Nd isotope composition of marine sediments as a tracer for iceberg provenance in the last glaciation

Abstract

OMEX core KC 024-19 was studied aiming at to assess the influence of climate changes on the origin and transport of the sediments of the Galician continental slope, in the last 40 thousand years. The sampled sediments are composed mostly of silt and clay, but also include a coarser-grained (sand-sized) fraction, corresponding essentially to foraminifera tests. Another remarkable feature is the occurrence of four depth intervals characterized by abundances greater than usual of relative large terrigenous clasts (considered as ice-rafted debris - IRD), related to melting of massive influxes of icebergs into the North Atlantic during the so-called Heinrich Events (HE). In order to obtain information on the origin of the detrital component of the sediments, 27 selected samples were submitted to a leaching procedure, to eliminate the biogenic fraction, and then analysed for Nd and Sr isotopes by TIMS, in the Isotope Geology Laboratory of the University of Aveiro. The obtained 143Nd/144Nd and 87Sr/86Sr ratios vary from 0.512072 to 0.511604 (Fig. 1) and from 0.732273 to 0.725140, respectively. Significantly, the lowest Nd isotope ratios were obtained in samples from HE layers (Fig. 1), namely in HE1 (~ 15 ka BP), HE2 (~ 24 ka BP) and HE4 (~ 38 ka BP). These results suggest a strong contribution of continental crustal sources significantly older than the Variscan basement for events HE 1, 2 and 4 (Fig. 2). The most likely provenance of the coarse clasts deposited during these three events lie probably in NE America, where Precambrian basement occupies large areas, and the carrier icebergs should be fragments of the Laurentide Ice Sheet. This provenance is probably related to extremely cool conditions. In contrast, the HE3 (~ 28 ka BP) layer displays Nd isotope ratios in the range of the compositions of the most common sediments in the core and, therefore, its IRD should have European source(s).