ALBERT

Description: Relatively high opal concentrations are measured in equatorial Atlantic sediments from the most recent deglaciation. To shed light on their causes, seven cores were analyzed for their content of siliceous (diatom, silicoflagellates, radiolarians, phytoliths and sponge spicules) and calcareous (coccolithophores) microfossils. An early deglacial signal is detected at the time of rising boreal summer insolation ca. 18 ka by the coccolithophores. The surface freshening is likely due to the rain belt associated with the intertropical convergence zone - ITCZ, implying its southward shift relatively to its present-day average positioning. The diatom assemblages corresponding to the following increase in diatom abundances ca. 15.5 ka suggest the formation of a cold tongue of upwelled water associated with tropical instability waves propagating westward. Such conditions occur at present during boreal summer, when southerly trade winds are intensified, and the ITCZ shifts northward. The presence of the diatom Ethmodiscus rex (Wallich) Hendey and the coccolithophore Florisphera profunda indicates a deep thermocline and nutrient enrichment of the lower photic zone, revealing that Si-rich southern sourced water (SSW) likely contributed to enhanced primary productivity during this time interval. The discrepancies between the maximum opal concentrations and siliceous marine microfossils records evidence the contribution of freshwater diatoms and phytoliths, indicative of other processes. The definition of the nature of the opal record suggests successive productivity conditions associated with specific atmospheric settings determining the latitudinal ITCZ positioning and the development of oceanic processes; and major oceanic circulation changes permitting the contribution of SSW to marine productivity at this latitude.

Description: A series of catastrophic iceberg discharges termed Heinrich events punctuated the last ice age in the North Atlantic. During Heinrich events, coarse terrigenous debris released from the drifting icebergs and preserved in deep-sea sediments serves as an indicator of their passage. Quantifying the vertical flux of ice-rafted debris (IRD) in pelagic sediments can resolve questions regarding the timing and variation in ice sheet calving intensity. In this study, 230Thxs-based IRD flux was measured throughout the last glacial period in a deep-sea sediment core from the western North Atlantic (EW9303-37JPC, 43.68°N, 46.28°W, 3981 m), and complemented during Marine Isotope Stages (MIS) 1-3 by measurements from DY081-GVY01 (50.16°N, 45.51°W, 3721m) in the Labrador Sea. The cores are downstream from the Hudson Strait, a leading candidate for the conduit of the icebergs from the Laurentide ice sheet (LIS). We compare our results with the directly equivalent existing data in the eastern North Atlantic, and show that EW37JPC and DY001GVY have higher IRD fluxes during all Heinrich events, notably including 3 and 6. This study demonstrates that the Laurentide played a role in all Heinrich events and raises the likelihood that a single mechanism can account for their genesis.

Description: The Atlantic Meridional Overturning Circulation (AMOC) helps regulate the transport and storage of a large reservoir of heat and carbon, and therefore plays an essential role in climate change past and present1. Authigenic uranium deposition in deep-sea sediments is a sensitive redox tracer2,3 that can shed light on bottom water oxygen, carbon storage and, in certain circumstances, water mass distributions in the deep ocean4-7. Here, we combine new and published authigenic uranium data to reconstruct oxygenation since the last ice age in the east and west basins of the North Atlantic Ocean. Overall, the glacial deep North Atlantic was less well oxygenated compared to the Holocene, suggesting a higher ocean carbon inventory and important contribution to the global sequestration of CO2. We find that lower oxygen levels occurred in the deepest locations in the western basin, while substantially lower-oxygen waters were present throughout the northeast Atlantic. Our findings indicate that lower oxygen levels and correspondingly greater carbon storage were persistent features of the last glaciation and deglaciation in the deep North Atlantic Ocean, particularly in the eastern basin. This zonal emphasis is likely related to the farther advance and greater infilling in the east of deep waters originating from the Southern Ocean.