Fischer, Hubertus

Fundel, Felix

Köhler, Peter

Schmitt, Jochen

English

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6301 LD 000701.07.2001 -31.06.2006PALEOCLIMATIC CHANGES IN THE GLOBAL CARBON CYCLEH. Fischer, P. Köhler, F. Fundel, and J. SchmittAlfred Wegener Institute for Polar and Marine Research, PO Box 12 06 51, D-27515 Bremerhavenhufischer@awi-bremerhaven.de; http://www.awi-bremerhaven.deKey words: carbon cycle modeling, ice cores, paleoclimate, aerosol, CO21 SummaryWithin RESPIC (young scientist Research group for Earth climate System reconstructions onPolar Ice Cores) new δ13CO2 and aerosol paleorecords derived from the EPICA (EuropeanProject for Ice Coring in Antarctica) ice core currently drilled in Dronning Maud Land(DML), Antarctica, are reconstructed. For this purpose novel analytical methods andmodeling approaches are developed. The expected high accuracy data in conjunction with ourmodel approach allows to pinpoint major drivers of the global carbon cycle on glacial/interglacial timescales. First results showed high interannual and decadal variability in sea saltand marine biogenic aerosol during the Holocene, in part related to the Antarctic CircumpolarWave. Carbon cycle modeling studies on biospheric changes from 30,000 yrs before present(BP) to the preindustrial era, point to a substantial reorganization of the terrestrial biospheresuperimposed on long-term variations in ocean/atmosphere carbon fluxes.2 Aim of the research in the framework of DEKLIMThe overarching goal of RESPIC is the quantitative understanding of paleoatmosphericchanges over the last glacial cycle and here especially the coupling of paleoclimate and theglobal carbon cycle. To this end new high-resolution atmospheric paleorecords derived fromthe EPICA DML ice core are derived which set the boundary conditions of temporal changesin the global carbon cycle. Those records will comprise isotopic measurements on δ13CO2 - anurgently lacking constraint on marine and biospheric carbon fluxes - and multivariatechemical aerosol records providing information on aeolian nutrient input into the SouthernOcean as well as on marine biogenic aerosol species, hence potential information on temporalchanges in the biological productivity of the Southern Ocean ecosystem. While the first ofthese tasks requires the development of a novel sublimation extraction technique andestablishing a high-precision isotope analysis for very small air samples, the latter task reliesheavily on state-of-the-art time series analysis and the understanding of atmosphericcirculation in the past. Quantitative interpretation to pinpoint the major drivers of changes inatmospheric CO2 is assured by the development of a new multiboxmodel of the global carboncycle. Model applications require also the assimilation of proxydata from other paleoarchives.With these interdisciplinary objectives RESPIC represents an important contribution to theresearch topics of DEKLIM such as the influence of climate variations on global nutrientcycles, the paleoclimatic and paleoatmospheric variability and the understanding of thephysical processes which link the two as well as the synthesis of models and paleoclimaticdata.3 Recent and completed activitiesDuring the last field season (where HF had the duty of field leader and chief scientist) ice coredrilling in DML was very successful and proceeded another 1100 m down to a depth of1550 m covering the time period until approximately 55,000 yrs BP. The core drilled in2002/2003 was completely retrograded to Germany and will be available for the joint core64Figure 1. Left: Setup of BICYCLE. Right: Cumulative carbon fluxes from atmosphere/biosphere to the oceanfrom simulations of the terrestrial biosphere only. A: total carbon. B: isotopic δ13C signal. Simulations of case I(enhanced pCO2 fertilization) and case II (enhanced temperature sensitivity).processing of the EPICA community this year. Last years core, covering the time intervalfrom 7500 BP to the present, was completely processed in 2002. Among others, thisprocessing provided seasonally resolved aerosol records using a contamination freecontinuous melt water flow analysis (CFA) for selected ion concentrations. In parallel anautomated melt water sampling was established providing about 7000 discrete samples for ionchromatography (IC) analysis in annual resolution shared by different European labs. ICanalysis provides concentrations on sea salt (Na+, Cl- ), mineral dust (Ca2+, K+), biogenicaerosol species (SO42-, methanesulfonate (MSA), NH4+), volcanic deposition (F-, SO42-) andreactive nitrogen (NO3-). Up to now samples for the time period from 170 AD to the presentand from 4600-7200 BP have been evaluated at the Alfred-Wegener-Institute (AWI). Aftercompletion of the CFA records, they will be the basis for precise stratigraphic dating of thecore at least for the Holocene, where seasonal cycles in the chemical species can be clearlyresolved. This has been already accomplished for the last 2000 years. Further preliminarydating was performed by counting annual cycles in the solid state conductivity record whichis available for the full core down to 1550 m. In the course of the core processing sampleshave been taken every 12th meter (equivalent to a temporal resolution of 300 yrs) for carbonisotopic analyses within RESPIC. In addition to the EPICA deep ice core studies our chemicalinvestigations on shallow ice cores from DML have been extended. In a cooperation with theCNRS, Grenoble, sulfate isotope ratios are determined to quantify the stratospheric influenceon sulfate fluxes onto DML.The development of a sublimation extraction system for air entrapped in bubble and clathrateice has proceeded. Quantitative sublimation extraction of CO2 from 10 g of ice is sufficientfor δ13CO2 measurements using our new gas chromatography isotope ratio mass spectrometry(GC-irmMS) method and 20-40 times less than required for conventional methods. Firstδ13CO2 measurements showed that further refinements are necessary. Based on δ13CH4measurements performed with this method an accuracy for δ13CO2 of 0.05 ‰ is feasible.For the interpretation of ice core data on atmospheric δ13C and CO2 the simple ocean-atmosphere-biosphere box model BICYCLE (Fig. 1) has been developed (Köhler et al.,2003). The model includes mass balance equations for the carbon stocks of the biosphericcompartments, for dissolved inorganic carbon, total alkalinity, phosphate (chosen as thelimiting nutrient) and oxygen in 10 oceanic reservoirs (five surface, two intermediate, threedeep), for atmospheric CO2 in the atmosphere, and the13C and 14C isotopic characteristics inall of them. The globally averaged terrestrial biosphere is divided into seven compartmentsrepresenting C3 and C4 ground vegetation, trees, and soil carbon with different turnovertimes. Most importantly, our model parametrizes the physiological response of photosynthesis65Figure 2. ACW signal as archived in sea salt records from DML: a) correlation/regression analysis of Na+concentrations and sea level pressure data for the spring season; b) spectral analysis of the Cl- data for the period176-1950 AD as shown in c).and respiration on changing atmospheric CO2 levels and temperature. We also take intoaccount the consumption of atmospheric CO2 and the transfer of bicarbonate to the ocean dueto continental weathering as well as deep-sea sediment/ocean exchange and carbonateaccumulation in coral reefs and on banks and shelves in the coastal zone.4 Principal results4.1 Southern Ocean atmosphere variabilitySpecial emphasis has been put on the evaluation of the interannual and decadal variability inSouthern Ocean atmosphere dynamics as reflected in sea salt aerosol export onto DML. Usingspatial correlation of chemical ice core data and meteorological reanalysis data (Fig. 2a) itwas possible to show, that sea salt export is modulated by the Antarctic Circumpolar Wave(ACW). Sea level pressure anomalies induced by the ACW create a blocking high pressureridge north-northeast of the EPICA drill site, leading to diversion of circumpolar cyclonesinto the interior of DML and hence to increased sea salt concentration. This interannualvariability exhibits a pronounced periodicity of 4-5 yrs as related to the circumpolarprogression of the ACW over the last 40 yrs. Spectral analysis on the sea salt records derivedfrom the deep DML ice core shows that this cycle (Fig. 2b) has been also prevalent over thelast 2000 years as well as during the time interval 4600-7200 yrs BP (Fischer et al., 2003b).Further efforts have been made in quantifying spatial and temporal effects on aerosoldeposition in the study area (Göktas et al., 2003). The postdepositional loss of acidic aerosoltracers such as HCl and HNO3 from the snow pack has been quantified. In addition marinebiogenic MSA shows an increasing loss with lower snow accumulation obliterating itsinterannual variability (Weller et al., 2003). Nevertheless MSA exhibits pronounced changeson decadal time scales, which together with variations in biogenic sulfate can be related toeither atmospheric transport and/or variations in source strength. Furthermore, we were ableto derive an independent volcano chronology over the last 2000 years using our stratigraphicdating, considerably increasing the dating accuracy of volcanoes in the penultimatemillennium where historic documentation is increasingly sparse and uncertain (Traufetter etal., 2003).4.2 Quantification of changes in the global carbon cycleIn a first step we ran our model in a “biosphere only” mode, driven by atmospheric CO2 andδ13C as well as temperature, sea level and land surface changes from the Last Glacial66Maximum (LGM) to the Holocene to determine which forcing factors are appropriate toexplain observed changes in the biosphere (Köhler and Fischer, 2003). Former forcingstrengths of this type of model on recent climate changes were too large to explainglacial/interglacial variations. In our model the terrestrial carbon stock at the LGM consists ofabout 1600 PgC, 600 PgC less than in preindustrial times, which agrees very well with otherestimates of the changes in biomass from the LGM to the Holocene. During the transition theoceanic release of carbon is in phase with the atmospheric pCO2 record, but four times largerthan the pCO2 increase due to the build-up of the terrestrial stocks.5 Main conclusionsEvaluation of continuous aerosol records shows that the seasonal and interannual variabilityin snow chemistry and Southern Ocean atmosphere dynamics is well archived in DML iceduring the Holocene. For the first time it could be shown, that the ACW is not only aprominent phenomenon of the last decades but was persistent throughout most of theHolocene. The partial loss of MSA and other acidic aerosol species from the snow packcompromises their quantitative interpretation, however, MSA exhibits still a pronounceddecadal variability, which may be related to climatic changes.Calculated changes in the isotopic signature of oceanic δ13C correspond well with data andsuggest not only a dominant role of the biosphere during the stable climate conditions such asLGM and the Holocene (Fischer et al., 2003a), but also a relevant influence on δ13C(atm)during the transition. Further insight is expected from coupled ocean/biosphere/atmospheremodel runs as well as new high-precision records of δ13C to be measured within RESPIC.6 Planned activitiesThe RESPIC project will continue as described in the proposal. The EPICA drilling in DMLwill proceed in austral summer 2003/04 to 2400 m (app. 160,000 yrs BP). The coreprocessing of the DML core will take place in the summer following the respective fieldseason. First results will be presented on the ESF Conference “Polar regions and Quaternaryclimate” to be held in October in San Feliu, Spain, where HF is member of the organizingcommittee as well as solicited speaker. In 2005 the second ESF conference of this series willbe held. In 2004 the PAGES meeting will be held in Bejing, where HF has also been invitedas speaker. Intensified collaboration is envisaged with the carbon modeling group at UniBern, where PK will spend a few months as exchange scientist in 2004.7 Cooperation within DEKLIM and with other programsD. Wolf-Gladrow, AWI; G. Lohmann, Uni Bremen; H. von Storch, GKSS; Antje Schwalb,Uni Braunschweig; M. Wahlen, Scripps Institution of Oceanography, UCSD; EPICA: Aconsortium of institutions from 11 European countries.8 Policy relevance and applicationReliable projections of future changes in the global carbon cycle and climate requires aninside knowledge of the processes which govern such changes. Paleoclimatic reconstructionsrepresent the test bed for such modeling approaches and give clues about the processesinvolved in global change. However, changes in the global carbon cycle overglacial/interglacial timescales have so far eluded a quantitative explanation. Accordingly,RESPIC is expected to make an important contribution to the validation of carbon cyclemodeling and the understanding of the coupling between climate and the atmosphere.The development of high-precision mass spectrometric techniques for very small gas samplesmay result in industrial applications of this new techniques. In addition, this new techniqueopens the door to a multitude of geoscientific and atmospheric studies where most oftensample sizes are strongly limited.67RESPIC related publicationsFischer, H., Wahlen, M., and Smith, J., 2003a, Reconstruction of glacial/interglacial changes in the global carboncycle from CO2 and δ13CO2 in Antarctic ice cores, Memoirs of the National Institute for Polar Research, inpress.Fischer, H., Göktas, F., Oerter, H., Weller, R., and Miller, H., 2003b, Prevalence of the Antarctic CircumpolarWave over the last two millennia in Dronning Maud Land ice, submitted to Nature.Göktas, F., Fischer, H., Oerter, H, Weller, R. Sommer, S., and Miller, H., 2003, A glacio-chemicalcharacterisation of the new EPICA deep drilling site on Amundsenisen, Dronning Maud Land, Antarctica.Annals of Glaciology, 35, 347-354.Köhler, P., and Fischer, H., 2003, Simulating changes in the terrestrial biosphere over glacial/interglacialtimescales, submitted to Global Biogeochemical Cycles.Köhler, P., Fischer, H., Zeebe, R.E., and Wolf-Gladrow, D.A., 2003, A coupled ocean-atmosphere-biospherebox model to investigate glacial/interglacial dynamics of the global carbon cycle, in preparation.Traufetter, F., Oerter, H., Fischer, H., Weller, R., Graf, W., and Miller, H., 2003, Spatio-temporal variability involcanic sulphate deposition over the past 2 kyr in ice cores from Amundsenisen, Dronning Maud Land,Antarctica, submitted to Journal of Glaciology.Weller, R., Traufetter, F., Fischer, H., Oerter, H., and Miller H., 2003, Post depositional losses of methanesulfonate, nitrate, and chloride at the EPICA deep-drilling site in Dronning Maud Land, Antarctica, inpreparation.

Paleoclimatic changes in the global carbon cycle

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