Sr Isotope Ratios (87Sr/86Sr) in Water and Fish Otoliths as Estuarine Salinity Tracers: Case Studies from Three NW African Rivers

Sebastian N Höpker,Ishmael Y Nuworkpor,Henry C Wu,Oumar Sadio,Hildegard Westphal,Peter Merschel,Friedrich Lucassen,Timothée Brochier,Simone A Kasemann

doi:10.1007/s12237-021-01041-x

Abstract

Variations of strontium isotope ratios (87Sr/86Sr) in river systems are increasingly utilised to geochemically trace origin and movement patterns of migratory fish species. Accretionary calcified structures, such as otoliths, preserve 87Sr/86Sr signatures of the surrounding water during a fish’s lifetime. In this study, we present 87Sr/86Sr measurements of water samples and catfish otoliths collected in the estuaries of the Sine-Saloum (Senegal), the Gambia River (The Gambia), and the Volta River (Ghana) to assess their systematics and relationships with salinity. The three rivers possess distinct hydrological properties resulting in variable degrees of correlations between 87Sr/86Sr and salinity. The Gambia River (87Sr/86Sr of ~ 0.71209) proved exceptionally preconditioned for the approach due to well-defined geochemical end-members, allowing for quantitative estimates of salinity based on otolith 87Sr/86Sr measurements. The Volta River (~ 0.71392) presents a more complex case due the possible influence of multiple water sources to the main channel, while the inverse salinity gradient and excessive evaporation in the Sine-Saloum estuary (~0.70915) impede any significant correlations between 87Sr/86Sr and salinity. Bulk otolith 87Sr/86Sr values in the Gambia River and Volta River clearly depicted a mixed influence of seawater and riverine compositions, strongly encouraging the application of this approach for geochemical fingerprinting of critical NW African species.

Highlights

Strontium isotope ratios (87Sr/86Sr) preserved in natural materials reflect the available Sr sources during their formation, enabling the use of 87Sr/86Sr signatures as chemical tracers for a range of applications
In the Gambia River, water exhibited a strong correspondence between theoretically and empirically determined relationships of 87Sr/86Sr and salinity, demonstrating the potential of 87Sr/86Sr as a quantitative proxy when Sr behaves conservatively, and seawater and freshwater end-members contributing to estuarine mixing are isotopically distinct and well-characterised
Calculated and observed 87Sr/86Sr relationships of water in the Volta estuary were likewise correlated, but showed larger discrepancies, potentially linked to an additional unknown source of Sr to the water. 87Sr/86Sr measurements of catfish otoliths from both estuaries clearly depicted a combined influence of respective freshwater and seawater end-members, and served to demonstrate that the reliability of 87Sr/86Sr-based salinity reconstructions is best at low salinities

Summary

Introduction

Strontium isotope ratios (87Sr/86Sr) preserved in natural materials reflect the available Sr sources during their formation, enabling the use of 87Sr/86Sr signatures as chemical tracers for a range of applications. The marine Sr budget and isotopic composition of seawater varied during Earth’s history in response to climatic and/or tectonic forcings that largely control the supply and geochemistry of continental inputs (Banner 2004; Jones et al 2014; McArthur et al 2001; Mokadem et al 2015). The modern 87Sr/86Sr value of approximately 0.70918 ± 0.00001 (2σ; Faure and Mensing 2005), which presents a more conservative estimate representative of a wide range of measurements from different oceanic regions with variable analytical precision (e.g., Ando et al 2010; Elderfield 1986; El Meknassi et al 2018; Farrell et al 1995; McArthur et al 2001; Mokadem et al 2015), has not significantly changed over the past ∼40,000 years (Mokadem et al 2015)

Methods

Results

Discussion

Conclusion