ALBERT

Description: Our understanding of the deglacial evolution of the Antarctic Ice Sheet (AIS) following the Last Glacial Maximum (26,000-19,000 years ago) is based largely on a few well-dated but temporally and geographically restricted terrestrial and shallow-marine sequences. This sparseness limits our understanding of the dominant feedbacks between the AIS, Southern Hemisphere climate and global sea level. Marine records of iceberg-rafted debris (IBRD) provide a nearly continuous signal of ice-sheet dynamics and variability. IBRD records from the North Atlantic Ocean have been widely used to reconstruct variability in Northern Hemisphere ice sheets, but comparable records from the Southern Ocean of the AIS are lacking because of the low resolution and large dating uncertainties in existing sediment cores. Here we present two well-dated, high-resolution IBRD records that capture a spatially integrated signal of AIS variability during the last deglaciation. We document eight events of increased iceberg flux from various parts of the AIS between 20,000 and 9,000 years ago, in marked contrast to previous scenarios which identified the main AIS retreat as occurring after meltwater pulse 1A and continuing into the late Holocene epoch. The highest IBRD flux occurred 14,600 years ago, providing the first direct evidence for an Antarctic contribution to meltwater pulse 1A. Climate model simulations with AIS freshwater forcing identify a positive feedback between poleward transport of Circumpolar Deep Water, subsurface warming and AIS melt, suggesting that small perturbations to the ice sheet can be substantially enhanced, providing a possible mechanism for rapid sea-level rise.

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.

Description: The Indo-Pacific Warm Pool (IPWP) contains the warmest surface ocean waters on our planet. Changes in the extent and position of the IPWP likely impacted the tropical and global climate in the past. To put recent ocean changes into a longer temporal context, we present new paleoceanographic sea surface temperature reconstructions from off Papua New Guinea (RR1313-23PC: 4.4939°S, 145.6703°E, 712 m water depth) which is at the heart of the Western Pacific Warm Pool (WPWP), which is the warmest region within the IPWP, across the last 17,000 years. A new surface temperature dataset from the northeast South China Sea is also presented (ODP1144: 20.053°N, 117.4189°E; water depth 2037 m). In both locations we use Mg/Ca measurements on G.ruber s.s. (white) to calculate sea surface temperatures.

Description: We provide global fields of simulated ocean velocity in zonal (netCDF variable UKO) and meridional (netCDF variable VKE) direction at a depth of 420 m. Six climate states are covered in the data set: 1. data set PI_mpiom_UKO_VKE_timmean_420m.nc: pre-industrial (PI) control state (representative for 1850 AD) as used in the publications by Stepanek and Lohmann (2012) and Zhang et al. (2013). The respective data is courtesy of Zhang et al. (2013) 2. data set LGM_mpiom_UKO_VKE_timmean_420m.nc: a climate state of the Last Glacial Maximum (simulation LGM-W by Zhang et al., 2013), representative for 21 kiloyears (ka) before present (BP) 3. data set Plio_mpiom_UKO_VKE_timmean_420m.nc: a climate state of the Mid-Pliocene Warm Period (simulation experiment 2 by Stepanek and Lohmann, 2012), that covers the time from 3.29 - 2.97 million years (Ma) BP 4. data set MIO_mpiom_UKO_VKE_timmean_420m.nc: a climate state representing conditions of the early to middle Miocene (23 to 15 Ma BP) including a regional bathymetry reconstruction (15 Ma) of the North Atlantic / Arctic Ocean by Ehlers and Jokat (2013) and considering 450 ppm of carbon dioxide (simulation EO450 by Stärz et al., 2017) 5. data set MioW_mpiom_UKO_VKE_timmean_420m.nc: a climate state representing conditions of the early to middle Miocene (23 to 15 Ma BP) including a regional bathymetry reconstruction (15 Ma) of the Weddell Sea (Huang et al., 2014) and considering 450 ppm of carbon dioxide in the atmosphere (simulation MIOW_450 by Huang et al., 2017) 6. data set MioW_PIS_mpiom_UKO_VKE_timmean_420m.nc: a climate state similar to 5. but with PI (278 ppm) carbon dioxide concentration and prescribed modern ice sheets (simulation MIOW_PIS by Huang et al., 2017) All data sets represent climatological annual averages over a time period of 100 years. The oceanography is based on climate simulations performed with the Community Earth System Models (COSMOS) that consist of the atmosphere general circulation model ECHAM5 (Roeckner et al., 2003), internally coupled to the land surface and terrestrial carbon cycle model JSBACH (Raddatz et al., 2007) in T31 resolution (3.75°x3.75°) with 19 vertical levels on a hybrid sigma-pressure coordinate, and the ocean general circulation model MPIOM (Marsland et al., 2003) on a bipolar curvilinear GR30 grid with a formal resolution of 3.0°x1.8° and 40 z-coordinate levels. Exchange of momentum, mass, and energy between the atmosphere and ocean domain is enabled via the OASIS3 coupler (Valcke et al., 2003). For all simulations, ocean characteristics (including sea ice) and properties of atmosphere and land (including the land carbon cycle and dynamic vegetation) are computed based on the prescribed climate forcing (concentration of atmospheric trace gases, configuration of the Earth's orbit) and boundary conditions (land surface elevation, ice sheets, ocean bathymetry, and land sea mask). For details of the utilized boundary conditions and climate forcing refer to the original publications describing the data (Stepanek and Lohmann, 2012; Zhang et al., 2013; Huang et al., 2017; Stärz et al., 2017). In case of analyzing velocities of the Arctic Ocean, note that for few grid cells in the northernmost data row (89.5°N), between 280°E and 293°E, there is a data artifact in variable VKE. This artifact is a side effect of rotating and interpolatiing velocities from the curvilinear model grid to a standard NSWE coordinate system; this artifact has not been removed from the data sets.

Description: There is compelling evidence that episodic deposition of large volumes of freshwater into the oceans strongly influenced global ocean circulation and climate variability during glacial periods (Maslin et al., 1995, doi:10.1029/94PA03040; Kageyama et al., 2013, doi:10.5194/cp-9-935-2013). In the North Atlantic region, episodes of massive freshwater discharge to the North Atlantic Ocean were related to distinct cold periods known as Heinrich Stadials Maslin et al., 1995, doi:10.1029/94PA03040; Kageyama et al., 2013, doi:10.5194/cp-9-935-2013; Böhm et al., 2013, doi:10.1038/nature14059). By contrast, the freshwater history of the North Pacific region remains unclear, giving rise to persistent debates about the existence and possible magnitude of climate links between the North Pacific and North Atlantic oceans during Heinrich Stadials (Praetorius and Mix, 2014, doi:10.1126/science.1252000; Menviel et al., 2014, doi:10.1002/2013PA002542). Here we find that there was a strong connection between changes in North Atlantic circulation during Heinrich Stadials and injections of freshwater from the North American Cordilleran Ice Sheet to the northeastern North Pacific. Our record of diatom δ18O (a measure of the ratio of the stable oxygen isotopes 18O and 16O) over the past 50,000 years shows a decrease in surface seawater δ18O of two to three per thousand, corresponding to a decline in salinity of roughly two to four practical salinity units. This coincided with enhanced deposition of ice-rafted debris and a slight cooling of the sea surface in the northeastern North Pacific during Heinrich Stadials 1 and 4, but not during Heinrich Stadial 3. Furthermore, results from our isotope-enabled model (Werner et al., 2016, doi:10.5194/gmd-9-647-2016) suggest that warming of the eastern Equatorial Pacific during Heinrich Stadials was crucial for transmitting the North Atlantic signal to the northeastern North Pacific, where the associated subsurface warming resulted in a discernible freshwater discharge from the Cordilleran Ice Sheet during Heinrich Stadials 1 and 4. However, enhanced background cooling across the northern high latitudes during Heinrich Stadial 3 -the coldest period in the past 50,000 years (North Greenland Ice Core Project members, 2004, doi:10.1038/nature02805) -prevented subsurface warming of the northeastern North Pacific and thus increased freshwater discharge from the Cordilleran Ice Sheet. In combination, our results show that nonlinear ocean–atmosphere background interactions played a complex role in the dynamics linking the freshwater discharge responses of the North Atlantic and North Pacific during glacial periods.