Question

A. How can the stable isotope 1"N be used to determine trophic level (ie. position a "food chain")? B. How can the radioisotope 234Th be used to measure particle flux from the euphotic zone to the deep ocean?

Answer 1

A)

Use of stable isotopes in the investigation of biological,ecological, and conservation issues has become increasingly popular in the past two decades. Analysis of trophic structure has been founded on an observation that $\delta$ ¹⁵N values increase in a consistent fashion with trophic level increases. Comparison of consumers and their resources indicates that $\delta$ ¹⁵N values increase approximately 3.4‰ with each trophic level.Therefore, by analyzing the $\delta$ 15N values of various organisms within an ecosystem, it may be possible to elucidate part of the trophic structure of the ecosystem .

B)

Thorium-234 is the particle-reactive daughter of uranium-238 which is highly soluble in seawater . Due to the significantly longer half-life of ²³⁸U relative to ²³⁴Th, the two isotopes are in equilibrium (activity ²³⁴Th/²³⁸U =1) when there are no net removal or addition processes acting on either species.

The primary use of this isotope system in the ocean has been to quantify upper ocean particle export .

Particles in the surface ocean from biological production or aeolian deposition scavenge ²³⁴Th. If these particles sink to deeper depths on time scales faster than ²³⁴Th ingrowth, they create a deficit of ²³⁴Th relative to ²³⁸U (²³⁴Th/²³⁸U 1 ) in the upper ocean.This deficit can be quantified and used to derive the flux of ²³⁴Th on sinking particles. Further-more, the sinking fluxes of other particulate species such as carbon, minerals, or trace elements can be estimated by multiplying the ²³⁴Th flux and the ratio of the species of interest to ²³⁴Th on the particles .

²³⁴Th has also been used to identify regions of intense particle remineralization, often immediately below the euphotic zone, when ²³⁴Th/²³⁸U is 1 .