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

摘要

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 Nd-143/Nd-144 ratios (epsilon(Nd)). 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 epsilon(Nd) 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; similar to 23 Ma). We compare our epsilon(Nd) 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. epsilon(Nd) values for all of our fish teeth, foraminifera, and Fe-Mn oxyhydroxide samples are extremely unradiogenic (epsilon(Nd) approximate to -15); much lower than the epsilon(Nd) for deep waters of modern or Oligocene-Miocene age from the North Atlantic (epsilon(Nd) approximate to -10) and South Atlantic (epsilon(Nd) approximate to -8). This finding suggests that partial dissolution of detrital particulate material from the Amazon (epsilon(Nd) approximate to 18) strongly influences the epsilon(Nd) values of deep waters at Ceara Rise across the OMT. We conclude that terrestrially derived inputs of Nd can affect epsilon(Nd) 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. (C) 2016 The Authors. Published by Elsevier B.V.