Radiocarbon Date List XI: Radiocarbon Dates from Marine Sediment Cores of the Iceland, Greenland, and Northeast Canadian Arctic Shelves and Nares Strait

Radiocarbon Date List XI contains an annotated listing of 178 AMS radiocarbon dates on samples from marine (169 samples) and lake (9 samples) sediment cores. Marine sediment cores, from which the samples for dating were taken, were collected on the Greenland Shelf, Baffin Bay, and the Eastern Canadian Arctic shelf. About 80% of the marine samples for dating were collected on the SW to N Icelandic shelf. The lake sediment cores were collected in northwestern Iceland. For dating of the marine samples, we submitted molluscs (117 samples), benthic and planktic foraminifera (45 samples), plant macrofauna (3 samples), and one serpulid worm. For dating of the lake cores, we submitted wood (8 samples) and one peat sample. The Conventional Radiocarbon Ages range from 294±9114C yr BP to 34,600±640 14C yr BP. The dates have been used to address a variety of research questions. The dates constrain the timing of high northern latitude late Quaternary environmental fluctuations, which include glacier extent, sea level history, isostatic rebound, sediment input, and ocean circulation. The dates also allowed assessment of the accuracy of commonly used reservoir correction. The samples were submitted by INSTAAR and affiliated researchers

The configuration, sensitivity and rapid retreat of the Late Weichselian Icelandic ice sheet

The fragmentary glacial-geological record across the Icelandic continental shelf has hampered reconstruction of the volume, extent and chronology of the Late Weichselian ice sheet particularly in key offshore zones. Marine geophysical data collected over the last two decades reveal that the ice sheet likely attained a continental shelf-break position in all sectors during the Last Glacial Maximum, though its precise timing and configuration remains largely unknown. Within this context, we review the available empirical evidence and use a well-constrained three-dimensional thermomechanical model to investigate the drivers of an extensive Late Weichselian Icelandic ice-sheet, its sensitivity to environmental forcing, and phases of deglaciation. Our reconstruction attains the continental shelf break across all sectors with a total ice volume of 5.96×105km3 with high precipitation rates being critical to forcing extensive ice sheet flow offshore. Due to its location astride an active mantle plume, a relatively fast and dynamic ice sheet with a low aspect ratio is maintained. Our results reveal that once initial ice-sheet retreat was triggered through climate warming at 21.8 ka BP, marine deglaciation was rapid and accomplished in all sectors within c. 5 ka at a mean rate of 71 Gt of mass loss per year. This rate of ice wastage is comparable to contemporary rates observed for the West Antarctic ice sheet. The ice sheet subsequently stabilised on shallow pinning points across the near shelf for two millennia, but abrupt atmospheric warming during the Bølling Interstadial forced a second, dramatic collapse of the ice sheet onshore with a net wastage of 221 Gt a−1 over 750 years, analogous to contemporary Greenland rates of mass loss. Geothermal conditions impart a significant control on the ice sheet's transient response, particularly during phases of rapid retreat. Insights from this study suggests that large sectors of contemporary ice sheets overlying geothermally active regions, such as Siple Coast, Antarctica, and NE Greenland, have the potential to experience rapid phases of mass loss and deglaciation once initial retreat is initiated

The Influence of Subpolar Gyre Dynamics on Centennial to Millennial Scale Holocene Climate Variability in the High-Latitude North Atlantic

Holocene paleoclimatic reconstructions provide longer time series than instrumental records, allowing us to examine climate variability under more extreme boundary conditions. I reconstructed temperature and δ18Osw using paired measurements of Mg/Ca ratios and δ18Ocalcite of planktonic and benthic foraminifera at sites in the subpolar North Atlantic that are sensitive to Subpolar Gyre (SPG) dynamics today. Glacial freshwater from the final stages of the decay of the Laurentide ice sheet influenced SPG dynamics and was routed via the SPG throughout the subpolar basin. Near Iceland we recorded the 8.2 ka cooling event and the freshwater spike from the catastrophic outburst flood of proglacial lakes Agassiz and Ojibway using paired Mg/Ca and δ18Ocalcite of the benthic foraminifer Cibicides lobatulus. This evidence for a brief cooling and freshening is supported by a coeval increase in arctic benthic foraminifera and a decrease in biogenic carbonate. Through the Holocene, the SPG warmed and the δ18Osw became heavier (saltier), suggesting a progression towards a contracted gyre. This is in sharp contrast to the established view of the rest of the North Atlantic, which was warmest during the early-mid Holocene. We attribute the SPG warming trend to a weakening of the NAO-like circulation in response to increasing winter insolation through the Holocene. Overlying this general trend were abrupt shifts that cannot be explained by changes in insolation. Between 10,000 and 8,000 cal yr BP, my study sites recorded cold temperature and light δ18Osw values, suggesting an extended SPG. Between 8,000 and 6,000 cal yr BP, the SPG changed shape but was still extended. This change coincided with the onset of Labrador Sea water formation, which today is associated with intensified SPG circulation. Between ~6,000 and 3,800 cal yr BP, I see a strong influence of Atlantic water on the SW Iceland shelf, consistent with a contracted SPG. After 4,000 cal yr BP, freshwater from the Arctic Ocean appeared on the SW Iceland shelf. The water column as recorded in benthic and planktonic foraminiferal assemblages went from well mixed in the early Holocene to stratified in the late Holocene. Overall my records show that the SPG played an important and, in some ways, surprising role in the Holocene climate evolution of the North Atlantic

The Influence of Subpolar Gyre Dynamics on Centennial to Millennial Scale Holocene Climate Variability in the High-Latitude North Atlantic

Holocene paleoclimatic reconstructions provide longer time series than instrumental records, allowing us to examine climate variability under more extreme boundary conditions. I reconstructed temperature and δ18Osw using paired measurements of Mg/Ca ratios and δ18Ocalcite of planktonic and benthic foraminifera at sites in the subpolar North Atlantic that are sensitive to Subpolar Gyre (SPG) dynamics today. Glacial freshwater from the final stages of the decay of the Laurentide ice sheet influenced SPG dynamics and was routed via the SPG throughout the subpolar basin. Near Iceland we recorded the 8.2 ka cooling event and the freshwater spike from the catastrophic outburst flood of proglacial lakes Agassiz and Ojibway using paired Mg/Ca and δ18Ocalcite of the benthic foraminifer Cibicides lobatulus. This evidence for a brief cooling and freshening is supported by a coeval increase in arctic benthic foraminifera and a decrease in biogenic carbonate. Through the Holocene, the SPG warmed and the δ18Osw became heavier (saltier), suggesting a progression towards a contracted gyre. This is in sharp contrast to the established view of the rest of the North Atlantic, which was warmest during the early-mid Holocene. We attribute the SPG warming trend to a weakening of the NAO-like circulation in response to increasing winter insolation through the Holocene. Overlying this general trend were abrupt shifts that cannot be explained by changes in insolation. Between 10,000 and 8,000 cal yr BP, my study sites recorded cold temperature and light δ18Osw values, suggesting an extended SPG. Between 8,000 and 6,000 cal yr BP, the SPG changed shape but was still extended. This change coincided with the onset of Labrador Sea water formation, which today is associated with intensified SPG circulation. Between ~6,000 and 3,800 cal yr BP, I see a strong influence of Atlantic water on the SW Iceland shelf, consistent with a contracted SPG. After 4,000 cal yr BP, freshwater from the Arctic Ocean appeared on the SW Iceland shelf. The water column as recorded in benthic and planktonic foraminiferal assemblages went from well mixed in the early Holocene to stratified in the late Holocene. Overall my records show that the SPG played an important and, in some ways, surprising role in the Holocene climate evolution of the North Atlantic

Holocene changes in marine productivity and terrestrial organic carbon inputs into an Icelandic fjord: Application of molecular and bulk organic proxies

This study examines the dynamics of organic carbon contributions from different sources to the sediments of a ~39 m core from Ísafjarðardjúp Fjord, Northwest Iceland, throughout the Holocene. Furthermore, it shows that the variability of terrestrial organic carbon (OCterr) and marine organic carbon (OCmar) is linked to palaeoclimatic change throughout the Holocene. glycerol-dialkyl-glycerol-tetraether (GDGT), alkenone, n-alkane, total OC and total nitrogen analyses were conducted on 326 samples to yield high-resolution branched versus isoprenoid tetraether index (BIT-index), n-alkane/alkenone index and C/N ratio records from ~10,800 to ~300 cal. a BP. These records were used to estimate the OCterr and the OCmar contributions to the sediments. Three different approaches of estimating the OCterr contribution yield different relative amounts, but similar long-term trends. These results indicate that the combination of biomarker records is a good approach to reconstruct OCterr contributions but also highlight the strengths and weaknesses of the individual biomarkers. The OCterr contribution to the total OC inventory continually increases throughout much of the Holocene but does not rise above 30%. It seems to have been driven by changing climate rather than changing sedimentation rates, and during the late Holocene, anthropogenic activity may have been an influence. The reconstructed OCmar contribution to the sediment was used to model changes in palaeoproductivity throughout the Holocene. These changes were likely forced by changes in nutrients supplied both by the catchment area and the Irminger Current

North Atlantic Holocene climate evolution recorded by high-resolution terrestrial and marine biomarker records

Holocene climatic change is driven by a plethora of forcing mechanisms acting on different time scales, including: insolation, internal ocean (e.g. Atlantic Meridional Overturning Circulation; AMOC) and atmospheric (e.g. North Atlantic Oscillation; NAO) variability. However, it is unclear how these driving mechanisms interact with each other. Here we present five, biomarker based, paleoclimate records (air-, sea surface temperature and precipitation), from a fjordic sediment core, revealing North Atlantic terrestrial and marine climate in unprecedented detail. The Early Holocene (10.7e7.8 kyrs BP) is characterised by relatively high air temperatures while SSTs are dampened by melt water events, and relatively low precipitation. The Middle Holocene (7.8e3.2 kyrs BP) is characterised by peak SSTs, declining air temperatures and high precipitation. A pronounced marine thermal maximum occurs between ~7 e5.5 kyrs BP, 3000 years after the terrestrial thermal maximum, driven by melt water cessation and an accelerating AMOC. The neoglacial cooling, between 5.8 and 3.2 kyrs BP leads into the late Holocene. We demonstrate that an observed modern link between Icelandic precipitation variability during different NAO phases, may have existed from ~7.5 kyrs BP. A simultaneous decoupling of both air, and sea surface temperature records from declining insolation at ~3.2 kyrs BP may indicate a threshold, after which internal feedback mechanisms, namely the NAO evolved to be the primary drivers of Icelandic climate on centennial time-scales

Studie zu Fragestellungen im Zusammenhang mit der Zwischen- und Endlagerung von abgereichertem Uran (Anfall, Menge, Wiederverwendungs- und Entsorgungsmoeglichkeiten) Abschlussbericht. Berichtszeitraum: 1.3.1985 - 28.2.1986

TIB: RO 3190 (137) / FIZ - Fachinformationszzentrum Karlsruhe / TIB - Technische InformationsbibliothekSIGLEDEGerman

Mg/Ca-temperature calibration for the benthic foraminifera Melonis barleeanum and Melonis pompilioides

An important tool for deep-sea temperature reconstruction is Mg/Ca paleothermometry applied to benthic foraminifera. Foraminifera of the genus Melonis appear to be promising candidates for temperature reconstructions due to their wide geographical and bathymetric distribution, and their infaunal habitat, which was suggested to reduce secondary effects from carbonate ion saturation (Δ[CO3 2−]). Here, we make substantial advances to previous calibration efforts and present new multi-lab Mg/Ca data for Melonis barleeanum and Melonis pompilioides from more than one hundred core top samples spanning in situ bottom temperatures from −1 to 16 °C, coupled with morphometric analyses of the foraminifer tests. Both species and their morphotypes seem to have a similar response of Mg/Ca to growth temperature. Compilation of new and previously published data reveals a linear dependence of temperature on Mg/Ca, with a best fit of Mg/Ca (mmol/mol) = 0.113 ± 0.005 ∗ BWT (°C) + 0.792 ± 0.036 (r2 = 0.81; n = 120; 1σ SD). Salinity, bottom water Δ[CO3 2−], and varying morphotypes have no apparent effect on the Mg/Ca-temperature relationship, but pore water Δ[CO3 2−] might have had an influence on some of the samples from the tropical Atlantic