The silicon stable isotope distribution along the GEOVIDE section (GEOTRACES GA-01) of the North Atlantic Ocean

Jill N Sutton,Maribel I García-Ibáñez,Gregory F De Souza,Christina L De La Rocha

doi:10.5194/bg-15-5663-2018

Abstract

Abstract. The stable isotope composition of dissolved silicon in seawater (δ30SiDSi) was examined at 10 stations along the GEOVIDE section (GEOTRACES GA-01), spanning the North Atlantic Ocean (40–60∘ N) and Labrador Sea. Variations in δ30SiDSi below 500 m were closely tied to the distribution of water masses. Higher δ30SiDSi values are associated with intermediate and deep water masses of northern Atlantic or Arctic Ocean origin, whilst lower δ30SiDSi values are associated with DSi-rich waters sourced ultimately from the Southern Ocean. Correspondingly, the lowest δ30SiDSi values were observed in the deep and abyssal eastern North Atlantic, where dense southern-sourced waters dominate. The extent to which the spreading of water masses influences the δ30SiDSi distribution is marked clearly by Labrador Sea Water (LSW), whose high δ30SiDSi signature is visible not only within its region of formation within the Labrador and Irminger seas, but also throughout the mid-depth western and eastern North Atlantic Ocean. Both δ30SiDSi and hydrographic parameters document the circulation of LSW into the eastern North Atlantic, where it overlies southern-sourced Lower Deep Water. The GEOVIDE δ30SiDSi distribution thus provides a clear view of the direct interaction between subpolar/polar water masses of northern and southern origin, and allow examination of the extent to which these far-field signals influence the local δ30SiDSi distribution.

Highlights

Proxies of nutrient utilization, such as the silicon stable isotopic composition (δ30Si) of diatom silica, provide a means of reconstructing the past behaviour of marine nutrient cycles, giving insight into the strength of the biological pump in the past, and its influence over atmospheric concentrations of CO2
The stations located to the east of the Mid-Atlantic Ridge (MAR; STN 01, STN 13, STN 21, STN 26, and STN 32) show dissolved silicon (DSi) increasing in concentration from < 10 to 20–50 μM below about 2000 m (Fig. 2a)
This difference relates to the distribution of water masses in the northern North Atlantic, with the predominance of the most egregiously Sipoor northern-sourced water masses (LSW, Iceland– Scotland Overflow Water (ISOW)/North East Atlantic Deep Water (NEADW), Denmark Strait Overflow Water (DSOW)) predominating in the western Atlantic while abyssal layers in the eastern Atlantic have had more of a contribution from Si-rich southern-sourced waters (LDW)

Summary

Introduction

Proxies of nutrient utilization, such as the silicon stable isotopic composition (δ30Si) of diatom silica, provide a means of reconstructing the past behaviour of marine nutrient cycles, giving insight into the strength of the biological pump in the past, and its influence over atmospheric concentrations of CO2. Significant progress has been made in this regard by fifteen years’ worth of work in the Southern Ocean (Varela et al, 2004; Cardinal et al, 2005; De La Rocha et al, 2011; Fripiat et al, 2011), in the North, Equatorial and South Pacific (De La Rocha et al, 2000; Reynolds et al, 2006; Beucher et al, 2008, 2011; de Souza et al, 2012a), and recently in the Arctic Ocean (Varela et al, 2016), in conjunction with various models (De La Rocha and Bickle, 2005; Reynolds, 2009; Coffineau et al, 2014), not the least of which are global circulation models (Wischmeyer et al, 2003; de Souza et al, 2014, 2015; Holzer and Brzezinski, 2015). For many deep waters of the ocean, the mixing between water masses of vastly different origin (and different δ30SiDSi) as they circulate through the ocean basins plays a much greater role than the dissolution of sinking biogenic silica in setting geographic patterns in deep ocean δ30SiDSi (de Souza et al, 2014)

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Discussion

Conclusion