Oceanic conditions in the eastern equatorial Pacific during the onset of ENSO in the Holocene

Records from South America show that modern ENSO (El Nino-Southern Oscillation) did not exist 7000 cal yr B.P. and has developed progressively since then. There has been little information available on oceanic conditions in the eastern equatorial Pacific (EEP) to constrain explanations for ENSO onset. We report quantitative observations on thermocline and mixed-layer conditions in the EEP during ENSO start up. We found important changes in both the thermocline and the mixed layer, indicating increased upwelling of cooler waters since 7000 cal yr B.P. This resulted from change in the source and/or properties of waters supplying the Equatorial Undercurrent, which feeds upwelling along the equator and the Peru margin. Modeling shows that ENSO is sensitive to subsurface conditions in the eastern equatorial Pacific and that the changes in the thermocline we observed were driven by extratropical processes, giving these a role in conditioning the development of ENSO. This is in contrast to models that call for control of equatorial Pacific oceanography by tropical processes only. These infer stronger upwelling and cooler surface waters for the EEP during the mid-Holocene, which is not supported by our results.     

Late Pleistocene history of the North Pacific: Evidence from a quantitative study of radiolaria in core V21-173

Objective quantitative estimates of paleo-oceanographic conditions in the North Pacific can be made by analyses of radiolarian assemblages. With appropriate computation, transfer functions developed in a study of surface sediments can be used to estimate oceanographic conditions in cores containing late Pleistocene radiolarian faunas. Analysis of core V21-173 indicates that conditions as warm as the Holocene were rare during the past 800,000 yr, and that the region experienced marked near-surface temperature drops correlative with Caribbean and continental records for the past 250,000 yr. A major world-wide warm event at about 400,000 yr is also indicated.     

Radiolaria as paleoclimatic indicators: Pleistocene climatic fluctuations in the equatorial Pacific Ocean

Late Pleistocene radiolarian assemblages were examined in two cores (V24-58 and RC11-209) from the eastern equatorial Pacific Ocean, using Nigrini's 1970 method of recurrent group analysis. The radiolarian “climatic index” curves obtained by this analysis were in close agreement with fluctuations in carbonate levels established previously for the same cores. This correspondence suggests that equatorial radiolarian assemblages changed significantly in response to Pleistocene climatic oscillations. The radiolarian “climatic indices” of the two cores showed close agreement with each other. Our data indicate that equatorial radiolarian assemblages may prove useful for regional correlations between cores, and for paleoclimatic interpretations.     

The relation of the southern trade winds to upwelling processes during the last 75,000 years

In the equatorial Pacific, between the Galapagos Islands and the coast of South America, two kinds of upwelling of oceanic waters occur. One is related to coastal upwelling and the other to surfacing of the Equatorial Undercurrent. Both of those processes are associated with the development of the southeast trade winds blowing in this area. Coastal upwelling is increased when the trade winds are intensified, and the surfacing of the Equatorial Undercurrent occurs when the trades weaken. The development of coastal upwelling and the surfacing of the Equatorial Undercurrent are inferred from the radiolarian assemblages in the sediments. The abundance of quartz, opal, and radiolarian assemblages in the deep-sea sediments of this area, as well as the distance from the sample locations to land and to the quartz source, is correlated with the intensity of the trade winds (in February and August) through multiple regression analysis. The chronostratigraphy of core V1929 (3°35′S, 83°56′W), used in this study, is inferred on basis of its δ¹⁸0 record. During the last 75,000 years, the fluctuations in intensity of the trade winds have been concurrent with or preceded the fluctuations in the amount of ice stored on the continents. In general, the wind velocity of the winter trades has been intensified during cool climatic stages of the earth (δ¹⁸0 stages 4 and 2) and they have been relaxed during warm stages (δ¹⁸0 stages 3 and 1). Seasonal contrast of the trade winds has also fluctuated within time, having been relatively high during the upper part of δ¹⁸0 stage 3.     

Diatoms in Southeast Pacific surface sediments reflect environmental properties

Diatom abundance and assemblage composition determined for 47 surface sediment samples from the Southeast Pacific (50°S–15°N), combined with existing data for the Peru and Chile margins, demonstrate responses to regional temperature, upwelling, and productivity. High diatom abundances (# valves/g) mark the eastern equatorial Pacific upwelling and the coastal upwelling areas, in particular the upwelling centers off Peru. Freshwater diatoms reflect the low-salinity tongue off the Chilean fjords. Diatom species composition distinguishes between coastal and eastern equatorial Pacific upwelling conditions, and records sea-surface temperatures. Q-mode factor analysis defines five floral assemblages. Factors 1 and 4 determined by the genus Chaetoceros (F1) and Thalassionema (F4) reflect coastal and equatorial upwelling conditions, respectively. Factors 2 and 3 characterized by the genus Thalassiosira and Azpetia nodulifera can be associated with El Niño conditions. A 5th factor, described by Paralia sulcata, records a near-shore upwelling center off Point Concepción, central Chile. Statistical transfer functions relate diatom species percentages to sea-surface temperature and productivity with error estimates of ±0.9 °C and ±23 gC/m² yr, respectively, and provide new tools for estimating past temperature and productivity along the west coast of South America.     

热带东太平洋晚第四纪的古海洋及古气候记录

热带东太平洋8001柱状样晚第四纪以来放射虫标志种Buccinospharea invaginata和Collosphaera tuberosa的研究发现, 它们的初现面(FAD) 出现于柱深22cm和45cm处, 年龄分别为0.21Ma和0.42Ma.它们的初现面年龄与SPECMAP氧同位素记录结合为该柱建立了地层框架.该柱放射虫丰度、生源蛋白石和有机碳含量、粒度的分析显示, 大约在氧同位素11、9~8.4、5~3和1期, 高的放射虫总丰度、第四纪常见种丰度和始新世-渐新世种丰度大致对应于高的中值粒径和平均粒径, 以及大约氧同位素5期以来增加的有机碳含量, 表明此期间贸易风增强, 搬运的陆源颗粒增大, 北赤道洋流加强, 导致上升流增强, 表层水生产力提高, 同时, 南极底层流的活动也加强.大约在氧同位素10、8.3~6和2期, 低的放射虫总丰度、第四纪常见种丰度和始新世-渐新世种丰度大致对应于减小的中值粒径和平均粒径, 反映此期间贸易风减弱, 搬运的陆源颗粒减小, 北赤道洋流减弱, 导致上升流消失, 表层水生产力降低, 同时, 南极底层流的活动也减弱.      

Pacific Pleistocene paleoclimatic stratigraphies: A comparative analysis of results

Global atmospheric and oceanic circulation effects of expansion of continental ice sheets initiated upwelling in the western equatorial Pacific and simultaneously intensified upwelling in the eastern equatorial Pacific; contraction of the ice sheets reversed the process. Published Pleistocene paleoclimatic stratigraphies correlated across the entire equatorial Pacific exhibit eight such cycles in the Brünhes epoch. The extrapolated chronostratigraphies for the equatorial Pacific compare favorably with published paleoclimatic schemes for the southeastern, eastern, and northern Pacific Ocean. The Southern Ocean record exhibits fewer, and possibly also consistently older, climatic variations. A time progression or lack of synchrony of marine Pleistocene climatic events is not inconsistent with modified “Milankovitch” hypotheses, such as that involving Antarctic ice-sheet instability.     

Paleoceanography of the Santa Barbara Basin during the last 8000 years

The varved sediments of the Santa Barbara Basin off southern California, offer a unique opportunity to study the changes in oceanographic conditions of this nearshore area during the last 8000 yr. Quantitative analysis of Radiolaria found in recent surface sediment samples from the eastern North Pacific allows the identification of four “assemblages” which can be related to the physical oceanography of the California Current. Two assemblages are associated with the southerly flowing California Current, one with the main stream of the current (California Current Assemblage) and the other with the offshore flow along northern California (Central Assemblage). The two other assemblages are associated with the subtropical region of the eastern North Pacific (Subtropical Assemblage) and one found mostly off the coast of Baja California (Baja Assemblage). Analysis of the Radiolaria found in the varved sediments of a core from the Santa Barbara Basin give an 8000-yr continuous record of these four assemblages. The California and Baja Assemblages show only minor fluctuations in their importance in the sediments of the Santa Barbara Basin. The California Assemblage, however, shows a steady increase during this time period. Prior to 5400 yr B.P. the Radiolaria were predominately subtropical in character, whereas after 5400 yr B.P. the Central Assemblage becomes more important. Since 5400 yr B.P. most of the changes in the radiolarian fauna consist of fluctuations in the importance of these two assemblages. Past sea-surface temperatures for the month of February were calculated using the transfer function technique of J. Imbrie and N. G. Kipp (1971, In “The Late Cenozore Glacial Ages” L. K. Turekian, Ed.), (Chap. 5, Yale Univ. Press, New Haven, Conn.). The time series of paleotemperature estimates show major changes in the average February temperature of Santa Barbara Basin waters. The range of estimated temperatures (12°C) exceeds that of the historical observations of February temperatures in the Santa Barbara Basin but does not exceed the observed range for the California Current region. The intervals from 800 to 1800 yr B.P. 3600 to 3800 yr B.P. and 5400 to the end of the record appear to have been generally warmer than today. Comparison of the Holocene record of alpine glacial advances with the radiolarian assemblage and paleotemperature time series shows that the initiations of advances was coincident with a decrease in sea-surface temperatures and an increase in the importance of the Central Assemblage in the Santa Barbara Basin. The terminations of these advances were not marked by any consistent characteristic in the Santa Barbara Basin time series.     

A prolonged warm and humid interval during marine isotope stage 13–15 as revealed by hydrographic reconstructions from the South China Sea (IMAGES MD972142)

Variations of orbital-driven insolation, ice volume, and greenhouse gas effects have been proposed as major controlling factors in determining the timing and amplitude of Quaternary cyclic climate changes. However, it remains to be determined how the internal feedback in the tropical atmosphere and ocean and the coupling between the low- and high-latitude systems may have produced instability or non-cyclic changes in the long-term climate evolution. Such “abnormalities” have been reported increasingly from paleoclimatic reconstructions in East Asia and the western Pacific for a prolonged warm and humid climate interval during marine isotope stage (MIS) 13–15, ～475–610 kya. To better address the climate abnormality in MIS 13–15 that has been observed in the western Pacific, here we report high-resolution late Quaternary planktic foraminifer faunal abundance and faunal sea surface temperature (SST) records from the International Marine Past Global Change (IMAGES) program core MD972142, which was retrieved from the southeastern South China Sea (SCS). Our results indicate that the faunal assemblages and SSTs in the southeastern SCS express a substantially prolonged, unusual warm interglacial-type climate condition in MIS 13–15. The climate was abnormally warm during the cold MIS 14. Our study also suggests a lowering of sea surface salinity (SSS) during MIS 13–15. While the western Pacific climate experienced a persistently warm and humid period at MIS 14, a “normal” cooling (～2 °C) condition on the surface of the eastern equatorial Pacific existed concomitantly. While assessing possible interpretations of this “abnormal” climate interval in MIS 13–15, our study indicates that an enhanced interhemispheric and/or longitudinal temperature gradient across the basin-wide Pacific cannot be ruled out. A change in the sensitivity of the East Asian Monsoon (EAM) that controlled temperature and precipitation patterns in East Asia and the western Pacific is probably also important or responsible for this climate abnormality. Based on our evidence, we suggest that the tropical dynamics would have played a role in the climate abnormality in MIS 13–15, through maintaining or even increasing the longitudinal SST gradient in the equatorial Pacific, which may have intensified the low-latitude trade winds in the eastern component of the Walker Circulation that drove a longer duration and/or stronger intensity of the summer vs. winter EAM.     

A transfer function for estimating paleoceanographic conditions based on deep-sea surface sediment distribution of radiolarian assemblages in the South Atlantic

A quantitative analysis of radiolarian species in 57 deep-sea surface sediment samples from the South Atlantic Ocean produced four geographically distinct assemblages (tropical, polar, gyre margin, and subtropical). The distributions of these assemblages or factors coincide with present-day patterns of sea-surface temperatures or water masses.These four assemblages were used to construct a transfer function relating radiolarian distribution in the surface sediments to present-day winter and summer temperatures using standard regression techniques. As a test of the quality of this transfer function, temperatures were estimated on surface sediment samples from the eastern South Pacific. The temperatures produced by the transfer function compared favorably with the observed (present-day) winter and summer sea-surface temperatures at these sites.     

南海表层沉积物中放射虫多样性与丰度的分布与环境

采用定量分析方法, 较系统地探讨了整个南海不同环境表层沉积物中放射虫的分布特征及其环境指示意义.结果显示: 南海表层沉积物中放射虫的种类多样性和个体丰度随着水深而增加, 表明南海较深水处的放射虫壳体溶解现象不明显; 上升流活动和海底火山喷发有利于放射虫的大量繁殖, 产生区域性的沉积富集; 反之, 较深水的陆坡区沉积物中出现放射虫种类与丰度异常减少, 可能属浊流搬运再沉积的结果, 物源来自陆架或上陆坡; 种类组成以热带-亚热带特征为主, 还包含了一定数量的冷水种或极区种, 表明在特定海区的古环境分析中采用单一指标判断可能会造成误解.沉积物中放射虫的综合指标较好地反映了南海的生态与沉积环境特征.      

Comparison of glacial and interglacial oceanographic conditions in the South Atlantic from variations in calcium carbonate and radiolarian distributions

Temperature estimates produced by a radiolarian-based transfer function, factor distributions of radiolarian assemblages, and variations in calcium carbonate were used to reconstruct the oceanographic conditions in the South Atlantic during the last glacial maximum (18,000 yr B.P.). This study suggests that while the position of the Subtropical Convergence at 18,000 yr B.P. was very similar to its present position, the Antarctic Polar Front shifted northward 1° to 3° of latitude in the eastern South Atlantic and 3° to 5° of latitude in the western South Atlantic. The largest temperature changes occurred in the subantarctic region and along the eastern portion of the Subtropical Gyre.     

Late Pleistocene-Holocene oceanographic changes in the Northeastern Pacific

The distributions of the radiolarian assemblages in the Northeastern Pacific Ocean were determined and correlated with the average summer temperature of the near surface waters of this region. These assemblages were compared with those in three sediment cores taken beneath the Transition Zone waters. This comparison indicates that the assemblage off Oregon at the last maximum cold interval (24,000 yr B.P.) was like that now found off southern Alaska. The correlation of the radiolarian assemblages with temperature gives an estimate of 11°C for the average summer temperature at that time. This is approximately 4°C cooler than present day conditions in the area. Superimposed on the general warming trend that began 24,000 y.a., there are minor oscillations in the assemblages which correspond to estimates of temperature change of about 2°C in the Pleistocene and about 1°C in the Holocene. In the Holocene, these minor warm intervals appear to be approximately synchronous with advances in mountain glaciers.     

东亚冬季冷暖变化与El Ni?o事件

近十年来,东亚冬季气温明显升高。本文分析证明,东亚温度变化与赤道东太平洋和北太平洋重要洋流区的海表面温度(SST)有密切的遥相关。如果赤道东太平洋SST异常偏高(或异常偏低),即出现埃尔尼诺(或反埃尔尼诺)事件,则在其后7～24个月中,我国华北和东北将出现暖冬(或冷冬),渤海冰情将出现轻冰年(或重冰年)。据此建立的回归预报方程,对1988/1989年和1989/1990年冬季渤海海冰的预报结果完全正确。     

南海表层沉积物中放射虫的组合特征与海洋环境

南海拥有较齐全的边缘海地理地貌与生态环境, 沉积物中保存的放射虫种类繁多, 个体丰富.本文采用数理统计与聚类分析方法, 对表层沉积物中放射虫的组合特征进行了较为详细的定量分析, 以期获得可靠的放射虫组合与分布的特征.聚类分析结果显示, 放射虫组合在南海表层沉积物中的分布大致可区分为5种类型: 陆架浅水区型、陆坡珊瑚礁区型、中央海盆区型、南部富营养区型和夏季上升流区型, 它们分别对应于特有的海洋环境, 并由含有某些优势种类的放射虫群落所组成.进一步分析还表明, 南海的北部、中部和南部分别表现为3个不同生态环境的海区: 北部海区受陆源影响较为明显; 中部海区被中央水团、海底火山活动和夏季上升流活动所控制; 南部海区总体上属于典型的珊瑚礁海洋环境, 生态与沉积环境明显优越于北部海区.南海表层沉积物中放射虫的组合分布特征较好地响应和反映了所在海区的生态环境与沉积条件, 是探讨古海洋环境的重要依据.      

A comparison of radiolarian and foraminiferal paleoecology in the Southern Indian Ocean: New evidence for the interhemispheric timing of climatic change

The deglacial transition between oxygen-isotope Stages 6 and 5e (about 127,000 yr B.P.) is marked by both oxygen isotopic depletion and estimated sea-surface temperature (SST) increase in two subantarctic Indian Ocean cores. The data show synchroneity between warming of foraminifera-based SST estimates and depletion of δ¹⁸O, but an earlier warming trend on the basis of radiolarian SST estimates. These data have been previously interpreted to indicate that the high-latitude Southern Ocean warms prior to significant melting of glacial-age ice sheets. Comparison of core-top assemblages with surface and subsurface conditions in the Southern Indian Ocean reveals that (1) a three-part foraminiferal zonation reflects the surface hydrographic regime, with abrupt faunal transitions at two major fronts: the Subtropical Convergence (STC) and the Antarctic Polar Front (APF); and (2) a two-part radiolarian zonation coincides with a two-part subsurface hydrographic regime, with an abrupt faunal transition corresponding to the southern terminus of subtropical lower water (STLW) between the STC and the APF. It is suggested that shifts of these surface and subsurface regimes are recorded by these foraminiferal and radiolarian assemblages. In this interpretation, the observed lead of radiolarian SST with respect to δ¹⁸O indicates an early response to a southward shift of STLW, while the later foraminiferal SST warming indicates a southward shift of the STC. Thus, the origin of the Southern Hemisphere SST lead may be related to STLW, which emanates from the subtropical gyres, rather than the polar regions.     

Global surface temperature in relation to northeast monsoon rainfall over Tamil Nadu

The local and teleconnective association between Northeast Monsoon Rainfall (NEMR) over Tamil Nadu and global Surface Temperature Anomalies (STA) is examined using the monthly gridded STA data for the period 1901–2004. Various geographical regions which have significant teleconnective signals associated with NEMR are identified. During excess (deficient) NEMR years, it is observed that the meridional gradient in surface air temperature anomalies between Europe and north Africa, in the month of September is directed from the subtropics (higher latitudes) to higher latitudes (subtropics). It is also observed that North Atlantic Oscillation (NAO) during September influences the surface air temperature distribution over north Africa and Europe. Also, the NAO index in January shows significant inverse relationship with NEMR since recent times. The central and eastern equatorial Pacific oceanic regions have significant and consistent positive correlation with NEMR while the western equatorial region has significant negative correlation with NEMR. A zonal temperature anomaly gradient index (ZTAGI) defined between eastern equatorial Pacific and western equatorial Pacific shows stable significant inverse relationship with NEMR     

Plio-Pleistocene palaeoceanography of the tropical Indian Ocean (ODP Hole 709C): Radiolarian and CaCO3 evidence

Sediments spanning the Olduvai subchron (1.79–1.95 Ma) of ODP Hole 709C in the tropical Indian Ocean were sampled at 5-cm intervals. High-resolution quantitative radiolarian and CaCO₃% data have been used as proxies in interpreting palaeoceanographical conditions, an a comparison made with previously obtained oxygen isotope data. Radiolarian data were subjected to principal components analysis, resulting in four varimax factors. Factor 1 accounted for the greatest variation and comprised well-know warm-water and cold-water taxa, with high +ve and - ve scores respectively, which were used in the construction of a temperature index. Results indicate a tripartite temporal division of Olduvai palaeoceanography. In the early Olduvai, prior to 1.895 Ma, and the late Olduvai, approximating the Plio-Pleistocene boundary at 1.81 Ma, divergence-driven equatorial upwelling and high-productivity coincide with isotope-defined ‘interglacials’, and with generally low CaCO₃%. However, during the mid-Olduvai, upwelling, productivity and sedimentation rates were all reduced. These changes in palaeoceanography are thought to be linked directly to eccentricity modulated variations in the precession of the Earth's orbit. The early and late Olduvai coincide with periods of minimum eccentricity, when precessional reinforcemnet of the southwest monsoon is weakest, coastal upwelling along the Oman margin is reduced, and zonal wind-driven equatorial divergence upwelling is increased. The mid-Olduvai, however, corresponds with an eccentricity maximum, during which monsoonal activity is stronger and equatorial divergence upwelling weak.     

Modeling North American Freshwater Runoff through the Last Glacial Cycle

The Northern Hemisphere ice sheets decayed rapidly during deglacial phases of the ice-age cycle, producing meltwater fluxes that may have been of sufficient magnitude to perturb oceanic circulation. The continental record of ice-sheet history is more obscured during the growth and advance of the last great ice sheets, ca. 120,000–20,000 yr B.P., but ice cores tell of high-amplitude, millennial-scale climate fluctuations that prevailed throughout this period. These climatic excursions would have provoked significant fluctuation of ice-sheet margins and runoff variability whenever ice sheets extended to mid-latitudes, giving a complex pattern of freshwater delivery to the oceans. A model of continental surface hydrology is coupled with an ice-dynamics model simulating the last glacial cycle in North America. Meltwater discharged from ice sheets is either channeled down continental drainage pathways or stored temporarily in large systems of proglacial lakes that border the retreating ice-sheet margin. The coupled treatment provides quantitative estimates of the spatial and temporal patterns of freshwater flux to the continental margins. Results imply an intensified surface hydrological environment when ice sheets are present, despite a net decrease in precipitation during glacial periods. Diminished continental evaporation and high levels of meltwater production combine to give mid-latitude runoff values that are highly variable through the glacial cycle, but are two to three times in excess of modern river fluxes; drainage to the North Atlantic via the St. Lawrence, Hudson, and Mississippi River catchments averages 0.356 Sv for the period 60,000–10,000 yr B.P., compared to 0.122 Sv for the past 10,000 yr. High-amplitude meltwater pulses to the Gulf of Mexico, North Atlantic, and North Pacific occur throughout the glacial period, with ice-sheet geometry controlling intricate patterns of freshwater routing variability. Runoff from North America is staged in the final deglaciation, with a stepped sequence of pulses through the Mississippi, St. Lawrence, Arctic, and Hudson Strait drainages.     

Late Quaternary Upwelling Intensity and East Asian Monsoon Forcing in the South China Sea

Foraminifera from two cores off eastern Vietnam and the northwestern Philippines, where modern summer and winter monsoon-driven upwelling occurs in the South China Sea, respectively, were analyzed to evaluate the changes in paleoproductivity and upper water structure over the last 220,000 yr. We observed enhanced organic carbon flux and a shoaled thermocline when upwelling intensified off eastern Vietnam during interglacial ages and off the northwestern Philippines during glacial ages. This indicates that the East Asian summer monsoon increased while the winter monsoon decreased during interglacial ages. Particularly, the upwelling reached a maximum off eastern Vietnam during late marine isotopic stage (MIS) 5 and off the northwestern Philippines during MIS 2, implying that the summer monsoon decreased gradually since MIS 5 while the winter monsoon displayed an opposite trend. The variations in upwelling proxies exhibit a distinct cyclicity with frequencies near 41,000 yr and 23,000 yr off eastern Vietnam, in contrast to a strong frequency peak near 100,000 yr off the northwestern Philippines. We suggest that the East Asian summer monsoon has been forced by changes in solar insolation associated with precession and obliquity, while ice-volume forcing is probably a primary factor in determining the strength and timing of the East Asian winter monsoon but with less important insolation forcing.