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 2004 Lecture 15 Other Tracers I. Paleoproductivity and related issues A. Organic carbon preservation: preserved organic carbon is only 1-2% of new production flux. If the fraction preserved is constant, then the accumulation rate of organic carbon is a measure of changes in new production. BUT… B. Barium: sinking organic matter is enriched in Ba, with a (roughly) constant Ba:org C ratio. Ba is preserved in sediments as BaSO4 to a much higher extent than organic carbon (up to 40%). Then it might serve as a better-preserved paleoproductivity indicator. Selective extraction of barite (denser than most other mineral phases…) provides a complement to bulk Ba, as well as a carrier phase for other elements (Ra, Sr, etc.) C. Nitrogen Isotopes and "nutrient utilization efficiency". Phytoplankton preferentially take up 14NO3- (by about 5‰). Uptake in a closed system proceeds according to Rayleigh distillation, with progressive enrichment of 15N in the photosynthetic product. In a high productivity environment with low nitrogen utilization efficiency, δ15N of the photosynthetic product is depleted relative to seawater δ15N by about 5‰. In an oligotrophic environment “what goes up comes down”, so the isotopic composition of the integrated photosynthate is the same as in the upwelling NO3-. Although there is significant further isotopic shifts on sinking particulate matter and during diagenetic transformation on the seafloor, the bulk organic matter reflects trends in nitrogen utilization in surface waters.2-5.00.05.010.015.020.025.00.00 0.20 0.40 0.60 0.80 1.00 1.20fraction remaining in solutionIsotope ratio of residual seawaterIsotope ratio of accumulated organic matterIsotope ratio of instantaneous biological mattersource seawater D. Pa/Th: production ratio (atoms) = 24.5 (0.093 activity ratio). Because Th is scavenged more readily by falling particulate matter (res. time 20-60 yrs), Pa (res. time 200-500 yrs) tends to migrate to sites of high productivity, resulting in higher Pa/Th ratios in sediments under high productivity areas. Complication: deep water movements are also a factor; high-production sites within a basin compete with each other. II. Boron isotope paleo-pH: fractionation of δ11B due to isotopic fractionation between B(OH)3 and (OH)4- B3 Image removed due to copyright considerations.4 III. δ13C of organic matter: paleo pCO2 or what?: The Rubisco enzymatic pathway can be limited by available free CO2 within a cell. As aqueous CO2 becomes limiting, the isotopic composition of organic matter becomes more limited by transport into the cell, hence shifting the isotopic composition towards that of the free aqueous CO2. One complication in using bulk organic matter as such a tracer is that different compounds have different δ13C compositions; this effect can be minimized by using single-compound δ13C. Another complication: there is also a clear ell size/growth rate effect as well as an external aqueous CO2 effect. c IV. Silica tracers: δ180, δ13C, δ15N, Ge/Si, δ30Si V. Isotopic-faunal paleosalinities VII. Dinoflagellates as biotic tracers References: Altabet M. A., Deuser W. G., Honjo S., and Stienen C. (1991) Seasonal and depth-related changes in the source of sinking particles in the North Atlantic. Nature 354, 136-138. Image removed due to copyright considerations.Source: Honisch et al. (2005).5 Altabet M. A. and Francois R. (1994) Sedimentary nitrogen isotopic ratio as a recorder for surface ocean nitrate utilization. Glob. Biogeochem. Cycles 8, 103-116. Bidigare R. R., Fluegge A., Freeman K. H., Hanson K. L., Hayes J. M., Hollander D., Jasper J. P., King L. L., Laws E. A., Milder J., Millero F. J., Pancost R., Popp B. N., Steinberg P. A., and Wakeham S. G. (1997) Consistent fractionation of 13C in nature and in the laboratory: Growth-rate effects in some haptophyte algae. Glob. Biogeochem. Cycles 11, 279. Dymond J., Suess E., and Lyle M. (1992) Barium in Deep-Sea Sediment: A Geochemical Proxy for Paleoproductivity. Paleoceanogr. 7, 163-182. Francois R., Altabet M. A., Yu E.-F., Sigman D. M., Bacon M. P., Frank M., Bohrmann G., Bareille G., and Labeyrie L. D. (1997) Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period. Nature 389, 929. Ganeshram R. S., Pedersen T. F., Calvert S. E., and Murray J. W. (1995) Large changes in oceanic nutrient inventories from glacial to interglacial periods. Nature 376, 755-758 Martin E. E., MacDougall J. D., Herbert T. D., Paytan A., and Kastner M. (1995) Strontium and neodymium istopic analyses of marine barite precipitates. Geochim. Cosmochim. Acta 59, 1353-1361 Paytan A., Kastner M., Martin E. E., Macdougall J. D., and Herbert T. (1993) Marine barite as a monitor of seawater stronium isotope composition. Nature 366, 445 Sanyal A., Hemming N. G., Hanson G. N., and Broecker W. S. (1995) Evidence for a higher pH in the glacial ocean from boron isotopes in foraminifera. Nature 373, 234-236. Shemesh A., Charles C. D., and Fairbanks R. G. (1992) Oxygen isotopes in biogenic silica: global changes in ocean temperature and isotopic composition. Science 256, 1434-1436. Sigman D. M., Altabet M. A., McCorkie D. C., Francois R., and Fischer G. (1999) The δ15N of nitrate in the Southern Ocean: consumption of nitrate in surface waters. Glob. Biogeochem. Cycles 13, 1149. Sigman D. M., Altabet M. A., McCorkle D. C., and Gaillard J.-F. (1999) The isotopic composition of diatom-bound nitrogen in Southern Ocean sediments. Paleoceanogr. 14, 118. Spivack A. J., You C.-F., and Smith H. J. (1993) Foraminiferal boron isotope ratios as a proxy for surface ocean pH over the past 21 Myr. Nature 363,