Small perturbations
to the hydrological
cycle cause large
changes in global
climate & ocean
circulation
Geochemical proxies of North
American freshwater routing during
the Younger Dryas cold event
Carlson et al. 2007 PNAS
Kelly Robinson
Geological Oceanography
Spring 2008
Image credit: NASA/JPL http://www.nasastockphotography.com
Questions
• What role does freshwater play in forcing
abrupt climate change?
Neogloboquadrina pachyderma
Globigerina bulloides
Image credit: Mary W. Silver, UC Santa Cruz
Methods
• Mg/Ca, U/Ca, 87Sr/86Sr & δ18O used as tracers of
changes in source & flux of surface waters
– Increase in Mg/Ca Æ decrease in SST
• Did eastward routing with an increase
freshwater flux into the St. Lawrence River
cause the Younger Dryas (YD) cold
event?
– Increase U/Ca Æ increase flux of oxygenated,
particulate-rich riverine water into estuary
– Increase 87Sr/86Sr Æ Weathering of old bedrock
– Depletion of δ18O Æ decrease in salinity
Atmospheric-ocean coupling
Pleistocene temperature
variability
North Atlantic Meridional
Overturning Circulation
YD winters in northern Europe
26-28°C colder than today
Image credits: Alley, R.B. 2000. The Younger Dryas cold interval as viewed from central Greenland. Quaternary
Science Reviews 19: 213-226; Marika Holland http://www.asp.ucar.edu/colloquium/2000/Lectures/holland.html
Lake Agassiz before & after
catastrophic flood
• 9500 km3 of water released to the N. Atlantic
Broecker 2003 Science 300(6):1519-1522
1
Younger Dryas
Precambrian
Drainage of St. Lawrence Basin
doubled…
Paleozoic
1.35 x 106 km2 Æ 3.13 x 106 km2
Mesozoic
a) Dinoflagellate cyst SST
reconstruction
Canadian freshwater
sources have δ18O of (-25‰)
Æ freshwater flux into
estuary would result in
18O values
deplete
Aburptδ0.32‰
decrease in
δ13C indicates increased
flux of 12C-enriched
Increase
of ∆Mg/Ca
freshwater
run-off by
2.5 mmol/mol indicates
routing of Canadian
Canadian
plain riversrunhave
off Æ
Rivers
draining
[U]
10-20x
greater
than [U]
have
0.6-1.0
in
the[Mg]
St. Lawrence
mmol/kgpre-YD
higher than [Mg]
system
of St.increase
Lawrence
Rivery
Rapid
at 12.5
kyr
BPsystem
reflect changes in ratio
due to increased weathering
over the older western
Canadian Shield bedrock
Cenozoic
b) N.pachyderma ( ),
seawater ( ), & G.ruber ( )
End YD ice margin
c) N.pachyderma δ13C
record ( )
12.5 kyr BP ice margin
YD drainage area
d) Mg/Ca ( ) & ∆Mg/Ca ( ) of
G. balloides
e) U/Ca of G. balloides ( )
Pre-YD drainage area
Increased freshwater discharge
from the St. Lawrence River
f) 87Sr/86Sr of G. balloides ( )
Proxies of AMOC & freshwater routing
• 0.07 Sv increase in
discharge explains
initial ∆Mg/Ca signal
at start of YD
• Subsequent flux 0.06
Sv explains additional
∆Mg/Ca increase
1 Sv = 106 m3 s-1
Discharge increased at start of YD, then
decreased around 12 kyr during YD
• Decreased fw during
YD due to diversion to
Arctic Ocean
– warming of N. Atlantic
SST
– increased SE Asian
monsoon intensity
– cooling over Antarctica
– increase in AMOC
• Increased fw after 12
kyr Æ water routed to
St. Lawrence River
– Decrease in AMOC
Summary
• First direct evidence of eastward routing of surface waters
from western Canada
• Retreat of Laurentide ice shelf doubled freshwater
discharge basin of St. Lawrence River
• Timing of routing & magnitude of freshwater flux sufficient
to induced changes in Atlantic Meridional Overturning
Circulation
– Initial 0.06 ± 0.02 Sv + During 0.06 ± 0.01 Sv = Total 0.12 ± 0.02 Sv
– Salinity of mixed-layer drops 4.1 psu
• Tight coupling between changes in freshwater fluxes,
Atlantic circulation, and climate illustrate sensitivity of
climate systems to hydrological cycle
2
Objectives
High resolution stratigraphy of the
Mediterranean outflow contourite
system in the Gulf of Cadiz during
the late Pleistocene: The impact
of Heinrich events
• Describe high-resolution seismic stratigraphy of a
“Mediterranean-forced” Contourite Depositional
System as it relates to millennial scale climate &
oceanographic changes
• Propose a model for changes of Mediterranean
Outflow Water pattern into the Gulf of Cadiz
during Late Pleistocene Heinrich events
Llave et al. 2006 Marine Geology
Heinrich Events
• Melting armadas of glaciers from the
Laurentide ice shelf
• Large injection of freshwater into N.
Atlantic Æ sluggish conveyor circulation
• Glacial cooling in Mediterranean & Atlantic
Ocean off of the Iberian Margin
Methodology
• Ultrahigh-resolution seismic data
• Piston & gravity cores
– Calypso giant piston cores
– standard gravity cores
• Stable oxygen (δ18O) isotopes
– temperature & salinity
• Radiocarbon dating of cores using
foraminifera tests
Record sediment
recovery of 64.4m!
Image credit: http://www.univ-brest.fr/IUEM/Universite_flottante/calypso.html
A) General circulation pattern
A)
B) Western Iberian margin circulation
patterns where Heinrich event studies
have been carried out.
B)
Extensive sampling & survey’s of the Gulf of Cadiz
3
Seismic profiles
Core MD-36
• Moderate-high amplitude
reflectors in a wellstratified parallel pattern in
upper layers
Spatial distribution & main depocenters of the
Late Pleistocene-Holocene sesmic unit
Core MD-41
• Semi-transparent, low
amplitude & not well
stratified pattern in lower
layers
Heinrich events
• Indicated by
peaks in lithic
particles (quartz)
• Minimum 5% of
grains were
dolomite from
Laurentide Ice
Sheet
MD-36
MD-36
Indicators of low
current velocity…
MD-36
• Mud dominant
MD-41
MD-41
• Sand content of
surface layers lower
compared to upper layer
samples of MD-36
MD-41
Sand
• Sand layers less
frequent than in MD-36
Sand
What the seismic profiles tell us…
• Unit-a
– transparent mud seismic facies
• Unit-b
– transparent facies at the base & more reflective
(coarser, silty sediments) near the top
• Unit-c
– most reflective seismic facies (silty/sandy sediments)
intercalated with mud layers
• Unit-d
Sand
Sand
– very transparent due to extensive muddy horizons at
top layers of the cores
– reflective facies (silty/sand) at the top of the subunit
4
Late Pleistocene climatic curve
MD-36
MD-41
Cool-warm transitions
demarked by increase
in δ18O values
Changes from cool to warm conditions are
accompanied by a...
Æ decrease in the sedimentation rate in
the deeper core (●)
Æ slight increase in the shallow core (□)
Heinrich events
Shallow core (MD9923-41)
Deep core (MD9923-36)
Palaeocirculation during…
A) Glacial conditions when MOW is
denser & deeper
B) Interglacial conditions when MOW
is less dense and current shoals
Glacial MOW circulation
Cool & dry Mediterranean
Higher salinity & more dense
water mass
Deep flowing MOW interacts
strongly with seafloor at
greater depths beyond shelf
break
Transport & deposition of
coarser material (sand) on
middle slope
Interglacial MOW circulation
Summary
Warm & (wet?) Mediterranean
climate
• Close connection between the North Atlantic and
the hydrological conditions of the Gulf of Cadiz
Lower salinity & less dense
water mass
• Evidence of the influence of climate variability on
slope sedimentation & MOW circulation
MOW interacts strongly with
seafloor at shallower depths
close to the shelf break
Transport & deposition of
coarse material at the shelf
break
– Cool Æ major contouric sedimentation on the shelf slope
due to intensification of the deep MOW
– Warm Æ contouric sedimentation occurs at shallower
depths on shelf due to stronger upper MOW
• Spatial occurrence of sandy contourites was
controlled by variations in the position & current
strength of the MOW
5
Overall conclusions
• Increased freshwater input into the
northern Atlantic can significantly alter
ocean circulation & the global climate
– Younger Dryas Æ riverine discharge
– Heinrich events Æ melting glacier armadas
• Far-field impacts include changes in the
strength & position of the Mediterranean
Outflow Waters (MOW)
6