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The late Holocene sedimentary record of the northern California continental shelf archives the combined influences of fluvial and oceanic mechanisms of land to ocean sediment flux, and provides perspective on the cumulative impacts of major floods and land-use change in the drainage basin during historical time. Piston cores collected on the shelf (50-150 m water depths) off the Eel and Klamath Rivers were analyzed to identify lithologies indicative of depositional processes over the past 5 k.y., and dated using [sup.137]Cs and [sup.14]C methods to resolve spatial patterns in sediment accumulation rates. Sediment accumulation rates averaged over the past several millennia display along- and across-shelf gradients that reflect interactions between the shelf flow field and antecedent tectonically produced topography. The highest rates (3-6 mm/yr; 0.4-0.8 g/[cm.sup.2]/yr) are associated with structural lows on the middle shelf, whereas the lowest rates (0.2-0.6 mm/yr; 0.03-0.05 g/[cm.sup.2]/yr) occur at structural highs and at the shelf edge. This accumulation pattern is spatially coincident with postglacial sediment thicknesses on the shelf, and with depocenters that form on shorter time scales in response to oceanic flood events. The sediment column exhibits an upward-fining sequence consistent with increased bypass of river silt and clay as delta plains progressively filled during the Holocene transgression. Sedimentary event beds indicative of major river floods, including two probable hyperpycnal discharges of the Eel River within the past 1 k.y., are preserved. Stratigraphic evidence of accelerated sediment delivery to the shelf during historical time, in the form of increased burial of fluvial mud relative to fine-grained sand, is superimposed on this natural record. This change is concordant with the regional history of forest clearing and related land-use practices in the watershed after the early 1800s, and appears to record the combined impacts of timber harvesting and several extreme floods after ca. A.D. 1950. Keywords: continental shelf sedimentation, fluvial systems, northern California, late Holocene, [sup.14]C geochronology, marine sedimentology.

Continental-shelf deposits off the Eel River, north-coastal California, document a recent increase in the magnitude and frequency of major hydrologic floods ([greater that or equal to] 10 yr recurrence interval). The shelf record reveals a sudden, three-fold increase in sedimentation rate ca. 1954 and a concomitant increase in the frequency of preserved flood beds. Comparison of sedimentary and river-discharge records reveals that major floods after ca. 1950 had a more pronounced effect on coastal sediment delivery and accumulation offshore than previous recorded events of similar magnitude, and that stratigraphic preservation of flood events is highly dependent on flood frequency and net sedimentation rate. We contend that this change in marine sedimentation is a response to documented climatic phenomena that have increased the frequency of major floods throughout the western United States during the past half century, together with intrabasinal impacts of extreme floods in 1955 and 1964. Anthropogenic increase in watershed-sediment production is a probable secondary factor. Keywords: northern California, climatology, floods, marine sediments, geochronology.

Analysis of severely, moderately and negligibly polluted sediments from the Palos Verdes margin shows that sediments near the outfall are mixed at intensities of 10 cm to the second power/yr, and bioturbation intensities are five times more rapid at sites away from the outfall. Unimpacted sites have intermediate rate of mixing intensities. There is negligible variability in trophic groupings along the enrichment gradient. Sublethal pollution effects and inhibition by a polychaete, Mediomastus sp., seems to suppress mixing intensities near the outfall.

A new deliberate tracer technique helps analyze the temporal and particle-size-dependent bioturbation processes in Massachusetts Bay. Tracer data indicate two types of nonlocal bioturbation, reverse conveyor-belt transport and head-down deposit feeding. Subsurface peaks are explained by a particle caching process involving an unidentified macrofaunal species. Neutron activation analysis helps determine the vertical profiles of the tracer.

A particle-labeling method was developed for both biological and physical sediment transport studies. The method involves labeling natural sediments from the area of interest with noble metals such as gold or silver, which are found only in trace levels in sediments. Labeled sediments are then thermally diffused into the cyrstalline matrix of ambient sediment grains. Subsequent dissolution and concentration enables detection of gold or silver particles using instrumental neutron activation analysis. The method was successfully tested in field studies.

The major concepts inherent in studies of organism-substrate interrelationships as a whole are critiqued. Various modes, styles, processes and products of marine sediment bioturbation are described. Shortcomings of the present state of research knowledge on this topic are discussed. Listed are 154 references. (CW)

Preservation potential of sedimentary event layers (e.g., ash layers or tempestites) is a function of net sedimentation rate and biogenous mixing rate. Previous studies that have used an inverse Peclet number to scale the problem in terms of advection or diffusion are in error due to the improper inclusion of the surface mixed layer thickness as the sole length scale. Herein, focus is on time scales. If the transit time (i.e., time required to advect the signal through the biologically active zone) is less than the dissipation time (i.e., time required to destroy the signal), then some fraction of the original signal is preserved. A simple quantitative model, based on the diffusion analogy of sediment mixing, estimates the actual percentage of preserved signal under various combinations of sedimentation rate, mixing rate, and event layer thickness.