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 (εNd). 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 εNd 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 εNd 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. εNd values for all of our fish teeth, foraminifera, and Fe–Mn oxyhydroxide samples are extremely unradiogenic (εNd≈−15); much lower than the εNd for deep waters of modern or Oligocene–Miocene age from the North Atlantic (εNd≈−10) and South Atlantic (εNd≈−8). This finding suggests that partial dissolution of detrital particulate material from the Amazon (εNd≈−18) strongly influences the εNd values of deep waters at Ceara Rise across the OMT. We conclude that terrestrially derived inputs of Nd can affect εNd 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.
Highlights
The weathering and transport of continental rock substrate is a major source of dissolved neodymium to the oceans (Goldstein and Jacobsen, 1987)
Samples spanning the Oligocene–Miocene transition (OMT) were selected from sediment cores recovered from Ocean Drilling Project (ODP) Leg 154, Site 926, Hole B (3◦43.148 N, 42◦54.507 W, ∼3600 m water depth; Leg 154 Shipboard Scientific Party, 1995), situated approximately 800 km to the northeast of the mouth of the River Amazon (Fig. 1)
Despite its slightly shallower depth, we assume that the εNd of seawater at Site ALV539 is typical of northern sourced deep water to Ceara Rise
Summary
The weathering and transport of continental rock substrate is a major source of dissolved neodymium to the oceans (Goldstein and Jacobsen, 1987). Deep water masses formed in the Southern Ocean have higher εNd (between −7 and −9; Piepgras and Wasserburg, 1987; Jeandel, 1993; Stichel et al., 2012) due to the contribution of young mantle-derived material surrounding the Pacific Ocean that mixes with Atlantic waters in this region. Records of seawater εNd values recorded in marine sediments have been widely used to identify the source of the overlying water masses (e.g. Scher and Martin, 2004; Piotrowski et al, 2005; Bohm et al, 2015; Lang et al, 2016)
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