Michel Fontugne

Glacial‐interglacial variability in denitrification in the World's Oceans: Causes and consequences

The late Quaternary history of water-column denitrifcation of the eastern Pacific margins and the Arabian Sea is reconstructed using sedimentary δ15N measurements. The δ15N values in six piston cores raised from these regions show remarkably similar cyclic variations, being heavy (9–10.5‰) during the interglacials and 2–3‰ lighter during the glacials. This implies that denitrification in these regions decreased substantially during the glacial periods. The glacial decline in denitrification is attributed to reduced upwelling and flux of organic material through the oxygen minimum zone. Since water-column denitrification in these areas accounts for about half of the fixed-nitrogen loss in the modern ocean, the inferred decrease in denitrification should have increased the oceanic nitrate inventory during glacial periods. Because nitrate is a limiting nutrient, oceanic productivity and attendant changes in CO2 may therefore have been modulated on glacial-interglacial timescales by variations in the oceanic NO3 content.

On the late Pleistocene-Holocene sapropel record of climatic and oceanographic variability in the eastern Mediterranean

Sapropels and intercalated marls in a piston core from the eastern Mediterranean are chemically and mineralogical distinct: kaolinite, smectite, and total S contents, Fe/A1, Ba/A1, Co/A1, Mo/A1, and V/A1 are higher, whereas quartz, Mg calcite, illite contents, Si/A1, Ti/A1, K/A1, Rb/A1, and Zr/A1 are lower in the sapropels. Missing and “ghost” sapropels are identified by mineralogical and chemical properties that are not prone to diagenesis. Primary production was higher (Ba/A1) and bottom water and/or interstitial oxygenation was lower (Co/A1, Mo/A1, Ni/A1, and V/A1) during sapropel formation. Wind speeds (quartz/clay, Si/A1, and Zr/A1) and bottom water salinities (Mg calcite/calcite) were higher during periods of marl formation. Sapropels represent a fundamentally different sedimentary facies whose formation is linked to changes in the hydrological balance in the basin, driven by precessionally modulated changes in monsoon strength and subtropical precipitation changes, which altered circulation, production, and sediment source areas.

Iron control of past productivity in the coastal upwelling system off the Atacama Desert, Chile

[1]xa0Biogenic opal and organic carbon vertical rain rates in sediment cores reveal a strong cyclicity in the productivity of the upwelling system off presently arid northern Chile during the last 100,000 years. Changes in productivity are found to be in phase with the precessional cycle (∼20,000 years) and with inputs of iron from the continent. During austral summer insolation maxima, increased precipitation and river runoff in the region appear to have brought high inputs of iron, mainly from the Andes, to the coastal ocean enhancing primary productivity there. We interpret our results as providing evidence for iron control of past productivity in this upwelling system and for a tight link between productivity and orbital forcing at midlatitudes.

Spatial variations in nutrient utilization, production and diagenesis in the sediments of a coastal upwelling regime (NW Africa): Implications for the paleoceanographic record

A biogeochemical study of recent (multicores) sediments of the northwest African slope was undertaken to understand how the sediment composition varies with respect to the location of core sites relative to the centers of coastal upwelling, and how this has affected the palaeoceanographic record. Sedimentary organic carbon contents are inversely correlated with the nitrogen isotopic composition (δ 15 N), high C organic concentrations and low δ 15 N occurring at proximal (shallow) sites and the opposite at distal (deep) ones. These spatial differences are interpreted to result from higher relative nutrient utilization and a decrease in production as waters are advected offshore from the zone of upwelling. Highest C organic contents also correlate positively with highest concentrations of redox-sensitive elements (U, Mo and S) that are fixed diagenetically in the sediments. These results suggest that the sedimentary regime at a fixed position depends on the spatial location of the productive areas relatively to a given core site. Downcore records of Zr/Al, Ti/Al, mean grain size of the terrigenous fraction, δ 15 N, C organic , biogenic Ba, U, Mo and sulfur at a single site on the slope are interpreted to reflect glacial-interglacial changes in the core location relative to the coastline (sea-level effect), and hence changes in production as the area of coastal upwelling moved on- and offshore as sea-level changed, as well as undoubtedly changes in upwelling intensity through wind forcing. Further studies are needed to fully understand the interrelationships of all these processes, which are required for building more reliable paleoceanographic-paleoclimatic records.