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MIT OpenCourseWare http://ocw.mit.edu 12.740 PaleoceanographySpring 2008For information about citing these materials or our Terms of Use, visit: http://ocw.mit.edu/terms.1PALEOCEANOGRAPHY 12.740 SPRING 2006 Lecture 9 DEEP-OCEAN PALEOCEANOGRAPHY AND OCEAN CHEMISTRY: LINKS TO ATMOSPHERIC CARBON DIOXIDE and 14C/12C I . Deep water paleoceanography A. In the 1970’s, it was hoped that benthic foraminiferal populations could be used in "Imbrie-Kipp" mode to reconstruct deep ocean physical and chemical properties. This idea has been long on hope and slim on results, however. 1. There are many more species of benthic foraminifera, but these are scarce compared to planktonic foraminifera (B/P ~ 1%). Studies of benthic foraminifera require much larger samples (10g or more) and take much longer to process and identify. 2. Benthic foraminifera do show large-scale variations throughout the present ocean, and temporal changes are recorded in sediment cores. The first studies were of necessity limited to small regions, and the temptation to overdo the planktonic analogy was great; hence depth-correlated changes in the modern ocean were at first attributed to temperature. (e.g. see Streeter, Schnitker studies). 3. Subsequent work has shown that, unlike planktonic foraminifera, correlations with temperature do not hold globally. This lack of correlation probably is due to the extremely small range of variation of temperature (a few °C) in the deep ocean. There have been attempts to attribute the observed variations to other deep-water parameters (e.g. oxygen content, carbonate saturation, or even the vaguer term "Uvigerina water" which implies that something in the bottom water controls the Uvigerina population). So far, none of these attempts has a "residence time" of more than a few years. So far, benthic foraminifera have defied a systematic generalization of the factors controlling their species composition. A recent paper by P. Loubere characterizes benthic foram census data from the Pacific in terms of surface productivity and deep water oxygen content. 4. It is likely that factors other than deep water properties influence the benthic foraminiferal populations: e.g., food (from the surface) - a correlation between benthic foram flux and productivity has been observed; sedimentation of non-food constituents; other aspects of the sedimentary environment (deep currents, grain size, etc.). One question of some interest to this "other factors" issue is: where do benthic foraminifera live? If they live at some depth in the sediment, the chemical environment they respond to can be different from those at the bottom of the ocean: while T is the same, the chemical composition of the water, the food sources, oxygen, etc. may differ significantly from that of the bottom water. (To be expanded upon later.) 5. Few paleoceanographic interpretations based on benthic foraminifera populations have survived more than a few years. One partial exception is the study of Streeter and Shackleton (1979) who used glacial increases in the Uvigerina abundance to argue that North Atlantic Deep water ceased or drastically diminished. Although the extent of the decrease is now thought to be considerably less than the2foraminiferal populations would suggest, it is agreed that there was less North Atlantic Deep Water in the deepest North Atlantic during glacial times, and so at least the sign was right! Images removed due to copyright considerations.Source: Streeter and Shackleton, 1979, figure 1 and 2.4B. Carbon isotopes as a tracer of deep ocean paleoceanography 1. The distribution of δ13C in the modern ocean is linked to oxygen and nutrients. a. Organic matter is depleted in 13C. This depletion occurs mainly during the enyzymatic conversion of inorganic CO2 (itself somewhat depleted related to total dissolved CO2, which is mainly HCO3-) to organic matter by plants, mainly a kinetic effect. Much of the carbon uptake by marine photosynthetic organisms is achieved by the transport of free aqueous CO2 (which is depleted in 13C by ~10‰) across cell membranes (although there are also some ways to use anionic HCO3- directly). Furthermore, because CO2(aq) concentrations are low (10-20 µmol/kg), the cell membrane environment can be depleted in CO2(aq) during rapid photosynthesis because the conversion of HCO3- to CO2. is relatively slow. Enzymatic (Rubisco) and other kinetic effects within plants add to this 12C depletion, resulting in the δ13C of marine organic ranging from -20‰ (tropical and subtropical waters) to -30‰ (Antarctic). b. When this organic matter sinks into the deep ocean, it quantitatively decomposes (~99% in the whole water column; ~80% in the upper 500 m), releasing the bound C, N, and P in the same ratio, and consuming O2 from the water in the process. The CO2 released during this respiration process is depleted in 13C, so that the δ13C of the deeper water lower than that of the original surface waters. A plot of δ13C vs. P for the modern ocean is linear. c. Hence it is possible to use the δ13C distribution in the ocean as a tracer of oceanic water masses. 2. Because δ13C is recorded in the shells of planktonic and benthic foraminifera, it is possible to determine the nutrient distribution in past oceans. a. Complication: δ13C in forams is not in equilibrium or even close to it; in fact, different species have significantly different offsets from the δ13C of the water.5 b. Complication: at least some species (and perhaps all) (especially Uvigerina) appear to have variable offsets from bottom water composition related to the productivity of the overlying surface ocean. Image removed due to copyright considerations.Source: Duplessy et al. (1984).6Images removed due to copyright considerations.Source: Zahn et al. (1987).7c. Complication: where in the sediment do foraminifera live? If they live at some depth in the sediment, δ13C can be significantly different from that of the bottom water. d. Complication: The average δ13C of the ocean is not constant, due to a significant transfer of terrestrial reduced organic into the oceans during glacial periods (tropical aridity and destruction of high-latitude hardwood forests: Shackleton (1977). This means that a local downcore records of δ13C have a large (and often dominant) component due to changes in the ocean average δ13C, and it is necessary to somehow correct for this in determining the


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