University:
Massachusetts Institute of Technology
Course:
12.740 | Paleoceanography
Academic year:

2023
Views:

117

Pages:

4
Author:

rumendracontium

1 PALEOCEANOGRAPHY 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 14 NO3- (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 15 productivity environment with low nitrogen utilization efficiency, δ N 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 Isotope ratio of residual seawater Isotope ratio of accumulated organic matter Isotope ratio of instantaneous biological matter 25.0 20.0 15.0 10.0 source seawater 5.0 0.0 0.00 0.20 0.40 0.60 0.80 1.00 1.20 fraction remaining in solution -5.0 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 B(OH)4- 3 Image removed due to copyright considerations. 4 Image removed due to copyright considerations. Source: Honisch et al. (2005). 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 cell size/growth rate effect as well as an external aqueous CO2 effect. 30 IV. Silica tracers: δ180, δ13C, δ15N, Ge/Si, δ Si V. Isotopic-faunal paleosalinities