Soils and Sediments: Mineralogy and Geochemistry
Such conditions result in a set of unique and properties that can be evaluated by the analysis of color. Therefore, this work aimed a to compare results using visual assessment versus spectrophotometry, b to relate spectrophotometric measurements to sediment composition of Amazonian wetlands, and c to assess the application of such techniques in palaeoecological studies of Amazon wetlands.
Materials and Methods. These mangroves are restricted to the coastline, colonizing intertidal flats of wider tidal channels. This region presents total annual precipitation of 2, mm and average temperature about The annual precipitation is 2, mm and average temperature is Sampling and Facies Description. Sediment cores were collected to a depth of 1 m using a Russian Sampler, and consisted of grayish mud and yellowish sand with 1 kg each. Following the proposal of Walker , facies analyses included descriptions of lithology and structures. X-ray radiographs aided in the identification of sedimentary structures.
The sedimentary facies were codified following Miall Visual assessments of color were made in the laboratory under diffuse natural daylight using Munsell color charts Munsell Color Additionally, the data were compared with automatic conversions between the color space models provided by BabelColor 3.
The repeatability of color measurements, determined by the standard deviation of successive measurements, was less than 0. Mineralogical and chemical analysis. The results were interpreted using the X'Pert HighScore 2. The chemical composition was analyzed from a 0.
The average contents of major components and trace elements were normalized to the Upper Continental Crust Wedepohl , Post-Archean Australian Shales Turekian and Wedepohl and other tidal mud flats colonized by mangrove vegetation Djuwansah ; Costa et al. Similarity analyses between chemical data were performed by Single Linkage and Pearson product-moment correlation coefficient? Comparison between Munsell notation and Cielab color measurements. Sedimentary facies, color description, mineralogy and geochemistry. The herbaceous plain deposit presents mud with fresh roots of herbaceous plants facies Mbh; Table 1 , and slight to intense bioturbation with dwelling structures of benthic fauna.
The minerals included quartz, kaolinite, anatase, muscovite, albite and K-feldspar Figure 2A. In general, the trace elements occurred in a homogeneous pattern along the core, except for Ba and Sr that slightly decreased from the bottom to the top. Also, Ba content was higher than in other tidal mud flats in the Amazon Table 3. SiO 2 and Al 2 O 3 are the most abundant components, comprising quartz, diatom shells Costa et al. Other studies focused on the tidal flats of the northern Brazilian coast have also shown the predominance of quartz and kaolinite, but also smectite, illite, pyrite, jarosite, halite, muscovite, feldspar, albite, vermiculite and vermiculite-chlorite Costa et al.
Mangrove deposit, Soure site. The mangrove core consisted of mud with lenses of rippled sand facies Hl; Table 1 that indicate low energy flow with mud deposition from suspension and periodic sand inflows, mostly through the migration of isolated ripples.
This core also displayed convolute lamination, and many bioturbation features facies Mb and Sb; Table 1 such as roots, root marks and dwelling structures produced by benthic fauna. The minerals identified included quartz, kaolinite, anatase, muscovite, albite, K-feldspar and pyrite. However, pyrite was only found in facies Hl Figure 3A. The trace elements occurred in a homogeneous pattern along the core, with the exception of Ba and Sr which decreased slightly from facies Hl to Sb.
Total organic carbon and sulfur contents decreased from 2. SiO 2 and Al 2 O 3 were the most abundant components, in the form of quartz, clays, feldspars and micas. The presence of quartz, kaolinite and anatase, and the contents of Fe 2 O 3 and TiO 2 suggest weathered products of Barreiras and Post-Barreiras sediments as a partial source to the deposit e.
Rossetti ; Rossetti et al. However, the presence of albite and muscovite indicates the contribution of Precambrian crystalline rocks from the Central Brazil Shield e. Costa et al. This deposit presented massive and cross-laminated sands facies Sm and Sc. The upper segments consisted of laminated mud facies Ml and peat material facies Pt , as shown in Table 1. In the color analysis, four segments were recognized visually, but only three zones of similar color were identified through spectrophotometric measurements.
The color zones closely followed changes in sedimentary facies Figure 4. However, pyrite was restricted to facies Ml and Pt Figure 4A. They also contained lesser amounts of P 2 O 5 0. The contents of trace elements except Zr increased, toward facies Pt. Furthermore, P 2 O 5 , CaO and MgO also increase toward the top of the core and their coefficients with total organic carbon 0. Cielab color system and its relationship with wetland sediments. The color of Amazon wetland sediments studied here can be attributed to the mineralogical and chemical composition of the sediment samples, including quartz, iron oxides and oxyhydroxides, free ferrous iron, as well as organic carbon content Figure 2 , 3 , 4 and 5.
Lalonde et al. Indeed, oxidation of the rhizosphere by leakage of oxygen from roots of adapted species occurs when more oxygen is supplied than required for root respiration, thereby forming oxidized rhizospheres. Additionally, prolonged exposure of tidal flats may also allow the development of such alternations of color zones Tinner Therefore, extended? The comparison between visual analysis based on Munsell and instrumental analysis based on CIELAB system revealed a generally biased color perception of mottlings by human eye. However, without mottling effect, the color zones of sediment cores closely follow changes in sedimentary facies.
Based on spectrophotometric measurements, the sediment colors of Amazon wetlands are directly related to its mineralogical and chemical composition including quartz, iron oxyhydroxides, and organic carbon content. Regarding the organic carbon, total organic carbon contents between 0. Therefore, spectrophotometric measurements can provide significant information about substrate conditions during plant development and the formation of peat deposits.
The BabelColor Company. Balsam, W. Marine Geology , The effects of water content on diffuse reflectance spectrophotometry studies of deep-sea sediment cores. Bezerra, P. Costa, M. Geological information system. Brazilian Geological Service. Debret, M. Earth-Science Reviews , Alteration of soil properties through a weathering sequence as evaluated by spectral reflectance.
Soil Science Society of America Journal , Djuwansah, M. Mangroves de la zone equatorial. Estude sedimentologique, mineralogique et geochimique. PhD Thesis. Fernandes, R. Fisch, G. Acta Amazonica , Gallo, M. Generation of over tides and compound tides in Amazon estuary. Ocean Dynamics , Role of organic matter in obliterating the effects of iron on spectral reflectance and colour of Brazilian tropical soils. International Journal of Remote Sensing , Mid and late Holocene sedimentary process and palaeovegetation changes near the mouth of the Amazon River.
The Holocene , Henderson, R. Soil aeration and plant productivity. In: Rechcigl, Jr. Hering, E. Outlines of a theory of the light sense. The Budahn and Schmitt  and Olivarez et al. We calculated source material contributions upper continental crust versus oceanic crust using the Budahn and Schmitt  method for two samples TT and TT studied by Olivarez et al.
Vincent Island [ Heath et al. Interestingly, this approach suggests that ashes from Dominica and St. Lucia appear to have had little influence on the genesis of western Atlantic soils, even on Barbados, a point we discuss in more detail later. All of the Barbados soil profile 3B samples contain more than A pure carbonate substrate was not sampled from Barbados, so we use the average composition of several Exuma Cays corals.
For simplicity, it is assumed that all of the Ca in these soils is from the carbonate. The Barbados soil profile 3B used in modeling is the average of five estimated soil compositions taken from various depths. In fact, only one soil was modeled without any compositional adjustment. The carbonate compositions used in these calculations were the average of several Florida Keys and Exuma Cays corals. Because most of the trace element contents in the carbonates are extremely low see discussion above on composition of the reef carbonates , the addition of carbonate to the soil acts simply as dilution and thus has no appreciable effect on the relative abundances of other elements analogous to the use of ternary plots above.
The distinct trace element abundance patterns for the Barbados soil and the New Providence Island soil are striking in Figure The same two parent materials dominate the soils of the Bahamas, although there are differences between islands. Acropora palmata , Acropora cervicornis , and Montastrea annularis are the three most commonly occurring corals in the Pleistocene reefs of Barbados [ Mesolella , ]. Concentrations of REE in all three coral species are very low and would require considerable reef dissolution to produce the observed concentrations of REE in Barbados soils.
This would require complete dissolution of the entire reef, plus dissolution to depths tens or hundreds of meters below sea level. This physical impossibility is analogous to Birkeland 's  calculations for a residual origin for soils on Rota Island in the Marianas chain, requiring complete island dissolution.
We conclude that reef dissolution plays, at most, a minor role in soil genesis on Barbados. Nevertheless, complications arise with the presence of negative Ce anomalies in some Barbados soils soil profiles 5B, 8A and 11 that are not present in either of the eolian parent materials. Thus it is possible that the negative Ce anomalies in the older Barbados soils reflect some influence from tephras that have negative Ce anomalies, although none of the tephras analyzed in the present study exhibit this pattern. Alternatively, negative Ce anomalies also have been reported in Atlantic Ocean corals, a reflection of small amounts of REE incorporation from seawater into the aragonite structure [ Sholkovitz and Shen , ].
However, as discussed above, all corals we analyzed show positive Ce anomalies and detrital coral fragments present in the soils would likely show this opposite effect. Furthermore, we found no correlation between negative Ce anomalies in the soils and carbonate content. Nesbitt and Markovics  interpreted negative Ce anomalies in a weathering profile developed on granodiorite in Australia to be the result of an intermediate stage of chemical weathering.
However, the process by which this takes place on Barbados is not clear and deserves further study. The bivariate and ternary plots, considered as a whole, suggest that Barbados soils originated from more than one parent material. Lucia tephras or from a mix of African dust and St. Vincent tephras. The interpretation that is consistent with all plots is derivation from a mix of St. Vincent volcanic materials and African dust. In an independent calculation, the MRA geochemical model integrating all immobile elements indicates that Barbados soils are derived from a combination of St. These results are consistent with the more subjective interpretation of the bivariate and ternary plots and are also consistent with the isotopic data of Borg and Banner .
Vincent, based on historic eruptions in and [ Carey and Sigurdsson , ; Sigurdsson , ]. During much of the Pleistocene, St. Vincent has been quite active volcanically [ Briden et al. Thus Barbados may have received small but frequent amounts of tephra from St. Vincent over much of the Pleistocene. In contrast, eruptions from the more silicic volcanic centers such as Dominica and St.
Lucia are larger and show up as layers in cores Figure 7 , but these eruptions may be far less frequent. Furthermore, Barbados appears to lie at the southern margin of the dispersal plumes from these centers Figure 6. This pattern suggests derivation solely or at least mostly from either or both Mississippi valley loess or African dust. If there was any volcanic influence on these soils, it has been overwhelmed by the inputs of dust. This trend is unexplained and needs more study.
Soils on the Exuma Cays, though farther south, seem to be derived from about equal parts African dust and Mississippi River valley loess. We have no explanation for this unexpected result and suggest that more work on soils of the southern Bahamas is required.
The Florida Keys and Bahamas are, however, well within the zone of the easterly trade winds that carry African dust to the region Figure 1 [ Prospero , ].
Soils and Sediments Mineralogy and Geochemistry
Studies at an inland site in Florida show that substantial concentrations of aerosol occur only when African dust is present over the region [ Prospero et al. The results of the Landing et al. As discussed earlier, soils on the Florida Keys are very thin, likely due to erosion by tropical storms and hurricanes for thousands of years.
Many of the Quaternary limestones on these islands are of exceptionally high purity and it is unlikely that the soils are derived solely or even mostly from insoluble residues of the local carbonate substrate. Measurement of REE and other trace elements in representative, unrecrystallized corals indicates that these components have very low concentrations.
If residual accumulation were the main mode of soil genesis, improbably large amounts of carbonate dissolution would be required to explain the observed soils. Furthermore, geomorphic considerations eliminate other local, noncarbonate bedrock sources, such as Tertiary sedimentary rocks from the Scotland District on Barbados. Tephras from the Lesser Antilles island arc St. Vincent, Dominica, and St. These observations are consistent with previous studies and indicate that volcanic materials in this island arc system have a wide range of mixed oceanic and continental crustal components.
Consideration of the REE plots, bivariate and ternary plots of immobile elements, and geochemical modeling suggests that Barbados soils have developed mostly from volcanic ash from nearby St. Vincent and secondarily, but significantly, from African dust. A possible minor contribution from distal Mississippi valley loess is apparent in the geochemical modeling results.
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We interpret these data to indicate that neither volcanic ash nor carbonate residue has been an important parent material in the genesis of these soils. Geochemical modeling indicates that soils on the Florida Keys and Bahamas have developed mostly from African dust, but with a significant component of distal Mississippi valley loess. This interpretation supports the recent modeling results of Mahowald et al. Nevertheless, the modeling results of Mahowald et al. We also document here the possibility that distal Mississippi valley loess may have been transported much farther from its source than previously supposed.
Furthermore, because of the constraints given by the ages of the underlying reef limestones on Barbados, our results indicate that African dust and North American loess? We hypothesize that other localities in the region may have been similarly affected. Soils on other Caribbean islands, northern South America [ Swap et al. Although we cannot say just how widely African dust has influenced soil development across this region, the results presented here suggest that LRT eolian inputs may have greater significance for pedogenesis than previously thought.
Muhs and J. Prospero's and S. Carey's work is supported by the National Science Foundation. Our thanks also go to Marith Reheis, Rich Reynolds, Jimin Sun, Suzanne Anderson, and a critical but very helpful anonymous reviewer for very useful reviews on an earlier version of the paper. Auxiliary material files may require downloading to a local drive depending on platform, browser, configuration, and size.
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Share Give access Share full text access. Share full text access. Please review our Terms and Conditions of Use and check box below to share full-text version of article. Figure 1 Open in figure viewer PowerPoint. Map of North America and the Caribbean basin with islands studied. Also shown are the distribution of loess from compilation of Muhs and Bettis  and approximate extent of African dust in summer, based on — TOMS satellite data and studies by Prospero and Carlson  , Prospero  , Perry et al. Study Areas 2. Barbados  Barbados is situated in the western Atlantic Ocean approximately km east of the Lesser Antilles island arc Figures 1 and 2.
Figure 2 Open in figure viewer PowerPoint. Tertiary sedimentary rocks crop out in the stippled area. Shown also is the Kitridge Point dust trap sampling locality of Prospero . Terrace names are from Bender et al. Figure 3 Open in figure viewer PowerPoint. Topographic profile and ages of reef terraces in the Holetown area of Barbados and soil profiles studied. Modified from Muhs . Bahamas  The Bahamas are islands composed primarily of carbonate reefs, carbonate oolitic marine sediments, and carbonate oolitic eolianites of Quaternary age see Figure S1.
Figure 4 Open in figure viewer PowerPoint. Map showing the Florida Keys and Bahamas area; islands in bold type are localities where soils were sampled. Figure 5 Open in figure viewer PowerPoint. Stratigraphy and ages of eolianite and paleosol sections studied on a and b New Providence Island and c the Exumas Cays, Bahamas. Stratigraphy of the Lyford Cay section is modified from Garrett and Gould  ; stratigraphy of other sections is from this study. U series ages are from Muhs et al.
Volcanic Ash Tephra  Given the amount and longevity of volcanic activity in the Lesser Antilles island arc [ Briden et al. Figure 6 Open in figure viewer PowerPoint. Maps redrawn from Carey and Sigurdsson [ , ]. Figure 7 Open in figure viewer PowerPoint. Core stratigraphy and faunal zones are from Sigurdsson and Carey  ; age estimates are from Reid et al.
Figure 8 Open in figure viewer PowerPoint. Map of the Mississippi River valley, showing distribution of loess and last glacial paleowinds compiled by Muhs and Bettis  and locations of lower Mississippi River valley loess analyzed in this study. Samples are loesses from the C horizons of soils studied by Muhs et al. African Dust  A third potential source of soil parent material in the study region is African dust, carried west across the Atlantic Ocean Figures 1 and 9.
Figure 9 Open in figure viewer PowerPoint. Principles of Element Mobility as a Function of Ionic Potential  The use of REE and other trace elements for provenance studies is dependent on an assumption of relatively low mobility during chemical weathering and pedogenesis. Empirical Test of Element Mobility on Barbados  We acknowledge that some intermediate ionic potential elements may be mobile in certain field settings, although we maintain that these elements are the least mobile constituents in soils.
Figure 10 Open in figure viewer PowerPoint. Chronofunctions of various element concentrations in individual soil horizons from Barbados along with best fit regression equations. See text for discussion of terrace ages. Reef Corals  Carbonates in coral terraces of Barbados have low amounts of detrital, noncarbonate impurities see reviews by Muhs et al.
Figure 11 Open in figure viewer PowerPoint. Volcanic Ash  Analyses of fresh and weathered Pleistocene, Holocene, and historic ignimbrites and scoria from St. Figure 12 Open in figure viewer PowerPoint. For geologic setting of samples, see Hay [ , ]. Figure 13 Open in figure viewer PowerPoint. Figure 14 Open in figure viewer PowerPoint. African Dust  African dust, like lower Mississippi River valley loess, also shows much more uniform REE patterns than volcanic materials from the Lesser Antilles island arc Figure Figure 15 Open in figure viewer PowerPoint. Figure 16 Open in figure viewer PowerPoint.
Figure 17 Open in figure viewer PowerPoint. Figure 18 Open in figure viewer PowerPoint. Figure 19 Open in figure viewer PowerPoint. Lucia, African dust, and lower Mississippi River Valley loess. Figure 20 Open in figure viewer PowerPoint. Ternary plots of relative abundances of Sc, Th, and La, showing fields defined by range of variability for various soil parent materials, and circles showing values for soils from a Barbados and b the Florida Keys and Bahamas.
Data from St. Vincent are from this study and also Turner et al. Figure 21 Open in figure viewer PowerPoint. Ternary plots of relative abundances of Sc, Th, and Zr divided by 10 , showing fields defined by range of variability for various soil parent materials, and circles showing values for soils from a Barbados and b the Florida Keys and Bahamas.
Figure 22 Open in figure viewer PowerPoint. Ternary plots of relative abundances of Cr divided by 5 , Th, and Nd, showing fields defined by range of variability for various soil parent materials, and circles showing values for soils from a Barbados and b the Florida Keys and Bahamas. Note that a single sample from Dominica falls within the range of African dust. Figure 23 Open in figure viewer PowerPoint. Note scale difference between Figures 23 top and 23 bottom.
Geochemical Modeling of Soil Parent Materials Using Multilinear Regression Analysis  An alternative to the bivariate and ternary plots of immobile element ratios is to integrate a number of elements in a geochemical modeling approach. Figure 24 Open in figure viewer PowerPoint. Plots comparing the measured and modeled abundances of REE and other immobile trace elements for two soils, a one from Barbados uppermost horizon of profile 3A and b one from New Providence Island, Bahamas modern soil A horizon , demonstrating the good agreement between measured values and the MRA geochemically modeled values see text for discussion.
Figure 25 Open in figure viewer PowerPoint. Plot showing the relative contributions of source sediments for soils on the Florida Keys, Bahamas, and Barbados using the MRA geochemical modeling approach. Note that St. Vincent volcanics contribute no significant mass to the soils of the Florida Keys and Bahamas and that tephras from Dominica and St.
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Lucia make no significant contribution to soils on any island. Discussion 7. Reef Coral Residuum as a Soil Parent Material  The new data on concentrations of REE and other trace elements in corals that occur commonly in Barbados reefs allow us to provide a rigorous test of the contribution of carbonate to REE in soils via residual accumulation after coral dissolution.
Acknowledgments  D. Supporting Information Auxiliary material for this article contains three figures and two tables. Additional file information is provided in the readme. Filename Description jgrfsupreadme. Photographs of field settings. Photographs of soils on Barbados. Photographs of soils on the Florida Keys. Geochemical data.