Relation of sortable silt grain-size to deep-sea current speeds: Calibration of the ‘Mud Current Meter’

Abstract

Fine grain-size parameters have been used for inference of palaeoflow speeds of near-bottom currents in the deep-sea. The basic idea stems from observations of varying sediment size parameters on a continental margin with a gradient from slower flow speeds at shallower depths to faster at deeper. In the deep-sea, size-sorting occurs during deposition after benthic storm resuspension events. At flow speeds below 10-15 cm s-1 mean grain-size in the terrigenous non-cohesive 'sortable silt' range (denoted by SS , mean of 10-63 μm) is controlled by selective deposition, whereas above that range removal of finer material by winnowing is also argued to play a role. A calibration of the SS grain-size flow speed proxy based on sediment samples taken adjacent to sites of long-term current meters set within ~100 m of the sea bed for more than a year is presented here. Grain-size has been measured by either Sedigraph or Coulter Counter, in some cases both, between which there is an excellent correlation for SS (r = 0.96). Size-speed data indicate calibration relationships with an overall sensitivity of 1.36 ± 0.19 cm s-1 /μm. A calibration line comprising 12 points including 9 from the Iceland overflow region is well defined, but at least two other smaller groups (Weddell/Scotia Sea and NW Atlantic continental rise/Rockall Trough) are fitted by subparallel lines with a smaller constant. This suggests a possible influence of the calibre of material supplied to the site of deposition (not the initial source supply) which, if depleted in very coarse silt (31-63 μm), would limit SS to smaller values for a given speed than with a broader size-spectrum 2 supply. Local calibrations, or a core-top grain-size and local flow speed, are thus necessary to infer absolute speeds from grain-size. The trend of the calibrations diverges markedly from the slope of experimental critical erosion and deposition flow speeds versus grain-size, making it unlikely that the SS (or any deposit size for that matter) is simply predicted by the deposition threshold. A more probable control is the rate of deposition of the different size fractions under changing flows over several tens of years (the typical averaging period of a centimetre of deposited sediment). This suggestion is supported by a simple depositional model for which the deposited SS is calculated from measured currents with a sizevarying depositional threshold. More surficial sediment samples taken near long-term current meter sites are needed to make calibrations more robust and explore regional differences.

Evidence for Stable Holocene Basin‐Scale Overturning Circulation Despite Variable Currents Along the Deep Western Boundary of the North Atlantic Ocean

“…We have produced records of two dynamical proxies for deep circulation from core KN140‐2‐51GGC (hereafter, 51GGC): the ratio of excess sedimentary 231 Pa/ 230 Th, considered a tracer of basin‐scale overturning rate and net export of deep and intermediate waters in the North Atlantic (e.g., Gherardi et al, ; Henry et al, ; Lippold et al, , ; McManus et al, ; Rempfer et al, ) and sortable silt mean size ( ), used as a proxy for changes in bottom‐current velocity and sorting capacity (e.g., Bianchi & McCave, ; Hall et al, ; McCave, ; McCave et al, , ; Praetorius et al, ; Thornalley et al, ). Together these provide a multiproxy picture of intermediate‐depth circulation within the DWBC through the Holocene.…”

“…Sediment grain size analyses provide a proxy for local bottom current velocity changes, based on the potential of faster currents to deposit coarser particles. “Sortable silt,” often defined as the 10‐ to 63‐μm fraction of the sediments (McCave et al, ), is considered the size distribution most likely to be transported and/or deposited by typical deep current speeds (McCave, ; McCave et al, ). Increases and decreases in sortable silt mean size ( ) can be interpreted as reflecting increased and decreased bottom current velocity, respectively (Bianchi & McCave, ; Hall et al, ; Hoogakker et al, ; Praetorius et al, ; Thornalley et al, , ).…”

Sea Surface Temperature Variability on the SE‐Greenland Shelf (1796–2013 CE) and Its Influence on Thrym Glacier in Nørre Skjoldungesund

“…However, it plays a crucial role in heat exchange between the ocean currents, the atmosphere and the glacier (Murray et al, ). The mean grain size of SS ( is commonly used as a proxy for current strength near the sea bed (Bianchi & McCave, ; McCave et al, , ) and has also been used as a proxy for water renewal rate in Sermilik Fjord (Andresen, Schmidt, et al, ). We use the as an indicator to investigate changes in the fjord water circulation in Nørre Skjoldungesund, which can be affected by enhanced meltwater discharge from Thrym Glacier or by external shelf currents.…”

Investigating the environmental drivers of deep‐seafloor biodiversity: A case study of peracarid crustacean assemblages in the Northwest Atlantic Ocean

“…This relationship may reflect differences in particle‐size preference among benthic taxa (Etter & Grassle, ; Flach & Thomsen, ; Leduc, Rowden, Probert, et al, ). Average sediment particle size, which is closely related to near seabed current speed (McCave, Thornalley, & Hall, ), is also thought to influence deep‐sea macrobenthic assemblage characteristics. This is because it can influence a suite of secondary factors relevant to benthic communities, including changes in oxygen concentration with sediment depth, ease of burrow/tube construction, level of physical support provided, and identity of effective feeding strategies.…”