Seasonal dispersal of fjord meltwaters as an important source of iron and manganese to coastal Antarctic phytoplankton

Kiefer O Forsch,Katherine A Barbeau,Robert M Sherrell,David Burdige,Maria Vernet,Vincent J Roccanova,Kaixuan Bu,Lisa Hahn-Woernle

doi:10.5194/bg-18-6349-2021

Abstract

Abstract. Glacial meltwater from the western Antarctic Ice Sheet is hypothesized to be an important source of cryospheric iron, fertilizing the Southern Ocean, yet its trace-metal composition and factors that control its dispersal remain poorly constrained. Here we characterize meltwater iron sources in a heavily glaciated western Antarctic Peninsula (WAP) fjord. Using dissolved and particulate ratios of manganese to iron in meltwaters, porewaters, and seawater, we show that surface glacial melt and subglacial plumes contribute to the seasonal cycle of iron and manganese within a fjord still relatively unaffected by climate-change-induced glacial retreat. Organic ligands derived from the phytoplankton bloom and the glaciers bind dissolved iron and facilitate the solubilization of particulate iron downstream. Using a numerical model, we show that buoyant plumes generated by outflow from the subglacial hydrologic system, enriched in labile particulate trace metals derived from a chemically modified crustal source, can supply iron to the fjord euphotic zone through vertical mixing. We also show that prolonged katabatic wind events enhance export of meltwater out of the fjord. Thus, we identify an important atmosphere–ice–ocean coupling intimately tied to coastal iron biogeochemistry and primary productivity along the WAP.

Highlights

Warm atmospheric temperatures are accelerating glacial retreat and increasing meltwater discharge, rapidly changing Earth’s cryosphere (Mouginot et al, 2019; Rignot et al, 2013)
Increased silicic acid (Si) concentrations, with respect to nitrate, within the inner fjord are driven by dissolution of biogenic silica sediments, or weathering of the bedrock by contact with the 11 marine-terminating glaciers feeding into Andvord Bay since Si-to-N ratio is highly correlated with meltwater fraction (MWf) below the surface layer (Hawkings et al, 2018; Ng et al, 2020)
The export of biogenic particles from the surface showed a distinct seasonality indicated by increased chlorophyll a pigment content in seafloor surface sediments in fall (Ziegler et al, 2020), as well as higher respiration rates from chamber incubation experiments in the fall compared to spring, no indication of sulfate reduction was observed in sediment box and Kasten cores (2.3 m long), suggesting that oxygen, nitrate, and metal oxides were sufficient to oxidize organic matter within the upper sediments (Craig Smith, personal communication, 2018)

Summary

Introduction

Warm atmospheric temperatures are accelerating glacial retreat and increasing meltwater discharge, rapidly changing Earth’s cryosphere (Mouginot et al, 2019; Rignot et al, 2013). Direct measurements of Fe in heavily glaciated fjords reveal that up to 90 %–99 % of dissolved Fe (dFe) originating from glaciers is removed upon mixing with seawater due to estuarine-type removal processes, including precipitation of insoluble oxyhydroxides, adsorption to the surfaces of particles, and aggregation of colloids and particles (Boyle et al, 1977; Schroth et al, 2014) Together, these processes are known as scavenging and constitute a major control on the distribution of Fe in the ocean by converting soluble forms of Fe into colloidal aggregates and particles (Wu et al, 2001). Constraints on the flux of newly delivered glacial Fe that escapes this sink and is transported across continental shelves will enable

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