Dummann, Wolf

Flögel, Sascha

Herrle, Jens O.

Hofmann, Peter

Latif, Mojib

Park, Wonsun

Steinig, Sebastian

Wagner, Thomas

OceanRep

Fourth International Conference on Earth System Modelling4ICESM-132, 2017© Author(s) 2017. CC Attribution 3.0 License.Early Cretaceous Climate and South Atlantic opening in the Kiel ClimateModelS. Steinig (1), S. Flögel (1), W. Park (1), M. Latif (1,2), W. Dummann (3), P. Hofmann (3), T. Wagner (4), andJ.O. Herrle (5)(1) GEOMAR Helmholtz Centre for Ocean Research Kiel, Wischhofstr. 1-3, D-24148 Kiel, Germany, (2) Kiel University,Christian-Albrechts-Platz 4, D-24118 Kiel, Germany, (3) Institute of Geology and Mineralogy, University of Cologne,Zülpicher Str. 49a, D-50674 Cologne, Germany, (4) Sir Charles Lyell Centre, School of Energy, Geoscience, Infrastructureand Society, Heriot-Watt University, Edinburgh, EH14 4AS, UK, (5) Institute of Geosciences, Goethe-University Frankfurt,Altenhöferallee 1, D-60438 Frankfurt am Main, GermanyPaleoceanographic data indicate large scale perturbations of the Aptian-Albian (∼125 to ∼110 Myrs) greenhouseclimate associated with severe changes of the marine carbon cycle. At the same time, the ongoing break-up ofGondwana and the related opening of the South Atlantic and Southern Ocean led to the emergence of young oceanbasins, characterised by vast shelf areas and limited circulation. Several studies relate these evolving basins andtheir restricted environments to periods of increased black shale formation and carbon burial with a particular im-portance of the developing South Atlantic.We test the hypothesis that the development and destruction of marine carbon sinks in the South Atlantic is con-trolled by the opening of several key ocean gateways. For this purpose we tigthly combine new geochemical proxydata from several drill sites with a joint physical and biogechemical modelling approach to detect regional changesin carbon sequestration and assess their influence on the global carbon cycle.In a first step we employ the Kiel Climate Model, a coupled atmosphere-ocean-sea ice general circulation model,under Early Aptian (120 Myrs) boundary conditions, including elevated pCO2 levels of 1200 ppm, a reduced solarconstant and changes in the land-sea and vegetation distribution. We investigate the simulated surface climate andlarge-scale dynamics and test the sensitivity of the ocean circulation to different bathymetries of key stages of theSouth Atlantic opening.Due to the large contrast between the applied initial conditions and the Early Cretaceous boundary conditions theadjustments in the global density stratification took nearly 10,000 model years. This was in part caused by a strongvertical salinity gradient induced by large surface freshwater fluxes in the early model years. The gradual build-upof a global overturning circulation only slowly redistributed higher saline waters from deeper levels to the surface,decreasing vertical density gradients. Steady state global mean surface air temperatures are elevated by nearly 10◦C compared to a pre-industrial simulation and reach 23.5 ◦C. The surface warming is mainly radiatively drivenby the higher atmospheric pCO2 levels (∼70% of the warming) and surface albedo changes (∼30% of the warm-ing). Weaker tropical deep convection reduces low-latitude cloud cover compared to the present day and leads to anet cloud-induced small warming in the tropics. The absence of polar ice caps prevents high-latitude atmosphericsubsidence resulting in thicker, low-level clouds that reflect incoming shortwave radiation and consequently coolthe surface. These cloud radiative feedbacks contribute to maintain low and mid-latitude meridional temperaturegradients similar to today. High latitude surface warming is attributable to significantly decreased Antacrtic eleva-tion levels and surface albedo reductions caused by the ice-free polar regions. The simulated Arctic surface climateshows pronounced variability on multi-decadal and multi-centennial time scales associated with periods of drasticreduction in wintertime deep convection and high-latitude heat transport.

Early Cretaceous Climate and South Atlantic opening in the Kiel Climate Model

Early Cretaceous climate and South Atlantic opening in the Kiel Climate Model (AOGCM) S. STEINIG1, S. FLÖGEL1, W. PARK1, W. DUMMANN2, P. HOFMANN2, T. WAGNER3, J. O. HERRLE4 1GEOMAR, Helmholtz Centre for Ocean Research Kiel, Wischhofstrasse 1-3, D-24148 Kiel, Germany 2Insitute of Geology and Mineralogy, University of Cologne, Zülpicher Str. 49a, D-50674 Cologne, Germany 3Sir Charles Lyell Centre, School of Energy, Geoscience, Infrastructure and Society, Heriot-Watt University, Edinburgh, EH14 4AS, UK 4Institute of Geosciences, Goethe-University Frankfurt, Altenhöferallee 1, D-60438 Frankfurt am Main, Germany   Paleoceanographic data for the Early Cretaceous (i.e. Aptian-Albian) greenhouse show large scale perturbations of the global ocean-climate system linked to severe changes in the marine carbon cycle (e.g. Jenkyns, 2010). At the same time the ongoing break-up of Gondwana and the associated opening of the South Atlantic and Southern Ocean led to the emergence of young ocean basins, characterised by vast shelf areas and limited circulation. Several studies relate these evolving basins and their restricted environments to periods of increased black shale formation and carbon burial (Trabucho-Alexandre et al., 2012; McAnena et al., 2013). Model results underline the importance of the opening South Atlantic, indicating that up to 16% of global carbon burial was deposited in a region covering only 1% of the global ocean surface (McAnena et al., 2013). Within this project we target the question whether carbon burial in the Cretaceous South Atlantic influenced or even triggered global climate perturbations. In order to asses the timing and magnitude of carbon burial we use a combined approach of physical and biogeochemical modelling which is constrained by a set of new multiproxy geochemical data provided by project partners at the University of Cologne (see abstract of Dummann et al.).  Oceanic gateways, such as the Falkland and Walvis Ridge Gateways in the Cretaceous South Atlantic, excert an important influence on carbon burial in young ocean basins. This is because progressive gateway opening enhances supply and recycling of nutrients to and within the ocean basin and the exchange of water masses across the gateways, thereby controlling the production and preservation of organic matter across the basin.  In order to simulate the evolution of the Falkland Gateway we will use the “Kiel Climate Model” (KCM), a coupled Atmosphere-Ocean General Circulation Model, with different sets of South Atlantic geometry and bathymetry to assess their influence on the oceanic circulation. A box model is then used to generate a transient response of key biogeochemical cycles from the KCM time slices and allow for quantification of carbon burial in the basin at different phases of the opening. In a first step we use the KCM under Early Cretaecous boundary conditions, representing the Aptian period including a modified geography, a reduced solar constant and a pCO2 of 1200 ppm (Hong & Lee, 2012). First results for the open South Atlantic set-up show a global mean surface temperature of 23.4 °C, an enhanced hydrological cycle and a significantly stronger horizontal atmospheric circulation. Driven by these atmospheric fields, an intensified oceanic surface circulation in the tropical and subtropical region developed that supported high surface temperatures and low salinities, and a stable stratification of the shallow water column, preventing any deep convection. Modelling suggests that the global meridional overturning circulation was restricted to the upper 2000 m.    References: Hong, S. K. and Lee, Y. I. (2012), Evaluation of atmospheric carbon dioxide concentrations during the Cretaceous, Earth and Planetary Science Letters, Volumes 327–328, pp. 23-28, ISSN 0012-821X Jenkyns, H. C. (2010), Geochemistry of oceanic anoxic events, Geochem. Geophys. Geosyst., 11, Q03004 McAnena, A., Flögel, S., Hofmann, P., Herrle, J., Griesand, A., Pross, J., Talbot, H., Rethemeyer, J., Wallmann, K. and Wagner, T. (2013): Atlantic cooling associated with a marine biotic crisis during the mid-Cretaceous period, Nature Geoscience, Vol. 6(7), pp. 558-561 Trabucho-Alexandre, J., Hay, W. and De Boer, P. (2012): Phanerozoic environments of black shale deposition and the Wilson Cycle, Solid Earth, Vol. 3(1), pp. 29-42                 

Early Cretaceous climate and South Atlantic opening in the Kiel Climate Model

http://oceanrep.geomar.de/45272/1/IODP_Kolloquium_Heidelberg_2016_poster_abstract_ssteinig.pdf

Early Cretaceous climate and South Atlantic opening in the Kiel Climate Model

Abstract

Similar works

Full text

Available Versions

OceanRep

OceanRep