Icebergs, sea ice, blue carbon and Antarctic climate feedbacks

Abstract

Sea ice, including icebergs, has a complex relationship with the carbon held within animals (blue carbon) in the polar regions. Sea-ice losses around West Antarctica's continental shelf generate longer phytoplankton blooms but also make it a hotspot for coastal iceberg disturbance. This matters because in polar regions ice scour limits blue carbon storage ecosystem services, which work as a powerful negative feedback on climate change (less sea ice increases phytoplankton blooms, benthic growth, seabed carbon and sequestration). This resets benthic biota succession (maintaining regional biodiversity) and also fertilizes the ocean with nutrients, generating phytoplankton blooms, which cascade carbon capture into seabed storage and burial by benthos. Small icebergs scour coastal shallows, whereas giant icebergs ground deeper, offshore. Significant benthic communities establish where ice shelves have disintegrated (giant icebergs calving), and rapidly grow to accumulate blue carbon storage. When 5000 km² giant icebergs calve, we estimate that they generate approximately 10⁶ tonnes of immobilized zoobenthic carbon per year (t C yr^-1). However, their collisions with the seabed crush and recycle vast benthic communities, costing an estimated 4 × 10⁴ t C yr^-1 We calculate that giant iceberg formation (ice shelf disintegration) has a net potential of approximately 10⁶ t C yr^-1 sequestration benefits as well as more widely known negative impacts.This article is part of the theme issue 'The marine system of the West Antarctic Peninsula: status and strategy for progress in a region of rapid change'.

Keywords: Southern Ocean; benthos; blue carbon sink; climate change; iceberg A68; phytoplankton.

Figure 1.

Monitoring sea ice and iceberg activity in West Antarctica. (a) Monitoring sites, using automated cameras (black circles) and manual observations (white circle), of sea-ice extent in time and space. Monitoring sites of both sea-ice extent and annual iceberg scouring (red circles) at Rothera (bottom red circle) and Carlini (top red circle) [9,21]. (b) Relationship between duration of fast-ice cover each year and ice scouring on the seabed at Rothera and Carlini (inner bay in orange, outer bay in red) stations. The y-axis shows the proportion of the monitored seabed from a 5–25 m depth hit by icebergs. Pearson's correlation line (−0.86, p < 0.001) drawn through Rothera data only.

Figure 2.

Iceberg impacts on seabed blue carbon. (a) Iceberg grounded at South Georgia in 2004; photo courtesy of US National Ice Center. Sites 11, 12 and 13 are benthic image and trawl samples. The island in the image (left) is South Georgia and iceberg size is 75 × 41 km. (b) Correlation between sea-ice duration and blue carbon storage on the seabed around southern Adelaide Island, West Antarctic Peninsula (from data in Barnes [6,9] ). Zones are zoobenthic carbon; significant increase with increased fast-ice duration (A), no significant change (B,D), significant decrease with increased fast-ice duration (C) and depth zone where there is too little known for meaningful analysis (zone of ignorance).

Figure 3.

Giant icebergs in the Southern Ocean. (a) Distribution of calved (circles) and calving (squares) giant icebergs (greater than 30 km²) around Antarctica, in which the symbols are giant icebergs. The calved icebergs are further categorized as grounded (red), roaming free on the continental shelf (black) or in deep water (blue), or stuck in fast ice (dark purple). (b) Is the UK's Halley VI research station set to become the first giant-iceberg-based research station?

Figure 4.

Hotspots and tracks of giant icebergs. (a) Hotspots of giant icebergs grounding on continental shelf around Antarctica from 2014 to 2017 (see http://www.polarview.aq/antarctic for data). (b) Historic tracks of giant icebergs, together with the major current systems (Antarctic Circumpolar Current (ACC, large arrows) and Antarctic Counter Current (ACoC, small arrows)) influencing these tracks. Historic iceberg information provided by US National Ice Center from Southern Ocean iceberg tracking database (see http://www.natice.noaa.gov/doc/Notice_Iceberg_Tracking_Criteria.pdf).

Figure 5.

Trends in benthic blue carbon change. Change in blue carbon benthic standing stock (tonnes per square kilometre) around the Southern Ocean. Each cell is 3° × 3°. Cell data based on: (i) yellow–red are increases from direct (increased standing stock from sampling benthos) and indirect (iceberg grounding data) evidence streams, Fillinger et al. [19], Barnes [5,9], Barnes & Sands [9]; (ii) white with question mark are samples awaiting analysis (from Antarctic Circumnavigation Expedition cruise 2016–2017) and expected samples from British Antarctic Survey JR17004 scientific cruise; (iii) light to dark blue are decreases from giant iceberg grounding probabilities given by US National Ice Center (figure 4a).