Transcript Document

PALAEOCLIMATE CHANGE (SOES 3015)
Convenor
Prof Paul A. Wilson (PAW)
Other Teaching Staff: Prof. Eelco Rohling(EJR) Dr Bob Marsh (RM) & Dr Sam Gibbs (SG)
Synopsis:
In a series of seven lectures we examine mechanisms of palaeoclimate change in the earth-ocean-atmosphere
system from those operating on short time-scales (eg. abrupt climate change, orbitally paced cycles) to those
operating at multi-million year time-scales (eg. tectonic controls on atmospheric carbon dioxide levels and
configuration of oceanic gateways). Nine lectures are devoted to developing an understanding of the workings
of the palaeoclimate 'tool-kit' and some of the key insights that these proxies have provided on palaeoclimate
change in the past 25 years of research. Three lectures deal with climate change on the anthropogenic timescale linking the past with the future and introducing the IPCC view and ensemble climate modeling. Six
thematic lectures “extreme climates” discuss some of the most dramatic events in climate history seen on
Earth in the past 150 million years such as the dramatic global warming and cooling events that apparently
occurred across the P/E and E/O boundaries respectively.
Professor Eelco Rohling
(Co-convener) SOES 3015
Palaeoceanography (micropalaeontology
and stable isotope geochemistry)
The lectures are accompanied by five practicals designed to help you obtain the
working understanding of the 'tool-kit' that you will need to perform well in the exam.
Aims of Course:
• Our main aim is to ensure that, by the time that you graduate from this institution you
are ready to engage in the public climate change debate and carbon economy
challenge in a knowledgeable way. To meet this objective we aim:
• To provide a general introduction to the study of natural and anthropogenic climate
change in the ocean atmosphere system on historical, millenial, orbital (10 to 100
ka) and tectonic (Ma) time-scales.
• To focus on records of natural climate change locked mostly in marine sediments
and ice sheets.
• To introduce students from a diverse range of backgrounds to active research
questions and area of scientific controversy in this most inter-disciplinary areas of
ocean and earth science.
• In all of the above, to give particular attention to palaeoclimate research areas of
particular strength within SOES.
Learning Outcomes:
At the end of the course you should:
• Have developed a comprehensive understanding of the nature of the modern field of
palaeoclimate change, and, in particular, have gained an appreciation of climate in
the 4th dimension- time.
• Be conversant with the rapidly expanding “tool-kit” and explanatory models that have
been developed to tackle patterns of climate change at different time-scales over the
past 150Ma.
• Have built up an extensive knowledge of case study examples from the research
literature which will serve as a means for you to critically evaluate published models.
• Be confident in the interpretation of palaeoclimate records but realistic as to the
limits of your analysis.
• Be able to find your way through the modern literature jungle in the field of
palaeoclimate.
• Be ready to engage in the public climate change debate and carbon economy
challenge in a knowledgeable way.
In all aspects of the above lectures merely serve as the starting point. If you are
reading for your degree, as the phrase implies, you must, repeat must, read around
the subject.
This is a final level course so there is no “umbrella” text book. Even if there were,
knowing it from cover to cover would probably only get you a (UK) lower
second. This is particularly true for SOES 3015 because palaeoclimate
change is such a young, rapidly evolving subject that nobody has attempted
a comprehensive treatment of the subject since the 1980’s. Final level
courses are advanced in their aims and are targeted to stretch students.
Students should read from a selection of texts and relevant journals. In the
reading lists the key references are marked by an asterisk.
Journals
Where possible we recommend short snappy articles published in the likes of
Nature, Science and Geology but crucial material can often only be found in
more specialist journals such as Paleoceanography, Palaeogeogr.
Palaeoclimatol.& Palaeoecol; Global Biogeochemical Cycles; J.
Foraminiferal Research; Marine Micropaleontology; Earth Planetary Science
Letters etc.
Palaeoclimate Change SOES 3015
Oxygen isotopes I:
(PAW)
Lecture outline:
• Geochemical proxies
-
Introduction to concept of geochemical proxies
Oxygen, its isotopes, notation & standards
• The hydrological cycle
-
Evaporation
Condensation
• Ice core records
-
Brief introduction
www.oceanography.ac.uk
(1) Geochemical proxies
•
•
Introduction to concept of geochemical proxies
Oxygen, its isotopes, notation & standards
(i) Introduction to concept of geochemical proxies
Que: What is a geochemical proxy & why do we use them?
Ans: A chemical signature present in a potential archive that can be used
to track palaeoenvironmental change. One advantage of geochemical proxies is
that they provide at least semi-quantitative constraints.
• There are many potential archives and most involve some kind or coring to
produce a time series record (eg. tree rings, ice cores, lake & ocean sediments).
• Palaeoceanographers are mostly interested in ocean sediments. What
comprises these sediments? Non-biogenic- clays, aeolian dust, ice-rafted
debris (IRD). Biogenic- CaCO3 & SiO2(H2O).
having heard all about the utility of carbonate microfossils such as calcitic
planktonic foraminifera it is no surprise to learn that the isotopes of oxygen &
carbon have been a focus of much work over past 50 years. More recently,
geochemists & palaeoceanographers have expanded into other less obvious
areas (eg. Sr & B isotopes; Mg/Ca, Sr/Ca, U/Ca ratio).
(ii) Oxygen, its isotopes, notation & standards
Oxygen: 3 stable isotopes:
16O
= 99.63%; 17O = 0.0375%; 18O = 0.1995% (abundances)
Isotopes = different varieties of the same chemical element whose atomic
structure have a common number of protons and electrons but a different
number of neutrons in the nucleus. Thus, they have a common atomic number
(O= 8, if written, as a subscript) but a different atomic mass (O= 16, 17 or 18,
written as a superscript). Mass differences cause fractionation*
*any process that causes isotope ratios to differ between phases.
Eg. when carbonate precipitates from water an isotope exchange reaction occurs:
1/3 CaCO163 + H2O18 = 1/3 CaCO183 +
H2O16
which means that the resulting carbonate has an isotopic composition that
is different but related to that of the parent water. This turns out to be very useful
to us but first we have to learn more about notation:
[CaCO183] 1/3 [H2O16]
The equilibrium constant for the above reaction is: K =
[CaCO163] 1/3 [H2O18]
which can be written as:
K=
[CaCO183] / [CaCO163] 1/3
[H2O18] / [H2O16 ]
which is the ratio of 18O/16O in the carbonate phase divided by the
ratio in the water:
Rc
K=
=  (fractionation factor)
Rw
where Rc & Rw are the 18O/16O ratios of carbonate & water
If the two isotopes behave exactly alike, then K=1 and  =1. But we
know from lab expts that  depends on temperature:
For calcite  = 1.0286 @ 25 °C
Given that >1, the calcite is preferentially enriched in 18O
In fact, we can say that calcite grown in equilibrium with water at 25°C
is enriched in 18O relative to the water by 28.6‰ (parts per thousand
or “mil”).
To make life easy we quote 18O/16O ratios relative to a standard value
(determined ages ago) and multiply by a thousand
18
16
18
16
18O = [ ( O/ O) spl – ( O/ O) std
(18O/16O) std
] x 1000
We quote to:
• SMOW (standard mean oceanographic water) when we measure 18O
in water
• PDB K belemnite, (Peedee Fm., South Carolina) when we measure
18O in CO3
(18OSMOW = 1.03086 18OPDB + 30.86)
which reflects the fact that  >1 (see above)
Que: So how is any of this useful to us?
Ans: We can use 18O values to:
• trace ’s in palaeo sea water 18O- ice volume & salinity
• estimate temperature of formation- exploit the fact that 
’s in predictable way with ’s in temperature
• caveats- ‘vital’*, ecological§ & diagenesis¶
*non-equil. behaviour- recall that we are using biogenic calcite
§ depth habitat, ontogeny
¶ post-depositional alteration is a fact of life
But first we need to understand how 18O’s evolve through the
hydrological cycle
(2) The hydrological cycle
• Evaporation
• Condensation
(i) Evaporation
Isotopic exchange at air-sea interface (evaporation)
H216Owater + H218Oair = H218Owater + H216Oair
18
16
Fractionation Factor:
 = [ O/ O] water ~ 1.0092 @ 25 ºC
illustrates:
[18O/16O] air
• preferential uptake of 16O with evaporation
• remaining water enriched in 18O
• important to palaeo sea water 18O & thus ice volume & salinity records
Que: Why does 16O evaporate preferentially?
Ans: Vapour pressure* of different isotopic molecules of water is inversely
proportional to mass  H216O has a significantly greater vapour pressure
than H218O.
*pressure exerted by molecules in vapour state at equilibrium with molecules
in the liquid state- thus a measure of the tendency of water to exist in the
gaseous or vapour state
(ii) Condensation
Fractionation processes during water droplet formation are the same as above but
operate in reverse.
Thus, when raindrops form in a cloud by condensation of water vapour, the liquid
phase is enriched in 18O such that the isotopic composition of the first raindrops
is similar to that of ocean water.
This means that the condensation of water in equilibrium with water vapour and its
subsequent removal from a cloud can be described by a Rayleigh distillation eqn:
where:
R
= f (-1)
R0
R = 18O/16O ratio of the remaining vapour
R0 = 18O/16O ratio of the vapour before condensation starts,
f = fraction of vapour remaining &
 = isotope fractionation factor = Rl/Rv.
Now, if we convert the isotope ratios R & R0 to  notation:
(18O)v + 1000
R
R0 = (18O)0 + 1000
= f (-1)
solve for 18Ov = [(18O)0 + 1000] f (-1) – 1000
and assume:
= 1.0092 (~20°C)
(18O)0= -9.2‰
then we can plot 18Ov as a function of the fraction of water vapour remaining (f)
Result: we see that the 18O value of the
remaining vapour decreases
(ie. vapour becomes progressively enriched
in 16O) as condensation progresses.
similarly, we can plot the value of the
condensate in equil. with vapour (18Ol) as a
function of the fraction of water vapour
remaining:
18Ol = (18Ov + 1000) – 1000
Result: the precipitation falling from the
cloud also decreases (becomes enriched
n 16O) through time.
Therefore, the result of isotopic fractionation during
evaporation from sea surface and condensation of vapour in
clouds through the global water cycle is that fresh (meteoric)
water becomes progressively isotopically “light”
From: University of Southampton
In fact, if we measure 18O in mean annual precipitation globally we find
strong linear correlations w/ air temperature or latitude:
This reflects the fact that both the
magnitude of rain-out (progressive
condensation) and isotopic fractionation
factor increase with decreasing
temperature (increasing latitude).
18Omean = 0.695T –13.6 (Dansgaard ‘64)
Both figures: Reproduced by permission of American Geophysical Union:
Rozanski, K., Isotopic Patterns in Modern Global Precipitation. (1993)
Geophysical Monograph v. 78. Copyright [1993] American Geophysical Union
“Noise” in these
correlations arises from
5 effects:
• seasonality effects@mid- to high
latitudes ( in Temp &
source of water vapour)
• continentality effect- more –ve
values in-land- progressive
removal of 18O
• altitude effects- more –ve values
up-slope progressive removal of
18O (ocean is source of water
over continents)
Reproduced by permission of American Geophysical Union: Rozanski, K.,
Isotopic Patterns in Modern Global Precipitation. (1993) Geophysical
Monograph v. 78. Copyright [1993] American Geophysical Union
• amount effectsin tropics strong
inverse relationship between
P (mm) & 18Owhy?
‘cos big stormsare unusually “thorough”
in terms of rain-out
& convect to great altitudes
Reproduced by permission of American Geophysical Union: Rozanski, K., Isotopic Patterns in Modern Global
Precipitation. (1993) Geophysical Monograph v. 78. Copyright [1993] American Geophysical Union
“
• the snow effect- snow is even
lighter than “should be” for its
temperature (greater 
Reproduced by permission of American Geophysical Union: Rozanski, K.,
Isotopic Patterns in Modern Global Precipitation. (1993) Geophysical
Monograph v. 78. Copyright [1993] American Geophysical Union
(3) Ice core records
(i) A brief introduction
It follows from the above (inverse relationship between fractionation factor &
temperature) that, to first approximation, 18O snow in polar ice caps and
“alpine” glaciers reflects temperature- strongly –ve w/ significant variations
on seasonal to geological timescales.
This discovery has given birth to an exciting whole new field of
palaeoclimate research in the past 20 years: ice core records- precipitation,
atmospheric dust &, atmospheric gases (ancient air trapped in bubbles).
The field- unusual ‘cos all pioneers were European. Ice cores drilled @
Greenland- “GRIP & GISP”, Antarctic “Vostok”.
The importance- initially 18O snow –LGM v. obvious providing irrefutable
evidence of rapid and pronounced N. hemisphere climate change (~10°C
within 100yr). Now main focus is on records of atmospheric gases
Timescales- past 10ka almost as good as tree rings- count layers, >10 ka
measure conductivity by ~80ka ± ~5ka
An example of ice
core records:
Reprinted by permission from Macmillan Publishers Ltd: Asynchrony of Antarctic and Greenland climate change during the lastglacial period
Blunier, T., Chappellaz, J., Schwander, J., Dällenbach, A., Stauffer, B., Stocker, T.F., Raynaud, D., Jouzel, J., Clausen, H.B., Hammer, C.U.,
Johnsen, J.L., Nature, v. 394, p. 739-743. Copyright (1998) Not under CC licence.
pCO2 and relative change in air temperature:
Vostock ice-core records, Antarctica
Reprinted by permission from Macmillan Publishers Ltd: Climate and atmospheric history of the past 420,000 years from the Vostok ice core, Antarctica, Petit, J.R., Jouzel, J., Raynaud,
D., Barkov, N.I., Barnola, J-M., Basile, I., Bender, M., Chappellaz, J., Davis, M., Delaygue, G., Delmotte, M., Kotlyakov, V.M., Legrand, M., Lipenkov, V.Y., Lorius, C., PÉpin, L., Ritz, C.,
Saltzman, E., Stievenard, M., Nature, v. 399, p. 429-436. Copyright (1999) Not under CC licence.
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