Age determination of sediments of Site LO09-14 (Table 1)

A composite record (LO09-14) of three sediment cores from the subpolar North Atlantic (Reykjanes Ridge) was investigated in order to assess surface ocean variability during the last 11 kyr. The core site is today partly under the influence of the Irminger Current (IC), a branch of the North Atlantic Drift continuing northwestward around Iceland. However, it is also proximal to the Sub-Arctic Front (SAF) that may cause extra dynamic hydrographic conditions. We used statistical methods applied to the fossil assemblages of diatoms to reconstruct quantitative sea surface temperatures (SSTs). Our investigations give evidence for different regional signatures of Holocene surface oceanographic changes in the North Atlantic. Core LO09-14 reveal relatively low and highly variable SSTs during the early Holocene, indicating a weak IC and increased advection of subpolar water over the site. A mid-Holocene thermal optimum with a strong IC occurs from 7.5 to 5 kyr and is followed by cooler and more stable late Holocene surface conditions. Several intervals throughout the Holocene are dominated by the diatom species Rhizosolenia borealis, which we suggest indicates proximity to a strongly defined convergence front, most likely the SAF. Several coolings, reflecting southeastward advection of cold and ice-bearing waters, occur at 10.4, 9.8, 8.3, 7.9, 6.4, 4.7, 4.3 and 2.8 kyr. The cooling events recorded in the LO09-14 SSTs correlate well with both other surface records from the area and the NADW reductions observed at ODP Site 980 indicating a surface-deepwater linkage through the Holocene.

A 300 kyr record of upwelling off Oman during the Late Quaternary: evidence of the Asian southwest monsoon

In the northwest Arabian Sea upwelling occurs each summer, driven by the strong SW monsoon winds. Upwelling results in high biological productivity and a distinctive assemblage of plankton species in the surface waters off Oman that are preserved in the sediments along the Oman continental margin, creating a geologic record of monsoon-driven upwelling. Sediments recovered from the Oman continental margin during Ocean Drilling Program leg 117 provide an opportunity to examine how upwelling has varied during the late Quaternary, spanning a longer interval than piston cores recovered prior to the ODP cruise. Variations in foraminifer shell accumulation and in the relative abundance of Globigerina bulloides indicate dominant cycles of variation at 1/100 kyr, the dominant frequency of glacial-interglacial variations, and at 1/23 kyr, the frequency of precessionally driven cycles in seasonal insolation. The strongest monsoon winds (indicated by increased upwelling) occurred during interglacial times when perihelion was aligned with the summer solstice, an orbital change that increased the insolation received during summer in the northern hemisphere. During glacial times upwelling was reduced, and although the precessional cycles were still present their amplitude was smaller. At both frequencies the upwelling cycles are in phase with minimum ice volume, evidence that glacial-interglacial climate changes also include changes to the climate system that influence the low-latitude monsoon. We attribute the decrease in the monsoon winds observed during glacial times to changes in bare land albedo over Asia and/or to changes in the areal extent and seasonal cycle in Asian snow cover that decrease the summer land-sea temperature contrast. Other mechanisms may also be involved. These new upwelling time series differ substantially from previous results, however the previous work relied on cores located farther offshore where upwelling is less intense and other physical mechanisms become important. Our results support the observations derived from atmospheric general circulation models of the atmosphere that indicate that both glacial boundary conditions, and the strength of summer insolation are important variables contributing to cycles in the monsoon winds during the late Quaternary.

Geochemistry of sediments of stage 5 of the Nordic seas

Stable isotope, foraminifera and ice rafted detritus (IRD) records covering the last interglacial (the Eemian) from 7 sediment cores in a transect from the Norwegian to the Greenland Sea are presented. The percentages of Neogloboquadrina pachyderma (s.) and Globigerina quinqueloba, foraminiferal content, and to some extent planktonic stable isotope records, demonstrate marked, regional changes in surface water conditions. Importantly, the variability in the abundances of subpolar foraminifera and foraminiferal content are not coherent, implying that these two types of proxies fluctuated independently of each other and most likely reflect changes in sea surface temperature and surface water carbonate productivity, respectively. Paleoceanographic reconstructions demonstrate significant movements of the oceanographic fronts. At the warmest periods, the Arctic front was located far west of the present-day location, at least within the Iceland Sea region. At 126-125 ka, this was most probably due to a stronger or more westerly located Norwegian current. Within the later warm intervals, higher heat flux to the western part of the basin reflects a combination of a stronger Irminger current and/or a weaker east Greenland current. During the main cold spell at ~124 ka, a diffuse Arctic front had a more southeasterly location than today, and intrusion of Atlantic surface waters was probably limited to a narrow corridor in the Eastern Norwegian Sea. A general correspondence between minima in sea surface temperatures and light benthic delta18O may indicate enhanced influx of freshwater to the basin within the cold events. At least in the Norwegian Sea, we find some evidence that the changes in surface water conditions are associated with changes in deep water ventilation. The majority of the fluctuations may be related to occasional breakdown or reduction of the thermohaline circulation within the Nordic seas. In the earliest Eemian, this could result from meltwater forcing. During the remaining part of the last interglacial the fine balance between temperature and salinity, which the deep water formation is depending on, may have been disturbed by periodic increases in fresh water supply or variable influx of warm Atlantic surface waters.

Macerals in surface sediments of the Laptev Sea (Arctic Ocean)

Surface sediments from the Laptev Sea and adjacent continental slope were studied for their composition of particulate organic matter (OM) by means of maceral analysis. The composition of macerals in sediments gives information about the environment, terrigenous supply from the hinterland, and marine OM. With reference to their biological sources, we distinguish between terrigenous and marine macerals. We found that the particulate OM in the surface sediments of the Laptev Sea is predominantly of terrigenous origin (mean: 78%). However, distinct variations exist when looking in detail. In the shelf area, sediments may contain up to 99% terrigenous OM. Freshwater algae occur directly north of the river mouths, reflecting the strong fluvial influence. Relatively high amounts of marine OM (20-40%) are restricted to the upper continental slope, the Vilkitsky Strait and west of the New Siberian Islands, explained by increased surface-water productivity due to increased fluvial nutrient supply, open-water conditions, and phytoplankton blooms at the ice-edge.

Distribution of clay minerals in surface sediments of the Eurasian Arctic Ocean

Clay-mineral distributions in the Arctic Ocean and the adjacent Eurasian shelf areas are discussed to identify source areas and transport pathways of terrigenous material in the Arctic Ocean. The main clay minerals in Eurasian Arctic Ocean sediments are illite and chlorite. Smectite and kaolinite occur in minor amounts in these sediments, but show strong variations in the shelf areas. These two minerals are therefore reliable in reconstructions of source areas of sediments from the Eurasian Arctic. The Kara Sea and the western part of the Laptev Sea are enriched in smectite, with highest values of up to 70% in the deltas of the Ob and Yenisey rivers. Illite is the dominant clay mineral in all the investigated sediments except for parts of the Kara Sea. The highest concentrations with more than 70% illite occur in the East Siberian Sea and around Svalbard. Chlorite represents the clay mineral with lowest concentration changes in the Eastern Arctic, ranging between 10 and 25%. The main source areas for kaolinite in the Eurasian Arctic are Mesozoic sedimentary rocks on Franz-Josef Land islands. Based on clay-mineral data, transport of the clay fraction via sea ice is of minor importance for the modern sedimentary budget in the Arctic basins.