Schulz, Michael
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Schulz, Michael
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Schulz, Michael
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Item type:Publication, On the stability of the Atlantic Meridional Overturning Circulation and its climate impact during Late Neogene(2014-09-30); ; ; This thesis focused on the Atlantic Meridional Overturning Circulation (AMOC) stability and global climate responses of an altered AMOC in the late Neogene by performing freshwater hosing/extraction computer simulations, using both intermediate complexity climate models and state-of-the-art coupled climate models. Two time slices were studied: 1) 38 kya (kilo years ago), during Heinrich event 4, and 2) around 4-5 million years ago, when the Panamanian Seaway finally gradually closed. Chapter 2 demonstrated a very sensible 38 kya global climate as response to external freshwater forcing in the North Atlantic and the AMOC is in a monostable mode during cold stadials. What is more, a threshold behaviour was observed in both Greenland temperature and Northern Hemisphere ice cover with regard to the AMOC maximum in the simulations, which indicates a system transition between a mild interstadial state and a cold stadial state. The global imprint of the AMOC change was also investigated in detail. Applying the Analysis of Variances (ANOVA) statistical method, several locations with high sensitivity to the AMOC strength change were captured at the surface, e.g. the Southern Indian Ocean and the North Atlantic. It was estimated the AMOC experienced a deduction of 10.2±1.8 Sv of its circulation strength from warm interstadials to cold stadials and this decrease in vigour was stronger from interstadial states to Heinrich stadials, with a larger estimated uncertainty (Chapter 3). By model inter-comparison, chapter 4 emphasized that the closure of the Panamanian Seaway could lead to an enhancement of the AMOC and also causes a shoaling of the equatorial Pacific thermocline. The change in the thermocline depth in the Equator is then preconditioned to the present Pacific cold tongue state. Wind stress feedback was able to amplify this process.doctoral thesis434 122 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Atlantic Meridional Overturning Circulation during the Last Glacial and Deglacial: Inferences from the Atlantic Tropical Thermocline Temperature and Seawater Radiocarbon Activity(2013-05-24); ; ; This thesis aims to investigate variations of the last glacial and deglacial Atlantic Ocean circulation, with emphasize on the Atlantic deepwater ventilation rate during the Last Glacial Maximum (LGM). The highlights of the thesis are as the followings: (1) the modern frontal zone between the North Atlantic and the South Atlantic central waters off NW Africa shifted to the south during the last glacial and deglacial time periods. (2) The radiocarbon-based circulation age of the deep Atlantic (> 1500 m) was estimated to be less than 400 years during the LGM, which was equal to or less than its pre-bomb value. Therefore, the LGM deep Atlantic was inferred to be well ventilated. (3) The abyssal Atlantic (below 2500 m) prior to the LGM was more depleted in radiocarbon activities by 400 per mil than the contemporary atmosphere. A significant change in the structure of the carbon cycle could have importantly contributed to extremely high atmospheric radiocarbon activities prior to the LGM.doctoral thesis591 187 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Planktonic foraminifera as signal carriers in paleoceanography: from modern biology to proxy application in the western South AtlanticProxies derived from planktonic foraminifera shell chemistry and assemblage composition allowed to assess past changes in surface hydrography. However, the usefulness of these planktonic foraminifera proxies relies on the understanding of their biology and species-specific environmental preferences. Despite the importance of these evaluations regarding foraminiferal biology, informations about the species reproductive cycle, calcification depth and seasonal fluxes still not well constrained. Therefore, this thesis focuses on the assessment of planktonic foraminiferal biology (e.g. reproduction and calcification depth) and on paleoceanographic application of proxies derived from these organisms in marine archives located at the western South Atlantic, which is a key region due to the presence of boundary currents that transport heat to high latitudes.doctoral thesis501 342 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Ice Sheet evolution in Patagonia and Northern Europe during the last ice age - Insights from numerical modellingDuring the Last Glacial Maximum (LGM, ~23–19 ka), large ice sheets covered much of the Northern Hemisphere and southern South America. Geochronological data indicate contrasting behaviours between the Patagonian Ice Sheet (PIS) and the Eurasian Ice Sheet Complex (EISC): the PIS reached its maximum during Marine Isotope Stage (MIS) 3, while the EISC expanded to its maximum during the LGM. This dissertation employs numerical ice sheet modelling (SICOPOLIS) and surface mass balance (SMB) analysis to explore the climatic conditions influencing these ice sheets across the last glacial cycle. Results show the PIS advanced during MIS4 and late MIS3–MIS2, driven by variations in integrated summer energy at mid-latitudes, with millennial-scale fluctuations linked to Dansgaard-Oeschger events. The study reveals that climate model limitations, such as low resolution and oversimplified topography, lead to inaccurate PIS simulations, often misrepresenting ice growth patterns. Offshore SST records from the southeastern Pacific more accurately capture the MIS3 expansion than Antarctic core-derived indices. For the EISC, SMB simulations highlight the dominant role of summer temperature in shaping maximum extent. Regional analysis shows strong correlations between SMB in different ice sheet sectors and climate conditions over the Greenland-Iceland-Norwegian and Labrador Seas. These findings underscore the need for high-resolution climate models and robust proxy integration to refine reconstructions of past ice sheet dynamics.doctoral thesis125 70 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Modeling insights into arctic climate and cryospheric characteristics during past interglacials(2024-09-20); ; ; ; Ongoing anthropogenic climate change is accelerating the loss of both land ice from the Greenland ice sheet (GrIS) and sea ice in the Arctic, underscoring the urgency to improve our understanding of the mechanisms threatening these critical climatic systems. Utilizing a modeling-centric approach, this thesis examines the past warm interglacials Marine Isotope Stage 5e (MIS-5e; ca. 130-115 ka) and MIS-11c (ca. 430-395 ka). MIS-11c in particular is regarded as enigmatic, with very warm boreal summer temperatures for a long period suggested by proxy records despite relatively weak solar forcing. However, time-slice simulations of this period suggest that orbital forcing conditions actually reduce atmospheric heat fluxes over Greenland and act as a negative feedback to further warming. Climate forcing data from these MIS-11 simulations were then used to conduct an ensemble of GrIS simulations using an ice model with a focus on identifying key uncertainties. Constraining these results using ice-core data suggests that the GrIS conservatively contributed at least 3.2 to 4.6 m of sea-level rise during MIS-11. Moreover, the lapse-rate methodology that was applied for correcting surface temperatures exhibited the greatest influence on these results, and a data-based spatially and temporally varying lapse rate performs best. Finally, the behavior of summer Arctic sea ice was examined in both MIS-5e and MIS-11c, with ice-free conditions only being achieved in early MIS-5 (127 ka). Summer ice levels exhibited a strong inverse relationship to Integrated Summer Energy in both interglacials, and interannual variability of ice bore a strong relationship to the mean-state area. Specifically, variability maximized for intermediate ice-areas, with both high- and low-ice states exhibiting limited variability. This suggests that the present century will see increasingly variable summer ice pack as mean-state ice levels approach those seen in the “intermediate” time-slice simulations.doctoral thesis155 115 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Modeling the Spatial and Temporal Variability of Marine Radiocarbon Reservoir Ages in the Late Quaternary(2008-03-18); ; ; In paleoceanography and paleoclimatology radiocarbon is frequently used to date carbonaceous samples of the past 50,000 years and as a proxy to reconstruct circulation states of the ocean. Temporal variations of the atmospheric radiocarbon concentration, caused by a varying radiocarbon production rate or changes in the global carbon cycle, lead to dating uncertainties and also limit the usability of radiocarbon as an oceanographic proxy. In order to analyze and correct the influence of varying atmospheric radiocarbon on the ocean, radiocarbon has been included as a tracer to a global climate model (University of Victoria Earth System Climate Model). This model was forced by the reconstructed atmospheric radiocarbon production rate, to predict differences between atmospheric and marine radiocarbon age, the so-called reservoir age, spatially and temporally.doctoral thesis505 142 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Antarctic ice-sheet expansions in the Middle Miocene and Pliocene(2008-12-12); ; ; The ice sheet-climate model developed in this study describes the Antarctic ice sheet. It is forced by energy and mass balances and includes oxygen isotopes as a passive tracer.Simulating the Middle Miocene climate transition showed that a decrease in atmospheric CO2, crossing a threshold of about 400 ppm, followed by a minimum in summer insolation, initiated a large-scale ice-sheet expansion.Experiments further confirm the validity of the often applied ratio of a 1 permille increase in oxygen-isotope composition for a global sea-level lowering of 100 m. For the mid-Pliocene, the ice-sheet component was forced by temperatures and accumulation rates from a comprehensive climate model. The closure of the Panamanian gateway in the mid-Pliocene was found to induce an intensification of the meridional circulation with a cooling over Antarctica as result. In turn, this cooling forced the Antarctic ice-sheet to expand. From the examples discussed in this study it can be concluded that changes in temperature, due to atmospheric CO2 as well as due to tectonic forcing, have a large impact on the extent of the Antarctic ice sheet.doctoral thesis560 131 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Assessing dissolution in the tests of planktonic foraminifera using computed tomography (CT): potential for improving paleoceanographic reconstructions(2010-11-15); ; ; An empirical dissolution index for planktonic foraminifera, XDX, was established based on the appearance of tests in computed tomography (CT) scans (Chapter 4). Calibrations between XDX and Delta[CO32-], Delta-mass (mass lost to dissolution) and Delta Mg/Ca (reduction in Mg/Ca due to dissolution) allow estimates of deep ocean Delta[CO32-] and also dissolution-corrected test mass and Mg/Ca (Chapter 5). Delta[CO32-] reconstructed for the deep (4,157 m) tropical western Indian Ocean (WIND28K) increased by ~25 µmol/kg over Termination I and ~15 µmol/kg over Termination II while dissolution maxima coincided with transitions to colder stages (Chapter 6). XDX-corrected Mg/Ca-derived SSTs were coolest during MIS 3. Minimum test mass in the corrected record coincided with the lowest temperatures. Comparison of two methods of cleaning foraminiferal tests from a depth transect (Chapter 7) found no offset in Mg/Ca between methods for G. sacculifer and P. obliquiloculata. Mg/Ca was decreased by reductive cleaning by ~4% for G. ruber and ~10% for N. dutertrei. Offset in Mg/Ca between the two methods was insensitive to preservation state.doctoral thesis725 183 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Using Lipid Biomarkers and their Isotopic Composition to reconstruct the Late Pleistocene Paleoclimate of the Amazon Basin(2017-03-13); ; ; This thesis deals with the reconstruction of the late Pleistocene climate and vegetation history of the Amazon basin and is made up of three manuscripts: In the first manuscript biomarker based paleosalinity proxies are tested. It is shown that the stable hydrogen isotope composition of palmitic acid reflects the salinity conditions in the Amazon Outflow Plume, while the stable hydrogen isotope composition of alkenones is influenced by factors other than salinity. The second manuscript deals with the transport of long-chain n-alkanes in the Amazon River and its main tributaries. It is demonstrated that long-chain n-alkanes mainly originate from the lowland basin, contrasting the dominant inorganic sediment source in the Andes. The third manuscript deals with the application of plant wax proxies in a marine sediment core covering the late Pleistocene. It is shown that forest vegetation persisted through drier conditions during the last glacial maximum, while minor savanna expansion occurred during Heinrich Stadials.doctoral thesis370 176 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Global assessment of species-specific habitats of planktonic foraminifera : an ecosystem modeling approach(2017-12-19); ; ; Over the last few million years, the Eartha s climate system has changed continuously on decadal to millennial time scales. Past climate conditions have been reconstructed based on fossil evidence of marine microorganisms, such as planktonic foraminifera. Planktonic foraminifera exhibit species-specific seasonal production patterns and different preferred depth habitats. To precisely reconstruct past climate conditions these spatial and temporal variations within the individual species distribution have to be considered. In this regard, an ecosystem modeling approach can help to gain a better knowledge about species-specific habitat shifts under climate change. In this study, a planktonic foraminifera model is used to predict monthly concentrations of the colder-water species Neogloboquadrina pachyderma, Neogloboquadrina incompta, and Globigerina bulloides, and of the warm-water species Globigerinoides ruber (white) and Trilobatus sacculifer throughout the world ocean. In particular, the seasonal distribution of the polar species N. pachyderma in the surface mixed layer of the North Atlantic Ocean during the last glacial period was investigated. In response to changes in the sea ice cover and food supply, the peak timing of N. pachyderma is shifted from the last glacial period to modern conditions by several months. However, for a more realistic simulation of species-specific habitats, the planktonic foraminifera model PLAFOM was adapted to allow for resolving the vertical dimension. This new model version estimates the foraminiferal biomass of the colder- and warm-water species as a function of temperature, nutrition, competition, and in particular light. To predict the species concentration over different water depths the model code of the improved version of the planktonic foraminifera model was added to the code trunk of the ocean component of a global earth system model. This model produces seasonally and vertically coherent distribution patterns that are in good agreement with available observations without any explicit parameterization in the vertical dimension regarding their ontogeny. The colder-water species exhibit a seasonal cycle in their depth habitat in the polar and subpolar regions: during the warm season they occur at mid-depth, while during the cold season they ascend through the water column and are found in the near-surface layer. The warm-water species show a less variable depth habitat and occur almost consistently close to the sea surface throughout the year in the tropics and subtropics. This emergence of species-specific depth habitats in the model that are consistent with available observations indicates that the population dynamics of planktonic foraminifera species may be driven by the same factors. Here the impact of global warming on the speciesa spatial and seasonal distribution patterns has been investigated. In response to changes in the temperature and food supply, the habitat range as well as the peak timing of both the colder-water and warm-water species will likely shift. In general, planktonic foraminifera do not respond uniformly to climate change due to their different ecological preferences. Their habitat is altered in time and space, and depending on the ambient conditions either warm-water or colder-water species benefit strongly from these changes. Knowing how individual planktonic foraminifera species adapt to changing environmental conditions can help to obtain more precise estimates of the geological past and can provide implications for future climate change.doctoral thesis730 248