Beschreibung: Coinciding with global warming, Arctic sea ice has rapidly decreased during the last four decades and climate scenarios suggest that sea ice may completely disappear during summer within the next about 50-100 years. Here we produce Arctic sea ice biomarker proxy records for the penultimate glacial (Marine Isotope Stage 6) and the subsequent last interglacial (Marine Isotope Stage 5e). The latter is a time interval when the high latitudes were significantly warmer than today. We document that even under such warmer climate conditions, sea ice existed in the central Arctic Ocean during summer, whereas sea ice was significantly reduced along the Barents Sea continental margin influenced by Atlantic Water inflow. Our proxy reconstruction of the last interglacial sea ice cover is supported by climate simulations, although some proxy data/model inconsistencies still exist. During late Marine Isotope Stage 6, polynya-type conditions occurred off the major ice sheets along the northern Barents and East Siberian continental margins, contradicting a giant Marine Isotope Stage 6 ice shelf that covered the entire Arctic Ocean.

Beschreibung: 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.

Beschreibung: Here we publish simulated early-middle Eocene annual mean ocean temperature, salinity, density, mixed layer depth and current velocities (at the depth of ~1500 m) with different gateways configurations in the North Atlantic region. The details of original paleogeography, compiled by GETECH, are provided in Vahlenkamp et al. [2018, doi:10.1016/j.epsl.2017.12.016]. The CO2 level is set to 840 ppm, while the obliquity to a minimum value. All data are averaged over the period of 100 years. All simulations were run with COSMOS (ECHAM5/MPIOM/OASIS3). The atmosphere component ECHAM5 was run in the resolution of T31/L19, while the ocean model MPIOM has a formal resolution of ~3.0°x1.8°. OASIS3 is a coupler between the atmosphere and ocean components. The gateway alterations with respect to original paleogeography include an opening of the Arctic and Tethyan Seaways, as well as changing the depth of the Greenland-Scotland Ridge to 50 m and 200 m [Vahlenkamp et al., 2018, doi:10.1016/j.epsl.2018.06.031]. We publish also one experiment with higher, 1000 ppm CO2 level. In order to compare our simulated temperatures with the data, we have collected 30 Eocene temperature reconstructions from literature. We compared these reconstructions with the zonal surface air temperatures as well as zonal surface ocean temperatures (annual, boreal winter and summer) from our base simulation. All data used are provided in this dataset.

Beschreibung: We provide the results of 11 Late Cretaceous climate simulations (Tab. 1 in Niezgodzki et al. [2019, doi:10.1016/j.gloplacha.2019.03.011]) produced with COSMOS in a coupled atmosphere-ocean configuration. Five of these experiments use a 3 x pre-industrial (PI) CO2 level (840 ppm) while 6 of them were run with 4xPI CO2 (1120 ppm). The experiments with the same CO2 levels differ by gateway configurations between the Arctic Ocean and North proto-Atlantic basin. In spin-up experiments we employ Maastrichtian (~70 Ma) paleogeography of Markwick and Valdes [2004, doi:10.1016/j.palaeo.2004.06.015]. More information about model scenarios and model set-up can be found in Niezgodzki et al. [2019, doi:10.1016/j.gloplacha.2019.03.011]. Here we publish simulated winter (DJF) surface temperatures (tsurf) and salinity (SAO), averaged March-April (Ma-Ap) sea surface temperature (THO) and SAO and monthly sea-ice compactness (SICOMO) of each experiment. Additionally, for two gateway configurations we show DJF and summer (JJA) 10m meridional (v10) and zonal (u10) wind speeds as well as JJA shortwave net surface radiation (srads), net clear sky surface radiation (srafs), longwave net surface radiation (trads) and clear sky surface radiation (trafs).

Beschreibung: Paleo-climate records and geodynamic modelling indicate the existence of complex interactions between glacial sea level changes, volcanic degassing, and atmospheric CO2, which may have modulated the climate system's descent into the last ice age. Between ~85-70 ka, during an interval of decreasing axial tilt, the orbital component in global temperature records gradually declined, while atmospheric CO2, instead of continuing is long-term correlation with Antarctic temperature, remained relatively stable. Based on novel global geodynamic models and the joint interpretation of paleo-proxy data as well as biogeochemical simulations, we show that a sea level fall in this interval caused enhanced pressure-release melting in the uppermost mantle, which may have induced a surge in magma and CO2 fluxes from mid-ocean ridges and oceanic hotspot volcanoes. Our results reveal a hitherto unrecognised negative feedback between glaciation and atmospheric CO2 predominantly controlled by marine volcanism on multi-millennial (suborbital) timescales of ~ 5,000-15,000 years.

Beschreibung: Little is known about the climate evolution on the Kamchatka Peninsula during the last glacial?interglacial transition as existing climate records do not reach beyond 12 ka BP. In this study, a summer temperature record for the past 20 kyr is presented. Branched glycerol dialkyl glycerol tetraethers, terrigenous biomarkers suitable for continental air temperature reconstructions, were analyzed in a sediment core from the western continental margin off Kamchatka in the marginal northwest Pacific (NW Pacific). The record suggests that summer temperatures on Kamchatka during the Last Glacial Maximum (LGM) equaled modern temperatures. We suggest that strong southerly winds associated with a pronounced North Pacific High pressure system over the subarctic NW Pacific accounted for the warm conditions. A comparison with an Earth system model reveals discrepancies between model and proxy-based reconstructions for the LGM temperature and atmospheric circulation in the NW Pacific realm. The deglacial temperature development is characterized by abrupt millennial-scale temperature oscillations. The Bølling?Allerød warm phase and the Younger Dryas cold spell are pronounced events, suggesting a connection to North Atlantic climate variability.

Beschreibung: This study mainly focuses on the millennial-scale climate variability influence on the Northern Hemisphere ice sheet evolution during the past glacial-interglacial cycles. We use the 3D thermodynamical Parallel Ice Sheet Model (PISM) version 0.7.3 to simulate the ice sheets. An index method is used to generate the climate between these states to force the model. In this method, the time-dependent climate evolution is represented by the combination of a time series and two extreme climate conditions. We conducted two experiments. In the first experiment, the ice sheet model is forced with climate that contains millennial-scale oscillations (GL_hf). In the other experiment, the millennial-scale oscillations are removed (GL_lf). The results show that the simulated ice sheet volume is significantly reduced when forced with large amplitude climate fluctuations compared to the case without. Sensitivity experiments are also conducted to see how the amplitude of climate noise affects the simulated ice sheets.

Beschreibung: During the last glacial termination, the upper North Pacific Ocean underwent dramatic and rapid changes in oxygenation that lead to the transient intensification of oxygen minimum zones (OMZs), recorded by the widespread occurrence of laminated sediments on circum-Pacific continental margins. We present a new laminated sediment record from the mid-depth (1100 m) northern Bering Sea margin that provides insight into these deglacial OMZ maxima with exceptional, decadal-scale detail. Combined ultrahigh-resolution micro-X-ray-fluorescence (micro-XRF) data and sediment facies analysis of laminae reveal an alternation between predominantly terrigenous and diatom-dominated opal sedimentation. The diatomaceous laminae are interpreted to represent spring/summer productivity events related to the retreating sea ice margin.We identified five laminated sections in the deglacial part of our site. Lamina counts were carried out on these sections and correlated with the Bølling–Allerød and Preboreal phases in the North Greenland Ice Core (NGRIP) oxygen isotope record, indicating an annual deposition of individual lamina couplets (varves). The observed rapid decadal intensifications of anoxia, in particular within the Bølling–Allerød, are tightly coupled to short-term warm events through increases in regional export production. This dependence of laminae formation on warmer temperatures is underlined by a correlation with published Bering Sea sea surface temperature records and d18O data of planktic foraminifera from the Gulf of Alaska. The rapidity of the observed changes strongly implies a close atmospheric teleconnection between North Pacific and North Atlantic regions.We suggest that concomitant increases in export production and subsequent remineralization of organic matter in the Bering Sea, in combination with oxygen-poor waters entering the Being Sea, drove down oxygen concentrations to values below 0.1ml/l and caused laminae preservation. Calculated benthic–planktic ventilation ages show no significant variations throughout the last deglaciation, indicating that changes in formation rates or differing sources of North Pacific mid-depth waters are not prime candidates for strengthening the OMZ at our site. The age models established by our correlation procedure allow for the determination of calendar age control points for the Bølling–Allerød and the Preboreal that are independent of the initial radiocarbon-based chronology. Resulting surface reservoir ages range within 730–990 yr during the Bølling–Allerød, 800–1100 yr in the Younger Dryas, and 765–775 yr for the Preboreal.