Changes in the Western Boundary Undercurrent Outflow since the Last Glacial Maximum, from smectite/illite ratios in deep Labrador Sea sediments

Abstract

High-resolution mineralogical studies were performed on late glacial and deglacial sediments from two deep piston cores from the Labrador Sea, located at the inlet (SW Greenland Rise) and outlet (Labrador Rise) of the Western Boundary Undercurrent (WBUC) gyre. At the two sites, smectites transported from the eastern Iceland and Irminger basins by the WBUC are observed. Clay mineral changes are used as proxies for the paleointensity reconstruction of the WBUC. On the Greenland Rise, a clay mineral index (smectite/illite (S/I) ratio) is defined. A S/I ratio of ∼1 characterized the Last Glacial Maximum. It increased after ∼17 ka. and reached a maximum value of 4 during the early Holocene. The mineralogical changes are gradual and do not show any reversal during the Younger Dryas. This pattern, which is confirmed by first-order estimations of smectite and illite fluxes, suggests gradually increasing sedimentary fluxes and WBUC intensity since the Last Glacial Maximum. A peak in the velocity of the WBUC at ∼9 ka, as recorded by clay assemblages, is consistent with other regional studies based on pollen, foraminifera, or grain-size measurements. A massive dilution of smectites by illite and chlorite (S/I ≈ 3) occurs at ∼8.5 ka. It corresponds to a period of rapid sediment accumulation and reflects an intensified illite-rich detrital supply by meltwaters from the southern Greenland Ice Margin. On the Labrador Rise, the smectite content varies between 20 and 60% with no obvious trend through time. The mineralogical composition is strongly influenced by ice-rafted deposition and by the abundance of fast deposit units (cf. Heinrich layers in the North Atlantic) which contain abundant detrital carbonates spilled-over from the North-West Atlantic Mid-Ocean Channel. In such layers, smectites are present but are diluted by the addition of illites, chlorites, and kaolinites. This provides evidence for a discrete and continuous WBUC supply of fine particles from the Irminger and Iceland Basins as far as the southeastern part of the Labrador Basin. Early deglacial smectite-rich layers (up to 60%) are also observed at this site. They indicate an increase in the outflow of the WBUC at ∼13.5 ka. (Bölling-Alleröd), as previously reported from grain size or foraminiferal assemblage studies.