Reconstructing regional to local scale patterns of fresh water input into the Eocene North Sea Basin

Understanding past Earth system processes is an essential component of placing current climate changes in context and testing climate model output. In this context, reconstructing patterns in marine or terrestrial temperatures is as important as understanding the mode of the hydrological cycle. However, there is only limited research on past hydrological systems, as the interplay between evapotranspiration, precipitation and runoff makes comprehensive reconstructions challenging. To overcome this issue, the spatiotemporal distribution of precipitation and riverine runoff can be estimated by reconstructing the dynamics of enhanced fresh water input into marginal sea.

Here, we focus on the hydrological conditions of shallow marine areas around the southern paleo-North Sea during the Eocene, including the Paris, Hampshire and Belgium Basins.  We utilised fossil bivalve shells (Venericor planicosta and Crassatella ponderosa) as archives as they can be used to reconstruct both long-term climate changes due to their widespread abundance and temporal continuity in the geological record as well as short-term variability via the time-distinct (sub-seasonal) layering of their shells. We reconstruct fresh water flux to these basins at different spatiotemporal resolutions, ranging from regional differences across millions of years to (sub-)seasonally resolved local variations. Depending on the targeted spatiotemporal resolution, different proxy systems were used. For the reconstruction of large scale changes in the input of terrigenous material by riverine runoff, Ba/Ca and ⁸⁷Sr/⁸⁶Sr are employed. In addition, δ¹⁸O and Δ₄₇+Δ₄₈ measurementswere conductedto detect sub-annual changes in the isotopic composition of the sea water by isotopically lighter fresh-water influx.

Result exhibit regionally specific Ba/Ca and ⁸⁷Sr/⁸⁶Sr values for each of the examined basins, generally reflecting the hinterland geology. However, these values show time dependent variations throughout the Eocene, suggesting variable degrees of terrigenous input by varying riverine runoff. The detected changes in riverine runoff are generally congruent to the depositional evolution of the basins, as derived from the sedimentary record, revealing less terrigenous input signal with increasing open marine conditions and vice versa.  A specimen of V. planicosta from the Paris Basin, showing a distinct riverine signal in Ba/Ca and ⁸⁷Sr/⁸⁶Sr, was used for combined δ¹⁸O and Δ₄₇+Δ₄₈ measurements to identify seasonal variations in the oxygen isotopic composition of the sea water (δ¹⁸O_SW). The resulting δ¹⁸O_SW values show a minimum seasonal variability of 0.9‰ and an enhanced fresh water input during the summer.

These results shed new light on the hydrological conditions in Western Europe during the Eocene and show how different proxy systems can be interlinked to reconstruct basin hydrology on different spatiotemporal scales.