Late Quaternary spatial and temporal variability in Arctic deep-sea bioturbation and its relation to Mn cycles
Changes in intensity and composition of bioturbation and trace fossils in deep-sea settings are directly related to changes in environmental parameters such as food availability, bottom water oxygenation, or substrate consistency. Because trace fossils are practically always preserved in situ, and are often present in environments where other environmental indicators are scarce or may have been compromised or removed by diagenetic processes, the trace fossils provide an important source of paleoenvironmental information in regions such as the deep Arctic Ocean. Detailed analysis of X-ray radiographs from 12 piston and gravity cores from a transect spanning from the Makarov Basin to the Yermak Plateau via the Lomonosov Ridge, the Morris Jesup Rise, and the Gakkel Ridge reveal both spatial and temporal variations in an ichnofauna consisting of Chondrites, Nereites, Phycosiphon, Planolites, Scolicia, Trichichnus, Zoophycos, as well as deformational biogenic structures. The spatial variability in abundance and diversity is in close correspondence to observed patterns in the distribution of modern benthos, suggesting that food availability and food flux to the sea floor are the most important parameters controlling variations in bioturbation in the Arctic Ocean. The most diverse ichnofaunas were observed at sites on the central Lomonosov Ridge that today have partially ice free conditions and relatively high summer productivity. In contrast, the most sparse ichnofauna was observed in the ice-infested region on the Lomonosov Ridge north of Greenland. Since primary productivity, and therefore also the food flux at a certain location, is ultimately controlled by the geographical position in relation to ice margin and the continental shelves, temporal variations in abundance and diversity of trace fossils have the potential to reveal changes in food flux, and consequently sea ice conditions on glacial–interglacial time scales. Down core analysis reveal clearly increased abundance and diversity during interglacial/interstadial intervals that were identified through strongly enhanced Mn levels and the presence of micro- and nannofossils. Warm stages are characterized by larger trace fossils such as Scolicia, Planolites or Nereites, while cold stages typically display an ichnofauna dominated by small deep penetrating trace fossils such as Chondrites or Trichichnus. The presence of biogenic structures in glacial intervals clearly show that the Arctic deep waters must have remained fairly well ventilated also during glacials, thereby lending support to the hypothesis that the conspicuous brown layers rich in Mn which are found ubiquitously over the Arctic basins are related to input from rivers and coastal erosion during sea level high-stands rather than redox processes in the water column and on the sea floor. However, the X-ray radiograph study also revealed the presence of apparently post-sedimentary, diagenetically formed Mn-layers which are not directly related to Mn input from rivers and shelves. These observations thus bolster the hypothesis that the bioturbated, brownish Mn-rich layers can be used for stratigraphic correlation over large distances in the Arctic Ocean, but only if post sedimentary diagenetic layers can be identified and accounted for in the Mn-cycle stratigraphy.