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Changes in the ocean iron cycle could help explain the low atmospheric CO2 during the Last Glacial Maximum (LGM). Previous modeling studies have mostly considered changes in aeolian iron fluxes, although it is known that sedimentary and hydrothermal fluxes are important iron sources for today's ocean. Here we explore effects of preindustrial-to-LGM changes in atmospheric dust, sedimentary, and hydrothermal fluxes on the ocean's iron and carbon cycles in a global coupled biogeochemical-circulation model. Considering variable atmospheric iron solubility decreases LGM surface soluble iron fluxes compared with assuming constant solubility. This limits potential increases in productivity and export production due to surface iron fertilization, lowering atmospheric CO2 by only 4 ppm. The effect is countered by a decrease in sedimentary flux due to lower sea level, which increases CO2 by 15 ppm. Assuming a 10 times higher iron dust solubility in the Southern Ocean, combined with changes in sedimentary flux, we obtain an atmospheric CO2 reduction of 13 ppm. The high uncertainty in the iron fluxes does not allow us to determine the net direction and magnitude of variations in atmospheric CO2 due to changes in the iron cycle. Our model does not account for changes to iron-binding ligand concentrations that could modify the results. We conclude that when evaluating glacial-interglacial changes in the ocean iron cycle, not only surface but also seafloor fluxes must be taken into account.

A three-dimensional, process-based model of the ocean's carbon and nitrogen cycles, including 13C and 15N isotopes, is used to explore effects of idealized changes in the soft-tissue biological pump. Results are presented from one preindustrial control run (piCtrl) and six simulations of the Last Glacial Maximum (LGM) with increasing values of the spatially constant maximum phytoplankton growth rate μmax, which accelerates biological nutrient utilization mimicking iron fertilization. The default LGM simulation, without increasing μmax and with a shallower and weaker Atlantic Meridional Overturning Circulation and increased sea ice cover, leads to 280 Pg more respired organic carbon (Corg) storage in the deep ocean with respect to piCtrl. Dissolved oxygen concentrations in the colder glacial thermocline increase, which reduces water column denitrification and, with delay, nitrogen fixation, thus increasing the ocean's fixed nitrogen inventory and decreasing δ15NNO3 almost everywhere. This simulation already fits sediment reconstructions of carbon and nitrogen isotopes relatively well, but it overestimates deep ocean δ13CDIC and underestimates δ15NNO3 at high latitudes. Increasing μmax enhances Corg and lowers deep ocean δ13CDIC, improving the agreement with sediment data. In the model's Antarctic and North Pacific Oceans modest increases in μmax result in higher δ15NNO3 due to enhanced local nutrient utilization, improving the agreement with reconstructions there. Models with moderately increased μmax fit both isotope data best, whereas large increases in nutrient utilization are inconsistent with nitrogen isotopes although they still fit the carbon isotopes reasonably well. The best fitting models reproduce major features of the glacial δ13CDIC, δ15N, and oxygen reconstructions while simulating increased Corg by 510–670 Pg compared with the preindustrial ocean. These results are consistent with the idea that the soft-tissue pump was more efficient during the LGM. Both circulation and biological nutrient utilization could contribute. However, these conclusions are preliminary given our idealized experiments, which do not consider changes in benthic denitrification and spatially inhomogenous changes in aeolian iron fluxes. The analysis illustrates interactions between the carbon and nitrogen cycles as well as the complementary constraints provided by their isotopes. Whereas carbon isotopes are sensitive to circulation changes and indicate well the three-dimensional Corg distribution, nitrogen isotopes are more sensitive to biological nutrient utilization.

Changing the supply of silica to the ocean may alter pCO2 levels. The increase in dust delivered to the ocean during glacial times increased the availability of silica for biological uptake. The increased silica levels shifted species composition: Diatom populations increased and coccolith populations decreased. Decreasing the population of coccoliths decreased the flux of calcite to the sediments, which, in turn, lowered pCO2 levels enough to explain the glacial-interglacial pCO2 transition. Furthermore, the contemporary increase in dust delivered to the ocean's mixed layer may be removing significant amounts of carbon dioxide from the atmosphere at present. To set the stage, this silica hypothesis is compared with the iron fertilization and nitrogen fixation hypotheses.

We present δ13C values for phytol, an algal biomarker, which document up to 7 per mil isotopic enrichment during the IronEx II iron fertilization experiment. We evaluate these data using a laboratory-derived 13C fractionation model and show this variability is largely the result of elevated growth rates. Isotopic enrichment and stimulation of growth rate were accompanied by a sevenfold increase in the export of particulate organic carbon as estimated from 234Th activities. This is the first direct evidence that enhanced productivity following iron enrichment can lead to both increased export of organic matter and an associated isotopic signal in an algal biomarker. On the basis of these results, we propose biomarker isotopic data be used in conjunction with paleo-CO2 records to reconstruct paleogrowth rates. This approach provides a means to test for iron-stimulated changes in algal growth in sedimentary records.

[1] In the Southern Ocean, magnetic mineral concentration increases in glacial periods. The variation pattern closely resembles eolian dust flux records from Antarctic ice cores, but the cause of the linkage remains unclear, as the dust flux is too small for the source of terrigenous materials in the Southern Ocean. We have conducted an environmental magnetic study of late Pleistocene sediments from the south Indian Ocean to investigate the origin of the magnetic concentration changes. Biogenic magnetites can be detected using the characteristics of almost no magnetostatic interactions and narrow coercivity distribution, reflecting occurrence of single-domain magnetites in a chain. We interpret that a non-interacting component on first-order reversal curve diagrams and low-coercivity components with small dispersion from isothermal remanent magnetization (IRM) component analyses represent biogenic magnetites, and that the interacting and middle-coercivity components represent terrigenous maghemites. The ratio of anhysteretic remanent magnetization susceptibility to saturation IRM reflects relative abundance of the biogenic and terrigenous components. It was revealed that biogenic magnetites are a dominant constituent of the magnetic minerals. In glacials, the abundance of both biogenic and terrigenous components increased with increased proportions of the latter. Increased ocean productivity in glacials is suggested from increased proportions of biogenic magnetites with elongated morphologies, indicative of less-oxic conditions, and increased sedimentation rates. These observations suggest that the increased magnetic concentration in glacials in the Southern Ocean may be explained by iron fertilization; the production of biogenic magnetites was enhanced associated with increased ocean productivity, which was fueled by increased eolian dust flux.

[1] The valve area of Fragilariopsis kerguelensis, the most abundant diatom species in the Southern Ocean, strongly changes in size in response to varying conditions in the surface ocean. We examined the link, both in two iron fertilization experiments and in sediment samples covering several glacial Terminations, between size variability in this species and environmental conditions across the Antarctic Polar Front, including sea ice extent, sea surface temperature, and the input of eolian dust. The iron fertilization experiments show valve area to be positively correlated with iron concentrations in ambient waters, which suggests the possibility of a causal relation between valve size of Fragilariopsis kerguelensis and ambient surface water iron concentration. Larger valves are usually found during glacial times and thus seem to be related to lower sea surface temperature and wider sea ice coverage. Moreover, our results indicate that there usually is a strong correlation between larger valve size and increased input of eolian dust to the Southern Ocean. However, this correlation, obvious for the fertilization experiments and for glacial Terminations I, II, III, and V, does not seem to be valid for Termination VI, where size appears to be inversely correlated to dust input. Citation: Cortese, G., R. Gersonde, K. Maschner, and P. Medley (2012), Glacial-interglacial size variability in the diatom Fragilariopsis kerguelensis: Possible iron/dust controls?, Paleoceanography, 27, PA1208, doi:10.1029/2011PA002187.

[1] Several hypotheses have been put forward to explain the onset of intensive glaciations on Greenland, Scandinavia, and North America during the Pliocene epoch between 3.6 and 2.7 million years ago (Ma). A decrease in atmospheric CO2 may have played a role during the onset of glaciations, but other tectonic and oceanic events occurring at the same time may have played a part as well. Here we present detailed atmospheric CO2 estimates from boron isotopes in planktic foraminifer shells spanning 4.6–2.0 Ma. Maximal Pliocene atmospheric CO2 estimates gradually declined from values around 410 μatm to early Pleistocene values of 300 μatm at 2.0 Ma. After the onset of large-scale ice sheets in the Northern Hemisphere, maximal pCO2 estimates were still at 2.5 Ma +90 μatm higher than values characteristic of the early Pleistocene interglacials. By contrast, Pliocene minimal atmospheric CO2 gradually decreased from 310 to 245 μatm at 3.2 Ma, coinciding with the start of transient glaciations on Greenland. Values characteristic of early Pleistocene glacial atmospheric CO2 of 200 μatm were abruptly reached after 2.7 Ma during the late Pliocene transition. This trend is consistent with the suggestion that ocean stratification and iron fertilization increased after 2.7 Ma in the North Pacific and Southern Ocean and may have led to increased glacial CO2 storage in the oceanic abyss after 2.7 Ma onward.

Received 3 July 2008; revised 9 October 2008; accepted 27 October 2008; published 14 February 2009. [1] Paleoclimatic reconstructions have provided a unique data set to test the sensitivity of climate system to changes in atmospheric CO2 concentrations. However, the mechanisms behind glacial/interglacial (G/IG) variations in atmospheric CO2 concentrations observed in the Antarctic ice cores are still not fully understood. Here we present a new multiproxy data set of sea surface temperatures (SST), dust and iron supply, and marine export productivity, from the marine sediment core PS2489-2/ODP Site 1090 located in the subantarctic Atlantic, that allow us to evaluate various hypotheses on the role of the Southern Ocean (SO) in modulating atmospheric CO2 concentrations back to 1.1 Ma. We show that Antarctic atmospheric temperatures are closely linked to changes in SO surface temperatures over the last 800 ka and use this to synchronize the timescales of our marine and the European Project for Ice Coring in Antarctica (EPICA) Dome C (EDC) records. The close correlation observed between iron inputs and marine export production over the entire interval implies that the process of iron fertilization of marine biota has been a recurrent process operating in the subantarctic region over the G/IG cycles of the last 1.1 Ma. However, our data suggest that marine productivity can only explain a fraction of atmospheric CO2 changes (up to around 40‐50 ppmv), occurring at glacial maxima in each glacial stage. In this sense, the good correlation of our SST record to the EDC temperature reconstruction suggests that the initial glacial CO2 decrease, as well as the change in the amplitude of the CO2 cycles observed around 400 ka, was most likely driven by physical processes, possibly related to changes in Antarctic sea ice extent, surface water stratification, and westerly winds position.

[1] We add a prognostic biogeochemical model to the Bern3D ocean circulation model to test the impact of increased aeolian iron fluxes in various regions of the ocean on long time scales. Atmospheric CO2 is most sensitive when modern dust flux is increased 100-fold in the Southern Ocean for 1000 years, resulting in a reduction of 10 ppmv. Seeding the Indian Ocean and South Pacific results in increased export production and CO2 drawdown in the Southern Ocean due to interbasinal transport of iron. The non-sea-salt calcium record from Dome C, Antarctica, is used to scale aeolian iron deposition in the Southern Ocean in transient simulations over four Antarctic warm events of the last glacial period. Our results suggest changes in dust flux to the Southern Ocean played a limited role in modulating CO2 variations. The impact of iron fluxes on CO2 is dependent on parameter values chosen for the iron-binding ligand.

Several explanations for the 200 to 280 ppm glacial/interglacial change in atmospheric CO2 concentrations deal with variations in southern ocean phytoplankton productivity and the related use or nonuse of major plant nutrients. An hypothesis is presented herein in which arguments are made that new productivity in today's southern ocean (7.4 × 1013g yr−1) is limited by iron deficiency, and hence the phytoplankton are unable to take advantage of the excess surface nitrate/phosphate that, if used, could result in total southern ocean new production of 2−3 × 1015 g C yr−1. As a consequence of Fe-limited new productivity, Holocene interglacial CO2 levels (preindustrial) are as high as they were during the last interglacial (≈ 280 ppm). In contrast, atmospheric dust Fe supplies were 50 times higher during the last glacial maximum (LGM). Because of this Fe enrichment, phytoplankton growth may have been greatly enhanced, larger amounts of upwelled nutrients may have been used, and the resulting stimulation of new productivity may have contributed to the LGM drawdown of atmospheric CO2 to levels of less than 200 ppm. Background information and arguments in support of this hypothesis are presented.

[1] “Iron fertilization” has been previously recognized as a potential mechanism for enhanced organic matter burial in marine sediments. However, the singular view of iron as a control on primary production overlooks its role in sedimentary diagenesis, a factor that must be evaluated when considering organic matter accumulation. This study examines the role of iron as a buffer of pore water sulfide and its implications for marine organic matter burial. Biogeochemical model experiments indicate that dissolved sulfide buildup in surficial marine sediments is highly sensitive to reactive iron concentration. A reduction in reactive iron concentration can initiate dissolved sulfide accumulation, the consequences of which include inhibition of bioturbation/bioirrigation, a decrease in oxygen exposure time, and enhanced organic matter burial. Alternatively, an increase in reactive iron concentration can serve to decrease organic matter burial. The coupling of iron and phosphorous cycling within marine sediments provides an important positive feedback, and therefore this mechanism is designated the “sulfide buffer/phosphorous trap hypothesis.” Given sufficient organic carbon supply, carbonate-rich and opal-rich sediments should be especially prone to the development of sulfidic conditions because of a deficiency in terrigenous iron. Widespread chalk and marl deposition during the Cretaceous, in association with the evolutionary expansion of calcareous plankton, may have predisposed many benthic marine environments to the accumulation of toxic hydrogen sulfide and fostered the development of “oceanic anoxic events.” Comparison of model results with proxy data from oceanic anoxic event II (middle Cretaceous) suggests a complex role of iron as a control on both organic matter production and preservation.

[1] We use an Earth system model of intermediate complexity, CLIMBER-2, to investigate what recent improvements in the representation of the physics and biology of the glacial ocean imply for the atmospheric concentration. The coupled atmosphere-ocean model under the glacial boundary conditions is able to reproduce the deep, salty, stagnant water mass inferred from Antarctic deep pore water data and the changing temperature of the entire deep ocean. When carbonate compensation is included in the model, we find a CO2 drawdown of 43 ppmv associated mainly with the shoaling of the Atlantic thermohaline circulation and an increased fraction of water masses of southern origin in the deep Atlantic. Fertilizing the Atlantic and Indian sectors of the Southern Ocean north of the polar front leads to a further drawdown of 37 ppmv. Other changes to the glacial carbon cycle include a decrease in the amount of carbon stored in the terrestrial biosphere (540 Pg C), which increases atmospheric CO2 by 15 ppmv, and a change in ocean salinity resulting from a drop in sea level, which elevates CO2 by another 12 ppmv. A decrease in shallow water CaCO3 deposition draws down CO2 by 12 ppmv. In total, the model is able to explain more than two thirds (65 ppmv) of the glacial to interglacial CO2 change, based only on mechanisms that are clearly documented in the proxy data. A good match between simulated and reconstructed distribution of δ13C changes in the deep Atlantic suggests that the model captures the mechanisms of reorganization of biogeochemistry in the Atlantic Ocean reasonably well. Additional, poorly constrained mechanisms to explain the rest of the observed drawdown include changes in the organic carbon:CaCO3 ratio of sediment rain reaching the seafloor, iron fertilization in the subantarctic Pacific Ocean, and changes in terrestrial weathering.

[1] Diatom-bound 15N/14N was used to reconstruct the glacial nutrient status of the Subantarctic Zone in the Southern Ocean. Down-core records from both the Pacific and Indian sectors show δ15N of 5 to 6‰ during the Last Glacial Maximum and a decrease, coincident with the glacial termination, to values as low as 2‰. The effect of either diatom assemblage or physiological change on the diatom-bound 15N/14N is unknown and cannot yet be ruled out as a possible explanation for the observed change. However, the consistency between Indian and Pacific sector records and with other paleoceanographic data suggests that the glacial-interglacial difference in diatom-bound 15N/14N was driven by higher consumption of nitrate in the subantarctic surface during the last ice age. Such a change in nutrient consumption may have resulted from atmospheric iron fertilization and/or decreased glacial mixed layer depths associated with sea ice melting. Enhanced nutrient consumption in the glacial subantarctic would have worked to lower the concentration of CO2 in the ice age atmosphere. It also would have reduced the preformed nutrient content of the low-latitude thermocline, leading to decreases in low-latitude productivity, suboxia, and denitrification.

Integration of geophysical logs with sedimentological, biostratigraphic, and physical properties data from poorly recovered chert-rich sequences at Ocean Drilling Program Leg 198 Sites 1207 and 1213 allows us to propose an Aptian–Albian (Early Cretaceous) depositional history for Shatsky Rise (North Pacific Ocean). The logs indicate that the lower Aptian and lower Albian deposits are relatively lithified and unporous compared to contiguous sediments. These characteristics are best explained by increased siliceous cementation that resulted from an elevated flux of radiolarians to the seafloor. The intervals of increased biosiliceous production may record a biotic response to enhanced nutrient delivery, coeval with (but longer in duration than) oceanic anoxic events (OAEs) 1a (early Aptian) and 1b (early Albian). On Shatsky Rise, OAE 1a is represented by organic-carbon-rich sediments, but OAE 1b is not. Thus “black shale” deposition in the Pacific Ocean was only possible when a “carbon-burial threshold” was crossed. We speculate that this threshold was related to the balance between productivity and carbon oxidation and that, ultimately, the supply of biolimiting nutrients through hydrothermal alteration of basalt may have been the key to the development of “black shale” in the Pacific basin.

[1] Enhanced aeolian supply of iron to the biota of the Southern Ocean during glacial periods is suspected to be an important contributory mechanism to the concurrently low observed mixing ratios of atmospheric CO2. Declining rates of dust deposition prior to the glacial terminations may be critical in driving the initial deglacial rise in CO2, but the reasons behind the dust decline itself are as yet unknown. Here we show that the dust record from the Vostok ice core can be qualitatively derived from a few general assumptions regarding the formation and aging of Patagonian sources of aeolian material and the efficiency with which it is transported through the atmosphere. We suggest that during glacial periods this dust supply becomes particularly sensitive to changes in global climate and that, in turn, climate is responsive to the dust due to iron fertilization. This positive feedback may mean that during glacial periods the carbon cycle exhibits two quasi steady states, characterized by distinct CO2 concentrations. Recognition of this “glacial subcycle” may help to account for the timing and sequence of events at the terminations.