ABSTRACT
ANDREA ABELMANN, RAINER GERSONDE, GREGOR KNORR, XU ZHANG, BERNHARD CHAPLIGIN, EDITH MAIER, OLIVER ESPER, HANS FRIEDRICHSEN, GERRIT LOHMANN, HANNO MEYER, and RALF TIEDEMANN
3.1 INTRODUCTION
The past four climate cycles are characterized by a repetitive pattern of gradually declining and rapidly increasing atmospheric CO2 concentrations, ranging between ~180 p.p.m. during glacials and ~280 p.p.m. during interglacials [1]. Although multiple processes on land and in the ocean are involved in the modulation of the observed CO2 variability [2], physical and biological processes in the Southern Ocean (SO) have been identified to be the key in these changes [3]. This view is supported by the tight re-
lationship between CO2 and Antarctic temperature development [4]. Most important are changes in ocean ventilation/stratification, sea-ice extent, wind patterns, atmospheric transport of micronutrients (for example, iron) and biological productivity and export, according to proxy and modelbased studies [3, 5, 6, 7, 8, 9, 10, 11]. Despite the scientific progress, the different hypotheses on the SO’s sensitivity to modulate the carbon cycle and the identification of involved processes remain under debate. In the SO, the availability of silicon nutrients (Si), the consumption by primary producers (diatoms) and cycling pathways are key for effective carbon sequestration [8, 12]. The widespread deposition of biogenic opal, which consists primarily in diatoms but also in radiolarians and, to a minor extent, in sponge spicules, allows for the application of specific opal-based proxies to trace these processes and related environmental conditions. However, controversial views exist for the interpretation of the proxies used to trace past productivity and their impact on the carbon cycle [3, 5, 13]. Similarly, glacial-interglacial changes in surface ocean stratification, which control ocean atmosphere exchange and the availability of nutrients, have been discussed contentiously. This has resulted in different notions of the impact of physical and biological processes in ice-free and ice-covered areas on the glacial-interglacial climate evolution [3, 5, 13]. Isotope records of diatom-bound nitrogen (δ15N) are interpreted to indicate a lowproductivity glacial seasonal sea-ice zone (SIZ) resulting from constricted nutrient supply to the surface ocean, owing to permanent and enhanced near-surface stratification [3, 13]. Further information on surface water (euphotic zone) conditions comes from oxygen isotopes (δ18O) of diatoms, used to identify meltwater supply from the Antarctic continent [14, 15, 16, 17]. Silicon isotope (δ30Si) measurements on diatoms and sponge spicules provide insights into the development of silicon utilization in surface waters [18, 19, 20] and the silicon inventory of the deep ocean [19, 21, 22]. A yet unexploited window into subsurface and deeper water conditions presents the isotope signal from radiolarians (protozooplankton). In combination with the diatom isotope data, these signals provide an enhanced framework to detect changes of upper and lower water column conditions, and thus the pattern and glacial-interglacial variability of stratification and nutrient exchange.
