Benthic Foraminifera Accumulation Rates Research Articles

Biotic responses of deep-sea benthic foraminifera in the equatorial Indian Ocean during the Quaternary: Influence of the ballasting effect on organic matter by calcareous plankton skeletons

We investigated Quaternary benthic foraminifera in core GPC1803 in the eastern equatorial Indian Ocean since ~740 ka to examine the relationship between benthic foraminiferal fauna and ballasting of particulate organic matter (POM) by calcareous plankton skeletons. Because plankton skeletons affect POM sinking in the water column as ballasts, ballasting of POM by biominerals may play an important role to linkage between the surface and deep-sea paleoceanography. We consider this scheme, based on faunal analysis of benthic foraminifera. From multidimensional scaling (MDS) of fossil benthic foraminifera, we recognized two MDS axes. Based on the scores of MDS axis 1, we subdivided our study period into five intervals and identified two major faunal transitions at ~370 ka and ~ 210 ka. After ~210 ka, the food supply to the seafloor increased with an increase in the past flux of calcareous plankton skeletons. A positive correlation between the benthic foraminifera accumulation rate and the carbonate mass accumulation rate existed during ~600–370 ka and after ~210 ka. It is reasonable to suppose that the increase in calcareous plankton skeletons stimulated more efficient vertical transport of POM through the water column due to the positive ballasting effect on POM. The ~370–210 ka period seemed to represent a transitional phase in food supply. Prior to ~370 ka, alternations among three seasonal food supply-related species were generally observed along with glacial–interglacial variations probably under lower food supply, compared with the after ~210 ka period; however, alternations among these three species became obscure and intermittent after ~370 ka. The nearly continuous common occurrences of these seasonal food supply-related species before ~370 ka might have been enhanced by a lower past flux of calcareous plankton skeletons. Scenario of ballasting of POM by biomineral might explain long-term changes of carbonate corrosivity in the Indian Ocean, similarly in the Atlantic and eastern equatorial Pacific Oceans, during the mid-Brunhes dissolution interval. • Quaternary deep-sea benthic foraminifera in the equatorial Indian Ocean • Positive relationship between BFAR and carbonate MAR after ~210 ka • Common occurrences of seasonal food supply-related species before ~370 ka • Changes in the influence of ballasting of POM by calcareous plankton

Paleoceanography of the Strait of Hormoz and its link to paleoclimate changes since the mid-Holocene

This paper presents the paleocurrent evolution of the Persian Gulf deep water (PGDW) during the last 5000 years using faunal (foraminiferal assemblage) and non-faunal (magnetic susceptibility, sediment grain size, number of coarse relict particles and physicochemical characteristics) indicators from a 133 cm long core retrieved from the Strait of Hormoz (SH). Our results suggest that the precession-related insolation change and solar variability induced long-term and short-term sea surface temperature (SST) rise and fall resulted in variation of PGDW outflow strength. Two periods of decreasing SST (5000–4300 and 2000–1450 cal yr BP), accompanied by wetter conditions in the Middle East, are related to the reduction of the PGDW current velocity. A combination of high sedimentation rate (0.72 mm yr−1, due to high terrigenous input) with a relatively low bottom flow energy cause the settling of finer sediments providing a favourable environment for the expansion of benthic foraminifera (dominated by infaunal taxa). On the contrary, the PGDW current velocity strengthens in the periods of SST increase (4300–2000 and 1450 cal yr BP to present) with decreasing sedimentation rate (0.15–0.34 mm yr−1, due to low river discharge). During both periods of high-velocity bottom currents, the benthic foraminifera accumulation rate decreases. However, as the PGDW brings a relatively high amount of dissolved oxygen (3.25 mg/l in summer and 4.79 mg/l in winter) and phytodetritus particles, compared to the surrounding water, some particular foraminifera (especially robust suspension feeder epifaunal taxa) evolved. SST increase, intensified by anthropically caused global warming, may lead to PGDW intensification in the future.

Holocene variations in North Atlantic export productivity as reflected in bathyal benthic foraminifera

Abstract The subpolar NE Atlantic Ocean experiences seasonal fluxes of labile organic matter (phytodetritus) which are expected to strongly influence the composition of benthic foraminiferal assemblages and benthic foraminiferal accumulation rates. We studied export production over the last 12 kyr at a sampling resolution of approximately 250–300 years through an investigation of bathyal benthic foraminiferal assemblages (>63 μm) at Ocean Drilling Program (ODP) Site 980 on the Feni Drift (55°29′N, 14°42′W, 2179 m water depth). During the last 12 kyr, faunas at Site 980 were dominated (~75%) by Cassidulina obtusa, Nonionella iridea, Bolivina difformis, Trifarina pauperata, Alabaminella weddellensis, Stainforthia fusiformis, Cassidulina laevigata and Eilohedra vitrea. The absolute and relative abundances and diversity of these and other species varied significantly. In the interval ~12–10 ka, A. weddellensis, S. fusiformis and T. pauperata had higher % abundance (named here ‘H10 species’), but this is not reflected in a higher accumulation rate, suggesting that surface productivity was low, at highly variable conditions. Species at lower % abundance during this time include B. difformis, C. laevigata, C. obtusa, E. vitrea and N. iridea (so-called ‘L10 species’). The ‘8.2 ka cold event’ was characterized by increased carbonate dissolution (reflected in decreases in the absolute abundance, benthic foraminifera accumulation rate, weight % coarse fraction, and presence of poorly preserved/fragmented benthic foraminifera). Peaks in the relative abundance of species which, in our opinion, exploited phytodetritus (‘phytodetritus species’: N. iridea, A. weddellensis, C. obtusa, and rare Epistominella exigua) occurred at 8.0 ka, 7.0 ka, 6.3–5.6 ka, 4.7 ka, 4.3–3.4 ka and 2.4 ka. These peaks generally correspond to peaks in absolute abundance (number of specimens per gram, accumulation rate), indicating increases in the seasonality of export productivity. However, the ‘phytodetritus species’ do not covary in absolute and relative abundance over the studied interval, suggesting that they have somewhat different ecological requirements. There appears to be no simple relationship between changes in the degree of seasonality of export productivity (i.e., abundance of ‘phytodetritus species’) and records of palaeoclimatic/palaeoceanographic proxies, suggesting that bentho-pelagic coupling (arrival of food on the seafloor with local surface productivity) might not have been straightforward in this region. Site 980 is located in the hydrodynamically active area of Feni Drift, and during the Holocene, currents might have winnowed and removed fine-grained organic matter, making it unavailable to benthic organisms. Alternatively, there may have been changes in remineralization and/or mid-water competition for food, so that the fraction of the organic flux that reached the seafloor may have varied. Holocene benthic foraminiferal assemblages thus reflect highly dynamic conditions in export productivity and arrival of organic matter at the seafloor.

EVOLUTION OF EARLY PLEISTOCENE DEEP WATER CIRCULATION IN WESTERN PACIFIC: EVIDENCE FROM BENTHIC FORAMINIFERA

Benthic foraminifera picked out from the interval of 43.02~26.27 mbsf(meters below seafloor) of ODP 807(3°36.42′N,156°37.49′E,water depth 2 803.8 m,length 822.9 m)on the Ontong-Java Plateau,western Pacific was studied for understanding the changes in paleoproductivity and its relation with deep water circulation during the period of 2.5~1.6 MaBP.The benthic foraminifera accumulation rate(BFAR),Epifaunal/Infaunal ratio(E/I),percentage of Uvigerina spp.and total organic carbon(TOC%) were used as the proxies of paleoproductivity in this study.Results show that the change in paleoproductivity was generaly high in glacials but low in interglacials during the period of 2.5~1.6 MaBP.,except Bulimina alazanensis,of which the paleopruductivity was low in glacials but high in interglacials.It may suggest that B.alazanensis,opposite to the others,preferred a warmer environment with lower nutrients.The benthic foraminiferal fauna was dominated by Uvigerina spp.in the samples,indicating that the western Pacific deep water was under the control of the North Pacific Deep Water Mass during Early Pleistocene.

Early Oligocene glaciation and productivity in the eastern equatorial Pacific: Insights into global carbon cycling

The onset of sustained Antarctic glaciation across the Eocene‐Oligocene transition (EOT) marks a pivotal change in Earth's climate, but our understanding of this event, particularly the role of the carbon cycle, is limited. To help address this gap we present the following paleoceanographic proxy records from Ocean Drilling Program Site 1218 in the eastern equatorial Pacific (EEP): (1) stable isotope (δ18O and δ13C) records generated in epifaunal benthic foraminifera (Cibicidoides spp.) to improve (double the resolution) the previously published records; (2) δ18O and δ13C records measured on Oridorsalis umbonatus, a shallow infaunal species; and (3) a record of benthic foraminifera accumulation rate (BFAR). Our new isotope data sets confirm the existence at Site 1218 of a two‐step δ18O increase. They also lend support to the hypothesized existence of a late Eocene transient δ18O increase and early Oligocene Oi‐1a and Oi‐1b glacial maxima. Our record of BFAR indicates a transient (∼500 kyr) twofold to threefold peak relative to baseline Oligocene values associated with the onset of Antarctic glaciation that we attribute to enhanced biological export production in the EEP. This takes the same general form as the history of opal accumulation in the Southern Ocean, suggesting strong high‐to‐low‐latitude oceanic coupling. These findings appear to lend support to the idea that the EOT δ13C excursion is traceable to increased organic carbon (Corg) burial. Paradoxically, early Oligocene sediments in the EEP are extremely Corg‐poor, and proxy records of atmospheric pCO2 indicate a transient increase associated with the EOT.

The Late Quaternary evolution of water masses in the eastern Indian Ocean between Australia and Indonesia, based on benthic foraminifera faunal and carbon isotopes analyses

Benthic foraminifera and carbon isotopes from closely spaced samples taken from three deep-sea cores were analysed to reconstruct the palaeoceanographic evolution of the eastern Indian Ocean for the last 30 kyrs, with an extension back to 60 kyrs based on one core. Benthic foraminifera were studied by means of Q-mode Factor Analysis. The benthic foraminifera accumulation rate (BFAR) and the accumulation rates (AR) of Bulimina aculeata, Epistominella exigua and Uvigerina proboscidea were calculated for determining episodes of increased organic matter supply to the sea floor. The δ 18O and δ 13C records of Cibicidoides wuellerstorfi were measured from all 3 cores to gather information about past intermediate- and deep-water circulation and changes in sea-surface palaeoproductivity. The co-variance of the organic matter supply and dissolved-oxygen levels affected the distribution of benthic foraminifera. Below a depth of 1800 m, reduced deep-water circulation was recognised by a low δ 13C of C. wuellerstorfi and increased carbon-flux rate by a high BFAR and B. aculeata, E. exigua and U. proboscidea AR, as well as by a B. aculeata faunal dominance. A more oligotrophic environment was characterised by a low BFAR and B. aculeata, E. exigua and U. proboscidea AR. Active deep-water circulation was postulated with high δ 13C values for C. wuellerstorfi and by a faunal dominance by C. wuellerstorfi. At intermediate depths (∼ 1000 m) and south of 20°S, the presence of strong bottom currents and the lateral advection of small amounts of organic matter, favoured the suspension feeder C. wuellerstorfi. Under extremely high dissolved-oxygen levels, determined by the increased influence of the Antarctic Intermediate Water (seen through high δ 13C of C. wuellerstorfi) and a reduced supply of organic matter, Nummoloculina irregularis and Globocassidulina subglobosa dominated the benthic foraminifera assemblage. The reduction of oxygen levels, a more stratified water column and the Leeuwin Current flow, along its present pattern, favoured the species U. proboscidea and Bolivina robusta. Based on these observations, the following palaeoceanographic evolution of the eastern Indian Ocean is proposed: • For 60–35 kyr BP, conditions of higher productivity (compared to the Present) at the sea surface were suggested for the Banda Sea. • For 35– 15 kyr BP, high productivity still characterised the Banda Sea. Strong and oxygenated bottom currents were present offshore Western Australia. For the Last Glacial Maximum, a reduction of deep-water circulation characterised the eastern Indian Ocean, while more active circulation was recorded at intermediate depths. Productivity also increased offshore the north coast of Western Australia • For 15–5 kyr BP, a reduction in productivity levels over the Banda Sea is related to increased atmospheric precipitation, which led to the formation of a low-salinity water cap at the sea surface. Off the north coast of Western Australia, productivity levels remained similar to those recorded for the LGM, probably due to the nutrients injected into the sea by rivers. Off the northwest coast of Western Australia, a deepened nutricline prevented any increase of organic matter supply to the sea floor. • For 5 kyr BP–Present, in the Banda Sea, productivity levels are similar to the Present. Off the Western Australian coast, an increased influence of the oxygen-depleted Indonesian Intermediate Water and the Leeuwin Current engendered a more stratified water column, characterised by lower dissolved-oxygen levels.

Benthic foraminifera and bottom water evolution in the middle-southern Okinawa Trough during the last 18 ka

A piston sediment core E017 from the middle-southern Okinawa Trough was investigated. A preliminary study of the deep-water evolution since 18 cal. ka BP was performed based on the quantitative census data of benthic foraminiferal fauna, together with planktonic foraminiferal oxygen and carbon isotope, AMS(14)C dating, and the previous results achieved in the southern Okinawa Trough. The result shows that the benthic fauna was dominated by Bulimina aculeata (d'Orbigny), Uvigerina peregrina (Cushman), Hispid Uvigerina and Uvigerina dirupta (Todd) during the glaciation-deglaciation before 9.2 cal. ka BP, while Epistominella exigua (Brady), Pullenia bulloides (d'Orbigny), Cibicidoides hyalina (Hofker), Sphaeroidina bulloides (d'Orbigny) and Globocassidulina subglobosa (Brady) predominated the fauna in the post-glacial period after 9.2 cal. ka BP. The benthic foraminifera accumulation rate (BFAR), paleoproductivity estimates and benthic foraminiferal assemblage conformably indicate that surface water paleoproductivity and organic matter flux during the glaciation-deglaciation were higher than those of the post-glacial period in the middle-southern Okinawa Trough, and gradually enhanced from the southern to the central Okinawa Trough during the glaciation-deglaciation, which could be caused by the discrepancy of the terrigenous nutrients supply. High abundances of E exigua, an indicator of pulsed organic matter input, after 9.2 cal. ka. BP may indicate that the intensity of seasonally riverine pulsed flux during the post-glacial period was stronger than that of the glaciation-deglaciation period, and the seasonal influx in the central trough might be stronger than in the south. The temporal distributions of the typical species indicating bottom water oxygen content and ventilation condition show that the ventilation of the bottom water during the post-glacial period is more active than the glaciation-deglaciation, which reflects that the evolution of the intermediate and deep waters of the northwestern Pacific during the last glaciation has no evident influence on the deep-water of the middle-southern Okinawa Trough. Additionally, the variations in agglutinated benthic foraminiferal abundance and other carbonate dissolution proxies indicate that carbonate dissolution gradually increased since the last 18 ka in the Okinawa Trough and rapidly enhanced at 9.2 cal. ka BP. The modern shallow carbonate lysocline could form at 3 cal. ka BP.

Foraminiferal responses to upwelling variations in the South China Sea over the last 220 000 years

Late Quaternary benthic and planktonic foraminiferal and isotopic data were obtained from two sediment cores in the present upwelling areas of the South China Sea: Core 17928 off northwestern Luzon and Core 17954 off eastern Vietnam. Quantitative analyses of foraminifera in the two cores revealed the foraminiferal responses to paleoceanographic changes over the last 220 000 years. High productivity species such as Neogloboquadrina dutertrei, Bulimina acculeata, Uvigerina peregrina, Melonis barleeanus and Chilostomella oviodea displayed high abundance during interglacial periods in Core 17954 and during glacial periods in Core 17928. Correspondingly, variations in benthic foraminifera accumulation rates, the ratio of infaunal to epifaunal benthic foraminifera and the presence of the benthic foraminifers Cibicidoides, Miloilids and Oridorsalis umbonatus, all indicate that bottom water was oxygen-rich and oligotrophic during glacial periods off Vietnam (Core 17954) but such bottom conditions occurred during interglacial periods off Luzon (Core 17928). Sea-surface temperature results from planktonic foraminifera transfer function and variations in the abundance of planktonic foraminifera, typical tropical species Globigerina sacculifer, tropical–subtropical species Globorotalia menardii, Globigerinoides ruber, and Pulleniatina obliquiloculata, transition species Globorotalia inflata and Arctic species Neogloboquadrina pachyderma, indicate an increase in cool waters during marine isotope stage (MIS) 5, except for MIS 5.5, and glacial intervals in the cores 17954 and 17928, respectively. The ratios of mixed-layer/thermocline dwelling species and depth of thermocline (DOT) estimates using transfer function suggest that the DOT was shallower during the interglacial than the glacial periods off Vietnam (Core 17954), but the DOT was shallower during the glacial periods off Luzon (Core 17928). Our results show that in the glacial periods lower sea surface temperature, shoaled thermocline, and high productivity increased off northwestern Luzon (Core 17928), and that, on the contrary, similar event occurred in the interglacial off Vietnam (Core 17954). These differences in the upper water column structure and productivity in the two areas were induced by coastal upwelling, which in turn is driven by monsoons. The intensity of upwellings is also closely related to the evolution of the eastern Asian monsoon. Therefore, we conclude that differences in the upwelling intensity in the two sites indicate strengthening of winter and summer monsoon during the glacial and interglacial periods, respectively.

Last glacial paleoproductivity patterns in the eastern equatorial Pacific: benthic foraminifera records

This study explores the export-productivity changes for the eastern equatorial region between 100 and 115°W during the Last Glacial with the aid of benthic foraminifera accumulation rates (BFAR) from the well-dated Pleiades (PLDS) core collection (Berger and Killingley, 1981) and compares these results with western equatorial Pacific ones. We present some new data for the calibration between BFAR and the organic carbon flux to the sea-floor and discuss some of the limitations of using absolute values for this type of reconstruction. Contrary to expectation, not all export productivity to the deep ocean under the equatorial Pacific responds with the same magnitude to changes in ocean circulation during the Last Glacial period. Two export-productivity patterns emerge from this reconstruction based on benthic foraminifera accumulation. Sites slightly North of the eastern equatorial Pacific (≈1°N) show a minor increase in the organic carbon flux to the sea-floor, up to a factor of 1.2, while the reconstructed BFAR values are very similar to western equatorial Pacific ones. One of the implications of this finding is a decreased east–west equatorial export-productivity gradient relative to the modern one. In contrast, sites south of the equator (≈3°S) show more than a double-fold increase in the accumulation rates of benthic foraminifera, equivalent to an increase by a factor of 1.6–1.8 in the organic carbon flux to the sea-floor. Although there is not enough data to decide unambiguously among the different mechanisms that may have forced this pattern of export productivity, we propose that while upwelling at the equator may have been stronger during the Last Glacial, its east–west gradient was significantly decreased with respect to the modern one. Either a southern migration of the equatorial divergence zone or a relative decrease in the nutrient concentration of the upwelled waters 1°N of the equator may explain the observed lower export-productivity gradient. To the south, stronger advection of coastal upwelled nutrient-rich waters of southern origin may have enhanced the production and export of organic carbon to the deep sea, which fueled the observed higher abundance in benthic foraminifera.

Deep-sea benthic foraminifera and biogenic opal: Glacial to postglacial productivity changes in the western equatorial Pacific

The accumulation rate of benthic foraminifera in deep-sea sediments is strongly correlated with the productivity of the surface ocean. Core tops from diverse deep-sea settings straddling different export production levels are used in this study for the calibration between the organic carbon flux to the seafloor ( J sf-flux) and the benthic foraminifera accumulation rate ( BFAR). The correlation can be used to reconstruct organic carbon flux to the seafloor in the past, and hence the productivity of past oceans. These results are compared with the accumulation rates of carbonate and biogenic opal in the sediments. It is found that biogenic opal is more sensitive to productivity changes in the modern ocean than is carbonate. The appropriate equations are derived for the equatorial Pacific, using ☐ core material from Eurydice (ERDC), Indomed (INMD), and Pleiades (PLDS) expeditions. These equations are then applied to two selected time horizons in sediments from the Ontong Java Plateau, a western equatorial Pacific pelagic depth transect, the last transition ( LT), and the last glacial ( LG). It is found that productivity was higher during the last glacial relative to the late Holocene, by a factor between 1.6 and 2. During the last transition productivity was intermediate between glacial and recent values. We do not see any corresponding increase in the rates of accumulation of carbonate or biogenic opal during the last glacial. This observation questions the importance of ocean productivity as the main controlling factor of carbonate and biogenic opal accumulation rates back through geological time.

A note on the contribution of rime and surface hoar to the accumulation on the Ross ice shelf, Antarctica

Late Quaternary benthic and planktonic foraminiferal and isotopic data were obtained from two sediment cores in the present upwelling areas of the South China Sea: Core 17928 off northwestern Luzon and Core 17954 off eastern Vietnam. Quantitative analyses of foraminifera in the two cores revealed the foraminiferal responses to paleoceanographic changes over the last 220 000 years. High productivity species such as Neogloboquadrina dutertrei, Bulimina acculeata, Uvigerina peregrina, Melonis barleeanus and Chilostomella oviodea displayed high abundance during interglacial periods in Core 17954 and during glacial periods in Core 17928. Correspondingly, variations in benthic foraminifera accumulation rates, the ratio of infaunal to epifaunal benthic foraminifera and the presence of the benthic foraminifers Cibicidoides, Miloilids and Oridorsalis umbonatus, all indicate that bottom water was oxygen-rich and oligotrophic during glacial periods off Vietnam (Core 17954) but such bottom conditions occurred during interglacial periods off Luzon (Core 17928). Sea-surface temperature results from planktonic foraminifera transfer function and variations in the abundance of planktonic foraminifera, typical tropical species Globigerina sacculifer, tropical–subtropical species Globorotalia menardii, Globigerinoides ruber, and Pulleniatina obliquiloculata, transition species Globorotalia inflata and Arctic species Neogloboquadrina pachyderma, indicate an increase in cool waters during marine isotope stage (MIS) 5, except for MIS 5.5, and glacial intervals in the cores 17954 and 17928, respectively. The ratios of mixed-layer/thermocline dwelling species and depth of thermocline (DOT) estimates using transfer function suggest that the DOT was shallower during the interglacial than the glacial periods off Vietnam (Core 17954), but the DOT was shallower during the glacial periods off Luzon (Core 17928). Our results show that in the glacial periods lower sea surface temperature, shoaled thermocline, and high productivity increased off northwestern Luzon (Core 17928), and that, on the contrary, similar event occurred in the interglacial off Vietnam (Core 17954). These differences in the upper water column structure and productivity in the two areas were induced by coastal upwelling, which in turn is driven by monsoons. The intensity of upwellings is also closely related to the evolution of the eastern Asian monsoon. Therefore, we conclude that differences in the upwelling intensity in the two sites indicate strengthening of winter and summer monsoon during the glacial and interglacial periods, respectively.

Thermal conduction across fracture zones and the gravitational edge effect : LOUDEN K. E. and D. W. FORSYTH, 1976. J. geophys. Res., 81 (26): 4869–4874

Late Quaternary benthic and planktonic foraminiferal and isotopic data were obtained from two sediment cores in the present upwelling areas of the South China Sea: Core 17928 off northwestern Luzon and Core 17954 off eastern Vietnam. Quantitative analyses of foraminifera in the two cores revealed the foraminiferal responses to paleoceanographic changes over the last 220 000 years. High productivity species such as Neogloboquadrina dutertrei, Bulimina acculeata, Uvigerina peregrina, Melonis barleeanus and Chilostomella oviodea displayed high abundance during interglacial periods in Core 17954 and during glacial periods in Core 17928. Correspondingly, variations in benthic foraminifera accumulation rates, the ratio of infaunal to epifaunal benthic foraminifera and the presence of the benthic foraminifers Cibicidoides, Miloilids and Oridorsalis umbonatus, all indicate that bottom water was oxygen-rich and oligotrophic during glacial periods off Vietnam (Core 17954) but such bottom conditions occurred during interglacial periods off Luzon (Core 17928). Sea-surface temperature results from planktonic foraminifera transfer function and variations in the abundance of planktonic foraminifera, typical tropical species Globigerina sacculifer, tropical–subtropical species Globorotalia menardii, Globigerinoides ruber, and Pulleniatina obliquiloculata, transition species Globorotalia inflata and Arctic species Neogloboquadrina pachyderma, indicate an increase in cool waters during marine isotope stage (MIS) 5, except for MIS 5.5, and glacial intervals in the cores 17954 and 17928, respectively. The ratios of mixed-layer/thermocline dwelling species and depth of thermocline (DOT) estimates using transfer function suggest that the DOT was shallower during the interglacial than the glacial periods off Vietnam (Core 17954), but the DOT was shallower during the glacial periods off Luzon (Core 17928). Our results show that in the glacial periods lower sea surface temperature, shoaled thermocline, and high productivity increased off northwestern Luzon (Core 17928), and that, on the contrary, similar event occurred in the interglacial off Vietnam (Core 17954). These differences in the upper water column structure and productivity in the two areas were induced by coastal upwelling, which in turn is driven by monsoons. The intensity of upwellings is also closely related to the evolution of the eastern Asian monsoon. Therefore, we conclude that differences in the upwelling intensity in the two sites indicate strengthening of winter and summer monsoon during the glacial and interglacial periods, respectively.