Robert A. Wheatcroft

Deep-sea deposit-feeding strategies suggested by environmental and feeding constraints

The principle of lost opportunity from optimal foraging theory, coupled with recent information about fluxes in the deep sea, allows prediction of feeding behaviours potentially specific to deep-sea deposit feeders. One possible strategy, thus far documented only indirectly, is to ‘ squirrel ’ away rich food from the seasonal or episodic pulses that recently have been shown to fuel meiofaunal growth. Echiurans and sipunculids show morphological and faecal handling patterns consonant with this suggestion. Where it is prevalent, this foraging strategy can have profound effects on stratigraphy. Autocoprophagy is another expected behaviour across a wider taxonomic spectrum, but one that is especially difficult to document. The principle of lost opportunity also predicts highly selective ingestion, not necessarily accomplished by the assessment of individual particles but possibly through pit building in areas where fluids move near-bed material. Under many depositions regimes, small but abundant feeding depressions may be the primary sites where deposition occurs. Conversely, digestive utilization of heterogeneous refractory substrates like humic acids seems as unlikely as an effective municipal waste recycling system that starts with mixed garbage. High gut: body volume ratios in deep-sea deposit feeders, rather than representing an adaptation to use this heterogeneous and refractory end of the food spectrum, instead may allow (through greater residence time of ingested material) greater conversion and absorption of the labile fraction of sediments as it becomes scarcer. Intense natural selection for particle selection ability in fact is one possible reason for the prevalence of meiofauna in the deep sea, and for the diminutive size of macrofaunal taxa there. This selective pressure probably imposes a very restrictive bottleneck on the initial developmental stages of deposit feeders.

Oceanic flood sedimentation: a new perspective

Abstract A new type of flood, an oceanic flood, is recognized. In contrast to seasonal floods that characterize moderate to large rivers, oceanic floods occur over short time periods on small rivers. A key aspect of oceanic floods is that the river–ocean system responds to the same storm system. Therefore, sediment is delivered to the ocean under a narrow range of oceanographic conditions. Because large quantities of fine-grained sediment are rapidly introduced to the sea during oceanic floods, the sediment dispersal capacity of the coastal ocean is often overwhelmed. New phenomena (e.g., fluid-mud flows) may therefore arise. Oceanic flood sedimentation is likely to be globally significant, especially with respect to the supply of fine-grained material to slope environments. Long-term changes in precipitation intensity suggest that oceanic floods will be even more common in the future. Studies of oceanic flood sedimentation require novel sampling strategies, such as event-driven coring.

Post-depositional alteration and preservation of sedimentary event layers on continental margins, I. The role of episodic sedimentation

The degree of post-depositional alteration and hence the preservation potential of sedimentary event beds and transient signals is determined by the outcome of a ‘race’ between biological (and to a lesser extent physical) processes that conspire to destroy a signal and sediment accumulation which advects the layer or signal out of the surface mixing zone. Preservation potential specifically depends on the relative magnitudes of the (1) biological mixing intensity (sometimes parameterized as a biodiffusivity, Db), (2) mixing-layer thickness, (3) layer or signal thickness, and (4) sediment accumulation rate. These terms control the dissipation time (i.e. time required to destroy a signal) and the transit time (i.e. time necessary to advect a signal through the surface mixing layer). On fine-grained, upper continental margins in general, and the Eel River shelf (northern California) in particular, biological mixing intensity is high (i.e. Db ranges from 10 to 100 cm2/yr), mixing-layer thickness is large (>10 cm), and sediment accumulation rates are rapid (0.1–1 cm/yr). Despite the high sediment accumulation rates, transit times through the surface mixing-layer range from decades to a century. Signal dissipation times are considerably shorter: (1) <3 yr for beds up to 6 cm thick imaged in X-radiographs, and (2) <15 yr for the grain-size signature of beds up to 8 cm thick. Therefore event layers and their corresponding grain-size signature have a low probability of preservation. However, short-lived episodic sedimentation events (e.g. oceanic floods) can instantaneously advect material through the surface mixing layer, thereby preserving event beds and transient signals. On the Eel River shelf the sequential timing of episodic sedimentation events has exerted a first-order control on the resultant stratigraphic record (presence/absence of layers and grain-size fluctuations). Episodic sedimentation – a hallmark of continental shelf settings – is key to understanding strata formation and preservation on margins.

Preservation potential of sedimentary event layers

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.

Shelf record of climatic changes in flood magnitude and frequency, north-coastal California

Continental-shelf deposits off the Eel River, north-coastal California, document a recent increase in the magnitude and frequency of major hydrologic floods (≥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.

Dynamics of surficial trace assemblages in the deep sea

Abstract We present a steady-state model addressing factors that determine surfical trace concentration (fraction of sea floor covered by tracks and trails) relative to the areal density and composition of mobile epibenthic megafauna. Trace concentration is posited to be a direct function of the mean production rate of new traces and their mean residence time. Production rate, in turn, is governed by the density of trace makers, their average width and rate of movement, and the fraction of available (i.e. untracked) space on the sea floor. Trace-maker density is not solely a function of epibenthic megafaunal density, but is also influenced by the microscale roughness of the sediment surface; as roughness increases, a higher proportion of mobile epibenthos (although deforming the sediment surface) do not leave recognizable traces. Trace residence time is controlled by the combination of three destructuve mechanisms; diffusive sediment mixing by infauna, retracking by epifauna, and physical reworking. Explicit predictions of the model are supported by new survey and time-series data from two deep-sea sites that differ radically in epifaunal abundance, Santa Catalina Basin (1300 m, east Pacific) and the HEBBLE area (4800 m, west Atlantic). In the former, due to high sediment roughness, a large proportion of the abundant epibenthic megafauna do not produce recognizable traces; thus trace production rate is low and trace destruction is dominated by retracking. This combination results in a low mean trace concentration (1% of sea floor surface area) relative to the density of epibenthic megafauna (16.5 m −2 ). In the latter region, a much smoother sediment surface results in trace production by most epifaunal taxa. The extremely abundant and active infauna at this site, however, produce high sediment mixing rates and thus low trace residence times and concentrations (5% of sea floor surface area). Application of a steady-state model to data from the HEBBLE region is validated by solving for the unsteady case and showing that approach to equilibrium trace concentration essentially (95%) complete only 10 days after a benthic storm erases all traces. Previous studies of deep-sea lebensspuren and megafauna prove consistent with the model.

Late Holocene sediment accumulation on the northern California shelf: Oceanic, fluvial, and anthropogenic influences

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 137Cs and 14C 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/cm2/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/cm2/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.

Prediction of the fate of p,p'-DDE in sediment on the Palos Verdes shelf, California, USA

Abstract Long-term (60-yr) predictions of vertical profiles of p , p ′-DDE concentrations in contaminated bottom sediments on the Palos Verdes shelf were calculated for three locations along the 60-m isobath using a numerical solution of the one-dimensional advection–diffusion equation. The calculations incorporated the following processes: sediment deposition (or erosion), depth-dependent solid-phase biodiffusive mixing, in situ diagenetic transformation, and loss of p , p ′-DDE across the sediment-water interface by two mechanisms (resuspension of sediments by wave action and subsequent loss of p , p ′-DDE to the water column by desorption, and desorption from sediments to porewater and subsequent molecular diffusion to the water column). A combination of field measurements, laboratory analyses, and calculations with supporting models was used to set parameters for the model. The model explains significant features observed in measurements made every 2 years from 1981 to 1997 by the County Sanitation Districts of Los Angeles (LACSD). Analyses of available data suggest that two sites northwest of the Whites Point sewage outfalls will remain depositional, even as particulate supply from the sewage-treatment plant and nearby Portuguese Bend Landslide decreases. At these sites, model predictions for 1991–2050 indicate that most of the existing inventory of p , p ′-DDE will remain buried and that surface concentrations will gradually decrease. Analyses of data southeast of the outfalls suggest that erosion is likely to occur somewhere on the southeast edge of the existing effluent-affected deposit, and model predictions for such a site showed that erosion and biodiffusion will reintroduce the p , p ′-DDE to the upper layer of sediments, with subsequent increases in surface concentrations and loss to the overlying water column.

In situ measurements of near-surface porosity in shallow-water marine sands

An in situ resistivity profiler was developed to measure with minimal disruption, the near-surface porosity of shallow-water marine sands. Results from a siliciclastic site off NW Florida and two Bahamian carbonate sites (an ooid shoal and coral reef sand flat) suggest the following general features. First, there is a 5- to 15-mm thick zone of elevated porosity adjacent to the sediment-water interface. Porosity in this layer was from 0.05 to 0.25 (decimal porosity) greater than the subjacent values, and would be difficult to resolve using traditional measurement techniques. Second, average porosity at >10-mm depth was 0.38 /spl plusmn/ 0.01 at the siliciclastic site, 0.39 /spl plusmn/ 0.01 at the ooid shoal site, and 0.49 /spl plusmn/ 0.02 at the coral reef sand flat site; consistent with literature values. Third, individual profiles exhibited 0.05-0.15 fluctuations about the mean, with vertical length scales of 5-15 mm. These fluctuations may be the result of grain packing heterogeneities caused by hydrodynamic sorting during deposition and subsequent physical and biological mixing or could be artifacts caused by disruption of the grain framework. Fourth, ripple troughs at the siliciclastic sand site had a significantly higher near-surface porosity compared to ripple crests, due most likely to the presence of detrital material in the troughs.

The effects of wildfire on the sediment yield of a coastal California watershed

The occurrence of two wildfi res separated by 31 yr in the chaparral-dominated Arroyo Seco watershed (293 km 2 ) of California provides a unique opportunity to evaluate the effects of wildfi re on suspended-sediment yield. Here, we compile discharge and suspended-sediment sampling data from before and after the fi res and show that the effects of the postfi re responses differed markedly. The 1977 Marble Cone wildfi re was followed by an exceptionally wet winter, which resulted in concentrations and fl uxes of both fi ne and coarse suspended sediment that were ~35 times greater than average (sediment yield during the 1978 water year was 11,000 t/km 2 /yr). We suggest that the combined 1977–1978 fi re and fl ood had a recurrence interval of greater than 1000 yr. In contrast, the 2008 Basin Complex wildfi re was followed by a drier than normal year, and although suspended-sediment fl uxes and concentrations were signifi cantly elevated compared to those expected for unburned conditions, the sediment yield during the 2009 water year was less than 1% of the post–Marble Cone wildfi re yield. After the fi rst postfi re winters, sediment concentrations and yield decreased with time toward prefi re relationships and continued to have signifi cant rainfall dependence. We hypothesize that the differences in sediment yield were related to precipitationenhanced hillslope erosion processes, such as rilling and mass movements. The millennialscale effects of wildfi re on sediment yield were explored further using Monte Carlo simulations, and these analyses suggest that infrequent wildfi res followed by flincrease long-term suspended-sediment fl markedly. Thus, we suggest that the current approach of estimating sediment yield from sediment rating curves and discharge data— without including periodic perturbations from wildfi res—may grossly underestimate actual sediment yields.