Determining the role of retreating glaciers in the biogeochemical cycling of iron and macronutrients at the West Antarctic Peninsula

Abstract

The West Antarctic Peninsula (WAP) shelf is a highly productive region of the Southern Ocean. Primary productivity in the Southern Ocean modulates climate over glacial-interglacial cycles, subject to iron and light limitation. Across the WAP the impact of accelerating glacial retreat upon water column and shelf sediment biogeochemical cycles is changing, through interaction of meltwater and terrestrial material with shelf seawater. The alleviation of iron limitation in the Southern Ocean by Antarctic shelf-derived sediment fluxes of iron is well documented. However, the role of the predominantly colloidal and nanoparticulate flux of iron from Antarctic glaciers in Southern Ocean productivity is not well characterised.
Data presented here investigates the influence of rapidly retreating glaciers upon biogeochemical cycling of iron and macronutrients in three glaciated bays along the WAP. Utilising stable oxygen isotopes, salinity, and short-lived radium isotopes, I demonstrated that submarine melting at marine-terminating glaciers can drive intense mixing of sediment into the water column. Buoyant meltwater plumes can then entrain ambient seawater, rich in NO3-, PO43-, Si(OH4), and sedimentary material, upwards to the mixed layer. Where surface runoff is the dominant meltwater input, the coastal surface ocean is highly enriched in sediment, and up to 580 nM particulate iron. X-ray Absorption Near-Edge Spectroscopy revealed that glacially-derived particulate iron is mixed valence, with a mean of 40% iron(II) across the WAP. These particles were often found co-located with carbon aggregates, which can provide stabilisation of iron(II) minerals in oxic seawater. As particulate iron(II) is potentially more bioavailable, this may represent a bioavailable particulate iron supply to shelf surface waters. Observed transport of particulate iron in surface melt to the shelf edge suggests a high export potential of iron(II)-rich particles to commonly iron-limited regions.
Finally, experimental work indicates a previously underappreciated reservoir of readily dissolvable iron from the solid phase during resuspension of glacially-influenced surface sediments. After adding glaciogenic sediments to overlying bottom water, an immediate
dissolved iron release of ~65 – 175 nM and total dissolvable iron release of 30 – 100 μM was observed in three WAP bays, not accounted for by concurrent pore water iron inventories. Over 2 days, 4 – 12 nM of excess dissolved iron remained stable in solution, mostly accounted for by rapid exchange from solid phase to dissolved phase. Sediments from areas both proximal to glacier outflow downstream regions of Fe limitation, such as the northern WAP, should be the most important for the bioavailable supply of Fe to the Southern Ocean mixed layer. Overall, this thesis highlights key pathways for a significant and potentially bioavailable flux of iron from surface ocean meltwater to sediments, delivered by retreating glaciers.

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Identifiers

ORCID for Rhiannon Laila Caitlin Jones: orcid.org/0000-0002-3182-9685

ORCID for Amber Annett: orcid.org/0000-0002-3730-2438

ORCID for Maeve Lohan: orcid.org/0000-0002-5340-3108

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