The Long-Term Variation of Short-Term The Long-Term Variation of Short-Term Oceanographic Events Revealed by Stable Isotope Oceanographic Events Revealed by Stable Isotope Sclerochronology of Amino Acid Dated Bivalve and Sclerochronology of Amino Acid Dated Bivalve and
Brachiopod ShellsBrachiopod Shells
Richard A. Krause Jr.1, Susan L. Barbour-Wood1, Christopher Romanek2, Michał Kowalewski1, Marcello G. Simões3, and
John F. Wehmiller4
1 Virginia Tech, Dept. of Geosciences, Blacksburg, VA2 Savannah River Ecological Laboratory, Univ. of Georgia, Aiken, SC
3 Instituto de Biociencias, Universidade Estadual Paulista, Botucatu, Brazil4 Univ. of Delaware, Dept. of Geology, Newark, DE
The South Brazilian Bight Marine Province is Dominated by the South Brazil Current.
SACW
TWCW
Coastal water (CW)Tropical water (TW)
South AtlanticCentral Water (SACW)
11°C
13°C
15°C17°C
19°C21°C
NW SECoast Santos Basin
50 km500
400
300
200
100
0
Dep
th (
m)
Kowalewski et al., 2002, PalaiosThe South Brazil Current is a weak western-boundary current that carries warm subtropical water south, along the Brazilian Coast.
Present oceanographic conditions include seasonal coastal upwelling and non-seasonal freshwater runoff events (possibly linked to ENSO events).
South Brazil Current
shelf profile
Coastal Water: mean annual salinity = 33- 34‰mean annual temp. = >20°C
Tropical Water:mean annual salinity = 33- 34‰mean annual temp. = >20°C
South Atlantic Central Water:mean annual salinity = ~34- 35‰mean annual temp. = <20°C
How have these conditions fluctuated in the Holocene?
SACW
TWCW
Coastal water (CW)Tropical water (TW)
South AtlanticCentral Water (SACW)
11°C
13°C
15°C17°C
19°C21°C
NW SECoast Santos Basin
50 km500
400
300
200
100
0
Dep
th (
m)
Kowalewski et al., 2002, Palaios
Goals:To assess the long-term variation of short-term oceanographic events.
1) Seasonal Coastal Upwelling2) ENSO related freshwater runoff events
To understand the multi-centennial dynamics of ocean currents and marine bio-productivity.
South Brazil Current
shelf profile
Barbour-Wood et al., in review, Quaternary Research
Collections are from Ubatuba Bay, Southeast Brazilian Bight.
Shells collected as surficial bulk samples
Van Veen grabs and short dredges
Collection at multiple sitesThis talk will focus on shells from inner shelf (10m site)
Research focused on 2 speciesSemele casali
aragonitic bivalvethin shellinfaunal
Bouchardia roseacalcitic brachiopodrobust shellepifaunal
10 mm
10 mm
D/L aspartic acid (D/L Asp)
Shells sampled from same region to minimize intra-shell variability (Carroll et al, 2003)
Racemization analysis performed on gas chromatograph with 25 m Chirasil-Val column and flame ionization detector.
D/L Asp ratios are excellent predictors of 14C age for both species
Methods: Amino Acid Racemization Dating
0
0.1
0.2
0.3
0.4
0 1000 2000 3000 4000 5000 6000 7000
Calendar Years B.P.
D/L
As
p2
-Bouchardia rosea (Brachiopoda)-Semele casali (Bivalvia)
r2=0.97
r2=0.86
Barbour-Wood et al., in review, Quaternary Research
Brachiopods (n = 90)
0
5
10
15
20
0 1000 2000 3000 4000 5000 6000 7000 8000
Calendar Years BP
0
5
10
Bivalves (n = 71)
Nu
mb
er o
f sh
ells
Methods: Carbon and Oxygen Stable Isotopes
1 mm 1 mm 1 mm
Shell carbonate analyzed for carbon and oxygen isotopes using continuous flow isotope ratio mass spectrometry (CF-IRMS)
High spatial resolution sampling accomplished with a computer controlled micro-milling system
>Carbonate drilled from shells along transects parallel to external growth lines
>Sampling resolution: every 200-400 μm
Neither species exhibits an appreciable vital effect
NW SECoast Santos Basin
SACW
TWCW
Coastal water (CW)Tropical water (TW)
South AtlanticCentral Water (SACW)
11°C
13°C
15°C17°C
19°C21°C
50 km500
400
300
200
100
0
Dep
th (
m)
Kowalewski et al., 2002, Palaios
Possible short-term oceanographic patterns
1) Background Conditions: Coastal upwelling water masses and runoff water masses are absent.
Bottom Water dominated by:Coastal Water (CW) on inner shelfTropical Water (TW) of SBC on mid shelfSouth Atlantic Central Water (SACW) on outer shelf
Sea surface temperature (SST) shifting annually by no more than 6°C
Salinity varying by less than 1‰
South Brazil Current
shelf profile
NW SECoast Santos Basin
SACW
TWCW
Coastal water (CW)Tropical water (TW)
South AtlanticCentral Water (SACW)
11°C
13°C
15°C17°C
19°C21°C
50 km500
400
300
200
100
0
Dep
th (
m)
Kowalewski et al., 2002, Palaios
Possible short-term oceanographic patterns
2) Coastal Upwelling: Seasonal upwelling can become stronger as cyclonic meanders of the SBC coincide with wind-driven episodes of the offshore Ekman transport.
Pronounced events occur intermittently in austral summers
Result of such events:Inner shelf experiences seasonally elevated phytoplankton blooms associated with nutrient-rich waters (SACW)
Salinity may increase slightly (~1‰)
SST may drop notably (~6°C)
South Brazil Current
shelf profile
NW SECoast Santos Basin
SACW
TWCW
Coastal water (CW)Tropical water (TW)
South AtlanticCentral Water (SACW)
11°C
13°C
15°C17°C
19°C21°C
50 km500
400
300
200
100
0
Dep
th (
m)
Kowalewski et al., 2002, Palaios
Possible short-term oceanographic patterns
3) Freshwater Runoff / ENSO:
Cold, low salinity waters invade region intermittently from the south
Events correlated with Southern Oscillation Index (SOI), and thus suggest an ENSO link
Result of such events:Inner shelf experiences seasonally elevated phytoplankton blooms associated with nutrient-rich coastal runoff
Salinity will drop markedly
SST may drop slightly
South Brazil Current
shelf profile
CoastalUpwelling
FreshwaterRunoff
Salinity Sea Surface Temp.
Summary of physical characteristics of short-term events
Change relative to background conditions
slightincrease
largedecrease
largedecrease
slightdecrease
Background Conditions δ13C: 1-2‰ (Hudson, 1977)
δ18O: -1.3 to 2.2‰ (Schmidt et al, 1999)
Coastal Upwelling Decrease in δ13C from background Possible slight +/- excursion in δ18O
Freshwater Runoff / ENSO Decrease in δ13C from background Decrease in δ18O from background
What is the expected isotopic signature of each of the hypothesized patterns?
-3.00
-2.00
-1.00
0.00
1.00
-4.00 -2.00 0.00 2.00 4.00
minimum δ13C value for each shell
δ18O
2.00
0
1
2
3
0
1
0 5 10
Bivalve 2944 yrs. BP
δ13C
δ18O
-2
-1
0
0 5 10
δ18O
Distance from hinge (mm)
BackgroundConditions
Brachiopod 652 yrs. BP
-2
-1
0
1
2
δ13C
FreshwaterRunoff / ENSO
Coastal Upwelling
What is the expected isotopic signature of each of the hypothesized patterns?
??
-3
-2
-1
0
1
2
3
-4modern
0 100 200 300 400 500 600 700 1000 2000 3000 4000 5000 6000 7000
Years BP
δ13C
δ13C range may be a reliable event-indicator.
-1
0
1
2
0 1 2 3 4 5 6
δ13C
ave
rage
δ13C range
301-600601-900
01-300
>900
Age
-As range increases, δ13C average drops.
-Productivity events are often short-lived (captured within the 4-7yr lifetime of the organism).
- This phenomenon seems to be time-invariant.
YearsBefore
Present
δ13C
-4
-3
-2
-1
0
1
0 5 10 15
0 5 10 150
1
2
δ13C
0
1
2
3
0 5 10 15
0 5 10 150
1
2
0 5 10 150
1
2
0
1
2
0 5 10 15 20
0 5 10 15-3
-2
-1
0
1
2
0 5 10 150
1
2
3
-2
-1
0
1
2
0 5 10
0 5 10 150
1
2
0
1
2
0 5 10
0
1
2
3
0 5 10
0 5 10 150
1
2
0 5 10 150
1
2
3
Brachiopods Bivalves
100 200 300 400 500 600 700 1000 2000 3000 4000 5000 6000 7000
Selected δ13C profiles for the last 6000 years.
Distance from hinge (mm)
Distance from hinge (mm)
Distance from hinge (mm)
High / VariablePrimary
Productivity
Relatively Stable;Background Conditions
High / VariablePrimary
ProductivityRelatively Stable;
Background Conditions
Future ResearchSao Paulo
SouthAmerica
Rio deJaneiro
Ubatuba
Analyze additional shells:
>Coastal sites several sites; up to 30m
>Outer-Shelf sites 101 additional sites 60-808m
Analysis of trace elements (Mg, Mn, Sr, Ba)
High-resolution sampling (LA-ICPMS)
Allow better distinction between high-productivity eventsHigh productivity: High Ba content in shell carbonateLower SST: Lower Sr content in shell carbonate
Conclusions
• The Holocene history of short-term oceanographic events can be reconstructed using time series of surficially-collected invertebrate shells.
• The Southeast Brazilian Bight seems to have been characterized by discrete periods of elevated or variable bio-productivity.
• Amino-acid dated invertebrate shells can yield insight into Holocene climate and oceanographic patterns.
• As such, time series shown here may expand the arsenal of long-term climatic and environmental proxies.
10
20
30
0 2000 4000 6000 8000
# o
f s
hel
ls
Calendar Years BP
δ13C
-4
-3
-2
-1
0
1
0 5 10 15Distance from
hinge (mm)
0 100 200 300 400 500 600 700 1000 2000 3000 4000 5000 6000 7000
Years Before Present
δ13C
δ13C
δ13C
0
1
2
0 5 10 15
-4
-3
-2
-1
0
1
0 5 10 15
0 5 10 150
1
2
δ13C
-1
0
1
2
0 5 10 15
0
1
2
3
0 5 10 15
0 5 10 15
0
1
2
0 5 10 15
-2
-1
0
1
2
0 5 10 150
1
2
0 5 10 150
1
2
0 5 10 150
1
2
0
1
2
0 5 10 15 20
0 5 10 15-3
-2
-1
0
1
2
0 5 10 150
1
2
3
0
1
2
0 5 10
-2
-1
0
1
2
0 5 10
0 5 10 150
1
2
0 5 10 150
1
2
3
0
1
2
0 5 10
0
1
2
3
0 5 10
0 5 10 150
1
2
0 5 10 150
1
2
3
Brachiopods Bivalves
1
2
3
4
5
1000 2000
Average Sr/Ca
1
2
3
5 10 15
Average Ba/Ca
δ13C
ran
geδ18
O r
ange
What is the trace element signature for the hypothesized patterns?
Barium: Elevated levels associated with areas of high productivity.
Strontium: Widely used as a paleo-thermometer of sea-surface temperatures. Cooler temperatures cause a draw-down in Sr/Ca
High-productivity eventsupwelling or runoff
Cooler-temperature eventsupwelling
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