Distribution, thickness and origin of Heinrich layer 3 in the Labrador Sea

Abstract

The presence of Heinrich layer 3 (HL-3) in the northwest Labrador Sea has been debated in the literature. Calypso giant piston core MD99-2233, five new standard piston cores, and re-interpretation of 34 cores from previous cruises confirm the presence of HL-3 in the Labrador Sea. It is identified by high total carbonate concentration (up to 45%), an increase in coarse fraction content, and lighter δ¹⁸O values in polar species planktonic foraminifer Neogloboquadrina pachyderma (left-coiling) as low as 3.1‰. The age of HL-3 of ∼27 ka was bracketed in the various cores by about 50 ¹⁴C-accelerator mass spectrometer dates. Where it is present in ice-proximal regions, it consists of nepheloid-flow deposits at the base and mud turbidites at the top. The thickness of HL-3 varies between 4.8 m (proximal to Hudson Strait) and 0.9 m (distal), decreasing rapidly seaward. On the upper continental slope, HL-3 was too deeply buried to be sampled. Elsewhere, HL-3 is absent in some cores, probably due to slumping or erosion associated with sandy turbidity currents or debris flows.

Introduction

Late Pleistocene sediments from the North Atlantic Ocean contain a sequence of layers of ice-rafted debris (IRD) known as Heinrich layers [1], [2], which are interlayered with hemipelagic and pelagic sediments. The Heinrich layers document episodes of massive iceberg discharge from the Laurentide Ice-Sheet. Ages of Heinrich events have been determined in the North Atlantic Ocean by radiocarbon dating and extrapolation of sedimentation rates and are summarized by Bond et al. [3]. Heinrich layers 1 and 2 were first systematically described in the Labrador Sea by Andrews and Tedesco [4] and deeper Heinrich layers were reported by Hillaire-Marcel et al. [5], Hiscott et al. [6], and Andrews et al. [7]. The origin, presence, and distribution of Heinrich layer 3 (HL-3) in the Labrador Sea has been debated in the past. For example, Grousset et al. [8] stated that HL-3 is restricted to the central and eastern part of the northern North Atlantic Ocean. Based on a limited number of cores, Andrews et al. [7] and Kirby and Andrews [9] suggested that HL-3 is absent in the northwest Labrador Sea. In a re-examination of the cores from the North Atlantic studied by Bond et al. [10], Bond and Lotti [11] identified a narrow peak of carbonate grains at the level of HL-3 in Orphan Knoll core EW9303-GGC31 and subsequently found equivalent narrow detrital carbonate peaks in DSDP Site 609 and core V23-81 in the eastern North Atlantic [3] not seen in earlier work on those cores [10]. Hillaire-Marcel et al. [5] also noticed the presence of HL-3 in two cores (Hu90-13-12 and 13) from the Greenland slope.

In the northwest Labrador Sea, eight Heinrich events, H0 to H6 (including H5a [12]), have been identified between 10 and 60 ka by their known ages, distinct nepheloid-flow deposit structure, high total carbonate concentration, increased coarse fraction content, mostly light oxygen isotope ratios, and low magnetic susceptibility values [13]. Using 40 cores (Fig. 1; Table 1), 34 of which are from our core collection, this study attempts to delineate the regional extent of HL-3 and map its thickness. The significance of establishing the distribution and thickness of HL-3 in the Labrador Sea is that it would demonstrate the extent to which iceberg discharge was similar and from the same sources during different Heinrich events, and in particular whether Hudson Strait was a primary source also for HL-3. For the absence of HL-3 in certain cores, alternative causes rather than the lack of iceberg discharge from the Hudson Strait have been considered, which are discussed in this contribution.

HL-3 is identified by bracketing ¹⁴C-accelerator mass spectrometer (AMS) dates of 26 and 28 ka, by high total carbonate content (up to 55%) and high L-values of reflectance, low magnetic susceptibility, δ¹⁸O values mostly lighter than 3.1‰ in polar species Neogloboquadrina pachyderma (left-coiling; hereafter N. pachyderma (l)), and an increase in IRD [4], [13]. Sedimentologically, it is characterized by typical nepheloid-flow deposit structure [14], consisting of graded mud layers that contain dispersed IRD and are occasionally interrupted by thin laminae of IRD. The basal boundary with the underlying hemipelagic sediment is sharp (in some cores erosional). The upper boundary is either gradual to overlying hemipelagic sediments or in sharp contact with overlying parallel laminated, carbonate-rich, mud turbidites devoid of IRD.