Grain Size of the North-Atlantic Drifts Sediments: is the Gloria Drift a Contourite Drift?

Abstract:

Mean size of mineral particles of 10-63 fraction, so-called sortable silt mean size (SS) (McCave, 1995) and modes of grain-size distribution were used as proxies for reconstruction of paleocurrents intensity variations in the North Atlantic. It was assumed that the first mode (3-8 µm) is formed as the result of normal pelagic sedimentation and the second mode (10-30 µm) appears under the bottom currents influence. The sediments with bimodal grain-size distribution (the second mode varies from 10 to 28 µm) correlate with increased SS (up to 18-23 µm) in the Hatton and Snorry Drifts, indicating an increase in contour currents intensity during MIS 1, 3 and 5e. In contrast, there are no any relationships between grain size distribution (high SS values, appearance of bimodal distributions) and climatic cyclicity of variations in contour currents intensity at the Gloria Drift. Moreover, the Gloria Drift sediments differ from the contourite sediments of the Snorry and Hatton Drifts by shifting of the second mode toward the coarse particles (25-40 µm), higher sedimentation rates and higher IRD content. This evidence puts in doubt the contourite origin of the Gloria Drift.
At the same time, we have identified the similarity between the Gloria Drift sediments and IRD-containing hemiturbidites of Labrador Sea (Hesse and Khodabakhsh, 2006). Fine-grained sediment lofting has been inferred for ice marginal regions of the northwest Labrador Sea. Sediment failure on the Labrador Slope predominantly produces muddy turbidity currents, because the slope sediments are mud-dominated. Their deposits are the indicative muddy spill-over turbidites of the NAMOC levees and the levees of the tributaries to the NAMOC. Dispersal of the IRD throughout the graded mud layers is evidence that the two processes, ice rafting and the delivery of the fines by lofting, occurred simultaneously.

This work was supported by Russian Scientific Fund (grant No. 14-50-00095).