No abstract

Sea level rise, especially combined with possible changes in storm surges and increased river discharge resulting from climate change, poses a major threat in low-lying river deltas. In this study we focus on a specific example of such a delta: the Netherlands. To evaluate whether the country's flood protection strategy is capable of coping with future climate conditions, an assessment of low-probability/highimpact scenarios is conducted, focusing mainly on sea level rise. We develop a plausible high-end scenario of 0.55 to 1.15 m global mean sea level rise, and 0.40 to 1.05 m rise on the coast of the Netherlands by 2100 (excluding land subsidence), and more than three times these local values by 2200. Together with projections for changes in storm surge height and peak river discharge, these scenarios depict a complex, enhanced flood risk for the Dutch delta.

The stable isotope compositions of the planktonic foraminifera Globigerina bulloides, Globigerinoides ruber (white and pink varieties), Globigerinoides trilobus, Globorotalia inflata and Globorotalia truncatulinoides (right‐ and left‐coiling types) were examined as recorders of North Atlantic surface water properties based on 40 box‐core surface sediments between 60° and 30°N. While G. ruber (white and pink varieties) and G. trilobus mainly reflect summer surface water conditions in their oxygen isotope composition, G. bulloides reflects temperatures of the northward‐migrating spring bloom, February–March in the south to May–June in the north. Our data show that G. bulloides cannot be regarded as an indicator for summer temperatures as deduced from Duplessy et al. ’s data. Gt. inflata and Gt. truncatulinoides (right‐ and left‐coiling) build their shells in the coldest waters compared with the other species and reflect temperatures between 100 and 400 m water depth. The difference in oxygen isotope composition between G. bulloides and G. inflata serves as a proxy for water mass stratification. G. bulloides is the only species that gives a distinct pattern in its carbon isotopic composition showing a high correlation with the surface water phosphate values along the transect and may serve as a proxy for palaeonutrients and/or productivity.

Abstract. Two sediment cores from the Zaire Fan and the Angola Margin have been investigated for their composition of terrigenous and biogenic constituents, respectively. For the late Quaternary, kaolinite/feldspar ratios and variations of terrigenous element ratios of Zr, Ti, K, Rb, and Al reveal that the composition of Zaire River sediment load has fluctuated in tune with precessional variations of boreal summer insolation. In particular, the correspondence of high kaolinite/ feldspar and Al/K ratios with low-latitude insolation maxima strongly corroborates the assumption that west African monsoonal precipitation and chemical weathering was enhanced during periods of increased central African heating. The most striking feauture observed is that opal accumulation has been 2 to 10 times higher in Zaire Fan sediments than outside in continental margin sediments off Angola, although biogenic Ba and Corg fluctuations from both areas indicate that changes of total paleoproductivity were of the same magnitude in the Zaire River plume and off Angola. From this we infer that the contribution of biogenic opal production to total paleoproductivity has been significantly higher within the Zaire River plume than in the oceanic upwelling regime farther to the south off Angola over the last 200,000 years. The pattern of opal accumulation rates with respect to that of marine organic carbon implies that enhanced opal production off the Zaire River to a great extent was the result of additional fluvial supply of dissolved silica during humid climates characterized by more intense chemical weathering on the continent, while total paleoproductivity created by oceanic upwelling was high in periods of increased zonal trade wind intensity at precessional insolation minima and during cold, more arid glacial climate conditions. We presume that paleoproductivity off the Zaire was controlled by the following two sources of nutrients: (1) marine nutrients including nitrate and phosphate as well as the uptake of Ba on particulate Corg caused by upwelling, and (2) silica, mainly delivered by the Zaire River dissolved silicate. Hence our results underline the importance of dissolved silicate in large rivers for marine silicate cycling in the tropical to subtropical Atlantic at a millenial timescale.