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Abstract: The reinvigoration of overturning in the Southern Ocean is hypothesized to have returned CO2 from the deep ocean to the atmosphere at the end of the last ice age. Large peaks in opal accumulation have been put forward as evidence for an increase in wind driven upwelling between 10 and 15ka. Here, we use coupled nitrogen and silicon isotope records alongside opal accumulation rates to provide quasi-quantitative estimates of Southern Ocean nutrient supply, by upwelling, and nutrient utilization across this interval. Significant changes in the consumption of N and Si across the two opal accumulation peaks indicate major changes in both upwelling and nutrient demand. We find N and Si consumption to be relatively incomplete during peak opal accumulation at the onset of the deglaciation. This indicates that nutrient supply was significantly enhanced. The second deglacial peak in opal accumulation is associated with more complete Si consumption and variable N consumption. We suggest that this peak represents strong upwelling and more complete utilization of the available silicic acid pool. Differences between the Si and N responses during opal peaks may stem from decreasing iron availability across the glacial termination. The nutrient isotope evidence for excess nutrients during the deglaciation indicates that the high export productivity was insufficient to overcome the evasion of CO2 to the atmosphere as a result of physical circulation changes. Previous work has demonstrated that the reinvigoration of overturning circulation during the deglaciation causes a transient peak in nutrient supply to the low latitudes. This is supported by our data, which indicate that relatively high macronutrient concentrations were maintained in the Southern Ocean surface waters that are incorporated into mode waters despite high demand. [Copyright &y& Elsevier]

Abstract: Chloride and δ 18O compositions of interstitial water extracted from a long sediment core retrieved from the NW coast of the Black Sea allowed us to constrain the main hydrologic changes of the Back Sea during the Late Pleistocene and Holocene. Prior to its reconnection with the Mediterranean Sea (through the Marmara Sea) at approximately 9000 calendar yr before present (9kacalBP), the Black Sea has evolved as a fresh to brackish water lake. At the time of reconnection, hydrologic changes were drastic. Bottom water salinities changed from a few psu (practical salinity unit) to ∼22psu. Since solutes in the interstitial water column within sediments are advected and diffused the measured concentrations do not reflect those of past bottom waters. In order to reconstruct these former concentrations, we used an advection/diffusion model. Different scenarios of bottom water chloride and δ 18O variations were accounted for in this model in order to simulate “present day” vertical profiles for concentrations of interstitial water in order to compare them to measured ones. The comparison suggests that the glacial Black Sea was a homogeneous freshwater lake (with a δ 18O of ∼−10‰ and a salinity of ∼1psu). Modern hydrologic conditions would only have been reached at ∼2kacalBP, concomitant with the onset of coccolith-rich thin layers that characterize modern basin sediments. Such delayed salinization (over ∼7000yr) in the basin may have been due to higher precipitation during the early Holocene. We also simulated the impact of a catastrophic reconnection and a smoother reconnection. Both salinity scenarios lead to undistinguishable modelled “present day” profiles, indicating that the precise impact of the last reconnection was lost due to the advection/diffusion processes. [Copyright &y& Elsevier]

Abstract: Ocean variability at decadal time-scales remains poorly described partly because of the scarcity of high temporal resolution marine records. Here, we present a reconstruction of Sea Surface Temperatures (SSTs) over the past two millennia at unprecedented temporal resolution (2 to 5 years), from a marine core located off North Iceland. Alkenone paleothermometry was used to infer SST variability, and tephrochronology to build the age model. Spectral analyses of the SST signal indicate intermittent 20–25 year oscillations, with periods of strong and weak power, that are likely reflecting the ocean response to wind forcing, presumably the North Atlantic Oscillation (NAO). Warmer SSTs and paleo-magnetic proxy data, between 1000 and 1350 year A.D., overlapping the Medieval Warm Period (MWP), suggest enhanced heat transport across the Denmark Strait by the North Icelandic Irminger Current (NIIC). This is in contrast with the subsequent period, which includes the Little Ice Age (LIA), showing continuous cooling towards the 20th century. Reduced NIIC flow through the Denmark Strait likely resulting from higher freshwater and sea ice export from the Arctic would account for the observed colder conditions. [Copyright &y& Elsevier]