Influence of the Amazon River on the Nd isotope composition of deep water in the western equatorial Atlantic during the Oligocene–Miocene transition

Highlights • Records of seawater eNdeNd are highly unradiogenic across the OMT at Ceara Rise. • Strong influence of Amazon particulate Nd on seawater eNdeNd at Ceara Rise. • Point-sourced riverine Nd influences seawater far from continental shelf. • Regional sedimentary Nd flux can be confused with water mass mixing signals. Abstract Dissolved and particulate neodymium (Nd) are mainly supplied to the oceans via rivers, dust, and release from marine sediments along continental margins. This process, together with the short oceanic residence time of Nd, gives rise to pronounced spatial gradients in oceanic 143Nd/144Nd ratios (eNdeNd). However, we do not yet have a good understanding of the extent to which the influence of riverine point-source Nd supply can be distinguished from changes in mixing between different water masses in the marine geological record. This gap in knowledge is important to fill because there is growing awareness that major global climate transitions may be associated not only with changes in large-scale ocean water mass mixing, but also with important changes in continental hydroclimate and weathering. Here we present eNdeNd data for fossilised fish teeth, planktonic foraminifera, and the Fe–Mn oxyhydroxide and detrital fractions of sediments recovered from Ocean Drilling Project (ODP) Site 926 on Ceara Rise, situated approximately 800 km from the mouth of the River Amazon. Our records span the Mi-1 glaciation event during the Oligocene–Miocene transition (OMT; ∼23 Ma). We compare our eNdeNd records with data for ambient deep Atlantic northern and southern component waters to assess the influence of particulate input from the Amazon River on Nd in ancient deep waters at this site. eNdeNd values for all of our fish teeth, foraminifera, and Fe–Mn oxyhydroxide samples are extremely unradiogenic (eNd≈−15eNd≈−15); much lower than the eNdeNd for deep waters of modern or Oligocene–Miocene age from the North Atlantic (eNd≈−10eNd≈−10) and South Atlantic (eNd≈−8eNd≈−8). This finding suggests that partial dissolution of detrital particulate material from the Amazon (eNd≈−18eNd≈−18) strongly influences the eNdeNd values of deep waters at Ceara Rise across the OMT. We conclude that terrestrially derived inputs of Nd can affect eNdeNd values of deep water many hundreds of kilometres from source. Our results both underscore the need for care in reconstructing changes in large-scale oceanic water-mass mixing using sites proximal to major rivers, and highlight the potential of these marine archives for tracing changes in continental hydroclimate and weathering.