Geophysical Research Letters

Relative sea level variations in the Chukchi region - Arctic Ocean - since the late Eocene

Authors


Corresponding author: Anne Hegewald, Alfred Wegener Institute for Polar and Marine Research, Post Box 120161, 27515 Bremerhaven, Germany. (anne.hegewald@web.de)

Abstract

[1] Relative sea level (RSL) variations are a result of tectonic activity, changing of the water volume in ocean basins (e.g. due to increasing/decreasing of ice volume, evaporation) and variations in regional to global climate, which influence erosional processes and material transport. We present multi-channel seismic data combined with dated sediment horizons from the Chukchi Shelf, Arctic Ocean. Based on a series of prograding sequences in the upper 4 km of sediments and the method of seismic sequence stratigraphy, we introduce the first RSL curve for the Chukchi region, beginning in the late Eocene (40 Ma). The comparison of the Chukchi RSL curve with the global RSL curve shows that RSL lowering events in the Chukchi region do not correlate with global events for the Eocene/Oligocene - early Miocene. Between the Eocene/Oligocene and the late Oligocene, the Chukchi RSL variations were small (< 100 m). Since the late Oligocene the Chukchi RSL increased until the opening of the Fram Strait in the early Miocene. We show that the Chukchi RSL variations are representative for the Arctic Ocean, and conclude that the Arctic Ocean was an isolated basin for the Eocene/Oligocene - early Miocene.

1 Introduction

[2] Since the opening of the Amerasian Basin (Arctic Ocean) in Jurassic to early Cretaceous time [Grantz and May, 1982], several seaways connected the Arctic Ocean with the global oceans. In the Eocene, the Turgai Strait (located between Europe and Asia) formed a seaway between the Arctic and the Tethys Ocean, and was closed by marine regression in late Eocene or Oligocene time [Marincovich et al., 1990]. The marine regression is thought to be the result of a significant reorientation of relative plate motions, which occurred in the circum Arctic Ocean when seafloor spreading in the Labrador Sea terminated and the separation of NE Greenland from Svalbard began [Chalmers et al., 1993]. About 15 Ma later, the Fram Strait opened in the early Miocene and recently represents the only deep-water connection between the Arctic and Atlantic Ocean [Engen et al., 2008]. The youngest shallow seaway, the Bering Strait, connects the Arctic with the Pacific Ocean and opened in the Miocene/Pliocene [Gladenkov et al., 2002; Verhoeven et al., 2011]. For the late Eocene to the early Miocene, nothing is known about the existence of seaways to the Arctic Ocean or about the RSL variations in the High Arctic.

[3] We present five multi-channel seismic (MCS) lines and the depth-velocity models of two sonobuoys gathered by the Alfred Wegener Institute (AWI) during the ARK-XXIII/3 expedition in 2008 [Fig. 1a; Jokat et al., 2009]. For the MCS data acquisition a 3000 m long streamer (240 active channels, group interval of 12.5 m) and an air gun array of 4 G-Guns (total volume of 32 l, fired with 200 bar every 15 s) were used. The sonobuoy data were recorded in parallel to the MCS data (recording times 3 h to 5 h, respective offsets 21 km to 34 km between seismic source and sonobuoy). The age control was carried out using two further data sets (Fig. 1a): (1) biostratigraphy from five offshore exploration wells drilled northwest of the coast of Alaska [Sherwood et al., 2002], and (2) additional seismic reflection lines located on the southern Chukchi Shelf [Grantz et al., 2004; Dinkelman et al., 2008; Verzhbitsky et al., 2008; Drachev, 2011]. In total, six marker horizons with ages between Barremian/Hauterivian and Top Miocene were correlated to the AWI MSC data (Figs. 1b, 2a). The marker horizons in the AWI MCS lines were used to calculate the RSL variations for the Chukchi region.

Figure 1.

Map of the Chukchi region and dated marker horizons in the AWI MCS data. (a) Bathymetric map [IBCAO; Jakobsson et al., 2012] showing the location of the AWI MCS lines in the Chukchi region and the southern part of the Mendeleev Ridge. The additional seismic reflection lines were acquired by TGS-NOPEC, ION-GXT, USGS and WesternGeco. The five exploration wells were drilled by Shell and Chevron USA. (b) Six marker horizons were dated within the AWI MCS data using the age information from exploration wells and additional seismic reflection lines. Annotated ages are after Gibbard et al. [2010]; CAP – Chukchi Abyssal Plain, CB – Canada Basin, CP – Chukchi Plateau, CS – Chukchi Shelf, ESS – East Siberian Shelf, MR – Mendeleev Ridge.

Figure 2.

Three interpreted MCS lines from the Chukchi Shelf (Fig. 1). (a) Shelf part of line 60 with six dated horizons and the interval velocities of the two sonobuoys SB 6 and SB 7. (b) Upper 3 s TWT of line 60 showing a series of ten prograding sequences. Marker horizons are indicated by numbers 1 to 4. (c) Shelf part of line 70 showing four prograding sequences between the horizons Top Miocene and Top Oligocene. (d) Shelf part of line 40 showing the same four prograding sequences as line 70; LST – lowstand systems tract, TST – transgressive systems tract, HST – highstand systems tract.

2 Calculating Relative Sea Level Variations

[4] For the calculation of the RSL variations, the AWI MCS data were interpreted following the method of seismic sequence stratigraphy [Catuneanu, 2006]. This method is based on the identification of three geometric elements in the seismic data: (1) stratal termination (e.g. truncation; Figs. 2b-d, black arrows), (2) stratigraphic surfaces (e.g. sequence boundary; Figs. 2b-d, red dotted lines), and (3) systems tracts and sequences. The latter includes the lowstand, transgressive and highstand systems tracts (LST, TST and HST, respectively; Figs. 2b-d, colored surfaces). Hence, ten prograding sequences were identified (Figs. 2b-d) within the upper 3 s two-way time (TWT; equates to 4 km) of the Chukchi Shelf sediments.

[5] A RSL curve was constructed by first picking points of coastal onlap onto the margin slope [Figs. 2b-d, black dots; Vail et al., 1977a]. The TWT value of each point was converted to depth using the following equation, which was derived from sonobuoy data:

display math

[6] From the resulting depth values a linear trend was subtracted which represents a constant compaction based on the weight of deposited sediments and compaction of underlying layers. A constant compaction was modeled because of linear increase of seismic velocity with depth, resulting from sonobuoy data (Fig. 2a), and slight variation in sedimentation rate over time (Fig. 3). Finally, the magnitudes of the Chukchi RSL variations were estimated calculating the difference between the depths of each successive point of coastal onlap.

Figure 3.

Comparison of three Cenozoic RSL curves. Left graphs: global RSL curves [Vail et al., 1977b; Haq et al., 1987]. Right graph: RSL curve for the Chukchi region from this study. Dashed lines: RSL lowering events observed in both curves. Grey areas: two major lowering events of the RSL in the Chukchi region, but not in the global oceans; MBU – Mid Brookian Unconformity, PO – Pliocene.

[7] Minimum sedimentation rates (Fig. 3) were calculated by dividing the thickness of the shelf sediments between successive horizons with known ages by the time span.

[8] Regarding the uncertainties of the calculated RSL variations, the vertical resolution of our seismic data varies between 15 m below sea floor and 100 m in a depth of 8 km. The maximum error of the age model [Sherwood et al., 2002] is +/− 1.5 Ma [Gibbard et al., 2010]. The age uncertainty resulting from interpolation between the six marker horizons is less than 1 Ma.

3 Relative Sea Level Variations

[9] We identified ten prograding sequences on the distal Chukchi Shelf which reaches back to the late Eocene (Fig. 2b). Only four prograding sequences between Top Oligocene and Top Miocene could be observed on the distal Eastern Siberian Shelf (Figs. 2c-d). Below the Top Oligocene horizon, no more sequences could be identified because of faulting. The ten sequences on the distal Chukchi Shelf show different horizontal and vertical dimensions (Fig. 2b), which we infer were caused by the Cenozoic RSL variations (Fig. 3). The four sequences on the distal Eastern Siberian Shelf look more symmetrical with constant vertical and horizontal dimensions (Figs. 2c-d), which we interpret as an artifact due to the seismic reflection lines being oblique (rather than parallel) to the direction of progradation.

[10] Our Chukchi RSL curve is compared with two global RSL curves [Vail et al., 1977b; Haq et al., 1987; Fig. 3]. Neither the Vail et al. [1977b] nor the Haq et al. [1987] global RSL curves include data from the Arctic or Antarctic regions. The maximum RSL variations are about 400 m between the late Eocene and modern time (Fig. 3). From the late Eocene to the Eocene/Oligocene boundary the Chukchi and the global RSL are similar. A small relative marine regression in the late Eocene of less than 100 m can be observed in the three curves (Fig. 3). In this period (late Eocene-Eocene/Oligocene), the global climate became cooler [Zachos et al., 2001], the first large ice sheets appeared in Antarctica, and isolated glaciers evolved on East Greenland [Eldrett et al., 2007]. Until the Eocene/Oligocene boundary, the Turgai Strait connected the Arctic Ocean with the Tethys [Marincovich et al., 1990]. The minimum sedimentation rate on the Chukchi Shelf during the late Eocene was 5 cm/ky, and sediments of the HST were eroded during the lowering of RSL at the Eocene/Oligocene boundary (Fig. 2b; below horizon 3). This large Chukchi RSL lowering event of more than 200 m at the Eocene/Oligocene boundary is not observed in the global curves (Fig. 3). The Turgai Strait fell dry, resulting in a continental pathway between Europe and Asia [Hou et al., 2011]. At this time, a significant reorientation of relative plate motions occurred in the circum Arctic Ocean, when seafloor spreading in the Labrador Sea ended and the separation of NE Greenland from Svalbard was initiated [Chalmers et al., 1993]. Above all, the global temperature decreased by 4 degrees Celsius and the climate changed from global greenhouse to icehouse conditions [Zachos et al., 2001].

[11] In the following 15 Ma (Eocene/Oligocene - early Miocene), the Chukchi RSL variations are different in timing and magnitude compared with the global curves (Fig. 3). From the Eocene/Oligocene boundary to the middle Oligocene the Chukchi RSL variations were small, whereas the global curves show a pronounced relative marine transgression for the entire time period (Fig. 3). Since the late Oligocene, the Chukchi RSL rose about 100 m and increased an additional 300 m in the early Miocene (Fig. 3). In the middle Oligocene, the global RSL fell between 200 m [Haq et al., 1987] and more than 350 m [Vail et al., 1977b], and varied in short regressive and transgressive episodes with a total rise of about 150 m until the early Miocene (Fig. 3). During most of the Oligocene, the global climate was constant and cooler than in the Eocene [Zachos et al., 2001]. The consequence for the Chukchi Shelf were a low sediment influx with a minimum sedimentation rate of 4 cm/ky, and a continuous progradation of the Chukchi Shelf margin with no erosion of the shelf sediments until the Oligocene/Miocene boundary (Fig. 2b; between horizon 2 and 3). From the late Oligocene to the early Miocene, temperature decreased by about 2 degrees Celsius [Zachos et al., 2001]. In the early Miocene, the Chukchi RSL increased considerably (300 m), in contrast to the small rise observed globally (less than 50 m; Fig. 3). This large Chukchi relative marine transgression produced a pronounced erosion of the palaeo-shelf edge (Fig. 2b; below horizon 2). With the opening of the Fram Strait in the early Miocene [Engen et al., 2008], the Chukchi RSL fell dramatically by around 400 m (Fig. 3) and reconnected the Arctic Ocean to the global oceans (especially the Atlantic Ocean). In this period, the global RSL curves show a slow and continuous increase (Fig. 3). However, part of the shelf margin erosion may also be the result of a small regression near the top Oligocene horizon. That regression would fall between the slow rise in RSL during the latest Oligocene and the rapid rise in RSL during the early Miocene.

[12] The Chukchi RSL variations can be apply to the whole Arctic Ocean from the late Eocene, because Eocene-Oligocene age unconformities reported in sediments on the East Siberian Shelf [Sekretov, 2001], the Laptev Shelf [Franke and Hinz, 2005], and the Mackenzie Delta [Dixon et al., 1992], correlate with the RSL lowering events observed in the Chukchi curve. Moreover, the sediment influx in the Chukchi region was not influenced by e.g. large river systems, based on the low minimum sedimentation rates between 2 and 5 cm/ky since the late Eocene. Finally, geochemical analyses of the IODP core from ACEX expedition 302 to the central Lomonosov Ridge [Jakobsson et al., 2007] lead to the interpretation that the Arctic Ocean was predominantly isolated after the closure of gateways in the Eocene until the opening of the Fram Strait in the early Miocene [O'Regan et al., 2008]. In this period, large volumes of fresh water came from Canadian, Alaskan and Siberian rivers into the landlocked Arctic Ocean and reduced the salinity of the Arctic water [Jakobsson et al., 2007]. In the early Miocene, sediments on the central Lomonosov Ridge show erosion, redeposition of older sediment, and deposition of new sediment in a shallow water regime [März et al., 2011; Jakobsson et al., 2007], which correlates with our considerable RSL lowering at this time (Fig. 3). These observations suggest that our Chukchi RSL curve is representative of the entire Arctic Ocean, which we suggest was isolated from the global oceans from the Eocene/Oligocene boundary to the early Miocene (Fig. 3). Our observed RSL fall of 400 m in the early Miocene leads to the interpretation that the RSL in the isolated Arctic basin was higher than in the global oceans. With the opening of the Fram Strait, the Arctic Ocean water drained into the Atlantic Ocean. On the New Jersey Coastal Plain (northeast coast of the USA), an absolute sea level increase of about 15 m was observed at that time [18 Ma; Kominz et al., 2008].

[13] During Miocene times, when the Fram Strait deepened through seafloor spreading, the Arctic Ocean became a well-ventilated saline ocean because of the inflow of saline North Atlantic water [Jakobsson et al., 2007]. Since the opening of the Fram Strait, the variations of the Chukchi and global RSL show the same trend (Fig. 3). From early to middle Miocene, the Chukchi RSL increased about 150 m to 200 m in total (Fig. 3) and the Chukchi Shelf edge prograded continuously (Figs. 2b-d; above horizon 2), with a minimum sedimentation rate of 2 cm/ky. In this period the global temperatures increased until the middle Miocene climate optimum [Zachos et al., 2001]. Since the middle Miocene, the three curves show a general lowering trend, which correlates with the onset of global cooling and the evolution of partial and ephemeral ice sheets on the Northern Hemisphere (Fig. 3). At the Miocene/Pliocene boundary, the global and the Chukchi RSL fell approximately 100 m to 150 m (Fig. 3), and the Chukchi Shelf sediments were eroded (Figs. 2b-d; below horizon 1). In the Pliocene, the minimum sedimentation rate increased significantly to 10 cm/ky (5-fold increase relative to the Miocene). Corresponding results are reported from the Beaufort-Mackenzie area [McNeil et al., 2001]: regional uplift across the cratonic margin in the late Miocene, combined with eustatic lowstand, followed by tectonic quiescence and dry cool climatic conditions in the early Pliocene, produced widespread erosion and a 23-fold increase in the sedimentation rate relative to the early and middle Miocene. Furthermore, the Bering Strait between Alaska and Siberia opened and connected the Arctic Ocean with the Pacific Ocean [Gladenkov et al., 2002; Fig. 3]. Since the Pliocene, larger and permanent ice sheets covered the Arctic Ocean [Eldrett et al., 2007] and eroded the Chukchi Shelf sediments (Figs. 2c-d; below ocean bottom).

4 Conclusion

[14] We produce the first RSL curve for the Chukchi region, based on the interpreted AWI MCS data located on the Chukchi Shelf. Our Chukchi RSL curve is representative of the Arctic Ocean. Comparing the Chukchi and global RSL curves, we suggest that the Arctic Ocean was an isolated basin from the Eocene/Oligocene boundary to the opening of the Fram Strait in the early Miocene. The opening of the Fram Strait might have caused significant fresh water input into the North Atlantic Ocean and a consequent rise in the global RSL. Hence, the RSL in the isolated Arctic Ocean was higher than the global one.

Acknowledgments

[15] We thank the captain and the crew of R/V Polarstern as well as the seismic team during the ARK-XXIII/3 expedition for their excellent job. We acknowledge ION-GX Technology, the USGS and TGS-NOPEC for the insight in their seismic data to constrain the extrapolation of the logging information into our data. Furthermore, we thank Bernard Coakley, who led expedition MGL 11–12 (2011), for a first view of the MCS data, which are located in the “data gap” between the southern seismic network and the AWI MCS data.

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