Rock-magnetic measurements along with grain size, acid-insoluble residue (AIR), organic carbon (OC), CaCO3 and δ18O of the planktonic foraminifers of the sediments were determined for 15 gravity cores recovered from the western continental margin of India. Magnetic susceptibility (MS) values in the surficial sediments reflect the land-derived input and, in general, are the highest in terrigenous sediment-dominated sections of the cores off Saurashtra–Ratnagiri, followed by the sediments off Indus–Gulf of Kachchh and then Mangalore–Cape Comorin.
The down-core variations in mineral magnetic parameters reveal that the glacial sediments off the Indus are characterized by low MS values/S-ratios associated with high AIR-content, low OC/CaCO3 contents and relatively high δ18O values, while those off SW India are characterized by low MS values/high S-ratio% associated with low AIR content, and relatively high OC, CaCO3 and δ18O values. Conversely, the Early Holocene sediments of all cores are characterized by high MS values/S-ratio% associated with high AIR content, low OC, CaCO3 contents and gradually decreased δ18O values. These results imply that during the Last Glacial Maximum (LGM), the cores off northwestern India received abundant continental supply leading to the predominance of eolian/fluvial sedimentation. In the SW region the influence of hinterland flux is less evident during this period, but convective mixing associated with the NE monsoon resulted in increased productivity. During the early Holocene intense SW monsoon conditions resulted in high precipitation on land, which in turn contributed increased AIR content/MS values in the continental margin sediments. A shallow water core off Kochi further suggests that the intense SW monsoon conditions prevailed until about 5 ka. The late Holocene organic-rich sediments of the SW margin of India were, however, subjected to early diagenesis at different intervals in the cores. Therefore, caution is needed when interpreting regional climatic change from down-core changes in sediment magnetic properties. 相似文献
The Late Quaternary sea-ice history of the northeastern Japan Sea is discussed on the basis of the occurrence of dropstones
and ice-rafted debris (IRD) in fine sediment cores. IRD was found in all strata except those from the Holocene and oxygen
isotope stage 5.5. The largest expansion of sea ice was recognized at the last glacial maximum (LGM; oxygen isotope stage
2), when the southern margin of seasonal sea ice was probably located in the vicinity of the Matsumae Plateau. The margin
might occasionally have expanded further southward to off the Oga Peninsula. Sea ice expanded southward from mid-stage 5 to
the LGM in response to global cooling, but with much fluctuation. Sea ice remained during deglaciation until around 10 ka,
but after 10 ka it retreated northward rapidly in response to global warming and changes in surface water conditions. Greater
fluctuations in IRD were found in core GH95-1208 collected from off Rumoi, Hokkaido, Japan. More IRD was found in sediments
from late stage 3, late stage 5, and early stage 6. The fluctuations were not concordant with global climate changes (based
on the standard oxygen isotope curve), and may have been controlled by regional climate factors such as the strength of the
winter monsoon, which is related in turn to high-latitude atmospheric circulation.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
A large number of 3D deep seismic surveys in the Faroe-Shetland Channel gives continuous coverage over most of the region. These surveys were designed primarily to image depths in excess of 4 km, use low frequency sources and are recorded at low temporal sample rates. However, commercial 3D data can generate highly detailed images of the seabed due to the high spatial sample rate, typically 12.5 m. This is particularly true in waters below 200 m. Despite geophysical artefacts, the images reveal that there are a number of sedimentary processes at work adjacent to and within this channel. On the West Shetland Shelf, iceberg scouring and moraines reflect the impact of glaciation. On the West Shetland slope there is clear evidence for down-slope processes, such as debris flows, linear erosion channels, basal fans and (one case) slope failure. Along-slope processes are also active as indicated by the presence of sediment waves and contourite mounds. On the floor of the basin, polygonal cracking can be observed. The most spectacular feature appears to be the Judd Deeps, a system of cliffs approximately 200 m high and 40 km across. Traditionally, seabed investigation has been performed using high-resolution surveys. This study shows that deep exploration data can also provide useful images of the seafloor. 相似文献
This study was undertaken to elucidate the impact of early diagenetic processes on the accumulation of trace metals in Sapelo Island saltmarsh sediments as a function of time, space and sediment properties. Samples were collected from three sites in summer (May 1997) and winter (January 1998) along a transect from an unvegetated Creek Bank through a vegetated Tidal Levee to the vegetated midmarsh with evident lateral heterogeneity caused by hydrologic regime, macrophytes and microbial and macrofaunal activities. A suite of trace metals (As, Ba, Cr, Co, Cu, Cd, Mo, Ni, Pb, Th, Ti, U, V, Zn and Zr) was analyzed to obtain their depth-distribution at the three sites. Spatially marked differences were observed, that were primarily related to hydraulic flushing of trace metals away from the sites in high-energy regimes, rapid downward mixing and reworking of sediment via bioturbation, and below-ground degradation and production of Spartina biomass. Although sulfate reduction and the formation of acid volatile sulfide and pyrite were dominant processes throughout the marsh, the trace metal scavenging role of sulfides was not apparent. However, possible sulfurization of organic matter, leading to enhanced trapping of trace metals with organic carbon, may have played an important role in sequestration of trace metals.No similarity was observed visually between the depth trends of trace metals and sediment properties (grain size, iron-oxyhydroxide content, acid volatile sulfides and pyrite content) that are known to play a major role in trace metal partitioning. Only organic carbon content closely followed the trace metal profiles at all the three sites. Minor variation in depth-integrated sediment trace metal content was observed seasonally at each of the three sites. Furthermore, the depth trend of profiles of individual trace metals also did not vary significantly over the seasons either. 相似文献