Birgit Gaye-Haake

Millennial‐scale oscillation of denitrification intensity in the Arabian Sea during the Late Quaternary and its potential influence on atmospheric N2O and global climate

The intensity of denitrification in the Arabian Sea during the last 65 kyr is reconstructed using high-resolution δ 15 N records of three sediment cores in conjunction with other geochemical tracers for water column oxygenation and productivity. The results reveal a close link to the Greenland ice core record with low or absent water column denitrification during the Last Glacial Maximum, the stadials, and at the time of the Heinrich Events including the Younger Dryas. In contrast, denitrification was high during the Holocene and the interstadials. The intensification of denitrification is related to stronger SW monsoonal upwelling, which enhances organic matter flux and degradation, resulting in a strengthening of the midwater oxygen deficiency. Such a combination of enhanced upwelling and denitrification has also been implied for the Eastern Tropical North Pacific (ETNP), where these events occur during the Holocene and to some extent during the interstadials, too. Today, the Arabian Sea and the ETNP together contribute substantially to the global marine water column denitrification, and a significant fraction of the ocean-atmosphere N 2 O flux originates from these areas. Changes in N 2 O emissions from these areas could thus have effected the recently described stadial/interstadial variations in the atmospheric concentration of this greenhouse gas as deduced from ice cores. Moreover, denitrification is the major sink for oceanic nitrate and provides a primary control for the oceanic nutrient inventory, which in turn influences global primary productivity and CO 2 sequestration by the biological pump. Short-term switches between a nondenitrification mode and a denitrification mode in these marine regions therefore have an impact on global climate.

Experiments on the influence of sediment disturbances on the biogeochemistry of the deep-sea environment

In experiments with large undisturbed sediment cores, stirring the upper centimeters of sediment immediately released dissolved pore-water constituents such as heavy metals and nutrients. Rapid scavenging in the suspension after the disturbance was observed for cationic particle-reactive metals like Mn, Co, Ni, Zn, Cu, Cd, Pb, and Fe, while anionic metal species like Mo, V, and U showed neither strong release nor sorption. Microbial activity, fresh particulate organic carbon and nitrogen, and amino acids and hexosamines increased during the first days after disturbance. The ratios of C and N and of the amino acids also changed with time. Applicability of the results of laboratory experiments to in situ deep-sea conditions supports the necessity of in situ experiments in benthic chambers.