Near-bottom Current Speed Research Articles

Experimental variation of near-bottom current speeds and its effects on depth distribution of sand-living meiofauna

Meiofauna are eroded from sediments and transported in the water column. To examine how meiofauna respond to erosive currents, in situ current speeds were altered on an intertidal sandflat located in Bogue Sound, North Carolina, USA in July 1982. Replicates of two different weir designs were erected. One design increased measured current speed and the other, a control, did not. The depth distributions of meiofauna in each treatment and from undisturbed sediment were compared. There were few differences among treatments in depth distribution of the most abundant taxa: nematodes, turbellarians, and ciliates (>37-μm-long). However, a significant amount of sediment erosion (a mean of almost 0.5 cm in 2 h) occured in the increased flow treatment. Meiofauna responded to the loss of sediment with no discernable differences in depth distribution relative to the controls. There was some evidence that nematodes were less abundant and ciliates more abundant in the top 0.5 cm of the increased flow treatment. This may indicate that nematodes respond to fast surface current speeds by moving deeper into the sediments, while ciliates manage to remain near the surface even as sediment erodes. The entrainment of meiofauna may be passive but meiofauna clearly exert considerable behavioral influence over their susceptibility to entrainment.

Paleoceanographic implications of terrigenous deposits on the Maurice Ewing Bank, southwest Atlantic Ocean

Grain size distributions of sediment samples from the Maurice Ewing Bank (MEB) were utilized to identify styles of sediment transport and to estimate current speeds within the benthic boundary layer. These data allow for examination of the relationship between sedimentation on the bank and circumpolar currents. Causal relationships between glacial episodes, current intensification, and bottom current scour were previously postulated. The intent of this study was to test the validity of these suggested relationships. Modern sediment distribution on the MEB is controlled by water column winnowing, bottom currents, and variations in the flux of ice-rafted detritus and biogenic material, and is modified by mass-flow processes. Indirect determinations of current speeds, using well established curves relating current velocity to sediment transport, suggest uniform, near-bottom current speeds of approximately 6–15 cm s −1 over the bank. This velocity range is considered relative, not absolute. Nonetheless, these moderate velocities are supported by geostrophic velocity calculations. Grain size distributions of down-core samples from Plio-Pleistocene sandy units were examined in order to investigate the history of bottom current flow over the bank during this time. The lack of significant variability of grain size distributions down-core indicates that no major fluctuations in circumpolar current intensification have occurred since Plio-Pleistocene time, and, therefore, argue against a causal relationship between glacial conditions and deep-sea (geostrophic) current intensification. In addition, mass movement, mainly in the form of debris flows and turbidites, has resulted in interruptions in the sedimentary record, which in some cases could have been misinterpreted as unconformities. Our scenario for sedimentation over the bank differs from previous ones in that it calls for less dramatic variations in bottom current speeds during Pliocene—Holocene time.

Bottom-current speed in the Vema Channel recorded by particle size of sediment fine-fraction

The particle-size distribution of the carbonate-free silt fraction was determined in forty-two core-top samples from the east flank of the Vema Channel in order to allow comparison of the mean particle size with near-bottom current speed based on nearby CTD and current-meter observations. The silt mean particle size fluctuates insignificantly between 1450–3950 m but coarsens markedly below 4000 m under Antarctic-source bottom currents. The top of the zone of coarse sizes approximates the top of Lower Circumpolar Water (LCPW) at ∼4000 m; the steep gradient to coarsest sizes in the axis of the channel marks the transition to Antarctic Bottom Water (AABW). The vertical profile of mean particle size on the east flank correlates in general respects with the vertical velocity gradient in the channel axis although the core sites on the east flank are as far as ∼200 miles from the current-meter arrays. Nevertheless, this relationship is used to infer that the non-carbonate particle sizes deposited on the east flank of the channel may be an indication of bottom-current speed and, therefore, the mean size is used to reconstruct profiles of mean current speed for three time slices (18, 120 and 140 ka B.P.). During glacial isotopic stages 2 and 6 the particle sizes deposited on the east flank of the channel are characterized by finer sizes at nearly all depths and the zone of coarse sizes corresponding to LCPW and AABW deepens. During interglacial isotopic stage 5e, the particlesize profile is very similar to the modern profile; however, LCPW and AABW appear to be deeper than at present. The finer particle sizes deposited at nearly all depths during glacial periods indicates that the velocity of deep circulation in the Atlantic was reduced in response to the ice ages. Deep circulation during isotopic stage 5e closely resembled modern circulation; however, AABW production rates may have been less that at present.

Erosion and deposition on the eastern margin of the Bermuda Rise in the late Quaternary

A near-bottom survey has been made on the Eastward Scarp (32°50′N, 57°30′W) of the Bermuda Rise, which rises 1150 m above the 5500-m deep Sohm Abyssal Plain in the western North Atlantic. The survey reveals evidence of erosion and deposition at present and in the late Quaternary by the deeper levels of the westward flowing Gulf Stream Return Flow. Four distinct regions of increasing bed gradient show increasing sediment smoothing and scour in the transition from plateau to abyssal plain. Bedforms observed are current crescents, crag and tail, triangular ripples, elongate mounds, transverse mud ripples, lineations, and furrows ranging from 10 to 1 m or less in depth, decreasing generally with bed gradient. Measured near-bottom current speeds are up to 20 cm s −1. Temperature structure on the lower, steep, slopes suggests that detachment of bottom mixed layers may occur there. Extensive net erosion appears to be confined to the lower steep slopes of the scarp. Reflection profiles (4 kHz) show that there has been erosion in areas thinly draped with recent sediments and in areas that show development of small scarps. The distribution of subsurface acoustic characteristics of the region corresponds broadly to the areas characterized by bed gradient and distinct sedimentation conditions. Subsurface hyperbolae, possibly caused by buried furrows, show furrow persistence through several tens of metres of deposition. Erosion occurs up to the top of the scarp during episodes of presumed stronger currents, which may correspond with intensified circulation during glacials.

Current speeds near the ocean floor west of Oregon

Near-bottom current speeds were measured at distances of from 1–3 m above the ocean floor west of Oregon. The instrument used was a temperature-current probe designed to measure temperatures at 8 levels and current speeds at either 1 or 2 levels near the sea floor. Sampling was carried out at 6 positions. A distribution of recorded current speed versus water depth (from 725 to 2900 m) reveals a systematic and significant increase in current speed with decreasing depth. Mean current speeds for depths from 2700 to 2900 m were approximately 2 cm/sec with maxima of up to 6 cm/sec. Mean current speeds for continental slope stations, with depths from 725 to 1700 m, range from 5 to 20 cm/sec with maxima of 20–40 cm/sec depending on water depth.