ISSN 0001-4370, Oceanology, 2009, Vol. 49, No. 1, pp. 93–100. © Pleiades Publishing, Inc., 2009. Original Russian Text © A.G. Matul, A. Abelmann, D. Nürnberg, R. Tiedemann, 2009, published in Okeanologiya, 2009, Vol. 49, No. 1, pp. 101–109.
MARINE GEOLOGY
Stratigraphy and Major Paleoenvironmental Changes in the Sea of Okhotsk during the Last Million Years Inferred from Radiolarian Data A. G. Matula, A. Abelmannb, D. Nürnbergc, and R. Tiedemannb a Shirshov
Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia E-mail:
[email protected] b Alfred–Wegener Institute of Polar and Marine Research, Bremershaven, Germany c Leibnitz Institute Of Marine Research, Kiel, Germany Received July 9, 2007; in final form, February 4, 2008
Abstract—The radiolarian distribution is studied in Core IMAGES MD01-2415 (46-m-long) from the central Sea of Okhotsk. The obtained data made it possible to refine the regional biostratigraphy and document the major paleoenvironmental changes in the basin in the last million years. In total, 17 radiolarian datum planes are defined with 12 of them being new. Their number exceeds that previously established for different fossil groups in the Subarctic Pacific for this period. Radiolarian datum planes are usually confined to the main boundaries and Quaternary climatic events. The analysis of the radiolaria distribution reveals several major paleoenvironmental shifts in the sea that occurred 950, 700, and 420–280 ka ago and are correlative with regional and global phases of the Middle Pleistocene climatic revolution. DOI: 10.1134/S0001437009010111
INTRODUCTION
R/V Marion Dufresne in the summer of 2001 [6]. The core is 46.23 m long. The section is dominated by greenish gray and dark gray terrigenous sandy–silty– pelitic mud with an admixture of pebbles. Uniform terrigenous mud alternates repeatedly with biogenic (diatomaceous) silty–pelitic ooze. The section encloses numerous interbeds and lenses of volcanic ash from 2–3 to 15 cm thick. Radiolarians were studied in 288 samples (fraction >40 µm) taken with a step of 9 to 20 cm.
Micropaleontological studies of the stratigraphy and paleoceanography in the Sea of Okhotsk started in the 1950s based on foraminifers, diatoms, and palynological spectra [16, 18, 45]. These researchers established the succession of large climatostratigaphic intervals, which they correlated with the Quaternary glacial and interglacial periods, though without exact dating. In recent publications, their authors discuss the detailed climatostratigraphy of the Sea of Okhotsk for the last 200–350 ka based on the lithology, oxygen isotopes, and various microfossil groups [4, 5, 26–28]. The first high-resolution data on radiolarians in Pleistocene sediments of the last 350 ka were derived from the KOMEX Core LV28-42-4 [29]. In this work, we discuss the radiolarian assemblages studied in the IMAGES Core MD01-2415 comprising the last 1.1 Ma with several additional datum planes and significant changes in the radiolarian assemblages, which accord with the global and regional Quaternary climatic events. The distribution of radiolarians and the oxygen isotope profile are first correlated in the same long core for the Subarctic Pacific.
STRATIGRAPHY AND AGE OF THE SEDIMENTS IN CORE MD01-2415 The oxygen-isotope and sediment-color chronostratigraphy. The main method used for determining the age of the defined sedimentary beds in Core MD012415 (Fig. 1) was correlation of the oxygen isotope curve [39] with the standard astronomically calibrated oxygen isotope profile [30]. The oxygen isotopes were measured in the calcium carbonate of benthic foraminiferal tests [39]. For specifying the oxygen isotope stratigraphy, high-frequency cyclic variations of the properties of the sediments were used. The sediment color parameter, i.e., the distribution of the reflected light intensity in a certain area of the spectrum, is distinctly consistent in the “filtered” form with the precession cycle in the insolation record [39]. The geological record of Core MD01-2415 covers the last 1.1 Ma or marine isotopic studies (MIS) 1–31 [39]. The linear sedimentation rate in Core MD01-2415 averages 2 to 4 cm/ky, being notably higher during deglaciations
MATERIAL Core MD01-2415 (53°57.09′N, 149°57.54′E; water depth 822 m) was taken in the Sea of Okhotsk at the base of the continental slope by the IMAGES WEPAMA expedition during the cruise of the French 93
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and the Holocene (up to 6–8 and 22 cm/ky, respectively) [39].
from 15–20 to 40–55 taxa during the glacial and interglacial periods, respectively.
Stratigraphy and age of the sediments based on radiolarians. The glacial–interglacial intervals in Core MD01-2415 are well distinguishable based on the distribution of radiolarians (Fig. 1). The distinct peaks of the total radiolarian abundance in Quaternary sediments of the Northwest Pacific and Sea of Okhotsk are related to interglacial conditions that were favorable for the productivity of siliceous microorganisms [29, 34, 41]. The concentrations of radiolarians in sediments of Core MD012415 in the interglacial intervals amount to 90000– 150000 specimen/g.
The interglacial intervals show an increase in the concentration of specific radiolarian species such as, for example, Plagoniidae spp., one of the dominant taxa indicating high productivity in present-day radiolarian assemblages of the Sea of Okhotsk, and Amphimelissa setosa, indicating intense interaction between local cold and alien warm water masses [27]. The coincidence of the maximums of A. setosa relative to the concentrations and “light” δ18O values in the interval of MIS 5 to MIS 10 [29] allows us to consider variations in the distribution of this species as a reliable tool for defining the interglacial periods in Middle–Upper Quaternary sediments of the Sea of Okhotsk.
In the North Pacific, a higher diversity of radiolarians is characteristic of sediments with relatively thermophilic interglacial fauna [20]. In total, 110 radiolarian taxa were determined in the examined samples, which is lower as compared with the pelagic part of the North Pacific (140 taxa), where the Quaternary radiolarian fauna includes both arcto-boreal and subtropical species [21]. The diversity of the radiolarian assemblages in the sediments of Core MD01-2415 varies
The oxygen-isotope age of the sediments is confirmed by the radiolarian datum planes (Fig. 1). The basal part of the core is marked by the last occurrence (LO) level of Eucyrtidium matuyamai dated back to approximately 1 Ma ago [13]. This level corresponds to the MIS 31/32 boundary. The upper part of MIS 21 hosts the LO level of Phormostichoartis pitomorphus OCEANOLOGY
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Radiolarian datum planes in Core MD01-2415 Datum plane: (LO) last occurrence, (FO) first occurrence
Depth in core, m
LO Lychnocanoma nipponica sakaii LO Amphimelissa setosa LO Sphaeropyle langii LO Spongodiscus sp. LO Stylacontarium acquilonium FO Artobotrys borealis LO Tholospira sp. 2 FO Lychnocanoma nipponica sakaii FO Stylochlamydium venustum FO Tholospira sp. 2 FO Dumetum rectum LO Perichlamydium sp. LO Litheliidae spp. LO Phormostichoartus pitomorphus LO Antarctissa (?) sp. 3 LO Rhizoplegma sp. LO Eucyrtidium matuyamai
4.23 5.63 16.23 16.43 18.93 19.93 22.53 26.53 26.53 28.13 31.13 33.15 33.75 37.23 37.43 45.03 45.83
dated 825 ka ago [33]. The MIS 9–MIS 10 boundary is confirmed by the LO level of Stylacontarium acquilonium (age 329 ka ago) [29]. The position of oxygen isotope event 8.5 is documented by the LO level of Spongodiscus sp. dated back to 287 ka ago [29]. The lower part of MIS 4 and upper part of MIS 3 enclose LO levels of Amphimelissa setosa Cleeve and Lychnocanoma nipponica (72 and 28 ka ago, respectively) [29]. RADIOLARIAN DATUM PLANES FOR THE LAST 1.1 MY In Core MD01-2415, the radiolarian analysis revealed again 17 datum planes (table) , which exceeds significantly their number established previously for different microfossil groups in the North Pacific for the last 1.1 Ma: 10, 9, and 4 datum planes for radiolarians [33, 35, 37], diatoms [17, 43, 50], and nannofossils [22, 48], respectively. The last 1.1 My were characterized by rapid highamplitude climatic oscillations: regular alternation of glacial and interglacial conditions. It is logical to believe that they should be accompanied by transformations of the faunal communities. Studies [2, 3] revealed numerous mass extinctions and strong transformations of the radiolarian fauna during the Phanerozoic on the scale of hundreds and tens of millions of years. These events were associated with major climatic shifts [3]. For example, the onset of large-scale glaciation at the Neogene–Quaternary transition was accompanied by extinction of 93.1% of the species and 83.5% of the Cenozoic radiolarian genera. Afanas’eva OCEANOLOGY
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Age, ka 34 64 267 272 340 353 403 500 500 536 630 694 716 823 830 1061 1082
Position in the oxygen-isotope profile: (MIS) marine oxygen isotope stage or event End of MIS 3 MIS 4 Middle of MIS 8 MIS 8.5 MIS 9-IS/10 boundary End of MIS 10 Optimum of MIS 11 MIS 13.13 MIS 13.13 MIS 13/14 boundary End of MIS 17 Optimum of MIS 17 MIS 17/18 boundary End of MIS 21 End of MIS 21 End of MIS 31 MIS 31/32 boundary
and Amon [2] confirmed the assumption in [10] that “warming–cooling” pulses are the main moving force that stimulated ecosystems characterized by different biotas becoming permanently adapted to changeable natural settings. In our material, the last occurrence levels of radiolarians are confined to either the boundaries between oxygen isotope stages or isotopic extremums (table); i.e., species became extinct (a) at transitions between glacial and interglacial periods and vice versa and (b) during the interglacial optimum, short distinct warming, or glacial events. The transitions to interglacial periods are marked by extinctions of E. matuyamai, Litheliidae spp., and S. aquilonium. Perichlamydium sp. and Tholosira sp. 2 became extinct during the interglacial optima. Distinct “warm” isotope extremums inside interglacial and glacial MIS are marked by extinctions of Antarctissa (?) sp. 3 and P. pitomorphus. Spongodiscus sp. Rhizoplegma sp., S. langii, A. setosa, and L. nipponica sakaii disappear at transitions to glaciations and during their initial phases. The paleoclimatic confinement of the LO levels accords with the distribution of species in the Pleistocene record: radiolarians that became extinct at transitions to warm phases or during warm phases formed maximums of their relative concentrations in glacial intervals or beyond the optimal parts of interglacial periods. On the contrary, radiolarians that disappeared at transitions to glaciations or during glacial intervals were typical of interglacials. The sole exception is L. nipponica sakaii, which was characterized by maximal abundances during glacial periods and disappeared at the end of inter-
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glacial MIS 3. The sharp decrease in the content of this radiolarian species (from 20 to 1–2%) concentrations of these radiolarians are formed. It is conceivable that these species need the so-called “ecological time” for dispersal and adaptation to the local ecosystem [24]. Mass radiolarian species with datum levels. Only three species among the extinct and appearing taxa played a significant, sometimes dominant role in the assemblages within occurrence intervals: A. setosa, Spongodiscus sp., and S. venustum. The relative content of A. setosa is maximal in the interglacial sediments (Fig. 2). Its high concentration coincides with both the high total abundance of radiolarians and the maximal contents of biogenic silica during the Middle and Late Pleistocene interglacial periods. Judging from the data on recent radiolarians, A. setosa is most abundant in the Subarctic Atlantic on the Iceland Plateau characterized by high accumulation rates of radiolarians in the bottom sediments [7, 8]. This area is washed by highly productive mixed cold and saline water masses with vertically leveled parameters. The relative content of Spongodiscus sp. and S. venustum was higher during the glacial periods. In the present-day Northwest Pacific and Sea of Okhotsk, representatives of the family Spongidiscidae with S. venustum being among the most abundant species are characteristic of the surface water mass and survive significant variations in the temperature and salinity [16]; i.e., they are able to adapt to substantial changes in the hydrological parameters during unfavorable climatic periods. Their maximal abundance associates with the spring phytoplankton bloom [1]. Inasmuch as Spongodiscus sp. belongs to the family Spongidiscidae, we believe that the habitat conditions of this species and S. venustum are similar. The disappearance of Spongodiscus sp. from the assemblages coincides with the growth of the S. venustum role, which could occupy the ecological niche of the former. If the niche is generally preserved, when some Spongodiscidae species is replaced by another one, the probable cause for such a replacement could be an increase in the amplitude of the seasonal environmental fluctuations that is more suitable for the habitat conditions of S. venustum in the particular ecosystem. The cause could also be due to the reduced biogenic silica content in the surface waters since the skeleton of SponOCEANOLOGY
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godiscus sp. is substantially thicker as compared with that of S. venustum. The notable progressing decrease in accumulation of biogenic silica in the Subarctic Pacific from the Late Pliocene to the Late Pleistocene is reflected in both the radiolarian records [27] and in the variations of the total opal in the sediments [11, 12]. The silica productivity and general bioproduction fall in the northern periphery of the Pacific and are confirmed by the δ13ë shift in the benthic foraminifers [44] resulting from the enhanced Subarctic water stratification after 2.7 Ma ago [11]. MAJOR PALEOCEANOGRAPHIC CHANGES IN THE SEA OF OKHOTSK DURING THE LAST 1.1 MA The changes at approximately 950 ka ago. In the period of 1.1 Ma to 950 ka ago, the variations in the total abundance of radiolarians were insignificant. The first notable increase in the accumulation of radiolarians occurred 960–950 ka ago during MIS 25. The latter is manifested as the first distinct global interglacial stage of the last 1.1 My, which is well documented by the oxygen isotope curves [23]. Prior to this level, the distribution of radiolarians was characterized by frequent low-amplitude fluctuations sometimes inconsistent with the glacial cyclicity. After 950 ka ago, the maximums of the total diversity become long, highamplitude, and consistent with the glacial–interglacial cycles. The level 950 ka ago starts the first long continuous (approximately 140 ky long) period of high (at least 30–40%) concentrations of Cycladophora davisiana (Fig. 2), which characterizes a cold ventilated water mass similar in its hydrological parameters to the present-day Sea of Okhotsk intermediate water mass. The same level also marks the onset of periods with high maximums of the Lithocampe platycephala relative contents (>20%) usually coincident with interglacial stages during the terminal Early–initial Late Pleistocene. In the present-day North Pacific [42] and Sea of Okhotsk, this species is rare constituting 2.0–2.5% in several samples of surface bottom sediments [37]. The present-day area with high concentrations of L. platycephala in recent sediments is located in the Norwegian–Greenland Basin [7]. Its maximal abundances (>7–10%) are recorded in the eastern Iceland Plateau and eastern Greenland Basin [8]. The distribution area of this species reflects the interaction of cold Arctic and Subarctic waters, with warm North Atlantic waters being displaced toward the warm Norwegian Current. The surface water temperature in this area is 1– 4°ë in the winter and 4–10°ë in the summer, while the salinity is equal to the normal oceanic one (approximately 35‰ [9]. Similar to L. platycephala, the general growth in the contents and intermittent interglacial abundance maximums of Lithomelissa setosa and Trisulcus sp. begin at OCEANOLOGY
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the level of 950 ka ago. Among the planktonic communities of the present-day Sea of Okhotsk, L. setosa has not been found so far [1]; nevertheless, the relation between the maximum of this species in recent sediments (up to 2%) and the warm water from the Sea of Japan and the neighboring Pacific near the southern half of the Kurile Arc is clear [1]. The concentration of L. setosa in the sediments of the southwestern part of the sea varies from 10 to 24%, being 1–4% through its remainder. In the North Atlantic, the maximal relative content of this species (>10%) is confined to warm water masses of the north periphery of the temperate zone [8, 25]. In the northern Sea of Japan, L. setosa (= Lithomelissa sp.) occurs in plankton communities at depths of 0–200 m [14]. Trisculus sp. is rare in the Sea of Okhotsk; its content increases up to 2% in the southern part of the sea in the Kurile basin [37]. According to sedimentologic data, the level of 940 ka ago is marked by the growth in the concentrations of ice-rafted material, indicating enhanced activity of floating ice and, probably, icebergs, and an increase in the magnetic susceptibility, reflecting the intensified accumulation of terrigenous material. Beginning with MIS 25, biogenic material, including Corg and SiO2, was accumulated during brief, more intense than before, interglacial pulses (Fig. 1). The level of 950 ka ago represents the probable transition to (a) a long period of intense formation of a water mass similar to the present-day intermediate waters of the Sea of Okhotsk; (b) relatively high-amplitude glacial–interglacial fluctuations and more intense accumulation of biogenic material, including silica, during the interglacial periods; and (c) a stronger interglacial influence of the water masses entering presumably from the Sea of Japan and the southwestern Subarctic Pacific. Strong fluctuations in the surface water layer from well-stratified cold conditions and significant seasonal contrasts to less extreme though cold conditions began in the pelagic North Pacific approximately at that time [47]. The changes in the Sea of Okhotsk 950 ka ago correspond to the onset of the distinct Middle Pleistocene Revolution, which resulted in the replacement of the 41-ky cycle by the 100-ky one. The changes approximately 700 ka ago. In the lithophysical and geochemical parameters of the Core MD-2415 sediments, this level is indistinct except for the deglaciation at the MIS 18/17 transition [39]. The abundance and diversity of radiolarians show no notable fluctuations as well. At the same time, the distribution of some characteristic species demonstrates distinct changes. This level is marked by the reduction in abundance L. setosa, which begins to occur discretely. This species serves as an indication of warm water masses entering the basin from the Sea of Japan and the neighboring North Pacific. Higher in the section, the species appears in notable abundances only during interglacial periods and is missing in glacial intervals.
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Slightly earlier than in L. setosa 750–740 ka ago, the abundances of Stylatractus pyriformis, Echinomma delicatulum, and Pyloniidae spp. decrease as well (from 2–5 to 0–1%). Similar to L. setosa, the species S. pyriformis is presumably an additional indicator of the warm water influx from the Sea of Japan and the neighboring Pacific and its influence on the present-day conditions in the southwestern Sea of Okhotsk [1]. Recently, it has been found in plankton communities of the Oyashio Current mostly at depths of 0–200 m [40]. E. delicatulum is one of the typical elements of Boreal Pacific radiolarian assemblages [19]. Its maximal concentrations in recent sediments (up to 3–6%) are documented in the eastern half of the Subarctic Pacific, including the Gulf of Alaska (up to 8–12%); along the Aleutian Islands washed by the Alaskan Current; and the southern Bering Sea [42]. The species is also found at depths of 300–500 m in plankton communities of the Oyashio Current near Hokkaido [40]. The group of species Pyloniidae spp. represented in our material likely by juvenile specimens of Tetrapyle octacantha belongs to thermophilic elements of the Pacific. The last species, along with Octopyle stenozona, constitutes 15–29% of all the radiolarians in the subtropical and tropical zones of the Pacific, 5–8% in the mixing area of the Kuroshio and Oyashio currents, approximately 1.5% along the Subarctic Front, up to 2% near the North American coast, and 0–0.4% in the remainder of the Subarctic Pacific [42]. The decrease in their abundances presumably implies the reduced influence of warm waters from the Sea of Japan and North Pacific (from different depths) on the paleoenvironments in the central Sea of Okhotsk. Subsequently, such a notable influence occurred intermittently during the interglacial periods. Judging from the drastic decrease in the abundances of C. davisiana up to its occasional disappearance (Fig. 2), the level of 700 ka ago marks the onset of a long stage (almost 200 ky) of insignificant formation, if at all, of the water mass with parameters typical of the present-day intermediate water. Simultaneously, A. setosa permanently colonizes the Sea of Okhotsk to form its periodic interglacial maximums reflecting the wellmixed relatively cold conditions of the upper water column [27]. Precisely the dominant role of this species begins to determine the interglacial radiolarian assemblages in the Sea of Okhotsk during the Middle and Late Pleistocene. The level of 700 ka ago represents the probable transition to the (a) weaker and/or occasional influence of warm water masses from the Sea of Japan and the neighboring Pacific on the Sea of Okhotsk, (b) more distinct cyclicity in the development of the Sea of Okhotsk corresponding to the glacial–interglacial fluctuations, and (c) deeper mixing in upper water layers during interglacial optima. The last event is correlative with the onset of enhanced glacial–interglacial variations in the radiolarian assemblages and water paleotemperatures in the southern part of the Northwest Pacific after 800 ka ago [32]. In the period from 800 to 700 ka ago, the Subarctic Front
shifted southward by several degrees in the central part of the North Pacific [47]. The changes in the Sea of Okhotsk 700 ka ago correspond to the terminal phase of the fastest shift during the Middle Pleistocene Revolution. According to climatic modeling [49], 800 ka ago, short symmetrical low-amplitude glacial cycles gave way to wider sawtooth oscillations accompanied by the global ocean cooling and wider distribution of sea ice. The first pseudoperiodic fluctuations in the volume of continental ice with a cycle of approximately 100 ky were registered 600 ka ago [35]. Changes in the period of 420–280 ka ago took place approximately 400 and 300 to 280 ka ago. Beginning from approximately 420 ka ago, the long-period average mass accumulation rates of the fine sediment fraction (. 49. E. Tzipermann and H. Glidor, “On the Mid-Pleistocene Transition to 100-kyr Glacial Cycle and the Asymmetry between Glaciation and deglaciation times,” Paleoceanography 18, PA1001 (2003) doi: 10.1029/2001PA000627. 50. D. Winter, J. Arney, and S. W. Wise, Jr, “Upper Miocene–Pleistocene Diatom Biostratigraphy in the Northwest Pacific, ODP Leg 191,” in Proceedings ODP. Scientific Results. Vol. 191, Ed. by W.W. Sager et al. (2005) [Online], http://www-odp.tamu.edu/publications/191_SR/VOLUME/CHAPTERS/009.PDF>.
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