Contact: Prof. Dr. Martin Frank
Arctic Ocean circulation and climate, as we know them today, have not been the rule in the geological past but rather represent an exceptional situation. This was shown in several publications on geochemical analyses of unique marine sediments recovered in the central Arctic Ocean. These results show that the Arctic Ocean had surface water temperatures of up to 24°C at 55 million years ago. At 45 million years ago the occurrence of ice rafted debris shows that ice sheets must have already have existed around the Arctic Ocean at that timeand not only about 30 million years later as previously assumed. A major transition from oxygen-poor to well oxygenated deep waters 17.3 million years ago indicates that the Fram Strait, which is the only deep water connection of the Arctic Ocean with the Atlantic Ocean, already opened at this time and allowed the establishment of a well-ventilated ocean basin. Isotope geochemical results suggest that the Arctic deep circulation was strongly influenced by sea ice formation during most of the past 15 million years, including the glacial stages of the Late Quaternary, and was not predominantly controlled by inflowing Atlantic waters, as is the case today.
The Arctic Ocean only has a limited exchange with the global ocean, whereby the Fram Strait between Greenland and Svalbard is the only deep water connection to the Atlantic Ocean. It is this connection that allows the supply of oxygen to the deep Arctic Ocean. Most previous studies based on tectonic models suggested that the opening of the Fram Strait for deep water exchange between the two basins occurred at about 10 million years ago. This and the paleoceanographic and paleoclimatic evolution of the Arctic Ocean could not be further investigated because despite the fact that the Arctic Ocean has been a sensitive responder and potentially an important driver of global climate change, the Arctic’s pre-Quaternary oceanographic history (prior to about 500,000 years ago), including the transition from a “greenhouse” to an “icehouse” world, has until recently, been inaccessible due to a lack of continuous sedimentary records. This was mainly due to the technical difficulties to drill long sediment cores in an ice-covered ocean.
Unraveling the history of the Arctic Ocean
The central Arctic sediments recovered in summer 2004 during the Arctic Coring Expedition (ACEX, IODP Leg 3021) near the North Pole on the Lomonosov Ridge (87°5N, 137°E; 1,250m water depth) provided, for the first time, a continuous sedimentary archive, from which Neogene changes of Arctic oceanography and climate can be reconstructed. The expedition was carried out by drill platform supported by two ice breakers, which guaranteed that the platform could maintain its position without being displaced by the drifting sea ice (Fig. 1). The 428 m of recovered sediments reach back into the Late Cretaceous and for the first time allow investigation of the evolution of Arctic climate and ocean circulation (Fig. 2). These sediments are very difficult to date, in particular in the upper 200 m, where detrital sediments essentially barren of any fossils prevail. The sediments of the upper 150 m were dated at IFM-GEOMAR by means of the cosmogenic radionuclide 10Be, which has a half-life of 1.5 million years. The dating showed that the sediments have an age of 12.3 million years at this depth and that the sedimentation rates on the order of 14.5 m/million years were similar to open ocean sites outside the Arctic Ocean (Frank et al., 2008).
The oldest part of these records revealed shallow and brackish waters and relatively high prevailing temperatures on the order of about 15-20°C in the Arctic region until about 46 Million years ago, which peaked at 24°C during the Paleocene Eocene Thermal Maximum periods (Moran et al., 2006). More importantly, however, the record showed that a major cooling and the first evidence for sea ice, as documented by large ice rafted debris, occurred as early as 45 million years ago. This is about 30 million years earlier than previously assumed and about synchronous with the onset of Antarctic glaciation. Thus this finding required the revision of the major paradigm that Antarctic glaciation already occurred 45 million years ago due to the tectonic configuration and thermal isolation of Antarctica and instead suggests that high latitude glaciations were the consequence of a global cooling trend.
Opening of the Atlantic-Arctic Gateway
One of the foci of research on these sediments was the timing of the establishment of the Fram Strait oceanic gateway between the North Atlantic and the Arctic Ocean, which had previously only been accessible through tectonic modeling. Above a 26 million year hiatus the ACEX sediments have an age of 18.2 million years and are characterized by organic-rich sediments that also prevailed prior to the hiatus and document that the deep Arctic Ocean was not well-ventilated but was rather an almost enclosed brackish basin which more resembled a large lake. At a depth of 190 m and an age of 17.3 million years there is a sharp transition from the organic rich, black sediments to organic-poor brownish sediments. This color change documents an increased supply of oxygen and thus the onset of deep ventilation. This ventilation could only be established with a Fram Strait opened to an extent that allowed large amounts of salt-rich Atlantic waters to penetrate the Arctic Ocean and supply oxygen to intermediate and deep waters (Jakobsson et al., 2007). The early opening was conformed new tectonic information as well as a model constraining the minimum width of the opening strait to allow deep convection.
Evolution of Arctic Ocean circulation
Most of the deep Arctic Ocean today is filled with Atlantic waters exchanging through Fram Strait, which today has a sill depth of about 2,500 m. A pronounced and stable freshwater layer at the surface originating from inputs of the large Russian rivers almost completely prevents any significant deep water formation in the Arctic Ocean itself.
New results obtained from the ACEX core show that this situation was an exception rather than the rule for most of the past 15 million years (Haley et al., 2008). This conclusion was drawn on the basis of the seawater isotope ratio of the element neodymium (143Nd/144Nd) that was extracted from the sediments. The Nd, which has characteristic isotope ratios in rocks as a function of their type and age, is transported to the ocean through weathering, where it provides information on the sources of water masses. Surprisingly the results showed that the Nd isotope signature of the seawater was much higher (more radiogenic) than the present day values (Fig. 3), with the exception of the warm periods of the past 400,000 years (Fig. 4). Such signatures indicate a pronounced influence of the weathering of basaltic rocks but on the Circum-arctic landmasses such rocks only exist in the form of the Siberian “Putorana flood basalts”. From this geologically unique setting and taking into account the evolution of the continental ice sheets of the past 140,000 years, it was then possible to reconstruct the circulation history of the deep Arctic Ocean. The signature of the basalts can only have arrived at 1,000 m water depth in the central Arctic Ocean if vast amounts of new sea ice formed near the basalt areas in the Kara Sea area (Fig. 2).
During sea ice formation the salt of the sea water freezes out and is rejected, thereby forming highly saline brines, which were denser than the surrounding sea water. These brines sank and transported the dissolved Nd isotope signature of the basalts to the sea floor where the sediment cores were recovered. Further, the obtained Nd isotope variations imply that the inflow of Atlantic waters was significantly reduced during most of the past 15 million years and during the glacial periods of the past 400,000 years. This also implies that the formation sites of North Atlantic Deep Water (NADW), which has been a very important component of global ocean circulation and of heat transfer between low and high latitudes, were most of the time not located in the Norwegian-Greenland Sea, similar to today, but further south, similar to the glacial periods of the Pleistocene.
1The Arctic IODP (Integrated Ocean Drilling Programme) drilling project was coordinated by ECORD (European Consortium for Ocean Research Drilling). 17 European nations, which participate in IODP are members of this organisation. ECORD is also responsible fort he planning and coordination of special missions, for which normal drill ships cannot be used due to extraordinary conditions, such as was the case during the ACEX expedition. IN scuh cases mission specific platforms need to be used to reach the scientific goals
Frank, M., Backman, J., Jakobsson, M., Moran, K., O’Regan, M., King, J., Haley, B.A., Kubik, P.W. and Garbe-Schönberg, D. (2008): Beryllium isotopes in central Arctic Ocean sediments over the past 12.3 million years: Stratigraphic and paleoclimatic implications.- Paleoceanography, in press.
Haley, B.A., Frank, M., Spielhagen, R.F. and Eisenhauer, A. (2008): Influence of brine formation on Arctic Ocean circulation over the past 15 million years.- Nature Geoscience 1, 68-72.
Jakobsson, M., Backman, J., Rudels, B., Nycander, J., Frank, M., Mayer, L., Jokat, W., Sangiorgi, F., O’Regan, M., Brinkhuis, H., King, J., and Moran, K. (2007): The Early Miocene onset of a ventilated circulation regime in the Arctic Ocean.- Nature 447, 986-990.
Moran, K., Backman, J., Brinkhuis, H., Clemens, S.C., Cronin, T., Dickens, G.R., Eynaud, F., Gattacceca, J., Jakobsson, M, Jordan, R.W., Kaminski, M., King, J., Koc, N., Krylov, A., Martinez, N., Matthiessen, J., McInroy, D., Moore, T.C., Onodera, J., O’Regan, A.M., Pälike, H., Rea, B., Rio, D., Sakamoto, T., Smith, D.C., Stein, R., St. John, K., Suto, I., Suzuki, N., Takahashi, K., Watanabe, M., Yamamoto, M., Farrell, J., Frank, M., Kubik, P., Jokat, W. and Kristoffersen, Y. (2006): The Cenozoic palaeoenvironment of the Arctic Ocean.- Nature 441, 601-605.
