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

McCave, I. N. and Thornalley, D. J. R. and Hall, I. R. (2017) Relation of sortable silt grain-size to deep-sea current speeds: Calibration of the ‘Mud Current Meter’. Deep Sea Research Part I: Oceanographic Research Papers. ISSN 09670637 DOI

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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 View the MathML source, 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 View the MathML source 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 View the MathML source (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 sub-parallel 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 View the MathML source to smaller values for a given speed than with a broader size-spectrum 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 View the MathML source (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 View the MathML source is calculated from measured currents with a size-varying depositional threshold. More surficial sediment samples taken near long-term current meter sites are needed to make calibrations more robust and explore regional differences.

Item Type: Article
Uncontrolled Keywords: 2017AREP; IA73;
Subjects: 01 - Climate Change and Earth-Ocean Atmosphere Systems
Divisions: 01 - Climate Change and Earth-Ocean Atmosphere Systems
08 - Green Open Access
Journal or Publication Title: Deep Sea Research Part I: Oceanographic Research Papers
Identification Number:
Depositing User: Sarah Humbert
Date Deposited: 22 Aug 2017 13:08
Last Modified: 10 Mar 2021 22:27

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