Alpha Ridge Research Articles

Canadian expedition to study the Alpha Ridge complex: results of the seismic refraction survey

SUMMARY A crustal-scale seismic refraction survey has been conducted in the Arctic Ocean across the NE Alpha Ridge complex and its northern flank within the SE Makarov Basin. The data from the four reversed profiles have been analysed using 1-D and 2-D synthetic seismogram modelling schemes. Upper crustal velocities beneath these parts of the Alpha Ridge complex and Makarov Basin are surprisingly uniform at 5.0-5.2 km s-l, increasing smoothly and rapidly to ~6.5 km sK1 at depths of only 43 km. Below this level the velocities continue to increase relatively smoothly, reaching values of ~7.0 km s-l at depths ranging from 14 to 19 km. Mantle-type velocities of 23.0 km sK1 are found at depths of 36-44 km beneath some regions of the Alpha Ridge complex and 21-25 km beneath the SE Makarov Basin. Based on these results and other geophysical and geological data from the Arctic Ocean we interpret the Alpha Ridge complex as an Icelandic-type structure generated by Mantle Plume activity, and we suggest that the region of Makarov Basin surveyed is underlain by a thick oceanic

TWO CRETACEOUSSTELLARIMASPECIES:S. STEINYIANDS. DISTINCTA; THEIR MORPHOLOGY, PALAEOGEOGRAPHY AND PHYLOGENY

Stellarima steinyi (Hanna) Hasle & Sims, both in its resting spore stage and its presumed vegetative stage, is described from the late Cretaceous deposits of the U.S.S.R., the Alpha Ridge in the Arctic Ocean, and California, U.S.A. Stellarima distincta (Long, Fuge & Smith) Hasle & Sims is described from the late Cretaceous of California and comparisons are made with S. steinyi and briefly with the two living Stellarima species, S. microtrias (Ehrenb.) Hasle & Sims and S.slellaris (Roper) Hasle & Sims. Possible palaeoceanographic and evolutionary sequences are discussed.

Clay mineralogy of late Cenozoic sediments in the CESAR cores, Alpha Ridge, central Arctic Ocean

The clay-sized (< 2 μm) fraction of the silty and arenaceous lutites constituting CESAR cores 14 and 103 (Alpha Ridge, central Arctic Ocean) is composed predominantly of mica (40–60%), with subequal percentages (10–20%) of kaolinite and chlorite and lesser amounts (< 5%) of smectite, quartz, plagioclase, and potassium feldspar. Calcite and dolomite also occur, but only intermittently in the upper 1.2 m; dissolution is probably responsible for their absence in other units. The silty lutites have a constant mineralogy throughout the 4.5 m long (~ 4.25 Ma) section, whereas some of the arenaceous intervals in the upper 2.1 m have markedly higher amounts of kaolinite, calcite, dolomite, and, to a lesser extent, smectite. The silty lutites were most likely derived from the Beaufort Sea shelf during nonglacial periods, whereas the distinctive components in the sandy layers suggest that they were transported from the Canadian Arctic Islands and Greenland by glacial ice. The absence of kaolinite peaks in the lower half of the core implies that the western Arctic Islands were not glaciated prior to 2.1 Ma, a conclusion supporting previous findings that the climate of the Arctic was warmer in the Pliocene.

Magnetotelluric measurements over the Alpha Ridge

Magnetic and telluric field variations were recorded on the Arctic Ocean sea ice over the Alpha Ridge in April and May, 1983. The magnetotelluric measurements were made near the base camp of Operation CESAR (Canadian Expedition to Study the Alpha Ridge) to investigate the deep structure and electrical properties of the lithosphere and upper mantle beneath the ridge. Thirty days of usable data were obtained within the period range 120–20 000 s. During the experiment the base station drifted across a deep (2000 m) median valley or graben lying parallel to the ridge axis and then over a steep rise on its northern flank. One data set was obtained over the graben, and the remainder of the measurements were made as the station travelled back and forth over the rise. The directional properties of both the geomagnetic induction arrows and the magnetotelluric impedance tensors were consistent with changes in water depth along the drift path. One-dimensional inversions of E-polarization MT data revealed a resistive lithosphere about 70–85 km thick with a conductive (10 μm) upper mantle (asthenosphere?) below. Sea-water depths estimated in the inversion process agreed remarkably well with the known bathymetry, indicating that the ocean floor contained less than 100 m of highly conducting sediments along most of the drift path. Two-dimensional models required simulation of the topography of the entire section of the Alpha Ridge between the Canada and Makarov Basins in order that a satisfactory fit be obtained to the H-polarization data. Two-dimensional forward modelling indicates that the anisotropic character of the impedance measurements can entirely be attributed to the bathymetry of the ridge. The data provided no indication of structural inhomogeneity within the oceanic crust and lithosphere, consistent with results from companion seismic refraction and gravity surveys. The thickness of the lower conducting region was not resolved because reliable long period data ( T > 20 000 s) were not available for analysis. Impedance estimates at these periods were poorly determined and subject to rapidly rising tensor skew. They were also subject to bias errors caused by water movements, in particular the M 2 tide.

Maps of the Arctic Basin Sea Floor. Part II: Bathymetry and Gravity of the Alpha Ridge: The 1983 CESAR Expedition

The general scarcity of geophysical data in the Arctic Ocean Basin and the lack of knowledge about the evolution of the Amerasia Basin and of the nature and origin of the Alpha Ridge led, in 1983, to the undertaking of a multidisciplinary polar expedition, code-named CESAR 83 for Canadian Expedition to Study the Alpha Ridge. The expedition was supported by the Canadian Armed Forces, first by parachuting 18 airborne engineers onto the CESAR site to build a 1.6 km long airstrip on the pack ice, and then by deploying and two months later evacuating the expedition by military Hercules aircraft. .... One of the major CESAR accomplishments was a regional bathymetric and gravity survey over the Ellesmere Island continental shelf and eastern part of the Alpha Ridge. Using the CESAR data as well as all publicly available data collected over the past 35 years, 100 contour interval bathymetric maps and 5 mGal contour intervale gravity free-air anomaly maps were compiled. These extend from the Ellesmere Island coast to the 116 degrees W meridian. The sea floor maps depict the Alpha Ridge as a very broad mountain complex of rugged topography with ridges and valleys trending parallel to the ridge axis. ... Elliptically shaped positive anomalies centered over the continental shelf break suggest that the continental margin adjacent to the Alpha Ridge has the typical Atlantic-type structure characteristic of the rest of the North American polar margin. Preliminary interpretation of the gravity field indicates that the Alpha Ridge crust is composed of very thick rocks of laterally uniform density and composition. It is suggested that the eastern part of the Alpha Ridge may be a massive accumulation of mafic rocks of probable oceanic origin formed by volcanic activity. This article is identified as Part II of a series on Maps of the Arctic Basin Sea Floor and is preceded by a A History of Bathymetry and its Interpretation (Webster, 1983). Key words: CESAR 83 expedition, Alpha Ridge, bathymetry, gravity, sea ice SLAR imagery

The Alpha Ridge: Gravity, seismic and magnetic evidence for a homogenous, mafic crust

Gravity and bathymetric results from the 1983 Canadian Expedition to Study the Alpha Ridge (CESAR) have outlined positive free-air anomalies centred on the continental break off Ellesmere Island characteristic of normal Atlantic-type passive margins. These data confirm implications derived from depth-to-magnetic basement calculations that the ridge may not be structurally connected to the continent. Across the Alpha Ridge magnetic and gravity anomalies mimic the bathymetry. The magnetic anomalies apparently are not caused, to any great extent, by internal structures or magnetic reversals, but rather seem to result simply from variations in depths to a homogenous magnetic structure. The gravity anomalies across a 500 km wide section of the Alpha Ridge can be almost completely accounted for by topography, shallow sedimentary fill and a simple two-tier crustal model. This implies an extraordinary lateral density homogeneity unknown in continental structures of comparable size. Gravity models show the crustal thickness to increase gradually from 20 km at the Marvin Spur to 38 km at the ridge crest. A comparison of this model with a gravity model of the continental-type Lomonosov Ridge, which has a thickness of about 25 km, indicates that, at the same thickness of 25 km, the average crustal density of the Alpha Ridge is 0.08 Mg/m 3 greater. These gravity constraints, the unusually homogenous seismic velocity structure revealed by the CESAR studies, the homogeneous magnetic structure, and the extraordinary high intensity satellite magnetic anomaly associated with the Alpha Ridge, indicate that the ridge may be composed of a large pile of mafic rock, possibly unique on this planet.

First samples of acoustic basement recovered from the alpha ridge, Arctic ocean: New constraints for the origin of the ridge

The Alpha Ridge is one of three subparallel trending ridges that cut the Arctic Ocean. It is roughly Late Cretaceous to Eocene in age, and seismic refraction records suggest it comprises a thick sequence of oceanic crust. During the 1983 CESAR expedition 20 similar samples of acoustic basement were dredged from the walls of a major graben of the Alpha Ridge, at one site. These are the only basement samples ever recovered from the ridge and provide the first direct evidence for its nature, composition and possible origin. The basement samples are highly altered pyroclastic rocks composed almost entirely of basaltic volcanic clasts with little matrix. Although the rocks are highly altered, most primary textures and structures are preserved. Most clasts are highly amygdaloidal to scoriaceous, fine grained to glassy, and angular to subround with rare vesicle controlled boundaries. Little reworking is suggested because a single clast type predominates, many of the clasts are subangular, and any amount of reworking would result in destruction of the delicate scoriaceous clasts. Rare clinopyroxene phenocrysts comprise the only unaltered portion of the rocks. They are salitic in composition (Wo 49–53, En 32–41, Fs 11–15), with significant amounts of Ca, Al and Ti. Salitic clinopyroxenes are typical of alkalic basalts. Interpretation of the whole rock geochemistry based on relatively immobile elements, (Nb, Zr, Tio 2, and Y), and chondrite-normalized incompatible trace element and REE patterns indicates that the volcanic rock fragments are of alkalic basalt. Geochemical discriminators suggest a within-plate tectonic setting. Textural evidence suggests that the CESAR basement rocks were sampled from a rapidly emplaced submarine fallout deposit that was erupted at a depth at least less than 800 m and likely less than 200 m. High extrusive rates would have been required to build the ridge up to shallow depth prior to the cessation of volcanism. The alkalic affinity of the rocks strongly suggests that the Alpha ridge was not formed by volcanism at an island arc or a mature spreading centre. It is also unlikely that it formed as a “leaky” fracture zone. Alkalic basalts, however, are commonly associated with various types of oceanic aseismic ridges. It is suggested that the Alpha Ridge is an aseismic ridge that formed due to voluminous hotspot volcanism as spreading began in the Canada Basin. Such hotppot activity may have been responsible for initiating the rifting, breakup, and dispersal that eventually formed the Canada Basin.

Geological constraints for tectonic models of the alpha ridge

Initial interpretations of the CESAR geological samples are re-examined in light of new data from the Alpha Ridge and circum-Arctic region. A composite stratigraphy for the CESAR and Fletcher Island (T-3) pre-Neogene cores shows a sequence of Campanian-Maastrichtian organic-rich terrigenous mud overlain by Maastrichtian-Eocene biosiliceous marine deposits with a low organic content, terminating in volcanoclastic mudstone of Late Eocene age. CESAR core 6 contains a transition zone in which biosiliceous sediment is replaced by volcanoclastic and terrigenous sediment of Paleocene-Eocene age. Palynomorphs provide a Late Eocene age for the volcanic outcrop dredged from Northern Alpha Ridge. Textural and geochemical studies of laminated biosiliceous sediments were made with special techniques for quantitative analyses of very small samples (1–10 mg) and particle sizes of less than 5 microns. Results show that the laminated sediments were deposited very slowly in an oxidizing environment. Laminae in CESAR core 6 mainly reflect cyclical variations in the formation and/or accumulation of particulate iron, probably due to periodic hydrothermal venting. Absence of detrital sediment, sparsity of pyroclastic material and lack of diagenetic alteration of the biogenic sediments suggest that the eastern Alpha Ridge was not an area of major tectonic activity during the Eurekan Orogeny, from ca. 80-40 Ma.

Terrestrial heat flow from project CESAR, Alpha Ridge, Arctic Ocean

During two months in spring, 1983, a multidisciplinary study, project CESAR, was undertaken from the sea ice across the eastern Alpha Ridge, Arctic Ocean. In the geothermal program, 10 gradiometer profiles were obtained; 63 determinations of in situ sediment thermal conductivity were obtained with the same probe, and 714 measurements of conductivity using the needle probe method were obtained on nearby core. Weighted means of the thermal conductivity of the sediment are 1.26 W/mK ( in situ) and 1.34 W/mK (core), consistent with the compacted sediment encountered across the ridge and with the lithology. Calculated terrestrial heat flow values, corrected for the regional topography, range from 37 to 72 mWm −2; the average is 56+/−8 mWm −2. Some temperature and heat flow versus depth profiles exhibit non-linearities that can be explained by physically reasonable (but otherwise unsubstantiated) variations in bottom water temperatures preceding the measurements; models are hypothesized that reduce the curvatures. Two heat flow values considerably higher than others in the area may be explained by higher bottom water temperature over several years, while the low value is consistent with a recent deposition from a slump. This hypothetical modelling reduces the scatter of heat flows and reduces the average to 53+/−6 mWm −2. The CESAR heat flow is somewhat greater than expected for a purely continental fragment but is consistent with crust of oceanic origin. The heat flow is similar to values obtained in Cretaceous back-arc basins. Based on the oceanic heat flow-age relationship, the heat flow constrains the age of the ridge to 60–120 million years. The heat flow observed on other aseismic features in the world's oceans suggests that the Alpha Ridge has experienced no significant tectono-thermal event in the last 100 million years.

Alpha Ridge and iceland-products of the same plume?

Refraction results from the 1983 Canadian Expedition to Study the Alpha Ridge suggest a crustal model with many similarities to the region of Iceland. The overall character of the seismic sections, the derived velocity structure, the lateral dimensions of crustal transition zones and the general crustal thicknesses are comparable from the Arctic and Atlantic regions. Additional similarities are noted in the alkalic basalts, the subaerial to subaqueous, within-plate volcanic environment and the positive regional magnetic responses at satellite altitudes. Based on the geophysical semblances and the apparent evolutionary sequence of within-plate volcanism and tectonic events extending from the northern Ellesmere-Greenland area to Iceland, it is proposed that the Alpha Ridge and Iceland may have been affected by the same mantle plume.

Summary of Arctic Geophysics

The Cenozoic history of the Eurasian Basin is well understood because it involves the Eurasian (EU) and North American (NA) plates and is therefore constrained by data from more southerly regions and contains a readilly decipherable magnetic pattern. Reconstruction of the older portion of the Arctic Ocean is more difficult; however, information collected on ice station CESAR in 1983, interpreted in conjunction with regional geologic and geophysical data, provides insight into its oceanic affinities and age. A dredged outcrop of the Alpha ridge, consisted of weathered fragmental alkaline volcanic rocks. Refraction data reveal a thick crust nearly 40 km and a high velocity lower crust on this ridge. The basal layer velocity is typical of all plateaus known to be oceanic crust. It is hypothesized that Alpha Ridge therefore represents a late Cretaceous oceanic plateau. A more recent analogue is the Iceland-Faeroe Ridge. The magnetic information are shown to be consistent with this interpretation of oceanic crust. The Amerasia Basin is closed by rotating the Arctic-Alaska plate against NA during the Cretaceous. This reconstruction, its timing and its position are consistent with the geology of the Canadian Arctic Islands and Alaska.

Progress in understanding the age and origin of the alpha ridge, arctic ocean

Multiparameter geophysical measurements and geological samples from CESAR suggest that close to Canada the Alpha Ridge is oceanic in nature and was built in part by volcanic activity. It is unclear whether this part of the ridge formed in an intraplate or a plate margin environment. Estimates from paleontological, heat flow and magnetic data place the construction of a volcanic ridge within the Cretaceous period between about 120 and 80 Ma ago, the interval in which the Canada Basin seafloor formed but clearly before the creation of the Lomonosov Ridge. The place of the Makarov Basin in this chronology remains unclear.

Oceanic affinities of the Alpha Ridge, Arctic Ocean

Geological and geophysical information collected on the ice station CESAR have been integrated to develop an origin and history for the Alpha Ridge. The fossil assemblage recovered limits its age to no younger than Late Cretaceous. Fragmental alkaline volcanic rocks were dredged from the escarpments of the trough system. A high-velocity lower crust and depth of nearly 40 km to mantle is revealed by the refraction experiment. Cretaceous aseismic oceanic plateaus of the Pacific exhibit striking similarities to the Alpha Ridge. Plate reconstructions are presented that rotate the Arctic-Alaska plate away from the North American plate, with the Alpha-Mendeleev Ridge complex on a spreading centre being fed by a hot spot forming a feature similar to the Iceland-Faeroe Plateau.

Biological Selectivity of Extinction: A Link between Background and Mass Extinction

The phenomenon of non-random or selective survival across major extinction boundaries in the geologic past is poorly understood but increasingly recognized as a critical area for future research. A current hypothesis, developedfrom a comparison of extinction patterns among Late Cretaceous molluscs, is that biological adaptations of organisms effectual during normal times of earth history are ineffectual during times of mass extinction. Microsampling of laminated biosiliceous marine sediments from the highlatitutde Alpha Ridge (-86? N lat.) provides evidence of an actively exploited biological adaptation by Late Cretaceous planktonic diatoms. Distinctive laminae reveal a life-history strategy comprised of alternating planktonic and nonplanktonic stages. Contrary to the hypothesis that mass extinctions are indifferent to biological adaptations, these data indicate a link between selection for traits in normal paleoenvironmental settings and differential survival during an unexpected episode of paleoenvironmental deterioration during a mass extinction. Planktonic diatoms, adapted locally to surviving periods of stress by leaving the plankton environment, may have differentially survived a global crisis at the end of the Cretaceous as a consequence of a life-history trait. The interaction of this proximate biological adaptation and the postulated end-Cretaceous extinction mechanisms may have played a major role in the unexpectedly high survival of planktonic diatoms, documenting a clear example of causal dependency between a biologic character selected for during times of normal or background extinction and macroevolutionary survivorship during times of crisis.

On the origin of the chemical properties of the Arctic Ocean halocline

Profiles of chemical constituents in seawater above the Alpha Ridge in the central Arctic Ocean obtained during the Canadian Expedition to Study the Alpha Ridge (CESAR) support a model in which the halocline is maintained by dense, saline water produced on the continental shelves within the Arctic Ocean by the formation of sea ice. It is argued that nutrient regeneration on the continental shelves largely accounts for the nutrient maximum that has been observed in the upper halocline over a wide region of the Arctic Ocean, and that while the Bering Sea and North Altantic can be recognized as origins of halocline water, water masses from these regions undergo sufficient changes in their chemical properties within the Arctic Ocean significant that they no longer retain their identity in the halocline.

Cretaceous black mud from the central Arctic Ocean

Perhaps the oldest sediment yet recovered from the central Arctic Ocean is a black, organic carbon‐rich mud of late Campanian or Maastrichtian age. The sediment is known in a single piston core, F1‐533, taken from the eastern Alpha Ridge. The black mud has an average organic carbon content of 14 wt % and contains abundant woody plant fragments and pollen in addition to amorphous organic matter in a marine dinoflagellate‐rich matrix. Moderate hydrogen and high oxygen indices determined by Rock‐Eval pyrolysis suggest a substantial contribution of terrestrial plant debris to the marine sediment. The black mud is younger than organic‐rich strata deposited during the Oceanic Anoxic Events proposed for Aptian‐Albian and late Cenomanian to early Turonian times in the Atlantic and Pacific oceans. The high content of organic carbon and the abundance of terrestrial organic matter in the Arctic black mud suggest a marine setting influenced by an abundant supply of organic detritus rather than by a low level of oxygenation in the water column. Sediment in this core demonstrates that the central Arctic Ocean may have been in existence by the late Campanian to Maastrichtian and that the area of core F1‐533 probably was adjacent to an eroding vegetated land mass.

Amino acid epimerization in planktonic foraminifera suggests slow sedimentation rates for Alpha Ridge, Arctic Ocean

Interpretations of the palaeoclimatic and palaeooceanographic histories of the Arctic Ocean1,2 have been hampered by conflicting chronologies obtained using different dating methods. Magnetostratigraphic and micropalaeontological data1–4 have suggested a sedimentation rate of ∼1 mm kyr−1 whereas the rate indicated by amino acid epimerization analyses on foraminifera5 is an order of magnitude higher. To establish a firm chronology for cores from the Alpha Ridge, Arctic Ocean, and to help resolve the controversy between the ages assigned by different dating methods, we have carried out detailed amino acid epimerization analyses on planktonic foraminifera Neogloboquadrina pachyderma sinistral from a long gravity core. Assuming that the temperature in the Arctic Ocean did not exceed 3°C during the past 1 Myr, the alloisoleucine/isoleucine (alle/Ile) ratios obtained from this core suggest a sedimentation rate of∼1 mm kyr−1, supporting previous magnetostratigraphic and micropalaeontological studies.

Crustal structure of the northern Alpha Ridge beneath the Arctic Ocean

Beneath the Arctic Ocean, the Alpha Ridge complex is the principal tectonic feature between the Canada Basin and the Lomonsov Ridge. Here we report on analysis of seismic refraction data along the Alpha Ridge and within the basin area 175 km to the north (Fig. 1), which, together with geological and aeromagnetic constraints, indicates that the Alpha Ridge is characterized by regions of continental-like thickness but has probably resulted from an oceanic mode of development. The ridge flank thins towards the North Pole but may extend as far north as 88° N, and is therefore >500 km wide in the region near 110° W. Consequently, south of the pole along 110° W, typical oceanic crust beneath the eastern Makarov Basin may be present only locally, if at all. Similarities between the velocity structures of the Alpha Ridge and the Iceland area suggest that the Icelandic structure may be a currently active analogue for the Alpha Ridge.

“Arctic tides” by Rollin A. Harris (1911) revisited

Rollin A. Harris used a few scattered tide observations on the periphery of the Arctic Ocean to contradict Nansen's statement that the Arctic Sea was an open basin of deep water occupying all, or nearly all, of the large unexplored area (in 1911) of the Arctic. Harris concluded that there was either a land mass or some shallow water near the north pole. Fjeldstad disagreed with Harris in 1923, and in 1927, Sverdrup supported Fjeldstad, using additional tide and current observations to reach this conclusion. Later, about 1950, Soviet scientists discovered the Lomonosov and Mendeleyev ridges, and shortly thereafter, U.S. scientists found the Alpha Ridge, all three ridges lying at least partially in what was in 1911 unexplored territory. It appears that Harris' hypothesis was at least partially accurate.

Magnetostratigraphy and palynology demonstrate at least 4 million years of Arctic Ocean sedimentation

It has long been recognized1,2 that the Arctic Ocean has a major role in regulating the timing of Northern Hemisphere glaciations, global heat exchange and ocean circulation. Several recent studies3–7 have attempted to interpret the palaeoclimatic record in sediment cores from the Arctic Ocean, but conflicting ages have been reported for palaeoclimatic events dated by different methods3–6. To establish a consistent chronology for cores from the central Arctic Ocean, we performed detailed palaeomagnetic and palynological studies of three cores collected7 near the crest of the Alpha Ridge, and obtained radiocarbon dates from planktonic foraminifers isolated from 1-cm-thick intervals. Magnetic inclination values for these cores clearly record normal and reversed-polarity intervals that apparently correspond to the Brunnes and Matuyama magnetochrons; the longest core also includes polarity intervals typical of the Gauss and Gilbert magnetochrons. This chronology is confirmed by the first and last occurrences of pollen and dinoflagellate species that can be correlated with European8,9 and DSDP10–12 chronostratigraphies dated by standard biochronological methods. Micromass radiocarbon dates for foraminifers indicate a sedimentation rate of ∼1 mm per 1,000 yr. We believe that these data confirm a long (>4 Myr) chronology for the Alpha Ridge cores, the lithostratigraphy of which can be correlated over extensive areas of the western Arctic Ocean2–7, and more important, provide dinoflagellate and palynomorph data that indicate the validity of early chronology.