Abstract
AbstractEight ferromanganese crust samples spanning the complete depth range of Tropic Seamount in the northeast Atlantic were analyzed for Pb and Nd isotopes to reconstruct water mass origin and mixing over the last 75 Ma. Pb isotopes were determined by laser ablation multicollector inductively coupled plasma mass spectrometer (LA‐MC‐ICP‐MS), which enables the rapid production of large, high spatial resolution data sets. This makes it possible to precisely correlate stratigraphy between different samples, compare contemporaneous layers, and create a composite record given the abundance of hiatuses in crusts. Pb and Nd isotope data show the influence of various oceanic and continental end‐members in the northeast Atlantic Ocean. This reflects its evolution from a restricted, isolated basins in the Late Cretaceous with influxes from the Tethys Ocean, to an increasingly well‐mixed, large‐scale basin, with a dominant Southern Ocean signature until the Miocene. Less radiogenic Nd isotope signatures suggest Labrador Sea Water influenced the northeast Atlantic basin as early as 17–15 Ma, flowing through a northern route such as the Charlie‐Gibbs Fracture Zone. Pb and Nd isotopes highlight the increasing influence of Saharan eolian dust input about 7 Ma, imparting a less radiogenic excursion to the binary mixing between North Atlantic water masses and riverine discharge from West and Central Africa. This highlights the influence of eolian dust input on the open ocean Pb and Nd budget and supports an early stage of North African aridification in the Late Miocene. This signature is overprinted about 3 Ma to the present by a strong North Atlantic Deep Water signature following the onset of Northern Hemisphere glaciation.
Highlights
Paleoceanographic reconstructions rely on biological, physical, and geochemical tracers, such as temperature, salinity, and a variety of isotope systems reflecting the effects of geological and climatic phenomena
Pb isotopes were determined by laser ablation multicollector inductively coupled plasma mass spectrometer (LA‐MC‐ICP‐MS), which enables the rapid production of large, high spatial resolution data sets
The continuous Pb isotope LA profiles from crust samples spanning a depth range of 1,028–3,777 mbsl are shown on a schematic west‐east topographic section of Tropic Seamount (Figure 3)
Summary
Paleoceanographic reconstructions rely on biological, physical, and geochemical tracers, such as temperature, salinity, and a variety of isotope systems reflecting the effects of geological and climatic phenomena. Pb and Nd isotopes have been frequently used as tracers for studying past water mass mixing and for monitoring continental weathering, because their residence times (50–100 and 600–2000 years, respectively) are shorter than the homogenization time of the ocean (Cochran et al, 1990; Henderson & Maier‐Reimer, 2002; Tachikawa, Jeandel, & Roy‐Barman, 1999). This difference in residence time is recognized within the regional Nd isotope signature of water masses that influence different oceanic basins. Waters from the Pacific Ocean have a more radiogenic signature (−3 to −5 ƐNd), as a result of interaction with younger volcanic
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