ALBERT

Description: Ice sheets in the North American Arctic and, to a lesser extent, those in northern Eurasia calved large quantities of icebergs that drifted through Fram Strait into the Greenland Sea several times during the late Pleistocene. These icebergs deposited Fe oxide grains (45–250 mm) and coarse lithic clasts 〉250 mm matched to specific circum-Arctic sources. Four massive Arctic iceberg export events are identified from the Laurentide and the Innuitian ice sheets, between 14 and 34 ka (calendar years) in a sediment core from Fram Strait. These relatively short duration (〈1–4 kyr) events contain 3–5 times the background levels of Fe oxide grains. They began suddenly, as indicated by a steep rise in the number of grains matched to an ice sheet source, suggesting rapid purges of ice through Fram Strait, due perhaps to collapse of ice sheets. The larger events from the northwestern Laurentide ice sheet are preceded by events from the Innuitian ice sheet. Despite the chronological uncertainties, the Arctic export events appear to occur prior to Heinrich events.

Description: The influence of the overturning circulation on the anthropogenic carbon sink in the North Atlantic is investigated with a simple box model. The net air-sea flux of anthropogenic carbon in the North Atlantic is the result of two opposing fluxes: The first is the uptake caused by the disequilibrium between the rapidly rising atmospheric pCO2 and the dissolved carbon content in the ocean, depending mainly on the water exchange rate between mixed layer and interior North Atlantic ocean. Superimposed is a second flux, related to the northward transport of heat within the Atlantic basin, that is directed out of the ocean, contrary to conventional wisdom. It is caused by a latitudinal gradient in the ratio of seawater alkalinity to total dissolved inorganic carbon that in turn is related to the cooling and freshening of surface water on its way north. This flux depends strongly on the vertical structure of the upper branch of the overturning circulation and on the distribution of undersaturation and supersaturation of CO2 in Atlantic surface waters. A data-based estimate of anthropogenic carbon inventory in the North Atlantic is consistent with a dominance of the disequilibrium flux over the heat-flux-related outgassing at the present time, but, in our model, does not place a strong constraint on the net anthropogenic air-sea flux. Stabilization of the atmospheric pCO2 on a higher level will change the relative role of the two opposing fluxes, making the North Atlantic a source of anthropogenic carbon to the atmosphere. We discuss implications for the interpretation of numerical carbon cycle models.

Description: How well do we know the composition of oceanic crust? Countless studies have described the occurrence of mid-ocean ridge basalt (MORB) at spreading centers, and few would argue that the bulk composition of oceanic crust is other than basaltic. Nevertheless, silicic volcanism (〉55 wt.% SiO2) does occur along part of the northern East Pacific Rise (10.5°N; Thompson et al. [1989]),on the 095° propagator of the Galápagos Spreading Center [Clague et al., 1981],and was recently discovered on the Pacific-Antarctic Ridge (PAR) near its intersection with the Foundation seamount chain [Hekinian et al., 1997, 1999]. Silicic lavas were recovered from a 290-km-long section of the northern PAR adjacent to the active Foundation plume (Figure 1) during cruise 157 of the F/S Sonne, which took place in June and July 2001. Furthermore, widespread hydrothermal activity indicates that the volcanogenic massive sulfidesilicic lava association is not only restricted to subduction and back arc settings.