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