No abstract

Dust has the potential to modify global climate by influencing the radiative balance of the atmosphere and by supplying iron and other essential limiting micronutrients to the ocean. Indeed, dust supply to the Southern Ocean increases during ice ages, and 'iron fertilization' of the subantarctic zone may have contributed up to 40 parts per million by volume (p.p.m.v.) of the decrease (80-100 p.p.m.v.) in atmospheric carbon dioxide observed during late Pleistocene glacial cycles. So far, however, the magnitude of Southern Ocean dust deposition in earlier times and its role in the development and evolution of Pleistocene glacial cycles have remained unclear. Here we report a high-resolution record of dust and iron supply to the Southern Ocean over the past four million years, derived from the analysis of marine sediments from ODP Site 1090, located in the Atlantic sector of the subantarctic zone. The close correspondence of our dust and iron deposition records with Antarctic ice core reconstructions of dust flux covering the past 800,000 years (refs 8, 9) indicates that both of these archives record large-scale deposition changes that should apply to most of the Southern Ocean, validating previous interpretations of the ice core data. The extension of the record beyond the interval covered by the Antarctic ice cores reveals that, in contrast to the relatively gradual intensification of glacial cycles over the past three million years, Southern Ocean dust and iron flux rose sharply at the Mid-Pleistocene climatic transition around 1.25 million years ago. This finding complements previous observations over late Pleistocene glacial cycles, providing new evidence of a tight connection between high dust input to the Southern Ocean and the emergence of the deep glaciations that characterize the past one million years of Earth history.

[1] Paleoclimatic reconstructions have provided a unique data set to test the sensitivity of climate system to changes in atmospheric CO 2 concentrations. However, the mechanisms behind glacial/interglacial (G/IG) variations in atmospheric CO 2 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 CO 2 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 CO 2 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 CO 2 decrease, as well as the change in the amplitude of the CO 2 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.Citation: Martínez-Garcia, A., A. Rosell-Melé, W. Geibert, R. Gersonde, P. Masqué, V. Gaspari, and C. Barbante (2009), Links between iron supply, marine productivity, sea surface temperature, and CO 2 over the last 1

[1] The Sr/Ca ratio of coral aragonite is used to reconstruct past sea surface temperature (SST). Twentyone laboratories took part in an interlaboratory study of coral Sr/Ca measurements. Results show interlaboratory bias can be significant, and in the extreme case could result in a range in SST estimates of 7 C. However, most of the data fall within a narrower range and the Porites coral reference material JCp-1 is now characterized well enough to have a certified Sr/Ca value of 8.838 mmol/mol with an expanded uncertainty of 0.089 mmol/mol following International Association of Geoanalysts (IAG) guidelines. This uncertainty, at the 95% confidence level, equates to 1.5 C for SST estimates using Porites, so is approaching fitness for purpose. The comparable median within laboratory error is <0.5 C. This difference in uncertainties illustrates the interlaboratory bias component that should be reduced through the use of reference materials like the JCp-1. There are many potential sources contributing to biases in comparative methods but traces of Sr in Ca standards and uncertainties in reference solution composition can account for half of the combined uncertainty. Consensus values that fulfil the requirements to be certified values were also obtained for Mg/Ca in JCp-1 and for Sr/Ca and Mg/Ca ratios in the JCt-1 giant clam reference material. Reference values with variable fitness for purpose have also been obtained for Li/Ca, B/Ca, Ba/Ca, and U/Ca in both reference materials. In future, studies reporting coral element/Ca data should also report the average value obtained for a reference material such as the JCp-1.

Although sulfur is an essential element for marine primary production and critical for climate processes, little is known about the oceanic pool of nonvolatile dissolved organic sulfur (DOS). We present a basin-scale distribution of solid-phase extractable DOS in the East Atlantic Ocean and the Atlantic sector of the Southern Ocean. Although molar DOS versus dissolved organic nitrogen (DON) ratios of 0.11 ± 0.024 in Atlantic surface water resembled phytoplankton stoichiometry (sulfur/nitrogen~0.08), increasing dissolved organic carbon (DOC) versus DOS ratios and decreasing methionine-S yield demonstrated selective DOS removal and active involvement in marine biogeochemical cycles. Based on stoichiometric estimates, the minimum global inventory of marine DOS is 6.7 petagrams of sulfur, exceeding all other marine organic sulfur reservoirs by an order of magnitude.

No abstract

The Weddell Gyre (WG) is one of the main oceanographic features of the Southern Ocean south of the Antarctic Circumpolar Current which plays an influential role in global ocean circulation as well as gas exchange with the atmosphere. We review the state‐of‐the art knowledge concerning the WG from an interdisciplinary perspective, uncovering critical aspects needed to understand this system's role in shaping the future evolution of oceanic heat and carbon uptake over the next decades. The main limitations in our knowledge are related to the conditions in this extreme and remote environment, where the polar night, very low air temperatures, and presence of sea ice year‐round hamper field and remotely sensed measurements. We highlight the importance of winter and under‐ice conditions in the southern WG, the role that new technology will play to overcome present‐day sampling limitations, the importance of the WG connectivity to the low‐latitude oceans and atmosphere, and the expected intensification of the WG circulation as the westerly winds intensify. Greater international cooperation is needed to define key sampling locations that can be visited by any research vessel in the region. Existing transects sampled since the 1980s along the Prime Meridian and along an East‐West section at ~62°S should be maintained with regularity to provide answers to the relevant questions. This approach will provide long‐term data to determine trends and will improve representation of processes for regional, Antarctic‐wide, and global modeling efforts—thereby enhancing predictions of the WG in global ocean circulation and climate.

Abstract. Circumpolar Deep Water (CDW), locally called Warm Deep Water (WDW), enters the Weddell Gyre in the southeast, roughly at 25 • E to 30 • E. In December 2002 and January 2003 we studied the effect of entrainment of WDW on the fugacity of carbon dioxide (fCO 2 ) and dissolved inorganic carbon (DIC) in Weddell Sea surface waters. Ultimately the fCO 2 difference across the sea surface drives airsea fluxes of CO 2 . Deep CTD sections and surface transects of fCO 2 were made along the Prime Meridian, a northwestsoutheast section, and along 17 • E to 23 • E during cruise ANT XX/2 on FS Polarstern. Upward movement and entrainment of WDW into the winter mixed layer had significantly increased DIC and fCO 2 below the sea ice along 0 • W and 17 • E to 23 • E, notably in the southern Weddell Gyre. Nonetheless, the ice cover largely prevented outgassing of CO 2 to the atmosphere. During and upon melting of the ice, biological activity rapidly reduced surface water fCO 2 by up to 100 µatm, thus creating a sink for atmospheric CO 2 . Despite the tendency of the surfacing WDW to cause CO 2 supersaturation, the Weddell Gyre may well be a CO 2 sink on an annual basis due to this effective mechanism involving ice cover and ensuing biological fCO 2 reduction. Dissolution of calcium carbonate (CaCO 3 ) in melting sea ice may play a minor role in this rapid reduction of surface water fCO 2 .