886 research outputs found
Ocean gyres and abrupt change in the thermohaline circulation: a conceptual analysis
No full textThe implications are investigated of representing ocean gyre circulations by a diffusion term in the Stommel and Rooth box models of the thermohaline circulation (THC) in one and two hemispheres, respectively. The approach includes mostly analytical solution and study of the bifurcation structure, but also numerical integration and feedback analysis. Sufficient diffusion (gyre strength) eliminates multiple equilibria from either model, highlighting the need for accurate gyre circulation strength in general circulation models (GCMs) when considering the potential for abrupt climate change associated with THC shutdown.With diffusion, steady-state flow strength in the Rooth model depends on freshwater forcing (i.e., implied atmospheric water vapor transport) in both hemispheres, not only on that in the upwelling hemisphere, as in the nondiffusive case. With asymmetric freshwater forcing, two solutions (strong stable and weak unstable) are found with sinking in the hemisphere with stronger forcing and one solution with sinking in the other hemisphere. Under increased freshwater forcing the two solutions in the hemisphere with stronger forcing meet in a saddle-node bifurcation (if diffusion is sufficiently strong to prevent a subcritical Hopf bifurcation first), followed by flow reversal. Thus, the bifurcation structure with respect to freshwater forcing of the diffusive Rooth model of two-hemisphere THC is similar to that of the Stommel model of single-hemisphere THC, albeit with a very different dynamical interpretation. Gyre circulations stabilize high-latitude sinking in the Stommel model. In the Rooth model, gyre circulations only stabilize high-latitude sinking if the freshwater forcing is weaker in the sinking hemisphere than in the upwelling hemisphere, by an amount that increases with diffusion. The values of diffusion and freshwater forcing at which qualitative change in behavior occurs correspond to the range of the values used in and obtained with GCMs, suggesting that this analysis can provide a conceptual foundation for analyzing the stability of the interhemispheric THC, and also for the potential of the Atlantic THC to undergo abrupt change
Economic Potential of Conservation Farming Annual Winter Forages for the Stocker Cattle Grazing Enterprise
No full textThe objective was to determine the expected net value of a no-till forage production and grazing system. Reduction in fuel and machinery costs offset the costs of herbicide application. The net value of the no-till system is $31 per acre, and is quite sensitive to relative differences in cattle performance.Farm Management, Land Economics/Use,
On the composition of firn air and its dependence on seasonally varying atmospheric boundary conditions and the firn structure
No full textThe dependence of the firn air composition on seasonally varying atmospheric boundary conditions and the firn structure has been studied based on firn air samples from two Antarctic sites, Halley and Kohnen.
The Halley data set consists of six profiles sampled in a two month resolution in the year 2003. Settling of the firn and snow accumulation led to a shift of the depth levels of both, the firn air sampling setup and the thermistors. Prediction of this sample level shift with a 1–dimensional box model developed in the frame of this study coincides with the decrease of the analyzed diurnal temperature cycle.
By forcing the temperature–diffusion model with the surface temperature history of Halley, temperature oscillations in the firn column originating from seasonal atmospheric temperature variations are reasonably well reproduced. Thus, the dependence of the thermal conductivity on firn density as derived by Schwander et al. [1997] is sufficient also on the seasonal scale.
In order to simulate thermal fractionation of the different gas species, thermal diffusion factors αT are determined based on two different model approaches and the firn air data set: αT = 3.727 · 10−3 for the fractionation of 15N14N in N2, αT = 12.480·10−3 for 18O16O in air, αT = 7.321·10−3 for 16O2 in air, αT = 32.241·10−3 for 36Ar in air, αT = 45.516 · 10−3 for 40Ar in air, and 57.371 · 10−3 ≤ αT ≤ 67.811 · 10−3 for CO2 in air.
Seasonal oscillations observed in the firn air profiles of CO2 and the isotopic species δ15N, δ18O, and δ36Ar act as expected from theory during the entire year. The imprint of both, seasonal atmospheric temperature variations and atmospheric concentration oscillations is well reproduced by the diffusion–temperature model. Neither a seasonal variability of ±0.5 ppmv in the atmospheric CO2 concentration nor thermal diffusion forced by seasonal atmospheric temperature oscillations will lead to considerable changes of the CO2 concentration in air bubbles finally occluded in the ice.
An overestimation of the measured February amplitude of the isotopic profiles (δ15N, δ18O δ36Ar) by the diffusion–temperature model can be explained by surface convection. Based on a parametrisation approach of Severinghaus et al. [2001] and Kawamura et al. [2006], a convective column height of 0.28 ≤ zconv ≤ 0.43 m is derived for Halley firn of 2003. Convection turned out to be active between end of December and mid of February. The calculation of surface convection as Darcian air flow through the firn driven by a pressure field originating from dunes on the surface [Colbeck, 1989] confirms the size of the convective layer. However, the temporal forcing and monthly mean wind speeds measured in Halley do not clearly coincide.
Within the frame of thesis, firn and firn air have been sampled at the deep drilling site Kohnen in Austral summer 2005/06. Independently from the container type samples have a good quality with respect to the already analyzed species (isotopes, elemental ratios, CO2, noble gases and ozone depleting substances). The usage of a bladder tube of distinctly smaller diameter (by 15 mm) than the hole turned out to be problematic and thus, is not recommended for further experiments.
From the average of the 20 m temperature obtained from five borehole temperature profiles measured between November 2005 and January 2006, an annual mean temperature of –44.4°C could be derived for the Kohnen site.
Forcing the temperature model with atmospheric temperature data obtained from an automatic weather station near Kohnen Station leads to a good agreement between simulations and the bore hole temperatures. CO2 firn air data agree well with simulations based on the atmospheric CO2 history from South Pole and the measured temperatures. The best match between modeled and measured CO2 profiles was obtained by selecting the tortuosity parameters α and β to α = 0.85 and β = 2.5. The close–off density is determined to ρco = 825 kg m−3, which is slightly lower than the mean air isolation level of 833.4 kg m−3 derived according to Martinerie et al. [1992, 1994]. ρco corresponds to a close–off depth zco = 87.6 m. Effective diffusivity is determined to become terminated at the lock–in depth zli = 87.4 m. Kohnen firn has a non–diffusive zone of about 7 m length, where the deepest firn air sample is at zmax = 94.9 m.
CO2 firn air data agree with the simulation disregarding thermal diffusion rather than showing a thermal signal. Thus, Kohnen firn seems to have a convective zone attenuating the seasonal thermal amplitude.
The mean gas age at the close–off level is estimated by combining the mean gas ages of air originating from the open pore space and the air occluded in air bubbles. In order to determine the amount of occluded air, a mass balance is implemented in the diffusion–temperature model. Air enclosure is simulated from two different temporally constant enclosure functions. According to this approach a combined mean CO2 gas age at the close–off level of (65 +75/–50) or (74 +85/–56) years, respectively, is retrieved for present–day Kohnen firn. Depending on the enclosure functions the width of corresponding age distribution functions representing air occluded in air bubbles lies between ≈ 47 and ≈ 52 years. The width of the age distribution of the air originating from the open pore volume is ≈ 17 years.
Forcing effective diffusivity De with the open porosity sop instead of the total porosity s, leads to about 2 ppmv lower CO2 concentrations simulated for the depth range around and below the close–off region. Accordingly, the mean gas age is calculated to be 2 to 5 years lower than for De ∼ s.
The ice age at the close–off level is determined to (905 ± 7) years from assigning documented volcanic events to peaks in the electrical conductivity record of the ice. Based on this depth–to–age relation an annual mean accumulation of 65 kg m−2 a−1 could be derived for the Kohnen site.
The ∆age, i.e. the difference between ice age and combined mean gas age, varies between (831 +85/–56) and (840 +76/–51) years for Kohnen firn subjected topresent–day conditions.
To the best of current knowledge Kohnen firn provides the oldest firn air that has ever been sampled so far. The age of the firn air at zmax = 94.9 m is determined to (114 +13/–10) years. The occurrence of such old firn air is attributed to a distinct stratification of Kohnen firn, which originates from a wind driven surface orography and is preserved down to the close–off region
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Steady-state and transient modeling of tracer and nutrient distributions in the global ocean
No full textThe deep circulation model developed by Wright and Stocker has been used to represent the latitude-depth distributions of temperature, salinity, radiocarbon and color'' tracers in the Pacific, Atlantic and Indian Oceans. Restoring temperature and salinity to observed surface data the model shows a global thermohaline circulation where deep water is formed in the North Atlantic and in the Southern Ocean. A parameter study reveals that the high-latitude surface salinity determines the composition of deep water and its flow in the global ocean. Increasing Southern Ocean surface salinity by 0.4 ppt the circulation changes from a present-day mode where North Atlantic Deep Water is one where Antarctic Bottom Water is dominant. An inorganic carbon cycle with surface carbonate chemistry is included, and gas exchange is parameterized in terms of pCO{sub 2} differences. Pre- industrial conditions are achieved by adjusting the basin-mean alkalinity. A classical 2{times}CO{sub 2} experiment yields the intrinsic time scales for carbon uptake of the ocean; they agree with those obtained from simple box models or 3-dimensional ocean general circulation models. Using the estimated industrial anthropogenic input of CO{sub 2} into the atmosphere the model requires, consistent with other model studies, an additional carbon flux to match the observed increase of atmospheric pCO{sub 2}. We use more realistic surface boundary conditions which reduce sensitivity to freshwater discharges into the ocean. In a glacial-to-interglacial experiment rapid transitions of the deep circulation between two different states occur in conjunction with a severe reduction of the meridional heat flux and sea surface temperature during peak melting. After the melting the conveyor belt circulation restarts
Large-scale ocean circulation, air-sea gas exchange, and carbon isotopes in a three-dimensional, computationally efficient ocean model
No full textThis thesis focuses on the intermediate-complexity modelling of the large-scale ocean circulation and the oceanic carbon cycle. The main objective is the development of a model of intermediate complexity by combining, evaluating and extending existing formulations of the single components within a coherent setting. The model should be suited to perform long integrations over the timescales of interest in climate research in short periods of time, also permitting to employ Monte-Carlo and other ensemble techniques.
In Chapter 2 the development of the physical core of the Bern3D ocean model is presented. First, the frictional-geostrophic balance ocean model of Edwards et al. [1998] is reviewed. This model has been designed to be computationally efficient and is used as the oceanic com ponent in the coupled atmosphere - sea ice - ocean model "C-GOLDSTEIN" [Edwards and Marsh, 2005] and is also applied as the oceanic component of the grid-based Earth system model GENIE (http://www. genie. ac.uk). The Bern3D ocean model is largely based on the ocean model described by Edwards et al. [1998] with updates as described in Edwards and Marsh [2005]. First, the governing equations describing the large scale ocean circulation and tracer transport are introduced. Then, specific extensions and modifications of the original ocean model constituting the development of the physical core of the Bern3D ocean model are described in detail. The Bern3D ocean model features seasonal boundary conditions and a high vertical resolution. In comparison to the ocean model used in Edwards and Marsh [2005], the mixing scheme for isopycnal diffusion and eddy-induced transport has been enhanced and an altered convection scheme has been implemented. Further, results from the standard simulation and the sensitivity of the steady state circulation to the mixing parameters are examined and presented. Also the simulated barotropic transport for a simulation with opened Indonesian Passage and increased latitudinal resolution is presented and compared to the standard solution.
The large scale circulation and the resulting distribution of water masses as simulated with the Bern3D ocean model has been examined and probed using a set of tracers governed by decadal to multi-century timescales [Müller et al., 2006] (Chapter 3). Modelled tracers are the 39Ar/Ar ratio and natural radiocarbon, both influenced by radioactive decay with different mean life times, as well as bomb-produced radiocarbon, CFC-11, and anthropogenic carbon, all influenced by their transient atmospheric boundary conditions. For all of these tracers exist measurements or data-based reconstructions of their distribution inside the ocean. Simulated distributions and inventories of temperature, salinity and the other tracers are compared with data-based estimates and reasonable agreement is found for a simulation performed with the Bern3D ocean model tuned towards data-based metrics, including the natural radiocarbon signatures of typical water masses and the inventories of CFC-11 for the mid-l 990s in the Indopacific. The model has also been applied to examine the importance of different surface to-deep transport mechanisms for the simulated distribution of natural radiocarbon and the uptake of CFC-11. Deep equatorial upwelling has been found to be sensitive to the vertical model resolution, reduced deep equatorial upwelling strength is found for a higher vertical resolution. Furthermore, the carbon budget for the industrial period was closed by simulating an uptake of anthropogenic carbon by the ocean of 18.9 GtC and 19.7 GtC for the 1980s and 1990s, respectively, and inferring land-atmosphere fluxes, in agreement with data-based estimates.
The Bern3D ocean model has been complemented with standard formulations of the abiotic, organic matter, and calcite carbon cycle (Chapter 4), as formulated for the phase two of the Ocean Carbon-Cycle Model Intercomparison Project (OCMIP-2) [Orr, 2000]. Further, the formulations of the OCMIP-2 have been extended to include the radiocarbon fluxes in the organic matter and calcite cycles and the 13C fluxes for the whole oceanic carbon cycle formulation. Results have been compared with data-based estimates and are found to reasonably reproduce the main large-scale features as observed in the data. The oceanic carbon-cycle model component has been complemented with a 4-box formulation for the land biosphere following Siegenthaler and Oeschger [1987], in order to calculate the radiocarbon production rate by inversion of atmospheric Ll14C reconstructions from tree rings [Reimer et al., 2004, McCormac et al., 2004] with a global carbon cycle model.
The magnitude of the global mean air-sea gas exchange rate for CO2 has been discussed and debated over the last few decades in the scientific literature. In a contribution to this discussion (manuscript in preparation, Chapter 5) the natural radiocarbon distribution, basin-wide inventories of excess radiocarbon, the Earth system budget of radiocarbon, and basin-wide inventories of CFC-11 have been simulated using the gas-exchange formulation of the OCMIP-2 in an extensive sensitivity study varying the magnitude of the gas-exchange rate and oceanic transport strength. The results are compared to recent data-based estimates. It is demonstrated that the rate-limiting process for the uptake of excess radiocarbon is the gas-exchange rate and that the uptake of CFC-11 is dominated by oceanic transport and mixing. A target consisting of the data-based basin-wide inventories of excess radiocarbon in the Pacific, Indian Ocean, and Southern Atlantic for different points in time, the natural radiocarbon distribution in the surface ocean, and the Earth system budget of radiocarbon is defined and is best matched if the gas-transfer field from the OCMIP-2 is scaled down by (26 ± 16)%. Simulated column inventories are found to be similar in all basins, in contrast to data-based estimates, where the North Atlantic inventories are significantly higher than in the other basins. Estimates for inventories of excess radiocarbon, using the downscaled gas transfer rate of (15.7 ± 3.3) cm hr-1, are presented.
In a publication by Muscheler et al. [2007] (Chapter 6), the radiocarbon production rate records calculated by different carbon-cycle models and reconstructions of 10Be from ice cores are used together with records of the geomagnetic field intensity to infer the solar magnetic modulation, which is linked to the solar activity. The solar modulation parameter record is compared to irradiance records. In this publication, the radiocarbon production rate calculated with the Bern3D model is presented and compared with other radiocarbon production rate reconstructions using carbon-cycle models featuring a box model representation of the ocean. In comparison to the box models of the ocean, the Bern3D model provides a more realistic boundary condition at the surface ocean for the air-sea gas exchange of radiocarbon. ∆14C reconstructions for the northern and southern hemispheres, which differ owing to a gradient in atmospheric ∆14C at the equator, can thus be included as boundary conditions of the respective oceanic domains. In the box models of the ocean, which do not resolve the two hemispheres, the atmospheric ∆14C boundary condition has to be an averaged value of the two hemispheric records. The results using the different modelling approaches to calculate radiocarbon production are found to agree well.
An outlook regarding ongoing and possible future model development extending the Bern3D ocean model into various directions and applications thereof is given in Chapter 7. The Appendix details the construction of smooth atmospheric pC02 records as applied in the studies presented in this thesis and also their projection into the future in the form of idealised stabilisation scenarios as used for projections of climate change commitment using Earth system models of intermediate complexity for the upcoming Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC) and e. g. in Plattner et al. [submitted] and Knutti et al. [2005]. Furthermore, a publication [Muscheler et al., 2005] contributing to a discussion that was initiated following the publication of a paper by Solanki et al. [2004] is attached in the Appendix. There, using radiocarbon production rate based reconstructions of the solar modulation parameter, Muscheler et al. [2005] find that the solar modulation parameter was comparable to or even exceeds today's values during several periods in the last 500 years
Highly resolved d13C measurements on CO2 in air from Antarctic ice cores
No full textDie vorliegende Arbeit befasst sich mit der hochaufgelösten Messung des stabilen 13C Isotopes an CO2 aus Luft von antarktischen Eisbohrkernen. Die Motivation dies zu tun, gründet in den seit rund fünf Jahren vorliegenden hochaufgelösten CO2 Konzentrations–Rekonstruktionen der Atmosphäre, die bis ins letzte Glazial zurückreichen. Die detailierte Entwicklung des atmosphärischen CO2 Verlaufs, wie sie in den Zeitreihen von Indermühle et al., 1999; Indermühle et al., 2000; Monnin et al., 2001; Flückiger et al., 2002 zum Vorschein kommt, ist grösstenteils ungeklärt.
δ13C Messungen am CO2 haben das Potential die Prozesse, die zu einer CO2 Konzentrationsveränderung führen, aufzuschlüsseln. Die wichtigsten CO2 Quellen, der Ozean und die terrestrische Biosphäre, haben unterschiedliche δ13C Signaturen. Das in die Atmosphäre
gemischte oder daraus entfernte CO2 kann daher durch die Analyse seiner δ13C Signatur einer Quelle zugeordnet werden. Als Beispiel wurde dies für die CO2 Entwicklung der aktuellen Warmzeit von Indermühle et al. [1999] versucht. In seinen Schlussfolgerungen ordnete er den CO2 Anstieg ab der Mitte des Holozäns einem Einbruch der terrestrischen Biosphäre zu. Die wenigen δ13C Daten, die ihm zur Verfügung standen, liessen allerdigns viel Spielraum für Spekulationen. Zwei Jahre später stellten Broecker et al. [2001] das Indermühle–Szenario in Frage und präsentierten eine Alternative, die auf Beobachtungen grosser Karbonat Veränderungen im Ozean während des Holozäns beruht. Das alternative Szenario sieht den Ozean als Hauptquelle der in der Atmosphäre beobachteten CO2 Zunahme.
Eine neu zusammengestellte Messanlage erlaubte uns in den vergangenen drei Jahren, δ13C in der selben Auflösung zu messen, wie dies für die CO2 Konzentration möglich ist. Gemessen wurde das δ13C an zwei antarktischen Eisbohrkernen, die im Rahmen des European Project for Ice Core drilling in Antarctica(EPICA) geborgen wurden. Der eine Bohrkern entstammt der Bohrstation Dome Concordia (EDC) in der Ost–Antarktis, der andere der Station Kohnen Station in Dronning Maud Land (EDML).
Am EDC Eiskern wurde eine δ13C Zeitreihe über die letzten 18’000 Jahre erstellt und zusätzlich 15 Proben mit einem Alter von zwischen 20’000 und 40’000 Jahren gemessen. Die Resultate zeigen eine Reaktion des atmosphärischen δ13C auf die wichtigsten Änderungen im globalen Kohlenstoff Kreislauf, wobei diese Änderungen im Glazial und im Übergang ins Holozän hauptsächlich Variationen der Ozean Zirkulation zuzuschreiben sind. Im frühen Holozän wirkte der Aufbau der terrestrischen und/oder der marinen Biosphäre als Senke für CO2 und prägte die atmosphärische Konzentration, während in der zweiten Hälfte des Holozän der Ozean wieder eine wichtigere Rolle spielte. Die Resultate des zweiten Eiskerns EDML, zeigen eine ungleich grössere Streuung und lassen keine klare Interpretation zu. Möglicherweise liegt der Grund dafür bei Fraktionierungen beim Einschlussprozess der Luft ins Eis. Die höhere Niederschlagsrate als in Dome C und damit die grössere zeitliche Auflösung durch unser Messverfahren führen dazu, dass solche Variationen nicht geglättet werden und die Resultate daher stark streuen.This work deals with highly resolved measurements of the stable 13C isotope on CO2 from air of Antarctic ice cores. The motivation to do this bases on highly resolved atmospheric CO2 reconstructions available since about five years, reaching back to the last glacial. The detailed evolution of the CO2, as presented in the records of Indermühle et al., 1999; Indermühle et al., 2000; Monnin et al., 2001; Flückiger et al., 2002 are mostly unexplained.
δ13C measurements on CO2 have the potential to decrypt the processes leading to CO2 concentration changes in the atmosphere. The main CO2 sources, the ocean and the biosphere have different δ13C signatures. The admixed or removed CO2 in or from the atmosphere can be allocated to its source by analyzing the changes in the δ13C signature. As an example this was tried by Indermühle et al. [1999] for the Holocene with data from the Taylor Dome ice core. In the conclusions they attribute the CO2 concentration re–increase in the mid Holocene to a decrease in the terrestrial biosphere. The few δ13C data which were available to support his scenario left room for speculations. Two years later Broecker et al. [2001] challenged the Indermühle–scenario and presented an alternative, based on ocean carbonate data showing strong carbonate changes in the ocean. This alternative scenario attributed the atmospheric CO2 increase in the second halve of the Holocene to this ocean carbon reorganization.
A new measuring system allows us to produce δ13C results in the same resolution as for the CO2. We measured the δ13C at two different Antarctic ice cores, both drilled in the frame work of the European Project for Ice Core drilling in Antarctica (EPICA). The first core was drilled at Concordia Station (EDC) in Est–Antarctica, the second at Kohnen Station (EDML) in Dronning Maud Land.
At the EDC ice core we produced a record covering the past 18’000 years. Additionally 15 samples were measured in the glacial with ages between 20’000 and 40’000 years. The results show a reaction of the atmospheric δ13C on the main changes in the carbon cycle. These changes are supposed to be subject to changes in the ocean circulation for the last glacial and the last termination. In the early Holocene in contrary the growing terrestrial and/or marine biosphere seems to contribute dominantly to the atmospheric δ13C changes. In the younger Holocene the ocean plays a major role again. The results of the second core EDML show a much larger scatter, why a interpretation is much more delicate. Potentially fractionation effects at the enclosure process of the air in the ice are responsible for this low reproducibility. The higher accumulation rate at the EDML site and the small sample size of our system leads to a subseasonal time resolution. The enclosure fractionation thus is not smoothed and scatters the data
Holocene optimum events inferred from subglacial sediments at Tschierva Glacier, Eastern Swiss Alps
No full textThis study investigates the subglacial sedimentary archive at Tschierva Glacier, Eastern Swiss Alps. Subfossil wood remains found at the retreating glacier tongue indicate that their emergence results from recent transport from an upvalley basin. A confluence-basin-like structure was found to exist by georadar measurements underneath the present glacier. In combination with high resolution age determinations based on dendrochronology and radiocarbon dating it is implied that a retreated Tschierva Glacier allowed vegetation growth and sediment accumulation in that basin. Three periods of glacier recession were detected, which occurred around 9200 cal yr BP, from 7450 to 6650 cal yr BP and from 6200 to 5650 cal yr BP. These periods are called Holocene optimum events (HOE). Accordingly, an equilibrium line rise >220 m compared to the reference period from 1960 to 1985 was inferred from digital elevation models of former glacier extents. Since glacier mass balance depends on summer (June–July–August) temperature and precipitation, an equilibrium line altitude (ELA) rise of 220 m implies a summer temperature increase of about 1.8 °C assuming unchanged precipitation during the dated HOE. Alternative calculations point to probable temperature increase in a broad interval between +1.0 °C taking into account a precipitation change of −250 mm/a to +2.5 °C with +250 mm/a precipitation change, supporting earlier paleotemperature estimates. It is proposed that higher mean summer insolation caused a stronger seasonality during the mid-Holocene as compared to late Holocene conditions
The role of Southern Ocean processes in orbital and millennial CO2 variations - A synthesis
No full textRecent progress in the reconstruction of atmospheric CO2 records from Antarctic ice cores has allowed for the documentation of natural CO2 variations on orbital time scales over the last up to 800,000 years and for the resolution of millennial CO2 variations during the last glacial cycle in unprecedented detail. This has shown that atmospheric CO2 varied within natural bounds of approximately 170-300 ppmv but never reached recent CO2 concentrations caused by anthropogenic CO2 emissions. In addition, the natural atmospheric CO2 concentrations show an extraordinary correlation with Southern Ocean climate changes, pointing to a significant (direct or indirect) influence of climatic and environmental changes in the Southern Ocean region on atmospheric CO2 concentrations. Here, we compile recent ice core and marine sediment records of atmospheric CO2, temperature and environmental changes in the Southern Ocean region, as well as carbon cycle model experiments, in order to quantify the effect of potential Southern Ocean processes on atmospheric CO2 related to these orbital and millennial changes. This shows that physical and biological changes in the SO are able to explain substantial parts of the glacial/interglacial CO2 change, but that none of the single processes is able to explain this change by itself. In particular, changes in the Southern Ocean related to changes in the surface buoyancy flux, which in return is controlled by the waxing and waning of sea ice may favorably explain the high correlation of CO2 and Antarctic temperature on orbital and millennial time scales. In contrast, the changes of the position and strength of the westerly wind field were most likely too small to explain the observed changes in atmospheric CO2 or may even have increased atmospheric CO2 in the glacial. Also iron fertilization of the marine biota in the Southern Ocean contributes to a glacial drawdown of CO2 but turns out to be limited by other factors than the total dust input such as bioavailability of iron or macronutrient supply. © 2009 Elsevier Ltd. All rights reserved
Sensitivity of Nd isotopic composition in seawater to changes in Nd sources and paleoceanographic implications
No full textIt has been argued that past changes in the sources of Nd could hamper the use of the Nd isotopic composition (ϵNd) as a proxy for past changes in the overturning of deep water masses. Here we reconsider uncertainties associated with ϵNd in seawater due to potential regional to global scale changes in the sources of Nd by applying a modeling approach. For illustrative purposes we describe rather extreme changes in the magnitude of source fluxes, their isotopic composition or both. We find that the largest effects on ϵNd result from changes in the boundary source. Considerable changes also result from variations in the magnitude or ϵNd of dust and rivers but are largely constrained to depths shallower than 1 km, except if they occur in or upstream of regions where deep water masses are formed. From these results we conclude that changes in Nd sources have the potential to affect ϵNd. However, substantial changes are required to generate large-scale changes inϵNd in deep water that are similar in magnitude to those that have been reconstructed from sediment cores or result from changes in meridional overturning circulation in model experiments. Hence, it appears that a shift in ϵNdcomparable to glacial-interglacial variations is difficult to obtain by changes in Nd sources alone, but that more subtle variations can be caused by such changes and must be interpreted with caution
Even smaller than now - evidence for long lasting glacier retreat periods in the alps during the holocene
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