The sources of the glacial IRD in the NW Iberian Continental Margin over the last 40 ka

Abstract

In this work, the first results of ice rafted detritus (IRD) provenance related to Heinrich Events in the NW Iberian Margin are presented and discussed. This study is based on multiproxies but thoroughly analyzes Sr and Nd isotope ratios measured in the detrital fraction of the sediment core KC 024-19, from the Galician continental slope (NE Atlantic), aiming to unravel the sources of the terrigenous component of the sampled sediments. In most of the core layers, the isotope signatures, as expected, are consistent with an origin, ultimately, in the nearby Variscan continental crust. However, allochthonous contributions were also deposited, namely during the four last Heinrich Events (HE). The non-carbonate detrital fraction of the sediments deposited during the HE1 (in its upper part), HE2, and HE4 displays εNd much lower than usual, suggesting that those layers incorporate clasts dropped by icebergs (Ice-Rafted Detritus - IRD) whose sources must have been located in Archaean cratons, such as those of northeastern America and Greenland. The low εNd values combined with dolomite peaks support the hypothesis that icebergs fed by the Laurentide ice sheet and launched through the Hudson Strait played a major role in the deposition of IRD during HE1, HE2, and HE4. The HE1 layer seems to record a complex evolution of this event, since, at its base, the coarse-grained non-carbonate fraction displays relatively high εNd values, pointing to European/Icelandic sources for the icebergs in the initial stages of HE1. The IRD content of the HE3 contrasts significantly with the other HE layers and the non-IRD layers because it has an isotope signature characterized by low 87Sr/86Sr ratios and high εNd values. This suggests that the most exotic components of the HE3 layer may have come from East Greenland – Fram Strait and/or result from a mixture of materials from western European and more juvenile (Iceland – Faeroes) crustal sources.