Correcting for the influence of ice‐rafted detritus on grain size‐based paleocurrent speed estimates

Abstract

The grain size of deep-sea sediments provides an apparently simple proxy for current speed.However, grain size-based proxies may be ambiguous when the size distribution reflects a combination of processes, with current sorting only one of them. In particular, such sediment mixing hinders reconstruction of deep circulation changes associated with ice-rafting events in the glacial North Atlantic because variable ice-rafted detritus (IRD) input may falsely suggest current speed changes. Inverse modeling has been suggested as a way to overcome this problem. However, this approach requires high-precision size measurements that register small changes in the size distribution. Here we show that such data can be obtained using electrosensing and laser diffraction techniques, despite issues previously raised on the low precision of electrosensing methods and potential grain shape effects on laser diffraction. Down-core size patterns obtained from a sediment core from the North Atlantic are similar for both techniques, reinforcing the conclusion that both techniques yield comparable results. However, IRD input leads to a coarsening that spuriously suggests faster current speed. We show that this IRD influence can be accounted for using inverse modeling as long as wide size spectra are taken into account. This yields current speed variations that are in agreement with other proxies. Our experiments thus show that for current speed reconstruction, the choice of instrument is subordinate to a proper recognition of the various processes that determine the size distribution and that by using inverse modeling meaningful current speed reconstructions can be obtained from mixed sediments.

Minimal Holocene retreat of large tidewater glaciers in Køge Bugt, southeast Greenland

“…Current-sorting of marine muds has only been sparsely investigated in glaciomarine environments 37 as the deposition of large ice-rafted clasts can potentially interfere with the current-sorting signal by shielding underlying sediment from winnowing 38 – 41 . However, deposition of ice-rafted debris is only problematic when sedimentation rates are rapid (>1 cm yr −1 ); in areas with lower sedimentation rates well-sorted sediments can be produced under just a few days of fast-flow conditions (i.e.…”

The Holocene Labrador Current: Changing linkages to atmospheric and oceanographic forcing factors

“…Smear slide examination showed negligible amounts of biogenic carbonate and opaline silica, suggesting an insignificant or negligible influence of these particles to the estimate of the S̅S̅ record of core Hu06‐40. Hass [] demonstrated that in some circumstances the contribution of ice‐rafted detritus (IRD) led to variation in the S̅S̅ [ Jonkers et al ., ].…”

“…The mean size of the sortable silt (S̅ S̅ ) in core Hu06-40 is hypothesized as a proxy for the Labrador Current speed. Silt fractions (63-10 μm) of the terrigenous sediments can be sorted by the prevailing bottom currents of the ocean on the basis that these fractions are noncohesive and are therefore sorted by Hass [2002] demonstrated that in some circumstances the contribution of icerafted detritus (IRD) led to variation in the S̅ S̅ [Jonkers et al, 2015].…”

An Empirical Method to Predict Sediment Grain Size From Inorganic Geochemical Measurements

“…As an example, we tested the method on a sediment core containing variable amounts ice-rafted debris (IRD), that is, sediments transported by ice rafting (Almirantazgo fjord, Chile, 54°S; Figures S3 and S4; Bertrand et al, 2017). Ice rafting deposition results in the presence of large particles in the sediment and skews the entire grain-size distributions toward the coarse fraction (Hass, 2002;Jonkers et al, 2015), without necessarily significantly changing sediment geochemistry. In this case, the prediction accuracy decreases with increasing amounts of IRD ( Figure S4).…”