WEPAMA (IMAGES VII/2001)

Summary of the cruise

The WEPAMA Cruise from Port Hedland (Australia) to Kochi (Japan) on board the French research vessel "Marion Dufresne" took place between 30th April and 18th June 2001. The cruise was divided into two main parts:

• Leg 1 from Port Hedland (Australia) to Keelung (Taiwan)
• Leg 2 from Keelung (Taiwan) to Kochi (Japan)

The ship and scientific technology for the cruise were provided by the IFRTP (Institut Français de Recherche et Technologies Polaires), and the scientific program was strongly supported by a team from the IFRTP under the direction of Yvon Balut. The WEPAMA cruise was organized within the framework of the international IMAGES program, and was partly financed by complementary contributions from China, Germany, Japan and Taiwan. The scientific parties were composed of French, Taiwanese, German, Chinese, Japanese, Russian, Austrian, British, Australian, Belgian, Indonesian and American researchers and students.

The prime objective of the WEPAMA cruise was to recover high resolution seafloor archives to decipher past climatic and oceanographic changes on decadal to millennial timescales over periods ranging from 10 000 to about 1 000 000 years. The principal scientific goals were:

1. to better constrain the variability of the Western Pacific Warm Pool (WPWP), particularly with regard to temperature and geographical extension, on time scales ranging from decadal variations to Milankovitch-type forcing;
2. to better understand changes in heat and salt transfer from the Pacific to the Indian Ocean through the Indonesian archipelago, in relation to glacio-eustatic changes;
3. to analyze the evolution of the east Asian monsoon;
4. to study the origin, characteristics and variations through time of intermediate and deep waters in the marginal seas of the western Pacific (Celebes Sea, Timor Sea, South China Sea) in relationship to glacio-eustatic changes, global circulation evolution (Great Conveyor Belt) and regional climate modifications (i.e. evaporation-precipitation budget linked to the evolution of the east Asian monsoon).

To address these objectives, the WEPAMA cruise program focused on the retrieval of long piston cores and complementary gravity cores, multicores or boxcores in high sedimentation rates areas of the easternmost Indian Ocean-Timor Sea, the marginal Indonesian seas, the South China Sea, East China Sea, Japan Sea and the Sea of Okhotsk as well as from the subarctic northwest Pacific and the continental slope east off Japan . Continuous multibeam mapping or high resolution seismic profiling (3.5 kHz), as well as hydrographic profiling and water sampling were also undertaken. Plankton tows, core tops and sub-surface water samples were collected for the calibration of paleoceanographic and paleoclimatic proxies (SST, SSS reconstructions, Mg/Ca ratio, Uk37, benthic/planktonic foraminifer transfer functions). Measurement of physical properties (MST, colour reflectance), sediment description and photography were systematically performed on board.

Leg 1 Summary and Highlights

During Leg 1 of the WEPAMA cruise, we acquired a total of 26 giant piston cores (mean length 36m, longest core length 44.95m), 3 gravity cores, and 6 multi-cores from the easternmost Indian Ocean, the Banda Sea (Indonesian Archipelago), the northern continental margin of Irian Jaya, the Halmahera area, the Celebes Sea, the South China Sea (SCS), the Okinawa Trough, and the East China Sea. CTDs were deployed at 18 coring stations and plankton tows at 24 stations.

Easternmost Indian Ocean and Timor Sea
Cores were retrieved to study past changes in the Indonesian Throughflow and in particular productivity circulation changes in relation to glacio.eustacy (piston cores MD01-2376 to -2379, multi-cores MD122-MC01 to -MC03).

Banda Sea
Core MD01-2380 was obtained to study the past variability of the WPWP over the past few glacial-interglacial cycles. Sedimentary records from this site will also provide insight into past environmental changes driven by Pacific Intermediate Waters as well as by sea-level fluctuations.

Northern Continental Margin of Irian Jaya
The sedimentary records of gravity core MD01-2381G, piston cores MD01-2382 to -2385, and multi-cores MD122-MC04 to -MC06 will allow reconstruction of the long-term history of the Indo-Pacific Warm Pool and atmospheric convection system since the Last Glacial Maximum (by comparing oceanic SST records to inland temperature changes reconstructed from snow cover limits on Irian Jaya mountains).

Celebes Sea
We retrieved piston cores MD01-2387 and -2388 to study past changes in Pacific intermediate water variations, tropical precipitation and volcanic activity.

South China Sea
Piston cores MD012389 and MD012390, recovered from a water depth transect (1600-2600m) in the southern part of the SCS near the Nansha Islands, will allow study of past sea-level changes (a large part of the sediments originated from the subaerially exposed nearby shelf areas and coral reef banks), riverine inputs of fresh water (the area is close to the mean position of the ITCZ), and changes in WPWP temperature and productivity. Cores MD01-2391 to -2393, located on the eastern slope of the Wan-An Shallow to which the Mekong River supplies large amounts of terrestrial sediments, will allow to reconstruct changes in the summer monsoon over southeastern Asia. In the North of the SCS, piston cores MD01-2394 and -2395 were recovered from a water depth transect (1450-2100m) on the northeastern slope of Vietnam, southwest of the Xisha Islands to provide important controls for the western extremity of the SCS where local upwelling develops during the summer monsoon season. Finally, piston cores MD01-2396 and -2397 in the northernmost part of the SCS, where the Pearl River and other rivers supply abundant terrigenous sediments, will allow to reconstruct past changes in the Asian winter monsoon (surface water productivity at these sites is enhanced by winter monsoon wind stirring and mixing).

In the Okinawa Trough, we collected gravity core MD01-2402G and piston cores MD01-2398 to -2404 to decipher past changes in the Kuroshio activity and their effects in the latitudinal gradient of surface water heat transport (the gravity core will be used to date the opening of the Okinawa Trough). In the East China Sea, we retrieved gravity core MD01-2405G and giant piston core MD01-2406 to study the influence of climate and sea level changes on the deposition history of the East China Sea margin.

The 26 giant piston cores retrieved during the Leg 1 of WEPAMA cruise range from 24.04m to 44.95m in length, averaging nearly 37m. Biostratigraphic analyses performed on board allow calculation of preliminary sedimentation rates. Two coccolith events were used: the disappearance of Pseudoemiliania lacunosa at 460 ka and the first occurrence of Emiliania huxleyi at 260 ka. The occurrence of these two species was only studied in smear slides from the base (core catcher) of each core. Therefore, sedimentation rate estimates provide only upper or lower limit values. Based on biostratigraphic events, sedimentation rates range from <7 cm/kyr for Core MD012399 (age at base >460 ka) to more than 13 cm/kyr for Core MD012388 (age at base < 260 ka). For some cores with a basal age between 260 and 460 ka, the sedimentation rates may be up to 16 cm/kyr, based on the upper age limits given by the biostratigraphic markers (i.e for Cores MD012379, MD012393 and MD012397).

In addition to biostratigraphic data, physical properties and color reflectance measurements performed on board provide stratigraphic logs that can be interpreted in terms of glacial/interglacial oscillations, and can also be used to estimate sedimentation rates by correlation to oxygen isotopic reference curves (Imbrie et al., 1984; Bassinot et al., 1994). In most cases, it is within the color reflectance records that the glacial/interglacial oscillations can be more readily observed and correlated to isotopic reference curves. The physical property data obtained with the MSCL system (magnetic susceptibility, gammadensity, P-wave velocity) provide more complex, puzzling records. At this point, it is not possible to clearly decipher whether the difficulties in recognizing glacial/interglacial oscillations arise from the intrinsic complexity of the physical properties and their lack of clear correspondence to climatic forcing or whether these difficulties are due to problems encountered during MSCL analyses (see "MST" chapter, explanatory notes).

Shorebased analyses will be necessary to fully explore the relationships between physical properties, sediment types and climatic/oceanographic evolution. Correlation of color reflectance data (L* factor) to isotopic stratigraphy was attempted for Cores MD012376, MD012377, MD012378, MD012383, MD012385, MD012389, MD012390, MD012391, MD012392, MD012396, and MD012398. Correlations appear relatively unambiguous for Cores MD012377, MD012378, MD2389, MD2392 and MD2398. In general, there is good agreement between sedimentation rate estimates obtained with this method and estimates based on biostratigraphic data, with the exception of Core MD012377. For this core, biostratigraphy indicates a sedimentation rate higher than 13 cm/kyr, whereas color reflectance correlation to glacial/interglacial changes indicates ~8.3 cm/kyr). Cores MD012383 (~33.9 cm/kyr), MD012385 (~24 cm/kyr), MD012389 (~22.8 cm/kyr) and MD012391 (~16 cm/kyr) display the highest sedimentation rates, and are particularly suitable for high resolution studies on a centennial scale. Cores MD012383 and MD012385 were retrieved within and outside of the Cenderawasih Bay, respectively. Core MD012383 will provide a high resolution history of terrestrial runoff from Irian Jaya, and will offer a key record for assessing changes in atmospheric convection system associated with the Indo-Pacific Warm Pool. Core MD012385, located outside the Cederawasih Bay, will provide a clear high-resolution record of the Warm Pool evolution that is not overly affected by river runoffs. Core MD012389, located on the northern tip of the Sunda Shelf, will offer a high-resolution record of past sea-levels, fresh water riverine inputs, precipitation and tropical Pacific Warm Pool temperature and productivity. Sedimentation rates estimated from this core are in good agreement with those obtained in piston cores from the Marine Geological Survey of Guang-Zhou, China (~20cm/kyr). Finally, Core MD012391, located close to the Mekong River, will provide a high-resolution record of monsoon activity over southeastern Asia.

In conclusion, the first Leg of WEPAMA cruise (IMAGES VII) achieved its major objectives. Long sediment records (average length of 37m) were obtained in key areas with sedimentation rates usually around or greater than 10 cm/kyr, with the highest rates in the order of 20 to 30 cm/kyr. In addition, water sampling, plankton tow, multicore and box-core deployements will allow the precise calibration of proxies used for paleoclimatic and paleoceanographic reconstructions.

Leg 2 Summary and Highlights

Leg 2 of the WEPAMA cruise allowed 24 cores to be acquired from the East China Sea, the Japan Sea, the Okhotsk Sea, the Emperor Sea Mounts and along the east coast of the Japan . All of them were of good quality, with 11 reaching more than 40 m in length, and three reaching more than 50 m in length. Echosounding profiles (3.5 kHz) of exceptional quality recorded during the navigation allowed the coring sites to be selected with high precision.

Most of the cores were shared between working groups with similar interests and common research objectives.

East China Sea
Two cores (MD01-2405G and 2406) were recovered on the East China Sea shelf: one gravity core of 2.62 m, which sampled surface sediments, and a long core of 19.80 m, which reached the Younger Dryas discontinuity. These will allow the calibration of the seismic profiles acquired by IFREMER during the Dong Haï cruise (1996). A gravity core, MD01-2402G was recovered in order to sample ante-rift series for understanding the opening of the Okinawa Basin in its south west part.

In the south part of the East China Sea, two cores (MD012403, MD012404) taken from the upstream Kuroshio areas, will be used to study past changes in Kuroshio circulation patterns during the last 300,000 years. Estimated sedimentation rates are ~ 20-30 cm/kyr for cores MD012403 (36.30 m) and MD012404 (43.67 m), which will allow analyses of orbital and millennial-scale variations of the Kuroshio.

Japan Sea
Core MD012407 (56 m) and MD012408 (33 m) are ideal to investigate phase lag or lead of decadal-to millennial-scale climatic changes between the western Pacific margin and the northern North Atlantic region and to understand the ultimate driving force of the rapid climate changes. MD01-2407 (55.28 m) and MD01-2408 (33.38 m) were recovered from intermediate water depths of 930 m on the Oki Ridge and 800 m off Akita. The alternating layering of the pelagic light and dark colored silty clays appears to correspond to Dansgaard-Oeschger cycles. Due to abundant intercalated ash layers, the color changes can be more precisely linked to other regions by means of tephrachronology. Variations in the color reflectance records of cores MD012407 and 2408 suggest a range back to Oxygen Isotope Stages 17 and 7, respectively, which is consistent with biostratigraphic basal ages of 0.46 Ma and 0.30 Ma.

Sea of Okhotsk
Cores MD012409 (44 m core length) from the western Tsugaru Strait and Core MD012412 (58,11 m) from north of Honshu monitor fluctuations of the Tsushima Current (a branch of the Kuroshio) during the last glacial cycle and in particular during the Holocene, as they are located at the terminal end of the present Tsushima Current. Core MD01-2409 was recovered from a water depth of 970 m off Shimokita in the western North Pacific. The sediment consists mainly of olive gray-colored homogenous silty clay containing diatoms with minor associations of coccoliths and foraminifers and includes probably some slump deposit. Core MD01-2412 was recovered from a water depth of 1100 m off Shiretoko in the Sea of Okhotsk. The sediment is composed dominantly of homogeneous silty clay with organic-rich spots, ash spots and ash layers. Although not confirmed by shipboard biostratigraphy, the basal age of this core appears to be Oxygen Isotope Stage 5a, based on variations in color reflectance values.

Two long sediment cores MD01-2413 (46.55 m) and MD01-2414 (46.23 m) were recovered from the central Sea of Okhotsk. Their basal ages are 1 Ma and 1.2 Ma respectively, which will allow investigation of marine productivity development since the beginning of the Pleistocene. The IMAGES cores will thus complement the suite of long gravity cores (max. 11 m in length) from the joint German-Russian KOMEX project, which so far span only the last 350.000 years of paleoceanographic evolution in the Sea of Okhotsk. The long IMAGES cores provides the opportunity to investigate whether extreme productivity events are exclusively restricted to the last 600 kyrs, when global climate was dominated by 100 kyr-orbital cycles or whether they reach back to periods, when the Earth´s climate was dominated by 41 kyr-cycles.

Emperor Seamounts
Long cores from the Emperor Seamounts were retrieved to reconstruct the millennial-scale variability of North Pacific surface and deep/intermediate water circulation and its impact on global climate change. Coring sites were chosen in water depths less than 2400 m, along the north-south transect of the Emperor Sea Mounts to ensure good preservation of carbonate in the sediment. Three cores (MD012416-18) were retrieved between 51°-45°N. Core MD012416 (44.75 m) in the north will probably provide a record extending back to the top of the Jaramillo magnetic event. Core MD012417 (32.27 m) reaches back to the middle Pleistocene. Core MD012418 (43.68 m) in the south contains a purely pelagic record, which goes back to the top of the Miocene.

Preliminary results show that the siliciclastic sediment component rapidly decreases south of 49°N. The great abundance of siliceous microfossils characteristic of sediments near 50°N extends south of 45°N over the whole Plio-/Pleistocene. Some Pliocene laminated diatom mats were observed, hitherto only reported from the equatorial Pacific (Kemp et al., 1997). Abundant volcanic ash layers in the two northerly sites may allow large-scale stratigraphic correlation and provide information on the directions of winds and sea ice drift during cold stadials and glacial stages.

West Pacific, eastern coast of Japan
Cores (MD012410, MD012411, MD012419) taken from downstream areas of the Kuroshio show estimated sedimentation rates around 10 cm/kyr. These cores will allow orbital scale analyses and will also allow the variability in temperature and productivity of the Kuroshio to be studied on longer time scales, back to approximately 400,000 years. Cores MD012420 (8.98 m) and MD012421 (45.82 m) were recovered from a water depth of 2200 m off Kashima in the western North Pacific. Both cores consist mainly of dark olive gray-colored homogeneous silty clay, occasionally intercalated with ash layers. The basal age of Core MD01-2420 is estimated as younger than 0.090 Ma, based on biostratigraphy. This core may be correlated to an interval of Core MD01-2421 (from 5 to 14 m), based on variations in the a* value of the color reflectance. The basal age of MD01-2421 is estimated between 0.26 Ma and 0.46 Ma, based on nannofossil biostratigraphy.

This summarizing text is part of the cruise report.
The complete report (453 pages) is available at the IMAGES office.