Transcript PowerPoint file

The Long-Term Variation of Short-Term
Oceanographic Events Revealed by Stable Isotope
Sclerochronology of Amino Acid Dated Bivalve and
Brachiopod 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, VA
2 Savannah River Ecological Laboratory, Univ. of Georgia, Aiken, SC
3 Instituto de Biociencias, Universidade Estadual Paulista, Botucatu, Brazil
4 Univ. of Delaware, Dept. of Geology, Newark, DE
The South Brazilian Bight Marine Province is Dominated by
the South Brazil Current.
0
South Brazil
Current
Depth (m)
100
CW
21°C 19°C
17°C
15°C
200
13°C
300
400
500
Coastal water (CW)
Tropical water (TW)
11°C
South Atlantic
Central Water (SACW)
Present oceanographic conditions include
seasonal coastal upwelling and nonseasonal freshwater runoff events (possibly
linked to ENSO events).
SACW
50 km
NW
Coast
The South Brazil Current is a weak westernboundary current that carries warm
subtropical water south, along the Brazilian
Coast.
TW
SE
Santos Basin
Kowalewski et al., 2002, Palaios
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?
0
South Brazil
Current
Depth (m)
100
CW
21°C 19°C
17°C
15°C
TW
200
13°C
300
400
500
Coastal water (CW)
Tropical water (TW)
11°C
South Atlantic
Central Water (SACW)
SACW
50 km
NW
Coast
SE
Santos Basin
Kowalewski et al., 2002, Palaios
Goals:
To assess the long-term variation of short-term oceanographic events.
1) Seasonal Coastal Upwelling
2) ENSO related freshwater runoff events
To understand the multi-centennial dynamics of ocean currents and marine bioproductivity.
Collections are from Ubatuba Bay, Southeast Brazilian Bight.
Shells collected as surficial bulk
samples
Van Veen grabs and short dredges
Collection at multiple sites
This talk will focus on shells from inner
shelf (10m site)
Research focused on 2 species
Semele casali
aragonitic bivalve
thin shell
infaunal
10 mm
Barbour-Wood et al., in review, Quaternary Research
Bouchardia rosea
calcitic brachiopod
robust shell
epifaunal
10 mm
Methods: Amino Acid Racemization Dating
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
Bivalves (n = 71)
-Bouchardia rosea (Brachiopoda) r2=0.97
-Semele casali (Bivalvia)
D/L Asp2
0.3
0.2
r2=0.86
0.1
0
0
1000
2000
3000
4000
5000
6000
Calendar Years B.P.
Barbour-Wood et al., in review,
Quaternary Research
7000
10
Number of shells
0.4
5
0
Brachiopods (n = 90)
20
15
10
5
0
0
1000 2000 3000 4000 5000 6000 7000 8000
Calendar Years BP
Methods: Carbon and Oxygen Stable Isotopes
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
1 mm
1 mm
1 mm
Possible short-term oceanographic patterns
South Brazil
Current
0
CW
21°C19°C
17°C
15°C
Depth (m)
100
Bottom Water dominated by:
Coastal Water (CW) on inner shelf
Tropical Water (TW) of SBC on mid shelf
South Atlantic Central Water (SACW) on outer shelf
TW
Sea surface temperature (SST) shifting annually by no
more than 6°C
200
13°C
300
400
500
Coastal water (CW)
Tropical water (TW) 11°C
South Atlantic
Central Water (SACW)
SACW
50 km
NW
Coast
SE
Santos Basin
Kowalewski et al., 2002, Palaios
1) Background Conditions: Coastal upwelling
water masses and runoff water masses are
absent.
Salinity varying by less than 1‰
Possible short-term oceanographic patterns
South Brazil
Current
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
0
CW
21°C19°C
17°C
15°C
Depth (m)
100
Result of such events:
Inner shelf experiences seasonally elevated
phytoplankton blooms associated with
nutrient-rich waters (SACW)
TW
200
13°C
300
400
500
Coastal water (CW)
Tropical water (TW) 11°C
South Atlantic
Central Water (SACW)
SACW
SST may drop notably (~6°C)
50 km
NW
Coast
Salinity may increase slightly (~1‰)
SE
Santos Basin
Kowalewski et al., 2002, Palaios
Possible short-term oceanographic patterns
3) Freshwater Runoff / ENSO:
South Brazil
Current
Cold, low salinity waters invade region intermittently
from the south
Events correlated with Southern Oscillation Index
(SOI), and thus suggest an ENSO link
0
CW
21°C19°C
17°C
15°C
Depth (m)
100
TW
Result of such events:
Inner shelf experiences seasonally elevated
phytoplankton blooms associated with
nutrient-rich coastal runoff
200
13°C
300
400
500
Coastal water (CW)
Tropical water (TW) 11°C
South Atlantic
Central Water (SACW)
SACW
SST may drop slightly
50 km
NW
Coast
Salinity will drop markedly
SE
Santos Basin
Kowalewski et al., 2002, Palaios
Summary of physical characteristics of short-term events
Change relative to background conditions
Salinity
Sea Surface Temp.
Coastal
Upwelling
slight
increase
large
decrease
Freshwater
Runoff
large
decrease
slight
decrease
What is the expected isotopic signature of each of the
hypothesized patterns?
2.00
Background Conditions
δ13C: 1-2‰ (Hudson, 1977)
δ18O: -1.3 to 2.2‰ (Schmidt et al, 1999)
1.00
Coastal Upwelling
Decrease in δ13C from background
Possible slight +/- excursion in δ18O
0.00
δ18O
-1.00
Freshwater Runoff / ENSO
Decrease in δ13C from background
Decrease in δ18O from background
-2.00
-3.00
-4.00
-2.00
0.00
2.00
minimum δ13C value for each shell
4.00
What is the expected isotopic signature of each of the
hypothesized patterns?
Background
Conditions
Bivalve 2944 yrs. BP
3
Freshwater
Runoff / ENSO
Coastal
Upwelling
Brachiopod 652 yrs. BP
??
2
2
1
1
δ13C
0
0
δ13C
-1
-2
1
δ18O
0
0
0
5
10
-1
Distance from hinge (mm)
-2
0
5
10
δ18O
δ13C range may be a reliable event-indicator.
3
2
1
δ13C
0
-1
-2
-3
-4
modern
0
100
200
300
400
500
600
1000 2000 3000 4000 5000 6000 7000
700
Years BP
2
δ13C average
-As range increases, δ13C average drops.
-Productivity events are often short-lived
(captured within the 4-7yr lifetime of the
organism).
Age
0
1-300
301-600
601-900
>900
1
0
- This phenomenon seems to be timeinvariant.
-1
0
1
2
3
4
δ13C range
5
6
Selected δ13C profiles for the last 6000 years.
High / Variable
High / Variable
Relatively Stable;
Primary
Primary
Productivity
Productivity Background Conditions
Relatively Stable;
Background Conditions
δ13C
3
3
3
2
2
2
2
2
2
2
1
1
1
1
1
1
1
0
0
5
10
Years
Before
Present
0
15 0
5
100
10
200
0
15 0
5
300
10
400
0
20 0
15
500
0
0
5
10
600
15
0
5
700
10
15
0
0
5
0
10
5
10
1000 2000 3000 4000 5000 6000 7000
δ13C
3
2
2
2
2
2
2
1
1
1
1
1
1
1
0
0
0
15
-1
0
0
-1
-2
-2
0
-1
5
10
0
15 0
5
10
Brachiopods
Bivalves
-2
-3
-4
0
5
10
15
Distance from hinge (mm)
0
0
0
0
5
10
5
10
0
5
10
15
Distance from hinge (mm)
-3
15
Distance from hinge (mm)
15
5
10
Future Research
South
America
Rio de
Janeiro
Sao Paulo
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 events
High productivity: High Ba content in shell carbonate
Lower SST: Lower Sr content in shell carbonate
Conclusions
The Holocene history of short-term oceanographic events
can be reconstructed using time series of surficially30
collected invertebrate shells.
20
10
0
•
2000 4000 6000
Calendar Years BP
The Southeast Brazilian Bight seems to have been
characterized by discrete periods of elevated or variable
bio-productivity.
8000
1
0
δ13C
# of shells
•
-1
-2
-3
-4
0
•
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.
5
10
Distance from
hinge (mm)
15
δ13C
2
2
1
1
0
0
0
5
10
0
15
5
10
15
3
2
2
2
2
1
1
1
1
1
0
0
0
0
δ13C
0
-1
-1
-2
-2
-3
0
5
10
5
10
15
20
0
0
5
10
0
5
10
15
Brachiopods
Bivalves
15
-4
0
5
10
15
δ13C
3
3
3
2
2
2
2
2
2
2
1
1
1
1
1
1
1
0
15
0
0
15
-1
0
0
0
5
10
0
0
5
10
5
10
0
15 0
5
10
15
0
0
5
10
0
5
10
15
0
5
10
15
-2
0
δ13C
3
5
10
2
2
2
2
2
2
2
1
1
1
1
1
1
1
0
0
0
15
-1
0
0
5
10
0
15 0
5
10
-1
0
5
10
0
0
0
5
10
15
0
5
10
-2
15
-3
0
0
100
200
300
400
500
5
10
600
15
700
1000 2000 3000 4000 5000 6000 7000
Years Before Present
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
δ13C range
5
4
3
2
High-productivity events
upwelling or runoff
1
Cooler-temperature events
upwelling
δ18O range
3
2
1
5
10
15
Average Ba/Ca
1000
2000
Average Sr/Ca