Heinrich event imprints in the Okinawa Trough: evidence from oxygen isotope and planktonic foraminifera

Planktonic foraminiferal faunas, oxygen isotope and modern analog technique sea surface temperature records were obtained in piston core DGKS9603 (28degrees08.869'N, 127degrees16.238'E, water depth 1100 in) collected from the middle Okinawa Trough. During the last glaciation, four cold events were identified and correlate Heinrich events (HE) H2-5 of the last 45 ka. During the last deglaciation, core DGKS9603 has begun to be influenced by the Kuroshio since about 16 cal ka BP. Three weakenings of this warm current occurred at about 2.8-5.3, 11.4 and 15.5 cal ka BP respectively. Among the three fluctuations, the oldest one is synchronous with HE1 and could be a response to the strong cooling observed in the North Atlantic Ocean. The fluctuation occurring at about 11.4 cal ka ago corresponds to the Younger Dryas within the age error bars. Our observations provide new evidence that the HEs documented from Greenland and the northern North Atlantic had a global climatic impact. Changes in the intensity of the East Asian monsoon could be the main mechanism responsible for the paleoccanographic variations observed in the Okinawa Trough. (C) 2001 Elsevier Science B.V. All rights reserved.

Flux and fate of Yangtze river sediment delivered to the East China Sea

Numerous cores and dating show the Yangtze River has accumulated about 1.16 x 10(12) t sediment in its delta plain and proximal subaqueous delta during Holocene. High-resolution seismic profiling and coring in the southern East China Sea during 2003 and 2004 cruises has revealed an elongated (similar to 800 km) distal subaqueous mud wedge extending from the Yangtze River mouth southward off the Zhejiang and Fujian coasts into the Taiwan Strait. Overlying what appears to be a transgressive sand layer, this distal clinoform thins offshore, from similar to 40 in thickness between the 20 and 30 m water depth to < 1-2 in between 60 and 90 m water depth, corresponding to an across shelf distance of less than 100 km. Total volume of this distal mud wedge is about 4.5 x 10(11) m(3), equivalent to similar to 5.4 x 10(11) t of sediment. Most of the sediment in this mud wedge comes from the Yangtze River, with some input presumably coming from local smaller rivers. Thus, the total Yangtze-derived sediments accumulated in its deltaic system and East China Sea inner shelf have amounted to about 1.7 x 10(12) t. Preliminary analyses suggest this longshore and across-shelf transported clinoform mainly formed in the past 7000 yrs after postglacial sea level reached its mid-Holocene highstand, and after re-intensification of the Chinese longshore current system. Sedimentation accumulation apparently increased around 2000 yrs BP, reflecting the evolution of the Yangtze estuary and increased land erosion due to human activities, such as fanning and deforestation. The southward-flowing China Coastal Current, the northward-flowing Taiwan Warm Current, and the Kuroshio Current appear to have played critical roles in transporting and trapping most of Yangtze-derived materials in the inner shelf, and hence preventing the sediment escape into the deep ocean. (c) 2006 Elsevier B.V. All rights reserved.

Development of the East Asian monsoon: Mineralogical and sedimentologic records in the northern South China Sea since 20 Ma

We here reconstruct the past change of the East Asian monsoon since 20 Ma using samples from Ocean Drilling Program (ODP) Site 1146 in the northern South China Sea based On a multi-proxy approach including a monomineralic quartz isolation procedure, identification of clay minerals by X-ray Diffraction (XRD) and grain-size analysis of isolated terrigenous materials. Terrigenous supply to ODP Site 1146 was dominated by changes in the strength of multiple sources and transport processes. Grain-size data modeled by an end-member modeling algorithm indicate that eolian dust from the and Asian inland and fluvial input have contributed on average 20% and 80% of total terrigenous material to ODP Site 1146, respectively. Specifically, about 40-53% of the total (quartz+feldspar) and only 6-11% of the total clay is related to eolian supply at the study site. Detailed analysis of the sedimentary environment, and clay minerals combined with previous studies shows that smectite originates mainly from Luzon, kaolinite from the Pearl River and illite and chlorite from the Pearl River, Taiwan and/or the Yangtze River. The proportion and mass accumulation rate (MAR) of the coarsest end-member EM1 (interpreted as eolian dust), ratios of (illite+chlorite)/smectite, (quartz+feldspar)% and mean grain-size of terrigenous materials at ODP Site 1146 were adopted as proxies for East Asian monsoon evolution. The consistent variation of these independent proxies since 20 Ma shows three profound shifts in the intensity of East Asian winter monsoon relative to summer monsoon, as well as aridity of the Asian continent, occurred at similar to 15 Ma, similar to 8 Ma and the youngest at about 3 Ma. In comparison, the summer monsoon intensified contemporaneously with the winter monsoon at 3 Ma. The phased uplift of the Himalaya-Tibetan plateau may have played a significant role in strengthening the Asian monsoon at similar to 15 Ma, 8 Ma and 3 Ma. (C) 2007 Elsevier B.V. All rights reserved.

Paleoenvironmental change in the middle Okinawa Trough since the last deglaciation: Evidence from the sedimentation rate and planktonic foraminiferal record

Well-dated, high-resolution records of planktonic foraminifera and oxygen isotopes from two sediment cores, A7 and E017, in the middle Okinawa Trough reveal strong and rapid millennial-scale climate changes since similar to 18 to 17 thousand years before present (kyr B.P.). Sedimentation rate shows a sudden drop at similar to 11.2 cal. kyr B.P. due to a rapid rise of sea level after the Younger Dryas (YD) and consequently submergence of the large continental shelf on the East China Sea (ECS) and the retreat of the estuary providing sediment to the basin. During the last deglaciation, the relative abundance of warm and cold species of planktonic foraminifera fluctuates strongly, consistent with the timing of sea surface temperature (SST) variations determined from Mg/Ca measurements of planktonic foraminifera from one of the two cores. These fluctuations are coeval with climate variation recorded in the Greenland ice cores and North Atlantic sediments, namely Heinrich event 1 (H1), Bolling-Allerod (B/A) and YD events. At about 9.4 kyr B.P., a sudden change in the relative abundance of shallow to deep planktonic species probably indicates a sudden strengthening of the Kuroshio Current in the Okinawa Trough, which was synchronous with a rapid sea-level rise at 9.5-9.2 kyr B.P. in the ECS, Yellow Sea (YS) and South China Sea (SCS). The abundance of planktonic foraminiferal species, together with Mg/Ca based SST, exhibits millennial-scale oscillations during the Holocene, with 7 cold events (at about 1.7, 2.3-4.6, 6.2, 7.3, 8.2, 9.6, 10.6 cal. kyr BP) superimposed on a Holocene warming trend. This Holocene trend, together with centennial-scale SST variations superimposed on the last deglacial trend, suggests that both high and low latitude influences affected the climatology of the Okinawa Trough. (c) 2006 Elsevier B.V. All rights reserved.

Evolution and variability of the East Asian summer monsoon during the Pliocene: Evidence from clay mineral records of the South China Sea

The Late Pliocene is thought to be characterized by the simultaneous intensification of both the East Asian winter monsoon (EAWM) and East Asian summer monsoon (EASM). However, the evolution of the EASM during the Pliocene remains still controversial and only little is known about the dynamics of the EASM during the Pliocene on orbital time scales. Here we use clay mineral assemblages in sediments from Ocean Drilling Program (ODP) Site 1143 in the southern South China Sea (SCS) to obtain proxy records of past changes in the EASM climate during the Pliocene. Provenance analysis suggests that illite, chlorite and kaolinite originated mainly from the Mekong River drainage area. Smectite was derived mainly from the Indonesian islands. The kaolinite/illite ratio and the chemical index of alteration (CIA) of siliciclastic sediments allowed us to reconstruct the history of chemical weathering and physical erosion of the Mekong River drainage area and thus, the evolution of,the EASM during the Pliocene. Our clay minerals proxy data suggests a stronger EASM during the Early Pliocene than during the Late Pliocene. We propose that the long-term evolution of the EASM has been driven by global cooling rather than the uplift of the Tibetan Plateau. Spectral analysis of kaolinite/ illite ratio displays a set of strong periodicities at 100 ka, 30 ka, 28 ka, 25 ka, and 22 ka. with no clear obliquityrelated signal. Our study suggests that the Pliocene EASM intensity on orbital time scales is not only controlled by the Northern Hemisphere summer insolation, but also strongly influenced by equatorial Pacific ENSO-like ocean atmosphere dynamics. (C) 2010 Elsevier B.V. All rights reserved.

River mouth bar formation, riverbed aggradation and channel migration in the modern Huanghe (Yellow) River delta, China

This paper addresses the recent (1970s-1990s) processes of river mouth bar formation, riverbed aggradation and distributary migration in the Huanghe River mouth area, in the light of station-based monitoring, field measurements and remote sensing interpretation. The results show that the morphological changes of the river mouth bar have been closely associated with the largely reduced fluvial discharge and sediment load. Landforrn development such as bar progradation occurred in two phases, i.e. before and after 1989, which correspond to faster and lower bar growth rates, respectively. Fast riverbed aggradation in the mouth channel was strongly related to river mouth bar progradation. During 1976-1996, about 2.8% of the total sediment loads were deposited in the river channel on the upper to middle delta. Therefore, the river water level rose by a few meters from 1984 to 1996. The frequent distributary channel migration, which switched the radial channel pattern into the SE-directed pattern in the mid-1980s, was linked with mouth bar formation. Marine conditions also constrain seaward bar progradation. Furthermore, the history of river mouth bar formation reflects human impacts, such as dredging and dyking in order to stabilize the coastal area. (c) 2005 Elsevier B.V. All rights reserved.

Coherence between solar activity and the East Asian winter monsoon variability in the past 8000 years from Yangtze River-derived mud in the East China Sea

AMS(14)C dating and grain-size analysis for Core PC-6, located in the middle of a mud area on the inner shelf of the East China Sea (ECS), were used to rebuild the Holocene history of the East Asian winter monsoon (EAWM). The 7.5-m core recorded the history of environmental changes during the postglacial transgression. The core's mud section (the upper 450 cm) has been formed mainly by suspended sediment delivered from the Yangtze River mouth by the ECS Winter Coastal Current (ECSWCC) since 7.6 kyr BP. Using a mathematical method called "grain size vs. standard deviatioW', we can divide the Core PC-6's grain-size distribution into two populations at about 28 mu m. The fine population (< 28 mu m) is considered to be transported by the ECSWCC as suspended loads. Content of the fine population changes little and represents a stable sedimentary environment in accord with the present situation. Thus, variation of mean grain-size from the fine population would reflect the strength of ECSWCC, which is mainly controlled by the East Asian winter monsoon. Abrupt increasing mean grain size in the mud section is inferred to be transported by sudden strengthened ECSWCC, which was caused by the strengthened EAWM. Thus, the high resolution mean grain-size variation might serve as a proxy for reconstruction of the EAWM. A good correlation between sunspot change and the mean grain-size of suspended fine population suggests that one of the primary controls on centennial- to decadal-scale changes of the EAWM in the past 8 ka is the variations of sun irradiance, i.e., the EAWM will increase in intensity when the number of sunspots decreases. Spectral analyses of the mean grain-size time series of Core PC-6 show statistically significant periodicities centering on 2463, 1368, 128, 106, 100, 88-91, 7678, and 70-72 years. The EAWM and the East Asian summer monsoon (EASM) agree with each other well on these cycles, and the East Asian Monsoon (EAM) and the Indian Monsoon also share in concurrent cycles in Holocene, which are in accord with the changes of the sun irradiance. (c) 2005 Elsevier B.V. All rights reserved.

THE COMPOUND EYE AS AN INDICATOR OF AGE AND SHRINKAGE IN ANTARCTIC KRILL

Laboratory studies have shown that Antarctic krill (Euphausia superba) shrink if maintained in conditions of low food availability. Recent studies have also demonstrated that E. superba individuals may be shrinking in the field during winter. If krill shrink during the winter, conclusions reached by length-frequency analysis may be unreliable because smaller animals may not necessarily be younger animals. In this study, the correlation between the body-length and the crystalline cone number of the compound eye was examined. Samples collected in the late summer show an apparent linear relationship between crystalline cone number and body-length. From a laboratory population, it appears that when krill shrink the crystalline cone number remains relatively unchanged. If crystalline cone number is little affected by shrinking, then the crystalline cone number may be a more reliable indicator of age than body-length alone. The ratio of crystalline cone number to body-length offers a method for detecting the effect of shrinking in natural populations of krill. On the basis of the crystalline cone number count, it appears from a field collection in early spring that E. superba do shrink during winter.

Elemental carbon record of paleofire history on the Chinese Loess Plateau during the last 420 ka and its response to environmental and climate changes

An elemental carbon (EC) record, covering the last 420 ka, was reconstructed using chemical oxidation method on a loess and paleosol sequence from the Lingtai section on the Chinese Loess Plateau. The EC record reveals the paleofire history and its relationship with climate and vegetation changes on the Chinese Loess Plateau. Our results show that the EC abundance is generally higher in the paleosols than in the loess layers, showing a glacial/interglacial pattern, which is coincident with biomass changes. This variation pattern indicates that paleofires were intensified when biomass accumulated and climate changed abruptly especially from wet to dry conditions. The EC abundance increases sharply at similar to 130 kaB.P., indicating a dramatic change in the vegetation and climate variation patterns. The occurrence of a peak value with the highest average EC abundance in the Holocene may reflect the occurrence of a major climate event at similar to 6 kaB.P., and may also be partly due to more frequent anthropogenic fire usages. (c) 2007 Elsevier B.V. All rights reserved.

Main geochemical characteristics and key biogeochemical carbon processes in the East China Sea

Research related to carbon geochemistry and biogeochemistry in the East China Sea is reviewed in this paper. The East China Sea is an annual net sink for atmospheric CO, and a large net source of dissolved inorganic carbon to the ocean. The sea absorbs CO, from the atmosphere in spring and summer and releases it in autumn and winter. The East China Sea is a CO, sink in summer because Changjiang River freshwater flows into it. The net average sea-air interface carbon flux of the East China Sea is estimated to be about 4.3 X 10(6) t/y. Vertical carbon transport is mainly in the form of particulate organic carbon in spring; more than 98% of total carbon is transported in this form in surface water, and the number exceeds 68% in water near the bottom. In the southern East China Sea, the average particulate organic carbon inventory was about one-tenth that of the dissolved organic carbon. Research indicates that the southern Okinawa Trough is an important site for particulate organic carbon export from the shelf. The annual cross-shelf exports are estimated to be 414 and 106 Gmol/y for dissolved organic carbon and particulate organic carbon, respectively. Near-bottom transport could be the key process for shelf-to-deep sea export of biogenic and lithogenic particles.