Late Cretaceous—Paleogene paleogeography and paleocirculation: Evidence of north polar upwelling

Palaeogeography, Palaeoclimatology, Palaeoecology, 40 (1982): 135--165

135

Elsevier Scientific Publishing Company, Amsterdam -- Printed in The Netherlands

LATE CRETACEOUS--PALEOGENE PALEOCIRCULATION: EVIDENCE

PALEOGEOGRAPHY AND OF NORTH POLAR UPWELLING

JENNIFER A. KITCHELL and DAVID L. CLARK

Department of Geology and Geophysics, University of Wisconsin, Madison, WI 53706 (U.S.A.) (Received May 12, 1982)

ABSTRACT Kitchell, J. A. and Clark, D. L., 1982. Late Cretaceous--Paleogene paleogeography and paleocirculation: evidence of north polar upwelling. Palaeogeogr., Palaeoclimatol., Palaeoecol., 40: 135--165. Quantitative determination of biogenic silica in Late Cretaceous and Paleogene deep° sea sediment cores from the central Arctic Ocean provides evidence of open-ocean polar upwelling. The timing of polar upwelling coincides with periods of a weakened meridional thermal gradient, suggesting that heat transport to the poles by oceanic circulation may have been important. The timing of biogenic silica deposition in the Arctic precedes its deposition in both the Norwegian--Greenland Sea of the North Atlantic and the Bering Sea of the North Pacific. Tectonic events may be responsible for the timing and siting of sites of deposition of biogenic silica in high northern latitudes, particularly the tectonic evolution of sites of deep-water exchange between the Arctic and the world ocean. We outline three phases in the post-mid-Cretaceous history of silica deposition in high northern latitudes. During Phase I, the Arctic is a silica sink, with deep-water formation but with no deep-water outflow. The transition to Phase II is brought about by opening of the Svalbard--Greenland Strait to deep-water outflow from the Arctic to the Norwegian-Greenland Sea. The transition to Phase III is initiated by submergence of the Faroe-Iceland Ridge and deep-water outflow from the Arctic to the North Atlantic. Climatic conditions in the Arctic during Late Cretaceous and Paleogene time are predicted to have favored open-ocean upwelling due to a circulation pattern dominated by cyclonic conditions, resulting from the establishment of a semi-permanent atmospheric low over the Alpha Ridge. Bathymetry of the Alpha Ridge may have intensified paleo-upwelling.

INTRODUCTION Q u a n t i t a t i v e d e t e r m i n a t i o n o f b i o g e n i c s i l i c a in L a t e C r e t a c e o u s a n d P a l e o gene sediments from the central Arctic Ocean provides evidence of polar upw e l l i n g . T h e s e s e d i m e n t s a r e u n i q u e in t h a t t h e y r e p r e s e n t t h e m o s t n o r t h e r l y known sites of biogenic silica accumulation for any geologic time period, represent rare recovery of primary opal-A of Late Cretaceous age, and represent the only recovery to date of pre-Neogene sediment deposited within the c e n t r a l A r c t i c O c e a n . T i m i n g o f b i o g e n i c s i l i c a d e p o s i t i o n in t h e A r c t i c 0031-0182/82/0000-0000/$02.75 © 1982 Elsevier Scientific Publishing Company

136

precedes its known deposition in both the Bering Sea of the North Pacific and the Norwegian--Greenland Sea of the North Atlantic, suggesting a Cenozoic transfer of silica sinks in high northern latitudes. This paper examines paleogeographic and paleoclimatologic factors that may have permitted or promoted polar upwelling and silica deposition in the central Arctic during these time intervals, and influenced the transfer of high-latitude oceanic sites of silica deposition. Two problems requiring evaluation of paleogeographic reconstructions and paleocirculation models are addressed: (1) the sources of silica, and (2) the generating mechanisms of upwelling. We propose patterns of paleocirculation that are consistent with polar paleo-upwelling and with both the tectonic evolution of the Arctic, North Atlantic and North Pacific and the climatic evolution of the world ocean. In addition, we hope these empirical data of biogenic silica content will challenge climate dynamicists to construct deductive paleoclimate models that predict upwelling in the central Arctic for a time when the earth was more isothermal. METHODS

These Late Cretaceous and Paleogene Arctic sediments, originally described as tuffaceous (e.g. Clark, 1974, 1981), are accurately described as biogenic siliceous oozes. To determine quantitatively the biogenic opal content of these sediments (core 437, lat. 85°59.87'N, long. 129°58.76'W and core 422, lat. 84°53.48'N, long. 124°32.87'W), the normative calculation technique developed by Leinen (1977), and subsequently applied to deep-sea sediments by Leinen (1979) and Brewster (1980), was selected as most reliable for marine sediments of this age. The equation used to calculate biogenic opal content is derived from Si, A1, and Mg concentrations determined by atomic absorption spectrometry. The equation is based on Pacific sediments containing high montmoriUonite values. Montmorillonite is similarly a dominant clay mineral in these Arctic sediments, as determined by X-ray diffraction. The equation used is: 4.33 A1 (wt %) + 1.36 Mg2 (wt %) = non-biogenic silica (wt %) Biogenic silica is determined by subtracting non-biogenic, clay-bound silica from total silica concentrations, corrected for quartz values. The lack of narrowly restricted age dating precludes the possibility of determining changes in rates of accumulation of silica. Hence, four representative horizons, based on quantitative measures of siliceous microfossil densities per sampling interval of 5 cm, were selected for analysis. Of each sample 200 mg were digested in Teflon-lined stainless steel decomposition bombs at l l 0 ° C with aqua regia and hydrofluoric acid. The solutions were neutralized with boric acid, and analyzed for Si, A1, and Mg by atomic absorption spectrometry. Samples were run in triplicate.

137 RESULTS

Biogenic silica content and depositional environment The elemental concentrations of Si, A1 and Mg in weight % are given in Table I. Although b o t t o m sediments of the modern Arctic Ocean are impoverished in biogenic opal, generally containing less than 0.5% (Lisitsyn, 1972), the Cretaceous and Paleogene Arctic sediments yielded mean representative biogenic opal concentrations calculated on a bulk-sediment basis of 43.8% to 78.5%. Cretaceous biogenic opal concentrations exceed Paleogene concentrations. Optical examination of these sediments using light microscopy and electron microscopy reveals an exceptionally high concentration of siliceous microfossils, specifically diatoms, silicoflagellates, archaeomonads and ebridians. Calculated densities of siliceous microfossils average 100 to 400 X 106 specimens/g bulk sediment. Comparable densities in modern marine sediments are found in the Gulf of California, the southwest coast of Africa, and the Antarctic, associated with strong upwelling conditions (e.g., Calvert, 1966; Zhuze, 1972). X-ray diffraction determination of opal-A as the silica form in these sediments rather than opal-CT is consistent with the observed lack of carbonates. The transformation of biogenic opal-A to opal-CT is retarded b y low alkalinity (Kastner et al., 1977; Kagami, 1979). The absence of transformation of opal-A to opal-CT, despite the greater than 65 m.y. age of the Late Cretaceous opal, also requires a burial depth less than 300 m (Hein et al., 1978). Hall (1973, 1979) described inactive sediment waves on the Alpha Ridge as evidence of strong paleo-bottom circulation, most likely related to opening of the North Atlantic--Arctic connection. Hence, erosion of Tertiary sediment may account for the current shallow depth of burial (<0.5 m). Development of a strong western boundary current in the Antarctic has similarly been postulated to have removed Tertiary sediments at D.S.D.P. Site 275, where Late Cretaceous diatomaceous ooze is exposed on the ocean floor (Kennett et al., 1974). At this site, an unrecovered Pleistocene veneer was inferred from down-core contamination with Pleistocene foraminifera. Similarly, the degree of down-core contamination of these Arctic sediments TABLE I Mean elemental concentrations (corrected for salt content) and per cent biogenic silica Site

Interval (cm)

A1

Mg

Si

SiO:

Biogenic silica

437 437 422 422

79--81 175--177 186--188 302--304

1.87 1.29 2.11 2.16

0.87 0.50 0.67 0.41

42.35 33.94 31.31 26.13

87.67 70.26 64.80 54.08

78.51 64.36 54.95 43.77

138 by Pleistocene foraminifera is negligible (contra Clark, 1974), and does not require an interpretation of slumping. The scarcity of terrigenous detrital material in these Arctic sediments is consistent with deposition in a non,shelf environment. Terrigenous dilution of opal concentrations is characteristic of many shelf sediments. Terrigenous diluents, for example, are responsible for the low (~ 30%) opal values in sediments of the modern Bering Sea (Lisitsyn, 1972; Calvert, 1974). Peruvian coastal sediments similarly display low opal concentrations resulting from terrigenous dilution, despite their association with a highly productive area of local divergence (Molina-Cruz, 1977a, b). However, many of the diatom genera are believed to be neritic, although the deep basins of the Arctic are surrounded by expansive shelves. The general lack of burrowing in these Arctic sediments, and the preservation of fish vertebrae and scales, are consistent with deposition beneath a highly productive upwelling area (Diester-Haass, 1978). Sonobuoy refraction studies suggest that the Canada Basin of the Arctic Ocean was at least 1500 m deep in Late Cretaceous time (Grantz and Eittreim, 1979). The dearth of laminae argues against pronounced seasonality of depositional modes. Finally, high voltage electron microscopy reveals mechanical breakage of microfossils but little chemical dissolution. The location of coring sites in high northern latitudes, and their representational temporal records, are plotted in a series of paleoceanographic reconstructions for the past 70 m.y. (Fig.l). The available record includes D.S.D.P. sites in the Bering Sea of the North Pacific, U.S.G.S. sites in the central Arctic, and D.S.D.P. sites in the Norwegian--Greenland Sea of the North Atlantic. Only U.S.G.S. Sites 437 and 422 provide direct evidence of paleoceanographic conditions of the central Arctic. Site 437 is located on the flank of a graben structure on the Alpha Ridge (Hall, 1979). Present water depth is 1584 m. Core length is 282 cm. The upper 46 cm consists of Pleistocene sediment with calcareous microfossils; the underlying 236 cm consists of Late Cretaceous siliceous sediments. Dating of the siliceous sediments by silicoflagellates places the section in the Late Cretaceous (Campanian--Maastrichtian) Lyramula fulcata zone (Ling et al., 1973; Bukry, 1981). Site 422 is also located on the Alpha Ridge, 115 km from site 437. Present water depth is 2049 m. Core length is 364 cm, and includes an upper 12 cm of Pleistocene sediment containing calcareous microfossils overlying 352 cm of Paleogene siliceous sediments. The age of core 422 is definitely Paleogene, but assignment within the Paleogene is still tentative: the absence of any species of Naviculopsis, the diagnostic Late Paleocene silicoflagellate, suggests placement in the Early Paleocene (Danian) Corbisema hastata zone. For example, Naviculopsis danica and N. constricta comprise 60--88% of the Late Paleocene assemblage at Site 384 in the western Atlantic (Bukry, 1978). Similarly, the absence of Hemiaulus incurvus, the diagnostic Late Paleocene diatom species, despite a high diversity ofHemiaulus spp. in this core, suggests

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140

a pre-Late Paleocene age. However, the occurrence of the silicoflageUate

Dictyocha spinosa suggests a possible Eocene age (D. Bukry, pers. comm., 1981); the occurrence of the diatom Pyxilla suggests a post-Early Eocene (but Paleogene) age (A. Gombos, pers. comm., 1981). The recovery of extremely well-preserved and abundant siliceous microfossils of Late Cretaceous and Paleogene age in two cores from the same geographic region suggests that the central Arctic Ocean may be a good site for obtaining recovery, in a single core, of the siliceous record across the Cretaceous/ Tertiary boundary.

Temporal shifts of silica sinks: tectonic effects The timing of deposition of biogenic silica in the Arctic precedes its known accumulation in both the Norwegian-Greenland Sea of the North Atlantic and the Bering Sea of the North Pacific (Fig.2). In deep~sea sediments of high northern latitudes recovered to date, biogenic siliceous ooze uniquely represents the Arctic Late Cretaceous--Paleogene record, occurs during the Eocene to Pliocene record in the Norwegian-Greenland Sea (Schrader et al., 1976), and is dominant since the Late Miocene in the Bering Sea (Hein et al., 1978). Opal accumulation in high southern latitudes remained depressed until the Miocene (Leinen, 1979; Brewster, 1980). The depositional transition from siliceous biogenic sedimentation to calcareous biogenic sedimentation in the Arctic Ocean is out-of-phase with the depositional transitions in the Norwegian-Greenland Sea and the Bering Sea (Fig.3). When siliceous sediments are accumulating in the Arctic, non90°N •

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Fig.3. Distribution of major lithologies at selected sites in the Bering Sea (D.S.D.P. Leg 19), the Arctic Ocean, and the Norwegian--Greenland Sea (D.S.D.P. Leg 38). Several possible Late Cretaceous ages are shown for core 437 based on magnetic p o l a r i t y ; s e e t e x t for discussion of age assignment for core 422. M represents m u d s t o n e ; Ch represents chalk; G represents glacial sediments; C represents calcareous sediments. Data c o m p i l e d f r o m this study, Hein et al. (1978) and Schrader et al. (1976).

siliceous sediment (chalk) is being deposited in the North Pacific at Site 192. When siliceous sediments are being deposited in the Bering Sea (e.g., Sites 1 8 4 , 1 8 5 , 1 8 8 , 1 8 9 , 1 9 0 , 1 9 1 , 1 9 2 ) and the Norwegian--Greenland Sea (e.g., Sites 346 and 348) during the Plio-Pleistocene, the extensive Late Neogene coring record in the Arctic (see Clark et al., 1980) evidences the deposition of calcareous microfossils.

142

Berger (1970) described ocean basins as being of two types: estuarine, characterized by siliceous biogenic sedimentation, surface water outflow and deep-water inflow, and lagoonal, characterized by calcareous biogenic sedimentation, surface water inflow and deep-water outflow (Fig.4). The modern Atlantic Ocean is lagoonal, with deep-water outflow, whereas the modern Pacific Ocean is estuarine, with deep-water inflow. Thiede et al. (1980) suggested that the North Atlantic may have changed several times from a lagoonal to an estuarine mode of deep-water circulation, resulting in different modes of biogenic sedimentation. Similarly, there has been at least one major change-over of the Arctic Ocean, from an estuarine type of basin, characterizing the Late Cretaceous--Paleogene, to a lagoonal type of basin, characterizing the Plio-Pleistocene. Recovery of the Late Paleogene--Early Neogene sediment record of the central Arctic is requisite before more exact timing of this change-over can be determined. However, Fig.5 contrasts the modem exchange of surface and deep water between the Arctic and the world ocean with a possible Late Cretaceous circulation diagram. The major distinctions between the modern pattern of circulation and the paleocirculation pattern are the lack of deep-water exchange between the Arctic and North Atlantic in the Late Cretaceous as compared to today, and the presence of extensive exchange of surface water between the Arctic and epicontinental seaways in the Late Cretaceous. Tectonic events may be responsible for both the timing and the siting of sites of deposition of biogenic opal at high latitudes in the Northern Hemisphere. A causal relationship between the timing of siliceous deposition in the North Atlantic and development of bottom water exchange with the SILICA-RICH BASIN

SILICEOUS SEDIMENTS SILICA-POOR BASIN

Fig.4. Model of oceanic basins after Berger (1970). Mode of biogenic sedimentation determined by pattern of deep-water flow.

143 ARCTIC

NORT.'AC!!'A"T'C MODERN CIRCULATION EPEIRIC SEAS

LATE CRETACEOUS CIRCULATION Fi8.5. Schematic representation of m o d e r n c i r c u l a t i o n p a t t e r n and Late Cretaceous circul a t i o n pattern. A r r o w s denote d i r e c t i o n o f surface w a t e r and deep-water f l o w between

Arctic Ocean and world ocean. Shaded symbol represents siliceous sediment accumulation.

Arctic has been suggested (Berggren and HoUister, 1974; Tucholke and Vogt, 1979). Reich and Von Rad {1979) related the deposition of Late Eocene biogenic silica at D.S.D.P. sites in the northern North Atlantic to opening of the Norwegian--Greenland Sea-Arctic Ocean to the North Atlantic, and the resultant inflow of deep, cold bottom water. Similarly, a major change in circulation within the North Pacific has been proposed to account for the onset and maintenance of upwelling in the Bering Sea (FuUam et al., 1973; Hein et al., 1978). Schnitker (1980) has also proposed that the timing of biogenic silica accumulation in the circum-Antarctic may be due to midMiocene subsidence of the Iceland--Faroe ridge, when cold bottom water of the Northern Hemisphere began to flow southward to be upwelled in the circum-Antarctic (see also Blanc et al., 1980). The evolution of the Arctic Basin is coupled to the plate motions of both North America and Eurasia, leading Norris and Yorath (1981) to refer to the Arctic Basin as the "Rosetta Stone of plate tectonics". There is widespread consensus concerning the tectonic origin of the deep Eurasian Basin of the Arctic but widespread dissentience concerning the tectonic origin of the larger Amerasian Basin. The younger Eurasian Basin formed by sea-floor spreading along the Nansen Ridge. The age of initial spreading is Paleogene,

144

with estimates ranging from Early Paleocene to Late Eocene time (see Vogt and Ostenso, 1970; Pitman and Talwani, 1972; Feden et al., 1974; Dawes and Peel, 1981). The Lomonosov Ridge, which divides the Arctic into its two oceanic basins, is aseismic. The ridge is widely accepted as having rifted~ff the Siberian--Russian platform, first separating from Eurasia at about anomaly 24 time, i.e. Early Eocene Time (Vogt et al., 1979), although there is room for some preanomaly 24 sediment (Fig.6). Churkin and Trexler (1981)have suggested that the Lomonosov Ridge may be a micro~ontinental plate that rifted from the Kara and Barents Sea in response to opening of the North Atlantic. Blasco et al. (1979) reported the occurrence of mid-Cretaceous and Devonian microfossils (palynomorphs) from the Lomonosov Ridge, but

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145 proposed deposition of these sediments prior to separation of the ridge from the Barents shelf. The origin of the Alpha Ridge of the Amerasian Basin remains enigmatic. Vogt et al. (1979) suggested that the Alpha Ridge originated during the long mid-Cretaceous normal polarity epoch. The ridge has none of the requisite geophysical characteristics to be considered either an ancient nor an active spreading center (Dawes and Peel, 1981), although tectonic models have ascribed its origin to an extinct spreading axis (Vogt and Ostenso, 1970; Hall, 1973; Kerr, 1981), as well as a subduction zone (Herron et al., 1974). More recently, a plate capture model has been proposed by Churkin and Trexler (1980, 1981). According to this interpretation, the Amerasian Basin represents an isolated fragment of the Pacific Basin, acquired by the Arctic Ocean through circumpolar drift and microplate accretion (Fig.7). Circumpolar drift of the continents coupled with northward drift of the Kolyma plate resulted first in suturing of Kolyma with Siberia and then in suturing of Alaska and Chukotka with Eurasia. If this model is correct, the Alpha Ridge may represent a collision zone between the proto-Pacific plate and the Eurasian plate. Hence, the Alpha Ridge may adequately represent a central Arctic Ocean site for the Late Cretaceous--Paleogene time interval. B. E A R ~ CRETACEOUS

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Fig.7. Paleogeographic reconstructions of the Arctic according to plate capture model of Churkin and Trexler (1980). A. 175 m . y . B . 125 m . y . C . 50 m . y . D . Present.

146

The Verkhoyansk region of Siberia represents the plate boundary between Eurasia and North America. Compression in the Verkhoyansk region occurred in response to opening of the North Atlantic. This region of the Verkhoyansk Mountains was a deep oceanic environment throughout the Paleozoic and much of the Mesozoic (Fujita, 1978). Collisions occurred in Cretaceous time between the Kolyma plate and Siberia on the west, and between the Kolyma plate and North America on the east (Fig.8). The Yuzhniy Anyuy foldbelt represents the collision zone between two continental land masses, the Kolyma plate and the Chukotka--Brooks Alaska plate. This suture also closed in Late Cretaceous time (Fujita, 1978). Consequently, any site of deep-water exchange between the Arctic Ocean and the Pacific Ocean must have been pre-Late Cretaceous. Similarly, at the close of the Mesozoic, there was no deep-water exchange between the Arctic Ocean and the North Atlantic. But whereas there has been no subsequent evolution of sites of deep-water exchange between the Arctic and the Pacific, two sites of exchange of deep water between the Arctic and the Atlantic did tectonically develop during the Cenozoic. The Svalbard--Greenland Strait controlled the Arctic/Norwegian--Greenland Sea connection, and the Faroe--Iceland Ridge controlled the Arctic/North Atlantic connection (Fig.9). Consequently, if the Arctic Ocean may be

Fig.8. Tectonic regions o f Arctic Basin, northeastern Siberia, northwestern Alaska, and North Pacific. Plate boundaries discussed in text are shown by dashed lines. Redrawn from Fujita (1978) and Sweeney et al. (1978).

147

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termed the Rosetta Stone of plate tectonics, these two sites of deep-water exchange may be the decoders of Tertiary paleoceanography. Fig.10 summarizes the timing of emergent, shallow water, and deep-water conditions within the Arctic Eurasian Basin and the Norwegian--Greenland Basin, and in the three important straits linking the Arctic to the world ocean. Greenland was part of the Eurasian plate prior to anomaly 25 time (Late Paleocene), but by anomaly 19 time (Eocene), Greenland had become part of the North American plate (Pitman and Talwani, 1972; Dawes and Peel, 1981). Opening of the Norwegian--Greenland Sea by sea-floor spreading first began between anomalies 24 and 25 time (~Paleocene/Eocene boundary) (Talwani and Eldholm, 1977). The oldest oceanic sediments in the Norwegian-Greenland Sea are Early Eocene (Thiede, 1979). The lack of any older known deep-water sediments in this region suggests that only shallow seaways existed during Mesozoic and pre-Eocene time (Eldholm and Thiede, 1980; Vogt et al., 1981). Even after the Norwegian--Greenland Sea opened, however, the Svalbard-Greenland Strait and the Faroe--Iceland Ridge efficiently prevented the exchange of water between the Arctic and the North Atlantic. Timing of the opening of the Svalbard--Greenland Strait is important, because only then could there be deep-water exchange between the Arctic and Norwegian-Greenland Sea. The deep-water passage of the Svalbard--Greenland Strait did not exist prior to mid-Oligocene time; deep-water exchange between the Arctic and the Norwegian--Greenland Sea was not established until Early Miocene time, according to Eldholm and Thiede {1980). On the basis of

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E M E RG E N T

Fig.10. Summary presentation of emergent, shallow-water and deep-water conditions in the Arctic Eurasian Basin, Bering Strait, Norwegian--Greenland Basin, Svalbard--Greenland Strait and Faroe--Iceland--Greenland Ridge. Magnetic polarity time scale of Lowrie and Alvarez (1981). 1 = time of oldest magnetic anomaly; 2 = time of initial spreading in Norwegian Sea; 3 = time of initial spreading in Greenland Sea; "1 = timing of deep-water connection with Arctic; 4 = timing of active spreading. Data from Nilsen (1978); Vogt et al. (1979); Eldholm and Thiede (1980); Schnitker (1980).

planktonic microfossils, Schrader et al. (1976) concluded that emergent land blocked the connection until post-anomaly 13 time, or Early Oligocene. By Miocene time, biogenic sedimentation had become the dominant mode of deposition within the Norwegian-Greenland Sea. Biogenic oozes had replaced hemipelagic mudstones (Nilsen and Kerr, 1978}. After link-up of the Arctic and the Norwegian--Greenland Sea, there was as yet no link-up between the Arctic and the Atlantic, and consequently, between the Arctic and the world ocean. The Faroe--Iceland Ridge remained a barrier to the exchange of deep water between the Arctic and Atlantic Oceans. The Faroe--Iceland Ridge was emergent during the Early Tertiary, as evidenced b y lateritic paleosols on basalt at D.S.D.P. Site 336 (Nilsen, 1978). This site is located on the northern ridge flank of the Faroe--Iceland Ridge and indicates that a marine neritic environment was not established until Late Eocene time. Moreover, because Site 336 is not on the major platform of the ridge, its subsidence below sea level must have occurred at a later date. Subsidence in Late Oligocene time presumably allowed the flow of North Atlantic surface water into the Norwegian Sea, b u t as y e t no return flow of deep water (Schnitker, 1980). By mid-Miocene time, subsidence of the Faroe-Iceland Ridge had increased sufficiently to allow the return flow of deep water, at last establishing the connection of the Arctic with the world ocean, so that the Arctic could n o w function as the global heat sink, the site of formation of deep cold b o t t o m water (Schnitker, 1980; Eldholm and Thiede, 1980). Marine microfossils recovered at D.S.D.P. sites in the North Atlantic lend support to an interpretation of isolation of the Arctic and the Norwegian--Greenland Sea from the North Atlantic until at least Miocene time (Schrader et al., 1976). The timing of Arctic sources of b o t t o m water

149

reaching the western and northern North Atlantic has been correlated to a pronounced seismic reflection at the Eocene/Oligocene boundary (Miller and Tucholke, 1982). Evidence derived from oxygen isotope analyses of benthonic foraminifera from D.S.D.P. Site 116, located on the present path of Norwegian Sea overflow, indicates a late Middle Miocene time of initiation of deep-water outflow (Blanc et al., 1980). The Baffin Bay and Labrador Sea area had an independent history of seafloor spreading. Jackson et al. (1979) presented detailed magnetic and geophysical data in support of oceanic crust in central Baffin Bay. Opening of Baffin Bay by sea-floor spreading was apparently restricted to Eocene time (Gradstein and Srivastava, 1980). Vogt et al. (].981) placed the time of spreading in Baffin Bay at anomaly 24 time (Early Eocene). On the basis of paleogeographic reconstructions, Gradstein and Srivastava (1980) suggested there was opening in the southern Labrador Sea during the Late Cretaceous to Eocene period, with subsequent opening in the northern Labrador Sea during Paleocene to Eocene time. A narrow, shallow strait through the present Nares Strait may have connected the Labrador Sea to the Arctic Ocean during Cretaceous and Cenozoic time, but bathyal conditions did not develop in the Labrador Sea until post-Paleocene time (Gradstein and Srivastava, 1980). Any tectonic uplift in the Davis Strait, however, would have cut off circulation through such a narrow strait. The Arctic flushing model, developed by Thierstein and Berger (1978) and Gartner and Keaney (1978), requires a deep ( - 5 0 0 m) strait in the region of the Norwegian--Greenland Sea which briefly opened in latest Maastrichtian time. Thierstein and Berger (1978) have suggested the Labrador Passage as the most likely site of this flushing event. However, in the opinion of Eldholm and Thiede (1980), neither land geology nor plate geometry supports the notion of a deep site in the region of the Norwegian--Greenland Sea during the latest Cretaceous: both Svalbard and northeast Greenland were emergent, not only in Late Cretaceous time but also into Early Paleocene time. Sea-level fluctuations also influence inflow into an oceanic basin. A summary of Late Cretaceous and Paleocene transgressions and regressions in seven provinces that experienced connections with the Arctic Ocean is presented in Fig.ll. Matsumoto (1980) is of the opinion that the Late Cretaceous epicontinental transgression was globally extensive, with the regressive phase that occurred at the end of the Cretaceous taking place rapidly, and culminating in the Paleocene global regression. Jeletzky (1978), however, has argued for regional sea-level changes, due to local tectonism, in the Arctic-Canadian province, suggesting that eustatic sea-level changes did not greatly affect either transgressions or regressions in this region. These Arctic-Canadian regions were emergent in latest Cretaceous and Paleocene time. In northwestern Europe, peak transgressions occurred in the Turonian, Santonian, and Campanian. On the Russian platform, the most extensive transgression occurred in the Campanian; the Danian was a time of general regression (Naidin et al., 1980; Matsumoto, 1980). The Western Interior Seaway of

150

LATE Province

Ceo. lTur.

CRETACEOUS

Cao. lsan,

Cam. IMoa.

Arctic Canada Sverdrup Basin

J

Arctic Urals

~

- - '='=~-'~

Russia

~..~,- - ~ . . ~

N, C e n t r a l Asia

N.W,

Europe N. A m e r i c a

Interior Nonrnarine

....

Trans.

Reg.

Peak Trans.

-52

Fig.ll. Summary diagram of Late Cretaceous--Paleocene transgressions and regressions in Arctic and associated provinces. Arrows denote timing of maximum transgression. Data compiled from Jeletzky (1978), Naidin et al. (1980), Matsumoto (1980).

North America experienced a major transgression in the Early Cretaceous from the Arctic, and multiple transgressions in the Late Cretaceous with a major regression in Late Maastrichtian time (Matsumoto, 1980). In summary, we envision three phases in the post-mid-Cretaceous history of transfer of global silica in high northern latitudes (Table II). Tectonic events played a key role in the timing of these transfers, particularly b y the opening and closing of exchange routes for silica-rich deep waters. During Phase I, which spanned the Late Cretaceous and a part of the Paleogene, the Arctic Ocean was a silica sink. Deep-water formation occurred within the Arctic Basin. There was no deep-water outflow to the Atlantic or, presumably, to the Pacific. The duration of biogenic silica deposition within the Arctic basin is n o t yet fully known. We anticipate that future coring will bracket the time interval of Phase I. The transition to Phase II was brought a b o u t b y opening of the Svalbard--Greenland Strait to deep-water outflow. Deep-water outflow o f b o t t o m water, formed in the Arctic, now occurred from the Arctic to the Norwegian--Greenland Sea. The Norwegian--Greenland Sea became a silica sink, with siliceous deposition beginning in the Eocene. The transition to Phase III was brought about b y submergence of the Faroe-Iceland Ridge to bathyal depths. Deep-water outflow occurred from the Arctic and Norwegian--Greenland Sea to the North Atlantic, where it was transported to the circum-Antarctic and, indirectly, to the Pacific. Both the circum-Antarctic and the North Pacific became silica sinks during Phase III,

151 TABLE II Phases of transfer of silica in high northern latitudes Phase

Silica sink

Transitional tectonic event

I

Arctic Ocean

II III

Norwegian--Greenland Sea Bering Sea

opening of Svalbard-Greenland Strait submergence of Faroe-Iceland Ridge

with siliceous sedimentation beginning in the Miocene. To recapitulate, we postulate that a shift in the siting of silica deposition took place in high northern latitudes in response to changes in the sites of formation of deep water and the routes of deep-water outflow. The Arctic Ocean presumably remained the site of formation of deep b o t t o m water until formation of its ice-cover when the Norwegian Sea became the northern global site of deepwater formation.

Oceanic circulation and polar paleo-upwelling: climatic effects Paleoclimate modeling We would now like to direct our attention to the climatic conditions of the Late Cretaceous and Paleogene to determine the effects of climate on oceanic circulation in order to address the question: what permitted or promoted paleo-upwelling in the central Arctic? The global climate is comprised of interacting atmospheric, oceanic, continental, and cryospheric components. Feedback between these components may be negative or positive, instantaneous or time-lagged. Such complexities prohibit a comprehensive knowledge of present-day climate, and few numerical models have examined pre-Quaternary paleoclimates. Such inherent non-linearities first led Lorenz (1968, 1970) to ask whether long-term climate change may be due to complex internal dynamics rather than to variations in external agents. Several time
152 changes in the oceanic transport of energy. The results indicated that variations in sea level are more capable of forcing climatic change than are latitudinal positions of land areas, and that increased land area in the subtropics accounts for greater albedo changes than does snow cover in high latitudes. The authors postulated that the observed 100 m.y. global cooling trend since the mid-Cretaceous is the result of an increase in the proportion of land area in the subtropics brought about by sea-level changes and shifting paleopositions of continents. Thompson and Barron (1981) have performed a sensitivity analysis of a simple climate model to variations of the albedo component. The model runs specify Cretaceous conditions of reduced land area, low land albedos, diminished meridional thermal gradient, and no polar sea ice. By changing each variable sequentially, Thompson and Barton were able to quantitatively map the effect of each variable on the climate model. The results indicated that about 40% of the difference between the mid-Cretaceous and present
153

energy, Barron and Washington (1982), however, have demonstrated that sluggish currents are not a model property of the Cretaceous condition, due to the effects of increased water vapor in the atmosphere which acts to maintain the meridional gradient. Barron et al. (1981) also performed a sensitivity analysis of a climate model based on zonal energy balance. Mid-Cretaceous parameters of paleogeography were specified and mid-Cretaceous mean equatorial temperatures were estimated for model verification at 304°K, whereas mid-cretaceous mean polar temperatures were estimated at 288°K, substantially elevated over present
Polar upwelling Paleo-upwelling is deduced from biogenic siliceous sedimentation: the deposition of biogenic opal in quantity parallels areas of upwelling that act as silica sinks regardless of the source sites of silica input (Heath, 1974). Upwelling is not known to occur presently in the Arctic Ocean. The fortuitous recovery of two cores, located 115 km apart, both comprised of siliceous ooze but of different ages, suggests that the upwelling phenomenon was not short-lived. The lack of distinctive laminae argues against pronounced seasonality. Because of the central location of the Alpha Ridge within the Arctic Basin during the Late Cretaceous and Paleogene, we assume the upwelling d o c u m e n t e d b y the biogenic opal content of these cores represents openocean upwelling. Our interest is consequently directed toward the potential generating mechanism(s) of open-ocean upwelling over the Alpha Ridge during ice-free conditions. Several factors will be examined, including atmospheric--oceanic circulation, b a t h y m e t r y , and density stratification.

154

Isotopic data derived from benthic microfossils indicate that bottom water temperatures were elevated to 15°C in the Late Cretaceous and Paleocene (Shackleton and Kennett, 1975; Savin, 1977). Bottom water temperatures have been assumed to reflect surface water temperature at high latitudes. Thus, although the meridional thermal gradient was weaker when polar surface temperatures were high, the vertical thermal gradient at high latitudes may not have been much different in magnitude (Fig.12). We suggest that a north polar projection during the Late Cretaceous winter period would reveal a cyclonic circulation pattern. The low level of incident radiation of winter coupled with the small heat storage capacity of the surrounding land would result in a relatively cold land surface temperature; in contrast, the large heat storage capacity of the ocean is assumed here to

30

MAASTRICHTIAN (LATE CRET,)

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MIDDLE/LATE EOCENE

LATE OLIGOCENE

o

w rr

20

I.,(%

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i

30

MIDDLE MIOCENE

~

I

i

r

i

PLIO- P L E I S T O C E NE

20

10

0 N9( N

, 60, 30

0

3'0 GO 9 0 S LATITUDE

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3'0 6'0 9'0 S

F i g . 1 2 . S c h e m a t i c s u m m a r y o f s u r f a c e w a t e r a n d b o t t o m w a t e r t e m p e r a t u r e as a f u n c t i o n o f l a t i t u d e for a series o f t i m e intervals o v e r t h e p a s t 7 0 m . y . ; a d a p t e d f r o m Savin et al. (1975).

155

result in comparatively small seasonal temperature departures from an annualaverage value of about 15°C. The polar ocean surface temperature would therefore exceed the temperature of the surrounding land surface during winter periods b y an amount that may have approached 15°C. Large-scale land-ocean seasonal temperature contrasts of this magnitude are known to be associated with monsoonal circulations. For example, the low-level flow pattern over Asia is cyclonic in summer when the land is warmer than the ocean, and anticyclonic in winter when the land is colder than the ocean (Manabe et al., 1974). If the high-latitude circulation response to land-ocean temperature differences is analogous to that for middle latitudes (e.g., Asia), then a resultant cyclonic circulation pattern of surface waters would be counterclockwise (Fig.13). Divergence would occur in the central region of such an atmospheric low. Open-ocean polar upwelling is predicted. We propose, therefore, that climatic conditions may have favored the establishment and maintenance of a semi-permanent low situated over the Alpha Ridge during ice-free winter periods in the Late Cretaceous and Paleogene. ICE- FREE POLE

CYCLONIC CIRCULATION

ICE-COVERED POLE

ANTICYCLONIC CIRCULATION

I Fig.13. Polar atmospheric conditions, oceanic circulation patterns, and mode of upwelling predicted for ice-free versus ice-covered polar conditions. L represents atmospheric low; H represents atmospheric high.

156 During Cretaceous summer periods, the temperature of the land surface surrounding the Arctic Ocean would presumably warm to levels exceeding 15 ° C. With the land somewhat warmer than the ocean, an anticyclonic circulation (probably relatively weak based on A T ) w o u l d develop, resulting in oceanic convergence and the potential for coastal upwelling. Such a weak anticyclonic circulation pattern might have a superficial resemblance to the present pattern of weak anticyclonic flow (Fig.13). Coastal upwelling in summer could possibly have been important. Late Cretaceous silicoflagellates and diatoms, for example, are known in deposits from the Canadian Arctic Islands (Bukry, 1981). Paleobiogeographical evidence is consistent with a dominant cyclonic circulation pattern in the Arctic, at least during the Late Cretaceous. Patterns of faunal provinces in the Late Cretaceous predict a circumpolar flow from west to east, or counterclockwise. Kauffman (1975)discussed a circumpolar current system during the mid- and Late Cretaceous on the basis of faunal evidence, particularly the similarity between shallow benthic faunas in Europe, Greenland and America. He suggested that the direction of circumpolar flow was from west to east. Gordon (1973) also investigated Cretaceous circulation patterns, using the paleogeographic distribution of stenothermal marine organisms, and depicted the Arctic Ocean as circulating in a counterclockwise direction under the influence of polar westerly winds. Such counterclockwise circulation is consistent with an atmospheric low over the Arctic during the Late Cretaceous. Luyendyk et al. (1972) suggested that their "non~lacial" paleocirculation simulation of the Northern Hemisphere utilizing a belt of westerlies from 40 ° to 65 ° N lat. and a belt of easterlies from 65 ° N to the pole approximates midCretaceous to Eocene conditions. This frequently cited experimental simulation involving a heated rotating tank, however, failed to develop upwelling in the Arctic Ocean, and the resultant direction of circumpolar flow was from east to west. We suggest that these experimental results are of little heuristic value in understanding Arctic circulation patterns during the Late Mesozoic or Early Tertiary. It may be of interest, however, to attempt such experimental trials utilizing a different heating gradient, paleoceanographic reconstruction, and atmospheric simulation to determine the conditions under which polar upwelling and eastward circumpolar flow might occur. A second mechanism known to influence upwelling is bathymetry. Did the relief of the Alpha Ridge itself influence both the siting and the intensity of upwelling? Submarine topography is known to affect upweUing (Swift and Aagaard, 1976; Hartline, 1980). Maximum upwelling in the Peruvian system, for example, results from the presence of a seamount (PreUer and O'Brien, 1980). Bathymetry is a particularly effective mechanism at high latitudes where weak stratification allows even very deep bathymetry to "steer" currents. Moreover, Galt (1973) demonstrated numerically that bathymetry may be particularly important in controlling circulation patterns

157

at high latitudes where variation in the vertical component of the Coriolis force is negligible. In a barotropic model of Arctic circulation, Galt showed that a bathymetric feature such as the Lomonosov Ridge acts as a dynamic block between the two Arctic basins, preventing the spread of vorticity developed in one basin to propagate into the other basin (Fig.14). Galt also showed that when inflow and outflow are relatively weak, as they are in the Amerasian Basin t o d a y and as they presumably were before opening of the Svalbard--Greenland Strait, then oceanic circulation is wind
WIND-DRIVENCASE ~,, t

~

.. ,:: , '~!.....

~

:

:].

®

'ICE-DRIVEN CASE

....,""5

"

!il

~

-..t.

Fig.14. Influence of bathymetry on Arctic circulation pattern (after Galt, 1973). Lomonosov Ridge is dominant bathymetric feature of Arctic Basin.

158 Relative stratification of the water column is important in that stratification can isolate surface water from bottom water, so that bathymetry can no longer influence circulation (Hart, 1975). Today in the Arctic, the water beneath 300 m depth is essentially isothermal and isohaline (Hunkins, 1974). In the Late Cretaceous--Paleogene time interval, when the temperature of polar surface water was higher, the surface-to-bottom temperature gradient at the poles was presumably not increased over its present value. Changes in stratification intensity of Arctic waters due to salinity differences are more contentious than thermally induced stratification differences. However, if density stratification was greater in the Late Cretaceous due to more highly saline bottom water (see next section), then bathymetry would be a less important mechanism controlling circulation and upwelling in polar regions. In summary, more than one mechanism may have been responsible for generating and maintaining upwelling in the Arctic. Climate-controlled circulation was presumably the principal mechanism. Bathymetry may have been important, particularly in argumenting the intensity of upwelling. Greater stratification would have ameliorated the effects of bathymetry, whereas reduced stratification would have enhanced the effects of bathymetry. Finally, although we have been assuming that upwelling over the Alpha Ridge during ice-free conditions represented open-ocean upwelling, it is interesting to speculate whether the Lomonosov Ridge, prior to its separation from the Barents Shelf, acted as a coastline to the Alpha Ridge. Such speculation requires a better understanding of the temporal positioning of the Alpha Ridge with respect to the Lomonosov Ridge, and the bathymetry of the Alpha Ridge with respect to sea level during this time period.

An alternative hypothesis: the salinity motor The sites of formation of bottom water as well as the patterns of deepwater circulation have changed over geological time, in response to the forcing mechanisms of climate, tectonism, and paleogeography. An alternative hypothesis to account for the more isothermal conditions of the Late Cretaceous has been termed the warm saline bottom water hypothesis. Chamberlin (1906) first suggested that during times of global ice-free conditions deep oceanic circulation may have been driven not by thermal effects but by salinity effects, caused by evaporation in low-latitude basins and the formation of saline water that was denser than polar waters. In effect, the global circulation of haline deep water would be the opposite of what it is today, with low-latitude downwelling and high-latitude upwelling, and the historical record should contain evidence of reversals of direction and reversals of sources and sinks of deep water (Chamberlin, 1906; Berger, 1970). A haline circulation would presumably increase the amount of sensible heat transport by the oceans into high latitudes. Kraus et al. {1979) have proposed upwelling of warm saline deep water in high latitudes as partly responsible for the absence of polar ice (as reported by Barton et al., 1981).

159

The absence of any known site of deep-water exchange between the world ocean and the Arctic ocean during the ice-free time intervals of the Late Cretaceous and Early Paleogene makes deep-water penetration into the Arctic unlikely. Kraus et al. (1979), however, do suggest that a haline-circulation system could have penetrated the Arctic via the more shallow Ural passage and Thetys Sea. Brass et al. (1982) have proposed a model for the formation of warm saline b o t t o m water. The model suggests that at high latitudes surface waters were freshened, thereby preventing the formation of cold b o t t o m water b y cooling. Instead, b o t t o m water was produced b y the sinking of saline water formed in low-latitude marginal basins. Consequently, the size, siting and shape of these low-latitude marginal seas resulting from sea-level changes and plate tectonics were of particular importance in the forcing of climatic change. Barron (1980) has demonstrated a dramatic change during the past 100 m.y. in the area of shallow seas in low latitudes -- i.e. the sites for the production of warm saline b o t t o m water. Such a decrease in the potential area of deep-water production in low latitudes is both correlative with the observed isotopic cooling evidence and may suggest a transition from the production of deep water in low latitudes to its production in high latitudes. Berger ( 1 9 7 9 ) h a d previously suggested that the earth switched from a halinedriven to a thermal
160

faunal indication that the epicontinental seas were abnormally saline (Kaufman, 1979; Berger, 1979). CONCLUDING REMARKS In conclusion, we would like to consider the possible bistability of Arctic conditions and consequent characteristics. Kellogg (1979) has suggested that the Arctic ocean may have the capability to exist in only two stable states: ice-free and ice-covered. If the Arctic were ice-free, its albedo would be substantially lower so that it would absorb more solar radiation, making it difficult to become ice-covered. But if the Arctic were already ice-covered, then the low-salinity surface layer would act to prevent much heat flux from deep warmer water, making it more difficult to move away from an ice-covered condition. Such conditions of bistability involve a threshold condition that is unstable and can be visualized as a repellor, whereas b o t h ice-free and icecovered conditions may be visualized as attractors. Are there equilibrium conditions that result in either ice-free or ice-covered Arctic conditions with consequent atmospheric, oceanic, biogenic and sedimentological features? We have presented evidence that the Late Cretaceous--Paleogene ice-free polar period may be causally related to a dominant polar cyclonic circulation pattern, open-ocean polar upwelling, a polar silica sink, and biogenic siliceous sedimentation, whereas the Late Neogene ice-covered polar period may causally correspond to polar anticyclonic circulation, possible coastal upwelling, a silica conduit, and biogenic calcareous sedimentation. ACKNOWLEDGMENTS We thank J. Kutzbach, M. Leinen, N. Brewster, J. Parrish, A. Semtner, J. Galt, D. Bukry, A. Gombos for discussion; E. Barron, J. Kutzbach, and a n o n y m o u s reviewers for review; D. Olson for performing the silica analyses; H. Ris and S. W. Bailey for technical assistance. This study was supported by NSF Grant DPP-7926251. REFERENCES Augustsson, T. and Ramanathan, V., 1977. Radiative-convective model study of the CO2 climate problem. J. Atmos. Sci., 34: 448--451. Barton, E. J., 1980. Paleogeography and Climate, 180 Million Years to the Present. Dissertation, University of Miami, Miami, Fl., 270 pp. Barron, E. J. and Washington, W. M., 1982. Cretaceous climate: a comparison of atmospheric simulations with the geologic record. Palaeogeogr., Palaeoclimatol., Palaeoecol., 40:103--133 (this volume). Barron, E. J., Sloan, J. L. and Harrison, C. G. A., 1980. Potential significance of land--sea distribution and surface albedo variations as a climatic forcing factor: 180 m.y. to the present. Palaeogeogr., Palaeoclimatol., Palaeoecol., 30: 17--40. Barron, E. J., Thompson, S. L. and Schneider, S. H., 1981. An ice-free Cretaceous? Results from climate model simulations. Science, 212: 501--508.

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