Burial and infilling of a karst in Papua New Guinea by road erosion sediments

The anthropogenic impact on karst in Papua New Guinea is briefly introduced and a specific case is presented detailing the effect of road erosion sediments on a small karst. The karst is in the perennially humid tropics and covered with primary rain forest. The road was placed high above the karst on steep friable rock and traverses several of its catchments. The changes to and the rate of burial of parts of the karst and the infilling of the caves are described. The karst drainage has altered, and there is increased water storage. The sediment build-up ceased in less than a year due to vegetation and stabilization of the road embankments. It is concluded that any construction within a catchment leading to a karst should be assessed as to its impact on the karst.     

Terraced fills of Pleistocene and Holocene age in the Rheidol Valley,Wales

The legacy of repeated Pleistocene glaciations has endowed many Welsh river valleys with locally thick successions of glacial and alluvial sediments. Investigations of a well-preserved flight of terraced sediments with good exposures at Capel Bangor, on the Afon Rheidol, mid-Wales, has allowed its Quaternary valley fill stratigraphy to be examined in detail. Study has revealed five terraced fills consisting of seven distinct sedimentary units. These range from Late Devensian ice-contact and ice-marginal deposits, to Holocene high-sinuosity stream sediments with episodes of man-induced accelerated deposition of fine-grained alluvium, and to aggradation and subsequent incision associated with historic metal mining. Examination of general sedimentary properties (e.g., granulometry, sedimentary structures, terrace surface morphology) show both differences in the pattern and controls of deposition and also progressive changes over Late Devensian and Holocene times. The sediments of the Rheidol Valley record the response and subsequent recovery of a drainage basin to glaciation, and the increasing influence of man on sediment yields, channel processes, and sediment quality.     

Decadal to millennial cyclicity in varves and turbidites from the Arabian Sea: hypothesis of tidal origin

We have analyzed a Late Holocene record, almost 5000 years long, consisting of varved sediments deposited in the oxygen-minimum zone (OMZ) off Pakistan. We searched for cyclicity in the series of varve thickness (“varve” cycles), of unusually large excursions in varve thickness (“agitation” cycles), and of abundance of turbidites (“turbidite” cycles). We found the following high-frequency cycles (periods between 10 and 100 years) in one or several of the three types of series as follows: near 12.4, 14–15, 16.8, 18.6 (strong, agitation), 25–26 (strong, turbidite), 29–31 (strong, agitation), 39 (varve), 44 (strong, turbidite), 51–54 (strong, agitation), 56 (strong, varve), 64 (strong, turbidite), 69, 77 (strong, turbidite), 82 (very strong, agitation), and 95 years (strong, varve). Low-frequency cycles center around 99–115, 125 (very strong, varve), 164, 177, 202, 242–255 (strong, agitation and turbidite), 280 (strong, varve; doubled, turbidite), 340–370 and 460–490 years.Some cycles of varve thickness match the cyclicity of turbidite frequency (12.3, 14–15, 25–26, 245–255 years) but similarities between spectra are not striking. Taken as a whole, however, the sequence of cycles detected (by autocorrelation and standard Fourier analysis) seems to contain a large proportion of multiples of the basic tidal cycles 4.425 (lunar perigee cycle) and 9.3 years (lunar half-nodal cycle). This impression is supported by testing the three binned spectra for whole-number multiples and fractions as well as whole-number beat structure. We therefore propose that a large proportion of the cyclicity detected can be ascribed to tidal action. Our record also contains evidence for the presence of the 1470-year cycle previously reported from the glacial-age Greenland ice record. The main harmonics of this Greenland cycle can be tied to the pattern of periods seen in the varved sediments. We hypothesize that tidal action produces the cycle, and that the reason for its great length is the requirement that maximum tidal activity has to fall into a narrow seasonal window to be geologically effective.     

RIVERINE SEDIMENTS AND CHINESE COASTLINE CHANGES

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The distribution and geochemistry of some trace metals in the Bermuda coastal environment

Results of the analyses of the water column and sediments of the Bermuda Platform for Cd, Cu, Fe, Mn, Ni, Pb and Zn are presented. The major process controlling the water column concentrations is physical mixing of open ocean waters with inshore waters, which are polluted by a wide range of diffuse inputs. Sedimentation within the inshore waters plays a lesser, but significant, role as do fluxes from the sediments of Fe and Mn and possibly phytoplankton uptake of Zn. Concentrations within the sediments are controlled by the formation of trace metal enriched clay/organic particles in the inshore areas and their subsequent redistribution by sediment resuspension, except for Fe and Mn which are largely associated with clay lattices.     

Fine-grained submarine fans as possible recorders of long- and short-term climatic changes

Two major end-members of point-sourced submarine fans can be recognized: coarse-grained and fine-grained. The coarse-grained member is known from active margins where the sediment source is rather close to the coast. Once reaching the coast, most of the sediment goes into longshore transport and may move down a shelf depression to the slope. The movement of the sediment will gradually carve out a canyon. On the basin floor, a submarine fan is constructed that gradually progrades into the basin. Its thickness and sand/shale ratio decrease downdip.The fine-grained member is common for passive margins. It is a bypass system. The sediment originates far from the coast, the fluvial system is long and a major delta commonly results. Exposure of the wide shelf requires a relative lowering of sea level. Rapid aggradation of sediment near the shelfbreak results in high pore pressure that causes failure, followed by slumping and density flow transport. Deposition may commence at the base-of-slope. Leveed-channel transport is common on the mid-fan, while sheet-sand deposition takes place on the outer fan. Progradation of individual fan systems is rapid, followed by lateral switching to minimize bottom topography.Global and regional climatic changes cause variation in the growth/decay ratio of continental glaciers and in the type of fluvial effluent (hypopycnal and hyperpycnal outflows). Major global climate changes can cause ice ages, while smaller climate changes often influence transport and depositional variations. Shelf bypassing may result if hyperpycnal outflow conditions exist upon entering salt water. Although presently unknown, fine-grained submarine fans may provide data required to help analyze paleoclimate.     

Radiocaesium and 210Pb in Clyde sea loch sediments

Analysis of ²¹⁰Pb, ²²⁶Ra, ¹³⁷Cs and ¹³⁴Cs in short sediment cores provide first estimates of deposition rates in some Clyde sea lochs. The radio-caesium nuclides originate mainly in the liquid effluent released at distance from the Clyde by the Windscale nuclear fuel reprocessing plant and their concentrations in Clyde sediments provide information on (a) enrichment factors onto particulate matter, (b) surficial mixing coefficients and (c) sedimentation rates. A radiocaesium residence time in coastal waters of ca. 10³ years reflects the importance of scavenging by the high nearshore particulate flux. ²¹⁰Pb levels in sediments are controlled, in the unsupported fraction, by a major input sorbed on catchment particulates and, in the supported component, by ²²⁶Ra activities occasionally perturbed by unusually high surface values probably of planktonic origin. In one loch, detectable levels of ¹³⁴Cs and ⁶⁰Co are attributed to their discharge by nuclear submarines.