Formation of a basin-wide bottom nepheloid layer in the western Mediterranean after major deep water formation events Formation of a basin-wide bottom nepheloid layer in the western Mediterranean after major deep water formation events Durrieu de Madron X. 1, Puig P. 2, Schroeder K. 3, Salat J. 2, López-Jurado J.L. 4, Gasparini G.P. 3, Palanques A. 2, Emelianov M. 1, Karageorgis A.P. 5, Theocharis A. 5 1 CEFREM, 2 ICM-CSIC, 3 CNR-ISMAR, 4 IEO, 5 NCMR

Dense water formation processes Heat loss Open-Sea Convection Cascading : Down-slope cascade of dense water formed on shallow coastal zone Open-Sea convection : Deepening of the mixed layer thickness in the open ocean Strong variation on the interannual time-scale with formation of dense water reaching the deep layer of the basin (> 1800 m) Wind-driven mixing and cooling of the upper layer of the ocean during winter

Tramontane Mistral Dense water formation regions in the Gulf of Lion

Present monitoring of dense formation Since 1993 (water column) Since 2007 (water column) Since 2003 (near-bottom)

Dense shelf water cascading Temperature signature since 1993 Strong down- canyon currents Evidence of bottom sediment resuspension High suspended sediment fluxes

Open-sea convection Formation of deep water during winter 2009 Strong horizontal and vertical currents Evidence of bottom sediment resuspension Oct 09Nov 09Dec 09Jan 09Feb 10Mar 10

Recurrence of deep water anomalies in the Gulf of Lion Quasi-decadal recurrence of intense events, contributing to the renewal of deep western Mediterranean water. Pairs of extreme events. Preconditionning of the second winter is probably favoured by the strong cooling occuring during the first winter, hence allowing formation of dense water even with milder forcings

LIW WIW WMDW AW Clear water < 0.1 mg/L Water masses in the NW Mediteranean AW : Atlantic Water WIW : Winter Intermediate Water (e.g. mild cascading) LIW : Levantine Intermediate Water WMDW :Western Mediterranean Deep Water

Anomalies during winter 2005 Chl-a Fluorescence (µg l -1 ) Open-sea generation of dense water forming a distinct thermo-haline anomaly throughout the water column Near-bottom intrusion of the dense shelf water in the deep basin forming well defined thermo-haline, turbidity and fluorescence anomalies

Thickness of turbid anomaly Very thick (> 1000 m) anomaly associated to the newly-formed deep and bottom waters

Theta-S diagram : Identifying the water masses Isopycnals (lines of constant potential density anomaly Water Mass 1 Water Mass 2 Water Mass 3 Water Mass 4

26 April - 8 May 2005 Propagation of the bottom water and nepheloid layer : The 2005 event case 1 mg/l Spreading across the NW Mediterranean basin in few months LIW «Old» WMDW «New» WMDW «New» Bottom Water

28 June - 2 July mg/l Spreading across the NW Mediterranean basin in few months Fading of the anomaly within 2years - absent in 2004 Spreading across the NW Mediterranean basin in few months Fading of the anomaly within 2years - absent in 2004 Propagation of the bottom water and nepheloid layer : The 1999 event case Propagation of the bottom water and nepheloid layer : The 1999 event case LIW «Old» WMDW «New» WMDW «New» Bottom Water

Persistence of the 2005 anomaly and BNL until present 6 years1 mg/l CASC+OSC OSC

Suspended matter originating from the sediment are mostly composed of fine particles (clays, silts). Questions about suspended particles Flocculation favors the formation of large aggregates (~ µm) that have larger settling velocity than individual particles Turbulence in the bottom layer is able to maintain these particles in suspension for long times

CONCLUSIONS Local processes, widespread impact BNL scale in thickness with a thermo-haline anomaly generated by the mixture of dense waters formed during major events cascading and/or open-sea convection The thermo-haline anomaly and associated BNL can be still clearly detected, confirming that fine particles in deep BNLs can have residence times of several years Importance for the redistribution of elements (conta- minants, organic matter) associated to fine sediments Interest for real time data flow (e.g., acoustic transmission via existing surface buoy) Local processes, widespread impact BNL scale in thickness with a thermo-haline anomaly generated by the mixture of dense waters formed during major events cascading and/or open-sea convection The thermo-haline anomaly and associated BNL can be still clearly detected, confirming that fine particles in deep BNLs can have residence times of several years Importance for the redistribution of elements (conta- minants, organic matter) associated to fine sediments Interest for real time data flow (e.g., acoustic transmission via existing surface buoy)