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  • 1
    Publication Date: 2014-05-05
    Description: Atmospheric carbon dioxide (CO2) during the last deglaciation (∼18–10 kyr BP) switched around 14.6 kyr BP from a rather gradual rise to an abrupt jump, which is recorded in ice cores as an increase of 10 ppmv in less than two centuries. So far the source of that CO2 excursion could not be identified and the climatic implications are largely unknown. Here we use highly resolved U/Th dated atmospheric ∆14C from Tahiti corals as independent age control for CO2 changes. This provides a temporal framework to show that the northern high latitude warming into the Bølling/Allerød occurred quasi-synchronous to this CO2 rise within a few decades. Furthermore we show that an abrupt release (within two centuries) of long-term immobile nearly 14C-free carbon (∼125 PgC) from thawing permafrost might explain the observed anomalies in atmospheric CO2 and ∆14C, in line with CH4 and biomarker records from ice and sediment cores. In transient climate simulations we show that the abrupt carbon release in the northern high latitudes and associated CO2 changes bear the potential to modulate Antarctic temperature. These findings are in agreement with the observed onset of the Antarctic Cold Reversal about two centuries after the beginning of the Bølling/Allerød, as detected in independent annual layer-counted ice cores from both hemispheres. Based on the timing, magnitude, origin and the inter-hemispheric impact we speculate that this abrupt deglacial release of long-term stored carbon via thawing permafrost might have provided the final push out of the last ice age.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 2
    Publication Date: 2014-06-23
    Description: State-of-the-art general circulation models (GCMs) are tested and challenged by the ability to reproduce paleoclimate key intervals. In order to account for climate changes associated with soil dynamics we have developed a soil scheme, which is asynchronously coupled to a state-of-the-art atmosphere ocean GCM with dynamic vegetation. We test the scheme for conditions representative of a warmer (mid-Holocene, 6 kyr before present, BP) and colder (Last Glacial Maximum, 21 kyr BP) than pre-industrial climate. The computed change of physical soil properties (i.e. albedo, water storage capacity, and soil texture) for these different climates leads to amplified global climate anomalies. Especially regions like the transition zone of desert/savannah and taiga/tundra, exhibit an increased response as a result of the modified soil treatment. In comparison to earlier studies, the inclusion of the soil feedback pushes our model simulations towards the warmer end in the range of mid-Holocene studies and beyond current estimates of global cooling during the Last Glacial Maximum based on PMIP2 (Paleoclimate Modelling Intercomparison Project 2) studies. The main impact of the interactive soil scheme on the climate response is governed by positive feedbacks, including dynamics of vegetation, snow, sea ice, local water recycling, which might amplify forcing factors ranging from orbital to tectonic timescales.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 3
    Publication Date: 2014-06-23
    Description: The Greenland-Scotland Ridge (GSR) is a crucial hydrographic barrier for the exchange of water masses between the Polar Seas and the North Atlantic Ocean. Through the Miocene (5-23 Myrs; Myrs=million years ago), the Greenland-Scotland Ridge deepened at 18 Myrs and 15.5 Myrs, and again at 12.5 Myrs by changes of the Icelandic mantle plume activity, which has direct consequences for the evolution of Northern Component Water. In a sensitivity study, we investigate the effect of GSR depth variations with a global atmosphere-ocean-vegetation General Circulation Model. Oceanic characteristics of the quasi-enclosed Nordic Seas and Arctic Ocean are analyzed, as well as the critical depth threshold for the evolution of the North Atlantic Current and the East Greenland Current is examined and linked to changes in global ocean circulation.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 4
    Publication Date: 2014-09-04
    Description: During glacial periods of the Late Pleistocene, an abundance of proxy data demonstrates the existence of large and repeated millennial-scale warming episodes, known as Dansgaard–Oeschger (DO) events1. This ubiquitous feature of rapid glacial climate change can be extended back as far as 800,000 years before present (BP) in the ice core record2, and has drawn broad attention within the science and policy-making communities alike3. Many studies have been dedicated to investigating the underlying causes of these changes, but no coherent mechanism has yet been identified3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15. Here we show, by using a comprehensive fully coupled model16, that gradual changes in the height of the Northern Hemisphere ice sheets (NHISs) can alter the coupled atmosphere–ocean system and cause rapid glacial climate shifts closely resembling DO events. The simulated global climate responses—including abrupt warming in the North Atlantic, a northward shift of the tropical rainbelts, and Southern Hemisphere cooling related to the bipolar seesaw—are generally consistent with empirical evidence1, 3, 17. As a result of the coexistence of two glacial ocean circulation states at intermediate heights of the ice sheets, minor changes in the height of the NHISs and the amount of atmospheric CO2 can trigger the rapid climate transitions via a local positive atmosphere–ocean–sea-ice feedback in the North Atlantic. Our results, although based on a single model, thus provide a coherent concept for understanding the recorded millennial-scale variability and abrupt climate changes in the coupled atmosphere–ocean system, as well as their linkages to the volume of the intermediate ice sheets during glacials.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 5
    Publication Date: 2014-12-13
    Description: The subsidence history of the Greenland-Scotland Ridge (GSR) from subaerial to current ocean depths has been investigated by several studies, and the initial hydrographic opening of the North Polar Seas, “NPS” (Arctic Ocean, Nordic Seas) has been linked to major reorganizations of the global oceans and climate throughout the Neogene (23‒3 Myrs ago). However, the current understanding of the GSR subsidence affecting the hydrographic evolution of the Greenland Scotland Seaway and of the critical GSR depth providing effective water mass exchange between the oceans are largely based on conceptual models. Here, we emulate the GSR subsidence by means of a fully coupled ocean-atmosphere General Circulation Model (GCM) with integrated terrestrial vegetation dynamics (community of earth system models, COSMOS). The model setup comprise a global reconstruction of the mid-Miocene 20‒15 Myrs ago (continental geography, orography, bathymetry, ice-sheet geography and topography) and a change of CO2 levels in the atmosphere. Especially, we additionally integrated a high resolution bathymetric dataset for the area of interest (northern North Atlantic, GSR, Nordic Seas and the Eurasian Basin). In different experiments we deepen GSR depth levels by increments of 100 m, ranging from a quasi-enclosed North Polar basin to an open gateway configuration. We identify thresholds in hydrographic communication across the seaway and discuss consequences in climate change and ocean characteristics. Secondly, we use the model setup close to the allocated depth threshold and test the models sensitivity by changes of greenhouse gas concentrations within the spectrum of CO2 reconstructions. We find that a shift in the modeled climate by CO2 changes directly impact the exchange of water masses across the GSR. Based on our model results, we provide a mechanism on the hydrographic opening of the NPS by controls of tectonic activity and CO2.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 6
    Publication Date: 2015-12-09
    Description: During the past two decades, several atmospheric and oceanic general circulation models (GCMs) have been enhanced by the capability to explicitly simulate the hydrological cycle of the two stable water isotopes H218O and HDO. They have provided a wealth of understanding regarding changes of the water isotope signals in various archives under different past climate conditions. However, so far the number of fully coupled atmosphere-ocean GCMs with explicit water isotope diagnostics is very limited. Such coupled models are required for a more comprehensive simulation of both past climates as well as related isotope changes in the Earth’s hydrological cycle. Here, we report results of idealized North Atlantic freshwater hosing experiments performed with the Earth system model ECHAM5-JSBACH/MPIMOM. Both H218O and HDO and their relevant fractionation processes are included in all compartments and branches of the water cycle within this model. An idealized freshwater hosing experiment has been performed starting from both pre-industrial (PI) and Last Glacial Maximum (LGM) background conditions. Characteristics of the hosing experiment (duration: 150 yrs, amount: 0.2 Sv, O-18 composition: -30‰) have been chosen in accordance with previous modelling studies and available paleodata. Simulation results reveal a maximum isotopic enrichment of down to -6‰ in ocean surface waters at the end of the hosing experiment and a full recovering to the surface background state after a few centuries, as well as much longer response times in the deeper ocean. Over terrestrial surfaces, the fresh water hosing results in spatially varying isotope depletion in precipitation between -5‰ and +3‰ in agreement with data from various isotope records and previous modelling studies. In further model analyses we investigate how the relation between water isotopes and key climate variables, e.g. land and ocean surface temperatures, precipitation amounts, and oceanic salinity, might has changed for different regions of the Earth due to assumed intermittent North Atlantic fresh water input.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 7
    Publication Date: 2015-12-22
    Description: Evidence from the joint interpretation of proxy data as well as geodynamical and biogeochemical modeling results point to complex interactions between sea level drawdown, volcanic degassing, and atmospheric CO2 that hampered the climate system’s decent into the last ice age. Ice core data shows that atmospheric CO2 dropped abruptly into glacial Marine Isotope Stage (MIS) 4 at ~71 ka, while Antarctic temperatures display a more gradual decline between ~85 ka to ~71 ka across the MIS 5/4 transition. Based on 2D and 3D geodynamical simulations, we show that a ~60-100 m sea level drop associated with the MIS 5/4 transition led to a significant increase in magma and possibly CO2 flux at mid-ocean ridges (MOR) and oceanic hotspot volcanoes. The MOR signal is assessed with 2D thermomechanical models that account for mantle melting and resolve the flux of incompatible carbon dioxide. These models have been run at different spreading rates and integrated with the global distribution of opening rates to compute global variations in magma and CO2 flux across the MIS 5/4 transition. 3D plume models have been used to quantify the impact of a dropping sea level on oceanic hotspot melting and CO2 release. Here a wide range of simulations with differing plume fluxes, lithospheric thicknesses as well as speeds, and plume excess temperatures have been integrated with data from ~40 hotspots in order to compute a global signal. Biogeochemical carbon cycle modeling shows that the predicted increase in volcanic emissions is likely to have raised atmospheric CO2 by up to 15 ppmv, sufficient to explain the bulk of the decoupling between temperature and atmospheric CO2 during the global change to pronounced glacial conditions across the MIS 5/4 transition.
    Repository Name: EPIC Alfred Wegener Institut
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  • 8
    Publication Date: 2015-10-13
    Description: Changes in high latitude ocean gateways and atmospheric CO2 are thought to be main drivers of Cenozoic climate evolution during the last 65 million years. However as yet, especially the link between climate changes and the opening history of the North Polar Seas via the subsidence of the Greenland-Scotland Ridge is poorly understood. Here we use a coupled ocean–atmosphere general circulation model for early Miocene boundary conditions to reveal a threshold behaviour for the ventilation of the North Polar Seas controlled by the Greenland-Scotland Ridge subsidence. Our model simulations show that a deepening of the ridge from 200 to 300 meters below sea-level induces major reorganizations in North Atlantic-Arctic Ocean circulation with an abrupt regime shift from restricted estuarine conditions to a bi-directional flow regime similar to today. Close to critical gateway depths, additional scenarios with different atmospheric CO2 concentrations indicate that realistic Oligocene-Miocene CO2 changes actively modulate the transition between the two circulation regimes via the impact of the atmospheric hydrological cycle. Taking uncertainties in timing into account this suggest that tectonic changes starting at ~33-30 Myrs controlled the circulation of the Nordic Seas. Thereafter superposed changes in CO2 delayed an abrupt transition to a modern prototype North Atlantic-Arctic exchange by millions of years until CO2-levels finally dropped to preindustrial levels at ~25-24 Myrs. This concept and the associated mechanism bridges tectonic processes with much shorter time-scales in the coupled atmosphere-ocean system that differ by three orders of magnitude, which provides an unanticipated new perspective on abrupt climate changes during the Cenozoic era.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Conference , NonPeerReviewed
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  • 9
    Publication Date: 2017-02-08
    Description: Reduced surface–deep ocean exchange and enhanced nutrient consumption by phytoplankton in the Southern Ocean have been linked to lower glacial atmospheric CO2. However, identification of the biological and physical conditions involved and the related processes remains incomplete. Here we specify Southern Ocean surface–subsurface contrasts using a new tool, the combined oxygen and silicon isotope measurement of diatom and radiolarian opal, in combination with numerical simulations. Our data do not indicate a permanent glacial halocline related to melt water from icebergs. Corroborated by numerical simulations, we find that glacial surface stratification was variable and linked to seasonal sea-ice changes. During glacial spring–summer, the mixed layer was relatively shallow, while deeper mixing occurred during fall–winter, allowing for surface-ocean refueling with nutrients from the deep reservoir, which was potentially richer in nutrients than today. This generated specific carbon and opal export regimes turning the glacial seasonal sea-ice zone into a carbon sink.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 10
    Publication Date: 2015-11-04
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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