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Holocene Sediment Records Reveal Environmental Changes in East Antarctica
1. Old Dominion University
ODU Digital Commons
OEAS Faculty Publications Ocean, Earth & Atmospheric Sciences
2001
Holocene Sediment Records From the Continental
Shelf of Mac. Robertson Land, East Antarctica
Peter N. Sedwick
Old Dominion University, Psedwick@odu.edu
Peter T. Harris
Lisette G. Robertson
Gary M. McMurtry
Maximilian D. Cremer
See next page for additional authors
Follow this and additional works at: https://digitalcommons.odu.edu/oeas_fac_pubs
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Repository Citation
Sedwick, Peter N.; Harris, Peter T.; Robertson, Lisette G.; McMurtry, Gary M.; Cremer, Maximilian D.; and Robinson, Philip,
"Holocene Sediment Records From the Continental Shelf of Mac. Robertson Land, East Antarctica" (2001). OEAS Faculty
Publications. 106.
https://digitalcommons.odu.edu/oeas_fac_pubs/106
Original Publication Citation
Sedwick, P.N., Harris, P.T., Robertson, L.G., McMurtry, G.M., Cremer, M.D., & Robinson, P. (2001). Holocene sediment records from
the continental shelf of Mac. Robertson Land, East Antarctica. Paleoceanography, 16(2), 212-225. doi: 10.1029/2000pa000504
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2. Authors
Peter N. Sedwick, Peter T. Harris, Lisette G. Robertson, Gary M. McMurtry, Maximilian D. Cremer, and
Philip Robinson
This article is available at ODU Digital Commons: https://digitalcommons.odu.edu/oeas_fac_pubs/106
3. PALEOCEANOGRAPHY,
VOL. 16,NO. 2, PAGES212-225,APRIL 2001
Holocene sediment records from the continental shelf of
Mac. Robertson Land, East Antarctica
Peter
N. Sedwick,
1Peter
T.Harris,
•,2Lisette
G.Robertson,
• Gary
M. McMurtry,
3
Maximilian
D.Cremer,
3and
Philip
Robinson
4
Abstract. Geochemicalrecordsarepresented
for five sedimentcoresfrom basinson the continentalshelfof Mac.
Robertson
Land, EastAntarctica. The corescontain2-4 m thick sequences
of hemipelagic,siliceousmud andooze
(SMO) deposited
underseasonally
openmarineconditions.The innerandmiddleshelfSMO sequences
aremassive
darkolive greenmaterial,whereas
theoutershelfSMO sequences
aredarkolivematerialinterspersed
with lightolive
greenlayers~1-10 cm thick. The biogenic
materialis dominated
by marinediatomsincludingFragilariopsis
curta,
Fragilariopsis
cylindrus,
andChaetoceros
spp.in thedark-colored
SMO andCorethron
criophilum
in thelight-colored
layers.Radiocarbon
dates
suggest
thatthecores
provide
continuous
accumulation
records
extending
from< 1kyrbefore
present
(B.P.)back
asfaras4-15kyrB.P.,withestimated
accumulation
rates
of0.07-5mmyr'•. Thethree
core
records
fromthemiddleandoutershelfsuggest
sixepisodes
of increased
accumulation
of biogenic
materialat -5.5 kyrB.P.(all
threecores),1,2, and6.2kyrB.P.(twoof thethreecores),
and3.8and10.8kyrB.P.(onecore),mostof whichcoincide
with Corethron
layers.We interpret
these
features
astheresultof enhanced
diatomproduction
overtheoutershelf,
possibly
relatedto climaticwarmperiods.Theabsence
of suchfeatures
in theinnershelfcorerecords
is thought
to
reflectarelativelyconstant
levelof seasonal
diatom
production
in adjacent
waters
maintained
byacoastal
polynya.
1. Introduction
The Antarctic continentalshelf accountsfor a significant
fractionof SouthernOceanprimaryproduction
andis a major
areaof oceanic
deepwater
formation[Comiso
etal., 1993;Arrigo
et al., 1998a;Deacon, 1984;Orsi et al., 1999]. Algal production
in Antarcticshelfwatersmay thusplay a significant
role in the
biogeochemical
cyclesof carbonandsilicon,andin definingthe
composition
of oceanic
bottomwaters[Smith
andGordon,1997;
Nelson et al., 1996;Arrigo et al., 1999]. At present,little is
known aboutalgal productionand its relationship
to environ-
mental conditions on the Antarctic shelf during the late
Quaternary. This largely reflectsthe dynamicnatureof this
continental margin, where seafloor sedimentsare widely
reworked and redistributedby the action of ice and currents
[Dunbar et al., 1985; Andersonand Molnia, 1989;Harris and
O'Brien, 1996;Anderson,1999]. However,somefjordsandshelf
basins provide natural sediment traps, where there are
accumulations
of hemipelagic
sediments
derivedfromoverlying
watersand adjacentshelfareas[Domack,1982;Domackand
McClennen, 1996;Harris and O'Brien, 1996;Barkeret al., 1998;
Harris and O'Brien, 1998]. A numberof studieshavemadeuse
of sedimentcoresfrom suchlocationsto infer paleoenvironmen-
•AntarcticCRC, Hobart,Tasmania,Australia.
2Australian
GeologicalSurveyOrganization,
Hobart,Tasmania,
Australia.
3Department
of Oceanography,
University
of Hawaii,Honolulu,
Hawaii.
4Department
ofGeology,
University
ofTasmania,
Hobart,
Tasmania,
Australia.
Copyfight
2001bytheAmerican
Geophysical
Union.
Papernumber
2000PA000504.
0883-8305/01/2000PA000504512.00
talconditions
ontheAntarcticshelfduringtheHolocene
andlate
Pleistocene
[e.g.,Leventer
andDunbar,1988;Domack
etal.,
1993; Leventer et al., 1993, 1996; Shevenellet al., 1996;
Frignani
etal., 1998;Sedwick
etal., 1998;Cunningham
etal.,
1999;
Domack
andMayewski,
1999;
Domack
etal., 1999].
In theseinvestigations,
downcorechemical,
physical,
and
micropaleontological
datatogether
withradiocarbon
chronolo-
gies
have
been
used
toconstruct
regional
records
oftherelative
accumulation
of biogenic
versus
lithogenic
material,
fromwhich
paleoenvironmental
conditions
havebeeninferred. A mid-
Holocene
climaticwarming
hasbeenpostulated
onthebasis
of
sedimentrecordsfrom the continentalshelf of GeorgeV and
Ad61ie
Land,PrydzBay, andthewestern
RossSea[Domacket
al., 1991;Jacobson,
1997;Frignaniet al., 1998;Cunningham
et
al., 1999], whereassedimentsfrom the westernmarginof the
Antarctic Peninsula and the continental shelf of Mac. Robertson
Landcontainevidenceof century-to millennium-scale
variations
in accumulation
of biogenic
matterduringtheHolocene
[Domack
etal., 1993;DomackandMayewski,1999;Leventer
etal., 1996;
Sedwick
etal., 1998]. Suchchanges
havealsobeeninferredfrom
a high-resolution
OceanDrillingProgram
sediment
record
from
the PalmerDeep on the AntarcticPeninsula,whichcontains
evidence
of ~400, 200, and50-70 yearcyclesin accumulation
of
pelagicbiogenicmaterialaswell aslonger-term
paleoenviron-
mentalchanges,
includinga lateHolocene
neoglacial
period,a
mid-Holoceneclimatic optimum, an early Holoceneclimatic
cooling,anda latePleistocene
deglacial
episode
[Domack
etal.,
200•1.
Theseresults
naturally
raisequestions
concerning
theregional
coherence
of suchrecords
andthespatialscaleof theinferredpa-
leoenvironmental
variations,given that present-day
environ-
mentalconditions
on theAntarcticmargin,suchasseaice cover
andalgalbiomass,
areknownto be highlyvariable[Comiso
et
al., 1993;Arrigoet al., 1998b;Barkeret al., 1998;Parkinson,
1998]. Suchquestions
canonlybeaddressed
by examining
the
212
4. SEDWICK ET AL.: SEDIMENT RECORDS FROM THE EAST ANTARCTIC SHELF 213
coherence
of paleoenvironmental
records
fromtheAntarcticshelf
overa widerangeof spatialscales.Herewe present
geochemical
and sedimentological
recordsfor five sedimentcorescollected
from shelf basinson the continentalmargin of Mac. Robertson
Land, EastAntarctica(the Mac. RobertsonShelf),a regionwhich
sustains
relativelyhigh algalbiomass[Comisoet al., 1993] and
may be a significantsourceof AntarcticBottomWater [Orsi et
al., 1999]. Preliminaryanalysesof two of thesecoresindicated
significantdifferencesin the sedimentary
recordsfrom the inner
andoutershelfover distancesof < 100 km [Sedwicket al., 1998].
The new datapresented
hereindicatethattherehaveindeedbeen
significantsmall-scaleregionalvariationsin the accumulation
of
biogenicmaterialin the Mac. Robertson
Shelfbasinsduringthe
Holocene,but our resultsalsoprovideevidencefor millennial-
scale environmental variations in common with sedimentary
records
from otherpartsof theAntarcticmargin.
2. Materials and Methods
2.1. Study Area
The Mac. Robertson
Shelfextends
some400 km westof Prydz
Bay, East Antarctica,with a typicalwidth of 90 km (Figure 1).
Herethe continentalshelfis madeup of relativelyshallowbanks
< 200 m in depth,separated
by steep-sided
basins
andvalleysup
to 1200m in depthwhichareinterpreted
asrelictglacialtroughs
[O'Brien,et al., 1994;Harris and O'Brien, 1998]. Dense,high-
salinity shelf water forms along the shelf in association
with
coastalpolynyas,andthis watersinksand flows off the shelf,
probablyfilling the perched
basins[Bainesand Condie,1998;
Harris and O'Brien, 1998; Harris, 2000]. On the outer shelf and
uppercontinentalslopea large-scale
westwardflowing current
follows the AntarcticSlopeFront, extendingfrom the surface
watersto the seafloor,wherecurrentspeeds
up to severalmeters
per second
havebeenmeasured
[Smithet al., 1984;Harris and
O'Brien, 1998; Bindoffet al., 2000]. Coarse-grained
sandand
gravel depositsoccuron the shallowerareasof the outer shelf
andupperslope,whereas
finer-grained
mudsoccurin thedeep
basins,particularlyon theinnershelf[Harris and O'Brien, 1996,
1998]. These basinscontainup to severalmeters(at least) of
siliceous
mudandooze(SMO) deposited
underseasonally
open
marineconditions,
overlyingsandysilt andglacialmarinemuds
thatwereprobablydeposited
underor neara permanent
ice shelf
[Harris and O'Brien, 1998; Anderson, 1999]. The radiocarbon
agesof thesefacies in sedimentcoresfrom the Mac. Robertson
shelfsuggest
thatseasonally
openmarineconditions
commenced
around10-12 kyr before present(B.P.) on the outer shelf and
around6 kyr B.P. on theinnershelf,thedifferencerepresenting
the period over which a permanentice canopyretreatedacross
the continental shelf during the early Holocene [Harris and
O'Brien, 1998; $edwicket al., 1998].
2.2. SampleMaterials
The sedimentcoresusedin thisstudywerecollectedfrom the
Nielsen Basin (maximum water depth -1200 m) and Iceberg
Alley (maximumwater depth-500 m), which are two of the
largerbasinsontheMac. Robertson
Shelf(Figure1). The 9 cm
diametergravity coreswere collectedduringcruisesof RSV
Aurora Australis in 1993, 1995, and 1997. Five cores were
examined in this study (approximate water depths in
parentheses):
AA186-GC34(470 m) andKROCK-GCI (478 m)
from theoutershelfin IcebergAlley, AA149-GC2 (1100 m) and
KROCK-GC2 (1090 m) from the inner shelf in the Nielsen
Basin, and AA149-GC12 (626 m) from midshelfin the Nielsen
Basin(Figure1). Thesesitesareseparated
by distances
ranging
from -5 to 100 km. All of the cores contain continuous
sequences
of SMO rangingin thickness
from 267 to 374 cm
(Figure2). In coresKROCK-GC2 andAA149-GCI2 the SMO
unitsoverliesandyglacialmarinemuds. The SMO sequences
areprimarilya mixtureof diatomooze,fine-sand
to coarse-silt
quartz,andotherfine lithogenicmaterial.The SMO unitsof the
ii• AN•,-R•TICA
-....:,..-.-.:.-..-¾"-"--'---:-'....-"::•.....-"•i•?/-'.
::'-•'•i
500m •-:'--:•:•::!•!•i:•:"..i•?'•"
""•'"'"•"
' '
'--
'"••:-•...•i:--:.-'i::iiiii'"'"'""':':'"•:-
"•. -"..-'i•iiii"-"-'J•'"•!ii":"=""""'""•-
•:-.-'-•::ii:::..:-•--'•i'"'"'"•'"'•'"':"
' '"
"•-
- -
'"•---
-
-:•• :"-":•::
....................
:':•iii::"'"'"'•'"':'"•""--
-'•..---'•i•ii ,
'....:.:• 86 .... ':':•?'-:--':•..-"•i-..':ili•i
.......... -'--'":"---'""•--"•.-'•
.............
•F--'::•i•i•.:::?--'-":----"...-:•i•.::i•i?•-.-:::•i•i•?.•i•i
Maws•
•n MAC. ROBERTS
63øE 64
ø ......
'6•
ø --
-••6• •'_:
:1
water
depth: •1 0-200m,..-•>a00m 0 20 40 km
I I I
Figure 1. Map of theMac. RobertsonShelf,showingsedimentcollectionsites.
5. 214 SEDWICKET AL.:SEDIMENTRECORDS
FROMTHEEASTANTARCTICSHELF
10o
200
3OO
AA186-GC34
o 20
0 50 1O0
KROCK-GC1
o 20
50 100
AA149-GC2
o 20
5O 100
KROCK-GC2
o 20
5O 100
AA149-GC12
o 20
..
o 50 lOO
Grain size
% gravel
% •and
% mud
Magnetic
susceptibility
(xllY
6cgs)
o 2o
Facies
Corethron
layers
Holocene SMO
cross-bedded
sandy silt
glacial marine mud
ice-rafted
debris
Figure 2. Grainsize,magnetic
susceptibility
andfaciesclassification
forcoresin thisstudy.
innerandmiddleshelfcores(AA149-GC2,KROCK-GC2,and
AA149-GC12) are massive,
generallyfeatureless,
darkolive
green sediment, whereas the SMO units from the outer shelf
cores(AA186-GC34andKROCK-GC1)aredarkolivegreen
material
interspersed
withfluffy,lightolivegreen
bands,
which
range
in thickness
from~1to 10cm. Finer-scale
lightanddark
laminations
wereapparent
in coresAA186-GC34 andKROCK-
GC1 whenthey werefirst split,but thesefeatures
fadedafter
severalweeksof storage.
The cores were split and describedimmediatelyafter
collection,
thenwrapped
in polyethylene
andstored
at 2øC. The
coreswere subsampled
for variouschemicaland physical
measurements
in Hobart. Subsamples
weretakenover 10 cm
intervalsfor geochemical
analysis,
andthegeochemical
data
presented
in section3.2 represent
the depth-averaged
bulk
compositions
of these10cmthicksubsamples.
In addition,1-2
cmthicksubsamples
weretakenfromsel•ted depths
forradio-
carbon
datingandgrain-size
analysis,
1 cmthicksubsamples
weretaken
fromtheupper
5-10cmofthecores
forgamma
spec-
trometric
analysis,
and1 cm
3subsamples
weretaken
at 10cm
intervals
fordetermination
of drybulkdensity.Theuppermost
v-',--,,,• of the outer .•...,c
•,,•,, coressuffered
somecompaction
(< 10
cm)andminorstratigraphic
disturbance
aftercollection
owing
to
the high water contentof the sediment,but we have madeno
attempt to correct our data for these effects. In addition, the
uppermost
section
of coreKROCK-GC1shrank
in lengthfrom
100to ~90 cmpriorto subsampling
owingto waterlossduring
storage.In thepresentation
of dataforthiscore,average
sample
depths
between
0 and90 cmhavebeenmultiplied
by a factorof
100/90in anefforttocorrect
forthisshrinkage.
2.3. Core ChronologyandIsotopicAnalyses
The near absence of carbonate microfossils in the cores
precludes
useof standard
•5180
stratigraphy
and/or
radiocarbon
dating of calcium carbonate, whereas the likelihood of non-
uniform sedimentation
ratesand the presence
of significant
concentrations
of authigenic
uranium(seesection
3.2) precludes
the estimationof accumulation
ratesusingthe uraniumseries
radionuclides
226Ra,
23øTh,
and23•pa.
Theprimary
chronostrati-
graphic
toolwehaveusedin thisstudyisradiocarbon
dating
of
bulkorganic
carbon(typically1-2%by mass
in these
sediments),
which has been successfullyemployedin other studiesof
sediments
fromtheAntarcticcontinental
shelf[e.g.,Domacket
al., 1989; Leventeret al., 1996;Domacket al., 2001]. Radio-
carbon
agesweredetermined
byaccelerator
mass
spectrometry
at
eithertheAustralian
NuclearScience
andTechnology
Organiza-
tion (ANSTO) or theNew Zealandinstituteof Geological
and
NuclearSciences
(NZI). Radiocarbon
datesarereported
hereas
conventionalradiocarbonyearsbeforepresent,as definedby
Stuiver
andPolach[1977].The•5•3C
values
used
tocalculate
the
radiocarbonages were either measured(NZI analyses)or
6. SEDWICK ET AL.: SEDIMENT RECORDS FROM THE EAST ANTARCTIC SHELF 215
assumed
(ANSTO analyses),
with theassumed
valuesbasedon
•513C
measurements
ofsubsamples
fromcores
KROCK-GC1
and
KROCK-GC2 performedby the AustralianGeologicalSurvey
Organization.The errorsintroduced
in theradiocarbon
agesdue
totheuse
ofassumed
•5'3C
values
arelikelytobeless
than
the
analytical
uncertainties
inthe'4Cmeasurements,
given
therange
ofmeasured
•5•3C
values
(-24.1to-34.1%o).
Inaddition,
unsup-
ported
2'øPb
wasdetermined
incore
topsubsamples
bygamma
spectrometry
[McMurtry et al., 1995]in aneffortto evaluatethe
possible
lossof coretopmaterialduringsample
collection.
2.4. Physicaland GeochemicalMeasurements
Down coremagneticsusceptibility,
whichprovidesa relative
measureof ferromagnetic (i.e., lithogenic) mineral content
[Leventer
et al., 1996], wasdetermined
with a Battington
MS-2
magneticsusceptibility
meter. The coreswerealsoX-rayed in
orderto identify macroscopic
sedimentary
structures,
anddown
coresubsamples
were wet sievedto determinepercentage
gravel,
sand,andmud by dry weight[Harris and O'Brien, 1998]. The
followinggeochemical
measurements
wereperformedon the 10
cm thicksubsamples:
(1) bulkmajorandminorelements
(A1,Si,
Ti, Mn, Fe, Ni, Cu, Zn, Br, Mo, Ba, and U) were determinedin
crushed,60øC dried (and, for major elements,aleionized
water
washed)materialby X-ray fluorescence
spectroscopy,
following
a modification
of themethodof Shimmield[1984]; (2) biogenic
silica(opal),with assumed
composition
SiO2.0.4H:O,wasdeter-
minedin freeze-driedsedimentby themethodof Mortlockand
Froelich[1989]; and (3) totalorganiccarbon(TOC) wasdeter-
mined in crushed, 60øC dried, acid-treated, deionized water
washedmaterial usingan elementalanalyzerat the Australian
Geological
SurveyOrganization
or theUniversityof Tasmania.
Analyticaluncertainties,
as estimated
from repeatedmeasure-
mentsof in-housestandards,are presentedin Table 1 (the
geochemical
datapresented
hereareavailableelectronically
at
http
://www.antcrc.utas.edu.au/antcrc/research/sediment_web/data
/geochem.html).
3. Results
3.1. Core Preservationand Chronostratigraphy
Table 2 presents
the radiocarbon
agesand measured
or
assumed
•5'3C
values
of bulkorganic
carbon
in subsamples
from
thefivecores.Themeasured
•5'3C
values
aregenerally
consistent
withtherangeof-20 to -30%oreported
for Southern
Ocean
pelagic
phytoplankton
[Gibson
etal., 1999;
Poppetal., 1999],
although
slightly
lowervalues
(< -32%o)
intheupper
portion
of
core
AA149-GC
12may
reflect
the
presence
of'•C-depleted
relict
terrestrial
organic
matter
[Harrisetal.,1996].Downcore
radio-
carbon
ages
generally
increase
inaregular
fashion
(Figure
3)and
suggest
thatthecores
preserve
continuous
records
of sediment
accumulation
overtimeperiods
ranging
from-3.8 kyr(AA149-
GC2)to 15kyr(AA149-GC12).Ourinitialanalyses
ofcoretop
samples
fromcores
KROCK-GC1
andKROCK-GC2
detected
no
unsupported
•'øPb
[Sedwick
etal., 1998],
suggesting
theloss
of
sediments
corresponding
tothepast
~100-200
years
(atleast)
of
accumulation
duringcollection
of thesecores. Subsequent
analyses
(datanotshown)
indicate
lowlevels
of unsupported
•øPb
inonlytheupper
2cmofcores
AA186-GC34
andKROCK-
GC2,and
nounsupported
vøPb
incore
KROCK-GC1,
consistent
withsome
lossofcoretopmaterial
during
collection,
whereas
the
Table 1. Estimated
AnalyticalUncertainties
Species Uncertainty
a
AI 0.5
Si 0.2
Ti 5
Mn 10
Fe 2
Ni 10
Cu 10
Zn 1
Br 10
Mo 5
Ba 1
U 20
Opal 5
TOC 0.2b
aRelative standarddeviation on mean.
bAbsolutestandarddeviation (wt %).
lowto moderate
levels
of unsupported
21øpb
measured
in the
upper5 cmof cores
AA149-GC2andAA149-GC
12suggest
that
therewereno significantcoretoplosses.
In our studyregionthe radiocarbon
ageof organicmatterat
thesediment-water
interfaceis expected
tobegreaterthanzeroas
a resultof (1) thenonzeroradiocarbon
ageof thedissolved
inor-
ganiccarbonthatis converted
intoorganicmatterin theeuphotic
zone(assumed
to betheprincipalsource
of organic
carbonin our
sediment
cores),termedthereservoireffect,whichis ~1300years
in surfacewaters of the SouthernOcean [Gordon and Harkness,
1992;Berkmanand Forman, 1996];(2) bioturbation
in theupper
sedimentcolumn,whichverticallymixesmaterialoverdepthsof
the orderof 10 cm [Berner, 1980; Libes, 1992]; and (3) dilution
of fresh sedimentsby older, resuspended
particulatecarbon
[Harris et al., 1996]. In an effort to correctour radiocarbon-
basedchronologies
for thecombinedeffectsof theseprocesses,
we havesubtracted
1730radiocarbon
yearsfrom ourraw radio-
carbonages. This valueof 1730radiocarbon
yearsis theraw
radiocarbonage of a well-stratified, water-saturated,
surface
sediment
grabsample
(AA186-GB9)thatwasrecovered
nearthe
location of AA149-GC2 in the inner Nielsen Basin (E. Domack,
personalcommunication,
1997). The radiocarbon
ageof this
surfacesedimentsampleis assumedto be representative
of
surfacesediments
withintheshelfbasins
of ourstudyarea.
We recognize
thattherearea number
of significant
uncertain-
ties included in our radiocarbonage correction. One is the
possiblegeographicvariationin radiocarbonage of surface
sedimentsin theseshelf basins,which might be expected,for
example,
because
of differences
in theproportion
of resuspended
materialaccumulating
at differentlocations.Anotheruncertainty
is introducedby the likely variation in the reservoirage of
Antarctic waters between the Last Glacial Maximum and the
earlyHolocene,
whichmaybeof theorderof thousands
of years,
basedon our knowledgeof changesin the radiocarbon
age of
oceanic
deepwatersoverthisperiod[Samson,
1999;Sikesetal.,
2000] andgiventhatupwelleddeepwatersdominatetheradio-
carbon inventory of Antarctic surfacewaters [Berkmanand
Forman, 1996]. Yet an additional complicationto the radio-
carbonagecorrection
is introduced
by the presence
of bomb-
8. SEDWICK ET AL.: SEDIMENT RECORDS FROM THE EAST ANTARCTIC SHELF 217
o
50
lOO
15o
200
250
300
350
400
AA186-GC34
(outer shelf)
/ . ,
53.2
•40.6
KROCK-GC1
(outer shelf)
_-! _ ß . i _ ß
•' 77.4
crn.
kyY'
1
0.4
•-13.7
AA149-GC2 KROCK-GC2 AA149-GC12
(inner shelf) (inner shelf) (mid-outer shelf)
crn.
kyY
'1
48.6
crn.
kyr
-1
-17.0
_ ß _ ß _ ß _ ß
3crn.
kyY
'1
46.8
39.6
15.1
21.1
•
7'.0
'"•'•
uncorrected radiocarbon age (kyr B.P.)
Figure 3. Uncorrectedradiocarbonagesversusdepthand calculatedsedimentaccumulation
ratesbetweendated samples.
Sampleswereexcludedfrom accumulation
rate calculations
where(1) a sampleis olderthanthat immediatelybelow it (open
circles)or(2) a sample
ageis withinanalytical
uncertainty
of thecoretopsample
age(stippled
circles).
carbon
ages
of thecoretopsamples
range
from~50to900radio-
carbonyears, which may reflect the previouslymentioned
uncertainties
in ouragecorrection,
aswell assomelossof core
topmaterial,although
thelatteris onlylikely to be significant
(i.e., greaterthan --100-200 years)for coresAA186-GC34,
KROCK-GC1, and KROCK-GC2, on the basis of our excess
2•øPb
analyses.
Thecorrected
coretopageof KROCK-GC1,
if
duesolely
tocoretoploss,
suggests
that--70cmof theuppermost
sediment
columnis missing,usingourestimated
accumulation
rates. However, as noted above, these accumulation rate
estimatesand thusestimatesof core top lossesare very poorly
constrained.
Corechronologies
wereestablished
by linearinterpolation
of
the age versusdepthdataas shownin Figure3, usingthe
correctedradiocarbonagespresented
in Table 2. With the
exception
of coreAA149-GC12 (seebelow),the downcore
physicaldata(Figure2) andX-ray imagesprovideno clear
evidence
of graded
bedsor erosional
surfaces
withintheSMO
units,andcoarse
materialtypicalof shallower
areas
ontheshelf,
including
calcareous
biotawhichoccuron nearbyFramBank
[Rathburn
etal., 1997],israre. Thisandtheabsence
of signifi-
cantagereversals
orhiatuses
in thedowncoreradiocarbon
ages
argues
against
significant
episodic
downslopetransport
of
sedimentsinto these basins, or significant erosional events,
althoughthe sustainedtransportof fine sedimentsfrom the
shallow shelf areasinto thesebasinsis clearly an important
process
[Harris and O'Brien,1996,1998]. Thus,in ourinterpre-
tationof the down core geochemicaldata we assumethat each
core contains a continuous record of sediment accumulation that
hasnotbeensignificantly
disturbed
by slumps,
turbidityflows,or
erosionalevents. In the caseof core AA149-GC12, however, X-
rayimagesindicateripplecross
bedding
concentrated
between
50
and 150 cm depth,which alongwith radiocarbon
agereversals
(Figure
3) andrelatively
depleted
15•3C
values
(< 30%0)
between
40 and 120cm depth(Table2), suggest
thatthesediment
record
in this core may reflect episodesof down slope sediment
transport
byrelativelystrong
density
currents
[Harris,2000].
3.2. Physicaland GeochemicalData
The SMO units are madeup of ~70-95% mud and ~5-30%
sand (Figure 2) and contain 16-46% opal (mean is 37%) and
0.66-2.3% TOC (meanis 1.3%) by weight. Downcoremagnetic
susceptibility
valuesareuniformlylow (< 5 x 10'6cgs)and
featureless
within theSMO facies(Figure2), indicatinga paucity
of ferromagneticminerals and lithogenic material in general.
Microscopic examinationof core material indicatesthat the
majority of organic matter is associatedwith the remainsof
diatomsthat are typical of Antarcticcoastaland shelf waters,
with lithogenicparticles(fine sandto coarsesilt) accounting
for
the remainder of the SMO facies material. A detailed microfossil
studyof coresKROCK-GC1 andKROCK-GC2 [Taylor,1999;F.
Taylor and A. McMinn, Evideace from diatomsfor Holocene
climatefluctuationalongtheEastAntarcticmargin,submitted
to
The Holocene, 2000] indicatesthat the diatomsFragilariopsis
curtaandFragilariopsiscylindrusdominate
thedarkolivegreen
SMO, exceptnearthebaseof bothcoreswhereChaetoceros
spp.
restingspores
aredominant,whereas
thelighter-colored
bandsin
coreKROCK-GC1 are characterized
by an increased
abundance
of the diatomCorethron criophilurn. Preliminarymicroscopic
examinationof materialfrom core AA186-GC34 suggests
that
the light and dark bandingin this core reflectsdiatom species
9. 218 SEDWICK ET AL.: SEDIMENT RECORDS FROM THE EAST ANTARCTIC SHELF
assemblages
similarto thosein coreKROCK-GC1 (L. Armand,
personal
communication,
1999). Theoccurrence
of thelarger
visible
"Corethron
layers"
inthese
cores
isindicated
inFigure
2.
In examining
thepaleoenvironmental
record
preserved
inthese
cores,
specifically
therecord
of accumulation
of biogenic
versus
lithogenicmaterial,the physicalmeasurements
are of only
limiteduseatthelevelof resolution
used
in thisstudy
because
of
the relatively homogeneous
grain size distributionand the
relatively
lowandconstant
values
ofmagnetic
susceptibility.
We
havetherefore
focused
onchemical
proxies
for theaccumulation
of lithogenicand biogenicmaterial in an effort to infer the
sedimentation histories for these core sites. The chemical data
considered
herearedivided
intothefollowing
groups
onthebasis
oftheirutilityaspaleoenvironmental
proxies.
1. Ti, A1,andFe,themajorfractions
of whicharegenerally
associated
with lithogenic
materialin pelagicandhemipelagic
sediments[Calvert and Pedersen,1993;Kuntaret at, 1995].
Bulk sediment
concentrations
of these
elements
thusprovidea
relativemeasure
of theaccumulation
of lithogenic
material.
2. TOC andBr, whichproviderelativemeasures
of theaccu-
mulationof organicmatterin marinesediments.
TOC provides
a
directmeasureof the depositedorganicmatterremainingafter
diagenetic
remineralization,
whereas
Br is thoughtto beuniquely
controlledby the organicfractionin marinesediments
[Price et
al., 1970; Calvert andPedersen,1993].
3. Opal and total Si/A1, both of which provide relative
measures
of theaccumulation
of biogenicsiliceous
matterwhere
postdepositionalpreservationis high or relatively constant
[Charles et al., 1991; Mortlock et al., 1991; McManus et al.,
1995]. Opaldetermined
by themethod
of MortlockandFroelich
[1989]providesa directmeasure
of thisbiogenicsilica,whereas
total Si (determinedby X-ray fluorescence),
whennormalizedto
A1,providesan indirectestimateof thissamequantity,assuming
thatthemajorityof A1islithogenic.
4. Mo, U, and Mn, which changevalenceand are thusad-
sorbedor precipitated
in response
to changes
in sedimentary
re-
dox conditions. Sedimentary
enrichments
in Mo/A1, U/A1, and
MrdA1,relativeto crustalabundances
(assuming
all A1is litho-
genic),areindicativeof anoxic(sulfidic),suboxic,andoxiccon-
ditions,respectively
[CalvertandPedersen,
1993, 1996;Crusius
et al., 1996]. Thustheseelementsserveassensitive
proxiesof
sedimentaryredox conditions,with down core changesin A1-
normalized concentrationsindicating past variationsin redox
conditions,
asmay resultfrom variationsin theaccumulation
of
organicmatter.
5. Ni, Cu, and Zn, which form insoluble metal sulfides and
may thus precipitate in reducing sedimentswhere dissolved
sulfideis present[Calvert and Pedersen,1993]. Sedimentary
enrichments in Ni/A1, Cu/A1, and Zn/A1 relative to crustal
abundance
(assuming
all A1is lithogenic)
areindicativeof anoxic
(sulfidic)conditions,
asmay resultfrom increased
accumulation
of organicmatter.
6. Biogenic (or excess)Ba, calculatedas the difference
between
totalandlithogenic
Ba, wherelithogenic
Ba isestimated
fromA1usingtheaverage
crustalweightratioof Ba/A1(0.0075).
BiogenicBa is thoughtto bedeliveredto thesediments
asbarite
containedin organicmatterfrom the surfaceoceanandis well
preserved
in oxicsediments,
whereit serves
asa proxyforexport
production
in overlyingwaters[Dyntond
etal., 1992].
Thesechemicaldataarepresented
in Figure4, in whichbulk
sediment
concentrations
(dry weightbasis)areplottedagainst
the
agecorresponding
to theaverage
depthof eachsubsample
in the
core. Theseageswerelinearlyinterpolated
usingthechronologi-
cal schemedescribed
in section3.1. The periodsof deposition
integrated
bythese10cmthicksubsamples
rangefrom-20 years
(baseof AA149-GC12) to 1400years(upperportionof AA149-
GC2), basedon the estimatedsedimentaccumulationratesshown
in Figure3, with a typicaltemporal
resolution
of-200 yearsfor a
sediment
accumulation
rateof50cmkyr
'•.
4. Discussion
4.1. Down Core CompositionalRecords
The SMO unitsof thefive coresaregenerally
similarin terms
of bulkcomposition,
with coresKROCK-GC1 andAA149-GC12
displayingthegreatestcompositional
ranges,asis evidentin the
downcoreopal data (Figure4). Manganese
concentrations
are
uniformlylow in all of the cores,with Mn/A1 values(meanis
0.16+ 0.27)statistically
indistinguishable
fromtheaverage
shale
ratioof 0.08 [Wedepohl,1971], whereas
theratiosMo/A1 (mean
is1.3+0.6x 10
'4)and
U/A1
(mean
is0.9_+
0.3x 10
.4)
aresignifi-
cantly
enriched
relative
totheaverage
shale
ratios
of-0.3 x 10
'4
and-0.4 x 10
'4,respectively
[Wedepohl,
1971]. These
geo-
chemical trends together with shipboardobservationsof
hydrogen
sulfideodoruponrecovery
of thesecoresandgrab
sampleAA186-GB9 indicatethattheentiresediment
columnand
possiblythe sediment-water
interfacewere anoxic(sulfidic) at
eachof thesesites. This wouldact to limit bioturbation
by
benthicorganisms,
thus favoringthe preservation
of high-
resolution records of sediment accumulation. However, the
presenceof hydrogen sulfide in the sedimentcolumn also
precludes
theuseof biogenic
Ba asa productivity
proxy,since
dissolution
of sedimentary
bariteaccompanies
sulfatereduction
[Dyntondet al., 1992]. Indeed,thedowncorerecordsshowthat
biogenic
Ba doesnot varyin concert
with theotherproxiesof
organic
matteraccumulation,
withsome
calculated
values
being
closetoor lessthanzero(Figure4). A similarsituation
islikely
toapplyin sediments
fromotherbasins
ontheAntarctic
margin,
suchasthePalmerDeep,wheresedimentary
A1andBa concen-
trations
[Rodriguez
andDontack,
1994]sometimes
yieldnegative
values
for biogenic
Ba.
The geochemical
recordsof thetwo outershelfcoresAA 186-
GC34 and KROCK-GC1 (Figures4a and 4b) showseveral
prominentminima in the lithogenicelementsTi, A1, and Fe,
which are generallycoincidentwith small maximain the bio-
geniccomponents
TOC, Br, opal,andSi/A1andalsocoincident
with maxima in Mo/A1 and, in some cases, U/AI and Zn/A1.
Thesefeaturesareindicatedby thestippled
bandsin Figure4,
eachof whichis 500yearsin thickness.
We suggest
500yearsas
a conservative
estimate
of theuncertainty
in thetimingof these
chemicalfeatures,given that the subsamples
usedfor geo-
chemical analysis typically integrate several centuries of
accumulation
andthechronological
uncertainties
of 60-280years
in ourradiocarbon
dates. Theseminimain lithogenic
compo-
nentsare also generally coincidentwith down core minima in
sandcontent(Figure 2) and maximain watercontent(data not
shown). In coreAA 186-GC34, all of thesefeaturescoincidewith
light-colored
Corethronlayers,whereas
for KROCK-GC1, three
of fivelithogenic
minimacoincide
withCorethron
layers,
whilea
fourthoccursin theChaetoceroslayer nearthebottomof the
core. Theseobservations
immediately
suggest
thatthedowncore
10. AA
186-G
034
(outer
shelf)
2
corrected
3
radi•arbon
age(kyr
B.P.) 4
5 .......
.....
......
6
7 , , ,
o • • • •,o o ; • o ;o ;o o , • • • o ,'o ;o •o •oo ,oo •oo
(b) o
KFIOOK-•Ol
•
(outershelf)
4
corrected
radiocarbon 6
age (kyrB.P.)
10
12
(c) 0
AA149-GC2
(inner
shelf) 1
corrected
radiocarbon 2
age (kyr B.P.)
3
i i i i
0 2 4 6 8 10 0 I 2 0 20 40 0 I 2 3 4 0 10 20 30 40 0 100 200
ß •
I '
(d) o
1
KROCK-GC2
(innershelf) 2
corrected 3
radiocarbon
age(kyr
B.P.)4
5
(e) o
2
AA149-GC12
(mid-outer
shelf)
4
6
corrected
radiocarbon 8
age (kyr B.P.)
10
12
glacial marine mud
14
16
0 2 4 6 8 10 0 I 2 0 20 40 0 I 2 3 4 0 10 20 30 40 0 100 200
•:•. - . . , • • . • :' .... , ,. .,:....•,•
....
....... •
0 2 4 6 8 10 0 1 2 0 20 40 0 1 2 3 4 0 10 20 30 40 0 100 200
• Ti(•/o)x20 • TOC(•/•) • opal(•/•) • (Mo/AI)x104 ß (Zn/AI)x104 ßbiogenic
Ba(ppm)
o AI(•/•) • Br(ppm)x1000 o total
Si/AI • (U/AI)x104 o (Cu/AI)x104
ß Fe(•%) ß (MWAI)x10 ß (Ni/AI)x104
Figure 4.
11. 220 SEDWICK ET AL.: SEDIMENT RECORDS FROM THE EAST ANTARCTIC SHELF
sediment sources to Mac. Robertson Shelf basins
Iresuspension,
reworking
and
I .^.
r ................... Iice-rafted
debris
I
ß ,
Iaeolian
dust
J
sea surface
rainI
shelf basins
~500-1000m
Figure 5. Major sediment
sources
totheMac. Robertson
Shelfbasins.ModifiedfromFigure10ofHarris andO'Brien[1996],¸
Springer-Verlag
1996.
minima in lithogenicelementsmay recordenhanced
deposition
of pelagicbiogenicmaterial on the surrounding
shelf,perhaps
due to blooms of Corethron or Chaetoceros in these waters.
The inner shelf cores AA 149-GC2 and KROCK-GC2 contain
generallyfiner-grainedsediments
(Figure2) andhigherconcen-
trationsof TOC, opal,andMo (Figures4c and4d) thantheouter
shelfcores,suggesting
either enhanced
deposition
of biogenic
materialrelativeto lithogenicmaterialin theinnershelfbasinsor
enhanced
preservation
of organicmatterin theselocations,
which
would createmore reducingconditionsin the sedimentcolumn.
In contrast to the outer shelf cores, the down core records of the
two coresfrom the inner Nielsen Basin containno significant
minima in lithogeniccomponents.Rather,the relativepropor-
tionsof lithogenicandbiogenicmaterialwhichhasaccumulated
at thesesiteshasbeenremarkablyuniformduringthe middleto
lateHolocene,beforewhichthisareawasprobablycoveredby a
floatingice shelf [Harris and O'Brien, 1998]. In bothcores,
thereare significant
decreases
in TOC contentwith increasing
sediment
age,whichmayreflectanincrease
in theaccumulation
of organic
carbon
duringthelateHolocene
or,morelikely,since
there are no correspondingtrends in biogenic silica, the
progressivediageneticremineralizationof TOC within the
sediment
column[Berner,1980;DontackandMcClennen,1996].
Not surprisingly,
thechemicalrecordof coreAA149-GC12
(Figure4e), from midshelfof the NielsenBasin,is generally
intermediate between the inner and outer shelf core records.
Minima in lithogenic
components
arediscernible
at -6 kyr B.P.
andpossibly
---1kyr B.P., asaresmallmaximain thebiogenic
components,
andthereis a steady
decrease
in theproportion
of
lithogenic
components
from--.
11to6 kyrB.P. In contrast
tothe
othercores,Mo/A1 valuesare lessthanU/A1, suggesting
lower
concentrations
ofhydrogen
sulfide,
thuslessreducing
conditions,
within the sediment column at this site. This observation
suggests
a slower
accumulation
of organic
matter
atthislocation
duringtheHolocene
compared
withtheothercoresitesandis
consistent with the sediment accumulation rates calculated for
AA149-GC12,whicharegenerally
lessthanthose
calculated
for
contemporaneous
sections
of theother
cores
inthisstudy
(Figure
3). Thislikelyreflects
a lesser
degree
of sediment
focusing
(see
below)at thesiteof coreAA149-GC12, whichis notwithinthe
centralaxisof theNielsenBasin(Figure1), aswell asepisodic
ventilation
by sinking,
oxygenated
surface
waters
in thisareaof
the shelf [Harris, 2000].
4.2. Paleoenvironmental Interpretation
In orderto interpret
ourproxyrecords
for theaccumulation
of
biogenic and lithogenic material we must first considerthe
sources of the SMO facies sediments in the Mac. Robertson Shelf
basins.The majorsources
of sediments
in thesebasins
areshown
schematically
in Figure5, asproposed
by Harris and O'Brien
[1996]. Underseasonally
openmarineconditions
thebasins
will
have receivedsedimentsfrom overlyingwatersin the form of
pelagicrain (biogenicmaterial), ice-rafteddebris(lithogenic
material), glacial meltwater plumes(lithogenicmaterial),and
aeoliandust(lithogenic
material). In addition,
a mixtureof bio-
genicandlithogenic
materialwill havebeentransported
laterally
into the basins from the shallower areas of the shelf as a result of
Figure 4. Down coregeochemical
datafor thefive coresin thisstudy. Stippled
bands
indicateinferredproduction
episodes;
hatching
indicates
sandy
siltor glacialmarinefacies.Estimated
analytical
uncertainties
(+ 2(;) shown
by widthof symbols/bars
alongbottomaxis.
12. SEDWlCK ET AL.: SEDIMENT RECORDS FROM THE EAST ANTARCTIC SHELF 221
resuspension
and erosionby currents,icebergs,and turbidity
flows. Harris and O'Brien[1996] have notedthat net erosional
conditions exist over ~90% of the Mac. Robertson Shelf, where
coarse-grained
sediments
dominatetheshallowbanksandslopes,
whereasnetdepositional
conditions
existon theremaining10%
of the shelf area, as represented
by the accumulation
of fine-
grainedSMO deposits
in theshelfbasins. A similarsedimenta-
tion patternhasbeendescribed
for otherpartsof the Antarctic
margin, including the Ross Sea, the Northern Victoria Land
Shelf, andtheWilkes Land Shelf [Andersonet al., 1984;Dunbar
et al., 1985;Anderson,1999].
The generalabsenceof coarselithogenicmaterial,graded
beds, and erosional surfaces in our cores argues against
significant
lateraltransport
of sediments
intotheshelfbasins
by
icebergs,
slumps,andturbidityflows,exceptin thecaseof core
AA149-GC12, which showsevidenceof episodicdown slope
sediment
transport
bydensity
currents
(asdescribed
in section
3.1
andHarris [2000]). In addition,swellwavesandtidal currents
arethought
toplaya minorrolein reworking
shelfsediments
in
ourstudyregion[Harris and O'Brien,1998]. On thebasis
of
theseconsiderations
and the observations
of relatively strong
bottom currents on the Mac. Robertson Shelf [Harris and
O'Brien,1998]we suggest
thattheshelf-basin
SMO deposits
are
dominated
by fine-grained
materialwhichhasbeenwinnowed
from the shallow banksand slopes,mainly by the action of
currents,with a lessercontributionof sedimentsderived from
overlyingwaters. The majorcurrentinvolvedin thisprocess
would be the AntarcticCoastalCurrent,a strong,circumpolar
currentdrivenby theEastWind Drift andthedensitygradient
across
theAntarctic
SlopeFront[Sntith
etal., 1984;Harris and
O'Brien,1998;Bindoff
etal., 2000]. Current
measurements
from
the Mac. RobertsonShelf suggestthat this current flows
consistently
towardthewest,withmaximum
speeds
of 50-200
cms'• ontheoutershelf,withsomereduction
in strength
during
thewinterandspring[HarrisandO'Brien,1998]. Suchcurrent
speeds
wouldcertainlybe sufficient
to mobilizemudandfine
sandfrom the shallowbanksand slopes,which would be re-
deposited
in thedownstream
shelfbasins
andwouldaccount
for
the accumulation of finer sediments in basins on the inner shelf,
wherecurrentspeeds
areless.
Thiswestflowingcurrent
regimehasprobably
existed
onthe
Mac. Robertson
Shelfduringmostof theHolocene,
sothatthere
hasbeena roughlycontinuous
transport
of fine biogenicand
lithogenic
material
(i.e.,SMO)intothese
basins
fromtheshallow
shelf areas to the east. Thus the undisturbed sediments which
haveaccumulated
in thesebasins
duringtheHolocene
will most
likelyprovide
uswitha record
of theproduction
anddeposition
of finesediments,
bothbiogenic
andlithogenic,
overlargerareas
of theadjacent,
upstream
banks.Thisscenario
provides
a likely
explanation
fortheapparent
increases
in sediment
accumulation
rates
during
thecourse
of theHolocene
thatareindicated
bythe
datain Figure3. HarrisandO'Brien[1998]haveargued
thatthe
glacialicesheet
began
toretreat
fromitsgrounding
position
on
themiddleto outershelfat ~10o
12kyr B.P., andthatthecalving
frontofthefloating
icesheet
hadretreated
totheinnershelf,
near
the locationof KROCKoGC2,by ~6 kyr B.P. If so,thenthe
seasonally
ice-free
areaof theMac.Robertson
Shelfwouldhave
increased
significantlybetweenthe early and late Holocene,
whichwouldbeexpected
toallowthecurrent-driven
transport
of
fine material into the shelf basinsfrom progressivelylarger
"catchmentareas"on the shelf. In the remainingdiscussion,
we
assumethat the SMO sequences
containedin our five sediment
coresprovide a recordof the depositionand erosionof fine
sedimentson the shallowshelf areasimmediatelyto the east,
with the exceptionof the sectionof core AA149-GC12 that is
thought
tobedisturbed
by downslopesediment
transport.
Whatprocesses
mightexplainthedowncorevariations
in the
proportions
of lithogenicandbiogeniccomponents
preserved
in
our middle and outer shelf cores? There are two likely
alternatives.The firstis thatthesevariations
mayreflectchanges
in thecurrent-driven
transport
of biogenic
andlithogenic
material
into the basins from the shallow areas of the shelf, such that the
lithogenicminima in our coresmay recordperiodsof lower
currentspeeds,which favor the transportof only the finer bio-
genicmaterial,suchasdiatomfrustules,
fromtheshelfbanksinto
the basins. This would lead us to expect significantlylower
sediment
accumulation
ratesin association
with theselithogenic
minima; however, we see no evidence for such variations in
accumulationrates. Moreover, it is difficult to explain the
corresponding
down core variationsin diatom speciesabun-
dances,
specificallytheoccurrence
of Corethron-rich
layers,as
the result of changesin current-drivenresuspension.A more
likely alternativeis that the downcorevariationsin thepropor-
tionsof lithogenicandbiogenicmaterialmay reflectchanges
in
the productionand depositionof biogenic material over the
shallowshelfareas,suchthatthe lithogenicminimamay record
periodsof enhanceddiatomproductionin shelfwatersupstream
of the core sites.
A similar interpretationof down core minima in lithogenic
componentsand corresponding
variationsin diatom species
assemblages
hasbeenadopted
in otherstudies
of sediment
cores
from Antarcticshelfbasins[e.g.,Dontacket al., 1993;Leventer
et al., 1996;LeventerandDunbar,1996;DontackandMayewski,
1999;Dontacket al., 2001]. Thuswe interpretthe downcore
minimain lithogeniccomponents
in our middleandoutershelf
cores as indicative of "production episodes," representing
sustained
periodsof high exportproduction
by diatomsin the
shelfwatersimmediatelyto the eastof theseshelfbasins. The
increasedaccumulationof organicmatter in the shelf basins
associated
with suchproduction
episodes
wouldbe expectedto
createmorereducing,
sulfide-rich
conditions
withinthesediment
column,as suggested
by theMo/A1, U/A1 andZn/A1maxima,
which approximately coincide with minima in lithogenic
components. That most of thesechemicalfeaturesroughly
coincide with Corethron layers in coresAA186-GC34 and
KROCK-GC1 suggests
thattheseproduction
episodes
entailed
massivebloomsof thisdiatomspecies
in theoutershelfregion,
giventhatCorethroncriophilunt
is a relativelylightlysilicified
diatomspecies
thatis thought
to bepoorlypreserved
in seafloor
sediments
[Jordanet al., 1991;Leventeretal., 1993, 1996].
4.3. TimingandForcingofProduction
Episodes
We assume
that the transport
of fine sediments
from the
shallow shelf areasinto the basinsis relatively rapid, so that
within the resolution of our radiocarbon chronologiesthe
compositional
changes
preserved
in themiddle
andouter
shelf
coresare coevalwith depositional
changes
overtheupstream
areasof the Mac. RobertsonShelf. However, the actualduration
of theproduction
episodes
inferred
fromthese
corerecords
is
13. 222 SEDWICK ET AL.: SEDIMENT RECORDS FROM THE EAST ANTARCTIC SHELF
AA186-GC34 KROCK-GC1 AA149-GC12
corrected
radiocarbon
age (kyr B.P.)
2
4
6
8
10
12
i
I I I I I I
810
••)•
trecord
disturbed
by
J• t• Iepisødes
ofdown
slope
•.•' ?s'.'•' .• "* '<*•*"•
0 2 4 6 8 10 0 2 4 6 8 10
ß Ti (wt%)x20 ß Ti (wt%)x20 ß Ti (wt%)x20
o AI (wt%) o AI (wt%) o AI (wt%)
ß Fe (wt%) ß Fe (wt%) ß Fe (wt%)
, Corethron
layer , Corethron
layer
(3of29 layersshown) (alllayersshown)
Figure6. Comparison
oftimingofproduction
episodes
(stippled
bands)
recorded
intheouter
andmiddle
shelfcores.
highly uncertain. On the basisof our rathercoarse-resolution
radiocarbon
chronologies,
these
production
episodes
lasted
ofthe
order
of 100-1000
years,
although
such
sediment
layers
might
haveaccumulated
over muchshorterperiods,giventheir
relativelyhigh watercontent
andour limitedstratigraphic
resolution.
Wealso
note
thatmany
ofthelight-colored
layers
in
theoutershelfcores
areonly-1-3 cmin thickness,
in whichcase
it would
beunlikely
thatsuch
features
would
beresolved
byour
geochemical
data,whichapplyto 10 cm thickdowncoresub-
samples.Thustheproduction
episodes
whichwe haveinferred
from
ourdown
core
composition
data
probably
represent
onlythe
moresustained
and/orfrequent
episodes
of exportproduction
in
adjacentshelfwaters.
In Figure6, we comparethe approximate
timingof these
productionepisodes
in coresAA186-GC34, KROCK-GC1, and
AA149-GC12, as indicatedby concentration
minimain Ti, AI,
andFe andtheoccurrence
of Corethronlayersin theIceberg
Alleycores.Relativetoourcorrected
radiocarbon
chronology,
production
episodes
areindicated
ataround
1,2, and6.2kyrB.P.
in two of thethreerecords,whereasall threecorerecordsindicate
an episodeat -5.5 kyr B.P. Only core KROCK-GC1 shows
evidence
of production
episodes
at --.3.8and 10.8 kyr B.P.,
althoughtheseepisodes
may be obscured
or unresolved
in core
AA149-GC
12andtheAA186-GC34
record
extends
backonlyas
faras-6.2 kyr B.P. Theonlyotherreported
Holocene
sediment
recordsfrom this region of the East Antarctic shelf are those
inferredfrom microfossilassemblages
in two coresfrom Fram
Bank, -100 km to the eastof the Nielsen Basin, where there is
evidence
of enhanced
exportproduction
duringtheperiod-2.6-
3.4 kyr B.P. [Rathburnet al., 1997]. This periodof higher
production
doesnotcorrespond
withthetimingof theproduction
episodes
in our Mac. RobertsonShelfrecords;however,we note
thatRathburnet al. [1997]corrected
theirradiocarbon
agesby
subtracting1300 radiocarbonyears, accountingonly for the
reservoir
effect. Applyinga largerradiocarbon
agecorrection
of
theorderof 1700 years(seesection3.1) to the raw radiocarbon
datesof Rathburnet al. [1997] placesthe Fram Bank high-
productionperiodat -2.2-3 kyr B.P., whichthenoverlapswith
the -1.6-2.5 kyr B.P. production episodesindicated in our
IcebergAlley cores(Figure6).
On the basisof comparisonsof sedimentrecordsfrom the
AntarcticPeninsula
shelfwithNorthernHemisphere
paleoclimate
records
it hasbeensuggested
thatperiods
of elevatedproductiv-
ity inferred from the Antarcticmarinerecordsreflect global
warmcycleswith periodsof-400, 200, and50-70 yearssuper-
imposedupon longer-termperiodsof low-productivitycorre-
sponding
to globalcoolingevents[Leventeret al., 1996;Domack
and Mayewski,1999;Domacket al., 2001]. To date,the most
detailed Holocene sediment records from the Antarctic shelf
regionare thosepreservedin corescollectedfrom the Palmer
Deep,off theAntarctic
Peninsula,
duringOceanDrillingProgram
Leg 178. High-resolutionpaleoproductivity
recordshave been
derived
fromthese
cores
using
downcoremagnetic
susceptibility
measurements
and radiocarbondating [Dornacket al., 2001].
The production
episodes
inferredfromourMac. Robertson
Shelf
cores
AA 186-GC34,KROCK-GC1,andAA149-GC12(Figure6)
14. SEDWICK ET AL.: SEDIMENT RECORDS FROM THE EAST ANTARCTIC SHELF 223
areroughlycoevalwith high-productivity
periodsinferredfrom
thePalmerDeeprecords[Domacketal., 2001] aswell asa mid-
Holoceneproductivityhigh inferred from RossSea sediment
records
[Jacobson,
1997;Frignani et al., 1998;Cunninghamet
al., 1999],suggesting
thattheremayhavebeencircum-Antarctic
periodsof enhancedexportproduction
duringthe Holocenein
response
to globalclimaticforcing. However,suchcomparisons
mustbe madewith caution,andtheapparent
agreement
between
corerecordsfrom the Mac. Robertson
Shelf,thePalmerDeep,
and the Ross Sea may be simply fortuitous, given the
uncertaintiesin our radiocarbonchronologyand the limited
temporal
resolution
of ourgeochemical
data.,,
What weretheimmediateforcingmechanisms
responsible
for
theseinferred episodesof enhancedproductionon the Mac.
RobertsonShelf, and why are no such productionepisodes
indicatedby the inner shelf cores? The factorswhich control
algal export productionand communitystructurein Antarctic
watersare not well understood
but are likely to includevertical
stabilityof the upperwater column[Smithand Nelson, 1990;
Sakshaug
etal., 1991;Arrigo etal., 1998a,1999],seeding
by sea
icealgae[SmithandNelson,1986;LeventerandDunbar, 1996],
grazing by zooplankton[Lancelot et al., 1993; DiTullio and
Smith, 1996], light limitation due to sea ice cover and self
shading [Smith et al., 1996], availability of micronutrient
elementssuchasiron [Martin et al., 1990;Sedwickand DiTullio,
1997;Sedwick
et al., 2000], andmorerarely,depletionof macro-
nutrients[Trdguer and Jacques,1992]. In studiesof sediment
recordsfromthe AntarcticPeninsula
continental
shelf,periodsof
enhancedorganicmatter accumulationhave been attributedto
enhanced
biologicalproductionduringperiodsof warm and/or
lesswindyclimate,asa consequence
of decreased
seaice cover
and increasedwater column stratification,the latter resulting
from decreased
wind mixing, warmerseasurfacetemperatures,
and increasedmeltwater inputs [e.g., Leventer et al., 1996;
Domack and Mayewski, 1999]. In addition, an increased
abundanceof Corethron criophilum andChaetoceros resting
sporesin sedimentsfrom the Ross Sea and the Antarctic
Peninsula shelf has been ascribed to enhanced water column
stratification[Leventeret al., 1993, 1996].
In considering
possible
causes
for theproductivity
episodes
recorded in our cores from the Mac. Robertson Shelf, we note
thatmostof thesefeatures
mayreflectmassive
Corethron
blooms
in adjacent
shelfwaters,with theexception
of theproduction
episode
recorded
at~10.8kyrB.P.incoreKROCK-GC1,
which
apparentlyrecordsproductionof Chaetoceros restingspores
associated
with theretreatof a permanent
ice cover. In contrast,
the sediment cores from the inner Nielsen Basin suggest
relatively uniform exportproductionin upstreamshelf waters.
With theexceptionof thebaseof coreKROCK-GC2, biogenic
materialin theseSMO sequences
is dominatedby the diatom
Fragilariopsis curta, which is thoughtto be favoredin ice-
marginalenvironments
[Leventeret al., 1993]. Satelliteobserva-
tions obtainedin recent years have revealed the existencea
persistent
coastalpolynyaontheinnerMac. Robertson
Shelf,the
Cape Darnley polynya, which is probablymaintainedby the
presenceof icebergsgroundedon the shallowbanksoff Cape
Damley (~70øE) [Massomet al., 1998]. Satelliteadvanced
very
high resolutionradiometerimagesobtainedduring 1997-1999
(seehttp://www.antcrc.utas.edu.au/avhrr/mawson/archive/)
reveal
thatthe ice-freewatersof thispolynyaextendwestfrom Cape
Damleyasfar astheinnerNielsenBasinduringthespringand
summer,whereasthe outershelfwatersare coveredby packice
duringthespringandsometimes
intothesummer.If we assume
thata similarpatternof seasonal
icecoverhasexistedsincethe
middle Holocene, by which time the glacial ice shelf had
retreated to the inner continental shelf, then the location of the
Cape Darnley polynyaprovidesa likely explanationfor the
differences between the inner and outer shelf core records.
Duringthe middleto late Holocene,the ice-marginal,
well-
mixed open waters of the Cape Darnley polynya may have
favoredconsistent
interannual
production
by F. curtaduringthe
springandsummer,whichwouldhaveresultedin theconsistent
transport
of fine, biogenic-rich
sediments
intotheinnerNielsen
Basin by the relatively weak inner shelf bottom currents. In
contrast, the often ice-covered waters of the outer Mac. Robert-
son Shelf may have experiencedmuch more variable algal
productionduringthe springandsummerasa resultof interan-
nual variations in sea ice cover. Moreover, unlike the wind-
mixedopenwatersof thepolynya,theoutershelfwaterswould
bemorelikely to maintaina stableupperwatercolumnasa result
of enhancedsea ice melting during warmer years. In this
scenario,
algalproduction
andcommunity
composition
upstream
of the outershelfbasinswould havebeenstronglymodulatedby
the extent of sea ice melting during the spring and summer,
whereas
watercolumnconditions
andalgalproduction
to theeast
of theinnerNielsenBasinwere"buffered"
by thepresence
of the
Cape Darnley polynya. From theseconsiderations
we suggest
that the productionepisodes
recordedin our outershelfcores
fromIcebergAlley andmoreweaklyin themidshelfcorefrom
the Nielsen Basin record years of enhancedsea ice melting
duringclimaticwarmperiods,whenfactorssuchasa meltwater-
stratified water column [Leventer et al., 1996] and ice-derived
ironinputs[Sedwick
et al., 2000] stimulated
massive
bloomsof
diatoms,particularlyCorethroncriophilum,on theouterMac.
Robertson Shelf.
4.4. Conclusions and Future Work
Our interpretations of sediment records from the Mac.
RobertsonShelf basins are generally consistentwith other
Holocene
sediment
records
fromtheAntarctic
margin
inthatthey
provide evidence of millennial-scaleepisodesof enhanced
diatom productionin outer shelf waters,includinga mid-
Holocene"warm period"between~5 and 7 kyr B.P. Such
production
episodes
mayberelatedtoincreased
seaicemelting
duringglobal climatic warm periods,althoughlocal forcing
mechanisms
cannot
bediscounted
atthistime,giventhedearth
of
sediment
recordsfrom our studyarea. Our resultsalsoindicate
thatsmall-scale
regional
phenomena,
suchascoastal
polynyas,
may stronglyinfluencethe sedimentrecordsin Antarctic shelf
basins,which highlightsthe needfor carefulconsideration
of
regionalsediment
dynamics
in paleoenvironmental
studies
of the
Antarcticmargin. Furtherstudies
areclearlyrequired
in orderto
establish
theextentandtimingof Holocene
paleoenvironmental
changes
ontheEastAntarcticmarginand,at a morebasiclevel,
toclarifytherelationship
between
thesediments
accumulating
in
shelfbasinsandthe processes
occurringin overlyingwaters.
Suchinformation
canonlybeprovided
by increasing
thespatial
coverageand temporalresolutionof sedimentary
recordsfrom
this region for comparisonwith emerging high-resolution
paleoenvironmental
recordsfrom otherareas.
15. 224 SEDWlCK ET AL.: SEDIMENT RECORDS FROM THE EAST ANTARCTIC SHELF
Acknowledgements.
Theauthors
thank
PhilO'Brien
forcollecting
coresamples
andproviding
initialencouragement,
BobConnell
and
Suenor
Woonfor laboratory
assistance,
andFionaTaylorandLeanne
Armandfor diatomspecies
identification.
LesleyDowlingandRoger
Summons
of theAustralian
Geological
Survey
Organization
arethanked
forproviding
TOCand•5•3C
data.John
Anderson,
Eugene
Domack,
and
AmyLeventer
arethanked
for theirthorough
reviews,
whichgreatly
improved the manuscript. Uranium series measurementswere
accomplished
withtravel
funding
provided
bytheAustralian
Department
ofIndustry,
Science
and
Technology.
Thestaff
oftheAustralian
Nuclear
Science
andTechnology
Organization
AMS facilityarethanked
for
radiocarbon
analyses,
funded
bygrants
fromtheAustralian
Institute
of
Nuclear
Science
andEngineering.
Principal
funding
forthisworkwas
provided
bytheAntarctic
CRC.
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