Holocene changes in eastern equatorial Atlantic salinity as estimated by water

isotopologues

Guillaume Leduc (CEREGE, France)

Julian Sachs (University of Washington, USA)

Orest Kawka (University of Washington, USA)

Ralph Schneider (University of Kiel, Germany)

_______________________Goldschmidt 2015

Adams and Faure, 1997, QEN project

African climate change during the Holocene

Weldeab et al., 2007, GRL

Sea Surface Salinity: tracking the hydrological cycle

1/ Surface ocean d18O / salinity relationship in the Gulf of Guinea

Leduc et al., 2013, EPSL

Methods for estimating salinity

Weldeab et al., 2007, Science

2/ Foraminiferal Ba/Ca, new salinity proxy, is an alternative of isotopic-based salinity reconstructions

Methods for estimating salinity

Weldeab et al., 2007, Science

D Salinity ~ 6

D Salinity ~ 16

« Green Sahara »Application to the Gulf of Guinea

Modified from Schwab and Sachs, 2009, Org. Geochem.

C37 alkenonesC36 alkenoatesC38 alkenones

3/ Surface ocean dD (analogous to d18O)

Methods for estimating salinity

C37:2 alkenones dD

Leduc et al., 2013, EPSL

Another proxy, another story

dD of C32:7:

modern climate more humid in tropical Africa than duringthe Green Sahara episode

(quite impossible)

Leduc et al., 2013, EPSL

The isotopologues method: a new salinity indicator

Surface ocean dD is globally affected by fractionation processes analogous to those affecting surface ocean d18O

However hydrogen has a higher diffusivity than oxygen, and the two isotopic systems align along different regional meteoric water lines, inducing deuterium excess values dictated by freshwater budgets

Open-oceansurface waters

Evaporative closed basins(Mediterranean Sea)

Rohling, 2007, Paleoceanography

Theoretically, it is possible to combine surface ocean d18O and dD to estimate salinity. Both isotopic systems can be used to converge toward a solution which provides an estimation of freshwater fluxes proportional to ocean water.

Freshwater fluxes ultimately determine salinity. Using both surface ocean dD and d18O, we can recalculate salinity changes FS induced by freshwater fluxes:

Rohling, 2007, Paleoceanography

In such equation, the d18O and/or dD – salinity relationship is not used, so that such method avoids biases associated with temporal changes in the salinity/isotopic composition of seawater relationship.

Leduc et al., 2013, EPSL

The isotopologues method:

- Progressive aridification captured

- Salinity estimation during the mid-Holocene extremely low

Models incorporating the water isotopes: a diagnostic for the isotopologues method

The method overestimates Holocene salnity changes in Gulf of Guinea by ~150%!

LeGrande et Schmidt, 2011, Paleoceanography

Temporal changes in the isotope/salinity relationship

… how it likely occurred in the Gulf of Guinea over the last 7000 years

Leduc et al., 2013, EPSL

can theoretically be explained by changes in the d18O of

freshwater fluxes (rainfall/rivers)

Temporal changes in the isotope/salinity relationship

… how it likely occurred in the Gulf of Guinea over the last 7000 years

Leduc et al., 2013, EPSL

A systematic bias in isotopic proxies of precipitation? Some examples from the monsoon areas

… in search of paleo d18O of rainfall in the tropical band

MD03-2707

Lake M’Balang

Speleothemes

SpeleothemesSedimentary archives

Marzin and Braconnot, 2009, Climate Dynamics

A systematic bias in isotopic proxies of precipitation? Some examples from the monsoon areas

Leduc et al., 2013, EPSL

8,000 year-old giraffe rock carving in DaBous, Niger