ALBERT

Description: Changes in the Atlantic Meridional Overturning Circulation (AMOC) have commonly been invoked to explain the low‐frequency climate changes evident over millennial‐multidecadal timescales during the Holocene period. While there is growing evidence that deep ocean circulation varied on millennial timescales, little is known about ocean variability on shorter timescales. Here we use a marine sediment core (GS06‐144‐09MC‐D) recovered from a high accumulation rate site on the Gardar Drift in the Iceland Basin (60°19′N, 23°58′W, 2081 m) to reconstruct decadal‐centennial variability in the vigor of Iceland‐Scotland Overflow Water (ISOW) with the paleocurrent proxy "sortable silt" mean grain size (ss). Our (ss) record reveals that changes in ISOW vigor have occurred on multidecadal‐centennial timescales over the past ~600 years; similar timescales as documented in Atlantic Multidecadal Variability observations and reconstructions. Our findings support a link between changes in basin‐wide climate and deep ocean circulation.

Description: Deep ocean circulation has been considered relatively stable during interglacial periods, yet little is known about its behavior on submillennial time scales. Using a subcentennially resolved epibenthic foraminiferal d13C record we show that North Atlantic Deep Water (NADW) influence was strong at the onset of the last interglacial period and then interrupted by several prominent, centennial-scale reductions. These NADW transients occurred during periods of increased ice rafting and southward expansions of polar water influence, suggesting that a buoyancy threshold for convective instability was triggered by freshwater and circum-Arctic cryosphere changes. The deep Atlantic chemical changes were similar in magnitude to those associated with glaciations, implying that the canonical view of a relatively stable interglacial circulation may not hold for conditions warmer/fresher than at present.

Description: The Nordic Seas overflows are an important part of the Atlantic thermohaline circulation. While there is growing evidence that the overflow of dense water changed on orbital time scales during the Holocene, less is known about the variability on shorter time scales beyond the instrumental record. Here we reconstruct the relative changes in flow strength of Iceland-Scotland Overflow Water (ISOW), the eastern branch of the overflows, on multidecadal-millennial time scales. The reconstruction is based on mean sortable silt (SS) from a sediment core on the Gardar Drift (60°19′N, 23°58′W, 2081 m). Our SS record reveals that the main variance in ISOW vigor occurred on millennial time scales (1-2 kyr) with particularly prominent fluctuations after 8 kyr. Superimposed on the millennial variability, there were multidecadal-centennial flow speed fluctuations during the early Holocene (10-9 kyr) and one prominent minimum at 0.9 kyr. We find a broad agreement between reconstructed ISOW and regional North Atlantic climate, where a strong (weak) ISOW is generally associated with warm (cold) climate. We further identify the possible contribution of anomalous heat and freshwater forcing, respectively, related to reconstructed overflow variability. We infer that ocean poleward heat transport can explain the relationship between regional climate and ISOW during the middle to late Holocene, whereas freshwater input provides a possible explanation for the reduced overflow during early Holocene (8-10 kyr).

Description: Ocean Drilling Program (ODP) cores permit us to extend the study of millennial-scale climate variability beyond the time period that is generally accessible for piston cores (i.e., the last glacial cycle). ODP Leg 177 provided for the first time continuous high sedimentation rate cores along a north-south transect from 41°to 53°S across the main subdivisions of the Southern Ocean (Shipboard Scientific Party, 1999, doi:10.2973/odp.proc.ir.177.101.1999). The main purpose of this drilling was to investigate the Pleistocene and Holocene paleoceanographic history of this region, documented in the sedimentary records. ODP Sites 1094, 1093, 1091, and 1089 accumulated throughout the Pleistocene at rates 〉10 cm/k.y. and are the most detailed Pleistocene climatic records ever retrieved from the Southern Ocean. These sections provide a unique opportunity to fill an important gap in the knowledge of the paleoclimatic evolution of the high southern latitude regions. The composite sections at each site were generated shipboard using magnetic susceptibility, gamma ray attenuation (GRA) density, and reflectance data to correlate the drill holes and splice together an optimal (complete and undisturbed) record of the sedimentary sequence at each site. A preliminary magnetic polarity stratigraphy was generated on the 'archive' halves of the core sections from each hole, using the shipboard pass-through magnetometer after demagnetization at a single peak alternating field (Shipboard Scientific Party, 1999). During July 1998, we sampled core sections spanning the mid-Pleistocene interval (0.65-1.2 Ma) from Sites 1094, 1093, and 1091 at the ODP Bremen Core Repository and have since then analyzed the stable isotopic ratios of foraminifers in samples from Sites 1094 and 1091. Our goals for these studies are to establish detailed chronology for the mid-Pleistocene Southern Ocean records from Leg 177 using high-resolution stable isotope analyses, and furthermore, to trace the evolution of millennial-scale variability in proxy records from older glacial and interglacial periods characterized by higher-frequency variation. Here, we report on our stratigraphic results to date and describe the laboratory methods employed for sample preparation and stable isotope analysis. Furthermore, we provide tab-delimited text files of the age models.

Description: All data are from core ODP 1094 recovered from the Antarctic Zone of the Southern Ocean (Atlantic sector). Age model of ODP 1094 (1.5Ma) dδ18O, Mg/Ca, Mn/Ca and Mg/Ca-derived sea surface temperature and surface water d18O based on down core measurements of planktic Neogloboquadrina pachyderma (sinistral) from core ODP 1094 (downcore data and averaged for MIS). δ18O, Mg/Ca, Mn/Ca and Mg/Ca-derived bottom water temperature and bottom water d18O based on down core measurements of benthic Melonis pompilioides from core ODP 1094 (downcore data and averaged for MIS). δ18O of benthic Cibicidoides spp. from core ODP 1094.