J. M. Jouanneau; C. Latouche:

The Gironde Estuary

1981. 115 pages, 49 figures, 16x24cm, 450 g
Language: English

(Contributions to Sedimentary Geology, Volume 10)

ISBN 978-3-510-57010-2, paperback, price: 38.00 €

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Synopsis top ↑

This article presents a synthesis of sedimentological and geochemical work carried out on the Gironde estuary until 1978.

The drainage area of the Gironde estuary is the Aquitaine Basin. It is a Cenozoic sedimentary basin of about 80,000 km2 largely open to the ocean and edged by different massifs (Pyrenees, Massif Central). This basin profits from a temperate oceanic climate. The estuary is formed by the joining of two hydrologic, well individualized river systems, the Garonne and the Dordogne. As the development of local industrial planning is still rather limited, the equilibrium of the estuary does not appear to have suffered serious harm.

Various ecological aspects and properties of the present-day environment are described. pH of waters fluctuates between the extreme values of 7.4 to 8.5. The percentage of saturation of dissolved oxygen may vary (1976) between 10 and 130 o/oo, especially seasonally. The quantitative changes in nutrients result from the upstream dilution and oxidation of organic matter. Upstream, high levels of trace-metals associated with Suspended particles have been determined; further downstream, a drastic decrease of these metals is observed and in the mid-estuary heavy metals concentrations agree with world average values found in not significantly polluted systems. This upstream/ downstream decrease in metal concentrations in the suspended particulate matter is attributed to a mobilization of the metals which become solubilized from the suspended material. In order to precise these mechanisms, special methods were used,such as the double tagged method with radioactive isotopes and chemical sequential extraction.

The Gironde estuary, situated in a well known environment, appears to be a rich source of in- formation for specific estuarine studies, and for the evaluation of the substance transfer to the continent/ocean interface.

Table of Contents top ↑

Abstract 1
Preface 2
Introduction 4
1. The geographical, geological, climatic and economic background 5
1.1 The Aquitaine Basin 5
1.1.1 The geographical and geological background 5
1.1.2 The climatic background 5
1.2 The Gironde estuary 9
1.2.1 The geographical and morphological background 11 The riverine estuary 11 The estuary (sensu stricto)12 The mouth of the estuary 12
1.2.2 The geological background 12 The overall situation 12 The estuary (sensu stricto) 14 The estuary mouth 17
1.2.3 The human activities 20 Agriculture, fishing and tourism 20 Navigation, commerce and industry 21 The development of the Gironde 21 Human activities and the natural equilibrium of the estuary 22
2. The hydrodynamic, thermal and sedimentological regimes 23
2.1 The hydrodynamic regime 23
2.1.1 The hydrology of rivers flowing into the estuary 23 The Garonne 23 The Dordogne 23 Fluvial inlets in the Gironde 24
2.1.2 The estuarine tide and the velocity of currents 24 The development of currents during tides 26 The distribution of velocities as a function of the morphology of the estuarine bed 28
2.1.3 The salt intrusion 28
2.1.4 The residual circulation 29 The general characteristics of the circulation 30 Seasonal circulation 30
2.2 The thermal regime 34
2.2.1 Importance of the role of temperature 34 The biological implications 34 The hydrodynamic implications 34
2.2.2 Seasonal variations of temperatures 34
2.3 The sedimentary facies of the present-day estuary 35
2.3.1 Description of the facies 37 The riverine estuary 37 The upper estuary 37 The lower estuary (marine) 41 The mouth of the estuary 42
2.3.2 The granulometry, mineralogy and geochemistry of the
estuary floor sediment 43 Granulometry 43 Gross mineralogy 44 The organic materials 45 The distribution of chemical elements 45
2.3.3 The sedimentary facies and benthic organisms 47
2.4 Transports in suspension and carried by bed-load 50
2.4.1 The sediments in suspension 50 The solid discharge in suspension 50 The turbidity maximum 52 The fluid mud 53 The total mass of material in suspension 55 The composition of material in suspension 56
2.4.2 The sediments carried by bed-load transport 57 Direct observations: radioactive tracer enrichment 59 Indirect observations of present movements 60
2.4.3 Sedimentological relationships. The estuary-continental plateau 65 The suspensions 65 Sedimentary displacement by bed-load transport 68
2.5 The geological history of the Gironde and the evolution of the sedimentological regime 71
2.5.1 Facies and evolution of the littoral and estuary mouth from the
last glaciation until now 71 The evolution during the last glacial and the Holocene 71 Evolution during the historic period 73 The present-day evolution 77
2.5.2 The sedimentological evolution of the estuary 77 Evolution during the last glaciation and during the
Holocene 77 Developments since the 17th century 85
3. Properties of the environment 91
3.1 The physico-chemical parameters of water 91
3.1.1 The pH 92
3.1.2 Dissolved oxygen 92
3.1.3 The nutritive salts 92 Ammonia 92 Nitrates 93 Nitrites 93 Phosphates 94 Silica 94
3.2 Heavy metals 95
3.2.1 General distribution of heavy metals in the system 95 In the water 95 In the sediments 96 In suspension 96
3.2.2 Specific studies concerning the upstream/downstream decrease
of the content of heavy metals associated with suspended matter 98 Geochemical distribution of particulate zinc in the
suspended matter 99 Comparison of the quantities of particulate and dissolved oligo-elements100 Results attained by the double tracing of mud 102 Conclusions 102
4. Conclusions 102
Acknowledgements 104
Bibliography 105