Long term stability of deep current supplies ODSP 646

Forty Nd isotope signatures were measured on the fine fraction of one sediment core from the Southern Greenland rise (ODP-646) that span the last 360 ka (Fagel and Hillaire-Marcel 2006). These data track changes in the relative supply of fine particles carried into the deep Labrador Sea by the WBUC during the last four glacial-interglacial cycles (G-I).

Relative contribution of end-members 0% 20% 40% 60% 80% 100%

Relative contribution of end-members 0% 20% 40% 60% 80% 100%

Fig. 7. Estimated relative contributions of sediment supplies over the last 12 kyr at core MD99-2227 location. Mixing takes into account particle supplies from North American Shield (NAS), Mid-Atlantic volcanism (MAR), and young crusts (YC). (Data from Fagel et al. 2004.) During the last 6.5 kyr, Pb data distribution allows to decipher contributions from the Greenland Panafrican crust (GPC) and from the European Panafrican crust (EPC). The maximum contribution from East Greenland observed from 6 to 3 kyr could be related with the inception of DSOW-like driven supplies. (See Fagel et al. 2004 for details.)

Fig. 7. Estimated relative contributions of sediment supplies over the last 12 kyr at core MD99-2227 location. Mixing takes into account particle supplies from North American Shield (NAS), Mid-Atlantic volcanism (MAR), and young crusts (YC). (Data from Fagel et al. 2004.) During the last 6.5 kyr, Pb data distribution allows to decipher contributions from the Greenland Panafrican crust (GPC) and from the European Panafrican crust (EPC). The maximum contribution from East Greenland observed from 6 to 3 kyr could be related with the inception of DSOW-like driven supplies. (See Fagel et al. 2004 for details.)

143Nd/144Nd

0,51140 0,51160 0,51180 0,51200 0,51220 0,51240

40 60 80 120 140

180 200 220 240 260

900 940 960 1000 1060

1140 1200 1320 1400 1440

1720 1760 1820 2040 2080

2520 2560 2580 2640 2660

2760 2800 2840 2880 2920

3000 3020 3100 3120 3140

Core depth ^

0.511832

Fig. 8. Nd isotopic signature of the clay-size fraction of last climate cycle sediments from site ODP646 (data from Fagel and Hillaire-Marcel 2006). Stratigraphical Nd distribution according to age model. Note the systematic shift between the Nd signature of glacials and interglacials. Vertical lines represent the averaged signature for glacial samples, interglacial samples or for all the samples (global mean).

Core depth ^

0.511832

0.512050 0.512193

0.512050 0.512193

Composite depth (cm)

Fig. 9. Estimation of the relative contribution of the three identified end-members (NAS, MAR, YC) within the clayey supplies settled at ODP646 location. Results are expressed as total particle flux taking into account the contribution of each end-member, the clay abundance in the sediment, the sediment density and the sedimentation rate. (See Fagel and Hillaire-Marcel 2006 for calculation.)

Clay mineralogical changes are evidenced between glacials and interglacials. On average glacial clay assemblages display a pronounced drop in smectite (< 40%) balanced by an increase in illite and chlorite. Such mineralogical observation reflects relative changes between proximal (illite-rich) and distal (smectite-rich) supplies. Likely Nd signatures of clay-size fraction also present a systematic shift between glacials and interglacials (Fig. 8). Clay-size fractions from glacial sediments have the lowest Nd isotopic ratios. Like for MD99-2227 Nd data distribution are consistent with a three end-members mixing diagram. The relative contribution of NAS, YC and MAR are reported on Fig. 9. Supplies of young crustal particles were similar during glacial stages of oxygen isotope stages (OIS) 2, 6, and 10, whereas mean volcanic contributions decreased relative to old craton material, from OIS 10 to OIS 6 and from OIS 6 to OIS 2. Compared with other interglacials OIS 7 was marked by a higher contribution from young crustal material but a similar mantellic-derived supply. The old crust contribution (NAS) is twice higher during glacials than interglacials, replaced by MAR contribution during interglacials. Estimated sedimentary fluxes suggest rather higher proximal supplies related to glacial erosion than any dilution process (Fig. 9).

Over the studied interval fine particle supplies to the Labrador Sea were strongly controlled by proximal ice-margin erosion and thus responded to glacial stage intensity. In contrast, the WBUC-carried mantellic-derived supplies (MAR) from the Eastern basins did not change significantly throughout the last 360 kyr, except for a pronounced increase in surface-sediments that suggests unique modern conditions. Over glacial/interglacial Nd ratios in clay-size fraction provide useful information on relative sedimentary supplies from proximal sources, thus on glacial erosion rates. However distal WBUC-controlled inputs from the northern and eastern North Atlantic seem to have been less variable.

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