Age ka / Chapter 4 Implications of paleoceanography and paleoclimate 4.1 Paleoclimate expression 4.2 Implications of paleocirculation and tectonics 4.3 Paleoenvironmental reconstruction MD05-2901 (Liu et al., 2007) 0 50 100 150 200 250 300 350 400 450 18 O ( PDB) Kaolinite (%) -1.0-2.0-3.0-4.0 5 10 15 20 25 15 15 25 35 Smectite (%) 25 30 35 40 45 Illite (%) Chlorite (%) 20 25 30 1 2-4 5 6 7 8 9 10 11 12 1 MIS 4.1 Paleoclimate expression Clay minerals preferentially form through weathering and pedogenesis at the surface of the Earth. Clay minerals basically express the intensity of weathering, and especially of hydrolysis, in the land masses adjacent to sedimentary basins. The middle to late Quaternary alternation of glacial and interglacial periods generally correlates with an alternation of rather physically altered and rather chemically altered clay assemblages. Cenozoic sedimentary series frequently display a step-by-step increase of rock-derived minerals, namely illite, chlorite, feldspars, at the expense of Al-Fe smectite and often of kaolinite. Paleoclimate reconstructions from clay stratigraphic data may be envisaged in series as old as the late Precambrian, especially in regions marked by the absence of important post-sedimentary overburden and tectonic structuration, and where clay-rich sediments actively and regularly deposited at the periphery of variously weathered land masses. 2 University 1
However, Clay minerals, the climatic interest of which strongly depends on the precise knowledge of the petrographic context and geological history, generally display a regional signal only. On the other hand, clay mineral sequences do not depend on biotic or evolutional factors, and often not on physico-chemical post-depositional modifications. They occur virtually in all types of sedimentary rocks from both continental and marine environments, and provide an exclusive, integrated signal for land-surface climatic conditions with a fairly high sensitivity. 3 4 University 2
Intensity/point.s Clay mineralogical parameters, that may represent quantitative indicators of hydrolysis processes on land (hydrolysis +) Illite crystallinity: 10 Å width at half-height, N (+); Illite chemistry index: 5/10 Å peak-area ratio, N (+); Chlorite relative abundance: 4.7/5 Å peak-height ratio, N (-); Chlorite crystallinity: 4.7 Å peak sharpness, N (-); Smectite relative abundance: 17/10 Å peak-height ratio, G (+); Smectite crystallinity: 17 Å width at half-height, G (+); Kaolinite relative abundance: 3.57/3.53 Å peak-area ratio, N (+); 5 16000 12000 8000 4000 0 Illite (001) Illite (003)+Quartz (101) Smectite (001) Chlorite Kaolinite (001)+Chlorite (002) Chlorite (004) Kaolinite (002) (001) Illite (002) Chlorite (003) Quartz (100) Glycolated Heated (520ºC) Air-dried 5 10 15 20 25 30 2 º Illite crystallinity: 10 Å width at half-height, N (+); Illite chemistry index: 5/10 Å peak-area ratio, N (+); Chlorite relative abundance: 4.7/5 Å peak-height ratio, N (-); Chlorite crystallinity: 4.7 Å peak sharpness, N (-); Smectite relative abundance: 17/10 Å peak-height ratio, G (+); Smectite crystallinity: 17 Å width at half-height, G (+); Kaolinite relative abundance: 3.57/3.53 Å peak-area ratio, N (+); Multiple X-ray diffratogrammes of one typical sample 6 University 3
Sedimentological and mineralogical data on late Quaternary hemipelagites of the NW Mediterranean Sea 7 Conditions of application The possible existence of diagenetic changes, requiring moderate depths of burial (usually <2.5 km). Precise, discriminant and highly reproducible techniques in sample treatment and XRD processing. The mineralogical variations induced by climate changes during a period of 10 3 to 10 5 years often do not exceed 10-20% of the relative abundance of a given clay group. Paleoclimatic reconstructions should be based systematically on relative variations in the abundance of the different clay minerals, rather than on absolute values. Weathering processes are too weak or too strong to allow appreciable differentiation in the clay composition during successive climatic periods. If the sources of clay minerals correspond to distinct petrographic and pedologic features and have changed in the course of time, the climatic signal may be deformed or even obliterated. At short distances from land masses, especially in shallow-water environments, hydrodynamic constraints may induce clay mineral segregations that are of the same order of magnitude as those controlled by climate. Active meridian currents may determine the mixing of suspended material originating from different latitudes, and therefore the confusion of climatic signals borne by clay minerals. 8 University 4
Application for late Quaternary 9 Application for Cenozoic 10 University 5
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4.2 Implications of paleocirculation and tectonics Changes in sources or in current activity often determine modifications of the clay assemblage that are slight and of the same order of magnitude as mineralogical changes induced by climate variations. Tectonic phases generally determine important and long-lasting modifications in detrital clay assemblages, which differs from most effects induced by changes in climate or currents. Clay mineral suites in sediments constitute useful markers of tectonic activity or relaxation. Changes in detrital sources, in marine or aeolian currents and in tectonic activity induce modifications of clay successions that are either complementary or in opposition to those resulting from climate changes. The mineral modifications due to tectonics often obliterate the signals provided by other environmental factors. 13 Identification of sources 14 University 7
Identification of currents 15 Identification of tectonics 16 University 8
17 Mediterranean range, late Cenozoic 18 University 9
New Zealand region, Cenozoic 19 France, Jurassic-Cretaceous 20 University 10
Clay mineralogical changes induced by tectonic activity and recorded in sedimentary series are generally of large amplitude. The more active the tectonics and more hydrolyzing the climate, the more intense is the resulting clay diversification and the wider is the range of quantitative variations for each mineral group. 21 22 University 11
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4.3 Paleoenvironmental reconstruction Stratigraphic successions of clay mineral associations represent a useful tool to reconstruct the evolution of past environments, especially when they are combined with lithologic, geochemical and micromorphological data. Vertical movements of the Earth s crust are clearly reflected by successive clay associations. Horizontal movements of lithospheric plates are often registered by detrital clay successions. The morphologic evolution of the continental source regions responsible for the terrigenous input to adjacent marine basins fundamentally controls the pedologic or non pedologic origin of mineral suites. Specific paleoenvironmental conditions appear to have prevailed at certain geological periods, as documented by clay sedimentary successions. 25 North Atlantic Schematic stratigraphic distribution of clay mineral suites and geochemical parameters since late Jurassic. 26 University 13
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Significance of clay sources: illite and chlorite 29 Smectite 30 University 15
Palygorskites 31 Homework 32 University 16