Turbulent Mixing and the Formation of an Intermediate Nepheloid Layer Above the Siberian Continental Shelf Break

Kirstin Schulz,Andreas Rogge,Markus Janout,Stefan Büttner,Tom P Rippeth,Jens Hölemann

doi:10.1029/2021gl092988

Abstract

AbstractIntermediate nepheloid layers (INLs) form important pathways for the cross‐slope transport and vertical export of particulate matter, including carbon. While intermediate maxima in particle settling fluxes have been reported in the Eurasian Basin of the Arctic Ocean, direct observations of turbid INLs above the continental slope are still lacking. In this study, we provide the first direct evidence of an INL, coinciding with enhanced mid‐water turbulent dissipation rates, over the Laptev Sea continental slope in summer 2018. Current velocity data show a period of enhanced downslope flow with depressed isopcynals, suggesting that the enhanced turbulent dissipation is probably the consequence of the presence of an unsteady lee wave. Similar events occur mostly during ice free periods, suggesting an increasing frequency of episodic cross‐slope particle transport in the future. The discovery of the INL and the episodic generation mechanism provide new insights into particle transport dynamics in this rapidly changing environment.

Highlights

Particle transport pathways and organic carbon cycling in the Arctic Ocean are substantially different compared to the rest of the world's pelagic oceans (Hwang et al, 2008; Honjo et al, 2010)
At station S3, located at approximately 360 m water depth at the Laptev Sea continental slope, we observed an intermediate water layer characterized by unusually high particulate matter (PM) concentrations, along with strongly enhanced turbulent dissipation rates over the whole water column (Figure 1)
Mid-water turbulent dissipation rates at S3 are up to 10−7 W kg−1, two orders of magnitude higher than the values of 10−9 W kg−1 typically observed in intermediate water layers above the continental slope

Summary

Introduction

Particle transport pathways and organic carbon cycling in the Arctic Ocean are substantially different compared to the rest of the world's pelagic oceans (Hwang et al, 2008; Honjo et al, 2010). Particle and POC settling fluxes are strongly affected by sea ice melt, via the deposition of large under-ice algae biomass (Boetius et al, 2013), and the release of sediments from dirty sea ice (e.g., Krumpen et al, 2019) Both processes are episodic in time and are becoming increasingly important in the central Arctic Ocean due to intensified melt. In the vicinity of the basin margins, particle fluxes are dominated by lateral advection of resuspended lithogenic ballasted material from the continental slopes (Fahl & Nöthig, 2007; Forest et al, 2015, 2016; Honjo et al, 2010; Hwang et al, 2008, 2015; Osborne & Forest, 2016; Xiang & Lam, 2020) This principal transport pathway closely links the basin interior and the disproportionately large shelf sea areas of the Arctic.

Results

Discussion

Conclusion