Dr Susan Little GEOL0017
GEOL0017 Part 2 Assessed Practical
Please submit your answers to this exercise on Moodle by
1pm, Friday 5 April
This practical is designed to apply the principles of stable isotope geochemistry
introduced in Week 7 and studied for several different isotope systems in weeks 8–10.
It addresses the following learning outcomes:
 Understand and apply stable isotope notation.
 Use stable isotope ratios to constrain oceanic mass balance calculations.
The number of marks for each question is given in brackets, e.g., (2 marks). The total
available is 40 marks. This practical is worth 15% of your final mark for GEOL0017.
Part A: Stable Isotope Basics (16 marks)
Choose one stable isotope system. Complete the following exercise using your chosen
element as an example. Write down your chosen element below.
Chosen Element: ………………………
Grading Note: The mark for this part will be capped at 50% if you choose an element that we
study in class (any of: H, O, C, N, B, S, Mo, Cr)!
Complete the following tasks.
a) (2 marks) List the main isotopes of the element you have chosen, noting whether each
isotope is stable or radiogenic. Note: You may ignore short-lived radioisotopes.
b) (2 marks) Which stable isotope ratio is reported in published literature? If more than one
ratio is reported, write the ratio that you find reported most often.
c) (2 marks) Which isotopic reference standard is most commonly used?
d) (2 marks) Write down the definition of δ-notation for your element, using your answers to
part b) and c).
e) (8 marks) Find one recent paper (published since 2015) that reports data for your isotope
system. Briefly summarise the findings (Max. 200 words). Include the paper reference in
your answer.
Grading Note: You must not choose the same paper as another student. You should email
me (susan.little@ucl.ac.uk) with your chosen paper by Thursday 4 April (or earlier). If another
student has already selected the paper you have chosen, I will tell you to choose again. If you
do not tell me the paper you have chosen, and you select the same paper as someone else,
you will receive zero marks for this part.
Dr Susan Little GEOL0017
Part B: Oceanic mass balance (24 marks)
Residence time is an important concept in many aspects of Earth Sciences. It refers to the
average lifetime of a substance (e.g., sulfate) in a reservoir (e.g., the ocean) of interest.
Residence time is calculated using this formula:
τ= A
F¿
(Equation 1)
Where τ is residence time, A is the total amount of substance in the reservoir (e.g., in moles),
and Fin is the flux into the reservoir per unit time (e.g., mol yr-1).
a) (1 mark) Sulfate concentrations in the modern ocean are 28 mmol kg-1. There are 1.37 ×
1021 kg of water in the ocean. Calculate many moles of sulfate there are in the ocean.
b) (3 marks) Rivers are the main input flux of sulfate to the modern ocean, with a flux of 3 ×
1012 mol yr-1. Calculate the residence time of sulfate in the ocean. Is the residence time of
sulfate longer or shorter than the ocean mixing time?
Figure 1 shows a simple one-box model of the oceanic mass balance of sulfate.
Figure 1. One-box model of the oceanic mass balance of sulfate.
It is commonly assumed that the ocean is at ‘steady state’, meaning that the concentration
(and isotopic composition) of an element does not change through time. In practice, steady
state means that the oceanic input flux must equal the oceanic output flux:
ΣFin = ΣFout (Equation 2)
Where ‘Σ’ means ‘the sum of’, for example, ΣFout refers to the sum of all the output fluxes. i.e.,
For the box model of oceanic sulfate: ΣFout = Fevaporite + Fpyrite.
Equation (2) can be expanded to include the S isotope compositions of the input and output
fluxes, providing an additional constraint on oceanic mass balance:
Σ(Fin⋅δin) = Σ(Fout⋅δout) (Equation 3)
c) (2 marks) Write out equation (3) for the oceanic mass balance of sulfate and S isotopes as
illustrated in Figure 1. You should include the following terms in your equation:
Friver, Fevaporite, Fpyrite, δ34Sriver, δ34Sevaporite, δ34Spyrite
EVAPORITES
BIOGENIC PYRITE
RIVERS SEAWATER
SULFATE
Dr Susan Little GEOL0017
Flux magnitudes and δ34S values for the simple one-box model are given in Table 1:
Table 1: Simplified modern oceanic mass balance of sulfate and δ34S
Source or Sink Flux (mol yr-1) δ34S (‰)
Rivers 3 × 1012 +5.0
Biogenic Pyrite 2 × 1012 -2.5
Evaporites 1 × 1012 Part (e)
d) (2 marks) Are the fluxes for the S cycle given in Table 1 in steady state?
e) (3 marks) Solve Equation 3 (part c) to find the S isotope composition of evaporites.
It is generally assumed that evaporites record the isotopic composition of seawater sulfate
without fractionation, i.e., δ34Sevaporite = δ34Socean
f) (1 mark) Given this relationship, what is the S isotope composition of seawater sulfate
according to the simple box model?
Biogenic pyrite is formed via the process of bacterial sulfate reduction, which is associated
with a kinetic isotope fractionation. In the box model, pyrite is isotopically fractionated from
seawater sulfate by –22.5‰. i.e., Δ34Spyrite-ocean = –22.5‰.
g) (3 marks) Rewrite the oceanic mass balance equation from part (c), replacing δ34Sevaporite
and δ34Spyrite with terms containing δ34Socean.
Hint: You will first need to write an equation for δ34Spyrite in terms of δ34Socean.
h) (3 marks) Imagine there is a major tectonic event, which exposes a lot of pyrite minerals to
oxidative weathering, lowering the average δ34S of rivers to +1‰. The fluxes of sulfate to
and from the ocean don’t change. What is the new steady state δ34S of the ocean?
i) (6 marks) Describe three limitations of the one box model of the oceanic mass balance of
S. (Max. 200 words)