terrigenous origin of the elevated barium that is frequently observed within these samples, we also measured spatially-resolved strontium isotopic ratios (87Sr/86Sr) by LA-MC-ICP-MS.

The Ba/Ca record displays an invariant background with large, reoccurring peaks, indicating episodic enhanced riverine input into the basin. The periodic freshwater discharge is also visible within the isotope record that shows a variable δ18OSW (+2 to -4‰), pointing to major changes in salinity. These periods are preferentially linked to warmer temperature, therefore representing a primary wet season during summer times.