Deep‐water mass source and dynamic associated with rapid climatic variations during the last glacial stage in the North Atlantic: A multiproxy investigation of the detrital fraction of deep‐sea sediments

Abstract

In order to investigate North Atlantic Deep Water (NADW) dynamics and variability during the last glacial stage, a very high resolution multiproxy analysis of the detrital fraction of sediments deposited during Heinrich event 4 and Dansgaard‐Oeschger cycles 8 and 7 has been conducted on three deep‐sea cores. These cores are distributed along the path of the North Atlantic Deep Water from the Faeroe Shetland Channel to the Reykjanes ridge and the Irminger basin. The concentration in fine‐grained magnetites, the Ti‐content, and the smectite percentage within the clay minerals show similar and coeval fluctuations at each site and are comparable from one site to the other. This is the imprint of the detrital fraction originating from the northern basaltic provinces and transported to the studied sites. The average grain size in the fine fraction indicates a transport by deep‐sea currents. Therefore the observed fluctuations illustrate changes in the vigor of the bottom currents associated with the NADW with weak currents during cold periods (Heinrich 4 and stadials) and reactivation of the bottom circulation during interstadials.

Particle Size-Specific Magnetic Measurements as a Tool for Enhancing Our Understanding of the Bulk Magnetic Properties of Sediments

“…However, magnetic minerals do not always exist as discrete entities and can often occur as inclusions within polycrystalline clasts. For example, basaltic glasses can be dominated by SP-SD grains [61] and medium to coarse silts can be dominated by relatively fine PSD size grains [32] complicating the seemingly simple and often assumed [9,27,28,62] scaling of magnetic and sedimentary granulometry. Understanding how sediment and magnetic grain size relate to each other is therefore important for interpretation, however, it is rarely considered in routine magnetic studies.…”

“…This may be the correct assumption if magnetic fragments exist as discrete particles, but does less well in accounting for the potential of fine magnetic grains to be carried within larger polycrystalline clasts. These techniques also poorly characterize PSD grains often considering them as linear mixtures of SD and MD contributions in the bulk sediment [93,94] despite many natural samples being dominated by these magnetic grains [28,32,[117][118][119][120]. Furthermore, detailed coercivity measurements can often require an intensive measurement program, sometimes utilizing specialized equipment that can make processing a large number of samples difficult [53].…”

“…Incorporation of these different sources into sedimentary records has implications for their interpretation. For example, while magnetic grain size coarsening would accompany sedimentary grain size coarsening in Greenland sourced records and may be a useful paleo-current strength indicator [8,9,27,28], magnetic grain size proxies of Icelandic sourced sediment records would be less sensitive to changes in sediment grain size. [32].…”

“…In practice, NNA records that do not display strong changes in bulk magnetic concentration or the proportion of high coercivity minerals (e.g., hematite) are generally assumed to possess a relatively homogeneous ferrimagnetic dominated source signature. Commonly dominated by PSD grains, these sediments are often assumed to be composed of discrete particles (e.g., [9,22,28]) with many ferrimagnetically dominated bulk magnetic grain size records interpreted in terms of sediment transport. For example, Mrs/Ms and the ratio of the volume normalized susceptibility of ARM to volume susceptibility (κARM/κ) frequently used as proxies for variations in bottom current paleo-strength [26][27][28]62].…”

“…These properties are strongly sensitive to the abundance and type of Fe-bearing minerals, which are both ubiquitous in environmental systems and sensitive to environmental change, such that magnetic measurements offer the opportunity to study temporal and spatial environmental variability in a range of settings. As a result, magnetic properties have been exploited in a variety of terrestrial, lacustrine, and marine environments to map anthropogenic pollution [13][14][15], investigate erosion, sediment sourcing and historical land use change [1,11,[16][17][18], reconstruct paleorainfall estimates [19][20][21], map the incidence of ice rafted debris (IRD) [4,22,23] and dust [12,23,24] to the open ocean, understand the response of icesheets to climate [25], investigate ocean circulation [9,[26][27][28], and document the cyclicity of Pleistocene climate in loess [29] and ocean sediments [24].…”