Canada Abyssal Plain Research Articles

Warming and depth convergence of the Arctic Intermediate Water in the Canada Basin during 1985–2006

The warming of the Arctic Intermediate Water (AIW) is studied based on the analyses of hydrographic observations in the Canada Basin of the Arctic Ocean during 1985–2006. It is shown that how the anomalously warm AIW spreads in the Canada Basin during the observation time through the analysis of the AIW temperature spatial distribution in different periods. The results indicate that by 2006, the entire Canada Basin has almost been covered by the warming AIW. In order to study interannual variability of the AIW in the Canada Basin, the Canada Basin is divided into five regions according to the bottom topography. From the interannual variation of AIW temperature in each region, it is shown that a cooling period follows after the warming event in upstream regions. At the Chukchi Abyssal Plain and Chukchi Plateau, upstream of the Arctic Circumpolar Boundary Current (ACBC) in the Canada Basin, the AIW temperature reached maximum and then started to fall respectively in 2000 and 2002. However, the AIW in the Canada Abyssal Plain and Beaufort Sea continues to warm monotonically until the year 2006. Furthermore, it is revealed that there is convergence of the AIW depth in the five different regions of the Canada Basin when the AIW warming occurs during observation time. The difference of AIW depth between the five regions of the Canada Basin is getting smaller and smaller, all approaching 410 m in recent years. The results show that depth convergence is related to the variation of AIW potential density in the Canada Basin.

The hydrography of the southern Canada Basin, 2002

Physical and geochemical data collected in 2002 during a multi-disciplinary expedition across the southern Canada Basin are interpreted here to provide context for the biological program that included observations from the entire water column. The mixed layer was shallow (15–20 m) and depleted of nitrate, indicating it was the limiting nutrient. Below the mixed layer a strong, multi-layered halocline complex, consisting of Pacific-origin summer and winter waters and Atlantic-origin lower halocline water, occupied the water column down to ~200 m. Pacific and Atlantic-origin waters were shallower by 25–50 m in comparison to data collected in 1997/1998, suggesting a progressive decrease in Pacific-origin water extent. Fram Strait Branch waters were warmer everywhere in the southern Canada Basin, indicating the arrival of the pulse of anomalously warm Atlantic-origin water observed in the three upstream basins during the 1990s. The highest temperatures (1.13°C) were observed along the flank of the Northwind Ridge in the core of the topographically steered boundary current. Comparison with 1997/1998 observations showed that this pulse of anomalously warm water advanced ~430 km in 4 years with an apparent speed of ~0.34 cm/s. Observation of high temperatures at a station in the Canada Abyssal Plain suggest that the boundary current bifurcates north of the Northwind Ridge, with one branch continuing eastward into the basin and the other turning southward, following the ridge.

Character, paleoenvironment, rate of accumulation, and evidence for seismic triggering of Holocene turbidites, Canada Abyssal Plain, Arctic Ocean

Four box cores and one piston core show that Holocene sedimentation on the southern Canada Abyssal Plain for the last 8010 ± 120 yr has consisted of a continuing rain of pelagic organic and ice-rafted clastic sediment with a net accumulation rate during the late Holocene of ⩽10 mm/1000 yr, and episodically emplaced turbidites 1–5 m thick deposited at intervals of 830 to 3450 yr (average 2000 yr). The average net accumulation rate of the mixed sequence of turbidites and thin pelagite interbeds in the cores is about 1.2 m/1000 yr. Physiography suggests that the turbidites originated on the Mackenzie Delta or its clinoform, and δ 13C values of −27 to −25%. in the turbidites are compatible with a provenance on a delta. Extant displaced neritic and lower slope to basin plain calcareous benthic foraminifers coexist in the turbidite units. Their joint occurence indicates that the turbidites originated on the modern continental shelf and entrained sediment from the slope and rise enroute to their final resting place on the Canada Abyssal Plain. The presence of Middle Pleistocene diatoms in the turbidites suggests, in addition, that the turbidites may have originated in shallow submarine slides beneath the upper slope or outer shelf. Small but consistent differences in organic carbon content and δ 13C values between the turbidite units suggest that they did not share an identical provenance, which is at least compatible with an origin in slope failures. The primary provenance of the ice-rafted component of the pelagic beds was the glaciated terrane of northwestern Canada; and the provenance of the turbidite units was Pleistocene and Holocene sedimentary deposits on the outer continental shelf and upper slope of the Mackenzie Delta. Largely local derivation of the sediment of the Canada Abyssal Plain indicates that sediment accumulation rates in the Arctic Ocean are valid only for regions with similar depositional sources and processes, and that these rates cannot be extrapolated regionally. The location of an elliptical zone of active seismicity over the inferred provenance of the turbidites suggests that they were triggered by large earthquakes. Distal turbidite sediment accumulation rates were more than two orders of magnitude greater than pelagic sediment accumulation rates on the Canada Abyssal Plain during the last 8000 years. This disparity reconciles the discrepancy between the high accumulation rates assumed by some for the Arctic Ocean because of the numerous major rivers and large ice sheets that discharge into this small mediterranean basin and the low pelagic sedimentation rates that have been reported from the Arctic Ocean.

Magnetic spherules in Arctic Ocean sediments

Five types of spherule were found in Quaternary sediments from the Canada Abyssal Plain. They include: type A spherules composed of magnetite and hematite surrounding metallic iron cores, exsolution blebs, or veinlets (Ni, Fe, Co); type B, of nearly pure homogeneous magnetite; type C, displaying intergrowths of magnetite and hematite and containing minor Ti (< 1%); type D, composed predominantly of magnetite with significant amounts of Ti (up to 4.8%); and type E, consisting of ilmenite and rutile with Ti, Si, Fe, Mn, Al, K, and Mg as major elements and Cr, Ca, and Na as minor elements. Ni and Co were detected only in a small metallic veinlet (with Fe) surrounded by magnetite containing significant amounts of Ti.Three possible sources are considered: industrial, volcanic, and extraterrestrial. Industrial input is unlikely in view of the great distances to possible sources and the relatively large size of the particles. The high Ti content of most of the spherules suggests a volcanic origin. The five type B spherules are the only possible candidates for a cosmic origin.

Very low‐frequency reverberation in the Arctic Ocean

During March and April of 1978 a camp was established on the Arctic ice over the Canada Abyssal Plain, including a horizontal receiving array, a multichannel data handling and storage system, and a digitally operated data analysis system. With the use of TNT charges of up to 880 pounds, acoustic reverberation in the Arctic Ocean was measured. Emphasis in data analysis was placed on the frequency range centered on 10 Hz. Reverberant (scattering) returns were obtained for elapsed times up to and greater than about 30 min (ranges greater than about 1300 km). The receiving array was about six wavelengths long, and helped to identify various topographic features in the Arctic Ocean. Measurement results and theoretical models will be presented and discussed. [These efforts were undertaken jointly with The Woods Hole Oceanographic Institution, the Lamont‐Doherty Geological Observatory, and the Bedford Institute of Oceanography. Work supported by the Arctic Program Office of ONR.]

Pleistocene Turbidites of the Canada Abyssal Plain of the Arctic Ocean

ABSTRACT Turbidites comprise about 27% of the cored sediments in the northeast and southeast parts of the Canada Abyssal Plain but only 2.5% of the cores from the north central part of the Canada Abyssal Plain. Turbidites have not been identified in cores from other areas of the Arctic Ocean. The most common sedimentary structures in the Canada Abyssal Plain turbidites are plane-parallel laminated Bouma B or D units, ripple cross-laminated Bouma C units, massive Bouma A units, and load deformation structures. The bulk of the deposits are composed of silt-sized particles. The sand-sized fractions are mostly well-sorted very fine sands. The turbidite sand-sized fractions are composed dominantly of quartz, chert, and detrital calcite. The mineralogy of the turbidites is very homogenous but does not identify a particular dominant source area. An averaged sedimentation rate of 83 mm/l,000 years can be calculated for part of the Canada Abyssal Plain based on a C14 age date of 700 ± 100 years BP for Foraminifera found in a brown lutite layer in 34 cores. The higher concentration of turbidites in the northeast and southeast parts of the Canada Abyssal Plain indicates that the Canadian Archipelago was the most likely source area for the deposits. The turbidity current flows possibly originated as a result of seismic shocks, or due to oversteepening of slopes because of high sedimentation rates, or as a result of lowered sea level during major periods of deglaciation.

The Floor of the Arctic Ocean in Photographs

Over 2,000 usable bottom photographs have been taken in the western Arctic Ocean. The 87 stations cover the major geomorphic provinces of this part of the Arctic Basin, including the Alpha Cordillera, Mendeleyev Ridge, and Canada Abyssal Plain as well as smaller features. The ridge and plain provinces differ markedly in their bottom characteristics. Scattered rocks, living animals and indications of bottom current are most prevalent on the ridges. Trails are most abundant on the abyssal plains. The differences are attributed to bottom current distributions and turbidity currents. Bedrock outcrops were observed on the tops of two knolls on the Mendeleyev Ridge.

Nepheloid layers and bottom currents in the Arctic Ocean

Between 1965 and 1969, fifty-one profiles of light scattering were made in the central Arctic Ocean from Fletcher's Ice Island (T-3). The profiles, taken with an in situ photographic nephelometer extend from just below the surface to the bottom. Two distinctly different types of profiles were observed. At all stations the strongest scattering occurs near the surface, decreasing with depth in the upper layers. Over the Canada abyssal plain, light scattering is almost constant below 2000 meters, decreasing slightly with depth all the way to the bottom so that the bottom water is the clearest. Over the ridges and rises surrounding the Canada basin, however, scattering increases with depth below an intermediate scattering minimum. The zone of deep light scattering on the ridges and rises is called the bottom nepheloid layer. The bottom nepheloid layer is evidently caused by fine material that is maintained in suspension by the turbulent flow. Four spot measurements of bottom currents on the Mendeleyev ridge gave speeds of 4 to 6 cm/sec. One spot measurement over the Canada abyssal plain gave a speed of less than 1 cm/sec. This indication of swifter current speeds over the ridges is supported by bottom photographs in which animal tracks are much less evident on the ridges than on the Canada abyssal plain in spite of a greater abundance of life on the ridges. This is attributed to the higher current speeds, which obliterate the tracks. The observations suggest a counterclockwise deep circulation in the Canada basin with the currents confined primarily to the sloping margins of the basin. This pattern of deep circulation is in agreement with ideas and experiments on deep circulation with a concentrated source and distributed surface sink. Deep water enters the basin over a sill and leaves by upward diffusion through the halocline into the surface water, which then flows out of the basin.

Quaternary sedimentation in the Arctic ocean

A distinct boundary between sediment types usually occurs at a depth of about 10 cm in bottom cores raised from the Alpha Rise in the Arctic Ocean. The sediment between the tops of the cores and the 10 cm boundary is a dark brown, foraminiferal lutite mixed with ice-rafted sand and pebbles. The sediment between the 10 cm boundary and a depth of about 40 cm is a light brown sand with ice-rafted material but few Foraminifera. The 10 cm boundary apparently represents the most recent change in pelagic deposition in this region and must be connected with climatic changes. Foraminifera from a zone between 7 and 10 cm have been dated by the C 14 method as 25,000 ± 3000 and as 30,000 years BP in two different samples. The 10 cm boundary itself has been dated as 70, 000 years BP by a uranium series method. If these dates are accepted, a low sedimentation rate of 1 1 2 to 3 mm/1000 years is indicated for the Alpha Rise and for the Arctic Ocean as a whole if pelagic sedimentation has been uniform over the entire ocean. Cores from the Canada Abyssal Plain differ in character from the Alpha Rise cores consisting primarily of olive-gray lutite without Foraminifera or ice-rafted material. This sediment was probably deposited by turbidity currents. A 3 mm layer of dark brown, foraminiferal lutite occurs at the top of the Canada Abyssal Plain cores. This layer is similar to the upper layer in Alpha Rise cores and apparently represents continued pelagic deposition since the last turbidity current. Foraminifera from this upper 3 mm layer have been dated as 700 ± 100 years BP by the C 14 method. The conclusion is that pelagic sedimentation has continued unchanged in the Arctic Ocean from about 70,000 years ago to the present. This implies that the present ice cover has existed for that length of time.