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  18 O / 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  Salinity ~ 6  Salinity ~ 16 « Green Sahara » Application to the Gulf of Guinea

Modified from Schwab and Sachs, 2009, Org. Geochem. C 37 alkenones C 36 alkenoates C 38 alkenones 3/ Surface ocean  D (analogous to  18 O) Methods for estimating salinity

C 37:2 alkenones  D Leduc et al., 2013, EPSL

Another proxy, another story  D of C 32:7 : modern climate more humid in tropical Africa than during the Green Sahara episode (quite impossible) Leduc et al., 2013, EPSL

The isotopologues method: a new salinity indicator Surface ocean  D is globally affected by fractionation processes analogous to those affecting surface ocean  18 O 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-ocean surface waters Evaporative closed basins (Mediterranean Sea) Rohling, 2007, Paleoceanography

Theoretically, it is possible to combine surface ocean  18 O and  D 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  D and  18 O, we can recalculate salinity changes  S induced by freshwater fluxes: Rohling, 2007, Paleoceanography In such equation, the  18 O and/or  D – 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  18 O 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  18 O of rainfall in the tropical band MD Lake M’Balang Speleothemes Sedimentary 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