Editing IPCC:AR6/SROCC/Chapter-3 (section)

==== 3.3.3.3 Biogeochemistry ====

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Both polar ice sheets have the potential to release dissolved and sediment-bound nutrients and organic carbon directly to the surface ocean via subglacial and surface meltwater, icebergs, melting of the base of ice shelves (Shadwick et al., 2013 <sup>[[#fn:r1247|1247]]</sup> ; Wadham et al., 2013 <sup>[[#fn:r1248|1248]]</sup> ; Hood et al., 2015 <sup>[[#fn:r1249|1249]]</sup> ; Herraiz-Borreguero et al., 2016 <sup>[[#fn:r1250|1250]]</sup> ; Raiswell et al., 2016 <sup>[[#fn:r1251|1251]]</sup> ; Yager et al., 2016; Hodson et al., 2017), in addition to indirectly stimulating nutrient input via upwelling associated with subglacial meltwater plumes (Meire et al., 2016b; Cape et al., 2018 <sup>[[#fn:r1272|1272]]</sup> ; Hopwood et al., 2018 <sup>[[#fn:r1253|1253]]</sup> ; Kanna et al., 2018 <sup>[[#fn:r1254|1254]]</sup> ) (Figure 3.9). These nutrient additions stimulate primary production in the surrounding ocean waters in some regions ( ''medium confidence'' ) (Gerringa et al., 2012 <sup>[[#fn:r1255|1255]]</sup> ; Death et al., 2014 <sup>[[#fn:r1256|1256]]</sup> ; Duprat et al., 2016 <sup>[[#fn:r1257|1257]]</sup> ; Arrigo et al., 2017b <sup>[[#fn:r1258|1258]]</sup> ). There is also some evidence to support melting ice sheets as source of contaminants (AMAP, 2015 <sup>[[#fn:r1259|1259]]</sup> ).

In Greenland, direct measurements suggest that meltwater is a significant source of bioavailable silica and iron (Bhatia et al., 2013 <sup>[[#fn:r1260|1260]]</sup> ; Hawkings et al., 2014 <sup>[[#fn:r1261|1261]]</sup> ; Meire et al., 2016a <sup>[[#fn:r1262|1262]]</sup> ; Hawkings et al., 2017 <sup>[[#fn:r1263|1263]]</sup> ) but may be less important for the supply of bioavailable forms of dissolved nitrogen or phosphorous (Hawkings et al., 2016 <sup>[[#fn:r1264|1264]]</sup> ; Wadham et al., 2016 <sup>[[#fn:r1265|1265]]</sup> ), which often limit the integrated primary production during summer in fjords (Meire et al., 2016a <sup>[[#fn:r1266|1266]]</sup> ; Hopwood et al., 2018 <sup>[[#fn:r1267|1267]]</sup> ). The offshore export of iron, however, has been linked to primary productivity in surface ocean waters in the Labrador Sea (Arrigo et al., 2017b <sup>[[#fn:r1268|1268]]</sup> ) ( ''limited evidence, high agreement'' ).

Subglacial meltwater plumes from tidewater glaciers have emerged recently as an important indirect source of nutrients to fjords, by entraining nutrient-replete seawater (Meire et al., 2016b <sup>[[#fn:r1269|1269]]</sup> ; Meire et al., 2017 <sup>[[#fn:r1270|1270]]</sup> ; Cape et al., 2018 <sup>[[#fn:r1271|1271]]</sup> ; Hopwood et al., 2018 <sup>[[#fn:r1272|1272]]</sup> ; Kanna et al., 2018 <sup>[[#fn:r1273|1273]]</sup> ) ( ''medium evidence, high agreement'' ). There is ''medium evidence'' with ''high agreement'' that these upwelled nutrient fluxes enhance primary production in fjords over a distance of up to 100 km along the trajectory of the outflowing plume (Juul-Pedersen et al., 2015 <sup>[[#fn:r1274|1274]]</sup> ; Cape et al., 2018 <sup>[[#fn:r1275|1275]]</sup> ; Kanna et al., 2018 <sup>[[#fn:r1276|1276]]</sup> ). ''' '''

In Antarctica ''',''' there is ''medium evidence'' with ''high agreement'' that enhanced input of iron from ice shelves, glacial meltwater and icebergs stimulates primary production in polynyas, coastal regions and the wider Southern Ocean (Gerringa et al., 2012 <sup>[[#fn:r1277|1277]]</sup> ; Shadwick et al., 2013 <sup>[[#fn:r1278|1278]]</sup> ; Herraiz-Borreguero et al., 2016 <sup>[[#fn:r1279|1279]]</sup> ). Satellite observations and modelling also indicate variable potential for icebergs to fertilise the Southern Ocean beyond the coastal zone (Death et al., 2014 <sup>[[#fn:r1280|1280]]</sup> ; Duprat et al., 2016 <sup>[[#fn:r1281|1281]]</sup> ; Wu and Hou, 2017 <sup>[[#fn:r1282|1282]]</sup> ).

Dissolved nutrient fluxes from ice sheets may be increasing during high melt years (Hawkings et al., 2015 <sup>[[#fn:r1283|1283]]</sup> ). The dominant sediment-bound fraction, however, may not increase with rising melt (Hawkings et al., 2015 <sup>[[#fn:r1284|1284]]</sup> ). Thus, there is ''low confidence'' overall in the magnitude of the response of direct nutrient fluxes from ice sheets to enhanced melting.

Future predictions of nutrient cycling proximal to ice sheets is made more challenging by the landward progression of marine-terminating glaciers and the collapse of ice shelves (Cook et al., 2016 <sup>[[#fn:r1285|1285]]</sup> ). This has the potential to drive major shifts in nutrient supply to coastal waters (Figure 3.9). The erosion of newly exposed glacial sediments in front of retreating land-terminating glaciers (Monien et al., 2017 <sup>[[#fn:r1286|1286]]</sup> ) and changes in the diffuse nutrient fluxes from newly exposed glacial sediments on the seafloor (Wehrmann et al., 2014 <sup>[[#fn:r1287|1287]]</sup> ) may amplify nutrient supply, whilst other nutrient sources may be cut off (e.g., icebergs, upwelling of marine water; Meire et al., 2017 <sup>[[#fn:r1288|1288]]</sup> ) ( ''low confidence'' ) ''.''

There is ''medium evidence'' with ''high agreement'' that long-term tidewater glacier retreat into shallower water or onto land, a plausible scenario for about 55% of the 243 distinct outlet glaciers in Greenland (Morlighem et al., 2017 <sup>[[#fn:r1289|1289]]</sup> ), will reduce or diminish upwelling a source of nutrients, thereby reducing summer productivity in Greenland fjord ecosystems (Meire et al., 2017 <sup>[[#fn:r1290|1290]]</sup> ; Hopwood et al., 2018 <sup>[[#fn:r1291|1291]]</sup> ).

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'''Figure 3.9'''

<span id="potential-shifts-in-nutrient-fluxes-with-landward-retreat-of-marine-terminating-glaciers-a-at-different-stages-b-and-c."></span>
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'''Potential shifts in nutrient fluxes with landward retreat of marine-terminating glaciers (a) at different stages (b and c).'''
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[[File:458cf80bd938340c10b5033b1a5c10c3 IPCC-SROCC-CH_3_9.jpg]]

Potential shifts in nutrient fluxes with landward retreat of marine-terminating glaciers (a) at different stages (b and c).

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