bluuebunny/crossref_metadata_2025 huggingface dataset:
Influence of consumer-driven nutrient recycling on primary production and the distribution of N and P in the ocean
Authors: Nugraha A. , Pondaven P. , Tréguer P.
Published: April 2010

Abstract. In this study we investigated the impact of consumer-driven nutrient recycling (CNR) on oceanic primary production and the distribution of nitrogen (N) and phosphorus (P) in the deep ocean. For this purpose, we used and extended two existing models: a 2-box model of N and P cycling in the global ocean (Tyrrell, 1999), and the model of Sterner (1990) which formalised the principles of CNR theory. The resulting model showed that marine herbivores may affect the supply and the stoichiometry of N and P in the ocean, thereby exerting a control on global primary production. The predicted global primary production was higher when herbivores were included in the model, particularly when these herbivores had higher N:P ratios than phytoplankton. This higher primary production was triggered by a low N:P resupply ratio, which, in turn, favoured the P-limited N2-fixation and eventually the N-limited non-fixers. Conversely, phytoplankton with higher N:P ratios increased herbivore yield until phosphorus became the limiting nutrient, thereby favouring herbivores with a low P-requirement. Finally, producer-consumer interactions fed back on the N and P inventories in the deep ocean through differential nutrient recycling. In this model, N deficit or N excess in the deep ocean resulted not only from the balance between N2-fixation and denitrification, but also from CNR, especially when the elemental composition of producers and consumers differed substantially. Although the model is fairly simply, these results emphasize our need for a better understanding of how consumers influence nutrient recycling in the ocean.
Hydrological changes in the Ligurian Sea (NW Mediterranean, DYFAMED site) during 1995–2007 and biogeochemical consequences
Authors: Marty J. C. , Chiavérini J.
Published: April 2010

Abstract. Data obtained during the monthly cruises of the DYFAMED time-series study (northwestern Mediterranean Sea) in the period 1995–2007 were compiled to examine the hydrological changes and the linked evolution of some biogeochemical characteristics. A regular increase of temperature and salinity (0.005 °C y−1, 0.0022 psu y−1) was recorded in deep waters of the NW Mediterranean Sea (2000 m depth) during 1995–2005. In February 2006 an abrupt increase in T (+0.1 °C) and S (+0.03 psu) was measured as the result of successive intense winter mixing events during the 3 previous years. The February 2006 event led to the mixing of the whole water column (0 to > 2000 m) and increased salt and heat content of the Western Mediterranean Deep Water by mixing with saltier and warmer Levantine Intermediate Water. The deficit in fresh water inputs to the western Mediterranean basin in three successive years (2003–2005) was suspected to be the major cause of this event since an increase of salinity in surface waters was monitored during these years. The measured phytoplankton biomass was specifically high after the periods of intense mixing. Chlorophyll a integrated biomass reached 230 mg m−2 in 1999, 175 mg m−2 in 2003, and 206 mg m−2 in 2006. The high levels of biomass were related to the particularly high increases in nutrients content in surface layers following the intense water column mixing and the subsequent development of a diatom bloom (as seen by fucoxanthin content). The frequency of extreme events (high mixing, high nutrients, and high biomass) increased in recent years. Our results suggested that the NW Mediterranean Sea could have an increased productivity and was not deriving towards the decreased productivity predicted by models.
Trends of anthropogenic CO2 storage in North Atlantic water masses
Authors: Pérez F. F. , Vázquez-Rodríguez M. , Mercier H. , Velo A. , Lherminier P. , Ríos A. F.
Published: April 2010

Abstract. A high-quality inorganic carbon system database spanning over three decades (1981–2006) and comprising 13 cruises has allowed applying the φCT° method and coming up with accurate estimates of the anthropogenic CO2 (Cant) stored in the main water masses of the North Atlantic. In the studied region, strong convective processes convey surface properties, like Cant, into deeper ocean layers and confer this region an added oceanographic interest from the point of view of air-sea CO2 exchanges. Commonly, a tendency for decreasing Cant storage rates towards the deep layers has been observed. In the Iberian Basin, the deep waters (North Atlantic Deep Water) have low Cant values and negligible Cant storage rates, while the North Atlantic Central Water in the upper layers shows the largest Cant concentrations and capacity to increase its storage on a yearly basis (1.13±0.14 μmol kg−1 yr−1). This unmatched Cant storage capacity of the warm upper limb of the Meridional Overturning Circulation weakens towards the Irminger basin (0.68±0.06 μmol kg−1 yr−1) due to the lowering of the buffering capacity. The mid and deep waters in the Irminger Sea show rather homogeneous Cant storage rates (between 0.33 and 0.45 μmol kg−1 yr−1), whereas in the Iceland basin these layers seem to have been less affected by Cant. The Cant storage rates in the study region during the 1991–1997 high NAO (North Atlantic Oscillation) phase are ~48% higher than during the 1997–2006 low NAO phase that followed. This result suggests that a net decrease in the strength of the North Atlantic sink of atmospheric CO2has taken place during the present decade. The changes in deep-water ventilation together with a detrimental renewal of the main water masses are likely the main driving processes causing this weakening of the North Atlantic CO2sink.
Chemical composition of volatile and extractive compounds of pine and spruce leaf litter in the initial stages of decomposition
Authors: Isidorov V. A. , Smolewska M. , Purzyńska-Pugacewicz A. , Tyszkiewicz Z.
Published: April 2010

Abstract. A litter bag experiment was conducted to analyze changes in chemical composition in Scots pine (Pinus sylvestris) and Norway spruce (Picea abies) needle litter in the first stages of decomposition in natural conditions. The emission rates of monoterpenes and concentration of extractive secondary metabolites were determined five times over a 16-month period. It has been shown that pine and spruce needle litter emits monoterpene hydrocarbons into the gas phase with the rates comparable to those in emissions from live needles of these trees. This suggests that leaf litter is an important source of atmospheric terpenes. It has also been proved that the litter contains considerable amounts of non-volatile substances that can be precursors of oxidized volatile compounds formed as a result of enzymatic reactions. Non-volatile but water soluble secondary metabolites of the leaf litter may be involved in nutrient cycling and have an influence on soil community.
Hypoxia and cyanobacterial blooms are not natural features of the Baltic Sea
Authors: Zillén L. , Conley D. J.
Published: April 2010

Abstract. During the last century (1900s) industrialized forms of agriculture and human activities have caused extensive eutrophication of Baltic Sea waters. As a consequence, the Baltic Sea developed a hypoxic zone that has caused serve ecosystem disturbance. Climate forcing has also been proposed to be responsible for the reported trends in hypoxia (<2 mg/l O2) both during the last c. 100 years and during the Medieval Period. By contrast, investigations on the degree of anthropogenic forcing on the ecosystem on long time-scales (millennial) have not been thoroughly addressed. This paper critically examines evidence for anthropogenic disturbance of the marine environment beyond the last century through the analysis of the population growth, technological development and land-use changes in the drainage area. Natural environmental changes, i.e. changes in the morphology and depths of the Baltic basin and the sills, were probably the main driver for large-scale hypoxia during the early Holocene (8000–4000 cal. yr BP). We show that hypoxia during the last two millennia has followed the general expansion and contraction trends in Europe and that human perturbations have been an important driver for hypoxia during that time. Hypoxia occurring during the Medieval Period coincides with a doubling of the population (from c. 4.6 to 9.5 million), a massive reclamation of land in both established and marginal cultivated areas and significant increases in soil nutrient release. The role of climate forcing on hypoxia in the Baltic Sea has yet to be convincingly demonstrated, although it could have contributed to sustain hypoxia through enhanced salt water inflows or through changes in hydrological inputs. In addition, cyanobacteria blooms are not natural features of the Baltic Sea as previously hypothesized, but are a consequence of enhanced phosphorus release that occurs together with hypoxia.
Laboratory measurements of nitric oxide release from forest soil with a thick organic layer under different understory types
Authors: Bargsten A. , Falge E. , Huwe B. , Meixner F. X.
Published: April 2010

Abstract. Nitric oxide (NO) plays an important role in the photochemistry of the troposphere. NO from soil contributes up to 40% to the global budget of atmospheric NO. Soil NO emissions are primarily caused by biological activity (nitrification and denitrification), that occurs in the uppermost centimetres of the soil, a soil region often characterized by high contents of organic material. Most studies of NO emission potentials to date have investigated mineral soil layers. In our study we sampled soil organic matter under different understories (moss, grass, spruce and blueberries) in a humid mountainous Norway spruce forest plantation in the Fichtelgebirge (Germany). We performed laboratory incubation and fumigation experiments using a customized chamber technique to determine the response of net potential NO flux to physical and chemical soil conditions (water content and temperature, bulk density, particle density, pH, C/N ratio, organic C, soil ammonium, soil nitrate). Net potential NO fluxes (in terms of mass of N) from soils of different understories ranged from 1.7–9.8 ng m−2 s−1 (grass and moss), 55.4–59.3 ng m−2 s−1 (spruce), and 43.7–114.6 ng m−2 s−1 (blueberry) at optimum water content and a soil temperature of 10°C. The water content for optimum net potential NO flux ranged between 0.76 and 0.8 gravimetric soil moisture for moss, between 1.0 and 1.1 for grass, 1.1 and 1.2 for spruce, and 1.3 and 1.9 for blueberries. Effects of soil physical and chemical characteristics on net potential NO flux were statistically significant (0.01 probability level) only for NH4+. Therefore, the effects of biogenic factors like understory type, amount of roots, and degree of mycorrhization on soil biogenic NO emission are discussed; they have the potential to explain the observed different of net potential NO fluxes. Quantification of NO emissions from the upmost soil layer is therefore an important step to quantify soil NO emissions in ecosystems with substantial organic soil horizons.
On the origin of highly active biogeochemistry in deeper coastal sediments – inverse model studies
Authors: Holstein J. M. , Wirz K. W.
Published: April 2010

Abstract. In coastal sediments, zones with highly active diagenesis may be situated below the usually studied first decimeters. Pore water profiles from a backbarrier tidal flat in the German Wadden Sea display a SO4 minimum zone associated with a distinct NH4 peak at a sediment depth of around 1.5 m. Such evidence for significant degradation of organic matter (OM) is challenging our understanding of tidal flat biogeochemistry as little is known about processes that rapidly transfer reactive OM into layers far distant from the sediment-water interface. We here test and compare two different scenarios for OM transfer: scenario A assumes rapid sedimentation and burial of OM, scenario B assumes lateral advection of suspended POM. A diagenetic model is adapted to describe both hypotheses. Uncertain process parameters, in particular those connected to OM degradation and (vertical or lateral) transport are systematically calibrated using existing data. We found that both scenarios, advection and sedimentation, have solutions consistent with the observed pore water profiles. Constrained process parameters are within the range of reported values. Solutions to scenario B describing advective transport of particulate material are, however, rather improbable due to highly specific assumptions on the OM source and flow geometry. In the alternative deposition set-up, model simulations suggest that the source OM was deposited about 60 yrs earlier (1945). A mean sedimentation rate of approximately 2 cm yr−1 indicates substantial changes in near coast sediment morphology, since sea level rise is at a much lower pace. High sedimentation rates most probably reflect the progradation of flats within the study area. These or similar morphodynamic features also occur in other coastal areas so that regional values for OM remineralization rates may often be much higher than predicted from surface biogeochemistry.
The Arabian Sea as a high-nutrient, low-chlorophyll region during the late Southwest Monsoon
Authors: Naqvi S. W. A. , Moffett J. W. , Gauns M. U. , Narvekar P. V. , Pratihary A. K. , Naik H. , Shenoy D. M. , Jayakumar D. A. , Goepfert T. J. , Patra P. K. , Al-Azri A. , Ahmed S. I.
Published: April 2010

Abstract. Extensive observations during the late Southwest Monsoon of 2004 over the Indian and Omani shelves, and along an east-west transect reveal a mosaic of biogeochemical provinces including an unexpected high-nutrient, low-chlorophyll condition off the southern Omani coast. This feature, coupled with other characteristics of the system, suggest a close similarity between the Omani upwelling system and the Peruvian and California upwelling systems, where primary production (PP) is limited by iron. An intensification of upwelling, reported to have been caused by the decline in the winter/spring Eurasian snow cover since 1997, is not supported by in situ hydrographic and chlorophyll measurements as well as a reanalysis of ocean colour data extending to 2009. Iron limitation of PP may complicate simple relationship between upwelling and PP assumed by previous workers, and contribute to the anomalous offshore occurrence of the most severe oxygen (O2) depletion in the region. Over the Indian shelf, affected by very shallow O2-deficient zone, high PP is restricted to a thin, oxygenated surface layer probably due to unsuitability of the O2-depleted environment for the growth of oxygenic photosynthesizers.
Microbial conversion of inorganic carbon to dimethyl sulfide in anoxic lake sediment (Plußsee, Germany)
Authors: Lin Y.-S. , Heuer V. B. , Ferdelman T. G. , Hinrichs K.-U.
Published: April 2010

Abstract. In anoxic environments, volatile methylated sulfides including methanethiol (MT) and dimethyl sulfide (DMS) link the pools of inorganic and organic carbon with the sulfur cycle. However, direct formation of methylated sulfides from reduction of dissolved inorganic carbon has previously not been demonstrated. During examination of the hydrogenotrophic microbial activity at different temperatures in the anoxic sediment from Lake Plußsee, DMS formation was detected at 55 °C and was enhanced when bicarbonate was supplemented. Addition of both bicarbonate and H2 resulted in the strongest stimulation of DMS production, and MT levels declined slightly. Addition of methyl-group donors such as methanol and syringic acid or methyl-group acceptors such as hydrogen sulfide did not enhance further accumulation of DMS and MT. The addition of 2-bromoethanesulfonate inhibited DMS formation and caused a slight MT accumulation. MT and DMS had average δ13C values of −55‰ and −62‰, respectively. Labeling with NaH13CO3 showed that incorporation of bicarbonate into DMS occurred through methylation of MT. H235S labeling demonstrated a microbially-mediated, but slow, process of hydrogen sulfide methylation that accounted for <10% of the accumulation rates of DMS. Our data suggest: (1) methanogens are involved in DMS formation from bicarbonate, and (2) the major source of the 13C-depleted MT is neither bicarbonate nor methoxylated aromatic compounds. Other possibilities for isotopically light MT, such as demethylation of 13C-depleted DMS or other organic precursors such as methionine, are discussed. This DMS-forming pathway may be relevant for anoxic environments, such as hydrothermally influenced sediments and fluids and sulfate-methane transition zones in marine sediments.
Impacts of exotic mangrove forests and mangrove deforestation on carbon remineralization and ecosystem functioning in marine sediments
Authors: Sweetman A. K. , Middelburg J. J. , Berle A. M. , Bernardino A. F. , Schander C. , Demopoulos A. W. J. , Smith C. R.
Published: April 2010

Abstract. To evaluate how mangrove invasion and removal can modify benthic carbon cycling processes and ecosystem functioning, we used stable-isotopically labelled algae as a deliberate tracer to quantify benthic respiration and C-flow through macrofauna and bacteria in sediments collected from (1) an invasive mangrove forest, (2) deforested mangrove sites 2 and 6 years after removal of above-sediment mangrove biomass, and (3) two mangrove-free, control sites in the Hawaiian coastal zone. Sediment oxygen consumption (SOC) rates were significantly greater in the mangrove and mangrove removal site experiments than in controls and were significantly correlated with total benthic (macrofauna and bacteria) biomass and sedimentary mangrove biomass (SMB). Bacteria dominated short-term C-processing of added microalgal-C and benthic biomass in sediments from the invasive mangrove forest habitat. In contrast, macrofauna were the most important agents in the short-term processing of microalgal-C in sediments from the mangrove removal and control sites. Mean faunal abundance and short term C-uptake rates in sediments from both removal sites were significantly higher than in control cores, which collectively suggest that community structure and short-term C-cycling dynamics in habitats where mangroves have been cleared can remain fundamentally different from un-invaded mudflat sediments for at least 6-yrs following above-sediment mangrove removal. In summary, invasion by mangroves can lead to large shifts in benthic ecosystem function, with sediment metabolism, benthic community structure and short-term C-remineralization dynamics being affected for years following invader removal.
Sources and accumulation of organic carbon in the Pearl River Estuary surface sediment as indicated by elemental, stable carbon isotopic, and carbohydrate compositions
Authors: He B. , Dai M. , Huang W. , Liu Q. , Chen H. , Xu L.
Published: April 2010

Abstract. Organic matter in surface sediments from the upper reach of the Pearl River Estuary and Lingdingyang Bay, as well as the adjacent northern South China Sea shelf was characterized by a variety of techniques, including elemental (C and N), stable carbon isotopic (δ 13C) composition, as well as molecular-level analyses. Total organic carbon (TOC) content was 1.61±1.20% in the upper reach down to 1.00±0.22% in Lingdingyang Bay and to 0.80±0.10% on the inner shelf and 0.58±0.06% on the outer shelf. δ13C values ranged from −25.11‰ to −21.28‰ across the studied area, with a trend of enrichment seaward. The spatial trend in C/N ratios mirrored that of δ13C, with a substantial decrease in C/N ratio from 10.9±1.3 in the Lingdingyang Bay surface sediments to 6.5±0.09 in the outer shelf surface sediments. Total carbohydrate yields ranged from 22.1 to 26.7 mg (100 mg OC)−1, and typically followed TOC concentrations in the estuarine and shelf sediments, suggesting that the relative abundance of total carbohydrate was fairly constant in TOC. Total neutral sugars as detected by the nine major monosaccharides (lyxose, rhamnose, ribose, arabinose, fucose, xylose, galactose, mannose, and glucose) yielded between 4.0 and 18.6 mg (100 mg OC)−1 in the same sediments, suggesting that a significant amount of carbohydrates were not neutral aldoses. The bulk organic matter properties, isotopic composition and C/N ratios, combined with molecular-level carbohydrate compositions were used to assess the sources and accumulation of terrestrial organic matter in the Pearl River Estuary and the adjacent northern South China Sea shelf. Results showed a mixture of terrestrial riverine organic carbon with in situ phytoplankton organic carbon in the areas studied. Using a two end-member mixing model based on δ13C values and C/N ratios, we estimated that the terrestrial organic carbon contribution to the surface sediment TOC was ca. 57±13% for Lingdingyang Bay, 19±2% for the inner shelf, which decreased further to 4.3±0.5% on the outer shelf. The molecular composition of the carbohydrate in surface sediments also suggested that the inner estuary was rich in terrestrial-derived carbohydrates but that the contribution of terrestrial-derived carbohydrates decreased offshore. Terrestrial organic carbon accumulation flux was estimated as 1.37±0.92×1011 g yr−1 in Lingdingyang Bay, which accounted for 37±25% of the terrestrial organic carbon transported to the Bay. The burial efficiency of terrestrial organic matter was markedly lower than that of suspended particulate substance (~71%) suggesting that the riverine POC undergoes significant degradation and replacement during transportation through the estuary.
Side effects and accounting aspects of hypothetical large-scale Southern Ocean iron fertilization
Authors: Oschlies A. , Koeve W. , Rickels W. , Rehdanz K.
Published: April 2010

Abstract. Recent suggestions to slow down the increase in atmospheric carbon dioxide have included ocean fertilization by addition of the micronutrient iron to Southern Ocean surface waters, where a number of natural and artificial iron fertilization experiments have shown that low ambient iron concentrations limit phytoplankton growth. Using a coupled carbon-climate model with the marine biology's response to iron addition calibrated against data from natural iron fertilization experiments, we examine biogeochemical side effects of a hypothetical large-scale Southern Ocean Iron Fertilization (OIF) that need to be considered when attempting to account for possible OIF-induced carbon offsets. In agreement with earlier studies our model simulates an OIF-induced increase in local air-sea CO2 fluxes by about 60 GtC over a 100-year period, which amounts to about 40% of the OIF-induced increase in organic carbon export. Offsetting CO2 return fluxes outside the region and after stopping the fertilization at 1, 7, 10, 50, and 100 years are quantified for a typical accounting period of 100 years. For continuous Southern Ocean iron fertilization, the return flux outside the fertilized area cancels about 8% of the fertilization-induced CO2 air-sea flux within the fertilized area on a 100-yr timescale. This "leakage" effect has a similar radiative impact as the simulated enhancement of marine N2O emissions. Other side effects not yet discussed in terms of accounting schemes include a decrease in Southern Ocean oxygen levels and a simultaneous shrinking of tropical suboxic areas, and accelerated ocean acidification in the entire water column in the Southern Ocean on the expense of reduced globally averaged surface water acidification. A prudent approach to account for the OIF-induced carbon sequestration would account for global air-sea CO2 fluxes rather than for local fluxes into the fertilized area only. However, according to our model, this would underestimate the potential for offsetting CO2 emissions by about 20% on a 100 year accounting timescale. We suggest that a fair accounting scheme applicable to both terrestrial and marine carbon sequestration has to be based on emission offsets rather than on changes in individual carbon pools.
Response of heterotrophic and autotrophic microbial plankton to inorganic and organic inputs along a latitudinal transect in the Atlantic Ocean
Authors: Martínez-García S. , Fernández E. , Calvo-Díaz A. , Marañón E. , Morán X. A. G. , Teira E.
Published: April 2010

Abstract. Atmospheric nutrient deposition into the open ocean increased over the past decades as a result of human activity and water-soluble organic nitrogen accounts for up to 30% of the total nitrogen inputs. The effects of inorganic and/or organic nutrient inputs on phytoplankton and heterotrophic bacteria have never been concurrently assessed in open ocean oligotrophic communities over a wide spatial gradient. We studied the effects of potentially limiting inorganic (nitrate, ammonium, phosphate, silica) and organic nutrient (glucose, aminoacids) inputs on microbial plankton biomass, community structure and metabolism in five microcosm experiments conducted along a latitudinal transect in the Atlantic Ocean (from 26° N to 29° S). Primary production rates increased up to 1.8-fold. Bacterial respiration and microbial community respiration increased up to 14.3 and 12.7-fold, respectively. Bacterial production and bacterial growth efficiency increased up to 58.8-fold and 2.5-fold, respectively. The largest increases were measured after mixed inorganic-organic nutrients additions. Changes in microbial plankton biomass were small as compared with those in metabolic rates. A north to south increase in the response of heterotrophic bacteria was observed, which could be related to a latitudinal gradient in phosphorus availability. Our results suggest that organic matter inputs associated with atmospheric deposition into the Atlantic Ocean will result in a predominantly heterotrophic versus autotrophic response and in increases in bacterial growth efficiency, particularly in the Southern Hemisphere. Subtle differences in the initial environmental and biological conditions are likely to result in differential microbial responses to inorganic and organic matter inputs.
Population modelling of Acartia spp. in a water column ecosystem model for the Southern Baltic Sea
Authors: Dzierzbicka-Glowacka L. , Żmijewska I. M. , Mudrak S. , Jakacki J. , Lemieszek A.
Published: April 2010

Abstract. This paper describes numerical simulations of the seasonal dynamics of Acartia spp. in the Southern Baltic Sea. The studies were carried out using a structured zooplankton population model adapted to Acartia spp. The population model with state variables for eggs, nauplii, five copepodites stages and adults was coupled with a marine ecosystem model. Four state variables for the carbon cycle represent the functional units of phytoplankton, pelagic detritus, benthic detritus, and bulk zooplankton, which represent all zooplankton other than the structured population. The annual cycle simulated for 2000 under realistic weather and hydrographic conditions was studied with the coupled ecosystem–zooplankton model applied to a water column in the Gdańsk Gulf (Southern Baltic Sea). The vertical profiles of selected state variables were compared to the physical forcing to study differences between bulk and structured zooplankton biomass. The simulated population dynamics of Acartia spp. and zooplankton as one biomass state variable were compared with observations in the Gdańsk Gulf. Simulated generation times are more affected by temperature than food conditions except during the spring phytoplankton bloom. The numerical studies are a following step in understanding how the population dynamics of a dominant species in the Southern Baltic Sea interact with the environment.
A mechanistic account of increasing seasonal variations in the rate of ocean uptake of anthropogenic carbon
Authors: Gorgues T. , Aumont O. , Rodgers K. B.
Published: April 2010

Abstract. A three-dimensional circulation model that includes a representation of anthropogenic carbon as a passive tracer is forced with climatological surface fluxes. This simulation is then used to compute offline the anthropogenic ΔpCO2 (defined as the difference between the atmospheric CO2 and its seawater partial pressure) trends over three decades between the years 1970 and 2000. It is shown that the mean increasing trends in ΔpCO2 reflects an increase of the seasonal amplitude of ΔpCO2. In particular, the ocean uptake of anthropogenic CO2 is decreasing (negative trends in ΔpCO2) in boreal (austral) summer in the Northern (Southern) Hemisphere in the subtropical gyres between 20° N(S) and 40° N(S). In our simulation, the increased amplitude of the seasonal trends of the ΔpCO2 is mainly explained by the seasonal sea surface temperature (SST) acting on the anthropogenic increase of the dissolved inorganic carbon (DIC). It is also shown that the seasonality of the anthropogenic DIC has very little effect on the decadal trends. This study underscores the need for surface CO2 measurements that resolve the seasonal cycle throughout much of the extratropical oceans.
Significance of N2 fixation in dissolved fractions of organic nitrogen
Authors: Konno U. , Tsunogai U. , Komatsu D. D. , Daita S. , Nakagawa F. , Tsuda A. , Matsui T. , Eum Y.-J. , Suzuki K.
Published: April 2010

Abstract. Using the 15N2 tracer method and high-sensitivity δ15N analytical systems, we determined N2 fixation rates by dividing fractions into particulate organic nitrogen (PON: >0.7 μm) and dissolved organic nitrogen (DON: <0.7 μm). While N2 fixation in the DON fraction had been ignored in previous studies, we found significant N2 fixation signal in the DON fraction in our study. The areal N2 fixation rates estimated from the PON fractions varied from <1–160 μmol N m−2 d−1, and those estimated from the DON fractions ranged from <0.5–54 μmol N m−2 d−1. Thus, N2 fixation in the DON fractions accounted for 50% (ranging from <10% to 84%) of the total N2 fixation rates on an average. The new total N2 fixation flux, which includes fixation in DON fractions, has possibility to double the original estimates; therefore, the revised influx may reduce the imbalance in the global oceanic fixed nitrogen budget.
Shelf erosion and submarine river canyons: implications for deep-sea oxygenation and ocean productivity during glaciation
Authors: Tsandev I. , Rabouille C. , Slomp C. P. , Van Cappellen P.
Published: April 2010

Abstract. The areal exposure of continental shelves during glacial sea level lowering enhanced the transfer of erodible reactive organic matter to the open ocean. Sea level fall also activated submarine canyons thereby allowing large rivers to deposit their particulate load, via gravity flows, directly in the deep-sea. Here, we analyze the effects of shelf erosion and particulate matter re-routing to the open ocean during interglacial to glacial transitions, using a coupled model of the marine phosphorus, organic carbon and oxygen cycles. The results indicate that shelf erosion and submarine canyon formation may significantly lower deep sea oxygen levels, by up to 25%, during sea level low stands, mainly due to the supply of new material from the shelves, and to a lesser extent due to particulate organic matter bypassing the coastal zone. Our simulations imply that deep-sea oxygen levels can drop significantly if eroded shelf material is deposited to the seafloor. Thus the glacial ocean's oxygen content could have been significantly lower than during interglacial stages. Primary production, organic carbon burial and dissolved phosphorus inventories are all affected by the erosion and rerouting mechanisms. However, re-routing of the continental and eroded shelf material to the deep sea-sea has the effect of decoupling deep-sea oxygen demand from primary productivity in the open ocean. P burial is also not affected showing a disconnection between the biogeochemical cycles in the water column and the P burial record.
Technical Note: Determining the size-normalised weight of planktic foraminifera
Authors: Beer C. J. , Schiebel R. , Wilson P. A.
Published: April 2010

Abstract. The size-normalised weight (SNW) of planktic foraminifera, a measure of test wall thickness and density, is potentially a valuable palaeo-proxy for marine carbon chemistry. As increasing attention is given to developing this proxy it is important that methods are comparable between studies. Here, we compare SNW data generated using two different methods to account for variability in test size, namely i) the narrow (50 μm range) sieve fraction method and ii) the individually measured test size method. Using specimens from the 200–250 μm sieve fraction range collected in multinet samples from the North Atlantic, we find that sieving does not constrain size sufficiently well to isolate changes in weight driven by variations in test wall thickness and density from those driven by size. We estimate that these SNW data are associated with an uncertainty, or error bar, of about ±11%. Errors associated with the narrow sieve fraction method may be reduced by decreasing the size of the sieve window, by using larger tests and by increasing the number tests employed. In situations where numerous large tests are unavailable, however, substantial errors associated with this sieve method remain unavoidable. In such circumstances the individually measured test size method provides a better means for estimating SNW because, as our results show, this method isolates changes in weight driven by variations in test wall thickness and density from those driven by size.
Quantifying the effect of vegetation dynamics on the climate of the Last Glacial Maximum
Authors: Jahn A. , Claussen M. , Ganopolski A. , Brovkin V.
Published: April 2010

Abstract. The importance of the biogeophysical atmosphere-vegetation feedback in comparison with the radiative effect of lower atmospheric CO2 concentrations and the presence of ice sheets at the last glacial maximum (LGM) is investigated with the climate system model CLIMBER-2. Equilibrium experiments reveal that most of the global cooling at the LGM (-5.1°C) relative to (natural) present-day conditions is caused by the introduction of ice sheets into the model (-3.0°C), followed by the effect of lower atmospheric CO2 levels at the LGM (-1.5°C), while a synergy between these two factors appears to be very small on global average. The biogeophysical effects of changes in vegetation cover are found to cool the global LGM climate by 0.6°C. The latter are most pronounced in the northern high latitudes, where the taiga-tundra feedback causes annually averaged temperature changes of up to -2.0°C, while the radiative effect of lower atmospheric CO2 in this region only produces a cooling of 1.5°C. Hence, in this region, the temperature changes caused by vegetation dynamics at the LGM exceed the cooling due to lower atmospheric CO2 concentrations.
Paleoclimatic reconstructions in western Canada from boreholetemperature logs: surface air temperature forcing and groundwater flow
Authors: Majorowicz J. , Grasby S. E. , Ferguson G. , Safanda J. , Skinner W.
Published: April 2010

Abstract. Modelling of surface temperature change effect on temperature vs.~depth and temperature-depth logs in Western Canada Sedimentary Basin show that SAT (surface air temperature) forcing is the main driving factor for the underground temperature changes diffusing with depth. It supports the validity of the basic hypothesis of borehole temperature paleoclimatology, namely that the ground surface temperature is systematically coupled with the air temperature in the longer term (decades, centuries). While the highest groundwater recharge rate used in the modelling suggests that for this extreme case some of the observed curvature in the profile, could be due to groundwater flow, it is more likely that the low recharge rates in this semi-arid region would have minimal impact. We conclude that surface temperature forcing is responsible for most of the observed anomalous temperature profile.
Equatorial insolation: from precession harmonics to eccentricity frequencies
Authors: Berger A. , Loutre M. F. , Mélice J. L.
Published: April 2010

Abstract. Since the paper by Hays et al. (1976), spectral analyses of climate proxy records provide substantial evidence that a fraction of the climatic variance is driven by insolation changes in the frequency ranges of obliquity and precession variations. However, it is the variance components centered near 100 kyr which dominate most Upper Pleistocene climatic records, although the amount of insolation perturbation at the eccentricity driven periods close to 100-kyr (mainly the 95 kyr- and 123 kyr-periods) is much too small to cause directly a climate change of ice-age amplitude. Many attempts to find an explanation to this 100-kyr cycle in climatic records have been made over the last decades. Here we show that the double maximum which characterizes the daily irradiation received in tropical latitudes over the course of the year is at the origin in equatorial insolation of not only strong 95 kyr and 123 kyr periods related to eccentricity, but also of a 11-kyr and a 5.5-kyr periods related to precession.
Ice-driven CO2 feedback on ice volume
Authors: Ruddiman W. F.
Published: April 2010

Abstract. The origin of the major ice-sheet variations during the last 2.7 million years is a long-standing mystery. Neither the dominant 41 000-year cycles in δ18O/ice-volume during the late Pliocene and early Pleistocene nor the late-Pleistocene oscillations near 100 000 years is a linear ("Milankovitch") response to summer insolation forcing. Both responses must result from non-linear behavior within the climate system. Greenhouse gases (primarily CO2) are a plausible source of the required non-linearity, but confusion has persisted over whether the gases force ice volume or are a positive feedback. During the last several hundred thousand years, CO2 and ice volume (marine δ18O) have varied in phase at the 41 000-year obliquity cycle and nearly in phase within the ~100 000-year band. This timing rules out greenhouse-gas forcing of a very slow ice response and instead favors ice control of a fast CO2 response. In the schematic model proposed here, ice sheets responded linearly to insolation forcing at the precession and obliquity cycles prior to 0.9 million years ago, but CO2 feedback amplified the ice response at the 41 000-year period by a factor of approximately two. After 0.9 million years ago, with slow polar cooling, ablation weakened. CO2 feedback continued to amplify ice-sheet growth every 41 000 years, but weaker ablation permitted some ice to survive insolation maxima of low intensity. Step-wise growth of these longer-lived ice sheets continued until peaks in northern summer insolation produced abrupt deglaciations every ~85 000 to ~115 000 years. Most of the deglacial ice melting resulted from the same CO2/temperature feedback that had built the ice sheets. Several processes have the northern geographic origin, as well as the requisite orbital tempo and phasing, to be candidate mechanisms for ice-sheet control of CO2 and their own feedback.
The origin of the European "Medieval Warm Period"
Authors: Goosse H. , Arzel O. , Luterbacher J. , Mann M. E. , Renssen H. , Riedwyl N. , Timmermann A. , Xoplaki E. , Wanner H.
Published: April 2010

Abstract. Proxy records and results of a three dimensional climate model show that European summer temperatures roughly a millennium ago were comparable to those of the last 25 years of the 20th century, supporting the existence of a summer "Medieval Warm Period" in Europe. Those two relatively mild periods were separated by a rather cold era, often referred to as the "Little Ice Age". Our modelling results suggest that the warm summer conditions during the early second millennium compared to the climate background state of the 13th–18th century are due to a large extent to the long term cooling induced by changes in land-use in Europe. During the last 200 years, the effect of increasing greenhouse gas concentrations, which was partly levelled off by that of sulphate aerosols, has dominated the climate history over Europe in summer. This induces a clear warming during the last 200 years, allowing summer temperature during the last 25 years to reach back the values simulated for the early second millennium. Volcanic and solar forcing plays a weaker role in this comparison between the last 25 years of the 20th century and the early second millennium. Our hypothesis appears consistent with proxy records but modelling results have to be weighted against the existing uncertainties in the external forcing factors, in particular related to land-use changes, and against the uncertainty of the regional climate sensitivity. Evidence for winter is more equivocal than for summer. The forced response in the model displays a clear temperature maximum at the end of the 20th century. However, the uncertainties are too large to state that this period is the warmest of the past millennium in Europe during winter.
Exposure dating of Late Glacial and pre-LGM moraines in the Cordon de Doña Rosa, Northern/Central Chile (~31° S)
Authors: Zech R. , Kull Ch. , Kubik P. W. , Veit H.
Published: April 2010

Abstract. Despite the important role of the Central Andes (15–30° S) for climate reconstruction, knowledge about the Quaternary glaciation is very limited due to the scarcity of organic material for radiocarbon dating. We applied 10Be surface exposure dating (SED) on 22 boulders from moraines in the Cordon de Doña Rosa, Northern/Central Chile (~31° S). The results show that several glacial advances in the southern Central Andes occurred during the Late Glacial between ~14.7±1.5 and 11.6±1.2 ka. A much more extensive glaciation is dated to ~32±3 ka, predating the temperature minimum of the global LGM (Last Glacial Maximum: ~20 ka). Reviewing these results in the paleoclimatic context, we conclude that the Late Glacial advances were most likely caused by an intensification of the tropical circulation and a corresponding increase in summer precipitation. High-latitude temperatures minima, e.g. the Younger Dryas (YD) and the Antarctic Cold Reversal (ACR) may have triggered individual advances, but current systematic exposure age uncertainties limit precise correlations. The absence of LGM moraines indicates that moisture advection was too limited to allow significant glacial advances at ~20 ka. The tropical circulation was less intensive despite the maximum in austral summer insolation. Winter precipitation was apparently also insufficient, although pollen and marine studies indicate a northward shift of the westerlies at that time. The dominant pre-LGM glacial advances in Northern/Central Chile at ~32 ka required lower temperatures and increased precipitation than today. We conclude that the westerlies were more intense and/or shifted equatorward, possibly due to increased snow and ice cover at higher southern latitudes coinciding with a minimum of insolation.
Change in ice rheology during climate variations – implications for ice flow modelling and dating of the EPICA Dome C core
Authors: Durand G. , Gillet-Chaulet F. , Svensson A. , Gagliardini O. , Kipfstuhl S. , Meyssonnier J. , Parrenin F. , Duval P. , Dahl-Jensen D.
Published: April 2010

Abstract. The study of the distribution of crystallographic orientations (i.e., the fabric) along ice cores provides information on past and current ice flow in ice-sheets. Besides the usually observed formation of a vertical single maximum fabric, the EPICA Dome C ice core (EDC) shows an abrupt and unexpected strengthening of its fabric during termination II around 1750 m depth. Such strengthening has already been observed for sites located on an ice-sheet flank. This suggests that horizontal shear could occur along the EDC core. Moreover, the change in the fabric leads to a modification of the effective viscosity between neighbouring ice layers. Through the use of an anisotropic ice flow model, we quantify the change in effective viscosity and investigate its implication for ice flow and dating.
Climate of the Last Glacial Maximum: sensitivity studies and model-data comparison with the LOVECLIM coupled model
Authors: Roche D. M. , Dokken T. M. , Goosse H. , Renssen H. , Weber S. L.
Published: April 2010

Abstract. The Last Glacial Maximum climate is one of the classical benchmarks used both to test the ability of coupled models to simulate climates different from that of the present-day and to better understand the possible range of mechanisms that could be involved in future climate change. It also bears the advantage of being one of the most well documented periods with respect to palaeoclimatic records, allowing a thorough data-model comparison. We present here an ensemble of Last Glacial Maximum climate simulations obtained with the Earth System model LOVECLIM, including coupled dynamic atmosphere, ocean and vegetation components. The climate obtained using standard parameter values is then compared to available proxy data for the surface ocean, vegetation, oceanic circulation and atmospheric conditions. Interestingly, the oceanic circulation obtained resembles that of the present-day, but with increased overturning rates. As this result is in contradiction with the current palaeoceanographic view, we ran a range of sensitivity experiments to explore the response of the model and the possibilities for other oceanic circulation states. After a critical review of our LGM state with respect to available proxy data, we conclude that the oceanic circulation obtained is not inconsistent with ocean circulation proxy data, although the water characteristics (temperature, salinity) are not in full agreement with water mass proxy data. The consistency of the simulated state is further reinforced by the fact that the mean surface climate obtained is shown to be generally in agreement with the most recent reconstructions of vegetation and sea surface temperatures, even at regional scales.
Spatial structure of the 8200 cal yr BP event in northern Europe
Authors: Seppä H. , Birks H. J. B. , Giesecke T. , Hammarlund D. , Alenius T. , Antonsson K. , Bjune A. E. , Heikkilä M. , MacDonald G. M. , Ojala A. E. K. , Telford R. J. , Veski S.
Published: April 2010

Abstract. A synthesis of well-dated high-resolution pollen records suggests a spatial structure in the 8200 cal yr BP event in northern Europe. The temperate, thermophilous tree taxa, especially Corylus, Ulmus, and Alnus, decline abruptly between 8300 and 8000 cal yr BP at most sites located south of 61° N, whereas there is no clear change in pollen values at the sites located in the North-European tree-line region. Pollen-based quantitative temperature reconstructions and several other, independent palaeoclimate proxies, such as lacustrine oxygen-isotope records, reflect the same pattern, with no detectable cooling in the sub-arctic region. The observed patterns challenges the general view of the wide-spread occurrence of the 8200 cal yr BP event in the North Atlantic region. An alternative explanation is that the cooling during the 8200 cal yr BP event took place mostly during the winter and spring, and the ecosystems in the south responded sensitively to the cooling during the onset of the growing season. In contrast, in the sub-arctic area, where the vegetation was still dormant and lakes ice-covered, the cold event is not reflected in pollen-based or lake-sediment-based records.