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doi:10.2204/iodp.proc.302.105.2006

Timescale and sedimentation rates

Although the Arctic Basin is virtually landlocked and receives an abundant input of freshwaters from the surrounding continents (Aagard and Carmack, 1989), it has long been believed that the sedimentation rates in the Arctic Basin during the Quaternary and late Tertiary were ~1 mm/k.y. (Clark, 1970, 1971, 1974, 1990, 1996; Clark et al., 1980; Witte and Kent, 1988). These early estimates were based primarily on the paleomagnetic stratigraphy of relatively short gravity and piston cores. More recently, a review of these and other data (Backman et al., 2004), along with an evaluation of sedimentation rates based on a consideration of bedrock ages and total sediment thickness, gave estimates of long-term sedimentation rates that are ~10 times higher than previously thought (i.e., ~1 cm/k.y.). This estimate is borne out by inspection of Figure F7A, which shows ~410 m of sediment overlying what is believed to be the ~57 Ma rifting unconformity.

Here we consider all biostratigraphic, lithostratigraphic, and paleomagnetic data that provide time control in the sedimentary sections recovered during Expedition 302. All sites are located within 16 km of each other (Fig. F18A, F18B). Because of the very flat-lying section of relatively uniform thickness on the Lomonosov Ridge, all sites cored are correlated based on lithostratigraphy, chemostratigraphy, and the MSCL core logs. We treat all the material recovered as increments of a single section and calculate sedimentation rates for this integrated section as a whole.

Biostratigraphy

One reason that it has been difficult to establish sedimentation rates for the Arctic Ocean sediments is that there are very few microfossils preserved in the upper part of the sedimentary section (Backman et al., 2004). Carbonate microfossils are extremely sparse below the Holocene and disappear altogether within the Pleistocene. Siliceous microfossils are also absent below the uppermost thin layer of Holocene sediments, in which they are rare. Thus, it has been difficult to use biostratigraphy as a guide to aid interpretations of paleomagnetic stratigraphies. In the sections recovered during Expedition 302, we depend almost exclusively on organic microfossils (dinoflagellate cysts) for biostratigraphic control. This is especially true for the Neogene, where siliceous microfossils are absent and where carbonate microfossils are poorly preserved and largely absent.

In the Paleogene, dark organic-rich sediments were recovered that contained abundant siliceous microfossils (diatoms, silicoflagellates, and ebridians) in addition to rich dinoflagellate assemblages. In one short middle Eocene interval, a few specimens of radiolarians were also found; however, all the other assemblages indicate a predominantly brackish water environment in which radiolarians are not commonly present. The brackish water, high-latitude diatom, silicoflagellate, and ebridian assemblages are not well calibrated to the paleomagnetic timescale. Dinoflagellate stratigraphy, however, is well suited for more brackish water environments and has recently been tied to paleomagnetic records and to well-dated calcareous nannofossil stratigraphies in the Norwegian Sea and other high- to mid-latitude areas (Eldrett et al., 2004; Firth, 1996; Mudge and Bujak, 1996; Bujack and Mudge, 1994; Brinkhuis, 1994).

First and last occurrences of important organic microfossils are given in Table T27. The silicoflagellate biostratigraphic datums are given in Table T28. The diatom biostratigraphic datums are listed in Table T29, and the foraminifer and the calcareous nannofossil datums are given in Table T30.

Paleomagnetic stratigraphy

Paleomagnetic stratigraphy provides the chronostratigraphic context into which biostratigraphic datums are placed. However, biostratigraphy provides an important guide to the interpretation of paleomagnetic reversal patterns. This is especially true for sections in which sediment recovery is incomplete or where the paleomagnetic record is discontinuous. The paleomagnetic record for Expedition 302 is discontinuous because of incomplete core recovery and because some parts of the section (particularly the middle Eocene) do not retain a strong paleomagnetic signal. Although the inclination record is complicated, it has a characteristic pattern than can be correlated from core to core and site to site (see Fig. F3 in the “Expedition 302 summary” chapter). Thus, the complicated nature of the record does not appear to be caused by simple disturbance of the core or of the local sedimentary section. With less than complete recovery and sparse biostratigraphic control, establishing a paleomagnetic stratigraphy from inclination data is open to multiple interpretations (see “Paleomagnetism”). Here we use our “preferred” model of paleomagnetic reversal boundaries (Table T31). This model is used in the Neogene for estimating sedimentation rates (Fig. F19).

Sedimentation rates

All the paleomagnetic and biostratigraphic datums are plotted in Figure F19. In developing the models for average sedimentation rates in the combined Lomonosov Ridge section, we rely primarily on paleomagnetic data and dinoflagellate biostratigraphic datums. Linear sedimentation rates are calculated based on the most reliable data points. (Fig. F19).

The sediment surface is assumed to have “zero” age. This assumption is supported by the presence of modern nannofossils species in the near-surface sediments. The first million years of the section contain a few calcareous nannofossils and dinocyst datums (Tables T27, T30) and three paleomagnetic chron boundaries (Tables T31, T32). They lie on a nearly straight line when plotted versus depth (Fig. F19), with a sedimentation rate of 19.9 m/m.y. (Table T33).

There is a very low sedimentation rate estimated between the base of the Jaramillo and the top of the Olduvai Subchrons (2.1 m/m.y.) (Table T33); however, both the N. pachyderma datum (noted at ~21 mcd) (Table T30) and the Habibacysta tectata datum appear to fit well with the paleomagnetic interpretation.

Below this point, sedimentation rates vary from ~11 m/m.y. to >20 m/m.y. to the base of Chron C5AAn. Given the occasional poor preservation and rare occurrence of the organic microfossils, the paleomagnetic stratigraphy is in good general agreement with the biostratigraphy (Fig. F19). Below Chron C5AAn, sedimentation rates drop markedly from 7 m/m.y. to slightly more than 1 m/m.y. in lithostratigraphic Subunit 1/5.

Immediately below lithostratigraphic Subunit 1/5, the color abruptly turns to gray and the magnetic signal becomes weak and uninterpretable. The next older dinoflagellate datum identified (between Cores 302-M0002A-44X and 5X) is the LO of W. gochtii (Table T27) with an age of 27 Ma (late Oligocene); however, there are other unidentified species of dinoflagellates and pollen in this interval that suggest a Miocene age for this sample. In and below this level there are also reworked specimens of dinoflagellates that are clearly of late Eocene and even older ages.

Cores 302-M0002A-44X, 45X, and 46X give ample evidence of rapid changes in depositional environment with distinct breaks in layering and marked changes in color. Between Core 302-M0002A-44X (~195 mcd) and Core 48X (~212 mcd), biostratigraphic datums range in age from 27 to 45.5 Ma. This, together with the clear breaks in color and layering in the sediments, suggests that much of the section in this interval has been removed by erosion. Just below what appears to be an erosional unconformity in the base of Core 302-M0002A-46X, there are several last occurrence datums for dinoflagellate species and one well-constrained first occurrence datum for a silicoflagellate species (FO of C. hexacantha) (Locker, 1996). The oldest last occurrence datum in the group of dinoflagellate events is the LO of T. delicata (45.5 Ma) at ~212 mcd. The FO of C. hexacantha (44.1 Ma) is at ~215 mcd. In this case, where erosion and reworking may have affected the position of a last occurrence datum, we take the FO of C. hexacantha as the more reliable age marker. Using this datum and the next older dinoflagellate datum (the FO of Phthanoperidinium clithridium) at 46.2 Ma, we estimate a sedimentation rate of 28.6 m/m.y. for this middle Eocene part of the section. The latter dinoflagellate datum is found in both Holes M0002A and M0004A and is used along with other data to correlate the two sections.

The next older, well-dated biostratigraphic datums are the top and bottom of the Azolla spp. event (48.6 and 49.2 Ma, respectively) (Eldrett et al., 2004). Combined with the FO of P. clithridium datum, these closely spaced events give an average sedimentation rate of 10.5 m/m.y. (Fig. F19). Between the FO of Azolla spp. (49.2 Ma) and the LO of D. oebisfeldensis (52.9 Ma), the sedimentation rate falls to 7.9 m/m.y. (Fig. F19).

The next older well-dated events are the closely spaced FO and LO of A. augustum. These events span the PETM and are dated at 55 and 55.6 Ma, respectively. The PETM lies close to the oldest identified paleomagnetic chron datum (top of Magnetochron C25n) (Table T30) found in the section. A straight line connecting the LO of D. oebisfeldensis datum and the paleomagnetic datum passes very close to the A. augustum datums (especially the LO of A. augustum) and gives an average sedimentation rate of 20.3 m/m.y. (Fig. F19).

The top of Chron 25n lies within 3 m of dinoflagellate datums LO of Palaeohystrichophora infusorioides and LO of Chatangiella verrucosa complex (dated at 69.42 and 72.5 Ma, respectively) (Table T27), and thus marks a sharp break in the record. This hiatus is probably associated with the rifting unconformity estimated to be ~57 Ma and lies near Core 302-M0004A-35X at 405 mcd. At and below this depth, the identified dinoflagellate datums grow rapidly older, stretching well into the Cretaceous, and have an average sedimentation rate of ~1.6 m/m.y. (Fig. F19). This average rate is consistent with a very slowly subsiding or episodically uplifted continental margin on which the lack of accommodation space for sediment leads to frequent erosional episodes and very slow average sedimentation rates.

Conclusion

The debate about the true average sedimentation rate in the Arctic Basin has been resolved. Average sedimentation rates on the Lomonosov Ridge are on the order of 10–20 m/m.y. In the Pleistocene section (that part of the record most frequently studied prior to Expedition 302), sedimentation rates are close to 20 m/m.y. This is >20 times the rate commonly applied to the gravity and piston cores studied by Clark and his colleagues (Clark, 1970, 1971, 1974, 1990, 1996; Clark et al., 1980), and it is ~2.5 times the long-term average rate proposed by Backman et al. (2004). The difference between the long-term average and the shorter-term averages presented here result from the presence of at least one substantial break in the sedimentary record. There may be as much as 30 m.y. missing between the middle part of the middle Eocene and the middle to lower Miocene. Given the limited age control, there may be other gaps in the record that we have not detected.

The major hiatus above the middle Eocene divides the cored section: the upper half is a siliciclastic section, with a few preserved biogenic remains. The lower half, below ~217 mcd, is relatively rich in biogenic debris and generally has finer-grained siliciclastic material. The preservation of the organic content of these sediments is likely to have resulted from relatively high productivity and a strong pycnocline controlled by salinity. Both the upper and lower parts of the section have average sedimentation rates ranging from ~10 to >20 m/m.y.

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