1,721,233 research outputs found
Northern hemispheric land ice distribution outside of Greenland during the Quaternary
No full textFrom the combination of orbital theory with benthic δ18O it has been suggested which obliquity cycles led to interglacials during the Quaternary. Here, we use a model-based deconvolution of this benthic δ18O record to calculate northern hemispheric land ice outside of Greenland, from which an alternative distribution of glacial and interglacial periods is defined.
Model output is compared with independent reconstructions of δ18O_seawater, sea level and atmospheric CO2 concentrations.
All data plotted in the figures of the article:
Köhler, P. & van de Wal, R. S. W. Interglacials of the Quaternary defined by northern hemispheric land ice distribution outside of Greenland, Nature Communications, 2020, 11, 5124, doi:10.1038/s41467-020-18897-5.
are compiled here
MIS 5e relative sea-level changes in the Mediterranean Sea: Contribution of isostatic disequilibrium
Full text linkSea-level indicators dated to the Last Interglacial, or Marine Isotope Stage (MIS) 5e, have a twofold value. First, they can be used to constrain the melting of Greenland and Antarctic Ice Sheets in response to global warming scenarios. Second, they can be used to calculate the vertical crustal rates at active margins. For both applications, the contribution of glacio- and hydro-isostatic adjustment (GIA) to vertical displacement of sea-level indicators must be calculated. In this paper, we re-assess MIS 5e sea-level indicators at 11 Mediterranean sites that have been generally considered tectonically stable or affected by mild tectonics. These are found within a range of elevations of 2–10 m above modern mean sea level. Four sites are characterized by two separate sea-level stands, which suggest a two-step sea-level highstand during MIS 5e. Comparing field data with numerical modeling we show that (i) GIA is an important contributor to the spatial and temporal variability of the sea-level highstand during MIS 5e, (ii) the isostatic imbalance from the melting of the MIS 6 ice sheet can produce a >2.0 m sea-level highstand, and (iii) a two-step melting phase for the Greenland and Antarctic Ice Sheets reduces the differences between observations and predictions. Our results show that assumptions of tectonic stability on the basis of the MIS 5e records carry intrinsically large uncertainties, stemming either from uncertainties in field data and GIA models. The latter are propagated to either Holocene or Pleistocene sea-level reconstructions if tectonic rates are considered linear through time
Idealised steady-state and transient simulations of Miocene Antarctic ice-sheet variability using 3D thermodynamical ice-sheet model IMAU-ICE
No full textWe present results from simulations of the Miocene Antarctic ice sheet, that were performed using the 3D thermodynamical ice-sheet model IMAU-ICE (v1.1.1-MIO). Five steady-state present-day simulations were conducted for reference (PI_ref), and 12 experiments using Miocene settings. Each Miocene experiment comprises 11 steady-state and 4 transient simulations. In the README file, the experiments and simulations are listed.
IMAU-ICE was run using a 40x40km grid covering the Antarctic continent. Initial conditions were obtained from reconstructions of the Antarctic bathymetry and bedrock topography pertaining to 23 to 24 million years (Myr) ago (dataset doi:10.1594/PANGAEA.923109). The simulations were forced by climate input data obtained from GENESIS simulations with varying CO2 levels (280 to 840 ppm) and Antarctic ice sheet cover (no ice to a large East-Antarctic ice sheet), and with present-day insolation. We utilized a matrix interpolation method to construct the time-varying climate forcing, based on the prescribed CO2 levels and ice cover simulated by IMAU-ICE.
For each simulation, we provide the run script, 1D output variables including CO2 level and the sea level contribution of the Antarctic ice sheet, and 3D output variables including ice thickness, bedrock and surface height, surface mass balance, basal mass balance, ice velocities, and ice temperatures. For more information, please contact L.B. Stap at [email protected]
A reconstruction of temperature, ice volume and atmospheric CO2 over the past 40 million years
No full textKnowledge on past climate change largely emerges from sediment records drilled from the ocean floor and ice-core records from the Antarctic and Greenland ice sheets. From these records proxy data is obtained indicating changes in, for example, temperature, sea level and greenhouse gas concentrations. A key parameter that emerges from the sediment records is the oxygen isotope ratio, δ18O, from fossilised benthic foraminiferal shells. The benthic δ18O data serves as a proxy for changes in local deep-water temperatures and global ice volume. With an innovative modelling approach surface-air temperatures relative to present day are derived from changes in benthic δ18O. This temperature anomaly is used to derive the two contributions to the benthic δ18O data with ice-sheet models and a simple deep-water temperature coupling function. With multiple 1-D ice-sheet models a continuous record of temperature and sea level over the past 40 million years has been derived from the observed benthic δ18O. The obtained data is shown to compare well with observations and other modelling results. It is shown that both the relation of ice volume with temperature and ice volume with sea water δ18O are highly variable and not constant through time. Therefore the use of a global coverage of ice volume changes is important for interpreting the benthic δ18O records in a transient mode. Using the temperature reconstruction from the 1-D ice-sheet models, a variety of different CO2 proxies are tested for their coherence with the observed ice-core CO2 data. The most coherent records are selected to derive a self-consistent and continuous CO2 record over the past 20 million years. Moreover, the long-term climate sensitivity of temperature to CO2 changes is derived, which includes radiative forcing of CO2, short and fast feedbacks and a correction for other greenhouse gasses. The large sensitivity derived here implies that subtle changes in atmospheric CO2 could be related to the Mid-Pleistocene Transition and the initiation of NH glaciation. With comprehensive 3-D ice-sheet models a more in-depth analysis has been performed on ice volume changes over the past million years. For the first time the separate contributions of these four ice sheets have been calculated explicitly over this time interval. With respect to eustatic sea level, the large NH ice sheets are naturally responsible for the largest variability during the Plio-Pleistocene. Coherently, these ice sheets also provide the largest contribution to sea water δ18O. The Antarctic and Greenland ice sheets contribute about 10 % to changes in eustatic sea level. For sea water δ18O the contribution is even larger, between 10 and 20 %, mainly due to the relatively much lower δ18Oice values of the Antarctic ice sheet. With a simple linear forcing of temperature and/or sea level it is shown that ice volume on Antarctica is increased due to lowering of sea level, which is enhanced by decreasing temperatures. On the contrary, ice growth on Eurasia and North America starts when temperatures drop, due to a positive surface mass balance which is enhanced by lowering of sea level
A simulation of CO2, benthic δ¹⁸O, sea level and global temperature over the past 5 million years
No full textBerends_etal_2018_GMD_code
No full text<p>Source code of the ANICE2.1 ice-sheet-shelf model. For more information, please contact the main author at [email protected].</p>
Thousand years of winter surface air temperature variations in Svalbard and northern Norway reconstructed from ice-core data
No full textTwo isotopic ice core records from western Svalbard are calibrated to reconstruct more than 1000 years of past winter surface air temperature variations in Longyearbyen, Svalbard, and Vardø, northern Norway. Analysis of the derived reconstructions suggests that the climate evolution of the last millennium in these study areas comprises three major sub-periods. The cooling stage in Svalbard (ca. 800-1800) is characterized by a progressive winter cooling of approximately 0.9°C century-1 (0.3°C century-1 for Vardø) and a lack of distinct signs of abrupt climate transitions. This makes it difficult to associate the onset of the Little Ice Age in Svalbard with any particular time period. During the 1800s, which according to our results was the coldest century in Svalbard, the winter cooling associated with the Little Ice Age was on the order of 4°C (1.3°C for Vardø) compared to the 1900s. The rapid warming that commenced at the beginning of the 20th century was accompanied by a parallel decline in sea-ice extent in the study area. However, both the reconstructed winter temperatures as well as indirect indicators of summer temperatures suggest the Medieval period before the 1200s was at least as warm as at the end of the 1990s in Svalbard
A study of the dark region in the western ablation zone of the Greenland ice sheet
No full textThe western ablation zone of the Greenland ice sheet contains a region that is darker than the surrounding ice. This region is several tens of kilometres wide and stretched parallel to the margin of the ice sheet for more than 350 kilometres. The dark appearance implies low radiance and therefore low spectral albedos, leading to enhanced melting. An estimation of the influence of this dark region with a simple model shows that it can increase the local melt rate in this area by several tens per cent. Therefore, this dark region can significantly affect the total mass balance of the Greenland ice sheet. Satellite images reveal that the dark region is caused by outcropping layers of ice that contain more dust than the brighter surrounding ice. This dust was initially deposited in the accumulation zone of the Greenland ice sheet, transported through the ice sheet towards the margin and released in the ablation zone. Geochemical analyses of dust from the dark region and dust from brighter reference ice confirm this hypothesis and indicate a local source for the dust, probably the nearby tundra. In addition, abundant microorganisms were observed in the dark region. Part of these organisms formed granules together with the mineral dust. As the organic matter in the dust is known to have a high light absorbency, the dark region is not only caused by dust from the outcropping ice, but biological processes also contribute to the darkening of the surface. Finally, carbonaceous particles in the ice from the dark region reveal that the material is not modern, and settled on the accumulation zone during the Holocene, during periods of enhanced eolian activity. Therefore, dust fluxes towards the ice sheet in the past contribute to albedo variations in the ablation zone of the Greenland ice sheet at present and can enhance the melting of the Greenland ice sheet without external forcing
Surface mass balance along the K-transect in West Greenland
No full textA 21-year record is presented of surface mass balance measurements along the K-transect. The series covers the period 1990-2011. Data are available at 8 sites along a transect over an altitude range of 390 - 1850 m at approximately 67° N in West Greenland. The surface mass balance gradient is on average 3.8 x 10**-3 m w.e./m, and the mean equilibrium line altitude is 1553 m a.s.l. Only the lower 3 sites within 10 km of the margin experience a significant increasing trend in the ablation over the entire period
- …
