Discharge of dissolved black carbon from a fire-affected intertidal system

Thorsten Dittmar,a,* Jiyoung Paeng,b Thomas M. Gihring,b I G. N. A. Suryaputra,a andMarkus Huettelb

a Max Planck Research Group for Marine Geochemistry, University of Oldenburg, Institute for Chemistry and Biology of the MarineEnvironment, Oldenburg, Germany

bDepartment of Earth, Ocean and Atmospheric Science, Florida State University, Florida

Abstract

We report substantial tidal fluxes of dissolved black carbon (DBC) in a fire-affected marsh in the northern Gulfof Mexico. DBC was molecularly determined as benzenepolycarboxylic acids in a tidal creek, adjacent rivers, andthe coastal ocean. Supported by stable carbon isotope and in situ fluorescence measurements, three sources ofdissolved organic carbon (DOC) were identified that mixed conservatively in the coastal system: groundwaterfrom salt marshes, river water, and seawater. Groundwater was the main source of DBC to the creek. The highestDBC concentrations of up to 41 mmol C L21 (7.2% of DOC) were found in the creek at low tide, compared with ,18 mmol C L21 in all other samples. Over the studied tidal cycle, we determined a runoff (load per drainage area)of 3700 moles DBC (44 kg C) km22 of salt marsh. This is high compared with the Apalachicola River, where theannual DBC runoff is on the order of 104 mol (120 kg C) km22 yr21. In the marsh, it would require , 20 tidalcycles similar to the one that we studied to remove all black carbon produced during one fire event. Because aspring tide was studied, our estimate is as an upper limit. DBC is ubiquitous in the global ocean, and dissolutionand subsequent lateral transport appear to be important removal mechanisms for soil black carbon. Our study,which provides a snapshot in time and space, demonstrates that tidal fluxes may be primary carriers of DBC, andtherefore tidal pumping and groundwater discharge cannot be ignored in assessing the continental runoff of DBC.

The residues of biomass burning contain some of themost resistant organic compounds known on Earth’ssurface (Masiello 2004). Charred organic matter is oftenthe only identifiable remnant of prehistoric humansettlements (Cohen-Ofri et al. 2006), and virtually all soilsand sediments contain charcoal or soot, a testimony ofwidespread wildfires over the past millennia (Masiello andDruffel 1998; Forbes et al. 2006). Pyrogenic organic matter(also known as black carbon) is more resistant tobiomineralization compared with common biomolecules,and black carbon selectively accumulates in soils andsediments on time scales of decades and centuries (Forbeset al. 2006). In the long term, however, black carbon alsodecomposes, and after several millennia, only a fraction ofthe original pyrogenic matter is preserved in soils andsediments (Druffel 2004; Major et al. 2010). The molecularstructure and stability of black carbon in the environmentdepends largely on the charring temperature, which canbe highly variable in wildfires. Chars produced at lowtemperatures are less condensed and richer in polarfunctional groups than those produced at higher temper-atures (Kuo et al. 2008; Keiluweit et al. 2010; Schneideret al. 2010). Low-temperature chars cycle on the order ofdecades to centuries (Alexis et al. 2007; Zimmerman 2010).Graphite-like structures, on the other hand, are one of themost resistant forms of reduced carbon on Earth (Dickenset al. 2004). In addition, environmental factors and soilphysical properties determine the fate of black carbon insoils (Major et al. 2010). Due to the stability of blackcarbon in the environment, knowledge of the turnovermechanisms are crucial to our understanding of global

biogeochemical cycles. The production of black carbonhas been proposed as a potential geoengineering tool tosequester carbon from active cycles (Lehmann et al. 2006).However, because the driving forces behind black carbonturnover are not well-understood, any geoengineeringattempt involving black carbon could be a fruitlessendeavor in the long term.

In addition to mineralization, lateral transport in thelandscape can be a significant process for the removal ofblack carbon from soils (Mannino and Harvey 2004;Guggenberger et al. 2008; Major et al. 2010). Charcoalthat is produced at high temperatures has a low solubilityin water; however, after decades of microbial attack insoils, oxygen is introduced into the condensed aromaticstructure of charcoal. The resulting carboxylated molecularsubunits dissolve in water and can migrate through the soil(Hockaday et al. 2006; Cheng and Lehmann 2009). Thisprocess explains the presence of a pyrogenic component indissolved organic matter (DOM) in streams and estuaries(Kim et al. 2004; Mannino and Harvey 2004; Guggenbergeret al. 2008) that transport dissolved black carbon (DBC) tothe ocean, where it is ubiquitous. Even in the most remotedeep ocean basins, , 2% of DOM has a heat-inducedmolecular structure (Dittmar and Koch 2006; Dittmar andPaeng 2009). Solubilization and subsequent transport in thedissolved form could be one of the main removalmechanisms of black carbon in soils. DBC is expected tobe released slowly from soils over long periods after a fireevent. It is therefore likely that water runoff, in addition tomicrobial oxidation, is a major controlling factor in thelong-term release of black carbon from soils.

By far, the largest volume of water runoff on thecontinents occurs in the intertidal zone, whereas tidal* Corresponding author: tdittmar@mpi-bremen.de

Limnol. Oceanogr., 57(4), 2012, 1171–1181

E 2012, by the Association for the Sciences of Limnology and Oceanography, Inc.doi:10.4319/lo.2012.57.4.1171

1171

flushing floods and drains large vegetated areas once ortwice a day. Tidal exchange is so efficient that, in manycoastal areas, the flux of organic matter from intertidalvegetated areas exceeds riverine fluxes (Dittmar et al. 2001).For example, . 10% of the worldwide land–ocean flux ofDOM is derived from mangroves alone, even though thistropical intertidal biome covers only 0.1% of the landsurface (Dittmar et al. 2006). Material transport from saltmarshes, the dominant intertidal vegetation of subtropicand temperate climate zones, could be similarly efficient(Moran et al. 1991).

Here, we hypothesize that tidal fluxes can be a maincarrier of DBC. To test this hypothesis, we investigated thetidal pattern of DOM and its pyrogenic (black carbon)component in a tidal creek and adjacent estuary of theApalachicola River in the northeastern Gulf of Mexico. Theintertidal marsh system was affected by a major burningevent 3 yr prior to the sampling campaign. Deposition ofsoil-derived black carbon particles from the riverinecatchment area during past storm surges is another likelysource of black carbon in the marsh. Otherwise, the systemwas in relatively pristine conditions. There are no majorsettlements or industries in the immediate vicinity, and nomajor oil spills were reported in the area for the past decadesprior to our investigation. The Apalachicola River isFlorida’s largest river in terms of water discharge, and itdrains , 50,000 km2 (Edmiston 2008). Prescribed burning oflarge forested areas and marshes is a common managementpractice within the catchment. Therefore, it is reasonable toassume that the drainage areas of the studied tidal creek andrivers all contain a substantial standing stock of soil blackcarbon. The main goals of the study were to determine thepathways and to estimate the magnitude of DBC releasefrom the marsh system to the ocean.

Methods

Sampling—Sampling was performed during a spring tidecycle on 04–05 June 2007 in the Apalachicola NationalEstuarine Research Reserve. A boat operated by theReserve management team was used as an anchoredworking platform. The spring tide cycle was purposefullychosen because of the expected strong tidal flushing.During neap tides, the fluxes are largely driven by windand are less predictable. The samples were collected from atidal creek (Buzzard Bayou; 29u45.793 N, 84u59.625 W)that drains freshwater swamps and salt marshes andeventually links to the Little St. Marks River that splitsoff the Apalachicola River , 15 km before the latterreaches Apalachicola Bay (Fig. 1). At the sampling station,the creek had a width of 25 m and a maximum depth of2.25 m at high tide. The tidal range was 63 cm on the day ofsampling. The creek starts in the salt marsh , 1 kmupstream of our sampling site, and it has no links to othercreeks or rivers other than the connection to the Little St.Marks River. Minor sheet flow may temporarily link thecreek to the nearby Little St. Marks and St. Marks riversonly during extreme high tides and major rain events. Thetidal creek was sampled in its cross-sectional center, hourlyover 25 h at 10 cm below the water surface and , 60 cm

above the bottom. The riverine endmembers (Little St.Marks River and Apalachicola River) were sampled atsalinity between 1 and 2 during the ebb cycle of 05 June2007. A salinity of . 0 was purposefully chosen forendmember sampling to account for the potential physico-chemical influence of salinity on the DOM compositionand concentration. Samples were drawn with a peristalticpump through acid-rinsed polyethylene tubing and in-lineWhatman glass-fiber filters (GF/F) into precombustedglass bottles. The marine endmember was sampled on 13June 2007 at five stations along an offshore transect east ofApalachicola Bay during a cruise aboard R/V Seminole.Samples were taken at 0.5 m below the sea surface and 1 mabove the seafloor using a Niskin sampler. Samples werevacuum-filtered through Whatman GF/F filters into glassbottles. After filtration, all samples were immediatelyacidified with HCl (10 molar, analytical grade) to pH 5 2.

In situ measurements—To characterize the temporalchanges in the creek water associated with the change ofthe tides, several parameters were monitored at 2-minintervals. An in situ fluorometer (Wetlabs ECO Triplet) thatrecorded the concentration of chromophoric DOM(CDOM), using fluorescence measurements (Excitation:370 nm, Emission: 460 nm) was anchored next to the inletof the sampling tubing at , 60 cm above creek bottom. In

Fig. 1. The sampling area in the northern Gulf of Mexico.The satellite image was modified from www.bing.com. Numbersat the marine sampling stations refer to the results in Table 1.

1172 Dittmar et al.

addition, chlorophyll a (Chl a) fluorescence (Excitation:470 nm, Emission: 695 nm) and turbidity (light-scattering of660 nm) were monitored by the same instrument. The sensorwas calibrated by the manufacturer. CDOM concentrationsare expressed as quinine sulfate equivalents (QSE, mg L21).Salinity (calculated from the measured conductivity) andtemperature were recorded using an in situ sensor (Sea BirdSBE 16) at the same depth and location as the other sensors.The water level of the tidal creek was manually recordedevery 15 min using a fixed scale at the margin of the creek.The current direction was manually determined at the twosampling depths using floating objects (creek surface) andsubmerged discs (subsurface). In addition, vertical profiles ofcurrent direction and flow velocity were determined bylowering an acoustic Doppler velocimeter (Nortek VectorADV) through the water column of the creek. Thevelocimeter was lowered at 5-min intervals during periodsof strongest tidal currents and at 30-min intervals duringstagnating tides.

To estimate water flux, the water column was dividedinto two compartments: the surface layer (0–50 cm), andthe subsurface (. 50 cm). The water velocities measuredwith the acoustic Doppler velocimeter were averaged overthe respective depth interval for each time point. The cross-sectional area of the creek was measured once using agauge at a 1-m horizontal resolution and was recalculatedfor each time point based on the measured water depths.The water flux was obtained for each time point and depthcompartment by multiplying the respective values for watervelocity and cross-sectional area. The loads of DBC,dissolved organic carbon (DOC), and salt were estimatedby multiplying the respective concentrations by the waterflux. Concentrations were linearly interpolated between thehourly time points to obtain the same temporal resolutionas the current velocity data.

Dissolved organic carbon—DOC was determined using atotal organic carbon analyzer (Shimadzu TOC-VCPH).The analytical accuracy was tested daily against deepAtlantic seawater reference material and low-carbon waterprovided by the consensus reference materials program(D.A. Hansell, University of Miami, Florida). Proceduralblanks, including the filtration step, were obtained usingultrapure water. These blank samples did not contain anydetectable amounts of DOC. The detection limit for DOCwas 5 mmol L21, and the analytical accuracy (relative to thereference material) and precision (replicate injections) werewithin 6 1 mmol L21.

Dissolved black carbon (DBC)—For the determinationof black carbon, a wide range of different analytical methodscurrently exist, each addressing a different spectrum of theblack carbon continuum (Hammes et al. 2007). In this study,DBC was determined at the molecular level via thebenzenepolycarboxylic acid (BPCA) method (Dittmar2008). This method is the most sensitive and unambiguousmethod for the determination of black carbon in marineDOM. It was originally developed for coal and soil analyses(Glaser et al. 1998; Brodowski et al. 2005; Kawamura et al.2006) and later modified for the analysis of DOM (Dittmar

2008). This method specifically quantifies condensed aro-matic moieties, and black carbon-free biomass from algaeand vascular plants does not yield detectable amounts ofBPCAs (Dittmar 2008). Furthermore, the relative propor-tion of the eight different detectable BPCAs is indicative ofthe extent of condensation and size of the polycyclicaromates (e.g., char produced at 200uC is characterized byBPCAs with a low number of carboxylic groups, whereasbiomass charred at 1000uC mainly yields benzenehexacar-boxylic acid [Schneider et al. 2010]).

Prior to molecular analysis, DOM was isolated from thewater samples via solid-phase extraction (Dittmar et al.2008). One liter of filtered water was acidified with HCl (10molar, analytical grade) to pH 5 2 and passed via gravitythrough solid-phase cartridges (Varian Bond Elut PPL, 1 g).The cartridges were desalted with 0.01 mol L21 HCl, driedwith a stream of N2, and DOM was eluted with 6 mL ofmethanol (analytical grade). The DOC extraction efficiencywas determined for each sample after evaporating analiquot of the methanol extract to dryness, redissolving it inultrapure water, and determining the DOC concentrationin this solution. The ratio between extracted and total DOCconcentrations is considered the extraction efficiency. Onaverage among all samples (n 5 74), 65% (SE 6 1.8%) ofthe DOC could be recovered by the solid-phase extraction.

For BPCA analysis, 1 mL of the methanol extracts weretransferred into 2-mL glass ampoules, evaporated to drynessunder a N2 stream, and redissolved in 0.5 mL of HNO3

(65%). The ampoules were then sealed, placed in a stainless-steel pressure bomb, and kept for 9 h at 170uC in a furnace.After the ampoules had cooled, the nitric acid wasevaporated in a heating block at 50uC under a stream ofN2. The samples were redissolved in 250 mL of ultrapurewater, and aliquots were transferred into high-performanceliquid chromatography system (HPLC) autosampler vials.The recovery of BPCAs was complete during all analyticalsteps, including oxidation, as determined using a BPCAstandard solution (for details see Dittmar 2008). BPCAswere determined on a Shimadzu HPLC (10ADvp) equippedwith an autosampler, sample and column coolers, and aphotodiode-array light absorbance detector (RF-10AXL).BPCAs were separated on a Waters Sunfire C18 column(2.1 3 150 mm, 3.5 mm) using an aqueous-phase methanolgradient, as described by Dittmar (2008). The aqueous phaseconsisted of a tetrabutylammonium bromide solution(4 mmol L21) and sodium acetate (50 mmol L21). Theinjection volume was 4 mL. BPCAs were identified accordingto retention time and absorbance spectra (220–380 nm).Quantification was performed using the absorbance signal at240 nm and an external calibration. Benzenetricarboxylicacids (1,2,3-, 1,2,4-, 1,3,5-B3CA), benzenetetracarboxylicacids (1,2,4,5-, 1,2,3,4- and 1,2,3,5-B4CA), benzenepenta-carboxylic acid (B5CA), and benzenehexacarboxylic acid(B6CA) were determined.

The concentration of DBC (mol carbon L21) wascalculated based on the concentrations of the individualBPCAs (mol phenol L21) after Dittmar (2008): DBC 533.4 3 (B6CA + B5CA + 0.5 B4CA + 0.5 B3CA). Thisformula is based on proposed structures for DBC (Dittmar2008).

Intertidal dissolved black carbon 1173

Stable carbon isotopes—To obtain information on thesources of the extracted DOM, the stable carbon isotopecomposition of the solid-phase–extracted DOM wasdetermined. For this purpose, methanol extracts containing, 20 mg of DOC were pipetted into Zn caps and dried in anoven at 50uC. The Zn caps were then closed and analyzedon a Thermo Delta isotope-ratio mass spectrometerequipped with a high-temperature combustion unit at theNational High Magnetic Field Laboratory (Tallahassee,Florida). Stable carbon isotope ratios are expressed as d13Crelative to the PDB reference. Procedural blanks did notyield detectable amounts of carbon isotopes.

Statistical analysis—All statistical data given here arebased on a Pearson correlation analysis and geometricmean regression that were performed after confirming a

normal distribution of the data. Average values werecompared using a Student’s t-test. The software programSigmaPlot 10.0 was used for all statistical analysis.

Results

The water level showed a sinusoidal pattern with a tidalrange of , 0.6 m (Fig. 2). The current regime in the creekchannel was complex. Pronounced countercurrents wereobserved mainly before and after the second high tide, withstrong inflowing currents in the surface layer and outflow-ing currents in the subsurface layer. All detected waterproperties showed a clear tidal signature (Figs. 2, 3). Thesalinity and turbidity decreased during ebb and increasedduring flood, with highest values at high tide and lowestvalues at low tide. Salinity ranged between 6.8 and 21.5 in

Fig. 2. Time series of in situ measurements in the tidal creek. All sensors were located close to the middle of the creek. (A) Thecurrent profile was obtained with help of an acoustic Doppler velocimeter (ADV). White triangles on top indicate time points of ADVdeployment. Red colors indicate water currents toward Apalachicola Bay, and blue colors indicate water currents from the Bay into themarsh. The maximum deployment depth of the ADV was 60 cm above ground, a depth similar to where the other in situ sensors wereanchored. (B) Chromophoric dissolved organic matter (CDOM) data are color-coded according to the flow direction obtained from themanual measurements at the sampling points: blue indicates incoming tide; red indicates outflowing tide; and purple showsstagnating tide.

1174 Dittmar et al.

the creek. DOC, CDOM, and DBC showed an oppositetrend than salinity and exhibited the highest concentrationsat low tide. DOC concentrations ranged between229 mmol L21 and 608 mmol L21, and the concentrationof DBC ranged between 7 mmol L21 and 41 mmol L21 inthe tidal creek (Fig. 3). The majority of the waterbody ofthe tidal creek was stratified with respect to all of theaforementioned parameters. The surface layer was gener-ally characterized by lower salinity and higher concentra-tions of DOC, CDOM, and DBC compared with thesubsurface. Stable carbon isotope ratios (d13C) of DOMranged between 227.8% and 226.2% in the tidal creek.The highest concentration of Chl a (4.1 mg L21) wasobserved during the first high tide in the afternoon, whenthe waterbody was stagnating, whereas the lowest concen-trations (2.1 mg L21) occurred immediately prior to hightide in the mornings of both sampling days.

One half tidal cycle (ebb or flood) carried an average of5.8 3 105 m3 water. The flood tide carried 14% more waterthan the ebb tide, thereby resulting in a residual tidal fluxof water into the marsh of 7.8 3 104 m3. In the surfacelayer, there was a residual tidal flux into the marsh (9.8 3104 m3), whereas there was a residual tidal flux out of themarsh (22.0 3 104 m3) in the subsurface layer. In contrastto the water fluxes, the ebb tide carried 25% more DBCthan the flood tide, thereby resulting in a residual tidal fluxof 23.0 3 103 moles DBC out of the marsh. There was aconsistent residual tidal flux of DBC out of the marsh inboth the creek surface (21.2 3 103 moles) and subsurfacelayers (21.8 3 103 moles). The residual tidal fluxes of DOCand salt were small in comparison (i.e., the respective fluxesof the ebb and flood cycles differed by , 10%).

The DOC concentrations in Apalachicola River and LittleSt. Marks River were 224 mmol L21 and 206 mmol L21,respectively. The concentration of DBC was 18.1 mmol L21

in the Apalachicola River and 6.6 mmol L21 in the Little St.

Marks River. Along the offshore transect, the DOCconcentrations decreased from 116 mmol L21 near shore to88 mmol L21 offshore, while the DBC concentrationsdecreased from 2.9 mmol L21 to 0.6 mmol L21. Salinityincreased along the transect from 35.4 to 36.5.

Discussion

The tidal pattern of the water properties (Figs. 2, 3)indicates the inflow of freshwater and DBC into the creekduring ebb. The studied tidal creek has no direct above-ground connection to a river, and this inflow of freshwateris therefore due to groundwater discharge into the creek.The creek is enclosed by two river arms (Little St. MarksRiver and St. Marks River) that bifurcate from the samesource connecting to the Apalachicola River (Fig. 1). Freshgroundwater in the creek is thus most likely derived fromthe nearby river arms that surround the catchment area ofthe creek and not from long-distance transport of freshgroundwater toward the creek. During its passage throughthe sediments, the fresh groundwater mixes with estuarinewaters and discharges into the creek at low tide. A simplestscenario would be a conservative behavior of waterproperties during mixing in the sediments. However, amixing model with a riverine (DOC: 215 mmol L21) andmarine endmember (95 mmol L21) did not account for theobserved concentrations of DOC in the creek (Fig. 4). Atbrackish salinity conditions in the creek, DOC concentra-tions were . 600 mmol L21. Similarly, DBC concentrationsin the riverine and marine endmembers were , 18 mmol L21,whereas concentrations of up to 41 mmol L21 occurred inthe creek. Consequently, a considerable amount of DOCand DBC was added to the groundwater during its passagethrough the sediments and before it was discharged into thecreek.

In situ production within the creek may also releaseDOC to the water column during low tide conditions,although this explanation can largely be ruled out based onin situ fluorescence measurements. CDOM and DOCconcentrations were highly significantly correlated (p ,0.001, r 5 0.92, n 5 25), and DOC concentrations could bepredicted from CDOM concentrations with help of aquadratic mean regression (DOC 5 4.80 3 CDOM; SE ofslope: 0.33). The Chl a and DOM parameters were largelyuncoupled. In contrast to DOM, Chl a was mainlycontrolled by the current velocity and time of day andwas probably transported with estuarine waters into thecreek, or washed off creek banks during high tide. Thehighest concentration of Chl a was observed during hightide in the afternoon, when the waterbody was stagnating.Most remarkably, the minimum concentrations of CDOMand DOC coincided with this Chl a maximum. Thisuncoupling between Chl a and DOM shows that DOMwas not primarily produced in the water column but wastransported from an allochthonous source into the creek.This observation is consistent with the fact that DBC andbulk DOC exhibited synchronous tidal fluctuations with amaximum at low tide.

Turbidity may have interfered with the fluorescencemeasurements of CDOM and Chl a by shading of the

Fig. 3. Time series of dissolved black carbon and bulkdissolved organic carbon in the tidal creek in addition to thewater level.

Intertidal dissolved black carbon 1175

detector cell. However, total suspended-solid concentrationswere consistently , 10 mg L21, as indicated by the level ofred-light scatter (Boss et al. 2009). At this concentration ofparticles, interference with fluorescence detection is notexpected (Saraceno et al. 2009). The turbidity readingsindicated a higher colloidal load in the inflowing estuarinewater than in the groundwater, with only minor resuspen-sion and settling of particles.

A closer analysis of the mixing pattern indicates acomplex, but apparently conservative, mixing of threeDOC sources in the tidal creek (Fig. 4). During flood, therewas a flux of freshwater into the creek in the surface layerand a flux of seawater in the subsurface layer. Bothwaterbodies had low DOC concentrations, and duringflood they were well-separated and conservatively mixed inthe creek with brackish water (high DOC concentrations).During the ebb cycle, brackish water with high DOCconcentrations mixed conservatively with the fresh andmarine water that had previously entered the creek.

Conservative properties of DOM in the system were alsoapparent on a compositional level. Independent of the time

point and depth of sampling, the bulk DOC and DBC inthe tidal creek were highly significantly (p , 0.001)correlated (Fig. 5). At high tide, 3.2% of DOC was DBC,which was the same value as in the Little St. Marks River.At low tide, DBC increased to 7.2% of the bulk DOC. Allother values can be explained as a conservative mixture oflow-tide and high-tide waters. If the regression (DOC vs.DBC; Fig. 5) is extrapolated to infinite concentrations ofDOC and DBC, then the proportion of DBC to DOC (i.e.,the reciprocal of the slope shown in Fig. 5) reaches 8.1% (60.3 SE). Therefore, it can be concluded that DOC in thebrackish groundwater that discharges into the tidal creekcontained 7.2–8.1% DBC.

In Apalachicola River, a similar proportion of DBC(8.1%) was found. Interestingly, a similar ratio betweenDOC and DBC was also determined via the BPCA methodin a river that drained a tundra ecotone in Northern Siberia(Guggenberger et al. 2008; and by using the BPCA–blackcarbon conversation equation of Dittmar 2008). Because ofthe comparatively high concentrations of DBC in the creek,Apalachicola River water must be excluded as the main

Fig. 4. Mixing diagram for dissolved organic carbon. DOC* are concentrations calculatedfrom in situ fluorescence data (details are provided in the main text). DOC* is color-coded inaccordance with Fig. 2.

1176 Dittmar et al.

source of DBC in the creek. However, similar bodies ofgroundwater (in terms of DOC and DBC concentrations)may be the source of DBC to both the river and creek. Inaccordance with this observation, the composition of DBC,as indicated by the relative proportion of the individualBPCAs, also did not exhibit significant differences between

river and creek. The relative proportion of individualBPCAs is a measure of the molecular size of the DBC. Agreater number of carboxylic groups of a given BPCAcorrespond to a larger original molecule. The predominantBPCAs were B4CAs (43%, on average), followed by B5CA(31%), B3CA (15%), and B6CA (11%). The relativeproportion of the individual BPCAs is consistent withpublished BPCA data for coastal waters (Dittmar 2008).Similar proportions of the individual BPCAs were alsofound in the water-soluble fraction of wood char that wasaged for 10 yr in a soil, whereas the water-soluble fractionof freshly produced wood char contained mainly B3CA andsmall amounts of B4CA (Abiven et al. 2011).

The stable carbon isotope composition of DOM hasproven useful for the elucidation of the sources of organicmatter in coastal areas (Dittmar et al. 2009). In the studiedsystem, there are four major sources of organic matter thathave distinct isotopic signatures (the following referencevalues are from Dittmar et al. 2009). The catchment area ofthe Apalachicola River is dominated by plants with theC3 photosynthetic mechanism, and thus d13C values of, 228% are expected for riverine DOM. The Little St.Marks River and the tidal creek drain freshwater marshes(d13C < 214%) and salt marshes (d13C < 228%). In situproduction in Apalachicola Bay and the coastal zone isdominated by algae (d13C < 221%, or lower, dependingon the isotopic value of the inorganic carbon source) andsea grasses (d13C < 211%). The measured stable carbonisotope ratios, in combination with DBC concentrations,indicate the mixing of three endmembers in the tidal creek(Fig. 6): (1) DOM derived from salt marshes; (2) riverineDOM; and (3) marine DOM. Water from the marshes hadthe highest concentrations of DBC, whereas the river andmarine endmember waters had relatively low black carbonconcentrations. These observations confirm the resultsfrom the DOC mixing-model approach where salinity wasused as a conservative tracer (Fig. 4). Despite the presenceof C4 vegetation in freshwater marshes (Spartina grasses) inthe catchment of the Little St. Marks River and the tidalcreek, C4 plants did not have a significant imprint on theisotopic composition of DOM. We estimated the marineDOM component for each sample using the d13C values ofLittle St. Marks River (227.9%) and the marine samples(222.4%) as endmembers. The contributions of marineDOC to the total DOC were 7–16% and 11–31% in thecreek surface and subsurface layers, respectively, which isconsistent with the observed salinity stratification.

DBC may survive offshore transport and be distributedover large scales, as indicated by a significant correlationbetween salinity and DBC for the Gulf of Mexico samples(p , 0.001, n 5 10). Here, surface and subsurface con-centrations were not statistically different. For the same setof samples, bulk DOC and salinity were only weaklycorrelated (p , 0.05), thereby indicating a more dynamicbehavior of bulk DOC compared with DBC. The theoreticalfreshwater endmember in the Gulf of Mexico was calculatedbased on a linear regression analysis between DBC andsalinity. The theoretical freshwater endmember contained .38 mmol L21 of DBC with a probability of p . 0.95. Theconcentration of DBC in the Apalachicola River, which is by

Fig. 6. Stable carbon isotope ratios of dissolved organiccarbon vs. concentrations of dissolved black carbon.

Fig. 5. Dissolved black carbon vs. bulk dissolved organiccarbon. The regression data are derived from geometric meanregression analyses. Only samples from the tidal creek (black dots)were considered for the regression.

Intertidal dissolved black carbon 1177

Table

1.

Sa

lin

ity

,co

nce

ntr

ati

on

so

fd

isso

lved

org

an

icca

rbo

n(D

OC

),a

nd

dis

solv

edb

lack

carb

on

(DB

C),

d1

3C

of

DO

C,

an

dco

nce

ntr

ati

on

so

fin

div

idu

al

ben

zen

epo

lyca

rbo

xy

lic

aci

ds

(BP

CA

s).

‘—’:

no

da

taa

va

ila

ble

;‘s

fc’:

surf

ace

sam

pli

ng

;‘b

tm’:

bo

tto

m(s

ub

surf

ace

)sa

mp

lin

g.

Sta

tio

nD

epth

Tim

e(h

)S

ali

nit

yD

OC

(mm

ol

CL

21)

DB

C(m

mo

lC

L2

1)

d1

3C

(%)

1,2

,3-B

3C

A1

,2,4

-B

3C

A1

,2,4

,5-

B4

CA

1,2

,3,5

-B

4C

A1

,2,3

,4-

B4

CA

B5

CA

B6

CA

(nm

ol

ph

eno

lsL

21)

Tid

al

cree

ksf

c1

4:0

01

1.2

41

62

02

27

.15

27

92

03

11

53

72

51

92

Tid

al

cree

ksf

c1

5:0

01

1.0

36

92

02

27

.12

46

61

96

11

24

02

72

10

5T

ida

lcr

eek

sfc

16

:00

11

.33

66

24

—5

08

22

46

14

25

03

15

11

0T

ida

lcr

eek

sfc

17

:00

11

.63

26

20

22

7.1

43

74

21

61

23

42

27

09

3T

ida

lcr

eek

sfc

18

:00

11

.83

42

22

22

7.1

40

73

22

71

34

46

29

21

00

Tid

al

cree

ksf

c1

9:0

01

2.9

36

52

22

27

.04

47

32

30

13

54

72

95

10

3T

ida

lcr

eek

sfc

20

:00

12

.84

39

25

22

7.2

56

87

25

71

51

51

32

11

15

Tid

al

cree

ksf

c2

1:0

01

2.3

50

22

82

27

.31

49

02

92

17

95

83

83

14

2T

ida

lcr

eek

sfc

22

:00

11

.85

23

34

22

7.4

23

10

33

47

20

57

14

68

17

9T

ida

lcr

eek

sfc

23

:00

11

.05

62

40

—2

81

15

39

82

34

83

56

12

18

Tid

al

cree

ksf

c0

:00

10

.95

73

32

—3

11

24

36

44

97

14

68

17

3T

ida

lcr

eek

sfc

1:0

01

0.8

58

53

92

27

.43

11

19

39

71

83

80

56

02

13

Tid

al

cree

ksf

c2

:00

10

.86

08

——

——

——

——

—T

ida

lcr

eek

sfc

3:0

01

0.8

57

83

52

27

.53

01

23

38

34

97

75

20

19

4T

ida

lcr

eek

sfc

4:0

01

0.5

59

4—

——

——

——

——

Tid

al

cree

ksf

c5

:00

10

.95

87

41

22

7.4

29

11

74

06

23

78

35

78

22

4T

ida

lcr

eek

sfc

6:0

01

0.4

54

63

1—

19

10

33

19

16

26

94

27

15

7T

ida

lcr

eek

sfc

7:0

09

.53

22

14

22

7.1

41

61

16

09

13

61

73

51

Tid

al

cree

ksf

c8

:00

9.1

29

11

62

27

.23

16

11

72

10

23

72

06

63

Tid

al

cree

ksf

c9

:00

7.7

25

11

02

27

.43

24

81

25

66

29

12

53

1T

ida

lcr

eek

sfc

10

:00

6.8

23

97

22

7.4

27

40

90

48

22

84

22

Tid

al

cree

ksf

c1

1:0

06

.92

41

10

22

7.4

34

48

10

96

12

71

17

33

Tid

al

cree

ksf

c1

2:0

07

.12

57

12

22

7.4

35

54

13

57

33

31

50

44

Tid

al

cree

ksf

c1

3:0

08

.23

15

14

22

7.3

45

66

15

99

43

41

80

53

Tid

al

cree

ksf

c1

4:0

09

.33

55

—2

27

.8—

——

——

——

Tid

al

cree

kb

tm1

4:0

02

0.3

30

11

22

26

.62

83

61

29

65

11

16

45

7T

ida

lcr

eek

btm

15

:00

21

.02

68

13

22

6.3

10

51

11

66

71

21

81

71

Tid

al

cree

kb

tm1

6:0

02

1.2

27

01

32

26

.33

35

71

23

65

14

17

76

0T

ida

lcr

eek

btm

17

:00

21

.02

39

12

22

6.2

31

43

13

06

41

41

68

58

Tid

al

cree

kb

tm1

8:0

02

0.1

22

91

12

26

.22

63

81

18

61

12

15

85

5T

ida

lcr

eek

btm

19

:00

17

.82

75

17

22

6.8

32

47

17

19

81

52

36

86

Tid

al

cree

kb

tm2

0:0

01

3.7

41

42

82

27

.35

28

62

84

17

42

13

87

13

5T

ida

lcr

eek

btm

21

:00

12

.94

67

30

22

7.2

44

91

29

31

84

21

41

91

61

Tid

al

cree

kb

tm2

2:0

01

3.7

41

42

22

27

.24

55

72

34

14

61

73

04

10

8T

ida

lcr

eek

btm

23

:00

14

.93

65

21

22

7.0

39

70

22

01

39

15

29

11

06

Tid

al

cree

kb

tm0

:00

14

.43

62

20

22

6.9

35

66

20

01

30

14

27

59

5T

ida

lcr

eek

btm

1:0

01

3.8

36

12

12

26

.93

86

92

11

13

21

62

85

10

5T

ida

lcr

eek

btm

2:0

01

3.5

39

2—

——

——

——

——

Tid

al

cree

kb

tm3

:00

13

.43

76

—2

27

.3—

——

——

——

Tid

al

cree

kb

tm4

:00

13

.83

74

——

——

——

——

—T

ida

lcr

eek

btm

5:0

01

3.5

37

22

22

27

.14

36

22

15

13

52

03

12

11

3T

ida

lcr

eek

btm

6:0

01

3.4

36

9—

——

——

——

——

Tid

al

cree

kb

tm7

:00

14

.73

04

12

22

6.6

26

40

11

37

51

11

61

54

Tid

al

cree

kb

tm8

:00

15

.03

86

17

22

6.9

36

62

16

21

09

15

22

77

6T

ida

lcr

eek

btm

9:0

01

3.4

39

91

92

27

.03

56

11

79

12

11

52

63

87

1178 Dittmar et al.

far the largest river in the region, was only 18.1 mmol L21.The reasons behind this discrepancy remain speculative.One explanation would be the presence of a major brackishwater source of DBC in the region. The region east of thesampling area, Apalachee Bay, hosts one of the largest tidalmarshes in the Gulf of Mexico. These extensive marshes inApalachee Bay are subject to frequent burning (www.fws.gov/saintmarks/fire.html), and an extraordinarily largeblack carbon component has been observed in DOM fromthis bay compared with adjacent open waters (Dittmar2008). Outwelling of DBC from Apalachee Bay marshes istherefore a possible explanation for the observed pattern inthe offshore transect sampled in this study. Alternatively,the Apalachicola River may exhibit higher concentrationsof DBC during other seasons than sampled here, or DBCmay slowly degrade in the open ocean (e.g., by photochem-ical reactions), which could also cause an apparent increaseof the theoretical freshwater endmember.

For a first rough estimate of the load (discharge perdrainage area) of DBC in the Apalachicola River, weassume that our single time-point measurement is represen-tative for a full year. Given the event-driven nature of riverload, such an estimate is expected to provide an order ofmagnitude resolution. The annual average water runoff ofthe Apalachicola River is 690 L m22 yr21 (Edmiston 2008),resulting in a runoff of DBC of 1.25 3 104 mol km22 yr21.Although the margin of error of this estimate is unknown, itillustrates that only a fraction of the annual black carbonproduction is actually exported in the dissolved form.According to Alexis et al. (2007), one fire event in aFloridian oak scrub produces , 5.6 3 106 mol km22 ofrecalcitrant soil black carbon. If we assume a prescribedfire-return cycle of 11 yr (Alexis et al. 2007) and an area ofburning of . 20,000 km2 (estimate based on land cover;Edmiston 2008), the production rate of recalcitrant soilblack carbon is . 2 3 105 mol km22 yr21 in the catchmentof the Apalachicola River. Our estimates of the DBC runoffare two orders of magnitude higher than those determinefor a field experiment in a savannah soil (Major et al. 2010),where , 0.04% of experimentally added charcoal wasreleased as DOM in the course of 2 yr. The observed higherrunoff in Florida could be explained, first, by an increase ofthe soluble fraction of black carbon in the course ofdecadal-scale microbial degradation (Hockaday et al. 2007),and, second, by a higher precipitation and water runoff inFlorida compared with the savannah biome.

Over the studied tidal cycle, the runoff from the marshsystem (residual tidal flux per drainage area) greatlyexceeded the runoff from the riverine drainage area. Overthe tidal cycle, we determined a residual tidal flux of DBCof 23.0 3 103 moles out of the marsh. This residual tidalflux is the consequence of a considerable tidal asymmetrybecause the ebb cycle carried 25% more DBC than theflood cycle. This imbalance is not the consequence ofresidual tidal fluxes of water, because contrary to DBC,there was more water flowing into than out of the marsh.A residual tidal flux of water is commonly observed insingle-tidal cycles but when the residual tidal water fluxesare averaged over a larger number of tidal cycles, thisasymmetry disappears. The tidal creek drains an area of

Sta

tio

nD

epth

Tim

e(h

)S

ali

nit

yD

OC

(mm

ol

CL

21)

DB

C(m

mo

lC

L2

1)

d1

3C

(%)

1,2

,3-B

3C

A1

,2,4

-B

3C

A1

,2,4

,5-

B4

CA

1,2

,3,5

-B

4C

A1

,2,3

,4-

B4

CA

B5

CA

B6

CA

(nm

ol

ph

eno

lsL

21)

Tid

al

cree

kb

tm1

0:0

01

4.4

33

41

22

26

.42

64

41

24

80

11

17

15

5T

ida

lcr

eek

btm

11

:00

18

.22

42

82

26

.52

03

17

95

28

10

43

7T

ida

lcr

eek

btm

12

:00

16

.92

87

12

22

6.7

28

47

11

97

81

21

72

55

Tid

al

cree

kb

tm1

3:0

01

6.8

28

91

22

26

.83

14

41

17

76

11

16

15

2T

ida

lcr

eek

btm

14

:00

17

.22

96

13

22

6.7

26

43

13

08

31

11

79

57

Ap

ala

chic

ola

sfc

1.2

22

41

82

28

.03

28

02

09

66

34

25

27

9L

ittl

eS

t.M

ark

ssf

c2

.12

06

72

27

.92

33

77

84

11

57

92

2G

ulf

of

Mex

ico

1sf

c3

5.3

61

16

2.9

—1

51

53

11

84

37

10

Gu

lfo

fM

exic

o1

btm

35

.37

11

83

.9—

17

17

30

18

42

87

Gu

lfo

fM

exic

o2

sfc

36

.14

92

2.0

—8

10

22

12

42

56

Gu

lfo

fM

exic

o2

btm

36

.17

93

2.2

—9

11

24

14

42

77

Gu

lfo

fM

exic

o3

sfc

36

.28

87

1.1

—9

81

27

31

13

Gu

lfo

fM

exic

o3

btm

36

.32

87

0.8

—7

69

43

74

Gu

lfo

fM

exic

o4

sfc

36

.34

88

1.3

—8

91

59

31

43

Gu

lfo

fM

exic

o4

btm

36

.40

88

1.2

—7

81

48

31

33

Gu

lfo

fM

exic

o5

sfc

36

.42

92

0.6

22

2.5

66

74

15

2G

ulf

of

Mex

ico

5b

tm3

6.5

28

81

.32

22

.28

91

69

31

44

Table

1.

Co

nti

nu

ed.

Intertidal dissolved black carbon 1179

, 0.8 km2 upstream of the sampling station. When theresidual tidal fluxes of DBC are normalized to this area, therunoff is 23.7 3 103 moles carbon km22. This estimaterepresents a snapshot of a single tidal cycle, and may not berepresentative of long-term fluxes. Storm surges and neaptides may behave differently, and seasonal fluctuations mayalso occur. Because a spring tide situation was studied, ourestimate must be considered as an upper limit that isperhaps only surpassed by extreme storm surges. The blackcarbon production rate during fires in salt marshes isunknown. Tropical grassland fires produce , 7 3 104

moles black carbon km22 (Forbes et al. 2006). Conse-quently, it would require , 20 tidal cycles similar to the onewe studied to remove all black carbon produced during onefire event in the marsh system. Because the last reported fireevent occurred 3 yr before our study, deposition of soil-derived black carbon particles from the riverine catchmentarea during past storm surges is likely an additional sourceof black carbon in the marsh. Dissolution and subsequentlateral transport to the oceans appear to be importantremoval mechanisms for black carbon in soils. Our studyprovides a snapshot in time and space, but it clearly showsthat tidal pumping and groundwater discharge cannot beignored in assessing the continental runoff of DBC.

AcknowledgmentsWe thank the research coordinator of the Apalachicola

National Estuarine Research Reserve, L. Edmiston, his team,and M. Laschet for helpful discussions and extraordinary supportin the field. We appreciate the assistance of Y. Wang and Y. Wuwith stable isotope analysis at the National High Magnetic FieldLaboratory. We also thank the crew of the R/V Seminole (FloridaState University) for support in the field. We thank AssociateEditor P. Hernes and three anonymous reviewers for theirconstructive comments on the manuscript. This study wassupported by the National Science Foundation (Ocean Sciences726754 and Ocean Sciences 536431).

References

ABIVEN, S., P. HENGARTNER, M. P. W. SCHNEIDER, N. SINGH, AND

M. W. I. SCHMIDT. 2011. Pyrogenic carbon soluble fraction islarger and more aromatic in aged charcoal than in freshcharcoal. Soil Biol. Biochem. 43: 1615–1617, doi:10.1016/j.soilbio.2011.03.027

ALEXIS, M. A., AND OTHERS. 2007. Fire impact on C and N lossesand charcoal production in a scrub oak ecosystem. Biogeo-chemistry 82: 201–216, doi:10.1007/s10533-006-9063-1

BOSS, E., AND OTHERS. 2009. Comparison of inherent opticalproperties as a surrogate for particulate matter concentrationin coastal waters. Limnol. Oceanogr.: Methods 7: 803–810,doi:10.4319/lom.2009.7.803

BRODOWSKI, S., A. RODIONOV, L. HAUMAIER, B. GLASER, AND W.AMELUNG. 2005. Revised black carbon assessment usingbenzene polycarboxylic acids. Org. Geochem. 36: 1299–1310,doi:10.1016/j.orggeochem.2005.03.011

CHENG, C.-H., AND J. LEHMANN. 2009. Ageing of black carbonalong a temperature gradient. Chemosphere 75: 1021–1027,doi:10.1016/j.chemosphere.2009.01.045

COHEN-OFRI, I., L. WEINER, E. BOARETTO, G. MINTZ, AND S.WEINER. 2006. Modern and fossil charcoal: Aspects ofstructure and diagenesis. J. Archaeol. Sci. 33: 428–439,doi:10.1016/j.jas.2005.08.008

DICKENS, A. F., Y. GELINAS, C. A. MASIELLO, S. G. WAKEHAM,AND J. I. HEDGES. 2004. Reburial of fossil organic carbon inmarine sediments. Nature 427: 336–339, doi:10.1038/nature02299

DITTMAR, T. 2008. The molecular level determination of blackcarbon in marine dissolved organic matter. Org. Geochem.39: 396–407, doi:10.1016/j.orggeochem.2008.01.015

———, N. HERTKORN, G. KATTNER, AND R. J. LARA. 2006.Mangroves, a major source of dissolved organic carbon to theoceans. Glob. Biogeochem. Cycles 20: GB1012, 1–7,doi:10.1029/2005GB002570

———, AND B. P. KOCH. 2006. Thermogenic organic matterdissolved in the abyssal ocean. Mar. Chem. 102: 208–217,doi:10.1016/j.marchem.2006.04.003

———, ———, N. HERTKORN, AND G. KATTNER. 2008. A simpleand efficient method for the solid-phase extraction ofdissolved organic matter (SPE-DOM) from seawater. Limnol.Oceanogr.: Methods 6: 230–235, doi:10.4319/lom.2008.6.230

———, ———, AND R. JAFFE. 2009. Tools for studyingbiogeochemical connectivity among tropical coastal ecosys-tems, p. 425–456. In I. Nagelkerken [ed.], Ecologicalinteractions among tropical coastal ecosystems. Springer.

———, R. J. LARA, AND G. KATTNER. 2001. River or mangrove?Tracing major organic matter sources in tropical Braziliancoastal waters. Mar. Chem. 73: 253–271, doi:10.1016/S0304-4203(00)00110-9

———, AND J. PAENG. 2009. A heat-induced molecular signaturein marine dissolved organic matter. Nat. Geosci. 2: 175–179,doi:10.1038/ngeo440

DRUFFEL, E. R. M. 2004. Comments on the importance of blackcarbon in the global carbon cycle. Mar. Chem. 92: 197–200,doi:10.1016/j.marchem.2004.06.026

EDMISTON, L. 2008. A river meets the bay—a characterization of theApalachicola River and Bay system. Eastpoint (FL): Apala-chicola National Estuarine Research Reserve. Available fromhttp://www.dep.state.fl.us/coastal/downloads/management_plans/A_River_Meets_the_Bay.pdf

FORBES, M. S., R. J. RAISON, AND J. O. SKJEMSTAD. 2006.Formation, transformation and transport of black carbon(charcoal) in terrestrial and aquatic ecosystems. Sci. TotalEnviron. 370: 190–206, doi:10.1016/j.scitotenv.2006.06.007

GLASER, B., L. HAUMAIER, G. GUGGENBERGER, AND W. ZECH.1998. Black carbon in soils: The use of benzenecarboxylicacids as specific markers. Org. Geochem. 29: 811–819,doi:10.1016/S0146-6380(98)00194-6

GUGGENBERGER, G., AND OTHERS. 2008. Storage and mobility ofblack carbon in permafrost soils of the forest tundra ecotonein Northern Siberia. Glob. Change Biol. 14: 1367–1381,doi:10.1111/j.1365-2486.2008.01568.x

HAMMES, K., AND OTHERS. 2007. Comparison of quantificationmethods to measure fire-derived (black/elemental) carbon insoils and sediments using reference materials from soil, water,sediment and the atmosphere. Glob. Biogeochem. Cycles 21:GB3016, doi:10.1029/2006GB002914

HOCKADAY, W. C., A. M. GRANNAS, S. KIM, AND P. G. HATCHER.2006. Direct molecular evidence for the degradation andmobility of black carbon in soils from ultrahigh-resolutionmass spectral analysis of dissolved organic matter from a fire-impacted forest soil. Org. Geochem. 37: 501–510, doi:10.1016/j.orggeochem.2005.11.003

———, ———, ———, AND ———. 2007. The transformationand mobility of charcoal in a fire-impacted watershed.Geochim. Cosmochim. Acta 71: 3432–3445, doi:10.1016/j.gca.2007.02.023

1180 Dittmar et al.

KAWAMURA, K., A. OKUWAKI, T. V. VERHEYEN, AND G. J. PERYY.2006. Separation of aromatic carboxylic acids using quaternaryammonium salts on reversed-phase HPLC. 2. Application forthe analysis of Loy Yang coal oxidation products. Separ. Sci.Technol. 41: 723–732, doi:10.1080/01496390500527985

KEILUWEIT, M., P. S. NICO, M. G. JOHNSON, AND M. KLEBER. 2010.Dynamic molecular structure of plant biomass-derived blackcarbon (biochar). Environ. Sci. Technol. 44: 1247–1253,doi:10.1021/es9031419

KIM, S., L. A. KAPLAN, R. BENNER, AND P. G. HATCHER. 2004.Hydrogen-deficient molecules in natural riverine water sam-ples—evidence for the existence of black carbon in DOM. Mar.Chem. 92: 225–234, doi:10.1016/j.marchem.2004.06.042

KUO, L.-J., B. E. HERBERT, AND P. LOUCHOUARN. 2008. Canlevoglucosan be used to characterize and quantify char/charcoal black carbon in environmental media? Org. Geo-chem. 39: 1466–1478, doi:10.1016/j.orggeochem.2008.04.026

LEHMANN, J., J. GAUNT, AND M. RONDON. 2006. Bio-charsequestration in terrestrial ecosystems—a review. Mitigationand Adaptation Strategies for Global Change 11: 403–427,doi:10.1007/s11027-005-9006-5

MAJOR, J., J. LEHMANN, M. RONDON, AND C. GOODALE. 2010. Fateof soil-applied black carbon: Downward migration, leachingand soil respiration. Glob. Change Biol. 16: 1366–1379,doi:10.1111/j.1365-2486.2009.02044.x

MANNINO, A., AND H. R. HARVEY. 2004. Black carbon in estuarineand coastal dissolved organic matter. Limnol. Oceanogr. 49:735–740, doi:10.4319/lo.2004.49.3.0735

MASIELLO, C. A. 2004. New directions in black carbon organicgeochemistry. Mar. Chem. 92: 201–213, doi:10.1016/j.marchem.2004.06.043

———, AND E. R. M. DRUFFEL. 1998. Black carbon in deep-seasediments. Nature 280: 1911–1913.

MORAN, M., L. POMEROY, E. SHEPPARD, L. ATKINSON, AND R.HODSON. 1991. Distribution of terrestrially derived dissolvedorganic matter on the southeastern U.S. continental shelf.Limnol. Oceanogr. 36: 1134–1149, doi:10.4319/lo.1991.36.6.1134

SARACENO, J. F., B. A. PELLERIN, B. D. DOWNING, E. BOSS, P. A.M. BACHAND, AND B. A. BERGAMASCHI. 2009. High-frequencyin situ optical measurements during a storm event: Assessingrelationships between dissolved organic matter, sedimentconcentrations, and hydrologic processes. J. Geophys. Res.114: G00F09, 1–11, doi:10.1029/2009JG000989

SCHNEIDER, M. P. W., M. HILF, U. F. VOGT, AND M. W. I. SCHMIDT.2010. The benzene polycarboxylic acid (BPCA) pattern ofwood pyrolyzed between 200uC and 1000uC. Org. Geochem.41: 1082–1088, doi:10.1016/j.orggeochem.2010.07.001

ZIMMERMAN, A. 2010. Abiotic and microbial oxidation oflaboratory-produced black carbon (biochar). Environ. Sci.Technol. 44: 1295–1301, doi:10.1021/es903140c

Associate editor: Peter Hernes

Received: 04 November 2011Accepted: 27 February 2012

Amended: 13 April 2012

Intertidal dissolved black carbon 1181