Abstract
Relief of iron (Fe) limitation in the surface Southern Ocean has been suggested as one driver of the regular glacial-interglacial cycles in atmospheric carbon dioxide (CO2). The proposed cause is enhanced deposition of Fe-bearing atmospheric dust to the oceans during glacial intervals, with consequent effects on export production and the carbon cycle. However, understanding the role of enhanced atmospheric Fe supply in biogeochemical cycles is limited by knowledge of the fluxes and ‘bioavailability’ of atmospheric Fe during glacial intervals. Here, we assess the effect of Fe fertilization by dust, dry-extracted from the Last Glacial Maximum portion of the EPICA Dome C Antarctic ice core, on the Antarctic diatom species Eucampia antarctica and Proboscia inermis. Both species showed strong but differing reactions to dust addition. E. antarctica increased cell number (3880 vs. 786 cells mL-1), chlorophyll a (51 vs. 3.9 μg mL-1) and particulate organic carbon (POC; 1.68 vs. 0.28 μg mL-1) production in response to dust compared to controls. P. inermis did not increase cell number in response to dust, but chlorophyll a and POC per cell both strongly increased compared to controls (39 vs. 15 and 2.13 vs. 0.95 ng cell-1 respectively). The net result of both responses was a greater production of POC and chlorophyll a, as well as decreased Si:C and Si:N incorporation ratios within cells. However, E, antarctica decreased silicate uptake for the same nitrate and carbon uptake, while P. inermis increased carbon and nitrate uptake for the same silicate uptake. This suggests that nutrient utilization changes in response to Fe addition could be driven by different underlying mechanisms between different diatom species. Enhanced supply of atmospheric dust to the surface ocean during glacial intervals could therefore have driven nutrient-utilization changes which could permit greater carbon fixation for lower silica utilization. Additionally, both species responded more strongly to lower amounts of direct Fe chloride addition than they did to dust, suggesting that not all the Fe released from dust was in a bioavailable form available for uptake by diatoms.
Highlights
Over at least the last 800,000 years, glacial-interglacial cycles in earth climate have been coincident with and largely driven by regular 80–100 ppmv changes in the levels of atmospheric CO2 [1,2,3]
The ‘iron hypothesis’ suggested that enhanced dust flux to the oceans during glacial intervals could have acted to relieve Fe-limitation of phytoplankton in some areas of the surface oceans, thereby increasing nutrient utilization and carbon export [6,7]. This increased carbon storage in the deep ocean could in return have lowered atmospheric CO2 over glacial timescales [6]. This hypothesis centered on three High Nutrient Low Chlorophyll (HNLC) regions of the surface ocean where today vanishingly-low dissolved Fe concentrations limit growth, while upwelling ensures macronutrients are found in excess in surface waters but does not supply sufficient dissolved Fe to utilize these macronutrients [8]
This hypothesis has been somewhat superseded by later ideas which instead suggest a greater role for upwelling and circulation control on deep ocean carbon storage during glacial intervals [9,10,11], the most recent studies suggests that Fe-fertilization may still play an important role in moderating atmospheric CO2 on glacial and millennial timescales [12,13,14,15,16]
Summary
Over at least the last 800,000 years, glacial-interglacial cycles in earth climate have been coincident with and largely driven by regular 80–100 ppmv changes in the levels of atmospheric CO2 [1,2,3]. Understanding how atmospheric CO2 changes during glacial inception or termination has been a focus of much research, with a number of hypotheses put forward to explain the cycling [4,5] One such idea, the ‘iron hypothesis’ suggested that enhanced dust flux to the oceans during glacial intervals could have acted to relieve Fe-limitation of phytoplankton in some areas of the surface oceans, thereby increasing nutrient utilization and carbon export [6,7]. This increased carbon storage in the deep ocean could in return have lowered atmospheric CO2 over glacial timescales [6] This hypothesis centered on three High Nutrient Low Chlorophyll (HNLC) regions of the surface ocean where today vanishingly-low dissolved Fe concentrations limit growth, while upwelling ensures macronutrients are found in excess in surface waters but does not supply sufficient dissolved Fe to utilize these macronutrients [8]. This hypothesis has been somewhat superseded by later ideas which instead suggest a greater role for upwelling and circulation control on deep ocean carbon storage during glacial intervals [9,10,11], the most recent studies suggests that Fe-fertilization may still play an important role in moderating atmospheric CO2 on glacial and millennial timescales [12,13,14,15,16]
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