Biogenic Opal Content Research Articles

Holocene sedimentation in the Hupo Trough of the southwestern East Sea (Japan Sea) and development of the East Korea Warm Current

Various sediment properties, such as mean grain size, total organic carbon, total nitrogen, C/N ratio, CaCO3, and biogenic opal content, were analyzed for a box core (BC02; 45 cm long) and a gravity core (GC02; 628 cm long), which were collected from the western margin of the Hupo Trough located off the eastern coast of Korea. The study area has been affected by the East Korea Warm Current (EKWC), a branch of the Tsushima Warm Current (TWC). The analytical results obtained for BC02 and the upper part of GC02 were in agreement, affirming the core-top preservation of GC02. Based on the corrected calibrated AMS 14C dates, the sedimentation rate of GC02 changed abruptly at ~8.2 ka from ~4.0–10.2 cm/kyr in the lower part to ~56.6–91.0 cm/kyr in the middle to upper part. This corresponds to the lithologic change from sandy mud to mud sediments showing the mean grain size change from 6.9 to 46.0 μm. Diverse paleoceanographic proxies representing the surface water condition exhibited varying degree of change at ~8.2 ka, after which all the properties remain almost unchanged, implying stable and continuous depositional conditions following the complete development of the EKWC. Furthermore, it indicated that the sediment depositional conditions in the Hupo Trough in response to the EKWC might have stabilized at ~8.2 ka since the opening of the Korea Strait during the Holocene sea level rise. Moreover, microfossil data from previous studies on the establishment of the TWC in the East Sea (Japan Sea) support our interpretation that the sediment properties revealed the Holocene development of the EKWC in the Hupo Trough.

A multiproxy study of past environmental changes in the Sea of Okhotsk during the last 1.5 Ma

Long-chain diols have been detected in a wide range of environments and have been used to reconstruct past environmental changes, however only a few long-term records exist to date. Here we reconstructed past environmental changes in the central Sea of Okhotsk over the last 1.5 million years, covering the Mid-Pleistocene Transition (MPT). Sea surface temperatures (SST) reconstructed using the Long-Chain Diol Index (LDI) reflects glacial/interglacial changes. However, when compared with other organic paleothermometers (Uk′37 and TEXL86) the LDI-SST is lower during interglacials and similar or higher during glacials, possibly suggesting a shift of diol production season during interglacials. The LDI-SST does not change in periodicity around the MPT as observed for the TEXL86, likely due to this seasonal shift. Diatom productivity, as recorded by 1,14-diols and biogenic opal content, increased during the main deglaciations with a succession from Proboscia diatoms to diatoms with a more heavily silicified shell, confirming that primary productivity in the central Sea of Okhotsk is driven by sea-ice progress and retreat. In contrast to the LDI-SST, the 1,14-diols record shows a change in periodicity around the MPT from 41- to 100-kyr cycle, suggesting an influence of orbital parameters on diatom productivity. In the central Sea of Okhotsk, the relative amount of C32 1,15-diol (FC32 1,15), a proxy for riverine input, correlates with sea-level change with more riverine-derived material reaching the core site when the Amur River mouth is closer at lower sea-levels. In agreement, FC32 1,15 shows a change in periodicity during the MPT, with the appearance of a 100-kyr cycle. Our results show that the long chain diols can provide important paleoceanographic information in subpolar environments over long time scales, but that temperature reconstructions can be severely impacted by changes in seasonality.

Late Neogene sediment properties in the Wilkes Land continental rise (IODP Exp. 318 Hole U1359A), East Antarctica

On the basis of the variations of sediment properties such as biogenic opal, grain size, natural gamma radiation (NGR), and clay mineralogy, we differentiate two types of late Neogene marine sediments that record paleoclimate changes associated with the growth and retreat of the East Antarctic Ice Sheet (EAIS) and associated changes in sea ice cover. The first type of sediments are massive muds characterized by high biogenic opal (30~55%), low NGR (15~30 counts per second (cps)), large mean grain size (>8.3 μm), and low smectite/(illite + chlorite) [S/(I + C)] ratios ( ~0.2). We interpret these sediments as the result from deposition by either turbiditic or contouritic processes during glacial conditions. All these sediment properties indicate that the Wilkes Land continental rise recorded late Neogene paleoclimatic changes in terms of surface water productivity, sediment provenance and transport pathway, and depositional processes that are related to the growth and retreat of EAIS. In addition, the decreased maxima of biogenic opal content at Hole U1359A highlights the temporal climatic change between the late Pliocene (~2.0 Ma to ~3.7 Ma) and the early Pliocene to late Miocene (~3.7 Ma to ~6.3 Ma), confirming the important role of sea ice to the surface water productivity associated with the global cooling trend in the East Antarctica.

Stable silicon isotope signatures of marine pore waters – Biogenic opal dissolution versus authigenic clay mineral formation

Dissolved silicon isotope compositions have been analysed for the first time in pore waters (δ30SiPW) of three short sediment cores from the Peruvian margin upwelling region with distinctly different biogenic opal content in order to investigate silicon isotope fractionation behaviour during early diagenetic turnover of biogenic opal in marine sediments. The δ30SiPW varies between +1.1‰ and +1.9‰ with the highest values occurring in the uppermost part close to the sediment–water interface. These values are of the same order or higher than the δ30Si of the biogenic opal extracted from the same sediments (+0.3‰ to +1.2‰) and of the overlying bottom waters (+1.1‰ to +1.5‰). Together with dissolved silicic acid concentrations well below biogenic opal saturation, our collective observations are consistent with the formation of authigenic alumino-silicates from the dissolving biogenic opal. Using a numerical transport-reaction model we find that approximately 24% of the dissolving biogenic opal is re-precipitated in the sediments in the form of these authigenic phases at a relatively low precipitation rate of 56μmolSicm−2yr−1. The fractionation factor between the precipitates and the pore waters is estimated at −2.0‰. Dissolved and solid cation concentrations further indicate that off Peru, where biogenic opal concentrations in the sediments are high, the availability of reactive terrigenous material is the limiting factor for the formation of authigenic alumino-silicate phases.

Constraining the dating of late Quaternary marine sediment records from the Scotia Sea (Southern Ocean)

In Southern Ocean sediments south of the Antarctic Polar Front, the scarcity of calcareous microfossils hampers the development of sediment chronologies based on radiocarbon dating and oxygen isotope stratigraphy established from carbonate. In this study, radiometric dating, magnetic susceptibility (MS), biogenic opal content, diatom abundance fluctuation, and paleomagnetic information were investigated on a north–south transect of central Scotia Sea sediment cores to verify their reliability as stratigraphic tools in the study area. Radiocarbon dating on organic carbon humic acid fraction can be used to establish the stratigraphy of upper core sections, but regional comparison and correlation are needed to verify a possible bias by fossil carbon contamination. For the long-term stratigraphy, MS, which can be correlated to the Antarctic ice core dust/climate signal, represents the most valuable parameter. Fine-grained single domain magnetite, probably of biogenic origin, makes a significant contribution to the interglacial MS signal, while major contributions from detrital material affect the glacial MS record. The core from the southern Scotia Sea contains significant proportions of biogenic magnetite also in glacial sediments, suggesting depositional environments different from those of the northern Scotia Sea. Our data suggest low contributions of high-coercive minerals to the overall magnetic intensity of glacial and interglacial Scotia Sea sediments, which excludes dust as a main source of the magnetic signal. Opal content can be used to distinguish between cold and warm intervals for the past 300 thousand years. Abundance fluctuation patterns of diatom species Fragilariopsis kerguelensis and Eucampia antarctica are useful stratigraphic tools for periods back to Marine Isotope Stage (MIS) 6. The Mono Lake geomagnetic excursion is identified in Scotia Sea sediments for the first time. Possible correlations of ash layers are suggested between Scotia Sea sediments and East Antarctic ice cores. They have potential to serve as additional age markers for further studies in this area.

Paleoproductivity and paleoceanography of the last 4.3 Myrs at IODP Expedition 323 Site U1341 in the Bering Sea based on biogenic opal content

Site U1341 in the southern Bering Sea was drilled and cored down to 600 meters below sea-floor (mbsf) during Integrated Ocean Drilling Program (IODP) Expedition 323, covering a nearly complete record of the last 4.3 million years (Myrs). Analyses of the biogenic opal content of sediments at the site provide detailed and useful information on past biological productivity and paleoceanographic changes that occurred in the region including shifts in the oceanographic condition during the intensification of the Northern Hemisphere Glaciation (NHG) and the Mid-Pleistocene Transition (MPT). An overall decreasing trend in the %biogenic opal record, combined with evidence from microfossil assemblages, indicates a gradual shift in environmental conditions during the last 4.3Myrs, from warm and nutrient-rich conditions to cool conditions with sea-ice. On the other hand, biogenic opal mass accumulation rates (MAR) were high during 2.6–2.1Ma after the intensification of the NHG, unlike in the western North Pacific. High biological productivity during this specific interval is consistent with the results of previous studies in the other Marginal Seas, possibly suggesting that iron leakage from the Bering Continental Shelf occurred. After the MPT, the data suggest that there was sea-ice expansion and discharge of lithogenic matter during glacial periods, and high productivity during interglacial periods.

동남극 윌크스랜드 대륙대의 마이오세 중-후기 동안 생물기원 오팔 생산과 고기후 변화(IODP Exp 318 Site U1359)

A 450 m-long sediment section was recovered from Hole U1359D located at the eastern levee of the Jussieau submarine channel on the Wilkes Land continental rise (East Antarctica) during IODP Expedition 318. The age model for Hole U1359D was established by paleomagnetic stratigraphy and biostratigraphy, and the ages of core-top and core-bottom were estimated to be about 5 Ma and 13 Ma, respectively. Biogenic opal content during this period varied between 3% and 60%. In the Southern Ocean, high biogenic opal content generally represents warm climate characterized by the increased light availability due to the decrease of sea-ice distribution. The surface water productivity change in terms of biogenic opal content at about 10.2 Ma in the Wilkes Land continental rise was related to the development of Northern Component Water. After about 10.2 Ma, more production of Northern Component Water in the North Atlantic caused to increase heat transport to the Southern Ocean, resulting in the enhanced diatom production. Miocene isotope events (Mi4~Mi7), which are intermittent cooling intervals during the Miocene, appeared to be correlated to the low biogenic opal contents, but further refinement was required for precise correlation. Biogenic opal content decreased abruptly during 6 Ma to 5.5 Ma, which most likely corresponds to the Messinian salinity crisis. Short-term variation of biogenic opal content was related to the extent of sea-ice distribution associated with the location of Antarctic Polar Front that was controlled by glacial-interglacial paleoclimate change, although more precise dating and correlation will be necessary. Diatom production in the Wilkes Land continental rise increased during the interglacial periods because of the reduced sea-ice distribution and the southward movement of Antarctic Polar Front.

Biogenic CaCO3 and Opal Depositions and Their Latitudinal Comparison During the Past 600 ka in the Central Equatorial Pacific

The orbital-scale variations in biogenic CaCO_3 and opal abundance in two piston cores collected in the central equatorial Pacific (core PC5101 from a southern site at 2°N, and core PC5103 from a northern site at 6°N) were compared to assess latitudinal differences. The correlation between the oxygen isotope stratigraphy of planktonic foraminifera (＂Globigerinoides sacculifer＂) of PC5103 with the LR04 stacks provides the age of PC5103 to be approximately 950 ka. The age of PC5103 was further refined by correlating the CaCO_3 content with the well-dated core RC11-210. The age of PC5101 was also constrained by the same CaCO_3 chronostratigraphic correlation with RC11-210, resulting in an age of approximately 650 ka. Distinct orbital-scale series of CaCO_3 and opal variations appear to be parallel between the two cores during the past 600 ka, which are controlled mainly by eccentricity with an approximate periodicity of 100 ka. It is worth noting that the biogenic CaCO_3 and opal deposition patterns in the two cores differ between interglacial and glacial periods. During interglacial periods the biogenic opal content is higher in the southern core than in the northern core, which corresponds with the present-day condition. In contrast the CaCO_3 content is higher in the northern core, which is contradictory to the present-day northward decreasing CaCO_3 deposition pattern from the Equator. The collection site of PC5101 is approximately 350 m deeper than that of PC5103, which significantly promotes CaCO_3 dissolution and causes unexpectedly high CaCO_3 content at the northern site in contrast to the biogenic opal content.

Biogenic opal production changes during the Mid-Pleistocene Transition in the Bering Sea (IODP Expedition 323 Site U1343)

Biogenic opal content and mass accumulation rate (MAR) at IODP Expedition 323 Site U1343 were found to fluctuate consistently, generally being high under warm conditions and low under cold conditions during the last 2.4Ma. Continuous wavelet transform analysis of the normalized biogenic opal content indicates that export production in the Bering Sea varied predominantly at 41-ka periodicity before 1.25Ma, and shifted to 100-ka periodicity at the onset of the Mid-Pleistocene Transition (MPT; 1.25–0.7Ma). The 100-kacycles dominated until the Holocene. Export production in the Bering Sea decreased markedly in the Bering Sea two times during the MPT: the first occurred at the beginning of the MPT (1.25Ma) and the second in the middle of the MPT (0.9Ma). These decreases coincided with both increases in the relative abundance of sea-ice diatoms and decreases in the warm-water diatom species Neodenticula seminae, indicating that reductions in export production in the Bering Sea during the MPT were associated with climate cooling. Decreases in export production in the Bering Sea during the MPT were most likely associated with the increased influence of polar/Arctic domains on the high-latitude North Pacific.

Changes in biogenic opal productivity with Milankovitch cycles during the last 1.3 Ma at IODP Expedition 323 Sites U1341, U1343, and U1345 in the Bering Sea

Sediments from three Sites (U1341, U1343 and U1345) of IODP Expedition 323, located in the Bering Sea, were analyzed for variation of biogenic opal representing surface water paleoproductivity. Initial age models of three Sites (bottom age, U1341: 4.3 Ma, U1343: 2.4 Ma, U1345: 0.5 Ma) were established onboard by biostratigraphic and paleomagnetic investigations. The initial age models were tuned by δ18O stratigraphy, which was established on shore. Biogenic opal contents of sediments were analyzed for the interval of 1.3 Ma to the present. Generally, the %biogenic opal values at Site U1341 (Bowers Ridge) were significantly higher than those at the Beringian slope sites (U1343 and U1345). Higher natural gamma radiation (NGR) counts and magnetic susceptibility (MS) at the Beringian slope sites suggested a greater contribution of lithogenic matter than at Site U1341 on Bowers Ridge. The percentages of sea-ice diatoms at the Beringian slope sites are also higher than those at Bowers Ridge Sites. This is primarily attributed to the fact that the sediments from the Beringian slope area were significantly diluted by lithogenic matter derived from the Alaskan continent, and were largely affected by sea-ice cover. The cyclicity of biogenic opal accumulation rates at Site U1341 is determined to be about 160, 40, and 30 ky, whereas that at Site U1343 is determined to be about 330, 100, 40 and 20 ky. If these had been reflecting a global climate change, the cyclicity of biogenic opal contents at all sites would be consistently the same. However, the results do not show such a consistency. Obliquity is observed at both sites of the Bowers Ridge and Beringian slope. A strong eccentricity was observed only at U1343, but no signature for eccentricity was detected at U1341. Therefore, the Milankovitch cycle with 100 ky periodicity was only observed at the Beringian slope area. The results suggest that the %biogenic opal at Site U1341 indicated a local characteristic for biogenic productivity changes. Thus, the absence of the 100 ky cycle of biogenic opal at Site U1341 seems to be due to local phenomena such as water circulation, although the changes in biogenic opal productivity at Site U1341 are attributed to summer insolation.

Characterization of gas hydrate reservoirs by integration of core and log data in the Ulleung Basin, East Sea

Examinations of core and well-log data from the Second Ulleung Basin Gas Hydrate Drilling Expedition (UBGH2) drill sites suggest that Sites UBGH2-2_2 and UBGH2-6 have relatively good gas hydrate reservoir quality in terms of individual and total cumulative thicknesses of gas-hydrate-bearing sand (HYBS) beds. In both of the sites, core sediments are generally dominated by hemipelagic muds which are intercalated with turbidite sands. The turbidite sands are usually thin-to-medium bedded and mainly consist of well sorted coarse silt to fine sand. Anomalies in infrared core temperatures and porewater chlorinity data and pressure core measurements indicate that “gas hydrate occurrence zones” (GHOZ) are present about 68–155 mbsf at Site UBGH2-2_2 and 110–155 mbsf at Site UBGH2-6. In both the GHOZ, gas hydrates are preferentially associated with many of the turbidite sands as “pore-filling” type hydrates. The HYBS identified in the cores from Site UBGH2-6 are medium-to-thick bedded particularly in the lower part of the GHOZ and well coincident with significant high excursions in all of the resistivity, density, and velocity logs. Gas-hydrate saturations in the HYBS range from 12% to 79% with an average of 52% based on pore-water chlorinity. In contrast, the HYBS from Site UBGH2-2_2 are usually thin-bedded and show poor correlations with both of the resistivity and velocity logs owing to volume averaging effects of the logging tools on the thin HYBS beds. Gas-hydrate saturations in the HYBS range from 15% to 65% with an average of 37% based on pore-water chlorinity. In both of the sites, large fluctuations in biogenic opal contents have significant effects on the sediment physical properties, resulting in limited usage of gamma ray and density logs in discriminating sand reservoirs.

Contribution of changes in opal productivity and nutrient distribution in the coastal upwelling systems to Late Pliocene/Early Pleistocene climate cooling

Abstract. The global Late Pliocene/Early Pleistocene cooling (~3.0–2.0 million years ago – Ma) concurred with extremely high diatom and biogenic opal production in most of the major coastal upwelling regions. This phenomenon was particularly pronounced in the Benguela upwelling system (BUS), off Namibia, where it is known as the Matuyama Diatom Maximum (MDM). Our study focuses on a new diatom silicon isotope (δ30Si) record covering the MDM in the BUS. Unexpectedly, the variations in δ30Si signal follow biogenic opal content, whereby the highest δ30Si values correspond to the highest biogenic opal content. We interpret the higher δ30Si values during the MDM as a result of a stronger degree of silicate utilisation in the surface waters caused by high productivity of mat-forming diatom species. This was most likely promoted by weak upwelling intensity dominating the BUS during the Late Pliocene/Early Pleistocene cooling combined with a large silicate supply derived from a strong Southern Ocean nutrient leakage responding to the expansion of Antarctic ice cover and the resulting stratification of the polar ocean 3.0–2.7 Ma ago. A similar scenario is hypothesized for other major coastal upwelling systems (e.g. off California) during this time interval, suggesting that the efficiency of the biological carbon pump was probably sufficiently enhanced in these regions during the MDM to have significantly increased the transport of atmospheric CO2 to the deep ocean. In addition, the coeval extension of the area of surface water stratification in both the Southern Ocean and the North Pacific, which decreased CO2 release to the atmosphere, led to further enhanced atmospheric CO2 drawn-down and thus contributed significantly to Late Pliocene/Early Pleistocene cooling.

Alkaline leaching characteristics of biogenic opal in IODP drilled cores from the Bering Sea

Leaching characteristics of biogenic opal in alkaline solutions were studied for Bering Sea sediment samples from the cores obtained during Integrated Ocean Drilling Program (IODP) Expedition 323. Generally, biogenic opal content is measured after alkaline leaching using Na 2 CO 3 solution. Biogenic opal in some sediment samples could not be completely leached by Na 2 CO 3 solutions, particularly in the deeper sections depending on sites: below the depth of ca. 122 m (CCSF-A) with 1.1 Ma at Site U1341 and ca. 211 m (CCSF-A) with 0.8 Ma at Site U1343. Strong alkaline 2M and 4M NaOH solutions were used instead of the standard Na2CO3 solution. As a result of the time-series leaching analysis, silicon contribution rates from clay minerals and other detrital grains were estimated in each of the methods employing two different concentrations of NaOH solutions. This made possible to correct for silicon contribution of clay minerals and other detrital grains.

Quaternary primary productivity in Porcupine Seabight, NE North Atlantic

Biogenic opal and calcium carbonate contents in the Quaternary fine-grained sediments in the deep sea coral mound area (Hole U1317E and Hole U1318B drilled by IODP Expedition 307) in Porcupine Seabight, southwest off Ireland, were measured to estimate the primary productivity (Pp) of the surface seawater in NE Atlantic. The results from the two holes commonly show relatively high Pp values of 10–30 g cm−2 ky−1, having stratigraphic cyclicity reversely correlated with the δ13C profile. The high Pp could be attributed to the oceanic setting controlled by the Eastern North Atlantic (Central) Water and Shelf Edge Current, which have been strongly influenced by the obliquity forcing of the Milankovitch cycle. A positive covariance was observed between high Pp and the mound development in U1317E, implying that the organic matter was the principal food source for the mound coral community. It is proposed that the pelagic marl matrix of the mound sediments provides information of the surface water productivity that is useful for reconstructing the paleoceanography and the paleoclimate in the NW Atlantic.

Shift in biotic response of abyssal benthic foraminifera since MIS 7 in the eastern equatorial Pacific Ocean

We investigated the late Quaternary abyssal benthic foraminiferal faunas from the upper 209 cm of sediment core (KODOS PC5101) in order to understand the biotic response of abyssal benthic foraminifera to the glacial-interglacial cycle in the eastern equatorial Pacific Ocean. Three factor assemblages were identified in the benthic foraminiferal faunas of core PC5101: the common deep-sea fauna for which there is some seasonal food supply (Factor 1 assemblage), the fauna that suffer from possible carbonate undersaturation in the deep waters of the Southern Ocean or low food supply (Factor 2 assemblage), and the fauna for which there is seasonal food supply (Factor 3 assemblage). The low Horn’s index of overlap indicates the instability of benthic faunal association during the Marine Isotope Stage (MIS) 5. There are reasonably good correlations between the Factor 1 or 2 assemblages and the CaCO3 or biogenic opal content in the earlier part of the studied interval (early MIS 5 to MIS 7), whereas there is no significant correlation in the later part (MIS 1 to late MIS 5), except for a weak correlation between the Factor 1 assemblage and biogenic opal content. It is suggested that carbonate undersaturation at the sediment-water interface on the seafloor was one of major factors that influenced benthic foraminiferal fauna at the site of our study, particularly from early MIS 5 to MIS 7. However, additional factors also affected benthic foraminifera from MIS 1 to late MIS 5. For example, enhanced periodicity of the food supply from the surface ocean (i.e., seasonality or ENSO variability) might be another factor responsible for the shift in the biotic response of abyssal benthic foraminifera deposited after MIS 5.

High organic carbon deposition in the northern margin of the Aleutian Basin (Bering Sea) before the last deglaciation

High-resolution geochemical, isotope and elemental data from core PC23A in the northern margin of the Aleutian Basin (Bering Sea) were used to reconstruct distinct paleoceanographic features of the last deglaciation (pre-Boreal[PB], Bolling-Allerod[BA], Younger Dryas[YD]). The PB and BA intervals are characterized by increased siliceous (diatom) and calcareous (coccolithophores and foraminifers) productivity represented by high biogenic opal and CaCO3 contents, respectively. The enhanced productivity can plausibly be attributed to an elevated sea-surface nutrient supply from increased melt-water input and enhanced Alaskan Stream injection under warm, restricted sea-ice conditions. High Corg/N ratios and low δ13C values of sediment organic matter during the PB and BA intervals reflect the contribution of terrestrial organic matters. The PB and BA intervals were also identified by laminated sediment layers of core PC23A, characterized by high Mo/Al and Cd/Al ratios, indicating that the bottom water condition remained anoxic. High δ15N values during the same period were attributed mainly to the increased nutrient utilization and subsequent denitrification of seawater nitrate. Part of high δ15N values may also be due to incorporation of inorganic nitrogen in the clay minerals. It is worthy of note that high total organic carbon (TOC) deposition occurred approximately 3,000 years before onset of the last deglaciation. Simultaneous high Corg/N ratios and low δ13C values clearly suggest that the high TOC content should be related to terrestrial organic carbon input. Low δ15N values during the high TOC interval also confirm the contribution of terrigenous organic matter. Although abundant calcareous phytoplankton production under cold, nutrient-poor conditions represented by Baex data was reported for high TOC deposition preceding the last deglaciation in an earlier study of the Okhotsk Sea, the main reason for the enhanced TOC deposition in the Bering Sea is an increased terrigenous input from the submerged continental shelves (Beringia) with a sea-level rise; this is further supported by Al enrichment of bulk sediments during the high TOC deposition.

Variations of the oxygen minimum zone of the Okhotsk Sea during the last 50 ka as indicated by benthic foraminiferal and biogeochemical data

Benthic foraminiferal and sediment biogeochemical data (total organic carbon, calcium carbonate and biogenic opal contents) in two cores (1265 and 1312 m water depths) from the southeastern Sakhalin slope and one core (839 m water depth) from the southwestern Kamchatka slope were investigated to reconstruct variations of the oxygen minimum zone during the last 50 ka in the Okhotsk Sea. The oxygen minimum zone was less pronounced during cooling in the MIS 2 that is suggested to be caused by a maximal expansion of sea ice cover, decrease of marine productivity and increase of production of the oxygenated Okhotsk Sea Intermediate Water (OSIW). A two-step-like strengthening of oxygen minimum zone during the warmings in the Termination 1a and 1b was linked to (1) enhanced oxygen consumption due to degradation of large amount of organic matter in the water column and bottom sediments, originated from increased marine productivity and supply of terrigenous material from the submerged northern shelves; (2) sea ice cover retreat and reduction of OSIW production; (3) freely inflow of the oxygen-depleted intermediate water mass from the North Pacific.

Behavior of particulate materials during iron fertilization experiments in the Western Subarctic Pacific (SEEDS and SEEDS II)

Abstract During two mesoscale iron-enrichment studies in the northwestern subarctic Pacific (SEEDS in 2001 summer and SEEDS II in 2004 summer), particulate materials from the iron-induced phytoplankton bloom in the upper water column were monitored to analyze the export processes beneath the upper mixed layer, mainly with drifting sediment traps. We could not observe the total downward export process of the high accumulation of particulate organic carbon from the mixed layer induced by the large diatom bloom of SEEDS [e.g., Tsuda, A., Takeda, S., Saito, H., Nishioka, J., Nojiri, Y., Kudo, I., Kiyosawa, H., Shiomoto, A., Imai, K., Ono, T., Shimamoto, A., Tsumune, D., Yoshimura, T., Aono, T., Hinuma, A., Kinugasa, M., Suzuki, K., Sohrin, Y., Noiri, Y., Tani, H., Deguchi, Y., Tsurushima, N., Ogawa, H., Fukami, K., Kuma, K., Saino, T., 2003. A mesoscale iron enrichment in the western subarctic Pacific induces large centric diatom bloom. Science 300, 958–961] because the 2-week observation period was too short to examine the decline phase of the bloom. In contrast, in SEEDS II, the particulate organic carbon and particulate organic nitrogen were accumulated 123 and 23 mmol m −2 , respectively, in the mixed layer until day-15 (days from iron-enrichment), and then ca. 90% were removed from the mixed layer by day-25. The sediment traps at 40 m depth between day-15 and day-25 accounted for at least more than 35% of these particles. There was no large variation in chemical composition in settling particles above 100 m depth throughout the experimental periods both in SEEDS and SEEDS II. The content of biogenic opal remained more than 50% of all settling particles during SEEDS, while the content of biogenic calcium carbonate was relatively high, with a low biogenic opal content of consistently less than 30% during SEEDS II. These results suggest that high standing stock of seed population of diatoms before the iron fertilization, indicated by low C/Si ratio of particulate matter, is an important factor to induce the large diatom bloom in SEEDS.

Changes in biological production in the mixed water region (MWR) of the northwestern North Pacific during the last 27 kyr

Piston core PC6 (40°N, 143°E) provides the first record of biological production and alkenone sea surface temperature (SST) in the mixed water region (MWR) off Sanriku, Japan, in the northwestern North Pacific over the last 27 kyr. Biogenic opal content was low until the early Holocene, and rapidly increased at 6 calendar kiloyears before present (cal kyr BP). Low biological production in the MWR resulted from the reduced duration of the phytoplankton bloom during the glacial period (27–18 cal kyr BP), and from nutrient depletion, especially silica, during the deglaciation (15–9.5 cal kyr BP). In spite of the decreased amount of nutrients in the Oyashio water, biological production increased in the late Holocene (6 cal kyr BP–present). The data revealed that nutrients supplied from deeper levels by eddies played an important role in biological production in the MWR, contrary to the conventional concept that the Oyashio current has controlled productivity in this region. Alkenone-SST during the glacial period was on average 13.6 °C, which is close to the modern SST (14.6 °C) in June, but this timing is not consistent with the CLIMAP results (13.4–14.3 °C in August). This result indicates that alkenone-SST recorded not late spring but summer temperatures, owing to a bloom shift caused by surface water cooling during the last glacial period. Alkenone-SST was even lower during the deglaciation than during the LGM and reached a minimum (9.4 °C) at 14.0 cal kyr BP. δ 15N was very high during the deglaciation, implying that nutrients in the euphotic zone were depleted around the core site. Nutrient stress probably had a physiological impact on coccolithophorids and resulted in the apparent minimum alkenone-SST. These findings indicate that the reconstruction of SST by alkenones can sometimes be very complex and possibly misleading.

Comparison of Mg/Ca‐ and alkenone‐based sea surface temperature estimates in the fresh water–influenced Gulf of Guinea, eastern equatorial Atlantic

This study presents a comparison of sea surface temperature (SST) estimates based on Mg/Ca ratios of Globigerinoides ruber and alkenone unsaturation index (U37K′) in core sediment recovered from the Gulf of Guinea, eastern equatorial Atlantic. Mg/Ca‐ and U37K′‐based SST estimates yield fairly comparable results for the time interval 21,000–14,500 years and for the late Holocene. The early and middle Holocene, however, are largely characterized by a discrepant trend, with warm Mg/Ca and cold U37K′ based SST estimates. This discrepant SST trend is accompanied by low sea surface salinity estimates (high riverine runoff) and biogenic sediment, which is characterized by high biogenic opal content, low carbonate content, and relatively low alkenone concentration. We hypothesize that the discrepancy in the reconstructed SSTs during the middle and early Holocene presumably suggests a period of elevated riverine input of dissolved silica and dominantly siliceous phytoplankton bloom in a low saline and warm surface water, while alkenone producers were likely prevalent in a season of cold SST and low riverine silica input. This study suggests that changes in the local hydrography and nutrient input have strong influence on the U37K′‐based SST estimate that may be unraveled by combining different SST proxies.