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Mineralogical and geochemical changes in the sediments of the Okhotsk Sea during deglacial periods in the past 500 kyrs
Global and Planetary Change 53 (2006) 47 – 57 www.elsevier.com/locate/gloplacha
Mineralogical and geochemical changes in the sediments of the Okhotsk Sea during deglacial periods in the past 500 kyrs Ya-Jiun Liu a , Sheng-Rong Song a,⁎, Teh-Quei Lee b , Meng-Yang Lee c , Yaw-Lin Chen a , Huei-Fen Chen a,d a
d
Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC b Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC c Department of Science, Taipei Municipal University of Education, Taipei, Taiwan, ROC Institue of Applied Geosciences, National Taiwan Ocean University, Keelung, Taiwan, ROC Received 4 June 2004; accepted 20 January 2006 Available online 24 July 2006
Abstract Characterized by extended seasonal sea-ice cover, the Okhotsk Sea is widely considered one possible source of the North Pacific Intermediate Water (NPIW). Therefore, reconstructing the characteristics of the sediments that resulted from the ice-melting pulses of this northeastern Asian sea during glacial–interglacial cycles is crucial to understanding past climatic changes and NPIW formation in the northwest Pacific Ocean. Here, we produced the detailed mineralogy and geochemistry of the upper 20 m of the sediments of IMAGES Core MD012414, which was drilled in the central part of the Okhotsk Sea. This depth covers the last 500 kyrs. The mineralogical data show that in glacial periods, the sediments were predominantly composed of quartz and plagioclase with a volume of about 80%, indicating that they were mainly from the surrounding landmasses. During interglacial periods, however, biogenetic calcite and amorphous opal drastically increased from less than 5% to a remarkable 40–60%, while the element ratios Mg/Al, Ca/Al and Si/Al also had anomalous increases from 0.2 to 0.5, 0.12 to 1.2 and 3.5 to 10, respectively. These characteristics of the sediments in interglacial periods strongly suggest that the melting of permanent ice opened a gateway, thereby letting the northwest Pacific warmer water flow into the Okhotsk Sea, which subsequently increased biogenetic productivity during the deglacial periods. In addition, in early interglacial periods, the bottom water became anoxic, as evidenced by the presence of dolomite and the enrichment of the Mn/Al and P/Al ratios more than 25 to 200 times. The fact that these trends did not occur in stage 7 strongly suggests that stage 8 may have been a warmer glacial period. © 2006 Elsevier B.V. All rights reserved. Keywords: IMAGES; Okhotsk Sea; mineralogy; major chemistry; deglaciation
1. Introduction The Okhotsk Sea, a marginal sea in the northwest Pacific, borders on southeastern Siberia, the Kamchatka ⁎ Corresponding author. P.O. Box 13-318, Taipei, 106, Taiwan, ROC. Tel.: +886 2 33662938; fax: +886 2 23636095. E-mail address: [email protected] (S.-R. Song). 0921-8181/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2006.01.007
Peninsula, the Kurile Islands, Hokkaido and Sakhalin. Albeit in a temperate zone, it is characterized by extended sea-ice cover and seasonal changes (Kawahata et al., 2003). Sea-ice production makes the temperatures of the Okhotsk Sea water lower than those of the open Pacific Ocean water at the same latitude. Recently, it has been suggested that the cold shelf derived water (SDW) of the Okhotsk mixes isopycnally with the North Pacific water
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(NPW) which forms the Okhotsk Sea intermediate water (SOIW), or the Okhotsk Sea water mass (Wong et al., 1998). Also reported is that the SOIW leaves the Okhotsk Sea and plays a significant role in the formation of the North Pacific Intermediate Water (NPIW) (Takahashi, 1998; Talley, 1991; Yamamoto et al., 2002). Ice-creation in the Okhotsk Sea may dominate the strength of the NPIW and may even have an influence on global atmospheric circulation (Cavalieri and Parkinson, 1987; Honda et al., 1996). In this regard, during early deglaciation, primary productivity was possibly intensified due to an increased nutrient supply (Ternois et al., 2001). Given the importance of broad sea-ice cover, Shiga and Koizumi (2000) investigated the significant changes in seasonal sea-ice cover in the Okhotsk Sea throughout the last 21,500 yr B.P. Its importance notwithstanding, the paleoceanography of the Okhotsk Sea, when compared to that of other marginal seas, has been the focus of very few research studies. This can be attributed to the low carbonate contents in its sediments and the inaccessibility of the basin to foreign scientists (Gorbarenko et al., 1998). While earlier studies
have only dealt with the history of the Okhotsk Sea since the last glacial period (Gorbarenko, 1996; Gorbarenko et al., 2002a,b; Kawahata et al., 2003; Shiga and Koizumi, 2000; Ternois et al., 2001), one recent study has provided the longer records of the Okhotsk Sea that cover the past 200 kyrs (Gorbarenko et al., 2002b). To gain insight into an even longer evolution of the NPIW as well as the environmental changes in the Okhotsk Sea, the international IMAGES VII circum-Pacific initiative (WEPAMA) conducted long sediment core drilling (ca. 53.88 m long) in the central part of the Sea of Okhotsk (NW-Pacific marginal basin) in 2001 with a view to examining the Pleistocene evolution of intermediate water formation, surface water productivity and sea-ice coverage (Bassinot and Baltzer, 2002). Located at longitude 149° 34.80′E and latitude 53°11.77′N, Core MD012414 with the depth of 1123 m and the length of 53.88 m recorded the history of the Okhotsk Sea for the past 1.8 Myrs (Chou, 2003). In that records of the Okhotsk Sea covering a longer period of time have not been reported, Core MD012414 data are as much invaluable as they are unprecedented.
Fig. 1. Bathymetric map of the Okhotsk Sea. The pentagram shows the location of Core MD012414. (redrawn from the web site: www.nodc.noaa. gov/OC5/ okhotsk/bot_map.html).
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Detailed mineralogical and geochemical data for the upper 20-meter section of this core have been completely analyzed. With these and other available data, this paper focuses on: (1) climatic changes (glacial–interglacial cycles); (2) the peculiarities of the sedimentary environment; and (3) the evolution of the paleoceanography and the process of sedimentation in the Okhotsk Sea. 2. Method As shown in Fig. 1, Core MD012414 was drilled in the central Derugin Basin of the Okhotsk Sea (149° 34.80′E, 53°11.77′N). Although the core was drilled to a length about 53.88 m, the section between 150 and 211 cm was too soupy to be collected. This paper discusses the upper 20 m of the core and this represents the past 500 kyrs. Samples of 1 cm in thickness were collected every 5 cm and these were frozen-dried for the analyses of mineralogy and the major elements. Powder mounts were obtained by grinding bulk-dried sediments in a mortar, and they were then smeared gently on glass slides for the mineralogical analysis. The mineralogy of the sediments was analyzed by using a Science Mxp III X-ray diffractometer (MAC) with CuKα radiation. Scans of bulk powders were run at 35 kV and 15 mA over a scanning range of 3°–70°. Following the procedures of Fagel et al. (2003), the intensity of the reflections was measured and the correction factors for the semi-quantitative analysis were determined; the results are listed in Table 1. Boski et al. (1998) claimed that any abnormal increase in background around 4.04 Å can be regarded as opal. Typically, the normal background level is averaged by using samples from adjacent diatom-barren intervals. To examine the reliability of this method, mixtures with different ratios, i.e., 0.5:0.1, 0.5:0.2, 0.5:0.3, 0.5:0.5, 0.5:1 and 0.5:2, of opal-barren sediment and pure diatom clay containing more than 90% diatom, were measured. The results show that there is very good correlation with R2 about 0.99 (Fig. 2). The major elements, i.e., SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, K2O, Na2O and P2O5, were analyzed using the RIX-2000 X-ray Fluorescence System (Rigaku). Samples between 1383 and 1433 cm were collected at 5-cm Table 1 Picked reflections and the correction factor for calculating the mineral contents of Core MD012414 sediments Mineral
Quartz Feldspar Calcite Opal
d-spacing of 4.26 Å 3.18 Å peak Correction 100/35 2 factor
3.03 Å Abnormal increase in background around 4.04 Å 1.92 20
Fig. 2. Plot of the weight percentages of additional diatom clay versus measured opal contents from XRD. Each point is the average of four analyses.
intervals, while the others were collected at 20-cm intervals. Consistent with the method of Yang et al. (1996), the analyses were performed on glass beads made from lithium tetraborate flux with a flux to sample ratio of 10:1. The analytical precision and accuracy for each of the elements are shown in Table 2 (Lee et al., 1997; Song et al., 2004). 3. Results 3.1. Lithology and age model Based on shipboard lithological descriptions, the sediments in the upper 20 m of Core MD012414 are light olive to olive gray and are predominantly composed of diatombearing clayed silts with some sandy and ash spots plus drop stones (Fig. 3). Two layers of volcanic ash are found at the depths of 95 to 105 cm and 334 to 341 cm. The porosity and the color reflectance b-value (b⁎) of the sediments are also shown in Fig. 3. However, the sediments farther down at 158–200 cm are too watery to be measured. The porosity of the core is in the range of 40 to 50%, with the mean of 47%. Only the porosity of the 0- to 300-cm sediments is lower (20 to 40%). On the other hand, the color b⁎ of the upper 300 cm fluctuates between 10 and 4, while the other depths range from 4 to 6, and are protruded by diatom-rich sediments at 600 to 740 cm, 1300 to 1450 cm, 1620 to 1800 cm, 1920 and 1980 cm. Generally, few nanno-fossils are found in the MD012414 core sediments. The age model of the MD012414 core sediments was originally determined by magnetostratigraphy (Chou, 2003). The ages for the geomagnetic reversal boundaries were derived from the orbital-tuned time scale (Shackleton et al., 1990, 1995; Tiedemann et al., 1994). The variations in the intensity of the paleomagnetic field in the core sediments are highly correlated with the corresponding data for the North Pacific and North Atlantic (Chou, 2003)
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Table 2 Analytical precision and accuracy of each element measured by XRF Elements
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
Precision % b0.9 b1.7 b1.0 b1.0 b1 b1.2 b1.0 b1.9 b3.7 b2 Accuracy related to the rock 1.1 ± 0.7 2.0 ± 1.4 1.2 ± 1.1 0.7 ± 0.5 1.3 ± 2.8 1.1 ± 0.4 1.8 ± 1.8 2.3 ± 1.1 3.8 ± 1.5 3.3 ± 7.5 standard (%)
a
b
a
The precision was calculated from six measurements of NTU rock standards, namely NTUG-1 (n = 9),; NTUG-2 (n = 6); NTUG-3 (n = 6); NTUG-4 (n = 5),; NTUG-5 (n = 5),; and NTUG-6 (n = 5). b The accuracy = [STD(measured) − STD(recommend)] / STD(measured). The STDs are RGM-1 (n = 10) and W-2 (n = 10) which are the USGS rock standards, and NTUG-1(n = 6), NTUG-3 (n = 6), and NTUG-5 (n = 6) which are the NTU rock standards. n: the numbers were analyzed in the same sample.
and several marked points were used as age-controlled data. However, the age control points are absent between 1400 and 2875 cm.
Due to the discontinuity of the δ18O results for Core MD012415 which was raised near the location of our core (MD012414), Nürnberg and Tiedemann (2004) used the
Fig. 3. Lithological column, porosity and reflectance color b⁎ of sediments in the upper 20 m of Core MD012414.
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shipboard color b⁎ value as the tuning medium and established their age model. Turning to Core MD012414, the δ18O records of our core were not continuous. Therefore, in order to build the age model of our core since 800 kyrs, the color b⁎ data of our core (MD012414) were
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correlated with those of MD012415 which were taken as the target curve (Fig. 4a). It is very clear from Fig. 4b that the age-controlled points since 500 ka which were derived from the two independent age models, i.e., the paleomagnetic model and the color b⁎ model, are consistent.
Fig. 4. Age model of Core MD012414 sediments. a. Shipboard color b⁎ variations versus age of Cores MD012414 and MD201415 (Nürnberg and Tiedemann, 2004) since 800 kyrs. b. The age-controlled points derived from the paleomagnetic model and color b⁎ model versus depth since 500 kyrs. The vertical dashed line shows the volcanic layers deposited at 95 cm and 334 cm.
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Beyond this, the ages of the two ash layers calculated from each of these two age models are also highly consistent with the ages of the K0 and K2 ash layers, which erupted at 7.8 and 26 kyrs, respectively and which are widely distributed in the central part of the Okhotsk Sea (Gorbarenko et al., 2002b). 3.2. Mineralogy The mineral assemblage of the upper 20 m of the Core MD012414 sediments consists of quartz, plagioclase, calcite, halite, amorphous materials and hemipelagic clays, namely illite, chlorite, kaolinite and smectite. Clay minerals and halite are not considered in this paper since the amount of the sub-sample was too small for us to extract clay minerals, and halite was precipitated during the frozen-dried process. The contents of the minerals were established on the basis of time and are illustrated in Fig. 5. The quartz contents vary from 5% to 65% and are higher, about
40%–60%, during glacial stages 2, 3, 4, 6, 8, 10 and 12, but lower, about 5%–20%, during interglacial stages (Fig. 5a). This trend closely resembles the changes in magnetic susceptibility (Chou, 2003). Generally, the plagioclase contents range from 8% to 60% but they are relatively depleted at the ages of 115, 324–326 and 390–430 kyrs (Fig. 5b). Basically, the contents of opal are almost zero during glacial stages but are highly enriched (∼ 60%) during interglacial stages (Fig. 5c). The calcite contents exhibit sharp peaks during deglacial periods and the early periods of interglacial stages 1, 5, 9 and 11 but are barren at other times, even in the middle to late interglacial stages (Fig. 5d). Nevertheless, it is indeed worth noting that the calcite peaks do not occur during stage 7. Apart from this, and as shown in Fig. 5d, dolomite is deposited at 4–10, 127, 334, 454 and 529 kyrs, which correlate well with early interglacial stages 1, 5, 9, 11 and 13. After the appearance of dolomite, it is obvious that calcite blooms out.
Fig. 5. Mineral content versus the age of Core MD012414 sediments. a. quartz; b. plagioclase; c. opal; and d. calcite. The dark green dots represent dolomite. (For intepretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Element/Al ratios versus the age of Core MD012414 sediments: a. Si/Al; b. Mg/Al; c. Ca/Al; d. Mn/Al; and e.P/Al.
3.3. Geochemistry
4. Discussion
The relative weight percent of the major elements, namely SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, K2O, Na2O and P2O5, were analyzed using XRF and the analytical results are presented as Al-normalized values to minimize the dilution effects associated with biogenic components. The Si/Al ratios (Fig. 6a) which range from 5 to 16 peak at the depths of 643, 863, 1353 and 1673 cm, which respectively represent the ages of 116, 161, 318 and 407 kyrs. The Ca/Al and Mg/Al ratios vary from 0.2 to 1.5 and 0.2 to 0.5, respectively, and are positively correlated to each other. As for Ca and Mg, both have higher contents at ages 0–2.4, 12, 120, 161, 307–346, 407 and 440 kyrs, but not at the age of 488 kyrs when only Ca shows a peak (Fig. 6b and c). These peaks coincide with those of calcite. With regard to the Mn/Al ratios, most of the values are as low as 0.008, but some sharp increases occur at the ages of 0.67, 113–128, 161, 312–352, 439 and 488 kyrs (Fig. 6d). Equally interesting, the values of the P/Al ratios also increase significantly at the same points in time (Fig. 6e).
4.1. Sedimentation in the glacial Okhotsk Sea As shown in Fig. 5a and b, the sediments from Core MD012414 are predominantly composed of quartz and plagioclase—more than 80%; contrast this with only 20–30% in the early periods of interglacial stages. There are also minor calcite, halite, clay and amorphous material during glacial periods. Important to note here is that quartz and plagioclase are detritus minerals in the sediments. Since the Core is located in the central basin of the Okhotsk Sea, it is very reasonable to assume that the detritus sediments were dominant in the surrounding land masses, including in southeastern Siberia, the Kamchatka Peninsula, the Kurile Islands, Hokkaido and in Sakhalin. Accordingly, the transport and deposit of those sediments in the Okhotsk Sea may well have been from ice raft debris (IRD), ocean currents, eolian, and fluvial processes. Beyond a doubt, the detritus sediments deposited in the core region could not have originated in southeastern Siberia or Sakhalin. For one, the Amur River, the largest river that flows into the Okhotsk Sea, was frozen during
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glacial times (Grosswald and Hughes, 2002). Besides this, the southward flowing East Sakhalin Current carried most fine river suspension to the south, thus obstructing the eastward transport of fluvial material (Nürnberg and Tiedemann, 2004). On these grounds, it follows that the fluvial material from southeastern Siberia and Sakhalin was not the main source of the detritus sediments identified in the region of Core MD012414. Not to be ignored, however, is the possibility that the sediments from the Kamchatka Peninsula and the Kuril Islands, located in the east and southeast of the Okhotsk Sea, could have been transported to the core region. In essence, most of the Okhotsk Sea was covered with perennial ice during glacial times but the sea-ice partly melted during the summer season in the eastern part due to the inflow of the warmer North Pacific current (Shiga and Koizumi, 2000). The higher contents of plagioclase than of quartz in the Core (Fig. 5a and b) also lend further support to this notion. Nevertheless, not enough empirical evidence has been collected thus far and further study using geochemical methods to determine the source of the sediments is required before this possibility can be ruled out. On the issue of the IRD, drop stones can often be identified simply by the naked eye throughout the whole core and the weight percent of non-biogenic coarse sediments (N 250 μm) can be as high as 32%. These data indicate that ice rafting deposits are not at all uncommon in the Okhotsk Sea. Furthermore, eolian sediments are an important component of the total North Pacific sediments and, as such, cannot be excluded from this study especially in light of the solid evidence of the high value of hard isothermal remnant magnetization (HIRM) in sediments during glacial times (Chou, 2003). However, until now, it has been difficult, at best, to determine the percentage of eolian components, and again, further study is required. In substance, quartz and plagioclase in Core MD012414 were mainly transported by the IRD, eolian and ocean current processes. 4.2. Sedimentation in the interglacial and deglacial Okhotsk Sea As shown in Fig. 5c and d, large amounts of biogenetic opal were deposited during interglacial times. Meanwhile, a large amount of calcite also bloomed out in the juvenile interglacial stages, so much so that the total rose as much as 20 times (50%) more than in other periods. From the SEM observations, the sediments are predominantly composed of biogenetic skeletons, strongly suggesting that as opposed to being authigenic deposits, calcite and opal were more than likely derived from biogeneous sources. The trends of the Ca/Al and Mg/Al ratios are strikingly
similar to the trend of the calcite contents (Figs. 6c, b and 5d). Just as interesting, the Si/Al ratios peak when the opal contents reach their highest values (Figs. 6a and 5c). More to the point, based on the mineralogical and chemical composition data, it might be concluded that during deglacial periods, the dominant sediments in the Okhotsk Sea were likely governed by biological productivity. So much biogeneous sediments could have descended to the sea bottom that the contents of quartz and plagioclase were diluted. In fact, they must have decreased to as low as 5%– 20%. Noteworthy is that the results here closely parallel recent data pertaining to low magnetic susceptibility during deglaciation (Chou, 2003). The point has also recently been made that the bloom of biogenic productivity was probably related to changes in ice cover in the Okhotsk Sea (Gorbarenko et al., 2002a). Once the icepack melted, many nutrients originally trapped in ice bodies or buried in the shelf may have been released into the sea, thereby strongly activating biological productivity. This would explain why biological blooming events occurred in the sediments of Core MD012414, always with an abundance of calcite and opal. Biological blooming is easily detected in interglacial stages 1, 5, 9 and 11 (Fig. 5c and d). Against this, blooming events are not at all obvious in stages 3 and 7 when there is only 30% opal and minor calcite. Compared to other interglacial periods, there is strong evidence that stage 3 was not as warm as other interglacial periods (McManus et al., 1999) and if this was so, then the ice-melting pulse could not have been very strong or it may not have even occurred at all. This naturally implies that sea surface productivity was probably inhibited in stage 3. Turning to stage 8, abnormal biogenetic opal contents (around 20%) and excess calcite are identified and this suggests there was higher biological productivity in this stage than in other glacial stages (less than 10% opal). The higher level of productivity infers that the climate may not have been cold enough for the water to have formed permanent ice. Bearing these facts in mind, it seems justified to assume that the ice-melting pulses in interglacial stages 3 and 7 could not have taken place and neither could the episodes of bloom. 4.3. Changes in the bottom water conditions As shown in Fig. 6d and e, the Mn/Al ratios show an overwhelming increase from 0.008 to 0.02, 0.03, 0.04, 0.08 and even to 1.7 during deglaciation in stages 1, 5, 6, 9 and 11, respectively. The P/Al ratios synchronously show substantially higher values which are, in fact, highly consistent with the Mn/Al ratios. The values increase three- to fourfold over that of the normal value and, in those same stages,
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they are generally within the range of 0.01 to 0.04 but even reach 0.05. On account of the extremely positive abnormal conditions that occurred in the early period of stage 9, i.e. 334 kyrs (1413 cm), the samples were collected at denser intervals (5 cm) between 1383 and 1433 cm. In addition, except in stage 6, an extraordinarily high quantity of dolomite appears in those periods (Fig. 5d). To be sure, these characteristics underscore the fact either that the bottom water chemistry must have changed or that diagenetic fronts must have occurred during these periods. It is widely accepted that the element Mn is mobile under anoxic conditions, such as in Mn2+, and that it easily migrates along a pore water gradient, changing from an anoxic state to an oxic one. It is eventually precipitated as oxyhydroxides at an oxic/post-oxic boundary. In the process, the amount of Mn may actually be enriched several percentage points (Sawlan and Murray, 1983; Kadko and Heath, 1984). Permanent ice-melting during deglaciation must have led to a relatively higher sedimentation rate and, at the same time, an abundance of organic matter must have been deposited at the sea bottom. The oxidation of the organic matter may have exhausted dissolved oxygen, perhaps making the bottom water anoxic (Richards, 1965). In this event, the oxic/post-oxic boundary in the strata must have been characterized by redoxic sensitive element deposits, like Mn (Thomson et al., 1993, 1986; George et al., 1997); what's more, even Fe-bounded P could have been enriched (Tamburini et al., 2002). When the sediments accumulated on the sea floor, diagenesis would have also redistributed some of the elements and caused the elements Mn and P to concentrate. That is to say, oxygendepletion caused by a great deal of sedimentation and/or diagenesis may very well account for the fact that these elements are extremely enriched in the core sediments. It cannot be denied that the anomalous phenomenon that occurred in glacial stage 6 remains a mystery and further study is required. During deglacial periods, calcite dissolution and anoxic bottom water conditions may very well have favored the precipitation of dolomite. The truth of the matter is that the occurrence of the specific elements Mn and P along with dolomite is indicative of a reduction in the bottom water environment of the Okhotsk Sea during the early deglaciation period. The formation of dolomite at the sea bottom is generally associated with a bacterial reduction of sulfate and it requires a particular geochemical condition, i.e., a strong reduction in bottom water or anoxia (Tamburini et al., 2002). During the early deglaciation periods, the specific environment may have been induced by a great deal of ice-melting. The oxidation of organic matter consumes the dissolved oxygen in the bottom water, making it more corrosive and anoxic. Furthermore,
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carbonate dissolution induces a local positive CO2 pressure gradient. In such a reducing environment, dolomite and siderite are thermodynamically more stable phases than calcite (Sighinolfi and Tateo, 1998). 5. Conclusions Historically, there have been drastic differences between glacial and interglacial periods when it comes to the sedimentation processes in the Okhotsk Sea. On a broad scale, the derivation of the sediments has been two-fold: biogeneous and detritus sources. Quartz and plagioclase can be treated as detritus components, and calcite and opal as biogeneous in this area. During glacial times, the covering of extended ice restricted the activity of diatoms, coccoliths and foraminifera. As a consequence, the biogenetic components were depleted, and the terrigenous sediments, such as eolian sediments, IRD and detritus from the surrounding landmasses, dominated. Then, with the coming of warm interglacial times, parts of the perennial ice-cover melted. A great quantity of sediments with organic matter originally trapped in the ice and on the shelf or on the sea bottom was likely released into the Okhotsk Sea and these served as nutrients for the whole environment. In turn, biogenetic productivity must have been highly activated, as indicated by the apexes of calcite, opal, Mg/Al, Ca/Al and Si/Al in the interglacial periods. Large amounts of biogenetic sediments must have then descended to the sea floor, reducing the level of the detritus sediments to below 40%. However, the results of the analyses of stages 3 and 7 are quite different from those of the other interglacial stages. No apparent calcite or opal can be observed in either stage 3 or 7. It is well known that stage 3 was colder than other interglacial stages, suggesting the ice-melting pulse was probably debilitated. Hence, this accounts for the fact that the quantity of biogenetic sediments does not appear to have increased. The anomaly of stage 7 was probably attributable to other factors. Based on our mineralogical and other studies, stage 8 might have been warmer than the other glacial stages. As a result, the environmental changes from stage 8 to 7 might have been so gentle that bioproductive prosperity did not occur. Not only did the ice-melting pulse have an influence on productivity, but it also affected bottom water chemistry. With the melting of the ice-cover, it is believed that lots of sediments with abundant organic matter were released into the sea. The oxidation of the organic matter might have exhausted dissolved oxygen, causing the bottom water to become anoxic. In such a reduced situation, redoxic sensitive elements,
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such as the Mn2+ ion, would have become mobile in the seawater and would have finally precipitated at the oxic/post-oxic boundary. This explains why higher Mn/Al and P/Al ratios representing the oxic/post-oxic boundary can be observed during the deglaciation periods. Furthermore, the fact that dolomite precipitated also supports the notion of a reduced environment of bottom water in the deglacial periods of stages 1, 5, 9 and 11. Acknowledgements The authors appreciate the assistance of the Captain and all crew members of the R.V. Marine Dufresne during the IMAGES VII cruise in June 2001. They are also deeply indebted to Dr. Lee Chi-Yu of the Department of Geosciences, National Taiwan University, Taipei, Taiwan, R.O.C. for the XRF and XRD analyses. Thanks are also due to two anonymous referees and editor whose suggestions and valuable reviews greatly improved the manuscript. This study was partly supported by the Asian Paleo-Environmental Changes II (APEC II) of Academia Sinica and the National Science Council of the R.O.C. under grant NSC91-2116-M-002-020. References Bassinot, F., Baltzer, A., 2002. WEPAMA Cruise MD122/IMAGES VII on board R.V. “Marion Dufresne”. Institut Polaire FrançaisPaul-Emile Victor, pp. 41–43. Boski, T., Pessoa, J., Pedro, P., Thorez, J., Dias, J.M.A., Hall, I.R., 1998. Factors governing the abundance of hydrolysable amino acids in the sediments from the NW European continental margin (47–50°N). Prog. Oceanogr. 42, 145–164. Cavalieri, D.J., Parkinson, C.L., 1987. On the relationship between atmospheric circulation and fluctuations in the sea ice extents of the Bering and Okhotsk Seas. J. Geophys. Res. 92, 7141–7162. Chou, Y.-M., 2003. Magnetic study of core MD012414 from Okhotsk Sea - paleoclimate and paleoenvironment changes of Northeastern Asia since 1.8Ma (in Chinese), master thesis of Institute of geoscience, National Taiwan Normal University, R.O.C., 81 pp. Fagel, N., Boski, T., Likhoshway, L., Oberhaensli, H., 2003. Late Quaternary clay mineral records in central Lake Baikal (Academician Ridge, Siberia). Palaeogeogr. Palaeoclimatol. Palaeoecol. 193, 159–179. George III, W.L., Byorn, S., Brent, L.L., Paul, J.B., Norman, S., 1997. Interactions of manganese with the nitrogen cycle: alternative pathways to dinitrogen. Geochim. Cosmochim. Acta 61 (19), 4043–4052. Gorbarenko, S.A., 1996. Stable isotope and lithologic evidence of lateglacial and Holocene oceanography of the Northwestern Pacific and its marginal seas. Quat. Res. 46, 230–250. Gorbarenko, S.A., Chekhovskaya, M.P., Souhton, J.R., 1998. On the paleoenvironment of the central part of the Sea of Okhotsk during the past Holocene glaciation. Oceanography 38 (2), 277–280. Gorbarenko, S.A., Khusid, T.A., Basov, I.A., Oba, T., Southon, J.R., Koizumi, I., 2002a. Glacial Holocene environment of the Southeast-
ern Okhotsk Sea: evidence from geochemical and palaeontological data. Palaeogeogr. Palaeoclimatol. Palaeoecol. 177, 237–263. Gorbarenko, S.A., Nürnberg, D., Derkachev, A.N., Astakhov, A.S., Southon, J.R., Kaiser, A., 2002b. Magnetostratigraphy and tephrochronology of the upper Quaternary sediments in the Okhotsk Sea: implications of terrigenous, volcanogenic, and biogenic matter supply. Mar. Geol. 183, 107–129. Grosswald, M.G., Hughes, T.J., 2002. The Russian component of an Arctic ice sheet during the Last Glacial Maximum. Quat. Sci. Rev. 21 (1–3), 121–146. Honda, M., Yamazaki, K., Tachibana, Y., Takeuchi, K., 1996. Influence of Okhotsk sea-ice extent on atmospheric circulation. Geophys. Res. Lett. 23 (24), 3595–3598. Kadko, D., Heath, G.R., 1984. Models of depth dependent bioturbation at MANOP site H in the east equatorial Pacific. J. Geophys. Res. 89, 6567–6570. Kawahata, H., Oshima, H., Shimada, C., Oba, T., 2003. Terrestrialoceanic environmental changes in the southern Okhotsk Sea during the Holocene. Quat. Int. 108, 67–76. Lee, C.Y., Tsai, J.H., Ho, H.H., Yang, T.F., Chung, S.L., Chen, C.H., 1997. Quantitative analysis in rock samples by X-ray fluorescence spectrometer (I) major elements. Abstract of Annual Meeting of the Geological Society of China, Taiwan, pp. 418–420 (in Chinese). McManus, J.F., Oppo, D.W., Cullen, J.L., 1999. A 0.5-million-year record of millennial-scale climate variability in the North Atlantic. Science 283, 971–975. Nürnberg, D., Tiedemann, R., 2004. Environmental change in the Sea of Okhotsk during the last 1.1 million years. Paleoceanography 19, PA401. doi:10.1029/2004PA001023. Richards, F.A., 1965. Anoxic basins and fjords. In: Riley, J.P., Skirrow, G. (Eds.), Chemical Oceanography. Academic Press, London, pp. 611–646. Sawlan, J.J., Murray, J.W., 1983. Trace metal remobilization in the interstitial waters of red clay and hemipelagic marine sediments. Earth Planet. Sci. Lett. 64, 213–230. Shackleton, N.J., Berger, A., Peltier, W.R., 1990. An alternative astronomical calibration of the lower Pleistocene timescale based on ODP site 677. Trans. R. Soc. Edinb. Earth Sci. 81, 251–261. Shackleton, N.J., Hall, M.A., Pate, D., 1995. Pliocene stable isotope stratigraphy of site 846. Proc. ODP Sci. Results 138, 337–356. Shiga, K., Koizumi, I., 2000. Latest Quaternary oceanographic changes in the Okhotsk Sea based on diatom. Mar. Micropaleontol. 38, 91–117. Sighinolfi, G.P., Tateo, F., 1998. Mineralogical and geochemical criteria for distinguishing turbidite and hemipelagic pelites—the Maastrichtian of the northern Apennines. Sediment. Geol. 115, 301–313. Song, S.S., Liu, C.M., Chen, C.H., Lo, W., 2004. Pumice layers in marine terraces: implications for tectonic uplift rates on the east and northeast coasts of Taiwan over the last hundreds of years. Quat. Int. 115–116, 83–92. Takahashi, K., 1998. The Bering and Okhotsk Seas: modern and past paleoceanographic changes and gateway impact. J. Asian Earth Sci. 16 (1), 49–58. Talley, L.D., 1991. An Okhotsk Sea water anomaly: implications for ventilation in the North Pacific. Deep Sea Res. 38 (1), S171–S190. Tamburini, F., Huon, S., Stenmann, P., Grousset, F.E., Adate, T., Follmi, K.B., 2002. Dysaerobic conditions during Heinrich events 4 and 5: evidence from phosphorus distribution in a North Atlantic deep-sea core. Geochim. Cosmochim. Acta 66 (23), 4069–4083. Ternois, Y., Kawamura, K., Keigwin, L., Ohkouchi, N., Nakatsuka, T., 2001. A biomarker approach for assessing marine and terrigenous
Y.-J. Liu et al. / Global and Planetary Change 53 (2006) 47–57 inputs to the sediments of the Sea of Okhotsk for the last 27,000 ka. Geochim. Cosmochim. Acta 65 (5), 791–802. Thomson, J., Higgs, N.C., Jarvis, I., Hydes, D.J., Colley, S., Wilson, T.R.S., 1986. The behavior of manganese in the Atlantic carbonate sediments. Geochim. Cosmochim. Acta 50, 1807–1818. Thomson, J., Higgs, N.C., Croudace, I.W., Colley, S., Hydes, D., 1993. Redox zonation of elements at an oxic/post-oxic boundary in deepsea sediments. Geochim. Cosmochim. Acta 57, 579–595. Tiedemann, R., Sarnthein, M., Shackleton, N.J., 1994. Astronomic time-scale for the Pliocene Atlantic δ13C and dust f lux records of ODP Site 659. Paleoceanography 9, 619–638.
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Wong, C.S., Matear, R.J., Freeland, H.J., Whitney, F.A., Bychkov, A.S., 1998. WOCE line PIW in the Sea of Okhotsk, 2, CGCs and the formation rate of intermediate water. J. Geophys. Res. 103, 15625–15642. Yamamoto, M., Watanabe, S., Tsunogai, S., Wakatsuchi, M., 2002. Effects of sea ice formation and diapycnal mixing on the Okhotsk Sea intermediate water clarified with oxygen isotopes. Deep-Sea Res. I 49, 1165–1174. Yang, T.F., Yang, C.C., Lee, C.Y., Chung, S.L., Chen, C.H., 1996. NTUG rock standards for geochemical analysis. J. Soc. China 39 (3), 307–323.
Mineralogical and geochemical changes in the sediments of the Okhotsk Sea during deglacial periods in the past 500 kyrs Ya-Jiun Liu a , Sheng-Rong Song a,⁎, Teh-Quei Lee b , Meng-Yang Lee c , Yaw-Lin Chen a , Huei-Fen Chen a,d a
d
Department of Geosciences, National Taiwan University, Taipei, Taiwan, ROC b Institute of Earth Sciences, Academia Sinica, Taipei, Taiwan, ROC c Department of Science, Taipei Municipal University of Education, Taipei, Taiwan, ROC Institue of Applied Geosciences, National Taiwan Ocean University, Keelung, Taiwan, ROC Received 4 June 2004; accepted 20 January 2006 Available online 24 July 2006
Abstract Characterized by extended seasonal sea-ice cover, the Okhotsk Sea is widely considered one possible source of the North Pacific Intermediate Water (NPIW). Therefore, reconstructing the characteristics of the sediments that resulted from the ice-melting pulses of this northeastern Asian sea during glacial–interglacial cycles is crucial to understanding past climatic changes and NPIW formation in the northwest Pacific Ocean. Here, we produced the detailed mineralogy and geochemistry of the upper 20 m of the sediments of IMAGES Core MD012414, which was drilled in the central part of the Okhotsk Sea. This depth covers the last 500 kyrs. The mineralogical data show that in glacial periods, the sediments were predominantly composed of quartz and plagioclase with a volume of about 80%, indicating that they were mainly from the surrounding landmasses. During interglacial periods, however, biogenetic calcite and amorphous opal drastically increased from less than 5% to a remarkable 40–60%, while the element ratios Mg/Al, Ca/Al and Si/Al also had anomalous increases from 0.2 to 0.5, 0.12 to 1.2 and 3.5 to 10, respectively. These characteristics of the sediments in interglacial periods strongly suggest that the melting of permanent ice opened a gateway, thereby letting the northwest Pacific warmer water flow into the Okhotsk Sea, which subsequently increased biogenetic productivity during the deglacial periods. In addition, in early interglacial periods, the bottom water became anoxic, as evidenced by the presence of dolomite and the enrichment of the Mn/Al and P/Al ratios more than 25 to 200 times. The fact that these trends did not occur in stage 7 strongly suggests that stage 8 may have been a warmer glacial period. © 2006 Elsevier B.V. All rights reserved. Keywords: IMAGES; Okhotsk Sea; mineralogy; major chemistry; deglaciation
1. Introduction The Okhotsk Sea, a marginal sea in the northwest Pacific, borders on southeastern Siberia, the Kamchatka ⁎ Corresponding author. P.O. Box 13-318, Taipei, 106, Taiwan, ROC. Tel.: +886 2 33662938; fax: +886 2 23636095. E-mail address: [email protected] (S.-R. Song). 0921-8181/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gloplacha.2006.01.007
Peninsula, the Kurile Islands, Hokkaido and Sakhalin. Albeit in a temperate zone, it is characterized by extended sea-ice cover and seasonal changes (Kawahata et al., 2003). Sea-ice production makes the temperatures of the Okhotsk Sea water lower than those of the open Pacific Ocean water at the same latitude. Recently, it has been suggested that the cold shelf derived water (SDW) of the Okhotsk mixes isopycnally with the North Pacific water
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(NPW) which forms the Okhotsk Sea intermediate water (SOIW), or the Okhotsk Sea water mass (Wong et al., 1998). Also reported is that the SOIW leaves the Okhotsk Sea and plays a significant role in the formation of the North Pacific Intermediate Water (NPIW) (Takahashi, 1998; Talley, 1991; Yamamoto et al., 2002). Ice-creation in the Okhotsk Sea may dominate the strength of the NPIW and may even have an influence on global atmospheric circulation (Cavalieri and Parkinson, 1987; Honda et al., 1996). In this regard, during early deglaciation, primary productivity was possibly intensified due to an increased nutrient supply (Ternois et al., 2001). Given the importance of broad sea-ice cover, Shiga and Koizumi (2000) investigated the significant changes in seasonal sea-ice cover in the Okhotsk Sea throughout the last 21,500 yr B.P. Its importance notwithstanding, the paleoceanography of the Okhotsk Sea, when compared to that of other marginal seas, has been the focus of very few research studies. This can be attributed to the low carbonate contents in its sediments and the inaccessibility of the basin to foreign scientists (Gorbarenko et al., 1998). While earlier studies
have only dealt with the history of the Okhotsk Sea since the last glacial period (Gorbarenko, 1996; Gorbarenko et al., 2002a,b; Kawahata et al., 2003; Shiga and Koizumi, 2000; Ternois et al., 2001), one recent study has provided the longer records of the Okhotsk Sea that cover the past 200 kyrs (Gorbarenko et al., 2002b). To gain insight into an even longer evolution of the NPIW as well as the environmental changes in the Okhotsk Sea, the international IMAGES VII circum-Pacific initiative (WEPAMA) conducted long sediment core drilling (ca. 53.88 m long) in the central part of the Sea of Okhotsk (NW-Pacific marginal basin) in 2001 with a view to examining the Pleistocene evolution of intermediate water formation, surface water productivity and sea-ice coverage (Bassinot and Baltzer, 2002). Located at longitude 149° 34.80′E and latitude 53°11.77′N, Core MD012414 with the depth of 1123 m and the length of 53.88 m recorded the history of the Okhotsk Sea for the past 1.8 Myrs (Chou, 2003). In that records of the Okhotsk Sea covering a longer period of time have not been reported, Core MD012414 data are as much invaluable as they are unprecedented.
Fig. 1. Bathymetric map of the Okhotsk Sea. The pentagram shows the location of Core MD012414. (redrawn from the web site: www.nodc.noaa. gov/OC5/ okhotsk/bot_map.html).
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Detailed mineralogical and geochemical data for the upper 20-meter section of this core have been completely analyzed. With these and other available data, this paper focuses on: (1) climatic changes (glacial–interglacial cycles); (2) the peculiarities of the sedimentary environment; and (3) the evolution of the paleoceanography and the process of sedimentation in the Okhotsk Sea. 2. Method As shown in Fig. 1, Core MD012414 was drilled in the central Derugin Basin of the Okhotsk Sea (149° 34.80′E, 53°11.77′N). Although the core was drilled to a length about 53.88 m, the section between 150 and 211 cm was too soupy to be collected. This paper discusses the upper 20 m of the core and this represents the past 500 kyrs. Samples of 1 cm in thickness were collected every 5 cm and these were frozen-dried for the analyses of mineralogy and the major elements. Powder mounts were obtained by grinding bulk-dried sediments in a mortar, and they were then smeared gently on glass slides for the mineralogical analysis. The mineralogy of the sediments was analyzed by using a Science Mxp III X-ray diffractometer (MAC) with CuKα radiation. Scans of bulk powders were run at 35 kV and 15 mA over a scanning range of 3°–70°. Following the procedures of Fagel et al. (2003), the intensity of the reflections was measured and the correction factors for the semi-quantitative analysis were determined; the results are listed in Table 1. Boski et al. (1998) claimed that any abnormal increase in background around 4.04 Å can be regarded as opal. Typically, the normal background level is averaged by using samples from adjacent diatom-barren intervals. To examine the reliability of this method, mixtures with different ratios, i.e., 0.5:0.1, 0.5:0.2, 0.5:0.3, 0.5:0.5, 0.5:1 and 0.5:2, of opal-barren sediment and pure diatom clay containing more than 90% diatom, were measured. The results show that there is very good correlation with R2 about 0.99 (Fig. 2). The major elements, i.e., SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, K2O, Na2O and P2O5, were analyzed using the RIX-2000 X-ray Fluorescence System (Rigaku). Samples between 1383 and 1433 cm were collected at 5-cm Table 1 Picked reflections and the correction factor for calculating the mineral contents of Core MD012414 sediments Mineral
Quartz Feldspar Calcite Opal
d-spacing of 4.26 Å 3.18 Å peak Correction 100/35 2 factor
3.03 Å Abnormal increase in background around 4.04 Å 1.92 20
Fig. 2. Plot of the weight percentages of additional diatom clay versus measured opal contents from XRD. Each point is the average of four analyses.
intervals, while the others were collected at 20-cm intervals. Consistent with the method of Yang et al. (1996), the analyses were performed on glass beads made from lithium tetraborate flux with a flux to sample ratio of 10:1. The analytical precision and accuracy for each of the elements are shown in Table 2 (Lee et al., 1997; Song et al., 2004). 3. Results 3.1. Lithology and age model Based on shipboard lithological descriptions, the sediments in the upper 20 m of Core MD012414 are light olive to olive gray and are predominantly composed of diatombearing clayed silts with some sandy and ash spots plus drop stones (Fig. 3). Two layers of volcanic ash are found at the depths of 95 to 105 cm and 334 to 341 cm. The porosity and the color reflectance b-value (b⁎) of the sediments are also shown in Fig. 3. However, the sediments farther down at 158–200 cm are too watery to be measured. The porosity of the core is in the range of 40 to 50%, with the mean of 47%. Only the porosity of the 0- to 300-cm sediments is lower (20 to 40%). On the other hand, the color b⁎ of the upper 300 cm fluctuates between 10 and 4, while the other depths range from 4 to 6, and are protruded by diatom-rich sediments at 600 to 740 cm, 1300 to 1450 cm, 1620 to 1800 cm, 1920 and 1980 cm. Generally, few nanno-fossils are found in the MD012414 core sediments. The age model of the MD012414 core sediments was originally determined by magnetostratigraphy (Chou, 2003). The ages for the geomagnetic reversal boundaries were derived from the orbital-tuned time scale (Shackleton et al., 1990, 1995; Tiedemann et al., 1994). The variations in the intensity of the paleomagnetic field in the core sediments are highly correlated with the corresponding data for the North Pacific and North Atlantic (Chou, 2003)
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Table 2 Analytical precision and accuracy of each element measured by XRF Elements
SiO2
TiO2
Al2O3
Fe2O3
MnO
MgO
CaO
Na2O
K2O
P2O5
Precision % b0.9 b1.7 b1.0 b1.0 b1 b1.2 b1.0 b1.9 b3.7 b2 Accuracy related to the rock 1.1 ± 0.7 2.0 ± 1.4 1.2 ± 1.1 0.7 ± 0.5 1.3 ± 2.8 1.1 ± 0.4 1.8 ± 1.8 2.3 ± 1.1 3.8 ± 1.5 3.3 ± 7.5 standard (%)
a
b
a
The precision was calculated from six measurements of NTU rock standards, namely NTUG-1 (n = 9),; NTUG-2 (n = 6); NTUG-3 (n = 6); NTUG-4 (n = 5),; NTUG-5 (n = 5),; and NTUG-6 (n = 5). b The accuracy = [STD(measured) − STD(recommend)] / STD(measured). The STDs are RGM-1 (n = 10) and W-2 (n = 10) which are the USGS rock standards, and NTUG-1(n = 6), NTUG-3 (n = 6), and NTUG-5 (n = 6) which are the NTU rock standards. n: the numbers were analyzed in the same sample.
and several marked points were used as age-controlled data. However, the age control points are absent between 1400 and 2875 cm.
Due to the discontinuity of the δ18O results for Core MD012415 which was raised near the location of our core (MD012414), Nürnberg and Tiedemann (2004) used the
Fig. 3. Lithological column, porosity and reflectance color b⁎ of sediments in the upper 20 m of Core MD012414.
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shipboard color b⁎ value as the tuning medium and established their age model. Turning to Core MD012414, the δ18O records of our core were not continuous. Therefore, in order to build the age model of our core since 800 kyrs, the color b⁎ data of our core (MD012414) were
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correlated with those of MD012415 which were taken as the target curve (Fig. 4a). It is very clear from Fig. 4b that the age-controlled points since 500 ka which were derived from the two independent age models, i.e., the paleomagnetic model and the color b⁎ model, are consistent.
Fig. 4. Age model of Core MD012414 sediments. a. Shipboard color b⁎ variations versus age of Cores MD012414 and MD201415 (Nürnberg and Tiedemann, 2004) since 800 kyrs. b. The age-controlled points derived from the paleomagnetic model and color b⁎ model versus depth since 500 kyrs. The vertical dashed line shows the volcanic layers deposited at 95 cm and 334 cm.
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Beyond this, the ages of the two ash layers calculated from each of these two age models are also highly consistent with the ages of the K0 and K2 ash layers, which erupted at 7.8 and 26 kyrs, respectively and which are widely distributed in the central part of the Okhotsk Sea (Gorbarenko et al., 2002b). 3.2. Mineralogy The mineral assemblage of the upper 20 m of the Core MD012414 sediments consists of quartz, plagioclase, calcite, halite, amorphous materials and hemipelagic clays, namely illite, chlorite, kaolinite and smectite. Clay minerals and halite are not considered in this paper since the amount of the sub-sample was too small for us to extract clay minerals, and halite was precipitated during the frozen-dried process. The contents of the minerals were established on the basis of time and are illustrated in Fig. 5. The quartz contents vary from 5% to 65% and are higher, about
40%–60%, during glacial stages 2, 3, 4, 6, 8, 10 and 12, but lower, about 5%–20%, during interglacial stages (Fig. 5a). This trend closely resembles the changes in magnetic susceptibility (Chou, 2003). Generally, the plagioclase contents range from 8% to 60% but they are relatively depleted at the ages of 115, 324–326 and 390–430 kyrs (Fig. 5b). Basically, the contents of opal are almost zero during glacial stages but are highly enriched (∼ 60%) during interglacial stages (Fig. 5c). The calcite contents exhibit sharp peaks during deglacial periods and the early periods of interglacial stages 1, 5, 9 and 11 but are barren at other times, even in the middle to late interglacial stages (Fig. 5d). Nevertheless, it is indeed worth noting that the calcite peaks do not occur during stage 7. Apart from this, and as shown in Fig. 5d, dolomite is deposited at 4–10, 127, 334, 454 and 529 kyrs, which correlate well with early interglacial stages 1, 5, 9, 11 and 13. After the appearance of dolomite, it is obvious that calcite blooms out.
Fig. 5. Mineral content versus the age of Core MD012414 sediments. a. quartz; b. plagioclase; c. opal; and d. calcite. The dark green dots represent dolomite. (For intepretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
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Fig. 6. Element/Al ratios versus the age of Core MD012414 sediments: a. Si/Al; b. Mg/Al; c. Ca/Al; d. Mn/Al; and e.P/Al.
3.3. Geochemistry
4. Discussion
The relative weight percent of the major elements, namely SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, K2O, Na2O and P2O5, were analyzed using XRF and the analytical results are presented as Al-normalized values to minimize the dilution effects associated with biogenic components. The Si/Al ratios (Fig. 6a) which range from 5 to 16 peak at the depths of 643, 863, 1353 and 1673 cm, which respectively represent the ages of 116, 161, 318 and 407 kyrs. The Ca/Al and Mg/Al ratios vary from 0.2 to 1.5 and 0.2 to 0.5, respectively, and are positively correlated to each other. As for Ca and Mg, both have higher contents at ages 0–2.4, 12, 120, 161, 307–346, 407 and 440 kyrs, but not at the age of 488 kyrs when only Ca shows a peak (Fig. 6b and c). These peaks coincide with those of calcite. With regard to the Mn/Al ratios, most of the values are as low as 0.008, but some sharp increases occur at the ages of 0.67, 113–128, 161, 312–352, 439 and 488 kyrs (Fig. 6d). Equally interesting, the values of the P/Al ratios also increase significantly at the same points in time (Fig. 6e).
4.1. Sedimentation in the glacial Okhotsk Sea As shown in Fig. 5a and b, the sediments from Core MD012414 are predominantly composed of quartz and plagioclase—more than 80%; contrast this with only 20–30% in the early periods of interglacial stages. There are also minor calcite, halite, clay and amorphous material during glacial periods. Important to note here is that quartz and plagioclase are detritus minerals in the sediments. Since the Core is located in the central basin of the Okhotsk Sea, it is very reasonable to assume that the detritus sediments were dominant in the surrounding land masses, including in southeastern Siberia, the Kamchatka Peninsula, the Kurile Islands, Hokkaido and in Sakhalin. Accordingly, the transport and deposit of those sediments in the Okhotsk Sea may well have been from ice raft debris (IRD), ocean currents, eolian, and fluvial processes. Beyond a doubt, the detritus sediments deposited in the core region could not have originated in southeastern Siberia or Sakhalin. For one, the Amur River, the largest river that flows into the Okhotsk Sea, was frozen during
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glacial times (Grosswald and Hughes, 2002). Besides this, the southward flowing East Sakhalin Current carried most fine river suspension to the south, thus obstructing the eastward transport of fluvial material (Nürnberg and Tiedemann, 2004). On these grounds, it follows that the fluvial material from southeastern Siberia and Sakhalin was not the main source of the detritus sediments identified in the region of Core MD012414. Not to be ignored, however, is the possibility that the sediments from the Kamchatka Peninsula and the Kuril Islands, located in the east and southeast of the Okhotsk Sea, could have been transported to the core region. In essence, most of the Okhotsk Sea was covered with perennial ice during glacial times but the sea-ice partly melted during the summer season in the eastern part due to the inflow of the warmer North Pacific current (Shiga and Koizumi, 2000). The higher contents of plagioclase than of quartz in the Core (Fig. 5a and b) also lend further support to this notion. Nevertheless, not enough empirical evidence has been collected thus far and further study using geochemical methods to determine the source of the sediments is required before this possibility can be ruled out. On the issue of the IRD, drop stones can often be identified simply by the naked eye throughout the whole core and the weight percent of non-biogenic coarse sediments (N 250 μm) can be as high as 32%. These data indicate that ice rafting deposits are not at all uncommon in the Okhotsk Sea. Furthermore, eolian sediments are an important component of the total North Pacific sediments and, as such, cannot be excluded from this study especially in light of the solid evidence of the high value of hard isothermal remnant magnetization (HIRM) in sediments during glacial times (Chou, 2003). However, until now, it has been difficult, at best, to determine the percentage of eolian components, and again, further study is required. In substance, quartz and plagioclase in Core MD012414 were mainly transported by the IRD, eolian and ocean current processes. 4.2. Sedimentation in the interglacial and deglacial Okhotsk Sea As shown in Fig. 5c and d, large amounts of biogenetic opal were deposited during interglacial times. Meanwhile, a large amount of calcite also bloomed out in the juvenile interglacial stages, so much so that the total rose as much as 20 times (50%) more than in other periods. From the SEM observations, the sediments are predominantly composed of biogenetic skeletons, strongly suggesting that as opposed to being authigenic deposits, calcite and opal were more than likely derived from biogeneous sources. The trends of the Ca/Al and Mg/Al ratios are strikingly
similar to the trend of the calcite contents (Figs. 6c, b and 5d). Just as interesting, the Si/Al ratios peak when the opal contents reach their highest values (Figs. 6a and 5c). More to the point, based on the mineralogical and chemical composition data, it might be concluded that during deglacial periods, the dominant sediments in the Okhotsk Sea were likely governed by biological productivity. So much biogeneous sediments could have descended to the sea bottom that the contents of quartz and plagioclase were diluted. In fact, they must have decreased to as low as 5%– 20%. Noteworthy is that the results here closely parallel recent data pertaining to low magnetic susceptibility during deglaciation (Chou, 2003). The point has also recently been made that the bloom of biogenic productivity was probably related to changes in ice cover in the Okhotsk Sea (Gorbarenko et al., 2002a). Once the icepack melted, many nutrients originally trapped in ice bodies or buried in the shelf may have been released into the sea, thereby strongly activating biological productivity. This would explain why biological blooming events occurred in the sediments of Core MD012414, always with an abundance of calcite and opal. Biological blooming is easily detected in interglacial stages 1, 5, 9 and 11 (Fig. 5c and d). Against this, blooming events are not at all obvious in stages 3 and 7 when there is only 30% opal and minor calcite. Compared to other interglacial periods, there is strong evidence that stage 3 was not as warm as other interglacial periods (McManus et al., 1999) and if this was so, then the ice-melting pulse could not have been very strong or it may not have even occurred at all. This naturally implies that sea surface productivity was probably inhibited in stage 3. Turning to stage 8, abnormal biogenetic opal contents (around 20%) and excess calcite are identified and this suggests there was higher biological productivity in this stage than in other glacial stages (less than 10% opal). The higher level of productivity infers that the climate may not have been cold enough for the water to have formed permanent ice. Bearing these facts in mind, it seems justified to assume that the ice-melting pulses in interglacial stages 3 and 7 could not have taken place and neither could the episodes of bloom. 4.3. Changes in the bottom water conditions As shown in Fig. 6d and e, the Mn/Al ratios show an overwhelming increase from 0.008 to 0.02, 0.03, 0.04, 0.08 and even to 1.7 during deglaciation in stages 1, 5, 6, 9 and 11, respectively. The P/Al ratios synchronously show substantially higher values which are, in fact, highly consistent with the Mn/Al ratios. The values increase three- to fourfold over that of the normal value and, in those same stages,
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they are generally within the range of 0.01 to 0.04 but even reach 0.05. On account of the extremely positive abnormal conditions that occurred in the early period of stage 9, i.e. 334 kyrs (1413 cm), the samples were collected at denser intervals (5 cm) between 1383 and 1433 cm. In addition, except in stage 6, an extraordinarily high quantity of dolomite appears in those periods (Fig. 5d). To be sure, these characteristics underscore the fact either that the bottom water chemistry must have changed or that diagenetic fronts must have occurred during these periods. It is widely accepted that the element Mn is mobile under anoxic conditions, such as in Mn2+, and that it easily migrates along a pore water gradient, changing from an anoxic state to an oxic one. It is eventually precipitated as oxyhydroxides at an oxic/post-oxic boundary. In the process, the amount of Mn may actually be enriched several percentage points (Sawlan and Murray, 1983; Kadko and Heath, 1984). Permanent ice-melting during deglaciation must have led to a relatively higher sedimentation rate and, at the same time, an abundance of organic matter must have been deposited at the sea bottom. The oxidation of the organic matter may have exhausted dissolved oxygen, perhaps making the bottom water anoxic (Richards, 1965). In this event, the oxic/post-oxic boundary in the strata must have been characterized by redoxic sensitive element deposits, like Mn (Thomson et al., 1993, 1986; George et al., 1997); what's more, even Fe-bounded P could have been enriched (Tamburini et al., 2002). When the sediments accumulated on the sea floor, diagenesis would have also redistributed some of the elements and caused the elements Mn and P to concentrate. That is to say, oxygendepletion caused by a great deal of sedimentation and/or diagenesis may very well account for the fact that these elements are extremely enriched in the core sediments. It cannot be denied that the anomalous phenomenon that occurred in glacial stage 6 remains a mystery and further study is required. During deglacial periods, calcite dissolution and anoxic bottom water conditions may very well have favored the precipitation of dolomite. The truth of the matter is that the occurrence of the specific elements Mn and P along with dolomite is indicative of a reduction in the bottom water environment of the Okhotsk Sea during the early deglaciation period. The formation of dolomite at the sea bottom is generally associated with a bacterial reduction of sulfate and it requires a particular geochemical condition, i.e., a strong reduction in bottom water or anoxia (Tamburini et al., 2002). During the early deglaciation periods, the specific environment may have been induced by a great deal of ice-melting. The oxidation of organic matter consumes the dissolved oxygen in the bottom water, making it more corrosive and anoxic. Furthermore,
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carbonate dissolution induces a local positive CO2 pressure gradient. In such a reducing environment, dolomite and siderite are thermodynamically more stable phases than calcite (Sighinolfi and Tateo, 1998). 5. Conclusions Historically, there have been drastic differences between glacial and interglacial periods when it comes to the sedimentation processes in the Okhotsk Sea. On a broad scale, the derivation of the sediments has been two-fold: biogeneous and detritus sources. Quartz and plagioclase can be treated as detritus components, and calcite and opal as biogeneous in this area. During glacial times, the covering of extended ice restricted the activity of diatoms, coccoliths and foraminifera. As a consequence, the biogenetic components were depleted, and the terrigenous sediments, such as eolian sediments, IRD and detritus from the surrounding landmasses, dominated. Then, with the coming of warm interglacial times, parts of the perennial ice-cover melted. A great quantity of sediments with organic matter originally trapped in the ice and on the shelf or on the sea bottom was likely released into the Okhotsk Sea and these served as nutrients for the whole environment. In turn, biogenetic productivity must have been highly activated, as indicated by the apexes of calcite, opal, Mg/Al, Ca/Al and Si/Al in the interglacial periods. Large amounts of biogenetic sediments must have then descended to the sea floor, reducing the level of the detritus sediments to below 40%. However, the results of the analyses of stages 3 and 7 are quite different from those of the other interglacial stages. No apparent calcite or opal can be observed in either stage 3 or 7. It is well known that stage 3 was colder than other interglacial stages, suggesting the ice-melting pulse was probably debilitated. Hence, this accounts for the fact that the quantity of biogenetic sediments does not appear to have increased. The anomaly of stage 7 was probably attributable to other factors. Based on our mineralogical and other studies, stage 8 might have been warmer than the other glacial stages. As a result, the environmental changes from stage 8 to 7 might have been so gentle that bioproductive prosperity did not occur. Not only did the ice-melting pulse have an influence on productivity, but it also affected bottom water chemistry. With the melting of the ice-cover, it is believed that lots of sediments with abundant organic matter were released into the sea. The oxidation of the organic matter might have exhausted dissolved oxygen, causing the bottom water to become anoxic. In such a reduced situation, redoxic sensitive elements,
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such as the Mn2+ ion, would have become mobile in the seawater and would have finally precipitated at the oxic/post-oxic boundary. This explains why higher Mn/Al and P/Al ratios representing the oxic/post-oxic boundary can be observed during the deglaciation periods. Furthermore, the fact that dolomite precipitated also supports the notion of a reduced environment of bottom water in the deglacial periods of stages 1, 5, 9 and 11. Acknowledgements The authors appreciate the assistance of the Captain and all crew members of the R.V. Marine Dufresne during the IMAGES VII cruise in June 2001. They are also deeply indebted to Dr. Lee Chi-Yu of the Department of Geosciences, National Taiwan University, Taipei, Taiwan, R.O.C. for the XRF and XRD analyses. Thanks are also due to two anonymous referees and editor whose suggestions and valuable reviews greatly improved the manuscript. This study was partly supported by the Asian Paleo-Environmental Changes II (APEC II) of Academia Sinica and the National Science Council of the R.O.C. under grant NSC91-2116-M-002-020. References Bassinot, F., Baltzer, A., 2002. WEPAMA Cruise MD122/IMAGES VII on board R.V. “Marion Dufresne”. Institut Polaire FrançaisPaul-Emile Victor, pp. 41–43. Boski, T., Pessoa, J., Pedro, P., Thorez, J., Dias, J.M.A., Hall, I.R., 1998. Factors governing the abundance of hydrolysable amino acids in the sediments from the NW European continental margin (47–50°N). Prog. Oceanogr. 42, 145–164. Cavalieri, D.J., Parkinson, C.L., 1987. On the relationship between atmospheric circulation and fluctuations in the sea ice extents of the Bering and Okhotsk Seas. J. Geophys. Res. 92, 7141–7162. Chou, Y.-M., 2003. Magnetic study of core MD012414 from Okhotsk Sea - paleoclimate and paleoenvironment changes of Northeastern Asia since 1.8Ma (in Chinese), master thesis of Institute of geoscience, National Taiwan Normal University, R.O.C., 81 pp. Fagel, N., Boski, T., Likhoshway, L., Oberhaensli, H., 2003. Late Quaternary clay mineral records in central Lake Baikal (Academician Ridge, Siberia). Palaeogeogr. Palaeoclimatol. Palaeoecol. 193, 159–179. George III, W.L., Byorn, S., Brent, L.L., Paul, J.B., Norman, S., 1997. Interactions of manganese with the nitrogen cycle: alternative pathways to dinitrogen. Geochim. Cosmochim. Acta 61 (19), 4043–4052. Gorbarenko, S.A., 1996. Stable isotope and lithologic evidence of lateglacial and Holocene oceanography of the Northwestern Pacific and its marginal seas. Quat. Res. 46, 230–250. Gorbarenko, S.A., Chekhovskaya, M.P., Souhton, J.R., 1998. On the paleoenvironment of the central part of the Sea of Okhotsk during the past Holocene glaciation. Oceanography 38 (2), 277–280. Gorbarenko, S.A., Khusid, T.A., Basov, I.A., Oba, T., Southon, J.R., Koizumi, I., 2002a. Glacial Holocene environment of the Southeast-
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