Abstract
AbstractRadiogenic lead (Pb) and neodymium (Nd) isotope compositions extracted from authigenic phases in marine sediments are sensitive tracers to reconstruct past ocean circulation and water mass mixing. Chemical reductive leaching of hydrogenetic ferromanganese oxyhydroxides from bulk sediments is the most practical way to recover past seawater Pb and Nd isotope signatures in the Southern Ocean, due to the scarcity of alternative archives. However, the leached signal could be compromised if substantial quantities of Pb and Nd were released from non‐hydrogenetic sediment fractions during chemical extraction. Here we developed a very short 10‐s leaching method to extract reliable seawater Pb and Nd isotope signals from sediments in the Atlantic sector of Southern Ocean. The effect of a previously recommended MgCl2pre‐wash, the role of chelate ligands in the leaching solution and length of leaching time were investigated. The results show that 10‐s exposure time of sediments to reductive leaching extracted sufficient and more reliable hydrogenetic Pb and Nd compared with the commonly used 30‐min leaching approaches. The robustness of our improved leaching method was validated via direct comparison of Pb and Nd isotope signatures with actual seawater, porewater, and corresponding sediment leachates from three stations in front of the Antarctic Filchner‐Rønne Ice Shelf. Our findings also indicate that in contrast previously studied sites on the West Antarctic continental shelf, the bottom seawater Nd concentration is less elevated through benthic fluxes in the area of the southern Weddell Sea shelf.
Highlights
Radiogenic lead (Pb) and neodymium (Nd) isotope compositions have been successfully applied as sensitive and powerful palaeoceanographic proxies for the reconstruction of past circulation changes and water mass mixing for decades (Burton et al, 1997; Christensen et al, 1997; Foster & Vance, 2006; Frank, 2002; Huang et al, 2020)
Via analysis of regional seawater-derived Pb and Nd signatures in the Atlantic sector of Southern Ocean, we found that the sediments further away from Antarctica and volcanically active regions are better suited to preserve unaltered seawater Pb and Nd isotope signals, which strongly supports the unique possibility of tracing past water mass sourcing in the Southern Ocean with our analytical approach
We investigated the effects of MgCl2 pre-wash, presence or absence of chelate ligands and leaching time on extracted authigenic Nd and Pb isotope compositions in the Atlantic sector of Southern Ocean in order to optimize the leaching method
Summary
Radiogenic lead (Pb) and neodymium (Nd) isotope compositions have been successfully applied as sensitive and powerful palaeoceanographic proxies for the reconstruction of past circulation changes and water mass mixing for decades (Burton et al, 1997; Christensen et al, 1997; Foster & Vance, 2006; Frank, 2002; Huang et al, 2020). Substantial quantities of dissolved Nd are delivered to the oceans by sediment-bottom water exchange along continental margins (Lacan & Jeandel, 2005; Lacan et al, 2012; Pöppelmeier et al, 2020; Rempfer et al, 2011) or oceanic islands (Pearce et al, 2013; Rempfer et al, 2011; Stichel, Frank, Rickli, Hathorne, et al, 2012). In contrast to Nd, Pb has a much shorter residence time (50–200 years) (Cochran et al, 1990; Henderson & Maier-Reimer, 2002; Schaule & Patterson, 1981), allowing it to track local and generally rather proximal weathering inputs (Crocket et al, 2013, 2012; Gutjahr et al, 2009; Kurzweil et al, 2010)
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