ALBERT

Description: Cold-water corals are widely distributed along the Atlantic continental margin with varying growth patterns in relation to their specific environment. Here, we investigate the long-term development of cold-water corals that once thrived on a low-latitude (17°40′N) cold-water coral mound in the Banda Mound Province off Mauritania during the last glacial–interglacial cycle. U/Th dates obtained from 20 specimens of the cold-water coral Lophelia pertusa, revealed three distinct periods of coral growth during the last glacial at 65 to 57 kyr BP, 45 to 32 kyr BP and 14 kyr BP, thus comprising the cool periods of Marine Isotopic Stages (MIS) 2–4. These coral growth periods occur during periods of increased productivity in the region, emphasizing that productivity seems to be the major steering factor for coral growth off Mauritania, which is one of the major upwelling regions in the world. This pattern differs from the well studied coral mounds off Ireland, where the current regime predominantly influences the prosperity of the cold-water corals. Moreover, coral growth off Ireland takes place during rather warm interglacial and interstadial periods, whereas off Mauritania coral growth is restricted to glacial and stadial periods. However, the on-mound sedimentation patterns off Mauritania largely resemble the observations reported from the Irish mounds. The bulk of the preserved sediments derives from periods of coral growth, whereas during periods without corals hardly any net sedimentation or mound growth took place.

Description: Variations of intensity and composition of biogenic particle flux at the northern boundary of the present PolarFrontal Zone in the Atlantic sector of the Southern Ocean are indicators of major changes of paleoenvironmentalconditions on glacial/interglacial time scales during the Late Quaternary. In order to estimate those pastchanges, sediment accumulation patterns of two piston cores, one from just north and one just south of thepresent day position of the Subantarctic Front were reconstructed. Using the 230Thex method large contributionsof laterally supplied material were quantified and used to correct sediment accumulation rates. During the lastglacial focussing of biogenic opal-dominated material exceeded the original contribution from the surfacewater above by a maximum factor of 8.7. The initial activity ratio of 231Paex/230Thex was used as tracer forbiogenic particle flux and composition and indicates that during the glacial stages 2 and 4 the area of high opalproductivity was situated above the location of the southern core whereas the northern core has not beenreached by this northward shift during the last 130 kyr as shown by the pattern of focussing-corrected bulkaccumulation rates. If the position of the Antarctic Polar Front has remained at the northern boundary of thehigh opal productivity area during the last 130 kyr, the results suggest that was located exactly between thetwo core sites during glacial stages 2 and 4. A two-box modeling approach involving particle flux and boundaryscavenging intensity of 231Pa was applied to estimate the possible range of the 231Paex/230Thex ratio recordedin Southern Ocean sediments. Previous estimates on the export of 231Pa from the Atlantic into the SouthernOcean are corroborated but the model suggests a low sensitivity of the 231Paex/230Thex ratio in Southern Oceansediments to variations of the residence time of North Atlantic Deep Water in the Atlantic Ocean.

Description: Here we present the first high precision 231Protactinium measurements in a manganese crust applying thermal ionization mass spectrometry (TIMS) using the double filament technique. The detection limit using TIMS is at least one order of magnitude lower, the statistical uncertainty 6–8 times better than for α-spectrometry. Thus, older sections of manganese crust VA13/2 from the Northern Equatorial Pacific could be measured precisely for their 231Pa activity. Our results reveal significant variations in 231Paxs activity for the last 150 ka which corroborate existing α-spectrometric data. If the growth rate was constant between 0 and 450 ka, the protactinium flux from the water column into manganese encrustations must have been variable. Thus, 231Paxs is not suitable for dating marine Mn/Fe deposits.

Description: Extreme, abrupt Northern Hemisphere climate oscillations during the last glacial cycle (140,000 years ago to present) were modulated by changes in ocean circulation and atmospheric forcing. However, the variability of the Atlantic meridional overturning circulation (AMOC), which has a role in controlling heat transport from low to high latitudes and in ocean CO2 storage, is still poorly constrained beyond the Last Glacial Maximum. Here we show that a deep and vigorous overturning circulation mode has persisted for most of the last glacial cycle, dominating ocean circulation in the Atlantic, whereas a shallower glacial mode with southern-sourced waters filling the deep western North Atlantic prevailed during glacial maxima. Our results are based on a reconstruction of both the strength and the direction of the AMOC during the last glacial cycle from a highly resolved marine sedimentary record in the deep western North Atlantic. Parallel measurements of two independent chemical water tracers (the isotope ratios of 231Pa/230Th and 143Nd/144Nd), which are not directly affected by changes in the global cycle, reveal consistent responses of the AMOC during the last two glacial terminations. Any significant deviations from this configuration, resulting in slowdowns of the AMOC, were restricted to centennial-scale excursions during catastrophic iceberg discharges of the Heinrich stadials. Severe and multicentennial weakening of North Atlantic Deep Water formation occurred only during Heinrich stadials close to glacial maxima with increased ice coverage, probably as a result of increased fresh-water input. In contrast, the AMOC was relatively insensitive to submillennial meltwater pulses during warmer climate states, and an active AMOC prevailed during Dansgaard–Oeschger interstadials (Greenland warm periods).

Description: The neodymium isotope proxy has become a valuable tool for the reconstruction of past ocean water mass provenance and mixing. For its accurate application, knowledge about the origin and preservation of Nd in sedimentary archives is crucial. Recently, concerns have emerged regarding the applicability of neodymium isotopes as a conservative palaeo water mass tracer, given potential Nd fluxes from sediments into bottom waters (Abbott et al., 2015a) and inferred relabelling of ocean waters by settling detrital material (Roberts and Piotrowski, 2015). Consequently, a decoupling of water mass provenance and proxy variations may arise. We investigate the mobility of Nd around extreme detrital sedimentation events such as glacial ice rafting pulses and turbidite deposition in the Northeast Atlantic. The constructed records from sediment leachates span extreme Nd isotope variations including volcanic (εNd ∼ 0) and Laurentian (εNd ∼ −27) sources. We find that Nd was released into pore waters from reactive detritus inside some detrital layers during early diagenesis, thereby overprinting any archived bottom water Nd signature and precluding the reconstruction of past water mass provenance during the affected time intervals. However, we do not observe any definite indication of diffusive vertical migration of Nd into adjacent layers. Furthermore, bottom water Nd isotope signatures were not modified to a measurable degree by any potential benthic flux of Nd during the deposition of these detrital sediment layers. Consequently, the Nd isotope composition of the pelagic glacial Northeast Atlantic water masses were resilient to such episodic large detrital fluxes. Apart from extreme local sedimentation events, we confirm the presence of detritally overprinted deep waters north of 47°N during the peak glacial from comparison of Northeast Atlantic depth transects. We furthermore suggest that the sensitivity of deep waters to this overprinting effect increased during periods of reduced Atlantic Meridional Overturning Circulation and elevated ice rafting. Overall, our study demonstrates that a thorough evaluation of the proportion of Nd originating from physical water mass advection versus in situ chemical inputs is crucial for the reliable application of Nd isotopes as a water mass tracer.

Description: Highlights • Icelandic input of radiogenic Nd essentially limited to coastal waters • Offshore bottom water Nd isotope signatures consistent with conservative mixing of intermediate and deep water masses • Decreased bottom water Nd concentrations likely reflect removal by particle scavenging Radiogenic neodymium (Nd) isotopes have been widely used as a proxy for tracing present and past water masses and ocean circulation, yet relatively few data exist for seawater from the important deep water formation area around Iceland. We have analyzed the dissolved seawater Nd isotope compositions (expressed as ƐNd) of 71 seawater samples, as well as Nd concentrations [Nd] of 38 seawater samples, collected at full water column profiles from 18 stations in the shelf area off the southern coast of Iceland. The goal of this work was to determine to what extent weathering inputs from Icelandic basalts, which are characterized by a distinctly radiogenic ƐNd signature within the North Atlantic, contribute to the Nd isotope and concentration signatures of water masses in the northern Iceland Basin. Radiogenic ƐNd values of up to −3.5 and elevated concentrations of up to 21 pmol/kg compared to nearby open ocean sites were found in surface waters at shallow sites closest to shore and to river mouths of Iceland. This documents partial dissolution of highly radiogenic basaltic particles, which are transported northwards by the coastal currents. A comparable signal is not observed, however, in offshore surface waters likely as a result of the advection of surface currents mainly directed onshore, thus isolating these sites from Icelandic weathering contributions. The dominance of Subpolar Mode Waters and Intermediate Water unaffected by Icelandic contributions in the offshore study area is supported by unradiogenic ƐNd signatures between −15 and −12. In agreement with hydrographic data, highly radiogenic bottom waters at one site on the Iceland-Faroe Ridge (ƐNd = −7.5) reveal the presence of almost pure Iceland Scotland Overflow Water (ISOW) near its formation site further to the east. In bottom waters of all deeper offshore sites, the combination of depleted Nd concentrations and similar ƐNd values (averaging at ≃−11.75 for the R/V Poseidon data and ≃−11 for the R/V Thalassa data) confirms the rapid entrainment of Atlantic mid-depth and deep waters into the overflow waters, which is accompanied by near bottom Nd removal via particle scavenging. Overall, our findings demonstrate that at present, apart from the radiogenic isotope signature of ISOW itself, the direct contribution of radiogenic Nd originating from weathering of Iceland basalts to the water column of the Iceland Basin is limited. This supports the reliable application of ƐNd values to trace changes in the mixing of open North Atlantic water masses (including ISOW).