Transcript PowerPoint file - Geobiology Research Group

Evaluating the Holocene Dynamics of Oceanographic Processes in the Southeast Brazilian Bight Using Accumulations of Calcitic and Aragonitic Shells
Abstract Reference Number: 417
Introduction and Methods
R.A. Krause Jr.1, S.L. Barbour-Wood2, M. Kowalewski1, M.G. Simões3, D.S. Kaufmann4, C.R. Romanek5, J.F. Wehmiller6
In this study we are attempting to understand the Holocene
dynamics various oceanographic processes in the Southeast
Brazilian Bight (western South Atlantic). This region is
today characterized by a west boundary equatorial current,
ENSO/SOI-related coastal runoffs, and shelf-break and
coastal upwelling events. Using several time series of dated
shells of brachiopods and bivalve mollusks, we plan to
analyze multiple geochemical proxies (Sr/Ca, Mn/Ca, Mg/Ca,
Ba/Ca, δ13C, δ18O) using CF-IRMS (stable isotopes) and
ICP-MS laser ablation (trace elements). This will enable
evaluation of yearly to multi-centennial and multi-millennial
changes in the frequency and intensity of short term events
(ENSO/SOI, upwelling), the strength and position of the
South Brazil Current, and in shellfish bio-productivity.
1Department
1
2.00
0
-1
-1.00
1
18
δ O
Barbour Wood et al. (2006)
Quaternary Research
0
South Brazil
Current
FIGURE 3: Map showing the location of the South
Brazil Current. The study are is indicated by the red
box.
Oceanographic Setting
The south Brazilian Bight consists of an open
tropical shelf influenced by a weak western
boundary current, the South Brazil Current. The
shelf is dominated by three, fully oxygenated,
major water masses:
TW
19°C
17°C
15°C
restricted to the coast, above the thermocline
Tropical Water: This water mass is indicative
of the South Brazil Current. It flows poleward
and forms the upper water layers on the outer
shelf.
South Atlantic Central Water: This water
mass is usually restricted to the bottom-water
layers on the outer shelf.
13°C
Coastal water (CW)
Tropical water (TW)
11°C
South Atlantic
Central Water (SACW)
SACW
500
50 km
Kowalewski et al., 2002, Palaios
NW
SE
2
10
4
8
2
2
0
5
2
4
6
4
10
-4.00
-2.00
0.00
3
FIGURE 5: Age-frequency
distributions for all amino
acid dated shells.
Isotopic Variance
S. casali, 10m, n=38
δ13C 0
-1
-2
-3
-4 modern
Years BP
0
2
100
200
300
2
4
6
Age (kyrs)
2
1
Sr
3.6
3.4
3.2
400
500
600
2000
700
-2
FIGURE 6: Box and
whisker plots of δ13C and
δ18O values for individual
shells. Note the close
temporal relationship of
shells with a high variance
of these isotopic values.
-3
3
3
2
2
2
1
1
1
1
1
1
1
0
0
0
0
0
0
5
10
0
15
0
0
5
10
15
20
0
5
10
15
0
5
10
15
0
5
5
10
15
100
200
300
400
500
600
700
1000 2000 3000 4000 5000 6000 7000
L-Aspartic acid
D-Aspartic acid
COOH
COOH
death
C
8
H
H
C
H2N
aar
CH2
COOH
CH2
0
2
2
2
2
2
2
1
1
1
1
1
1
0
15
-1
0
0
-1
-2
-2
0
0
0
-1
-2
5
10
15
0
Brachiopods
Bivalves
-3
5
10
0
5
10
15
5
10
0
5
10
0
5
Distance from hinge (mm)
5
10
-3
0
-4
0
0
10
5
5
10
15
FIGURE 7: δ13C profiles from
selected shells plotted against
the age of the shell. Many shells
are indicative of background
conditions, but several other
shells show negative isotopic
excursions indicative of elevated
productivity events.
Mn/Ca
2000
1000
1900
1700
1500
1300
1100
900
700
Mn/Ca
Sr/Ca
6000
Sr/Ca
5000
4000
3000
2000
0
1
2
3
4
5
6
7
1000
0
8
0.5
Distance from hinge (mm)
25
5
6
Sr
Ba
Mg
4.5
5
5
1000
900
800
700
600
500
400
300
200
100
1000
900
800
700
600
500
400
300
200
100
Mg/Ca
Mn/Ca
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
6000
Mg/Ca
Mn/Ca
5000
4000
3000
2000
1000
1900
1700
1500
1300
1100
900
700
Sr/Ca
25
1
2
3
4
5
6
7
8
Sr/Ca
0
9
1
2
3
4
5
6
Distance from hinge (mm)
Bivalve 13053, 157 yrs.
Ba/Ca
20
FIGURE 10: The relationship between the average trace element
concentration of a shell and its age. The results of Reduced Major
Axis Linear Regressions for these data, as well as standard
deviation (SD), range and mean first difference, are shown below.
Mn
4
10
10
7
15
10
Bivalve 13054, 891 yrs.
Ba/Ca
5
30
3000
Mg/Ca
25
>10000
20
2500
n=46
α =0.05
3.5
15
Distance from umbo (mm)
4
3
20
20
2
3
2.5
Brachiopod 13008, 7176 yrs.
Ba/Ca
30
30
0
2
2
40
2000
1
1.5
Bivalve 13083, 1056 yrs.
Ba/Ca
50
3
0
1
Distance from umbo (mm)
3000
Distance from hinge (mm)
Results obtained so far indicate that upwelling activity and bio-productivity has fluctuated in this area
during the last millennium. Several centuries have higher variability in carbon and oxygen isotopes and
Ba/Ca ratios than most other centuries. Specifically, modern shells, and those from 500 to 600 years BP,
show a variability range of 4-5 per mil for carbon, which is twice the average from other centuries. These
findings could indicate that this upwelling cell has been only intermittently active for the past millennium.
This may have important implications for the recognition of ancient high-productivity systems as well as
for better understanding of the long-term dynamics of upwelling-supported high-productivity ecosystems in
the modern ocean.
Mg/Ca
1000
900
800
700
600
500
400
300
200
100
3000
4000
15
Preliminary Results
1400
1200
1000
800
600
400
200
Mg/Ca
5000
COOH
We are using shells of the calcitic brachiopod Bouchardia rosea and the aragonitic bivalve Semele casali
dredged from two upwelling-influenced sites along the inner shelf. We employed amino acid racemization
dating using aspartic acid calibrated with 24 AMS radiocarbon dates. This enables dating of individual
shells with very good precision. The resulting time series have nearly-complete centennial resolution for
the past millennium for each bio-mineralic system.
10
4000
4
Distance from hinge (mm)
Amino Acid Racemization Dating
15
5000
Bivalve 91076, 0 yrs.
Ba/Ca
25
15
6000
Age (kyrs)
Years
Before
Present
Scavenged
Elements
30
20
30
0
10
Recycled
Elements
20
5000
4500
4000
3500
3000
2500
2000
1500
1000
500
6000
Mg
2
15
25
-1
δ18O -1
2
10
0
Mn
Brachiopod 91026, 137 yrs.
Ba/Ca
30
5
2
5
6000
1
0
2
0
4000
Ba
Ba, Sr
FIGURE 8: Diagrammatic profiles of the concentration of trace elements in
seawater. Modified from Open University (2005), Marine Biogeochemical Cycles.
1
3
1
H2N
3
2
8
8
4
1
There are certain times in the
Holocene that are characterized
by a larger variation in δ13C.
Since these times are always
characterized by a drop in the
median δ13C value, we interpret
these events as high or variable
productivity events. Likely, they
are runoff events because of the
corresponding drop in δ18O.
B. rosea, 10m, n=71
6
FIGURE 9: Trace
element profiles for 6
of the 46 that have
been analyzed thus
far. Measurements
were typically taken
at a spacing of 250
μm in transects
perpendicular to
growth lines. In
several cases (shells
13053, 13054),
measurements were
taken at every visible
growth line, resulting
in much higher
resolution. We graph
here the mean of 12
distinct
measurements taken
at each spot (colored
line), along with ±
one standard
deviation (grey lines).
Bivalves are
indicated by blue
lines and
brachiopods by green
lines.
Mn
3
3
4
5
2
4
2
4.00
We consider background conditions to yield δ13C signatures of 1-2 per mil – the average observed for
marine carbonate shells. Background conditions should also be characterized by δ18O values of -1.3 to
2.2, according to the Global Seawater Oxygen 18 database.
Elevated productivity events, either upwelling or freshwater runoff, should be expressed as a notable
drop in the δ13C value for a shell. This is related to the preferential fractionation of Carbon 12 by
phytoplankton. As this productivity turns over and decays, organic matter will become oxidized through
microbial respiration to form dissolved inorganic carbon (DIC) that has a relatively low δ13C value,
compared to ambient marine conditions. Carbon isotope compositions of shell carbonate will reflect these
changes, since DIC is the predominant form of carbon used by mollusks and brachiopods in the secretion
of shell carbonate.
Upwelling and freshwater runoff events can be distinguished most reliably by their oxygen isotope
signature. Since upwelling produces a slight increase in salinity, while freshwater runoff produces a
marked decrease in salinity, the delta O18 values should be sufficiently distinct. The differences in sea
surface temperature between these events would probably not be sufficiently large to dampen-out the
salinity-induced signature.
8
6
2
2.00
minimum δ13C value for each shell
S. casali, 30m, n=37
6
-3.00
-2
δ13C
Frequency
12
δ18O
-1
-4 modern
B. rosea, 30m, n=32
20
18
16
14
12
10
8
6
4
2
10
Santos Basin
Coast
14
5
Distance from hinge (mm)
δ13C
400
0
Isotopic Expression of Oceanographic Conditions
Coastal Water: This water mass is generally
300
-2.00
0
FIGURE 2: Locality map showing the position of the two
sites. Shells were collected from each site using a Van
Veen grab sampler (1/40 m2), which collects the uppermost
several centimeters of substrate. Collection was done in
conjunction with an ongoing project of the Marine Ecology
Group of São Paulo State University.
200
Depth (m)
FIGURE 4:
Shelf profile
along with the
average
positions of the
three main
water masses.
The diagram
depicts an
upwelling
event.
δ18O
ppm
100
0.00
-2
Mg
Conservative
Elements
15
Age vs. Average
Age vs SD
Age vs. Range
Age vs. Mean Absolute
First Difference
Age vs. Average
Age vs SD
Age vs. Range
Age vs. Mean Absolute
First Difference
Age vs. Average
Age vs SD
Age vs. Range
Age vs. Mean Absolute
First Difference
Age vs. Average
Age vs SD
Age vs. Range
Age vs. Mean Absolute
First Difference
r
p
0.309 0.037
0.342 0.02
0.346 0.018
10
2000
5
1500
1000
0.318
-0.04
-0.11
-0.12
0.031
0.793
0.467
0.435
-0.06
0.269
0.515
0.465
0.696
0.071
3E-04
0.001
500
ppm
21°C
1.00
30 m
Background Conditions: During such times significant
coastal upwelling water masses and runoff water masses are
absent from most of the shelf. Background conditions are
characterized by sea surface temperatures varying by no more
than 6°C annually, and salinity in each water mass varying by
less than 1 per mil.
Intensified Coastal Upwelling: This is a seasonal
phenomenon in this region, usually occurring in austral
summers. However, in intermittent years it can be intensified
greatly when cyclonic meanders of the South Brazil Current
coincide with wind-driven episodes of offshore Ekman
transport. During such events, the inner shelf experiences
elevated phytoplankton blooms associated with the upwelling
nutrient-rich waters of the South Atlantic Central Water.
Also, salinity may drop slightly and sea surface temperatures
will drop notably.
Intensified Coastal Runoff: Large low-salinity plumes have
been observed in the study area, approaching from a large
estuary in the south. The frequency of these events has been
correlated with the Southern Oscillation Index, indicating a
possible link with El Niño events. During these events
salinity in the study area drops markedly and sea surface
temperatures may also drop somewhat. Such events also
coincide with increases in bio-productivity due to the
nutrient-rich runoff.
CW
δ13C
Log Average ppm
Identifying Oceanographic Conditions
Decrease in δ13C from background
Possible slight +/- excursion in δ18O
ppm
1
10 m
Decrease in δ13C from background
Decrease in δ18O from background
Manganese: Scavenged element (removed
from solution by adsorption onto larger
particle surfaces); principle source in this
region is eolian dust; elevated concentrations
could indicate freshwater runoff events.
Barium: A recycled, bio-intermediate
element; elevated concentrations indicative of
local dysoxia brought on by elevated
productivity in the water column.
Strontium: Has been used as a
paleotemperature indicator (lower values =
cooler waters), but the complexities of it’s
uptake into biogenic calcite or aragonite
make interpretations difficult.
Magnesium: Positively correlated with
temperature; Also affected by very low
salinity (< 10‰).
1000
900
800
700
600
500
400
300
200
100
6000
0.406
0.178
0.313
0.29
0.005
0.236
0.034
0.05
Mn/Ca
1400
1200
1000
800
600
400
200
1000
900
800
700
600
500
400
300
200
100
6000
Sr/Ca
5000
5000
4000
4000
3000
3000
2000
2000
1000
0
0.286 0.054
ppm
2
Coastal Upwelling
depth increasing
2
13
δ C
Freshwater Runoff / ENSO
ppm
Brachiopod 652 yrs. BP
??
Log Average ppm
Bivalve 2944 yrs. BP
3
Trace Elements
δ13C: 1-2‰
δ18O: -1.3 to 2.2‰
Freshwater
Runoff / ENSO
Coastal
Upwelling
concentration increasing
ppm
Background Conditions
Background
Conditions
Log Average ppm
FIGURE 1: Shells used in this study. The
aragonitic bivalve, Semele casali (left), and the
calcitic brachiopod Bouchardia rosea (right).
Log Average ppm
10 mm
of Geosciences, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061 USA
2Department of Geology, Colby College, Waterville, ME 04901 USA
3Instituto de Biociências, Universidade Estadual Paulista, Distrito de Rubião Junior, CP. 510, 18.618-000, Botucatu, São Paulo, Brazil
4Department of Geology, Northern Arizona University, Flagstaff, AZ 86011 USA
5Savannah River Ecology Laboratory, University of Georgia, Drawer E, Aiken, SC 29808 USA
6Department of Geology, University of Delaware, Newark, DE 19716 USA
0
0
Paper Number: PP43A-1233
1
2
3
4
5
6
Distance from umbo (mm)
7
0
Mg/Ca
Mn/Ca
Sr/Ca
1
2
3
4
5
6
7
8
Distance from umbo (mm)
9