PALEOCEANOGRAPHY,VOL. 3, NO. 5, PAGES539-550, OCTOBER1988

EARLY HISTORY OF THE ARCTIC OCEAN

David L. Clark

Department of Geology and Geophysics Univers!ty of Wisconsin, Madison

Abstrac_t. The Lomonosov Ridge divides Silicoflagellates of this core are either the Arctic Ocean into two basins with Paleocene or middle to late Eocene. The different histories. The older Amerasian sediment, like that of the Maastrichtian Basin consists of a ridge-basin complex cores, has been interpreted to represent reflecting multiple tectonic cycles that oceanic upwelling and high productivity. are poorly understood. The younger The older Arctic cores were recovered as Eurasian Basin includes the present Arctic a result of fortuitous sampling in areas Mid-Ocean Ridge. Its origin is linked to of thin late Cenozoic sediment. All other Cenozoic spreading. The entire older Arctic cores consist of late Cenozoic sediment record of the Arctic Ocean is sediment. The older sediment record is known from only four short piston cores, strong evidence of a Late Cretaceous to all recovered from the Alpha Ridge of early Paleogene Arctic Ocean with no the Amerastan Basin. The oldest sediment perennial ice and with high productivity recovered is a single core of black of dinoflageliates or siliceous organisms mud consisting of abundant terrestrial in water that circulated with lower- plant material deposited in a marine latitude oceans. Recent modeling environment. are late predicts some of the same climatic and Campanian. The black mud resulted from oceanographic events recognized in the rapid accumulation of organic matter oldest sediment of the Arctic Ocean. rather than from any of the well-kmown The sequence of events from a warm Arctic Cretaceous anoxic events. Two cores taken Ocean to an ice-covered condition involved 100 to 200 km from the black mud consist circulation changes, seasonality, and of late Maastrichtian siliceous ooze of uneven spreading rates that increased and siltcoflagellates. This sedi- North Atlantic-Arctic ventilation. ment has been interpreted to represent oceanic upwelling and high productivity ORIGIN AND STRUCTURE in the Late Cretaceous Arctic Ocean. A Paleogene core consisting of similar Geophysical observations related to biogenic silica has been recovered approx- the origin and structural history of imately 100 km from the Cretaceous cores. the Arctic Ocean Basin date back some 30 years, but it has only been during the Copyright !988 past few years that aerial surveys and by the American Geophysical Uniom. seismic reflection work from ice stations have contributed to a concordant data set Paper number 8P0390. for that history. We have learned that 0883- 8305 / 88/008P-o 3905 i0.00 the Lomonosov Ridge divides the Arctic 540 Clark: Early History of the Arctic Ocean

500KM I :.

Amerasian

Basin

NORW E GIA N/G REENLAND..j SEA J ...... Chukchi

ALASKA '• Fram Basin

•, Makarov Basin

Fig. 1. Topographic nomenclature for the central Arctic Ocean. Lomonosov Ridge divides the Arctic Ocean into two major basins, the Amerasian and Eurasian. The Amerasian Basin consists of four important features, the Makarov Basin, the Alpha Ridge, the Canada Basin, and the Chukchi complex. The Eurasian Basin includes the Fram Basin and opens into the Norwegian- Greenland Sea.

Ocean into two basins, and it also divides during the past 55 m.y. Work in progress the ocean's history into parts, one indicates that only 1 m or so of the 1-km complex and imperfectly understood and sediment cover from the deeper part of the the other somewhat less complex and basin has been recovered, and this is the better understood (Figure 1). record of less than 700,000 years of The Eurastan, or eastern Arctic Ocean Eurasian Basin oceanic history. Basin, is the better understood basin. The Amerasian Basin is older than the Most studies of this part of the ocean Eurasian part of the Arctic Ocean and concur on an early Cenozoic origin at structurally more complex (Figure 1), and approximately magnetic anonely 24, its origin is more speculative (see Clark probably at 55 Ma or perhaps slightly [1981] for a •eview of older theories). later [e.g., Jackson and Johnson, 1984]. It is composed of at least four separate Seafloor spreading initiated movement basin and ridge complexes (Figure 1) that away from the Nansen Ridge, and this may have had unrelated origins. A unified movement transported the Lomonosov Ridge synthesis of Arctic Ocean origin is not from a continental shelf (the Barents available because interpretations for one Shelf) location to its present position. Amerasian Basin structure often have been As a consequence the Eurasiau Basin was incompatible with theories for another formed. The Canadian Lomonosov Ridge [Vogt et al., 1984]. Experiment (LOREX) study provided addi- Recently, the LOREX and Canadran tional data that are consistent with Expedition to study the Alpha Ridge this theory [Morris et al., 1985]. In (CESAR) projects have provided data the Fram Basin portion of the Eurasian more or less compatible with seismic Basin adjacent to the Lomonosov Ridge, work in the Beaufort and Chukchi seas the crust is thinner and apparently easier that together may permit formulation of a to interpret than that of the Amerasiau coherent explanation for the origin and Basin side of the ridge, the Makarov Basin history of the Amerasian Basin. The data [Weber and Sweeney, 1985]. There is suggest that the major Amerasian Basin little indication of substantial struc- structure, the Canada Basin (Figure 1), tural modification of the Eurasian Basin originated through Early Cretaceous Clark: Early History of the Arctic Ocean 541 rifting and counterclockwise rotation that record of late Cenozoic is available. The separated parts of the northern Alaskan only oceanic constraints on the timing and northern Siberian crustal block from of the ortg•n of the various basins and the Canadian Arctic Islands [e.g., Jackson ridges that constitute the Amerastan Basin and Johnson, 1984]. The oceanic crust has are provided by the age of the oldest been covered by as much as 10 km of sedi- Arctic Ocean sediment known, and that is ment adjacent to Alaska, and this includes from the Alpha Rldge. turbid!res in the most recently depos!ted sediment. PALEOCEANOGRAPHY The history of the adjacent Alpha Ridge Introduction (Figure 1) is still incompletely known, , but it may have formed as thickened oceanic crust injected during Canada Basin Four short cores, three taken from spreading or as the result of hot spot T-3 and a single core from CESAR, compose acttv!ty that produced an oceanic plateau the total accesslble older history of the similar in structure to the Manthtkt Rise central Arctic Ocean (Figure 2). These [Jackson, 1985]. Additional modeling of cores were recovered from an area of CESAR data has resulted in comparison of several hundred square kilometers on the at least a part of the Alpha Ridge with northeastern part of the Alpha Ridge. The the structure of Iceland [Forsyth et al., cores have been studied extensively, and 1986]. According to this interpretation these studies have provided all that is the Alpha Ridge attains a thickness of known concerning the early history of the 40 km and a width of at least 500 km tn Arctic Ocean. The use of only four cores the region of 110*W, and this may preclude to interpret the paleoceanographic history the presence of normal oceanic crust of an area the size of the Arctic Ocean beneath the adjacent Makarov Basin. The should not be acceptable, but this is the velocity models support the Idea that the actual data base. modern Icelandic structure is an active analog of the Alpha R•dge. Altermately, Campantan the Alpha Ridge may have been part of the or•gtnal Lomonosov structure moved to its The oldest known central Arctic Ocean present position during Eurastan Basin sediment ts a black, organic-carbon-rich format!on and mod!f!ed near !ts present position by structural extension and maffc rock intrusion that separated t t from the LomonosovRidge [Weber and Sweeney, 1985]. According to the Weber-Sweeney theory, the keoo origin of the Makarov Basin (Figure 1) is Oo related to the structural extension that separated the Lomonosov and Alpha ridges and created an almost grabenltke structure between the ridges [Weber and Sweeney, 9851. Several theories exlst for the origin of the other major Amerastan Basin struc- ture, the Chukchi Plateau (Figure 1), but its origin remains enigmatic [Vogt et al., zooo• ...¾: 1982; Grantz and May, 1984; Jackson and Johnson, 1984]. Most •nterpretations Fig. 2. The oldest sediment recovered to assume a continental crust for most of the date tn central Arcttc Ocean: core 533, Chukcht block but differ on details such from 85005.9'N, 98017.8'W, at 1855 m, as the timing and type of movement of the 348 cm long, late Campantan; core 437, block during the format•od of the Canada from 85059.87'N, 129ø58.76'W, at 1584 m, Basin. 286 cm long, late Maastrtchttan; core 6 Most parts of the Areeras tan Basin (CESAR 6), from 85049.8'N, 109ø04.9W, at have at least I km of sediment, and th•s 1365 m, 305 cm long, late Maastrtcht•an; includes sediment as old as Cretaceous. and core 422, from 84ø53.48'N, Paleogene sediment is known. Middle 124032.87'W, at 2049 m, 364 cm long, Cenozoic sediment has not been recovered early Paleogene. Bathymet• ts tn in the central Arctic Ocean, but a good meters [from Clark et al., 1986]. 542 Clark: Early History of the Arctic Ocean mud of probable late Campantanage [Clark FI-533 and Byers, 1984; Clark etal., 1986]. The sediment has been recognized in a single Corg Tmax Hindex Oindex •t3Corg core, FL-533, taken in 1970 from ice 0 20 300 500 t50 ;550 50 250 -25 -35 island T-3. This core was taken from the crest of the Alpha Ridge in the area of 85øN. The core consists of 67 cm of black mud that is overlain by 20 cm of yellowish sediment, all Campantan (Figure 3). This Late Cretaceous sediment is overlain with obvious unconformtty by 261 cm of late Pltocene to Holocene sediment. The late Cenozoic component of this core consists of sediment that can be correlated with the standard ltthostratigraphy of the Pliocene to Holocene of the central Arctic Ocean [Clark etal., 1980; Mudte and Blasco, 1985]. Fig. 3. Core 533; 1tthology and organic Description geochemistry. The lowest 67 cm is organic-rich sediment, with the upper The lower 67 cm of black mud is part lamina ted. The 20 cm untt above homogeneous except for the upper 2 cm, black mud laminations is illtie-quartz- which is laminated. The sediment is silt- rich. General trends in organic and mud-sized with less than 5% detrital geochemical data of the Campantan core or sand-sized material. The black mud are indicated [from Clark etal., 1986]. averages 14% organic carbon by weight and is rich in woody fragments, leaf cuticles, , pollen and amorphous organic Cretaceous oceanic anoxic events, the matter. This organic component occurs presence of a Cretaceous black mud in along with marine dtnoflagellates that the central Arctic Ocean deserves special are abundant throughout the Campantan part attention. Other Cretaceous black muds of the core. The organic geochemtstr"f of have been recovered from Deep Sea Drilling the core has been described in detail Pro•ect (DSDP) sites in lower-latitude [Clark etal., 1986] (Figure 3). The oceans as well as from outcrops on the black mud is thermally immature with continents. Although the existence of respect to hydrocarbon generation and has oceanic anoxtc events is of special intermediate to high hydrogen and oxygen paleoceanographlc interest, the organic- indices. The samples have values between rich Cretaceous sediments are known to Type II and Type III organic matter and have resulted from a variety of causes. reflect the mixture of marine planktons These include poorly oxygenated bottom (dinoflagellates) and terrestrial plant water, expanded midwater oxygen mtntma, debris. There is a decrease in proportion and rapid turbidity related accumulations of marine organic matter preserved in the of organic matter. Also, th.e deposition black to the yellowish Campanian sediment. and preservation of the organic-rich Carbonate content of the sediment is less sediment apparently were most common than 1% throughout. during particular parts of the Cretaceous. The Campantan age is based on a study This includes the Aptian to Albian and the of an abundant dtnoflagellate assemblage. late Cenomantan to early Turontan. The Some 30 species are present, many of which Arctic Campanian black mud differs both were undescrtbed previously. The species in organic content and time framework from are most characteristic of the late the Cretaceous anoxlc events previously Campanian to Maastrtchttan interval of the described [e.g., Schlanger and Jenkyns, Late Cretaceous but include species that 1976]. The Arctic mud is younger than range into the Paleogene. The abundance the Cretaceous anoxic events, and the of Chatan•t.ell a species and the absence geochemical data support a mixed of typical late Maastrtchttan/Paleocene terrestrial plant and marine planktonic species are considered diagnostic for source. The ox•fgen level of the water the Campanian age [Clark etal., 1986]. column for t.he Arctic sediment was less Because of the reports of widespread important than the rapid transportation Clark: Early History of the Arctic Ocean 543 and burial of the terrestrial organic indicates that the bulk of this sediment debris [Clark etal., 1986]. is detrital illire and quartz with clay Because the Cretaceous Arctic Ocean and silt-sized 2M-muscovite. Organic was only approximately one-half the size content is very low (Figure 3). The of the present ocean, it is reasonable to abundance of the high-temperature musco- reconstruct the paleogeography during the vite is good indication of the detrital Campanian by closing up the Eurasian Basin origin of this component of the Campanian and then locating the site of FL-533 on sediment; aeolian or fluvial systems may this restructured base (Figure 4). Such be responsible for transportation of the an exercise results in the placement of particles to the marine environment. the Campanian mud some 500-600 km closer to the broad continental shelves of Summary Eurasia (and very close to Svalbard). If the Makarov Basin is a post-Cretaceous The single core is good evidence that basin, it is possible that Svalbard, or marine conditions existed in the Arctic some portion of the Greenland-Scandinavian Ocean by at least the late Campantan and block, could have been the source of the that the setting of FL-533 was adjacent to abundant plant debris in FL-533. For a Svalbardltke land mass that supplied example, on Svalbard, Early Cretaceous large quantities of terrestrial organic marine sequences grade upward into material to this part of the ocean. This Cretaceous and Paleogene detrital sedimentation resulted in the accumulation sediment-bearing coal. The abundant of sediment with 14% organic carbon. The terrestrial vegetation on Svalbard or a assemblage includes several Svalbardltke block may have been the Arctic species not reported elsewhere and source for the organic material of the also a host of species that have been Arctic sediment and resulted in rapid found in Campanian rocks in different transportation and burial of organics. parts of Earth. The Arctic waters The abundance of marine dtnoflagellates tu apparently freely circulated with the the sediment indicates that the sediment world ocean during this time. was formed by deposition of terrestrial debris in a marine matrix. Maas trichtian ,, ,, ,, , , ,, Proximity to a major land source may also explain the 20 cm of yellowish Cores FL-437 and CESAR 6 were recovered Campanian sediment that overlies the approximately 100 km apart on the Alpha black mud (Figure 3). This sediment is Ridge crest (Figure 2). The T-3 core a clayey mud with 2-3% sand-sized detrital (FL-437) was taken in 1969, and the CESAR particles that are mostly angular to 6 core in 1983. Both cores consist of rounded quartz. X ray diffraction btogentc silica; diatoms, stltcoflagel- lares, archaeomonads, and ebrtdians are abundant [Ling etal., 1973; Clark, 1974; Kitchell and Clark, 1982; Mudte and Blasco, 1985]. The cores have been determined to be Maastrtchtlan on the basis of well-preserved stlicoflagellates [Ling st al., 1973; Bukry, 1981, 1985], but a late Campantan age is favored on the basis of diatoms [Barron, 1985b]. The difference in Late Cretaceous age assign- ments with diatoms and silicoflagellates is not resolved. The lack of any com- parable high-latitude diatoms of Campantan or Maastrichtian age for comparison with the Arctic material is a problem [Barron, 1985b]. The Maastrichtian age is tenta- tively accepted on stratigraphic grounds as follows: Core FL-533, •tth late Fig. 4. Possible relationships of core Campanlan dinoflagellates, is a black mud 533 during the Campanian. This represents overlain by yellowish laminated sediment a pre-Eurasian Basin structure [from Clark (Fi•ure 3). This yellowish sediment is etal., 1986]. megascoptcally similar to the yellowish 544 Clark: Early History of the Arctic Ocean

•edtment of FL-437 and CESAR 6, but the interpretations on the study of CESAR 6, sediments of the different cores contain these students concluded that deposition different fossils; Campantan dtnoflagel- of the btogentc silica tn the CESAR core !ates occur in the yellowish sediment of was the result of slow deposition tn a FL-533, and Campantan or-Maastrtchttan nutrient-poor, offshore environment. This diatoms occur in the yellowish sediment of interpretation is influenced by the paleo- CESAR6 and FL-437. If the superposition magnetic measurements on CESAR 6 that of yellowish sediment overlying black mud suggest several episodes of normal and represents a widespread stratigraphic reversed polarity [Aksu, 1985]. The pattern, then the sediment of FL-437 absence of calcareous in the and CESAR6 may be younger than that of core also was a factor in the Mudte and FL-533. The stltcoflagellates are Blasco [ 1985] interpretation. Accordlng interpreted to be Maastrichtian by to their arguments, calcareous fossils different students [e.g., Ling et al., should be present in any upwelling ]973; Bukry, 1981, 1985]. The idea of a deposit. This argument is weakened by a restricted circulation of the Arctic Ocean later conclusion of the same authors that during the latest Cretaceous has been the absence of calcareous microfossils suggested [Bukry, 1981, 1985]. The theory in CESAR 6 is most likely due to post- on which this idea was based [Gartner and deposittonal dissolution [Mudie and Keany, 1978] has been challenged [Clark Blasco, 1985]. and Kitcheil, 1979, 1981; Watts et al., Another hypothesis to explain the rich 98o]. btogentc s{ltca in distinct light and dark layers is that of periodic transport of Description the skeletal material from other areas to the sites of FL-437 and CESAR 6. Strong The most comprehensive description but fluctuating currents could wash the of core FL-437 includes quantitative skeletal material of organic carbon as determinations of biogentc silica and well as result in layered sediment. interpretations of paleocirculatton that Upwelling, slow accumulation, and lateral suggest polar upwelling [Kitcheil and transport must each be considered as Clark, 1982]. Together with the evidence explanattons. of the same sediment in CESAR 6, this ts The upwelling model is supported by good evidence of Mesozolc btogenic silica recent modeling experiments that predict accumulation tn a limited area of the open-ocean upwelling in the Cretaceous Cretaceous central Arctic that preceded Arctic Ocean during the winter and coastal its known deposition tn the subpolar seas. upwelling during the Arctic summer Concentrations of siliceous microfossils [Parrish and Curtis, 1982; Barron, 1985a]. in the cores average 100 to 400 x 106 Also, CESAR 6 was opened just a few weeks specimens per gram of bulk sediment. after it was collected. The sediment These densities of btogentc silica compare showed distinctive layering, and lamina- favorably with siliceous deposits tn tions were sharp with distinctive colors. modern marine sediments of the Gulf of Laminations were counted following the California, the southwest coast of Africa, opening of the core. Below the disturbed and Antarctica, all of which are asso- top part of the core [Mudie and Blasco, ciated with strong upwelling conditions 1985] almost 900 distinctive layers were [Calvert, 1966; Zhuze, 1972]. Because the counted in approximately 200 cm. These oceans are undersaturated with respect to layers range from light yellow to darker St02, high productivity and rapid accumu- shades of yellow and gray. The colors lation are factors generally necessary for alternate between darker and lighter preservation of biogentc silica. These colors throughout the core. Because the conditions are commonly associated with dominant diatoms and stltcoflagellates upwelltngs. The idea of Maastrtchttan show only changes in abundance and not in polar ocean upwelling and consequent rapid taxonomy through the core, the laminations accumulation is supported by the absence could represent cyclic, even seasonal of any major evolutionary change in the deposition. Similar laminations in FL-437 siliceous assemblages from were much less distinctive when this core the bottom to the top of the two cores was opened, but these dark and light lami- [Barron, 1985b]. nations have been sampled separately. This interpretation has been challenged This microsampling of 48 laminations in by Mudte and Blasco [1985]. Basing their FL-437 revealed that there were layers Clark: Early History of the Arctic Ocean 545

are seasonal, the •900 laminations of CE•$AR6 ('200 cm) could represent '450 years. Th•s •nterpretat•on needs an extremely h•gh sedimentation rate, and It •s possible that the suspected cycl•c•ty may be based on some superseasonal cycle that represents a longer period of time. However, the absence of any evolutionary change in e•ther d•atoms or stl•cofla- gellates argues against an extremely long period of time for the sediment [Barron, 1985b]. In fact, Barron [1985b] concurs that the extreme high product•vity of d•atoms in CESAR 6 suggests that the laminat•ons are seasonal. Magnetic reversal stratigraphy that suggests that the Maastr•chtlan sediment of CESAR 6 •s several m•ll•on years old supports a slower sedimentation rate •nterpretat•on [Aksu, 1985]. Slow deposi- tion or periodic lateral transport are alternate hypotheses, but the evidence from the d•atom stratigraphy suggesting no evolution [Barron, 1985b] the presence of resting spores in darker layers and of mot•le vegetative cells •n l•ghter layers [K•tchell et al., 1987] (F•gure 5), the geochemical constraints for accumulation of thick deposits of b•ogen•c s•l•ca •n the world ocean [Broecker a•d Peng, !982], and favorable comparison of the sed•ment wt•h modern upwelllng deposits [Calvert, 1966; Zhuze, 1972] are strong arguments for an upwelling ortgln of the b•ogentc sediment of FL-437 and CESAR 6 [Mitchell et al., 1987].

Fig. 5. Diatoms and res ting spores from Summar• Maastrichttan core 437. (a) Motile vegetative skeletons of Hem•aulus sp., Although much of the d•scusston Trtnacria sp., Coscinodtscus sp., and available on the Cretaceous Arctic other species, Scanningglectron Ocean is concerned with age and mode of microscope (SEM) photograph X 2000. accumulation of the sediment, the real These species are from a light-colored s•gn•f•cance of the Cretaceous cores ts lamination in 437. (b) Resting spores their meaning •n paleoc•rculat•on and of centtic diatoms from darker colored paleoclimatology for that t•me Interval. lamination in 437, SEM photograph X 2000. The late Cretaceous Arctic Ocean had no perennial ice cover. It was an ocean that probably supported upwelling and the that consist almost exclusively of growth of abundant plants on Its shores. resting spores (maximum93.3%, N >200/ per Circulation w!th the lower-lat!tude ocean sample) and layers w•th few or no resting was good. spores but w•th vegetative cells (maximum 96.4%, N >200/ per sample) (F•gure 5) Paleogene. [K•tchell et al., 1987]. The vegetative cell lam•nae record sedimentation during a The youngest core that bears on the growth phase of active nutrient uptake and early history of the Arctic Ocean is T-3 photosynthesis, and the resting core FL-422 (Figure 6), located approxima- lamlnae record sedimentation during a tely 100 km from the older Cretaceous period of stress. If these laminations cores on the Alpha Ridge (Figure 2). This 546 Clark: Early History of the Arctic Ocean

argued that the stlicoflage!lates from this core are middle to late Eocene, while more recently Ling [1985], working with the same stltcoflagellates, argued that the core •s most likely Paleocene. Diatoms and other organisms have not been thoroughly studied. Until the age is resolved with diatoms, ebridians, or archaeomonads, FL-422 will be referred to as early Paleogene.

Description

Concentrations of stlicoflagel!ates and diatoms in FL-422 are the same as in FL-437, and this sediment, like that of FL-437, is interpreted to represent oceanic upwelling [Kitcheil and Clark, 1982]. Whether upwelling was coutinuous from the Maastrichttan through the Paleocene or Eocene or discontinuous, only occurring during short intervals of both the Maastrichtian and early Paleogene, is not known and probably will not be known until a complete core across the entire Mesozoic-Cenozoic transition is recovered.

Summ•

The anecdotal idea of a greatly restricted Late Cretaceous-early Paleogene Arctic Ocean circulation with that of lower latitudes [Gartner and Keany, 1978; Gartner and McGutrk, 1979; Bukry, 1985] is challenged by the widespread distribution of many of the diatoms and silicoflagel- late species identified in the Late Cretaceous-early Paleogene Arctic Ocean p-oi.-arity sediment cores. Fig. 6. Core 422, the early Paleogene sediment. The upper part of this core consists of laminae megascoptcally similar to laminations of Maastrichttan cores 437 and CESAR 6. Late Cenozoic sediment unconformably overlies Paleogene btogentc sediment. The magnetic signature and sampling interval are shown. ' ¾ ,. core, like FL-437 and CESAR 6, consists of biogenic silica, is yellowish in color, and is laminated [Clark, 1985]. In segascopic appearance it is very close to the older Cretaceous biogenic sediment of FL-437 and CESAR 6, but the silicofla- •tg. 7. •mte Cgetaceoum-em•ly Paleogeme gellares, diatoms, and other siliceous circulation of the Arctic Ocean before fossils of this core suggest either formation of the Eurasian Basin. Surface Paleocene or Eocene age sediment. The age water in the equatorial region provided has been debated among specialists working high-latitude heat flux [from Clark, with the same fossil groups. Bukry [1984] 19SS]. Clark: Early History of the Arctic Ocean 547

THE EARLY ARCTIC OCEAN AND THE WORLD OCEAN discontinuous upwelling [Kitehell et al., 1987]. Evidence for some form of season- C1 ima te ß ality, cited earlier, also suggests a unique feature of the early Arc tic Ocean. Cretaceous oceans had circulation Because Arc tic diatoms were adap ted to patterns with a very strong cross-latitude seasonal darkness and reduction of transport component that intensified and nutrient availability, their survival enhanced western boundary current heat strategy included rapid formation of transfer from the low to high latitudes res ting spores that would remove the (Figure 7) [Berggren, 1982]. The Creta- population from a seasonally darkened ceous palcogeography responsible for this and/or nutrient-poor photic zone. This circulation was complemented by high adaptation to Late Cretaceous seasonality atmospheric CO2 levels that together with may have served diatom populations well. the oceanic heat pump produced surface If the Cretaceous/Paleocene extinction water temperatures that may have been event [e.g., Alvafez et al., 1980] 15% higher than those of the present. included a period of darkness, from Berggren [1982] has estimated that Creta- whatever cause, Arctic Ocean diatoms may ceous Earth's thermal gradient may have have differentially survived as a con- been only 50% of that of the present. The sequence of their life-style that included deep-sea evidence of moderate Arctic Ocean seasonal adaptation to stress. A period climate during the Late Cretaceous to of darkness, from any cause, was a signal early Paleogene is supported by reports to leave the plankton environment and concerning Arctic Paleogene vertebrates resulted in the formation of deep pelagic [West et al., 1977], Arctic terrestrial or benthie resting spores that preserved and aquatic invertebrates, and plants the various populations during the times [Marincovich and Zinsmeister, 1985; Wolfe, of surface water s tress. Quant!tative 1985]. A question not completely resolved estimates of differential plankton sur- is the role of orbital forcing for the vival based on comparison of Maastrichtian Mesozoic-early Cenozoic Earth climate. and Danian assemblages indicate that 73% It is generally concluded that much of of the coccoliths, 85% of the radio- the Pleistocene climate cycle can be larians, and 92% of the foraminifera explained by orbital forcing. Why was genera became extinct. In contrast, only orbital forcing of less influence 65 m.y. 23% of diatoms, on the global scale, were ago? Evidently, the result of atmospheric affected. Most of the Arctic Ocean diatom chemistry and Cretaceous-early ?aleogene genera and species survived the Cretaceous oceanic and atmospheric circulation extinction almost unaffected [Kitcheil et produced climate patterns that suppressed al., 1987]. the extreme effects of orbital forcing in A significant conclusion of the late Cenozoic. Orbital effects of observations of the survival of diatoms mid-Cretaceous black shale cycles in the in the early Arc tic Ocean is that a lower latitudes may have been climate biological character may determine a related [Herbert and Fischer, 1986], but macroevolutionary pattern of differential evidently, Milankovttch factors did not survival [K!tchell et al., 1987]. These produce Mesozoic glaciatton. data question the recent hypothesis that mass extinctions are indifferent to biolo- Adaptark..on,t n High Lat!tude. s gical adaptations [Jablonski, !986]. High-latitude planktons that had developed Certainly, the ice-free Cretaceous to a seasonal strategy for survival were able early Paleogene Arctic Ocean did not exist to use this strategy for survival of independent of Earth's orbital effects extinction events, as well. [Herbert and Fischer, 1986]. There is no evidence for reduced obliquity of Earth's Summary ecliptic during the Late Cretaceous to early Paleogene [Harris and Ward, 1982]. Origin of the Arctic Ocean Basin is This was important for Arctic organisms. related to tectonic modification of the The control of Earth's ecliptic on photo- Alaskan-Siberian crustal blocks, possible period probably was the same as it is at hot spot activity, and seafloor spreading present, and organisms of the Arctic Ocean along the Nansen Ridge. The different of 70 to 50 Ma had to live with a seasonal basins and ridge structures that form the dark cycle as well as with a seasonally present Arctic Ocean are imperfectly 548 Clark: Early History of the Arctic Ocean

latitude oceans existed. Oceanic and ERNARYP•eietoc.n. urbidites atmospheric circulation patterns that QUAT-z Holocene • were responsible for the warm Earth were • glacial marine sediment altered during the Cenozoic. The Cenozoic o U.I o climate modification resulted in an z • elevated latitudinal heat gradient and a cooling of the Earth's poles. z • ,,• • O .A..cknowled..gements.Much of the research that forms the basis for this paper was supported by the Office of Naval Research, including support from current contract • J silica N00014-82-K0003. Jim Ling, David Bukry, Louis Maher, John Barron, Graham Williams, John Bennett, and David Goodman have made o •'= •' detritalillite& quartz important contributions to the study of "' • carbon rich sediment the older Arctic Ocean organisms. Charles Byers, Lisa Pratt, Christine Mennicke, Mark Langseth, S. W. Bailey, Steve Blasco, Fig. 8. Early sedimentary history of the and P. J. Mudie have made important Arctic Ocean. The oldest known sediment contributions to the study of the is a carbon-rich sediment and detrital Cretaceous-Paleogene sediment. The vision tlltte showing fluvial and aerial contri- and help of Art Lachenbruch and Vaughn bution to Arctic Ocean sediment during Marshall are greatly appreciated. the Late Cretaceous. Biogenic sediment accumulated as the result of upwelling REFERENCES during the latest Cretaceous and early Paleogene. No middle Cenozoic sediment Aksu, A., Paleomagnetic stratigraphy of known, but glacial marine sediment and the CESAR cores, Initial Geological turbidties characterize most parts of the Report on CESAR, edited by H. R. late Cenozoic Arctic Ocean. Jackson, P. J. Mudie, and S.M. Blasco, Geol. Surv. Pap. Geol. Surv. Can., 84'-2•, 101-114, 1985.' ...... understood. The oldest sediment known is Alvafez, L. W., W. Alvafez, F. Asaro, and Campantan black mud with a high organic H. V. Michel, Extraterrestrial cause for carbon content related to rapid transport the Cretaceous-Tertiary extinction, and burial of terrestrial plant material S_ctence, 208, 1095-1108, 1980. (Figure 8). Following this episode of Barron, E. J., Numerical climate modeling, black mud deposition, btogentc silica a frontier in petroleum source rock accumulated during the Campanian to prediction: Results based on Cretaceous Maastrtchtian (Figure 8). The enormous simulations, Am. Assoc. Pet. Geol. concentrations of diatoms and silico- flagellates in layers may have resulted Barron, J. A., Diatom btostratigraphy of from periodic lateral transport or slow the CESAR 6 core, Alpha Ridge, Initial deposition but are interpreted to repre- Geological Report on CESAR, edited by sent oceanic upwelling. During the early H. R. Jackson, P. J. Mudie, and Paleogene, the same kind of deposition S. M. Blasco, Geol. Surv. P.ap. Geol. was taking place, but whether upwelling Surv. Can., 84-22, 137-148, 1985b. was continuous from the Cretaceous to Berggren, W. A., Role of ocean gateways in climatic change, in Climate in Earth the Paleogene or sporadic is not known , (Figure 8). If darkness was part of the History, Studies in Geophysics, Late Cretaceous extinction event, survival pp. 118-125 National Academy Press, of Arctic diatom species may have been Washington, D.C., 1982. facilitated by formation of resting Broecker, W. S., and T.-H. Peng, Tracers spores, an adaptation to seasonal light in the Sea, p. 690, Eldigio Press, New change at high latitude. York, 1982. The early Arctic Ocean of the late Bukry, D., Cretaceous Arctic Mesozoic to early Cenozoic was an ice- stltcoflagellates, Geo Mar. Lett., 1, free, warmer water mass than that of the 57-63, 1981. present. Good circulation with the lower- Bukry, D., Paleogene paleoceanography of Clark: Early History of the Arctic Ocean 549

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