Glacial-interglacial Timescales Research Articles

Sustained sulfide oxidation by physical erosion processes in the Mackenzie River basin: Climatic perspectives

The chemical weathering of rocks with sulfuric acid is usually not considered in reconstructions of the past evolution of the carbon cycle, although this reaction delivers cations and alkalinity to the ocean without involvement of atmospheric CO 2 . The contribution of sulfuric acid as a weathering agent is still poorly quantifi ed; the identifi cation of riverine sulfate sources is diffi cult. The use of δ 34 S and δ 18 O of dissolved sulfate allows us to demonstrate that most of the sulfate in surface waters of the Mackenzie River system, Canada, derives from pyrite oxidation (85% ± 5%) and not from sedimentary sulfate. The calculated fl ux of pyrite-derived sulfate is 0.13 ◊ 10 12 mol/yr, corresponding to 20%‐27% of the estimated global budget. This result suggests that the modern global ocean delivery of sulfi de-derived sulfate, and thus chemical weathering with sulfuric acid, may be signifi cantly underestimated. A strong correlation between sulfi de oxidation rates and mechanical erosion rates suggests that the exposure of fresh mineral surfaces is the rate-limiting factor of sulfi de oxidation in the subbasins investigated. The chemical weathering budget of the Mackenzie River shows that more than half of the dissolved inorganic carbon discharged to the ocean is ancient sedimentary carbon from carbonate (62%) and not atmospheric carbon (38%). The subsequent carbonate precipitation in the ocean will thus release more CO 2 in the atmosphere-ocean system than that consumed by continental weathering, typically on glacial-interglacial time scales.

No change in the neodymium isotope composition of deep water exported from the North Atlantic on glacial-interglacial time scales

Quantifying past circulation is a vital part of testing our understanding of the modern and future climate system. The isotopic composition of neodymium (Nd) in marine precipitates has considerable promise as a recorder of past circulation patterns, but its robust application requires knowledge of the end-member compositions in order to correctly deconvolute a downstream signal. We show here, using in situ, high temporal resolution analyses of ferro-manganese crusts from the North Atlantic, that the Nd isotopic composition of deep water during times of much more extensive Northern Hemisphere ice cover was no different than the modern-day interglacial value. This result is surprising, but greatly simplifies the use of Nd isotopes as tracers of the strength and patterns of circulation in the Atlantic in the past.

Phytoliths as quantitative indicators for the reconstruction of past environmental conditions in China II: palaeoenvironmental reconstruction in the Loess Plateau

Quantitative reconstruction of the climatic history of the Chinese Loess Plateau is important for understanding present and past environment and climate changes in the Northern Hemisphere. Here, we reconstructed mean annual temperature (MAT) and mean annual precipitation (MAP) trends during the last 136 ka based on the analysis of phytoliths from the Weinan loess section (34°24′N, 109°30′E) near the southern part of the Loess Plateau in northern China. The reconstructions have been carried out using a Chinese phytolith–climate calibration model based on weighted averaging partial least-squares regression. A series of cold and dry events, as indicated by the reconstructed MAT and MAP, are documented in the loess during the last glacial periods, which can be temporally correlated with the North Atlantic Heinrich events. Our MAT and MAP estimations show that the coldest and/or driest period occurred at the upper part of L2 unit (Late MIS 6), where MAT dropped to ca 4.4 °C and MAP to ca 100 mm. Two other prominent cold-dry periods occurred at lower Ll-5 (ca 77–62 ka) and L1-1 (ca 23–10.5 ka) where the MAT and MAP decreased to about 6.1–6.5 °C and 150–370 mm, respectively, ca 6.6–6.2 °C and 400–200 mm lower than today. However, the highest MAT (average 14.6 °C, max. 18.1 °C) and MAP (average 757 mm, max. 1000 mm) occurred at Sl interval (MIS 5). During the interstadial of L1-4–L1-2 (MIS 3) and during the Holocene warm-wet period, the MAT was about 1–2 °C and MAP 100–150 mm higher than today in the Weinan region. The well-dated MAT and MAP reconstructions from the Chinese Loess Plateau presented in this paper are the first quantitatively reconstructed proxy record of climatic changes at the glacial–interglacial timescale that is based on phytolith data. This study also reveals a causal link between climatic instability in the Atlantic Ocean and climate variability in the Chinese Loess Plateau.

Negligible glacial–interglacial variation in continental chemical weathering rates

Chemical weathering of the continents is central to the regulation of atmospheric carbon dioxide concentrations, and hence global climate. On million-year timescales silicate weathering leads to the draw-down of carbon dioxide, and on millennial timescales chemical weathering affects the calcium carbonate saturation state of the oceans and hence their uptake of carbon dioxide. However, variations in chemical weathering rates over glacial-interglacial cycles remain uncertain. During glacial periods, cold and dry conditions reduce the rate of chemical weathering, but intense physical weathering and the exposure of carbonates on continental shelves due to low sea levels may increase this rate. Here we present high-resolution records of the lead isotope composition of ferromanganese crusts from the North Atlantic Ocean that cover the past 550,000 years. Combining these records with a simple quantitative model of changes in the lead isotope composition of the deep North Atlantic Ocean in response to chemical weathering, we find that chemical weathering rates were two to three times lower in the glaciated interior of the North Atlantic Region during glacial periods than during the intervening interglacial periods. This decrease roughly balances the increase in chemical weathering caused by the exposure of continental shelves, indicating that chemical weathering rates remained relatively constant on glacial-interglacial timescales. On timescales of more than a million years, however, we suggest that enhanced weathering of silicate glacial sediments during interglacial periods results in a net draw-down of atmospheric carbon dioxide, creating a positive feedback on global climate that, once initiated, promotes cooling and further glaciation.

A dynamic-flow carbon-cycle box model and high-latitude sensitivity

Most of the hypotheses put forward to explain glacial–interglacial cycles in atmospheric pCO2 are centred on Southern-Ocean-based mechanisms. This is in large part because: (1) timing constraints rule out changes in the North Atlantic as the trigger; (2) the concept of “high-latitude sensitivity” eliminates changes in the non-polar oceans as likely contenders. Many of the Southern-Ocean-based mechanisms for changing atmospheric pCO2 on glacial–interglacial time-scales are based on results from highly simplified box models with prescribed flow fields and fixed particulate flux. It has been argued that box models are significantly more “high-latitude sensitive” than General Circulation Models. In light of this, it is important to understand whether this high-latitude sensitivity is a feature common to all box models, and whether the apparent degree of sensitivity changes for different tracers and parameters. We introduce a new metric for assessing how “high-latitude sensitive” a particular solution is to perturbations. With this metric, we demonstrate that a given model may be high-latitude sensitive to certain parameters but not to others. We find that the incorporation of a dynamic-based flow field and a Michaelis–Menten type nutrient feedback can have a significant impact on the apparent sensitivity of the model to perturbations. The implications of this for current box-model-based estimates of atmospheric pCO2 drawdown are discussed.

Distribution of planktonic foraminifera: a modeling approach

Abstract

Introduction to “Progress towards reconstructing past climate in Central Eurasia, with special emphasis on Lake Baikal”

concentration will further act to exacerbate changesin biogeography, productivity, water quantity, andwater quality. Yet, in this region, both experimentswith quasi-realistic climate models and palaeoclimatereconstructions from proxies in natural archives areunderrepresented. Given the remoteness of CentralEurasia (far from ice sheet and oceanic influences), itis not prudent to simply extrapolate climate changesignals from the Atlantic or the Pacific regions.Hence, there is an important gap in our knowledgeof how large-scale climate processes, such as theNorth Atlantic Oscillation and interactions betweenthe Westerlies and monsoons, affect such continentalregions.Important to the concept of climate changestudies are palaeoecological programmes to testmodels, to improve model parameterisation, and toplace these recent trends in climate variability into theperspective of longer time scales. In this context, LakeBaikal, located in southern Central Siberia, is one ofthe most exciting lakes for climate change research, asshown by successful international scientific effortsduring the last 15 years under umbrella programmessuch as the Baikal International Centre for EcologicalResearch (BICER), and the Baikal Drilling Project(Baikal Drilling Project BDP-96 (Leg II) members,1997; Grachev et al., 1997, 1998; Kuzmin et al., 2001;Kashiwaya, 2003)–ICDP Programme and IGBPPAGES–PANASH initiatives. Previous studies cover-ing glacial–interglacial time scales clearly show theorbital control on the sediment parameters revealingpronounced precession and eccentricity cyclicity(Colman et al., 1995; Williams et al., 1997, Kashi-waya et al., 2001). However, the Lake Baikalsedimentary archive also has the potential to sensi-tively record climate changes at high temporalresolution (decadal to centennial) over more recenttime scales. Despite the intense concern over theenvironmental and ecological impacts of anthropo-

Deep flow in the Madagascar–Mascarene Basin over the last 150000 years

The SW Indian Ocean contains at least four layers of water masses with different sources: deep Antarctic (Lower Circumpolar Deep Water) flow to the north, midwater North Indian Deep Water flow to the south and Upper Circumpolar Deep Water to the north, meridional convergence of intermediate waters at 500-1500 m, and the shallow South Equatorial Current flowing west. Sedimentation rates in the area are rather low, being less than 1 cm ka(-1) on Madagascar Ridge, but up to 4 cm ka(-1) at Amirante Passage. Bottom flow through the Madagascar-Mascarene Basin into Amirante Passage varies slightly on glacial-interglacial time-scales, with faster flow in the warm periods of the last interglacial and minima in cold periods. Far more important are the particularly high flow rates, inferred from silt grain size, which occur at warm-to-cold transitions rather than extrema. This suggests the cause is changing density gradient driving a transiently fast flow. Corroboration is found in the glacial-interglacial range of benthic delta18O which is ca. 2 per thousand, suggesting water close to freezing and at least 1.2 more saline and thus more dense glacial bottom waters than present. Significant density steps are inferred in isotope stage 6, the 5e-5d, and 5a-4 transitions. Oxygen isotope data suggest little change by mixing in glacial bottom water on their northward path. Benthic carbon isotope ratios at Amirante Passage differ from glacial Southern Ocean values, due possibly to absence of a local productivity effect present in the Southern Ocean.

Glacial–interglacial contrast in climate variability at centennial-to-millennial timescales: observations and conceptual model

A set of published paleoclimate proxy records from the northern hemisphere, capturing different climate processes, is used to study glacial–interglacial differences in climate variability at centennial-to-millennial timescales during the past fifty thousand years. These proxy records reveal the existence of distinct oscillatory modes of the climate system. Glacial climate variability is dominated by a single mode, the Dansgaard–Oeschger cycles, composed of stadial and interstadial states. This glacial mode results in well-expressed covariations of the proxies, which are paced by a fundamental 1470-year signal. In contrast, there is no compelling evidence for a dominant and persistent centennial-to-millennial climate cycle during the Holocene. Interglacial climate variations seem to covary less pronounced than those of the last glacial period, suggesting the simultaneous activity of independent climate modes, each characterized by its own natural periods, between approximately 400–3000 years. A conceptual model is introduced to interpret this contrast in covariation at glacial–interglacial timescales. It is assumed that different climate modes can be represented by relaxation oscillators with different natural periods in the centennial-to-millennial band. Interactions among such oscillators may lead to a phase-synchronization and the development of a new climate mode with a joint frequency. We suggest that the coupled state with its synchronized dynamics resembles a glacial whereas the decoupled state represents an interglacial with its reduced covariations of climate fluctuations. The synchronization greatly enhances the frequency stability of the coupled system, and has the potential to reconcile the stability of the glacial 1470-year pacing cycle with an origin within the Earth's climate system.

Weddell Sea is a globally significant contributor to deep-sea sequestration of natural carbon dioxide

A mechanism of lateral transport of remineralized carbon from the subsurface Weddell Sea into the abyssal world oceans is presented and its impact is quantified. In the Weddell Sea interior, full remineralization of the export production occurs at shallow depths. This shallow, CO 2-charged water stands in isopycnal contact with the abyssal world ocean waters to the north of the subpolar Weddell Sea. Via isopycnal water transport, remineralized CO 2 is transferred and sequestered in the deep sea. The amount involved is 1.9×10 13 g C yr −1, which is equal to at least 6% of the presently estimated world-wide natural CO 2 sequestration in the abyssal oceans. It thus constitutes an important component of the lower limb of the global oceanic carbon cycle. There may be more regions like the Weddell Sea, where this mechanism could be active. It is likely to play a significant role on the glacial–interglacial time scale. During a glacial period, reduced CO 2 transport via the CIW would tend to increase the atmospheric partial pressure of CO 2 as opposed to the generally decreasing pCO 2 trend.

Evidence for a link between the flux of galactic cosmic rays and Earth's climate during the past 200,000 years

The possibility that galactic cosmic rays (GCR) influence the Earth's cloud cover and therefore have an important impact on the Earth's radiative climate forcing has become a leading candidate to explain the observed sun–climate connection. A correlation between GCR and low cloud cover has been ascertained in models and observations over the past few years. The deposition of cosmogenic radionuclides in ice cores and deep sea sediments can be used as a proxy for the past GCR-flux and provides an important tool to study the supposed GCR–climate connection on glacial–interglacial timescales. In this study, a record of geomagnetic paleointensity based on 10 Be from deep sea sediments is used as proxy for GCR-flux over the past 200,000 years. It is compared with climate records from marine, terrestrial and ice core archives. Our results are consistent with the GCR–climate theory and suggest the existence of a GCR–climate connection over the past 200,000 years.

Investigating the history of East Asian monsoon and climate during the last glacial–interglacial period (0–140 000 years): mineralogy and geochemistry of ODP Sites 1143 and 1144, South China Sea

Monsoon climate is an important component of the global climatic system. A comprehensive understanding of its variability over glacial–interglacial time scales as well as of its effects on the continent and in the ocean is required to decipher links between climate, continental weathering and productivity. A detailed multiproxy study, including bulk and clay mineralogy, grain-size analysis, phosphorus geochemistry (SEDEX extraction), organic matter characterization, and nitrogen stable isotopes, was carried out on samples from ODP Sites 1143 and 1144 (Leg 184, South China Sea), covering the past 140 000 years. We tentatively reconstruct the complex sedimentation and climatic history of the region during the last glacial–interglacial cycle, when sea-level variations, linked to the growth and melting of ice caps, interact with monsoon variability. During interglacial periods of high sea level, summer monsoon was strong, and humid and warm climate characterized the adjacent continent and islands. Clay minerals bear signals of chemical weathering during these intervals. High calcite and reactive phosphorus mass accumulation rates (MARs) indicate high productivity, especially in the southern region of the basin. During glacial intervals, strong winter monsoon provided enhanced detrital input from the continent, as indicated by high detrital MAR. Glacial low sea level resulted in erosion of sediments from the exposed Sunda shelf to the south, and clay mineral variations indicate that warm and humid conditions still prevailed in the southern tropical areas. Enhanced supply of nutrients from the continent, both by river and eolian input, maintained high primary productivity. Reduced circulation during these periods possibly induced active remobilization of nutrients, such as phosphorus, from the sediments. Intense and short cold periods recorded during glacial and interglacial stages correlate with loess records in China and marine climatic records in the North Atlantic, confirming a teleconnection between low- and high-latitude climate variability.

The climate of the Altiplano: observed current conditions and mechanisms of past changes

The large-scale controls on the climate of the South American Altiplano are investigated using local observations, reanalysis data and general circulation model experiments. The objective is to gain understanding of causes of climate variability and climate change by relating mechanisms that operate on timescales ranging from the intraseasonal to the glacial–interglacial. Our results suggest that, on all timescales, the climatic conditions on the Altiplano are closely related to the upper-air circulation, with an easterly zonal flow aloft favoring wet conditions and westerly flow causing dry conditions. Different factors influence the upper-air circulation on the different timescales. Intraseasonal variability is a reflection of the position and intensity of the Bolivian High, which is modulated by Rossby waves emanating from the midlatitude South Pacific. The annual cycle of dry winter and wet summer conditions is caused by the seasonal expansion of the equatorial easterlies in the upper troposphere, rather than direct insolation forcing over the Altiplano or moisture changes in the source area. Interannual variability is primarily related to changes in the mean zonal flow over the Altiplano, reflecting changes in meridional baroclinicity between tropical and subtropical latitudes, which in turn is a response to sea-surface temperature changes in the tropical Pacific. Orbitally forced changes in the land–sea contrast drive continental-scale circulation changes, which significantly alter the zonal flow over the Altiplano. On glacial–interglacial timescales, the contrast in heating between northern and southern hemispheres during the glacial leads to upper-air easterly anomalies throughout the tropics. On modern timescales the marked submonthly, seasonal and interannual changes of moisture over the Altiplano cannot be accounted for by moisture changes in the humid tropical lowlands. However, the model experiments suggest that cooler conditions during a glacial reduce moisture availability from the tropical lowlands, which counteracts the effect of the upper-level circulation, resulting in little overall change in precipitation. This observational and modeling analysis provides a physical framework for relating the mechanisms of both internal and forced climate change on the Altiplano on a wide range of timescales.

Two‐way interactions between ocean biota and climate mediated by biogeochemical cycles

AbstractSome of the two‐way interactions between ocean biota and climate are mediated by biogeochemical cycles that link the different components of the climate system. As suggested by proxy records extracted from ice and ocean cores, by recent measurements, and by numerical models, such two‐way interactions were likely major players in past climate variability on glacial—interglacial timescales, and may act to amplify or moderate an anthropogenically induced climate change in the near future. At present, our lack of understanding of these interactions hampers our ability to anticipate the consequences of possible anthropogenic climate change. In this article, we highlight some of the possible feedbacks between ocean biota and climate, reviewing some key biogeochemical processes and discussing mechanisms of two‐way interactions. We also outline the need and strategies for continuing research aimed at advancing our understanding of these feedbacks and discuss their significance.

Paleoproductivity in the southern Peru–Chile Current through the last 33 000 yr

Based on a multiparameter approach including organic carbon, biogenic opal, carbonate and the species composition of planktic foraminifera, this paper provides the first qualitative assessment of the history of paleoproductivity on glacial–interglacial time scales in the southern Peru–Chile Current (PCC) off Chile, which belongs to the least studied parts of the world ocean. During the Last Glacial Maximum (LGM) highest relative paleoproductivity of the last 33 kyr, indicated by high accumulation rates of organic carbon, biogenic opal and carbonate, has been found, contrasted by lowest values during the early and Middle Holocene. This shift from high to low productivity is accompanied by a major change in the species composition of planktic foraminifera from a dominance of Neogloboquadrina pachyderma (sin.) to the dominance of N. pachyderma (dex.). The temporal pattern of paleoproductivity off Chile is very similar to the history of the continental paleoclimate in the region, which is supposed to be driven by the position of the Southern Westerlies. This observation points to a functional relationship between the position of the Southern Westerlies and the history of paleoproductivity off Chile. Assuming that atmospheric and oceanographic circulation are closely linked, a northward displacement of the Antarctic Circumpolar Current during the LGM, in line with the northward movement of the Southern Westerlies, would bring the main nutrient source closer to our core sites resulting in increased productivity. However, based on the available data, it is not clear if the higher productivity in the southern PCC during the LGM reflects generally higher productivity during this time or only a regional displacement of the main productivity centers.

History and variability of Asian interior aridity recorded by eolian flux in the Chinese Loess Plateau during the past 7 Ma

Eolian flux in the Chinese Loess Plateau was reconstructed by measuring the dry bulk density and CaCO3 content of the late Cenozoic loess-paleosol-red clay sequences in the Lingtai profile. Comparison of eolian flux variation between the Lingtai profile and the ODP sites 885/886 in the North Pacific shows a significant wet-dry variability in addition to a gradual drying trend in the dust source regions in interior Asia. Especially, the increase of eolian fluxes from both continental and pelagic eolian sediments indicates a sharp drying of the dust source regions between 3.6 and 2.6 MaBP, which might be attributed to the tectonic uplift of the Tibetan Plateau, which cut down the moisture input to the interior Asia. The average value and variability of eolian flux are higher after 2.6 MaBP than before, which may be related to the Quaternary climatic fluctuations on the glacial-interglacial timescale after the commencement of major Northern Hemisphere Glaciations. The eolian fluxes of the Lingtai profile and Core V21-146 in northwest Pacific show a synchronous variation on the 10(4)-10(5) a timescale, indicating that the flux variations from both continental and marine records are closely correlated to the Quaternary climatic fluctuation forced by the ice volume changes on a global scale.

Isotopic tracing of the dissolved U fluxes of Himalayan rivers: implications for present and past U budgets of the Ganges-Brahmaputra system

U activity ratios have been measured in the dissolved loads of selected rivers from the Himalayan range, in Central Nepal, and from the Bangladesh, as well as in some rain waters. A few European and Asian rivers have also been analyzed for their U activity ratios. The data confirm the negligible effect of rainwater on the budget of dissolved U in river waters. The results also indicate that rivers on each Himalayan structural unit have homogeneous and specific U isotope compositions: i) (234U/238U) activity ratios slightly lower than unity in the dissolved load of the streams draining the Tethyan Sedimentary Series (TSS); ii) values slightly higher than unity for waters from the High Himalaya Crystalline (HHC) and the Lesser Himalaya (LH); iii) systematically higher (234U/238U) activity ratios for waters from the Siwaliks. Thus, U activity ratios, in association with Sr isotopic ratios, can be used to trace the sources of dissolved fluxes carried by these rivers. Coupling of U with Sr isotope data shows (1) that the U carried by the dissolved load of the Himalayan rivers mainly originates from U-rich lithologies of the TSS in the northern formations of the Tibetan plateau; and (2) that the elemental U and Sr fluxes carried by the Himalayan rivers at the outflow of the highlands are fairly homogeneous at the scale of the Himalayan chain. Rivers flowing on the Indian plain define a different trend from that of the Himalayan rivers in the U-Sr isotopic diagram, indicating the contribution of a specific floodplain component to the U and Sr budgets of the Ganges and the Brahmaputra. The influence of this component remains limited to 10 to 15 percent for the U flux, but can contribute 35 to 55% of the Sr flux. The variations of the Sr and U fluxes of the Ganges-Brahmaputra river system in response to climatic variations have been estimated by assuming a temporary cut off of the chemical fluxes from high-altitude terrains during glacial episodes. This scenario would significantly decrease the dissolved U flux of the Ganges-Brahmaputra river system and increase its U activity ratio. Such a climatic dependence of the Himalayan U flux could induce a periodic variation of the mean U activity ratio of the world rivers on glacial-interglacial timescales

Dust transport to Dome C, Antarctica, at the Last Glacial Maximum and present day

The Antarctic polar ice‐core records show large changes in the concentration of dust over glacial‐interglacial timescales. This paper explores how much of this variation is due to changes in the transport of dust from arid regions to the ice cores. Back trajectories, initialised from the site of Dome C ice‐core, Antarctica, are calculated using an offline trajectory code forced by modelled winds, output from the UKMO Unified Model running under present day and LGM boundary conditions. As well as comparing the present day and LGM back trajectories, their seasonal and interannual variability is also explored. The results suggest that the Patagonian provenance of the Dome C dust can be understood in terms of the atmospheric transport whereas the total change in dust concentration requires changes to the sources or sinks.

Speleothem evidence from Oman for continental pluvial events during interglacial periods

Growth periods and stable isotope analyses of speleothems from Hoti Cave in northern Oman provide a record of continental pluvial periods extending back over the past four of Earth’s glacial-interglacial cycles. Rapid speleothem growth occurred during the early to middle Holocene (6‐10.5 ka B.P.), 78‐82 ka B.P., 120‐135 ka B.P., 180‐200 ka B.P., and 300‐325 ka B.P. The speleothem calcite deposited during each of these episodes is highly depleted in 18 O compared to modern speleothems. The d 18 O values for calcite deposited within pluvial periods generally fall in the range of 24‰ to 28‰ relative to the Vienna Peedee belemnite standard, whereas modern speleothems range from 21‰ to 23‰. The growth and isotopic records indicate that during peak interglacial periods, the limit of the monsoon rainfall was shifted far north of its present location and each pluvial period was coincident with an interglacial stage of the marine oxygen isotope record. The association of continental pluvial periods with peak interglacial conditions suggests that glacial boundary conditions, and not changes in solar radiation, are the primary control on continental wetness on glacial-interglacial time scales.

Climatic effect of Southern Ocean Fe fertilization: Is the jury still out?

[1] One of the great puzzles in oceanography has been the reason for the existence of certain areas of the world ocean like the Southern Ocean and equatorial and north Pacific where there is an abundance of unused macronutrients essential for phytoplankton growth (such as nitrate and silicic acid) [de Baar and Boyd, 2000]. Although physical (solar insolation and ocean surface mixing) and grazing regimes must play a part in controlling phytoplankton standing stocks in these so-called ` high-nitrate low-chlorophyll'' (HNLC) regions, Martin suggested that growth limitation through insufficient availability of the micronutrient iron (Fe) might also exert an important control [Martin and Fitzwater, 1988]. Furthermore, he proposed that stimulation of biological productivity through increased aeolian Fe delivery to the Southern Ocean at the height of the last ice age (the Last Glacial Maximum (LGM)) might have led to the concurrently low levels of atmospheric CO2 observed [Martin, 1990]. The ` IronEx II'' open ocean Fe fertilization experiment demonstrated unequivocally the role that iron played in limiting phytoplankton growth (particularly of larger diatoms) in the equatorial Pacific [Coale et al., 1996]. More recently, a similar experiment (Southern Ocean Iron Release Experiment (SOIREE)) was carried out in the Southern Ocean, a region suspected to play a key role in controlling variability in the concentration of atmospheric CO2 over glacial-interglacial timescales [Knox and McElroy, 1984]. Here, too, an unambiguous response of the phytoplankton standing stock to the addition of Fe was observed [Boyd et al., 2000]. However, for these changes to produce a significant response in the concentration of atmospheric CO2, there must also be an increase in the export of carbon to the deep ocean. It is here that our understanding of biogeochemical cycling in the Southern Ocean and in particular of the potential relationships between iron supply and climate now appears to falter.