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How could a freshwater swamp produce a chemical signature characteristic of a saltmarsh? Terrence A McCloskey, Christopher G. Smith, Kam-biu Liu, Marci Marot, and Christian Haller ACS Earth Space Chem., Just Accepted Manuscript • DOI: 10.1021/ acsearthspacechem.7b00098 • Publication Date (Web): 17 Nov 2017 Downloaded from http://pubs.acs.org on November 20, 2017
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ACS Earth and Space Chemistry
How could a freshwater swamp produce a chemical signature characteristic of a saltmarsh?
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Author List: Terrence A McCloskey (corresponding author)
[email protected] U.S. Geological Survey Wetland and Aquatic Research Center Parker Coliseum Baton Rouge, LA 225-578-7491 (telephone) Christopher G. Smith United States Geological Survey Coastal and Marine Science Center 600 Fourth Street South St. Petersburg, FL 33701 Kam-biu Liu 1002-Y Energy, Coast, and Environment Building Department of Oceanography and Coastal Sciences Louisiana State University Baton Rouge, LA 70803 Marci Marot United States Geological Survey Coastal and Marine Science Center 600 Fourth Street South St. Petersburg, FL 33701 Christian Haller College of Marine Science University of South Florida 140 Seventh Avenue South St. Petersburg, FL 33701
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Abstract:
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Reduction-oxidation (redox) reaction conditions, which are of great importance for the soil
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chemistry of coastal marshes, can be temporally dynamic. We present a transect of cores from
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northwest Florida wherein radical post-depositional changes in the redox regime has created
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atypical geochemical profiles at the bottom of the sedimentary column. The stratigraphy is
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consistent across the transect, consisting of, bottom upward, carbonate bedrock, a gray clay, an
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organic mud section, a dense clay layer, and an upper organic mud unit representing the current
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saltwater marsh. However, the geochemical signature of the lower organic mud unit suggests
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pervasive redox reactions, although the interval has been identified as representing a freshwater
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marsh, an unlikely environment for such conditions. Analyses indicate that this discrepancy
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results from post-depositional diagenesis driven by millennial-scale environmental parameters.
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Rising sea level that led to the deposition of the capping clay layer, created anaerobic conditions
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in the freshwater swamp interval, and isolated it hydrologically from the rest of the sediment
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column. The subsequent infiltration of marine water into this organic material led to sulfate
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reduction, the buildup of H2S and FeS, and anoxic conditions. Continued sulfidation eventually
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resulted in euxinic conditions, as evidenced by elevated levels of Fe, S and especially Mo, the
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diagnostic marker of euxinia. Because this chemical transformation occurred long after the
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original deposition the geochemical signature does not reflect soil chemistry at the time of
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deposition and cannot be used to infer syn-depositional environmental conditions, emphasizing
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the importance of recognizing diagenetic processes in paleoenvironmental studies.
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Keywords: Florida, diagenesis, anoxic, euxinic, molybdenum, XRF, redox reactions, marsh
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ACS Earth and Space Chemistry
INTRODUCTION
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Bulk metal concentrations in sediments occur as a result of both authigenic and allogenic
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processes; proper identification of the processes contributing to the downcore presence, state and
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abundance of metals or suite of metals is an essential requirement for correctly interpreting the
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sedimentary data when reconstructing paleoenvironments. For example, across multiple river-
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dominated ocean margins, increase in allochthonous trace elements in surface and downcore
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sediments have been largely attributed to anthropogenic activity (1, 2), and subsequently used as
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marker horizons for industrial and similar anthropogenic activities (3). Anomalous metal
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concentrations and selected elemental ratios can identify the provenance of sediment, with the
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identification of allochthonous material used to indicate such processes as transport by extreme
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events (4-7) or the specific riverine source of sediments in coastal zones (8).
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The increasingly common use of such geochemical analyses has been driven, at least
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partially, by the development/technical improvement of portable XRF (pXRF) devices (9), which
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are capable of producing elemental concentrations much more quickly and cheaply than such
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traditional methods as flame Atomic Absorption Spectroscopy (fAAS), Inductively coupled
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plasma-mass spectroscopy (ICP-MS), or optical emission spectroscopy (ICP-OES). A recent
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review of articles published during the 12-month period from April 2014 to March 2015 that
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made use of this methodology contained >400 references (10). Geological studies have used
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pXRF data in a variety of ways, most commonly to distinguish sedimentary units, detect subtle
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differences within relatively homogenous intervals, and/or infer their provenance either by
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qualitative differences (11, 12), or by inclusion in statistical analyses such as Hierarchical Cluster
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Analysis (HCA) and Principal Component Analysis (PCA) (13, 14). A recent study used XRF data
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to generate transfer functions as a quantitative tool to reconstruct a drought record to 3000 BP
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(15)
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. In order to apply this technology in the context of paleoenvironmental reconstructions, it
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is essential to determine whether the elemental composition of the sedimentary material
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represents the environment at the time of deposition. It is important to note that pXRF data only
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reports elemental abundance, and not oxidation state. As a result, reduction-oxidation (redox)
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reactions can complicate pXRF-based interpretation of environmental conditions. In this paper
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we present sediment cores from the west central coast of Florida that illustrate the need to
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account for post-depositional processes when interpreting sedimentary geochemical data.
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METHODS AND MATERIALS
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Study site
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The Waccasassa River is a small river with a drainage basin of 1,600 km2 (16), and an
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average discharge 8.5 m3/s (17). It is situated along the middle section of the low-gradient, low-
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energy, sediment-starved Big Bend coast in west central Florida (Figure 1). Mean annual wave
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height is ~30cm with an average spring tidal range of ~90 cm (18). The mouth is located at
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~29.16N, 82.81 W, emptying into the Gulf of Mexico at the head of a shallow tide-dominated
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bay. The lower end of the river and the surrounding coastline are dominated by coast-parallel salt
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marshes up to 1.5 km wide. The sediment cover is thin, with the marsh surface typically 30 elements and converts cps into parts per million (concentration) using an internal
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calibration to certified solid standards NIST 2710a and 2711a. Analysis was conducted across
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three frequencies for 30 seconds each at 2 cm intervals down the length of each core immediately
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upon opening. We present the concentration data for S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Zn, Br,
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Rb, Sr, Zr, and Mo. Although assessments of µXRF have shown that absolute concentration data
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require additional pre-treatment for fully quantitative geochemical computations (e.g., elemental
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mass balance), relative concentrations are applicable to assess internal variability through time
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(20)
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(https://www.staff.ncl.ac.uk/stephen.juggins/software/C2Home.htm) to individual core profiles
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with standardized and centered variables to refine down-core patterns. Loss-on-ignition was
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conducted continuously down the center of each core at 1-cm resolution following the procedure
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of (21). Thirteen samples of terrestrial plant material were sent to the National Ocean Sciences
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Accelerator Mass Spectrometry (NOSAMS) Laboratory at Woods Hole Oceanographic
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Institution for radiocarbon dating. Radiocarbon dates were calibrated to calendar years, and
. As such, we apply PCA (C2 v1.7.5;
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median dates provided by Calib 7.1 (http://calib.qub.ac.uk/calib/calib.html) based on the Reimer
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et al. (2013) data set (22).
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RESULTS
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Stratigraphy
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The stratigraphy, which is very consistent across the ten cores (Figure 2), is comprised of
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five stratigraphic units, although the lowest two are not reached in all cores. From the bottom up
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the units are: a calcareous basal unit containing flakes of the limestone bedrock (Unit 1); a gray
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clay (Unit 2), a lower organic mud (Unit 3), a clay layer (gray at the top, blue at the bottom)
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(Unit 4), and an organic mud (present marsh surface) (Unit 5).
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Dates Radiocarbon dates obtained from the various stratigraphic units produced inconsistent ages
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across different cores. The radiocarbon dates (Table 1) associated with the transition from the
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lower organic unit to the blue clay range from 325±20 to 2,100±15. Median probability ages for
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the other five samples taken from just below this transition are 501, 666, 702, 971, and 1,624 cal
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yr BP. The radiocarbon age of samples taken from the bottom of the lower organic layer are
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2700±20 in core WC22 and 3,040±20 in core WC20 (Figure 2).
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LOI and elemental concentrations The LOI data display percentage of water (wet weight), organics, carbonates and
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residuals (dry weight) at cm resolution. Because the material was burned at both 550oC and
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1000oC, the residual curve corresponds almost entirely to percentage of siliclastic material,
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although negligible amounts water loss from the dehydration of clay mineral may be included.
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Presented are representative cores WC01 and WC05 which contain the same three stratigraphic
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units: lower (Unit 3) and upper (Unit 5) organic mud units, separated on both cores by a lower-
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organic clay interval (Unit 4) dominating the middle (~20-115 cm on WC01, and ~30-110 cm on
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WC05) (Figure 3).
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Because each stratigraphic unit possesses a unique chemical signature, as evidenced by
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changes in the elemental profiles at the stratigraphic boundaries (Figure 3), each unit can be
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considered a separate geochemical facies. The geochemical signature of each facies is consistent
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across cores, and differs markedly from the other units within a core, as demonstrated by the
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PCA biplots (Figure 4). Unit 5 is characterized by high values for Ca, Br, Sr, and Zn, and low
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values for S, Cl, Fe, and Co, with the samples located along the vectors associated with the
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elements Sr, Ca, and Br. Unit 4 is marked by generally high concentrations of K, Ti, V, Cr, Fe,
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Co, Rb, and Zr, with samples aligned along the Mn, Zr, K, Ti, and V axes, while Unit 3 has high
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concentrations of S, Cl, Mo, and low concentrations of K, Ca, Ti, V, and Sr, with samples
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located along the Mo, S, and Cl vectors. On the longer cores Unit 2 is characterized by high
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values of Mo, S, Cl, and, in some cores, Zr, K, Ti, and V; and Unit 1 generally has high
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concentrations of Ca, Mo, S, and Cl. Separation between units 3 and 4 is along the first principal
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component (mainly Mo, S, and Cl vs the terrestrial elements Mn, V, Ti, K, Zr), while the
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separation between those units and Unit 5 is along the second principal component, driven by Sr,
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Ca, and Br. Together, the first two principal components explain from 55-76% of the variability
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in the ten cores. The compositional (LOI data), stratigraphic, and elemental patterns of the
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individual stratigraphic units/geochemical facies are remarkably consistent across the entire set
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of cores, as are their positions and relationship on the PCA biplots (Figures 3, 4, SI-1-8).
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DISCUSSION X-ray diffraction analysis has shown that the mineralogy, which is dominated by a
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random-ordered, mixed-layer illite-smectite and kaolinite, is extremely similar for all facies (19,
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23)
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marsh sediments formed by reworked material that has been transported onto the marsh from the
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near-shore area, after which it cycles through a system of erosion and deposition (19, 23, 24). This
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constant reworking of the sediments likely accounts for our inconsistent chronology, although
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previous studies (19, 23) were able to date the Unit 3-4 transition at ~1800 cal yr BP, and the Unit
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4-5 transition at ~150- 500 cal yr BP.
. The authors suggest this indicates that all layers are sourced from the same material, with the
These same studies (19, 23) have reconstructed the environmental history of the site. Above
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the limestone bedrock (Unit 1); they have interpreted Unit 2, with abundant marine fossils, as a
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pre-Holocene shallow marine environment analogous to the current Florida shelf, and Unit 3 as a
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forested freshwater swamp, similar to that currently existing just landward of the present salt
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marsh. This interpretation was based on Unit 3’s fine-grained black mud, the presence of
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horizontal detrital wood fragments, fleshy root material, and total lack of marine organisms (19,
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23)
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either a patchy intertidal saltmarsh or salt flat, while Unit 5 represents the present Juncus-
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dominated saltmarsh. A nearly identical environmental history, including a similar succession
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from Pleistocene marine to a Holocene freshwater wetlands replaced by saltmarsh, has been
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documented from the lower reaches of the Suwannee River,
