Sedimentary and atmospheric sources of iron around South Georgia, Southern Ocean: a modelling perspective

Abstract

Abstract. In high-nutrient low-chlorophyll waters of the western Atlantic sector of the Southern Ocean, an intense phytoplankton bloom is observed annually north of South Georgia. Multiple sources, including shallow sediments and atmospheric dust deposition, are thought to introduce iron to the region. However, the relative importance of each source is still unclear, owing in part to the scarcity of dissolved iron (dFe) measurements in the South Georgia region. In this study, we combine results from a recently published dFe data set around South Georgia with a coupled regional hydrodynamic and biogeochemical model to further investigate iron supply around the island. The biogeochemical component of the model includes an iron cycle, where sediments and dust deposition are the sources of iron to the ocean. The model captures the characteristic flow patterns around South Georgia, hence simulating a large phytoplankton bloom to the north (i.e. downstream) of the island. Modelled dFe concentrations agree well with observations (mean difference and root mean square errors of ∼ 0.02 nM and ∼ 0.81 nM) and form a large plume to the north of the island that extends eastwards for more than 800 km. In agreement with observations, highest dFe concentrations are located along the coast and decrease with distance from the island. Sensitivity tests indicate that most of the iron measured in the main bloom area originates from the coast and very shallow shelf-sediments (depths < 20 m). Dust deposition exerts almost no effect on surface chlorophyll a concentrations. Other sources of iron such as run-off and glacial melt are not represented explicitly in the model, however we discuss their role in the local iron budget.

Mechanisms of dissolved and labile particulate iron supply to shelf waters and phytoplankton blooms off South Georgia, Southern Ocean

“…1) (Borrione et al, 2014;Nielsdóttir et al, 2012). Based on our study, the main DFe sources during this passage of the ACC were benthic release and vertical mixing, release of DFe from krill and krill faecal pellets, and supply of particles from run-off and glacial melt-water.…”

“…This indicates that DFe was quickly removed from ACC surface waters following passage of the island. However, previous studies in the region suggest DFe transfer beyond the shelf break of South Georgia (Borrione et al, 2014;Nielsdóttir et al, 2012). Nielsdóttir et al (2012) (2012) were also obtained during the summer season but indicated rapid reduction in concentrations through mixing with DFe-depleted ACC water, biological uptake, and/or particle scavenging (authigenic precipitation).…”

“…Aeolian flux model estimates, supplied by Borrione et al (2014) using a regional South Georgia model, suggested that up to 8 µmol Fe m −2 d −1 is delivered to the bloom regions downstream of South Georgia by dry and wet deposition. However, reliable dry-and wet-deposition estimates for the Southern Ocean are limited.…”

“…The blooms peak in austral summer (Borrione et al, 2014), stretch over an area of ∼ 750 000 km 2 (Atkinson et al, 2001;Korb et al, 2004), and are responsible for the largest dissolved inorganic carbon deficit reported within the Antarctic Circumpolar Current (ACC) (Jones et al, 2012(Jones et al, , 2015. As a consequence of the Fe fertilization, the waters in the vicinity of South Georgia support extensive phytoplankton blooms and a large biomass of zooplankton, fish, seabirds, and marine mammals, some of which are exploited commercially (Atkinson et al, 2001;Murphy et al, 2007).…”

“…The ACC surface waters are enriched in nitrate, phosphate, and silicic acid, but strongly depleted in most trace elements, notably Fe and manganese (Mn) (Browning et al, 2014). The large seasonal phytoplankton blooms downstream of South Georgia are thought to be supplied with Fe from the island during the passage of ACC waters (Borrione et al, 2014;Nielsdóttir et al, 2012).…”