Holocene evolution of deep circulation in the northern North Atlantic traced by Sm, Nd and Pb isotopes and bulk sediment mineralogy

Abstract

Bulk mineralogy, Sm, Nd and Pb elemental and isotopic compositions of the clay‐size fraction of Holocene sediments were analyzed in three deep North Atlantic cores to trace the particle provenance. The aims of the present paper are to identify the origin of the particles driven by deep currents and to reconstruct deep circulation changes over the Holocene in the North Atlantic. The three cores are retrieved in fracture zones; two of them are located in the Island Basin along the gyre of North Atlantic Deep Water, and the third core is located off the present deep circulation gyre in the Labrador Sea. Whereas sedimentary supplies in the Labrador Sea were constantly derived from proximal sources, the geochemical mixing trends in the Iceland Basin samples indicate pronounced changes in the relative contribution of continental margin inputs over the past 6 kyr. Supplies from western European margin that sharply increased at 6 kyr were progressively diluted by a larger contribution of Scandinavian margins over the last 3 kyr. Changes in composition of the particles imply significant reorganization of paleocirculation of the deep North Atlantic components in the eastern basins: mainly reorganizations for both Iceland‐Scotland Overflow Water and Norwegian Sea Overflow Water. Moreover the unusual Pb isotopic composition of the oldest sediments from the southern Iceland Basin indicates that distal supplies from Greenland margin were driven into the Iceland Basin, supporting a deep connection between Labrador Sea and Iceland Basin through the Charlie Gibbs Fracture Zone prior the Holocene Transition period.

Long-term variations in Iceland–Scotland overflow strength during the Holocene

“…Decreasing I-S overflow strength after ∼ 7 ka is consistent with changes inferred from faunal assemblage time slice data from the Faroe-Shetland region (Rasmussen et al, 2002) and is in excellent agreement with recent reconstructions of ISOW flow strength along Gardar Drift using grain size and magnetic mineral proxies (Kissel et al, 2013). The results of Kissel et al (2013) may hint at a slight plateau between ∼ 3 and 5 ka, which would agree with the suggestion of a significant reorganization of deep water circulation at ∼ 3 ka (Fagel and Mattielli, 2011); the uncertainty in our I-S overflow stack prevents us from confidently commenting on any such feature. We also observe that the changes in I-S overflow strength were closely coupled to regional climate on millennial timescales throughout the Holocene (Fig.…”

“…For example, records from sites influenced by LSW suggest that strong LSW formation similar to the present day started at ∼ 7-8 ka and then underwent subsequent millennial-scale oscillations in its strength (Hillaire-Marcel et al, 2001;Oppo et al, 2003;Olsen and Ninnemann, 2010). Data on the bulk mineralogy and geochemical provenance of clays have been used to argue for an increased contribution of ISOW to the deep North Atlantic at ∼ 6 ka, and a reinforcement of waters formed in the Norwegian Sea over the last 3 ka (Fagel and Mattielli, 2011). This interpretation is in apparent conflict with results from benthic foraminiferal isotopes, faunal proxies, grain size and magnetic susceptibility, which have been used instead to infer a decline in the influence of ISOW and Northeast Atlantic Deep Water (to which ISOW is a major contributor) over the past ∼ 6.5 ka, both in the deep Northeast Atlantic (Rasmussen et al, 2002;Hoogakker et al, 2011;Kissel et al, 2013) and Northwest Atlantic (Bilodeau et al, 1994;Hoogakker et al, 2011).…”

Long-term variations in Iceland–Scotland overflow strength during the Holocene

“…For example, records from sites influenced by LSW suggest that strong LSW formation similar to the present day started at ∼ 7-8 ka and then underwent subsequent millennial-scale oscillations in its strength (Hillaire-Marcel et al, 2001;Oppo et al, 2003;Olsen and Ninnemann, 2010). Data on the bulk mineralogy and geochemical provenance of clays have been used to argue for an increased contribution of ISOW to the deep North Atlantic at ∼ 6 ka, and a reinforcement of waters formed in the Norwegian Sea over the last 3 ka (Fagel and Mattielli, 2011). This interpretation is in apparent conflict with results from benthic foraminiferal isotopes, faunal proxies, grain size and magnetic susceptibility, which have been used instead to infer a decline in the influence of ISOW and Northeast Atlantic Deep Water (to which ISOW is a major contributor) over the past ∼ 6.5 ka, both in the deep Northeast Atlantic (Rasmussen et al, 2002;Hoogakker et al, 2011;Kissel et al, 2013) and Northwest Atlantic (Bilodeau et al, 1994;Hoogakker et al, 2011).…”

Holocene North Atlantic Overturning in an atmosphere‐ocean‐sea ice model compared to proxy‐based reconstructions

“…There are two categories of reconstructions of past North Atlantic Ocean circulation: surface and deep ocean reconstructions. This paper focuses on deep ocean circulation reconstructions, based on a number of proxies: ratios of carbon and oxygen isotopes (δ 13 C and δ 18 O) measured in shells of benthic foraminifera [ Oppo et al , ; Kleiven et al , ; Hoogakker et al , ], deep ocean sediment characteristics, such as the sortable silt (SS) mean grain size [ Bianchi and McCave , ; Hoogakker et al , ; Thornalley et al , ; Kissel et al , ], magnetic properties of grains (abbreviated as “ κ ”) [ Kleiven et al , ; Kissel et al , , ], geochemical data such as the Pa/Th ratio [ McManus et al , ; Gherardi et al , ] and bulk mineralogy and isotopic composition of sediments (such as Nd, Sm, and Pb) [ Fagel and Mattielli , ]. In our present study, we focus on SS, κ , and δ 13 C, because the first two proxies focus on the speed of bottom water masses, which are thought to relate to flow speed in our model, whereas the third one focuses on the activity of ventilation and therefore supply of deep water, which is thought to relate to convective activity and the meridional overturning strength in our model.…”