Constraints on the hydrological cycle in Antarctica during the Neogene
Rhian L. Rees-Owen*1, Robert J. Newton1, Ruza F. Ivanovic1, Julia Tindall1, Jane E. Francis2, James B. Riding, Alan Haywood1, Christopher H. Vane3, Raquel Lopez Dos Santos3
E-mail: [email protected]; 1University of Leeds, UK; 2British Antarctic Survey, UK; 3British Geological Survey, UK
Antarctic palaeoclimate in a warmer world
Transantarctic Mountains
Oliver Bluffs
Ross Ice Shelf
There is increasing evidence for a dynamic East Antarctic Ice Sheet
during warmer periods of the Miocene and Pliocene (eg. [1]). Much less is
known about terrestrial palaeoclimate on the Antarctic continent during
these periods of ice sheet retreat. Reconstructing past hydrological
change over the continent is a key part of understanding Antarctica’s
response to warmer worlds.
Exceptionally preserved fossil southern beech trees from Oliver Bluffs in
the Transantarctic Mountains (85° S) are a record of the last woody
vegetation on Antarctica. Although their age is uncertain, most
evidence suggests a pre-mid-Miocene age. Regardless, this site is a
unique opportunity to investigate Antarctic palaeohydrology
during ice sheet retreat in a warmer Neogene world.
Fig 3 Reconstructed ancient precipitation d18O against modern precipitaiton Antarctic
values (from Valsson-DeMotte 2006). Cellulose δ18O is strongly governed by relative
humidity so a conservative range was used for the reconstruction. A temperature range of
3-7°C was taken from our geochemical palaeothermometer at the site (not discussed here).
Results I: Reconstructing
18
precipitation δ O
•
•
1 cm
Fig 1 Map showing Oliver Bluffs in the Transantarctic Mountains (top); photo of
mummified wood fragment in the field (bottom).
Data-model approach
1. Tree ring oxygen isotopes → reconstruct ancient
precipitation δ18O
Oxygen isotopes in plants
The relationship between the oxygen isotope
composition (δ18O) of tree ring cellulose (1) is
controlled by:
• Relative humidity (2)
• Source water δ18O (3)
From this, we can reconstruct precipitation
δ18O (4) from measured cellulose δ18O [2; fig.
2]. This gives us insight into climatic
parameters such as
moisture transport and source water region for
the time period of interest.
•
•
Change is largely driven by
temperature [3].
•
Increased precipitation (fig. 4B),
changes to cyclonicity and a
weakened polar cell (fig. 4C-F) →
changes in source region and
internal moisture distribution.
Reconstructed ancient Antarctic precipitation is -10
to -30 ‰.
Comparison with modern Antarctic
precipitation isotopes at similar sites shows
an enrichment of ~8‰.
•
First observational evidence for a shifted
Antarctic hydrological in a warmer
Neogene world.
Observational data for precipitation
δ18O is broadly consistent with results
from oxygen isotope-enabled HadCM3
general circulation model (fig. 4A).
-10
Ancient precipitation δ18O is reconstructed from measured
oxygen isotope analysis of tree ring cellulose from the fossil
plants, using a range of environmental parameters.
•
Results II: Further
investigation using a
climate model
2. Oxygen isotope-enabled climate model →
understanding mechanisms
0
-20
-30
-40
-50
-60
Ancient
Modern
precipitation precipitation δ18O
δ18O
(>75°S, <700 m)
Modern
precipitation
δ18O
(>75°S)
Modelled
ancient
precipitation
δ18O
(HadCM3)
Caveats
•
Age uncertainty
•
Regional or local signal?
•
Large range of reconstructed
δ18O – due to conservative
relative humidity range
Conclusions and
implications
Fig 4 Results from an idealised palaeoclimate model run on
oxygen-isotope enabled HadCM3. Anomalies are palaeo
simulation minus pre-industrial control. a) Precipitation δ18O
anomaly b) Precipitation anomaly c) Wind vectors preindustrial, 100 m atmospheric level d) Palaeo wind vectors,
100 m atmospheric level e) Pre-industrial wind vectors 7000 m
f) Palaeo wind vectors 7000 m. Idealised palaeo scenario: 405
ppmv CO2; prescribed initial vegetation from PRISM3 with
MOSES2 dynamic vegetation; modern geography. Control: preindustrial simulation, CO2 280 ppmv
Tree ring isotope analysis and combined with
an oxygen isotope-enabled general
circulation model give us evidence for a
shifted Antarctic hydrological cycle in a
warmer Neogene world.
Initial model data indicates changes driven by
temperature, internal recycling and source.
Implications for ice sheet mass balance and sea
level calculations [4].
Fig 2 Cartoon of oxygen isotope fractionation in trees (adapted from [2]).
1. Cook et al, Nature, 2013; 2. Roden, Lin & Ehleringer, GCA, 2000; 3. Tindall & Haywood, Paleoceanography 2015; 4. Winnick and Caves, Geology, 2015.