Antarctic Climate Evolution

593 Pages·2008·40.1 MB·English

by  Fabio Florindo and Martin Siegert (Eds.)

Developments in Earth & Environmental Sciences, 8
ANTARCTIC CLIMATE
EVOLUTION
Edited by
FABIO FLORINDO
Istituto Nazionale di Geofisica e Vulcanologia, 00143 Roma, Italy
MARTIN SIEGERT
School of GeoSciences, Grant Institute, University of Edinburgh, Edinburgh
EH9 3JW, UK
Amsterdam – Boston – Heidelberg – London – New York – Oxford
Paris – San Diego – San Francisco – Singapore – Sydney – Tokyo
Preface
Antarctic Climate Evolution is the first book dedicated to understanding the
history of the world’s largest ice sheet and, in particular, how it responded
to and influenced climate change during the Cenozoic. To explain the story
of Antarctic ice and climate history, information on terrestrial and marine
geology, sedimentology, glacier geophysics, ship-borne geophysics, and
numerical ice sheet and climate modelling is presented within thirteen
chapters.
The book’s content largely mirrors the structure of the Antarctic Climate
Evolution (ACE) program (www.ace.scar.org), an international initiative of
the Scientific Committee on Antarctic Research (SCAR), affiliated with
the International Polar Year 2007–2009, to investigate past changes in
Antarctica by linking climate and ice sheet modelling studies with terrestrial
and marine geological and geophysical evidence of past changes. The
programme is designed to determine climate conditions and change in both
therecentpast(i.e.duringthelastglacialmaximum,whentemperatureswere
cooler than at present) and the more distant past (i.e. in the pre-Quaternary,
when global temperature was several degrees higher than it is today). This
new cross-disciplinary approach has led to a substantial improvement in the
knowledge base on past Antarctic climate and to our understanding of the
factors that have guided its evolution. This in turn has allowed us to build
hypotheses, examinable through numerical modelling, for how the Antarctic
climate is likely to respond to present and future global changes.
Most of the subcommittees in ACE have been responsible for individual
chapters, and in this way we have been able to cover the complete history of
theAntarcticIceSheetanditsclimateevolution.Thebookwillbeofinterest
to research scientists from a wide range of disciplines including glaciology,
palaeoclimatology, sedimentology, climate change, environmental science,
oceanography and palaeoentology. It will also be valuable as a supplemen-
tary text for undergraduate courses.
We are grateful to our many friends and colleagues for advice and
encouragementthroughthegestationofthebookoverthelast3years.Wealso
acknowledge input to the ACE initiative by a number of scientists (many of
them contributedtothis book),including P. Barrett,A.K.Cooper,J. Francis,
R. Gersonde, M.J. Hambrey, D.H. Harwood, A. Moldonado, D. Pollard,
xii Preface
D. Sugden, G. Villa, P.-N. Webb and G.S. Wilson. We are sure that the
chapter authors will join us in thanking the reviewers for their comprehensive
and valuable comments and suggestions. We acknowledge their very special
contributions to this book by naming them here: J. Evans, J. Francis, W.
Howard, L. Krissek, A. Mackintosh, C. O’Cofaigh, G. Orombelli, A.H. Orsi,
D. Pollard, C.A. Ricci, I.C. Rutt, E. Stump, C. Summerhayes and G.S.
Wilson.FinallywethankLindaVersteeg-buschman,FemkeWallienand Suja
Narayanaof ElsevierSciencefortheir supportin theproduction ofthis book.
Fabio Florindo
Martin Siegert
Rome and Edinburgh,
July 2008
Elsevier
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F.FlorindoandM.Siegert(Editors)
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DOI10.1016/S1571-9197(08)00001-3
Chapter 1
Antarctic Climate Evolution
Martin J. Siegert1,(cid:2) and Fabio Florindo2
1School of GeoSciences, Grant Institute, University of Edinburgh, The King’s
Buildings, West Mains Road, Edinburgh EH9 3JW, UK
2Istituto Nazionale di Geofisica e Vulcanologia, via di Vigna Murata 605, 00143
Roma, Italy
ABSTRACT
Centraltotheunderstandingofglobalenvironmentalchangeisanappreciationof
how the Antarctic Ice Sheet interacts with climate. To comprehend the processes
involvedonemustlookintothegeologicalrecordforevidenceofpastchanges.For
several decades international efforts have been made to determine the glacial and
climate history of Antarctica and the Southern Ocean. Much of this information
derivesfromstudiesofsedimentarysequencesdrilledinandaroundthecontinent.
In addition, there have been numerous terrestrial geological expeditions to the
mountains exposed above the ice surface usually close to the margin of the ice
sheet. Holistic interpretation of these data is now being made, and hypotheses on
thesizeandtimingofpastchangesinAntarcticaarebeingdeveloped.In2004,the
Scientific Committee on Antarctic Research (SCAR) commissioned a scientific
research programme on Antarctic Climate Evolution (ACE) to quantify the
glacialandclimatehistoryofAntarctica.Thisbookisaresultofthatprogramme,
and documents, for the first time, the state of knowledge concerning the ice and
climate evolution of the Antarctic continent and its surrounding seas through the
Cenozoic era.
1.1. Introduction
The Antarctic Ice Sheet has existed for approximately 35 million years, but it
has fluctuated considerably and has been one of the major driving forces for
(cid:2)
Correspondingauthor.Tel.:[phone];Fax:[phone];
E-mail:[email protected](M.J.Siegert).
2 M.J. Siegertetal.
Quat. Pleistocene
1.81 Ma
E Pliocene
N
E 5.33 Ma
G
O
E
N Miocene
Y
C R 23.03 Ma
ZOI TIA
O R Oligocene
CEN TE ENE 33.9 ± 0.1 Ma
G
O
E Eocene
A
L
A 55.8 ± 0.2 Ma
P
Palaeocene
65.5 ± 0.3 Ma
Figure1.1: GeologicaltimeperiodsduringtheCenozoicera.Dateslistedon
the right hand side are taken from Gradstein et al. (2004).
changesinglobalsealevelandclimatethroughouttheCenozoic(Fig.1.1).The
rates, size and frequencies of these fluctuations have been the subjects of
considerabledebate.Determinationofthescaleandrapidityoftheresponseof
largeicemassesandassociatedseaicetoclimaticforcingisofvitalimportance,
becauseice-volumevariationsleadto:(1)changingglobalsealevelsonascale
of tens of metres or more, and (2) alteration to the capacity of ice sheets and
seaiceasmajorheatsinks/insulators.Itisthusimportanttoassessthestability
of the cryosphere under a warming climate (IPCC, 2007), particularly as ice-
core records have yielded evidence of a strong correlation between CO
2
concentrations in the atmosphere and palaeotemperatures (Fig. 1.2). This
concernisjustifiedwhenCO levelsarecomparedwiththoseofthepast.Since
2
Antarctica is a major driver of Earth’s climate and sea level, much effort has
beenexpendedinderivingmodelsofitsbehaviour.Someofthesemodelshave
beensuccessfully evaluated againstmodernconditions. In 2004, modelling the
past record of ice-sheet behaviour in response to changes in climate (inferred
from ice cores, sedimentary facies and seismic data), palaeoceanographic
conditions (inferred from palaeoecology and climate proxies in ocean
sediments) and palaeogeography (as recorded in landscape evolution) was
seen as a critical next step, and became the focus of the ACE programme.
The ACE programme facilitates research in the broad area of Antarctic
climate evolution. The programme links new geophysical surveys and
Antarctic Climate Evolution 3
Figure 1.2: Variation in the Earth’s temperature during the last 65 million
years, based on reconstructions from deep-marine oxygen isotope records.
Future atmospheric temperature scenarios are based on IPCC (2001).
Greenhouse trace gas projections are shown at top of diagram. Given the
worse-casescenario,planetarytemperaturescouldincreasein100–300yearsto
alevelwhere,accordingtoourknowledgeofpreviousAntarcticglaciations,ice
cover on Antarctica could not be sustained. The representation of palaeo-
temperatures is adapted from Crowley and Kim (1995).
geological studies on and around the Antarctic continent with ice-sheet and
climate modelling experiments. The programme determines past climate
conditions and changes in both the recent past (i.e. during the Holocene,
prior to anthropogenic impacts as well as at the last glacial maximum, when
temperatures were cooler than at present) and the more distant past (i.e. in
the pre-Quaternary, when global temperature were several degrees warmer
4 M.J. Siegertetal.
thantheyaretoday).Thiscross-disciplinaryapproach,involvingclimateand
ice-sheet modellers, geologists and geophysicists, has led to a substantial
improvement in the knowledge base on past Antarctic climate, and
our understanding of the factors that have guided its evolution. This in
turn allows us to build hypotheses, examinable through numerical model-
ling, for how the Antarctic climate is likely to respond to future global
change.
1.2. Antarctic Glacial History
As is discussed in Chapter 7, the East Antarctic plate formed a significant
component of the Gondwanaland super-continent during the Jurassic. Since
180Ma, this continent broke up into what are recognised today as distinct
continental landmasses with the repositioning of Antarctica at southern
polar latitudes in the Early Cretaceous (ca. 120Ma). In spite of its polar
position, Antarctica is thought to have remained mostly ice-free, vegetated,
andwithmeanannualtemperaturesabovefreezinguntilthelatterhalfofthe
Cenozoic (around 34 million years ago, Fig. 1.1), whereupon the continent
became subject to repeated phases of glaciation at a variety of temporal and
spatial scales.
The southern continent and its surrounding ocean basins have been the
targetofnumerousscientificexpeditionsandseveralscientificdrillingproject
efforts that have led to significant advances in understanding of Cenozoic
climate evolution, oceanography, and biota of the Antarctic continent and
theSouthernOcean.Thedeep-oceanrecordsdocumentclearlythelong-term
cooling of climates over the past 50 million years and large-scale variability
in the last 3–5 million years. They also show events that are either abrupt or
brief(e.g.thePaleocenewarmingeventwithadurationoflessthan1million
years; the Middle-Eocene Climatic Optimum, MECO at ca. 41Ma), or are
marked by a distinct shift in the rate at which long-term changes occur (i.e.
middle-Miocene increased cooling trend). The explanation for these events
include changes in atmospheric gas concentrations (e.g. carbon dioxide and
methane), opening of gateways with enhanced ocean circulation, peaks in
orbital forcing resulting from Croll–Milankovitch cyclicities, interactions
with northern hemisphere glaciations and others. Scientific drilling on the
Antarctic continental shelf and upper slope, to examine the direct record of
glaciation, has been sparse and has had significant problems with recovery
(o20% in diamict) using current Integrated Ocean Drilling Programme
(IODP) techniques. Consequently, the linkages between Antarctic
Antarctic Climate Evolution 5
continental shelf and deep-ocean basin records are not well established and
the basic problem of ice-sheet history remains unsolved. Proxy measure-
ments (particularly oxygen isotopes) provide general details, but initiation,
growth and extent of the ice sheets still are debated.
1.2.1. Late Eocene-Early Oligocene Cooling
TheEocenetoearlyOligocene(focusofChapter8)representatimeofglobal
cooling which was marked by reorganisation of global ocean circulation
patterns and significant turnovers in the marine and terrestrial biota (e.g.
Berggrenand Prothero,1992)thatculminates in thedevelopmentof thefirst
Antarctic Ice Sheet and an important expansion of Antarctic ice volume.
Global deep-sea oxygen isotope records indicate that this long-term cooling
trend was not monotonic, but that it was interrupted by a series of abrupt
short-term (ca. 1 million years) excursions in d18O (Zachos et al., 2001).
Amongthese,theOi-1coolingevent(Milleretal.,1991)at33.55Mamarked
one of the most significant global climatic deteriorations in the Cenozoic in
response to the appearance of the first continent-wide glaciation in
Antarctica (e.g. Zachos et al., 1996). Coupled GCM/ice-sheet modelling has
alreadybeenusedtoshowthattheformationoftheEastAntarcticIceSheet
was triggered by a combination of gradual pCO lowering coupled with ice-
2
climate feedbacks and orbital-forcing-induced cooling, rather than by the
cooling associated with the opening of circumpolar seaways during the
earliest Oligocene (e.g. Kennett et al., 1974; DeConto and Pollard, 2003;
Lawver and Gahagan, 2003).
1.2.2. Oligocene–Miocene Boundary Mi-1 Glaciation
TheOligocene–Mioceneboundary(Chapter9)marksasignificanttransition
in the development of the Antarctic cryosphere, where small dynamic ice
sheetsofthelateOligocenerapidlyexpandedtocontinentalscaleintheearly
Miocene. The transition is recorded in benthic foraminiferal d18O records as
a positive 1.0 per mil shift, representing the first of the Miocene glaciations
(Mi-1). The climatic significance of this was first outlined by Zachos et al.
(2001) who recognised the coincidence of the Oligocene–Miocene boundary
and the Mi-1 isotope excursion with an unusual coincidence of low
eccentricity and low-amplitude variability in obliquity of the Earth’s orbit.
Mg/Ca reconstructions imply little orno changein temperatureand that the
ice-volume increase was equivalent to 90m of sea level lowering (assuming a