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Influence of terrestrial weathering on early diagenetic reactions in continental shelf sediments
Chemical Geology, 107 (1993)437-438 Elsevier Science Publishers B.V., Amsterdam
437
Influence of terrestrial weathering on early diagenetic reactions in continental shelf sediments R.C. Aller Marine Sciences Research Center, SUNY at Stony Brook, Stony Brook, NY 11794-5000, USA
(Received March 29, 1993; revised and accepted March 30, 1993 )
Much of the recent emphasis in studies of early diagenetic reactions is on the cycling of biogenic debris. Lithogenic particles, their properties and the processes that formed them are often given little consideration other than as a diluent. With the discovery of extensive hydrothermal systems, interest has also waned in the possible relationships of terrestrial weathering and potential reverse weathering processes during early diagenesis of marine sediments. Studies of Amazon Shelf deposits demonstrate that this lack of interest is unwarranted and that terrestrial weathering processes can apparently have major influences not only on authigenic silicate formation but on biogeochemical reaction balances and minor element export processes in continental shelf deposits. The Amazon River delivers ~ 1- 109 metric tonnes of sediment annually to the equatorial Atlantic (Meade et al., 1985 ). Although much of this bulk material experiences little alteration during transit from the Andes source (Gibbs, 1967), a portion is highly weathered and contains reactive Fe-, Mn-, and Al-oxides as particle coatings, ferricrete and lateritic debris. U p o n entering the Amazon Shelf region, these reactive components and surface-associated phases are exposed to seawater brine, mixed with labile planktonic organic matter, and undergo a variety of diagenetic reactions. Net rates of sediment ~v'CO2 formation
( ~ 5 5 _ 1 0 mmol m -2 day -1) imply that ~25% of water column net primary production is remineralized in the upper few meters of the shelf sea floor. In this respect the Amazon Shelf deposits are comparable to many temperate estuarine and shallow shelf regions. 02 uptake by sediments shows little seasonality and averages 12.8 + 4.5 m m o l m -~ day -1. Balances between 02 and C net fluxes and SCO2 production rates indicate that ~ 75% of sediment C is remineralized through anaerobic pathways and that authigenic carbonates are precipitated. Isotopic evidence and nutrient release ratios demonstrate that marine, not terrestrial, C compounds predominantly drive reactions. Unlike many nearshore sediments of comparable C remineralization rates, Fe and Mn reduction are apparently particularly important during C decomposition and dissolved Fe 2+ and Mn 2+ are extensively elevated ( ~ 0.5-1 m M ) in the upper ~ 1-2 m of the deposits. There is relatively little evidence of sedimentary S storage. C / S burial ratios (weight basis) are typically > 6 , significantly higher than the world average ( ~ 2.8) (Aller et al., 1986, 1991, 1992). Except in regions of sea-floor erosion, methanogenesis is limited to depths > 5 m within the seabed ( C H 4 generally < 100/~M) (Blair et al., 1990, 1992). The supply of highly weathered debris from the drainage basin, coupled with moderate marine C inputs, and massive physical rework-
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
438
ing of shelf sediments are apparently the primary causes of Fe, Mn dominance of early diagenetic biogeochemical cycles. A variety of minor and trace elements are strongly influenced by the Fe, Mn diagenetic cycles and are dramatically enriched in pore waters. These include I, As, and U, elements known to associate with Fe,Mn-oxides and subject to enrichment during intense weathering. Isotopic ratios of dissolved U in pore water demonstrate a dominant terrestrial rather than recycled marine source, suggesting that the carrier oxide phases and other associated elements are also of terrestrial rather than marine origin (B. McKee, pers. commun. ). The intense physical reworking of sediments results in export of dissolved minor elements into the water column and presumably the interior ocean. Other seawater components are clearly taken up by sediments and are involved in authigenic mineral formation. Distinct K +, F - , and Mg 2+ gradients into sediments are often present and dissolved Si concentrations are some of the lowest found in marine sediments. The F / K diffusive flux ratios are consistent with formation of Fe,Al-silicates. Progressive Mg enrichment of Fe-oxide particle coatings is also found as is X R D evidence of authigenic Fe,A1silicate formation (Rude and Aller, 1989, 1993 ). Preliminary mass balances indicate that several percent by weight of sediments may be authigenic silicates. Additional studies are underway to separate and characterize the authigenic mineral phases which are apparently forming. Diagenetic properties of Amazon shelf sediments are consistent with a strong influence of intense chemical weathering on early diage-
R.C. ALLER
netic reactions. Tropical drainage basins with sufficient particle residence times to promote residual oxide formation presumably favor such effects. Many of the reactions appear to have analogs in deep-sea sediments adjacent to hydrothermal areas, although Fe,Mn-oxides are not as abundant on the shelf. References Aller, R.C., Mackin, J.E. and Cox, R.T., 1986. Diagenesis of Fe and S in Amazon inner shelf muds: apparent dominance of Fe reduction and implications for the genesis of ironstones. Cont. ShelfRes., 6: 263-289. Aller, R.C., Aller, J.Y., Blair, N.E., Mackin, J.E., Rude, P.D., Stupakoff, I., Patchineelam, S., Boehme, S.E. and Knoppers, B., 1991. Biogeochemical processes in Amazon Shelf sediments. Oceanography, 4: 27-32. Aller, R.C., Xia, Q. and Rude, P.D., 1992. Early diagenesis and benthic carbon remineralization in Amazon Shelf muds. Eos (Trans. Am. Geophys. Union), 73: 268-269. Blair, N.E., Boehme, S.E., Gunn, J.M. and Mendlovitz, H.P., 1990. The production of dissolved inorganic carbon and methane in Amazon Shelf sediments. Eos (Trans. Am. Geophys. Union), 71: 1376. Blair, N.E., Gunn, J. and Aller, R.C., 1992. Methane and sediment dynamics on the Amazon Shelf. Eos (Trans. Am. Geophys. Union), 73: 269. Gibbs, R.J., 1967. The geochemistry of the Amazon River system: Part I. The factors that control the salinity and composition and concentration of suspended solids. Geol. Soc. Am. Bull., 78: 1203-1232. Meade, R.H., Dunne, T., Richey, J.E., de M. Santos, U. and Salatil, E., 1985. Storage and remobilization of suspended sediment in the lower Amazon River of Brazil. Science, 228: 488-490. Rude, P.D. and Aller, R.C., 1989. Early diagenetic alteration of lateritic particle coatings in Amazon Continental Shelf sediment. J. Sediment. Petrol., 59: 704716. Rude, P.D. and Aller, R.C., 1993. Fluorine uptake by Amazon continental shelf sediment and its impact on the global fluorine cycle. Cont. ShelfRes. (in press).
437
Influence of terrestrial weathering on early diagenetic reactions in continental shelf sediments R.C. Aller Marine Sciences Research Center, SUNY at Stony Brook, Stony Brook, NY 11794-5000, USA
(Received March 29, 1993; revised and accepted March 30, 1993 )
Much of the recent emphasis in studies of early diagenetic reactions is on the cycling of biogenic debris. Lithogenic particles, their properties and the processes that formed them are often given little consideration other than as a diluent. With the discovery of extensive hydrothermal systems, interest has also waned in the possible relationships of terrestrial weathering and potential reverse weathering processes during early diagenesis of marine sediments. Studies of Amazon Shelf deposits demonstrate that this lack of interest is unwarranted and that terrestrial weathering processes can apparently have major influences not only on authigenic silicate formation but on biogeochemical reaction balances and minor element export processes in continental shelf deposits. The Amazon River delivers ~ 1- 109 metric tonnes of sediment annually to the equatorial Atlantic (Meade et al., 1985 ). Although much of this bulk material experiences little alteration during transit from the Andes source (Gibbs, 1967), a portion is highly weathered and contains reactive Fe-, Mn-, and Al-oxides as particle coatings, ferricrete and lateritic debris. U p o n entering the Amazon Shelf region, these reactive components and surface-associated phases are exposed to seawater brine, mixed with labile planktonic organic matter, and undergo a variety of diagenetic reactions. Net rates of sediment ~v'CO2 formation
( ~ 5 5 _ 1 0 mmol m -2 day -1) imply that ~25% of water column net primary production is remineralized in the upper few meters of the shelf sea floor. In this respect the Amazon Shelf deposits are comparable to many temperate estuarine and shallow shelf regions. 02 uptake by sediments shows little seasonality and averages 12.8 + 4.5 m m o l m -~ day -1. Balances between 02 and C net fluxes and SCO2 production rates indicate that ~ 75% of sediment C is remineralized through anaerobic pathways and that authigenic carbonates are precipitated. Isotopic evidence and nutrient release ratios demonstrate that marine, not terrestrial, C compounds predominantly drive reactions. Unlike many nearshore sediments of comparable C remineralization rates, Fe and Mn reduction are apparently particularly important during C decomposition and dissolved Fe 2+ and Mn 2+ are extensively elevated ( ~ 0.5-1 m M ) in the upper ~ 1-2 m of the deposits. There is relatively little evidence of sedimentary S storage. C / S burial ratios (weight basis) are typically > 6 , significantly higher than the world average ( ~ 2.8) (Aller et al., 1986, 1991, 1992). Except in regions of sea-floor erosion, methanogenesis is limited to depths > 5 m within the seabed ( C H 4 generally < 100/~M) (Blair et al., 1990, 1992). The supply of highly weathered debris from the drainage basin, coupled with moderate marine C inputs, and massive physical rework-
0009-2541/93/$06.00 © 1993 Elsevier Science Publishers B.V. All rights reserved.
438
ing of shelf sediments are apparently the primary causes of Fe, Mn dominance of early diagenetic biogeochemical cycles. A variety of minor and trace elements are strongly influenced by the Fe, Mn diagenetic cycles and are dramatically enriched in pore waters. These include I, As, and U, elements known to associate with Fe,Mn-oxides and subject to enrichment during intense weathering. Isotopic ratios of dissolved U in pore water demonstrate a dominant terrestrial rather than recycled marine source, suggesting that the carrier oxide phases and other associated elements are also of terrestrial rather than marine origin (B. McKee, pers. commun. ). The intense physical reworking of sediments results in export of dissolved minor elements into the water column and presumably the interior ocean. Other seawater components are clearly taken up by sediments and are involved in authigenic mineral formation. Distinct K +, F - , and Mg 2+ gradients into sediments are often present and dissolved Si concentrations are some of the lowest found in marine sediments. The F / K diffusive flux ratios are consistent with formation of Fe,Al-silicates. Progressive Mg enrichment of Fe-oxide particle coatings is also found as is X R D evidence of authigenic Fe,A1silicate formation (Rude and Aller, 1989, 1993 ). Preliminary mass balances indicate that several percent by weight of sediments may be authigenic silicates. Additional studies are underway to separate and characterize the authigenic mineral phases which are apparently forming. Diagenetic properties of Amazon shelf sediments are consistent with a strong influence of intense chemical weathering on early diage-
R.C. ALLER
netic reactions. Tropical drainage basins with sufficient particle residence times to promote residual oxide formation presumably favor such effects. Many of the reactions appear to have analogs in deep-sea sediments adjacent to hydrothermal areas, although Fe,Mn-oxides are not as abundant on the shelf. References Aller, R.C., Mackin, J.E. and Cox, R.T., 1986. Diagenesis of Fe and S in Amazon inner shelf muds: apparent dominance of Fe reduction and implications for the genesis of ironstones. Cont. ShelfRes., 6: 263-289. Aller, R.C., Aller, J.Y., Blair, N.E., Mackin, J.E., Rude, P.D., Stupakoff, I., Patchineelam, S., Boehme, S.E. and Knoppers, B., 1991. Biogeochemical processes in Amazon Shelf sediments. Oceanography, 4: 27-32. Aller, R.C., Xia, Q. and Rude, P.D., 1992. Early diagenesis and benthic carbon remineralization in Amazon Shelf muds. Eos (Trans. Am. Geophys. Union), 73: 268-269. Blair, N.E., Boehme, S.E., Gunn, J.M. and Mendlovitz, H.P., 1990. The production of dissolved inorganic carbon and methane in Amazon Shelf sediments. Eos (Trans. Am. Geophys. Union), 71: 1376. Blair, N.E., Gunn, J. and Aller, R.C., 1992. Methane and sediment dynamics on the Amazon Shelf. Eos (Trans. Am. Geophys. Union), 73: 269. Gibbs, R.J., 1967. The geochemistry of the Amazon River system: Part I. The factors that control the salinity and composition and concentration of suspended solids. Geol. Soc. Am. Bull., 78: 1203-1232. Meade, R.H., Dunne, T., Richey, J.E., de M. Santos, U. and Salatil, E., 1985. Storage and remobilization of suspended sediment in the lower Amazon River of Brazil. Science, 228: 488-490. Rude, P.D. and Aller, R.C., 1989. Early diagenetic alteration of lateritic particle coatings in Amazon Continental Shelf sediment. J. Sediment. Petrol., 59: 704716. Rude, P.D. and Aller, R.C., 1993. Fluorine uptake by Amazon continental shelf sediment and its impact on the global fluorine cycle. Cont. ShelfRes. (in press).