Min-Te Chen

DEEP SEA AND LAKE RECORDS OF THE SOUTHEAST ASIAN PALEOMONSOONS FOR THE LAST 25 THOUSAND YEARS

Abstract High resolution paleoceanographic proxy records from a deep-sea core in the northern South China Sea (SCS) and palynological records from a lake core in central Taiwan have been measured for the last 25 kyr. The variations in these proxy records are interpreted in terms of the past variation in the Asian monsoons. The two consistent and complementary records suggest that the East Asian monsoon system has fluctuated significantly from a strengthened winter monsoon during the last glaciation (25,000-12,000 yr BP) through a moderate to weak winter and summer monsoons during the deglaciation (12,000-10,000 yr BP) to an enhanced summer monsoon in the Holocene. Strengthening of the winter monsoon during the glacial is deduced from decreases in winter SST and increases in productivity indicators, such the abundances of alkenones and total organic carbon in the SCS. A stronger summer monsoon during the Holocene is inferred from the increases in floral productivity, the sedimentation rate of peat, the deposition of storm-related clay layers in the lake sediment, and the higher abundance of thermophyte Castanopsis. The 25 kyr paleomonsoon history registered in the SCS core and the Taiwan lake sediments is in agreement with the climatic records of the Chinese Loess plateau.

A 450-kyr record of hydrological conditions on the western Agulhas Bank Slope, south of Africa

Abstract The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200–250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.

500 000-Year records of carbonate, organic carbon, and foraminiferal sea-surface temperature from the southeastern South China Sea (near Palawan Island)

High-resolution records of planktic foraminifer sea-surface temperature (SST) and biogenic sediment components of carbonate and total organic carbon (TOC) concentrations were determined in an IMAGES giant piston core spanning Vthe last 500 000 years, taken near the western slope of Palawan Island in the southeastern South China Sea (SCS). The records provide information of paleoceanographic and paleoclimatological variations linked to East Asian monsoon systems in the SCS, the largest marginal sea of the western Pacific. Constrained by planktic foraminifer (Globigerinoides ruber) oxygen isotope stratigraphies, the records show a lowering of faunal SSTby V3‡C during glacial stages, indicating significant cooling in the glacial western Pacific climate. In general, they show lowfrequency patterns with high SSTs, high carbonate content, and low TOC content during interglacial periods, and exhibit low SSTs, low carbonate content, and high TOC content during glacial periods. The carbonate content variations indicate that the sediment composition is mostly controlled by terrigenous inputs, which are associated with sea-level fluctuations in the SCS during past glacial^interglacial stages. The low SST and high TOC content indicate cooling and high productivity conditions in the surface oceans of the SCS, which also reflect a condition of intensified winter monsoon winds associated with glacial boundary conditions. Some rapid, high-frequency oscillations of the SSTand T OC found in the records are coincident with intervals of intensified winter or summer monsoons from the Arabian Sea, implying that the Asian monsoon systems had wider regional effects than previously assumed. Timeseries analyses reveal that variations in the SST, carbonate and TOC contents of this record contain statistically significant concentrations of variance at orbital frequency bands, namely 100 kyr 31 , 41 kyr 31 , and 23 kyr 31 , suggesting that both ice volume and orbital solar insolation changes are potential mechanisms for the SCS monsoon variations.

Ice‐volume forcing of winter monsoon climate in the South China Sea

High-resolution studies of a planktonic foraminifer core record from the South China Sea (SCS) (31KL: 18°45.4′N, 115°52.4′E, water depth 3360 m) reveal changes driven by ice-volume forcings in the climate of the East Asian monsoon in the western Pacific marginal sea during the late Quaternary. The analyses of planktonic foraminifer faunal abundance data from the core indicate significant variations in the relative abundances of the dominant taxa over the past 100,000 years since the isotope stage 5. The transfer function estimates of faunal sea surface temperatures (SST) correlate well with a long-term (104–105 years) trend of global glaciation. About 65,000 years ago, there was an observable change in the mode of SST variability as many low-latitude records have shown. These findings suggest that the SCS surface circulation and the East Asian winter monsoon systems are mainly driven by variations in global glaciation levels. The association of surface ocean cooling in the SCS with global climatic events suggests that fluctuations in the strength of the East Asian winter monsoon may be linked to shifts in the latitudinal position of the westerly winds and the Siberian high-pressure system.

Reconstructing the southern South China Sea upper water column structure since the Last Glacial Maximum: Implications for the East Asian winter monsoon development

difference in the stable oxygen isotopes (Dd 18 O) and Mg/Ca‐based temperatures (DT) of surface‐dwelling (G. ruber) and thermocline‐dwelling (P. obliquiloculata) planktonic foraminifera and the temperature difference between alkenone‐ and P. obliquiloculata Mg/Ca‐based temperatures to estimate the upper ocean thermal gradient at International Marine Past Global Change Study (IMAGES) core MD01‐2390. Estimates of the upper ocean thermal gradient were used to reconstruct mixed layer dynamics. We find that our Dd 18 O estimates are biased by changes in salinity and, thus, do not display a true upper ocean thermal gradient. The D To fG. ruber and P. obliquiloculata as well as the alkenone and P. obliquiloculata suggest increased surface water mixing during the late glacial, likely due to enhanced EAWM winds. Surface water mixing was weaker during the late Holocene, indicating a weaker influence of winter monsoon winds. The weakest winter monsoon activity occurred between 6.5 ka and 2.5 ka. Inferred EAWM changes since the Last Glacial Maximum coincide with EAWM changes as recorded in Chinese loess sediments. We find that the intensity of the EAWM and the East Asian summer monsoon show an inverse behavior during the last glacial and deglaciation but covaried during the middle to late Holocene.

25,000-year late Quaternary records of carbonate preservation in the South China Sea

Pelagic sediments recovered from the South China Sea (SCS) exhibit characteristic carbonate preservation fluctuations that are closely linked to major cycles of climatic change. Two deep sea cores (SCS 90-36: 17°59.70′N, 111°29.64′E, water depth 2050 m; and 31-KL: 18°45.4′N, 115°52.4′E, 3360 m) that are located respectively above and below the regional lysocline (∼ 3000 m) were selected for studying the late Quaternary pattern of carbonate preservation in the SCS. Preservation indices measuring changes in the abundances of dissolution-resistant planktonic foraminifers and coccoliths, and in ratios of foraminiferal fragments, benthonic foraminifers, and radiolarians, have revealed increased carbonate preservation during the last glacial period and/or during the last deglaciation (Termination I; ∼ 12,000 k.y.), but stronger carbonate dissolution during the interglacial period of the Holocene. Detailed comparisons of preservation and oxygen isotope records further indicate that the level of carbonate preservation increased steadily from 25 k.y. through Termination I and that preservation reached a maximum during Termination I. From Termination I into the Holocene, however, the level of preservation decreased gradually to a minimum. Both records exhibit noticeable glacial/interglacial variations (1f on the order of 104 years), while the higher resolution record of 31-KL records a series of higher frequency oscillations (1f on the order of 103 years). These data confirm the observation that maximal carbonate preservation tends to occur during the later half of glacial stages as shown previously in many Indian and Pacific deep sea core studies.

Recent planktonic foraminifers and their relationships to surface ocean hydrography of the South China Sea

The relative abundances of Recent planktonic foraminifers from surface sediments on the sea floor reveal relationships to hydrographic variables of the ocean surface. To investigate whether the eVect of the vertical structure of oceanic upper-layer environments is important to the changes in the abundance of planktonic foraminifers, a set of 173 coretop faunal data from modern South China Sea (SCS) surface sediments was compiled for comparison with direct observations of sea-surface temperature (SST ) and the depth of thermocline (DOT ). The coretops used in this study are distributed within the area of 25°N and 105°‐125°E, with water depths ranging from 68 to 3990 m. The relationships between the abundances of planktonic foraminifer species and hydrographic variables were examined using simple correlation analyses. In the analyses, four groups of planktonic foraminifers with diVerent ecological preferences were identified: Group I ‐ faunas primarily reliant on DOT, with positive correlations (N. dutertrei), Group II ‐ faunas reliant on both SST and DOT, showing negative correlations with SST and positive correlations with DOT (G. glutinata, P. obliquiloculata, and G. bulloides, Group III ‐ faunas reliant on both SST and DOT, showing positive correlations with SST and negative correlations with DOT (including G. sacculifer, G. menardii, and G. aequilateralis), and Group IV ‐ faunas reliant on both SST and DOT, with positive correlations (G. ruber). Though each of these species displays selectivity of ecological controls, these results indicate that in the SCS, correlations between faunal abundances of planktonic foraminifers and SST or DOT are about equally significant. This study also indicates a need for re-evaluating the relationships between planktonic foraminifer abundances and many other important hydrographic variables in the upper-layer of oceans on a more regional scale.

Warm pool hydrological and terrestrial variability near southern Papua New Guinea over the past 50k

Here,we presenta 50k archiveof U37′ seasurface temperature (SST), planktic foraminifera oxygen isotopes, and terrestrial input indicators including branched and isoprenoid tetraether (BIT) biomarkers, 232 Th activity, and non‐biogenic sediment components recorded in core MD052928 from the WPWP (near southern Papua New Guinea, PNG). The planktic foraminifer oxygen isotopes in the core show millennial‐scale changes indicating fresher seawater during the NH cold periods (i.e., Heinrich Events, HEs) and suggesting hydrological changes that are most likely linked to the strength of the boreal winter Asian‐Australian monsoon (AAM). Our observations are corroborated by evidence from the same core that indicates increased terrestrial input caused by higher precipitation on land and more river runoff from southern PNG during the cold periods. Consistent with other nearby hydrological records from land, our study indicates persistent millennial‐scale hydrological changes within the past 50k in the western tropical Pacific and Southeast Asia. The timing of the millennial‐scale changes appears to have been determined by the latitudinal displacement of the Intertropical ConvergenceZone(ITCZ)thatreflectsahistoryofheattrans

Marine records of East Asian monsoon variability over the past 5 Ma

Abstract Marine sedimentary cores retrieved from the western Pacific provide important clues for deciphering how the East Asian Monsoon (EAM) system has evolved during the past 5 Ma. Here we briefly review some recent progress on the reconstructions of the EAM based on marine SST (sea surface temperature), SSS (sea surface salinity), and productivity records from the SCS (South China Sea) and ECS (East China Sea) and their implications for EAM evolution and variability on tectonic, orbital and millennial timescales. This review highlights the importance of high resolution sampling on giant marine cores (such as cores collected with the International Marine Past Global Change, IMAGES program) that provide opportunities for better defining the timing and amplitude of the EAM variability expressed in marine records. We also discuss possible future directions of EAM palaeoclimatic and palaeoceanographic studies that require development of multiple new marine EAM proxies and a comparison of the marine records with the stalagmite records on land.

Sedimentology, foraminiferal assemblage, and isotope record of the Agulhas Bank region

The Agulhas Bank region, south of Africa, is an oceanographically important and complex area. The leakage of warm saline Indian Ocean water into the South Atlantic around the southern tip of Africa is a crucial factor in the global thermohaline circulation. Foraminiferal assemblage, stable isotope and sedimentological data from the top 10 m of core MD962080, recovered from the western Agulhas Bank Slope, are used to indicate changes in water mass circulation in the southeastern South Atlantic for the last 450 kyr. Sedimentological and planktonic foraminiferal data give clear signals of cold water intrusions. The benthic stable isotope record provides the stratigraphic framework and indicates that the last four climatic cycles are represented (i.e. down to marine isotope stage (MIS) 12). The planktonic foraminiferal assemblages bear a clear transitional to subantarctic character with Globorotalia inflata and Neogloboquadrina pachyderma (dextral) being the dominant taxa. Input of cold, subantarctic waters into the region by means of leakage through the Subtropical Convergence, as part of Agulhas ring shedding, and a general cooling of surface waters is suggested by increased occurrence of the subantarctic assemblage during glacial periods. Variable input of Indian Ocean waters via the Agulhas Current is indicated by the presence of tropical/subtropical planktonic foraminiferal species Globoquadrina dutertrei, Globigerinoides ruber (alba) and Globorotalia menardii with maximum leakage occurring at glacial terminations. The continuous presence of G. menardii throughout the core suggests that the exchange of water from the South Indian Ocean to the South Atlantic Ocean was never entirely obstructed in the last 450 kyr. The benthic carbon isotope record and sediment textural data reflect a change in bottom water masses over the core location from North Atlantic Deep Water to Upper Southern Component Water. Planktonic foraminiferal assemblages and sediment composition indicate a profound change in surface water conditions over the core site approximately 200-250 kyr BP, during MIS 7, from mixed subantarctic and transitional water masses to overall warmer surface water conditions.