ALBERT

Description: To understand Holocene climate evolutions in low-latitude region of the western Pacific, paired δ18O and Mg/Ca records of planktonic foraminifer Globigerinoides ruber (250-300 μm, sensu stricto, s.s.) from a marine core ORI715-21 (121.5°E, 22.7°N, water depth 760 m) underneath the Kuroshio Current (KC) off eastern Taiwan were analyzed. Over the past 7500 years, the geochemical proxy-inferred sea surface temperature (SST) hovered around 27-28 °C and seawater δ18O (δ18OW) slowly decreased 0.2-0.4‰ for two KC sites at 22.7° and 25.3°N. Comparison with a published high-SST and high-salinity equatorial tropical Pacific record, MD98-2181 located at the Mindanao Current (MC) at 6.3°N, reveals an anomalous time interval at 3.5-1.5 kyr ago (before 1950 AD). SST gradient between the MC site and two KC site decrease from 1.5-2.0 °C to only 0-1 °C, and δ18OW from 0.1-0.3‰ to 0‰ for this 2-kyr time window. The high SST and low gradient could result from a northward shift of the North Equatorial Current, which implies a weakened KC. The long-term descending δ18OW and increasing precipitation in the entire low-latitude western Pacific and the gradually decreasing East Asian summer monsoonal rainfall during middle-to-late Holocene is likely caused by different land and ocean responses to solar insolation and/or enhanced moisture transportation from the Atlantic to Pacific associated with the southward movement of ITCZ.

Description: An astronomically tuned late Quaternary planktic foraminiferal delta18O record of Site MD972142 (12°41.33'N, 119°27.90'E; 1557 m water depth) in the southeastern South China Sea was established. The difference in delta18O between MD972142 and ODP792 of the Sulu Sea is regarded as a maritime proxy of the summer monsoon intensity over the South China Sea and Southeast Asia. The profile of this maritime proxy matches well with the summer monsoon index obtained from the terrestrial record of Louchuan, central Chinese Loess Plateau. The amplified planktic delta18O signals of the South China Sea relative to the Sulu Sea record are partly caused by the changing intensities of the East Asian Monsoons at the glacial-interglacial time-scale throughout the late Quaternary.

Description: Significantly older 14C ages by 2500-7900 years are found for sedimentary total organic carbon (TOC) when compared to ages of codeposited surface-dwelling foraminifera in the southern Okinawa Trough. This age discrepancy increases with rising sea level since the Last Glacial Maximum. A progressive shift in TOC d13C toward more negative values with rising sea level reflects an increasing fractional contribution of terrestrial organics (soil organics, plant debris, and/or fossil organics) to the buried organic pool. Organic matter previously stored on the East China Sea shelf during sea level lowstand and riverine material from Taiwan may be the sources that cause the d13CTOC to shift to more terrestrial values. During the Holocene when sea level is above -40 m, d13CTOC values stabilize within a narrow range (-22.3 to -22.8 per mil) while age discrepancies continue to increase and less chemically weathered sediments are deposited. The increase in age discrepancy between TOC and foraminifera in the Holocene may be due to a wetter climate that drove higher rates of physical weathering on Taiwan and greater transport rates of fossil organic C-bearing lithogenic sediment to the ocean. The climate impact on the relative delivery of fossil and nonfossil TOC in depositional settings influenced by fluvial sources should be considered in interpretations of sedimentary C isotope records.

Description: The South China Sea is the largest marginal sea of southeastern Asia, lying presently under the influences of the Western Pacific Warm Pool and Asian monsoon systems. Sediment cores from this area provide high-resolution records for interpreting millennial- to centennial-scales paleoclimatic changes expressed in the western Pacific. Here we present results of high-resolution paleoceanographic data including planktic foraminifer fauna sea surface temperature (SST) anddepth of thermocline (DOT) estimates along with foraminifer stable isotopes, alkenone SST estimates analyzed froma core takenfrom the southern South China Sea (SCS) near Wan-An Shallow (IMAGES III 1997 cruise core MD97-2151). The intervals of the record presented here cover the past two glacial Terminations (centering at ab. 12,000 and 128,000 yrs B.P.). Our analyses of SST estimates by using planktic foraminifer transfer functions with paralleling measurements of alkenone SST methods all show events of rapid cooling reversals occurrring during the Termination I concurrent with the Younger Dryas (ab. 13-11 kyr B.P.), and Heinrich events reported previously from GISP2 ice core and North Atlantic core studies. Our reconstructions indicate also that theTermination I inthe southern SCS is characterized by a change of monsoon wind systems, with probably much stronger winter monsoon winds in the glacial period. We also found that during the Termination II, there was no such climatic reversal analogous to the Younger Dryas. During oxygen isotope stage 5, our estimates of SST and DOT, and abundances of deep-dwelling planktic foraminifer species all show large-amplitude variations, indicating an instability of monsoon climate during the interglacial period. Our studies also highlight the climatic teleconnections shown by the linkage of the SCS and other regional records for examples from the East China Sea and Chinese loess.

Description: X-ray fluorescence (XRF) core-scanning is a fast and nondestructive technique to assess elemental variations of unprocessed sediments. However, although the exposure time of XRF-scanning directly affects the scanning counts and total measurement time, only a few studies have considered the influence of exposure time during the scan. How to select an optimal exposure time to achieve reliable results and reduce the total measurement time is an important issue. To address this question, six geological reference materials from the Geological Survey of Japan (JLK-1, JMS-1, JMS-2, JSD-1, JSD-2, and JSD-3) were scanned by the Itrax-XRF core scanner using the Mo- and the Cr-tube with different exposure times to allow a comparison of scanning counts with absolute concentrations. The regression lines and correlation coefficients of elements that are generally used in paleoenvironmental studies were examined for the different exposure times and X-ray tubes. The results show that for those elements with relatively high concentrations or high detectability, the correlation coefficients are higher than 0.90 for all exposure times. In contrast, for the low detectability or low concentration elements, the correlation coefficients are relatively low, and improve little with increased exposure time. Therefore, we suggest that the influence of different exposure times is insignificant for the accuracy of the measurements. Thus, caution must be taken when interpreting the results of elements with low detectability, even when the exposure times are long and scanning counts are reasonably high.

Description: In order to investigate the paleoceanographic record of dissolution of calcium carbonate (CaCO3) in the central equatorial Pacific Ocean, we have studied the relationship between three indices of foraminiferal dissolution and the concentration and accumulation of CaCO3, opal, and Corg in Core WEC8803B-GC51 (1.3°N, 133.6°W; 4410 m). This core spans the past 413 kyr of deposition and moved in and out of the lysoclinal transition zone during glacial-interglacial cycles of CaCO3 production and dissolution. The record of dissolution intensity provided by foraminiferal fragmentation, the proportion of benthic foraminifera, and the foraminiferal dissolution index consistently indicates that the past corrosion of pelagic CaCO3 in the central equatorial Pacific does not vary with the observed sedimentary concentration of CaCO3. Although there is a weak low-frequency variation (~100 kyr) in dissolution intensity, it is unrelated to sedimentary CaCO3 concentration. There are many shorter-lived episodes where high CaCO3 concentration is coincident with poor foraminiferal preservation, and where, conversely, low CaCO3 concentration is coincident with superb foraminiferal preservation. Spectral analyses indicate that dissolution maxima consistently lagged glacial maxima (manifest by the SPECMAP delta18O stack) in the 100-kyr orbital band. Additionally, there is no relationship between dissolution and the accumulation of biogenic opal or Corg or between dissolution and the burial ratio of Corg/CINorg (calculated from Corg and CaCO3). Because previous studies of this core strongly suggest that surface water productivity varied closely with CaCO3 accumulation, both the mechanistic decoupling of carbonate dissolution from CaCO3 concentration (and from biogenic accumulation) and the substantial phase shift between dissolution and global glacial periodicity effectively obscure any simple link between export production, CaCO3 concentration, and dissolution of sedimentary CaCO3.

Description: Abstract: The Intertropical Convergence Zone (ITCZ) encompasses the heaviest rain belt on the Earth. Few direct long-term records, especially in the Pacific, limit our understanding of long-term natural variability for predicting future ITCZ migration. Here we present a tropical precipitation record from the Southern Hemisphere covering the past 282,000 years, inferred from a marine sedimentary sequence collected off the eastern coast of Papua New Guinea. Unlike the precession paradigm expressed in its East Asian counterpart, our record shows that the western Pacific ITCZ migration was influenced by combined precession and obliquity changes. The obliquity forcing could be primarily delivered by a cross-hemispherical thermal/pressure contrast, resulting from the asymmetric continental configuration between Asia and Australia in a coupled East Asian-Australian circulation system. Our finding suggests that the obliquity forcing may play a more important role in global hydroclimate cycles than previously thought.