Coral records of ENSO through the late Holocene

Kim CobbHussein Sayani

Georgia Inst. of Technology

Chris CharlesNiko Westphal

Scripps Inst. of Oceanography

Larry Edwards, Hai ChengUniversity of Minnesota

with thanks to ACS-PRF, NOAA, NCL, PARC, Cobb lab undergrads

Motivation: How is the tropical Pacific climate systemresponding to anthropogenic forcing?

Approach: Use well-dated, high-resolution paleoclimaterecords from the tropical Pacific to assess its response toknown natural and anthropogenic climate forcings.

Example:

El Nino-Southern Oscillation response to insolation forcingduring the current interglacial period (the “Holocene”)

Dai and Wigley, 2000

El Niño Temperature

El Niño Precipitation

“El Niño-Southern Oscillation”(ENSO)

ENSO is a climate phenomenon in the tropical Pacific which arisesfrom coupled interactions betweenthe atmosphere and ocean

ENSO impacts global climate every2-7 years

tropical Pacific climate variabilityover decades to centuries to millennia poorly constrained;20th century trends uncertain

Why tropical Pacific climate?

Vecchi et al, 2008

- climate models project widely divergent scenarios for tropical Pacific climate under greenhouse forcing

Climate modelresponse to

greenhouse forcing

Vecchi et al, 2008

- instrumental SST datasets contain trends of different signs no help

Instrumental SST trends

Ice age cycles are timed to “insolation” forcing

What happened to ENSO during the mid-Holocene?

Model: Clement et al., 2000; Lake P: Moy et al., 2002;

- climate models generally show reduced ENSO activity in response to insolation forcing…

BUT

are they correct?

- primary evidence of reduction comes from 2 lake and 1 marine sediment records (Moy et al., 2002; Conroy et al., 2008; Koutavas et al., 2006) but these do not agree!

A composite of available Holocene coral ENSO activity

Model: Clement et al., 2000; Lake P: Moy et al., 2002; Corals: Cobb et al., 2003; Tudhope et al., 2001; Woodruffe et al., 2003; McGregor and Gagan 2004; Correge et al., 2001

- scant fossil coral ENSO reconstructions during Holocene

- no statistically significant reduction in ENSO variance from fossil corals

- primary evidence of reduction comes from 2 lake and 1 marine sediment records (Moy et al., 2002; Conroy et al., 2008; Koutavas et al., 2006) but these do not agree!

150°E 180° 150°W 120°W

b

SS

T (

°C)

AVERAGECONDITIONS

EL NIÑO

SST, hydrology, and coral δ18O in the Line Islands

Coral oxygen isotopes (δ18O) decreases when warm (thermodynamics)Coral δ18O decreases when rainy (lower seawater δ18O)

Reminder: Palmyra coral δ18O is a sensitive proxy for ENSO

Cobb et al., 2003

Overlapping fossil coral δ18O records during last millennium

Cobb et al., in prep

Palmyra50 cores U/Th dated18 cores undated

Christmas25 cores U/Th dated51 cores undated

Fanning17 cores U/Th dated19 cores undated

The Line Islands Coral Collection

modern coresfrom threeislands

splice overlappingcores in lastmillennium

many coresin mid-Holocene

1 23

Strengths of Line Islands Fossil Corals

1. Modern coral δ18O has high correlations with NIÑO3.4 SST index

2. Overlapping fossil coral δ18O records highly correlated duringlast millennium; provide uncertainty estimates for single fossil coral reconstructions

3. New collection of >100 fossil corals extends back 7000 years (20-100yrs each)

New fossil coral δ18O records from Fanning: 6-7kybp

What do we see in the mid-Holocene fossil corals?

Scanning electron microscope (SEM) photos of 6ky fossil coral

- surface coatings represent no more than ~10% by mass- original coral geochemical signature likely preserved, not “altered”

A modern-young fossil coral comparison

Crespo et al., in prep

New Line Islands Fossil Coral Data no change in ENSO

- added 650 years of monthly-resolved data (red dots = our stuff)

- we have tripled the amount of fossil coral data available from the mid- to late- Holocene

Model: Clement et al., 2000; Lake P: Moy et al., 2002; Single-foram (purple): Koutavas et al., 2006Corals: Cobb et al., 2003; Tudhope et al., 2001; Woodruffe et al., 2003; McGregor and Gagan, 2004; Correge et al., 2001

What is the take-homefor climate scientists?

Conclusions

- No fossil coral support for reduced mid-Holocene ENSO activityclimate model response questionable, so 21st centuryENSO projections of same models questionable

- Reduced ENSO activity throughout late Holocene, combined with evidence for cooler and drier conditions

- explanation?- need more data from the tropical Pacific!

What’s next?

How much does seawater δ18Osw vary today across Line Islands?Postdoc Jess Conroy

Careful diagenetic investigations on fossil corals &Micro-scale Sr/Ca analyses of pristine sections of fossil coral skeletons

Student Hussein Sayani

We probably need many more corals! Koutavas-like approachapplied to 5-10yr-long sequences?