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doi:10.2204/iodp.sp.302.2004
SCIENTIFIC OBJECTIVES
There are two major objectives: understanding the paleoceanographic history and the tectonic evolution of the central Arctic Ocean.
- The history of Arctic paleoceanography is so poorly known that we can look at the recovery of any material as a true exploration that will, by definition, increase our knowledge and understanding of this critical region.
- The tectonic objectives are focused on ridge evolution. If proven to be a continental fragment, it represents truly unique global information on the relative strength of continental and oceanic lithosphere.
Paleoceanographic objectives
There are a number of specific paleoceanographic objectives, questions that can be framed on results from lower latitudes, for which we believe there are testable hypotheses and that fully fit the scientific objectives outlined in the IODP Initial Science Plan; we offer some examples of these below.
History of ice rafting
Recent drilling in the Norwegian, Iceland, Irminger, and Greenland Seas has shown that the first coarse-grained ice-rafted material seems to appear earlier off southern Greenland than in the Fram Strait - Yermak Plateau region (Thiede and Myhre, 1996). Does this trend continue into the central Arctic Ocean? Did the cooling and glacial inception occur earlier in the sub-Arctic than in the central Arctic or vice versa? These questions can be addressed only through sampling of central Arctic seafloor sediments. The presence or absence of ice-rafted material in a constrained stratigraphic context (see below) should directly address this issue.
Local versus regional ice-sheet development?
Drilling results from the Fram Strait and Yermak Plateau regions have shown a series of middle and late Miocene pulses of ice rafting (14 Ma, 10.8-8.6 Ma, 7.2-6.8 Ma, 6.3-5.5 Ma, and continuing in sediments younger than 5 Ma.) (Thiede and Myhre, 1996). Do these represent local Svalbard ice expansion events or can the events also be observed in the central Arctic? The resolution of this issue has important ramifications on the climatic history of the Arctic. Again the presence or absence of ice-rafted material in a constrained stratigraphic context should provide the means to determining the answer to this question.
Density structure of Arctic Ocean surface waters, nature of North Atlantic conveyor and onset of Northern Hemisphere glaciation
Aargard and Carmack (1994) proposed that the convective renewal rate and nature of large-scale North Atlantic/Nordic Seas circulation is dependent on the fresh-water supply from the Arctic Ocean. Driscoll and Haug (1998) also call upon changes in fresh-water input (from Siberian rivers) to facilitate ice formation and contribute to the onset of Northern Hemisphere glaciation. A decrease in fresh-water supply would move the present site of deep-water North Atlantic convection from the Greenland Sea into the central Arctic Ocean basins; this model implies a virtually ice-free Arctic Ocean. The contrast from ice-covered, well-stratified (oxygen-poor) Arctic Ocean waters to ice-free waters with free air-sea exchange (well oxygenated) will undoubtedly generate a recognizable signal in the sediments accumulating on the seafloor. A major change in river input should yield a strong sedimentological signal and deposit pollen and spores. These signals which can only be measured in the Arctic Basin should also be expressed in a number of other paleoceanographic proxies including, major and/or trace element geochemistry (i.e., MnO content), as well as in the isotopic composition of the calcareous benthic forams, if present.
Timing and consequences of the opening of the Bering Strait?
Consistent with the model of Aargard and Carmack, Stigebrandt (1981) suggests that a decrease in fresh-water supply combined with a shut-off of Bering Strait inflow would result in the virtual loss of sea ice. Classically, the opening of the Bering Strait has been recognized by a dramatic change in the composition of shallow-water marine faunas (e.g., Marincovich, et al., 1990) and in particular the influx of Pacific boreal molluscs to Iceland (Einarsson et al., 1967). Ice-rafted debris should reveal when sea ice first formed in the Arctic Basin. Is the timing of this first permanent sea-ice cover coincident with the arrival of the Pacific boreal molluscs to Iceland?
Land-sea links: response of Arctic to Pliocene warm events
Svend Funder and colleagues (1985) have demonstrated that northernmost Greenland was forested in the late Pliocene. Was this warm event local or regional? What was the Arctic Ocean doing at this time? Was biogenic carbonate preserved in the Arctic Basin at this time?
Development of deep Fram Strait and deep-water exchange between Arctic and GIN (Greenland, Iceland, Norway) seas/world ocean
The Fram Strait represents the only deep-water connection between the Arctic and the world ocean. The timing of the formation of this passage is critical to the development of global circulation models. Several reconstructions exist (based mostly on tectonic arguments, e.g., Lawver, et al., 1990, Eldholm et al., 1990, Kristoffersen, 1990b) that place opening at times ranging from early Oligocene to late Miocene. What would the effect of the outflow of Arctic bottom waters have on the environment within the Arctic Basin?
History of biogenic sedimentation
The four pre-Pleistocene cores from the Alpha Ridge (with ages of ~70 and ~35 Ma, respectively), all consist of black biosiliceous muds that indicate poorly ventilated bottom waters. Was the Arctic continuously biosiliceous and poorly stratified between 50 and 35 Ma? (Our drilling strategy will probably only take us back to the early Eocene). Plio-Pleistocene cores from Fram Strait and Yermak Plateau all contain biogenic carbonates. When did the transition from the dominance of biosiliceous sedimentation to carbonate-dominated sediments occur? Is this transition related to the strength of North Atlantic advection into the high latitudes?
Stratigraphic control
Dating of Arctic Ocean sediments offers a classic problem in stratigraphy. When considering the general lack of information about the composition and microfossil contents of “pre-Pleistocene” sediments in the central Arctic, it appears pointless to speculate about the abundance and preservation of the various microfossil groups (e.g., foraminifers, nannofossils, radiolarians, diatoms, silicoflagellates), although spores, pollen, and dinoflagellates are likely to occur consistently. Magnetostratigraphy and various isotopic methods (e.g., Sr, U-Pb) in combination with biostratigraphy should ensure adequate chronological control. The use of ion microprobe techniques will allow in-situ analysis of element and isotope compositions of geological samples on a micrometer scale. Zircon, monazite and sphene are routinely analyzed for U-Pb ages >20 Ma using ion mass-spectrometry, where ages are determined on individual grains, making the technique well suited for sediment core material.
We must take into account the possibility that foraminiferal calcite may be largely lacking in the Lomonosov Ridge sediments, either due to carbonate dissolution or to paleoecological exclusion, thus preventing us from applying the conventional paleoceanographic proxy methods provided by stable isotope and trace element analysis of foram shells. Still, we consider that the wide array of existing analytical techniques in sedimentology, sediment physical properties, geochemistry, and paleontology, which can be applied to the Lomonosov Ridge sediments will yield adequate answers to our key questions. Available paleoceanographic proxy indicators include, for example, Plio-Pleistocene biogenic carbonate, dinoflagellates, pollen and spores, silicoflagellates, diatoms, O-isotopes in biogenic silica, fishapatite stable isotopes, etc. Spectral signatures of sediment color banding and provenance studies of IRD are also useful tools for deciphering the Arctic paleoenvironmental puzzle.
Tectonic setting
The Lomonosov Ridge and the Eurasia Basin developed during the Late Cretaceous and Cenozoic, substantially expanding the Arctic Ocean basin and opening a deep-water connection to the North Atlantic. The Lomonosov Ridge has an asymmetric architecture expressed in its central part by strata prograding towards the Amerasian Basin. The topsets have been eroded away. The units are unconformably overlain by a several hundred metre thick drape of velocity <2 km/s (Jokat et al., 1992). In contrast, the Eurasia Basin side of the ridge is a steep terrace of narrow fault blocks which accommodate more than 4 km of vertical relief relative to basement of the Amundsen Basin (Poselov et al., 1998; Sorokin et al., 1998).
The ridge structure changes character from a main block in the central narrow part to a more broadly faulted area towards the Laptev Sea (Jokat, 1998) as well as the Greenland and Canadian margin (Coakley and Cochran, 1998). The central narrow part of the Lomonosov Ridge near the North Pole exhibits a strong uneven reflection below about 600 m of sediments (Kristoffersen, 1998). These reflections resemble the acoustic image of basalt flows that also have been interpreted to cover basement on the margin north of Franz Josef Land and Kvitøya (Baturin, 1987), and may suggest a more-or-less continuous basalt province between Franz Josef Land and Ellesmere Island during Cretaceous time (Kristoffersen, 1998). Interpretation of the late Paleozoic and Mesozoic paleoenvironment of the northern margin suggests that the area to the north of Svalbard and Franz Josef Land was for the most part elevated to or above sea level from the Permian through Cretaceous, except for the Early Triassic and Late Jurassic (Doré, 1991). Present geological information of pre-Cenozoic rocks from the Lomonosov Ridge is limited to piston core recovery (Eurasian flank near 89°N; Grantz et al., 1998; 2001) of monolithic rubble of indurated siltstone clasts containing reworked Devonian and Carboniferous spores, zircons of latest Permian age, and spores of a Jurassic and Cretaceous fern.
Tectonic objectives
The Lomonosov Ridge is more than 1500 km long and less than 150 km wide. If proven to be a continental fragment, it represents truly unique global information on the relative strength of continental and oceanic lithosphere. The olivine rheology of the oceanic lithosphere is estimated to be three times stronger than typical continental lithosphere that includes a 35 km-thick continental crust of predominantly quartz/plagioclase rheology (Vink et al., 1984). Juxtaposed oceanic and continental lithosphere in a tensional stress field would be weakest landward of the continental shelf edge (Lavier and Steckler, 1997; Steckler and ten Brink, 1986) and the Lomonosov Ridge may have formed as a result of this mechanism. The tectonic objectives for drilling on the Lomonosov Ridge are:
- To investigate the nature and origin of the Lomonosov Ridge by sampling the oldest rocks below the regional unconformity in order to establish the pre-Cenozoic environmental setting of the ridge.
- To study the history of rifting and the timing of tectonic events that affected the ridge.