Development of coccolithophore-based transfer functions in the western Mediterranean sea: a sea surface salinity reconstruction for the last 15.5 kyr

B Ausín,B Alonso,I Hernández-Almeida,M Grosjean,G Francés,F.-J Sierro,J.-A Flores

doi:10.5194/cp-11-1635-2015

Abstract

Abstract. A new data set of 88 marine surface sediment samples and related oceanic environmental variables (temperature, salinity, chlorophyll a, oxygen, etc.) was studied to quantify the relationship between assemblages of coccolithophore species and modern environmental conditions in the western Mediterranean Sea and the Atlantic Ocean, west of the Strait of Gibraltar. Multivariate statistical analyses revealed that coccolithophore species were primarily related to sea surface salinity (SSS), explaining an independent and significant proportion of variance in the coccolithophore data. A quantitative coccolithophore-based transfer function to estimate SSS was developed using the modern analog technique (MAT) and weighted-averaging partial least square regression (WA-PLS). The bootstrapped regression coefficient (R2boot) was 0.85MAT and 0.80WA-PLS, with a root-mean-square error of prediction (RMSEP) of 0.29MAT and 0.30WA-PLS (psu). The resulting transfer function was applied to fossil coccolithophore assemblages in the highly resolved (~ 65 years) sediment core CEUTA10PC08 from the Alboran Sea (western Mediterranean) in order to reconstruct SSS for the last 25 kyr. The reliability of the reconstruction was evaluated by assessing the degree of similarity between fossil and modern coccolithophore assemblages and by a comparison of reconstructions with fossil ordination scores. Analogs were poor for the stadials associated with Heinrich events 2 and 1 and part of the Last Glacial Maximum. Good analogs indicate a more reliable reconstruction of the SSS for the last 15.5 kyr. During this period, several millennial and centennial SSS changes were observed and associated with sea-level oscillations and variations in the Atlantic Water entering the Alboran.

Highlights

Coccolithophores are one of the major components of marine phytoplankton
We suggest that freshwater advection (FA) events would have resulted from the influx of fresher and colder Atlantic waters in the Alboran Sea related to the southeastward drifting of meltwater from the Labrador, Greenland and Iceland seas (Bond et al, 1997)
Multivariate statistical analyses show that the distribution of modern coccolithophore assemblages in the Atlantic Ocean, west of the Strait of Gibraltar, and the western Mediterranean was mainly influenced by annual mean salinity at 10 m depth

Summary

Introduction

Coccolithophores are one of the major components of marine phytoplankton. They are sensitive to changes in many environmental variables, such as nutrients, temperature and salinity, and are widely used in qualitative paleoenvironmental studies (Baumann et al, 2005; Guerreiro et al, 2013, 2014). These studies provide general insight into the response of coccolithophores to environmental variables, but quantitative studies (e.g. transfer functions) allow assessing these relationships in a more rigorous and clear manner. Giraudeau and Rogers (1994) used factor analyses and multiple regressions to estimate chlorophyll a from coccolithophore census counts in surface sediment samples

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