**4. The importance of life on polar seabeds to carbon storage and feedbacks on climate change**

The cold waters of polar oceans are the major marine sinks for atmospheric CO<sup>2</sup> but these are finite, likely diminishing and do not negatively feedback on global climate change. There is evidence to show that polar marine algal capture of CO<sup>2</sup> has increased with ice shelf loss [18], sea ice loss [25] and iceberg production [20]. It also seems likely that polar macroalgal production could increase spatial and temporal extent with exposure of new habitats, sea ice reduction and increased light energy reaching the shallows. These negatively feedback (mitigate) on climate change through increased capture of CO<sup>2</sup> with increasing atmospheric of CO<sup>2</sup> content. Only very small proportions of this captured CO<sup>2</sup> are genuinely sequestered, depending on how much reaches the seabed and how much is recycled in the microbial loop and reworked following bioturbation. All natural carbon sequestration is via burial, mainly at the seabed, where zoobenthic assemblages (consumers) live. They are an important part of the negative feedback on climate, as new and longer availability of phytoplankton is converted into increased growth (organic carbon to tissues and inorganic carbon to skeletons). The feedback value is complicated to measure because it is dynamic in space and time but also because of simultaneous positives and negatives. For example ice shelf loss leads to more open water, a) reducing albedo, thus potential to absorb more heat; b) reduces buttressing of ice sheets, thus potential for this to accelerate coastwards, c) increasing potential for gas exchange, d) generating new phytoplankton blooms, e) opening new habitat for zoobenthos and f) generating giant icebergs with ocean fertilization potential [17–20]. Even the latter components themselves each contain contrasting feedbacks on climate, for example calving of an giant iceberg such as that to break off Larsen C in 2017 may scour and recycle 4x10<sup>4</sup> tCyr−1 of benthic carbon but algal capture and seabed zoobenthic storage of new carbon contributes a net positive of 10<sup>6</sup> tons of carbon per year [17]. The magnitude of this negative feedback is probably similar to that of Arctic Taiga expansion [2], although this too has also complicating factors such as increased heat absorption and less than expected growth gains.

Sea-ice loss areas exceed 1,000,000 km<sup>2</sup> whereas ice shelf losses approximate to ~30,000 km<sup>2</sup> see http://nsidc.org/) so biological responses to these are the largest measured natural negative feedback on climate change. These are dwarfed as an organic carbon store by tropical forests, but these are not increasing as a result of climate change and thus not a negative feedback (their genuine sequestration potential is also low, as burial rates of carbon are very small except for water logged swamp forests). The magnitude of polar blue carbon negative feedback from sea ice losses depends on whether the carbon is calculated from primary production, secondary production, immobilized carbon or sequestered carbon. The sequestration value is considered to be as low as two orders of magnitude different along the cascade from algal production to buried sequestered benthos (**Figure 9**). Scaling up from regional samples suggests that between 2002 and 2015 the zoobenthic blue carbon negative feedback averaged ~10<sup>7</sup> T C in production, 4.5x10<sup>6</sup> T C in immobilization or 1.6x10<sup>6</sup> tons C in terms of sequestration [16] along the West Antarctic Peninsula continental shelf alone. Scaled up to the whole Antarctic continental shelf area (4.4x10<sup>6</sup> km2 ) the annual zoobenthic blue carbon feedback is estimated at 30-80x10<sup>6</sup> T C yr.−1 but including outer Subantarctic continental shelves, such as the Kerguelen Plateau doubles this [17], equivalent to 1–2% of global anthropogenic output. It is clear this feedback is dynamic, polar blue carbon storage has demonstrably increased in coincidence with climate-forced sea ice changes, at least around West Antarctica [26]. Global climate change, ozone losses and other indirect (e.g. non indigenous species invasions) or direct (e.g. harvesting) anthropogenic pressures have the potential to have major impacts on marine biodiversity [27], and thus considerably increase or decrease polar blue carbon.

and decreases (**Figure 8**). Around the Southern Ocean blue carbon increases are most associated with West Antarctic seas and decreases with the East Antarctic coasts [17] but the vast majority of all shelf carbon stocks and change is unknown. It seems likely that the biggest blue carbon changes are near coast caused by ice shelf collapse [18–20], iceberg scour [19] and glacier retreat driven sedimentation [22] but there could also be significant offshore change associated with a shifting seasonal sea ice margin [4, 24, 25]. Given the higher potential ectotherm growth performances at slightly higher sea temperatures [21] it also seems likely that the Arctic and subpolar regions are key areas to quantify blue carbon budgets for. Quantifying these becomes

(from the Antarctic circumnavigation expedition and future British Antarctic survey scientific cruises.

**Figure 8.** Trends in zoobenthic blue carbon accumulation around the Southern Ocean. The key to cell (3x3 degrees) colors are red (biggest increase) to blue (biggest decrease) [17]. Cells with question marks are samples awaiting analysis

one of the key steps in estimating biotic feedbacks on climate change.

is evidence to show that polar marine algal capture of CO<sup>2</sup>

**feedbacks on climate change**

50 Carbon Capture, Utilization and Sequestration

**4. The importance of life on polar seabeds to carbon storage and** 

The cold waters of polar oceans are the major marine sinks for atmospheric CO<sup>2</sup>

are finite, likely diminishing and do not negatively feedback on global climate change. There

but these

has increased with ice shelf loss

**Figure 9.** Cascade of blue carbon through trophic levels and states in polar waters, scaled to mean annual values. Data for the West Antarctic peninsula [16].
