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Abstract

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Samples and Analytical Methods
  5. 3. Results and Discussions
  6. 4. Concluding Remarks and Implications to the Glacial CO2 Cycle
  7. Acknowledgments
  8. References

[1] Biogenic opal and ice-rafted detritus (IRD) data from sediments in the Okhotsk Sea and the neighboring North Pacific revealed the remarkable reduction in opal production and southward advancement of sea-ice covered area during the last glacial maximum, resulting also southward shift of high biological productive area in the northwestern North Pacific. It implies that the substantial reduction in outflux of CO2 to the atmosphere in northwestern North Pacific and the pronounced increase in CO2 sequestering in temperate North Pacific. This could be an additional CO2 reduction mechanism of atmospheric CO2 in the last glacial period.

1. Introduction

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Samples and Analytical Methods
  5. 3. Results and Discussions
  6. 4. Concluding Remarks and Implications to the Glacial CO2 Cycle
  7. Acknowledgments
  8. References

[2] The opal accumulation rates from sediments in the subarctic Pacific Ocean [Haug et al., 1999] showed the remarkable reduction in the opal production 2.73 Myr ago, which is coincident with the intensification of Northern Hemisphere glaciation. This great cooling event induced strong oceanic stratification in this region, therefore, resulting in the decrease of opal productivity and atmospheric CO2 concentration at 2.73 Myr B.P. The reduction in opal production with the expansion in sea-ice covered area is found during the last glacial periods in the northwestern North Pacific in this study.

2. Samples and Analytical Methods

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Samples and Analytical Methods
  5. 3. Results and Discussions
  6. 4. Concluding Remarks and Implications to the Glacial CO2 Cycle
  7. Acknowledgments
  8. References

[3] We studied three piston cores, from the Okhotsk Sea XP98 PC1 (51° 00.9' N, 152° 00.5' E, 1107 m) and from the Emperor Seamount region in the northwestern North Pacific MR98-05 Sta. 4 (44° 47.2' N, 170° 04.6' E, 1856 m) and KH99-03 Sta. ES (49° 44.7' N, 168° 18.9' E, 2388 m).

[4] To obtain a fine vertical variability, opal (biogenic silica) was analyzed for by extracting with an alkaline solution [Mortlock and Froelich, 1989] at 1.2 cm intervals for the Okhotsk Sea core or 2 to 2.4 cm intervals for using the other cores. The ice-rafted detritus and dropstones in the samples were separated a 250 μm sieve and picked by hand during sample preparation. For three sliced samples of the Okhotsk Sea core, which are 62.2 to 64.6 cm, 121.0 to 123.5 cm, and 211.9 to 214.3 cm, mono-species planctonic foraminiferal shells (N. Pachyderma) were extracted and analyzed for the 14C ages using an accelerator mass spectrometer.

3. Results and Discussions

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Samples and Analytical Methods
  5. 3. Results and Discussions
  6. 4. Concluding Remarks and Implications to the Glacial CO2 Cycle
  7. Acknowledgments
  8. References

3.1. Opal and Normalized δ18O

[5] The Okhotsk Sea sediment core was 10.22 m long and covered one glacial-interglacial cycle (ca. 125,000 years) as determined from the radiocarbon data for the surface 200 cm, which indicated that the sedimentation rate was around 8 cm/kyr. The vertical profile of the opal concentration shows in Figure 1 together with normalized δ18O values of biogenic carbonate in the deep sea sediment. The periods of a higher opal concentration correspond to the Holocene, Stage 3, and interglacial Substages 5a, 5c, and 5e, which can be assigned by the higher δ18O values indicating a warmer climate. Conversely, the lower opal concentration periods correspond to the last glacial maximum, Stage 4, Substages 5b and 5d, with the colder climate being indicated by the lower δ18O values. The concentrations of biogenic opal in the Holocene sediments of the top 50 cm of the core agreed fairly well with those of V34-90 (48° 50'N, 150° 28'E, 1590 m) [Gorbarenko, 1996], collected at the similar latitude in the northwestern North Pacific.

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Figure 1. Vertical profile of concentration of biogenic opal in PC1 sediment from the Okhotsk Sea (b) compared with the normalized oxygen-isotope profiles [Martison et al., 1987] (a). Peaks are connected with dashed lines, corresponding number of isotopic events identified is noted. Radiocarbon ages are also shown as calendar ages with a reservoir correction of 900 years [Keigwin, 1998].

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[6] At the Emperor Seamount region of the northwestern North Pacific (Figure 2), the concentration of opal was low during the glacial ages and high during the interglacial ages. The decrease of opal concentration during last glacial maximum was also observed in the far northwestern Pacific from the core RAMA 44PC (53° 00'N, 164° 39'E, 2980 m) [Keigwin et al., 1992]. However, the fine structure was not as clearer as that of the Okhotsk Sea sediments due to the low time resolution. These results suggest that the trend in biological productivity can be extended to the northwestern North Pacific, at least north of 45° N. Biological productivity was contrarily reported high during the glacial ages at a location (34° 54.3' N, 179° 42.2' E, depth 3571 m) in the Hess Rise in the temperate North Pacific south of the present subarctic front [Kawahata et al., 2000]. It suggests that during the glacial ages there was a boundary in the North Pacific between 35 and 45° N and that biological productivity was low in the region north of that boundary. These results indicate that the productivity of biogenic opal during the warmer periods was higher than during colder periods, unless the concentration of opal was reduced only during the glacial ages or decreased with time due to dissolution after sedimentation. Significant alternation due to dissolution, however, is unlikely, because the vertical trend of the opal increased again in the last interglacial period, Stage 5, and its fine structure coincides well with the δ18O pattern, even though some of the opal arriving at the bottom may have dissolved to some extent in the near surface sediments. We can safely say that the production of opal as well as its sedimentation in the Okhotsk Sea and northwestern North Pacific was greatly responded to the biogenic opal concentrations in the sediments.

image

Figure 2. Vertical profiles of concentration of biogenic opal in sediment from two stations located in the Emperor Seamount region of the North Pacific compared with that at PC1 in the Okhotsk Sea. The magnetic susceptibility data obtained at Sta. 4 are also shown. Panels A, B, and C show opal concentration at PC1, at Sta. ES and at Sta. 4. Panel D shows the magnetic susceptibility at Sta. 4. The diamonds and squares in panels B and C indicate the layers containing IRD and dropstones, respectively. The corresponding peaks are connected with dashed lines.

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3.2. IRD Distribution

[7] We can add one further finding related to the sea-ice distribution of this region during the glacial ages, involving the IRD and dropstones found in the sediments. Both were continuously present in the Okhotsk Sea sediments below about 110 cm, although they were not generally found in the Holocene time slice above that depth. Some IRD and dropstones deposited during the glacial ages were found even in the Emperor Seamount sediments (Sta. 4) located at around 45° N in the North Pacific, and their magnetic susceptibilities were also high in this time slice (Figure 2). These observations suggest even more clearly than those previously reported [St. John and Krissek, 1999] that sea-ice covered the northwestern North Pacific more extensively during of the last glacial ages in winter than it does now.

3.3. Opal Production and its Paleocimatic or Paleoceanographic Implications

[8] What reduced the production of biogenic opal during the glacial ages? The present Okhotsk Sea is characterized by a well-developed seasonal ice cover, which marks the sea-ice southern limit in the world. Therefore, if the sea were covered by perennial sea-ice during the glacial ages, biological productivity could be low. Low productivity, however, is unlikely because the sedimentation rate did not decease, and IRD and dropstones were present, indicating melting of the ice in the region during glacial period. That the opal concentration matched well with the δ18O pattern and coincided with patterns outside the Okhotsk Sea is also strong evidence for the absence of perennial sea-ice. However, we do not totally exclude the possibility that it was present in some areas in the Okhotsk Sea during the glacial ages [Siga and Koizumi, 2000].

[9] Alternative explanation for the lowering of opal production during the glacial ages is the reduction of nutrient utilization efficiency. If biological productivity in the glacial northwestern North Pacific is limited only by the availability of micronutrients, such as iron, rather than that of the major nutrients supplied by upwelling of the deep water, the iron-limited environment induced reduction in nutrient utilization and decrease in productivity [Martin, 1990]. In the present Okhotsk Sea, the nutrient profiles during summer show nearly depletion of even dissolved silica in the surface mixed layer [Honjo, 1997]. This means that the utilization of dissolved silica is extremely high there at present. Furthermore, the input of dust and associated trace metals increased during the glacial period in the Okhotsk Sea [Ternois et al., 2001] and central North Pacific [Kawahata et al., 2000]. The decrease in nutrient utilization including dissolved silica is unlikely to have occurred under the abundant micronutrient condition such as Fe during the last glacial ages, and the decrease of the biogenic opal concentration is unlikely to have occurred without decrease in the nutrient supply to the surface. Therefore, the supply of nutrients should be less during the glaciation, so that productivity could be reduced. Since the nutrients in the present northwestern North Pacific are mostly supplied from upwelled Pacific Deep water, it seems that the upwelling could be extremely weakened in the northwestern North Pacific during the glacial ages. As the iron repletion in the surface water during the glacial ages induced a low-nitrate, high-silicate condition [Takeda, 1998], the surface water might have contained relatively depleted nitrate compared with dissolved silica.

[10] The following scenario is proposed to explain the decrease in the nutrient supply to the euphotic layer followed by biological productivity during the last glacial period in the northwestern North Pacific including the Okhotsk Sea. Starting with the glacial ages, more sea-ice was produced and spread more widely over the northwestern North Pacific [Morley, 1980; Morley and Hays, 1983] due to both the global cooling and the subsequent weakening of the Kuroshio Current flowing northward [Ujiié and Ujiié, 1999], resulting the enhanced salinity-based stratification of the surface water [Keigwin et al., 1992]. This stratification acted as a positive feedback process, increasing the amount of sea-ice. The brine excluded by the sea-ice formation increased the formation of intermediate water in the North Pacific. This intermediate water was more briny and contained less nutrients [Keigwin, 1998] and more dissolved oxygen [Cannariato et al., 1999]. At the same time, the upwelling of the Pacific Deep Water was weakened, and the upwelling region shifted southward. These changes in the oceanic environment of the glacial North Pacific reduced the dissolved silica content and biological productivity in the surface water.

4. Concluding Remarks and Implications to the Glacial CO2 Cycle

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Samples and Analytical Methods
  5. 3. Results and Discussions
  6. 4. Concluding Remarks and Implications to the Glacial CO2 Cycle
  7. Acknowledgments
  8. References

[11] Over the last 400,000 years, the concentration of atmospheric CO2 has ranged from around 200 ppm in the glacial ages to 300 ppm in the warmer periods [e.g., Petit et al., 1999]. Two mechanisms that might have lowered the CO2 level during the glacial ages have been proposed: an enhanced biological pump and reduced vertical mixing in high-latitude oceans [Sarmiento and Toggweiler, 1984; Siegenthaler and Wenk, 1984]. However, over the past decade, there have been many reports of lowered productivity in the Southern Ocean, which is a possible sink for atmospheric CO2 [e.g., Mortlock et al., 1991]. The increase in productivity in the Subantarctic Ocean and the decease in the exposure flux of CO2 from the Antarctic Ocean to the atmosphere have greatly contributed to the lowering of atmospheric CO2 [François et al., 1997]. The northwestern North Pacific, where much of the nutrient supply goes unused, is another possible sink for atmospheric CO2. Both the reduced leakage of CO2 from the ocean to the atmosphere due to salinity-based stratification in the northwestern North Pacific [Keigwin et al., 1992] and the increase in productivity in the temperate North Pacific [Kawahata et al., 2000] might have been significantly responsible for the glacial reduction in atmospheric CO2 concentration.

Acknowledgments

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Samples and Analytical Methods
  5. 3. Results and Discussions
  6. 4. Concluding Remarks and Implications to the Glacial CO2 Cycle
  7. Acknowledgments
  8. References

[12] We sincerely thank Dr. M. Kusakabe and Dr. H. Tokuyama, and also all the members of MAG in Hokkaido Univ. for helping with the sampling and for their comments. We also thank Dr. Nori Tanaka for providing many helpful suggestions on the manuscript. This research was supported by CREST and GCMAPS, founded by Japanese Science and Technology Corporation and Science Technology Agency, respectively.

References

  1. Top of page
  2. Abstract
  3. 1. Introduction
  4. 2. Samples and Analytical Methods
  5. 3. Results and Discussions
  6. 4. Concluding Remarks and Implications to the Glacial CO2 Cycle
  7. Acknowledgments
  8. References
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