Rapid Authigenic Ferric Clay Formation in Shallow Marine Tropical Sediments: Environmental Controls and Implications for Major Elemental Cycles
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Eleventh Annual V. M. Goldschmidt Conference (2001) 3733.pdf RAPID AUTHIGENIC FERRIC CLAY FORMATION IN SHALLOW MARINE TROPICAL SEDIMENTS: ENVIRONMENTAL CONTROLS AND IMPLICATIONS FOR MAJOR ELEMENTAL CYCLES. T.C.W. Ku1 and L.M. Walter, Department of Geological Sciences, 2534 CC Little Building, Univer- sity of Michigan, Ann Arbor, MI 48109, [email protected]. Introduction: The rapid formation of alumi- mole ratio of 2.1/ 1.0/ 1.2 and most likely grew syn- nosilicate minerals in marine sediments was proposed depositionally by incorporating terrestrially-derived Fe over 30 years ago as a major control on oceanic chem- and Al, seawater Mg, and pore water Si derived from istry [1]. This concept, reverse weathering, was so biogenic opal dissolution. Radiocarbon ages of mol- named because the chemical reaction (soluble cations + lusc shells indicate that these clays can be largely pyri- detrital clays à new aluminosilicate minerals + carbon tized in 5 kyr. If similar clay formation processes are dioxide) is the opposite of the weathering reaction on common in other tropical nearshore sediments, “reverse land. This hypothesis was invoked to balance steady weathering” could account for an estimated 36% of the state oceanic elemental cycles (Na, K, Mg, Si) because world’s dissolved riverine Mg+2 flux. Thus, the role of riverine inputs of these elements were much greater marine shelf clay mineral authigenesis needs to be than then-known outputs. However, direct evidence evaluated more thoroughly as an important control of for new aluminosilicate mineral formation was not ocean chemistry. found in major deltaic environments and the concept of reverse weathering was largely ignored [2, 3]. In addi- tion, the discovery of large scale hydrothermal cycling of elements at mid-ocean ridges could solve many seawater elemental budgets with invoking reverse weathering processes [e.g. 4]. Recent experiments and observations have shown that rapid clay formation in nearshore marine sedi- ments may be more important than previously thought [5,6]. In these studies, extensive Si-Al-Fe-K-Mg clay precipitation occurred on a variety of experimental mineral substrates (quartz, kaolinite, FeOOH coated quartz grains, glass beads, and diatom frustules) that were incubated in natural anoxic for up to 36 months at 28ºC. These results demonstrate that rapid authi- genic mineral formation removes major cations from seawater. In this study, pore water and sediment chemistries document authigenic clay formation in the San Blas Archipelago, Panama. These results provide new in- sights into the environmental conditions required for nearshore authigenic marine clay formation. Results and discussion: Pore water and sediment samples were obtained from diver-collected boxcores and pushcores from seven sites in the San Blas Archi- pelago, Panama. Sediments have high total iron con- centrations (up to 6.wt.%) and the dominant phase is a 7.1Å ferric clay. The bulk clay assemblage is highly Figure 1: SEM/EDS spectra (from boxed area) and susceptible to HCl-attack and comprises a large frac- SEM image of clay precipitation in the cross-lamellar tion (up to 5.4 wt. % Fe) of the total sediment. These microstructure of a recent aragonite marine gastropod authigenic clays coat grains, infill carbonate micro- from the San Blas Archipelago, Panama. pores, and replace faecal pellets and carbonate shells (Fig. 1). The mode of occurrence, structure, and ferric References: [1] MacKenzie F.T. and Garrels R. nature of the most abundant clay mineral identifies it M.. (1966) Am. J. Sci., 264, 507-525. [2] Russell, as odinite, a dioctohedral – trioctohedral Fe+3 rich 1:1 K.L. (1970) Geochim. Cosmo. Acta., 34, 893-907. clay mineral, which is the key member of the verdine [3] DeMaster D.J. (1981) Geochim. Cosmo. Acta,, 45, facies [7]. The verdine facies in San Blas occurs in 1715-1732. [4] Edmond et al. (1979) Earth and Plan. suboxic sediments supporting Fe reduction and in Sci. Let., 46, 1-18. [5] Michalopoulos P. and Aller R. some cases, the clays have been extensively converted C. (1995) Science, 270, 614-617. [6] Michalopoulos into pyrite. The HCl-soluble clays have a Fe/Mg/Al P. et al. (2000) Geology, 28, 1095-1098. [7] Bailey S.W. (1988) Clay Min., 23, 237-247..