Carbonate counter pump stimulated by natural iron fertilization in the Polar Frontal Zone

Abstract

The production of organic carbon in the ocean's surface and its subsequent downward export transfers carbon dioxide to the deep ocean. This CO 2 drawdown is countered by the biological precipitation of carbonate, followed by sinking of particulate inorganic carbon, which is a source of carbon dioxide to the surface ocean, and hence the atmosphere over 100-1,000 year timescales 1 . The net transfer of CO 2 to the deep ocean is therefore dependent on the relative amount of organic and inorganic carbon in sinking particles 2 . In the Southern Ocean, iron fertilization has been shown to increase the export of organic carbon 3-5 , but it is unclear to what degree this e ect is compensated by the export of inorganic carbon. Here we assess the composition of sinking particles collected from sediment traps located in the Polar Frontal Zone of the Southern Ocean. We find that in high-nutrient, low-chlorophyll regions that are characterized by naturally high iron concentrations, fluxes of both organic and inorganic carbon are higher than in regions with no iron fertilization. However, the excess flux of inorganic carbon is greater than that of organic carbon. We estimate that the production and flux of carbonate in naturally iron-fertilized waters reduces the overall amount of CO 2 transferred to the deep ocean by 6-32%, compared to 1-4% at the non-fertilized site. We suggest that an increased export of organic carbon, stimulated by iron availability in the glacial sub-Antarctic oceans, may have been accompanied by a strengthened carbonate counter pump.The biological carbon pump is the downward flux of particulate organic carbon (POC) from the surface to the deep ocean 6 . The fraction of settling POC that is not remineralized in the winter mixed layer sinks to depth, driving a reduction in surface ocean partial pressure of carbon dioxide (pCO 2 ) that is compensated by oceanic uptake of atmospheric CO 2 (ref. 7). The iron hypothesis 8 suggests that increased iron supply to the nutrient-rich, but irondeficient Southern Ocean, contributed towards the termination of glacial periods by enhancing phytoplankton growth and the biological carbon pump. Recent studies of natural systems 3,4 and artificial 5 iron fertilization experiments support this idea by demonstrating enhanced POC flux well below the mixed layer into the deep ocean 3-5 .Counteracting the organic carbon pump in terms of its influence on air-sea CO 2 exchange is the carbonate counter pump 9 . The precipitation of CaCO 3 shells by mainly coccolithophores, foraminifers (both calcite) and pteropods (aragonite), and the resulting particulate inorganic carbon (PIC) flux from the surface ocean, causes an increase in surface ocean pCO 2 (ref. 10) on timescales of the order of 100-1,000 years 1 . As a result of these opposing effects, particle flux studies addressing deep-ocean CO 2 sequestration need to discriminate between organic (soft-tissue) and inorganic carbon, with a strong focus on a relationship formalized as the rain ratio (POC:PIC; ref. 2). Despite its obviou...

Planktic foraminifer and coccolith contribution to carbonate export fluxes over the central Kerguelen Plateau

“…Given the good relationship between area and minimum diameter, this method is considered as an appropriate mean to characterize the test density (Beer et al, 2010b). We compared the Kerguelen dataset (station A3, AAZ) with previously published size and weight data using the same methodology from the Crozet Islands (Salter et al, 2014). Stations M10 and M5 are located in the PFZ .…”

“…Foraminifer quantification, morphometric measurements and weighing was performed following the methods outlined in Salter et al (2014). One 1/8 aliquot was sieved on a 63 mm mesh with tap water and the 463 mm fraction was dried overnight (40°C).…”

“…Therefore the PIC:POC ratio of exported particles is likely to have a significant impact on the atmosphere-ocean CO 2 fluxes from seasonal to geological timescales Sarmiento et al, 2002). More recently, in the Subantarctic Southern Ocean, the strong response of calcifying organisms to natural iron fertilization has been observed to increase the PIC:POC export ratio leading to a strong carbonate counter pump, lowering the efficiency of CO 2 sequestration by the biological carbon pump (Salter et al, 2014).…”

“…species composition, test size and test weight) with seasonal and annual geochemical budgets. In this context, the study by Salter et al (2014) quantified a carbonate counter pump effect accounting for 6-32% of measured POC fluxes with a notable contribution from foraminifer species (mainly Globigerina bulloides and N. pachyderma) in iron-fertilized waters downstream of the Crozet Islands. Several studies have reported geochemical transitions in particle stoichiometry across the Polar Front (Trull et al, 2001;Honjo et al, 2008), highlighting the importance of regional variability for a Southern Ocean carbonate counter pump that is partly linked to the biogeography of calcareous organisms (Salter et al, 2014).…”

“…Southern Ocean coccolithophore populations are dominated by the cosmopolitan species Emiliania huxleyi (Saavedra-Pellitero et al, 2014;Winter et al, 2014) that is thought to be the major component of the "great calcite belt" observed in the vicinity of the Subantarctic Front (SAF) and Polar Front (PF) Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/dsri (Balch et al, 2014). Several studies have reported modern planktic foraminifer abundances and fluxes in the Southern Ocean from net tows (Asioli and Langone, 1997;Mortyn and Charles, 2003;Bergami et al, 2009;Meilland, 2015) and sediment traps (Donner and Wefer, 1994;King and Howard, 2003;Northcote and Neil, 2005;Salter et al, 2014). Foraminifer assemblages are characterized by a southward dominance of polar species Neogloboquadrina pachyderma.…”