Depositional and post-depositional controls on magnetic signals from saltmarshes on the north-west...

Depositional and post-depositional controls on magnetic signalsfrom saltmarshes on the north-west coast of Ireland

ANDREW J. WHEELER*1, FRANK OLDFIELD 2 and JULIAN D. ORFORD**School of Geosciences, The Queen's University of Belfast, Belfast BT7 1NN, Northern Ireland, UK Department of Geography, University of Liverpool, PO Box 147, Liverpool L69 3BX, UK

ABSTRACT

Magnetic measurements were carried out on samples from two saltmarshes

deposited during the last century on the north-west coast of Ireland. Based on

pro®les from cliffed saltmarsh edge, mid-marsh and supratidal locations, the

processes that affect the generation and persistence of magnetic properties in

environments were investigated. This was done to help in understanding the

geochemical processes affecting north-west Irish coast saltmarshes and to

determine the frequency and provenance of sediment-laden ¯ooding events;

hence, disentangling the importance of clastic and organic contributions to

saltmarsh growth. Magnetic properties indicate depositional and post-

depositional effects varying on both temporal and spatial scales. The

interpretations presented here suggest that both biogenic and diagenetic

processes have a strong effect on the magnetic properties recorded: in the

former case, the growth of magnetotactic bacteria is the main process; in the

latter case, it is reductive diagenesis, associated with organic matter

decomposition, that is important. The biogenic and diagenetic effects overprint

and degrade the detrital magnetic signatures respectively. This suggests that

magnetic measurements within the context of north-west Irish coast

saltmarshes can provide limited information on clastic sediment input to these

environments. Comparisons between sites on the north-west coast of Ireland

show similar magnetic properties, despite differences in deposition rates,

implying that signi®cant post-depositional alteration of magnetic properties

occurs within the ®rst 20±40 years after deposition. Magnetic properties of

north-west coast Irish saltmarshes are put into a regional context by

comparison with magnetic pro®les from similar sites.

Keywords Environmental magnetism, magnetic grain-size, magnetotactic

bacteria, organic carbon, reductive diagenesis, saltmarsh.

INTRODUCTION

Environmental magnetism provides informationrelating to the physiochemical status of ironcompounds within sediment. This has proved tobe a powerful diagnostic tool in numerous depo-

sitional environments, with applications indetermining sediment source areas (e.g. Wallinget al., 1979; Old®eld et al., 1985; Yu & Old®eld,1989, 1993), assessing the affects of post-deposi-tional processes (Karlin & Levi, 1983; Can®eld &Berner, 1987; Karlin et al., 1987), palaeoclimaticstudies (Kent, 1982; Heller & Liu, 1986; Robinson,1986; Kukla et al., 1988), pollutant studies(Scoullos et al., 1979; Old®eld & Scoullos, 1984;Old®eld & Richardson, 1990) and for the pur-poses of correlation (Walden et al., 1987; Robin-son, 1993).

Present addresses: 1Coastal Resources Centre, Univer-sity College Cork, Old Presentation Buildings, WesternRoad, Cork, Ireland. (E-mail: [email protected])2PAGES, BaÈrenplatz 2, CH-3011 Berne, Switzerland

Sedimentology (1999) 46, 545±558

Ó 1999 International Association of Sedimentologists 545

Old®eld & Yu (1994) have reported on magneticmeasurements from recent saltmarsh and mud¯atsediments from the north-eastern sector of theIrish Sea. Their results show that most samplescontain two types of magnetic mineral assem-blage. Detrital magnetic minerals, including bothmagnetite and haematite, occur in the coarse silt/®ne±medium sand grades, whereas ®ne-grained,stable single-domain magnetite dominates the®nest silt and clay grades. Unpublished high-resolution transmission electron microscopy(TEM) micrographs (Z. Gibbs, pers. comm.,1998) con®rm that these ®ne magnetite grainsare biogenic in origin and have been produced bymagnetotactic bacteria (cf. Blakemore, 1975; Pe-terson et al., 1986). Unpublished results from thesame area show that older coastal sediments,dating from the period of the mid-Holocenemarine transgression, also contain high concen-trations of iron sulphides, including the stronglyferrimagnetic mineral greigite (Snowball &Thompson, 1988). These results are all from areaswhere the saltmarsh sediments are mostly muchless organic than the ones considered in thispaper. Several studies (e.g. Anderson & Rippey,1988; Williams, 1992) show that, in highlyorganic sediments and peats, magnetite dissolu-tion can take place within, at most, a few decades.This possibility is taken into account in thepresent study.

Physical processes affecting the saltmarshes onthe west coast of Ireland have been the focus of afew studies (Guilcher & King, 1961; Orford et al.,1996; Wheeler et al., 1996, 1999; Duffy & Devoy,1999) that have highlighted the effects of arestricted sediment supply, a propensity fororganic accumulation, and the frequency andmagnitude of storm activity as signi®cant controlson deposition. These controls are a re¯ection ofthe severity of the west coast climate, beingsubject to a high wind regime and consequenthigh-energy wave environment and storm surgepotential. For instance, a yearly average of 34 and34á5 days of gale force winds (17á2±20á7 m s±1)were recorded at Belmullet and Malin Head,respectively, between 1957 and 1984 (Rohan,1986). Offshore wave data from the OceanWeather Ship LIMA suggests a high-energy envi-ronment (average monthly signi®cant waveheight » 4 m) with an increase in sea surfaceroughness from 1975 to 1988 (Carter & Draper,1988; Bacon & Carter, 1991, 1993). Associatedwith high storm frequency are high averagerainfall measurements, e.g. 1498 mm year±1 be-tween 1951 and 1980 at Glenties, inland from

Bracky Bridge (principal site of present study;Rohan, 1986). The high rainfall generates apropensity for freshwater organic peat accumula-tion, which commonly, within Irish west coastsaltmarshes, results in organic facies in mid- tohigh-saltmarsh settings intercalated with marineclastic sediments seawards, a situation also facil-itated by a limited sediment supply at many sites.Gallagher et al. (1996) have shown that the highorganic content of Irish north-west coast salt-marshes has a recognizable control on geochem-ical processes.

This paper presents the results of magneticmeasurements from two saltmarshes on the north-west coast of Ireland in order to develop ourunderstanding of depositional and post-deposi-tional processes affecting saltmarsh developmentin this region further. Additionally, this studyassesses the applicability of magnetic measure-ments in organogenic saltmarshes as a method ofdetermining the role of clastic sedimentation insaltmarsh growth. Analysis of cores taken in low-to high-marsh settings from two saltmarshes andset within a chronostratigraphic context enablesus to determine the rates of post-depositionalchange and lateral variation in processes and todraw quasi-regional conclusions.

SITE DESCRIPTIONS

Two saltmarsh sites were studied on the north-west coast of Ireland (Fig. 1), the results of whichform the basis of other studies (Gallagher, 1995;Gallagher et al., 1996; Orford et al., 1996; Wheel-er et al., 1996, 1999).

Bracky Bridge

Bracky Bridge is located 55 km north of DonegalTown, near the village of Ardara (Fig. 1). Thesaltmarsh is at the head of a narrow estuary(Loughros Beg) 7á5 km from the open sea (Fig. 1).Loughros Beg narrows from 1á6 km at the mouthto 250 m at the head. The nearest predicted meanspring and neap tidal ranges are 3á5 m and 1á5 m,respectively, with the mean high water level atspring tide of 1á70 m OD (Ordnance Datum:Malin; data for Killybegs: Admiralty, 1995). Atlow tide, the intertidal areas are exposed to revealextensive sand and mud¯ats. The Bracky River¯ows into the head of the estuary and forms theeasterly and northerly limit to the site (Fig. 2)draining a mountainous catchment. The salt-marsh is subject to infrequent grazing by cattle.

546 A. J. Wheeler et al.

Ó 1999 International Association of Sedimentologists, Sedimentology, 46, 545±558

It possesses a distinct cliffed edge and increasesin elevation landwards, from 1á50 m OD to 2á56 mOD, allowing increased tidal inundation at thesaltmarsh edge. Juncus maritima covers large

areas of the saltmarsh, with saltmarsh grasses(e.g. Puccinellia maritima) dominant at moreseaward localities. Backmarsh areas, onlyinundated by high spring tides and storm surges,are dominated by bryophytes. Floral zonation ofthe saltmarsh is indistinct as a result of the highfreshwater input to the saltmarsh in the form ofrainfall and seepage.

Subsurface data from Bracky Bridge (Fig. 3),obtained using a gauge auger, reveal a sedimen-tary sequence of clays, silts and organic deposits,overlying a basement of sand, gravel or bedrock.The sand and gravel represent both foreshore andslope-wash deposits. In the southern half of thenorth±south transect, these sands and gravels areoverlain by clays grading up into organic depos-its. These units represent the former out¯ow ofthe Bracky River. The remaining upper lithologiesare of saltmarsh origin.

Map evidence (1:10 500 from the 1835, 1906 and1952 editions) shows a change in the course of theBracky River, with a minor outfall at the south ofthe saltmarsh and the main course throughthe middle of the saltmarsh in 1835. From 1906to the present, a more northerly course is shown.The cliffed edge has shown limited readjustmentsince 1835, but no major retreat (Wheeler et al.,1996). It should be noted that cliffed saltmarsh

Fig. 1. Location map showing all sites mentioned in the text. The principal sites of the present study are BrackyBridge and Streamstown. The small inserted box shows the offshore location of OWS Lima.

Fig. 2. Detailed site map of Bracky Bridge showing thelocation of auger holes used to de®ne the lithostratig-raphy and the core locations where magnetic determi-nations were performed. Also note that the previous(pre-1900) course of the Bracky River is currently oc-cupied by the southerly tidal channel. Dense stipplingcorresponds to tidal ¯at; lighter stippling to tidalchannels.

Magnetic signals from Irish saltmarshes 547


edges do not necessarily indicate erosion. Thepresence of saltmarsh vegetation facilitates anenhanced sedimentation rate in comparison withthe adjacent tidal ¯at with a difference in elevationgenerated between the two environments.Depending on sediment supply and wave energy,the morphology of the saltmarsh edge may takethree forms; ramped, cliffed or eroding cliff. Mapevidence shows that, despite the presence of acliffed saltmarsh edge, as a result of the local waveenergy and sediment supply, there has been nosigni®cant measurable retreat of the saltmarshedge over the last 150 years.

Streamstown

Streamstown is 12 km from Sligo town near thevillage of Ballysadare on the southern side ofBallysadare Bay (Fig. 1). Ballysadare Bay is nar-rowed at its mouth by a sand spit and dune system.The nearest predicted mean spring and neap tidalranges are 3á5 m and 1á5 m, respectively, withmean high water spring tides at 2á09 m OD (data forOyster Island, Sligo Harbour: Admiralty, 1995). Atlow tide, the intertidal areas of the bay are exposedto reveal extensive tidal ¯ats. The saltmarsh ismature, existing between 1á07 m OD and 2á19 mOD, with highest elevations near the distinct

cliffed saltmarsh edge. New saltmarsh is accretingat the base of this cliff. The landward margin of thesaltmarsh is bounded by a road at the supratidallimit (Fig. 4). Juncus maritima is the predominantvegetation type, although a ¯ora more diverse thanthat at Bracky Bridge exists as a result of lack ofgrazing. Again, ¯oral zonation is not apparent.

Cross-sections from Streamstown (Fig. 5) showa saltmarsh sequence resting on coarser-grainedintertidal deposits. The saltmarsh sequence isgenerally represented by silty clay grading up intoclayey peat. Laminated sediment exists along thecliffed saltmarsh edge, where the lithologies aregenerally coarser grained (e.g. ST5 and ST3). Onthe north of the site, the laminated sediment atthe saltmarsh edge forms a `pseudo-chenier'structure, with the saltmarsh elevation up to70 cm higher than the landward saltmarsh (e.g.ST5 and STA), re¯ecting the area of maximumdeposition. Map evidence (1:10 500 from the1836 and 1913 editions) implies limited readjust-ment to the saltmarsh edge at Streamstownwithout signi®cant erosion of the northern edge.Recent embryonic saltmarsh growth seaward ofthe eastern cliffed saltmarsh edge is also appar-ent, suggesting ®lling of the embayment. Pub-lished cartographic evidence is not available after1913.

Fig. 3. North±south and east±west borehole transects revealing the lithostratigraphy at Bracky Bridge. Boreholelocations are shown in Fig. 2.



METHODS

Blocks and cores were extracted from cliffed edge,mid-marsh and backmarsh/supratidal settings

(Figs 2 and 4) and levelled to OD (Malin) usingdifferential GPS (Leica System 200) to an accura-cy of �1 cm. Reconstruction of site stratigraphieswas generated from the detailed description andcorrelation of borehole (auger) data, including anappreciation of both sediment type and sedimen-tary structures. Blocks and cores were describedusing the Troels-Smith (1955) scheme of sedi-ment description.

Dating of sections (BBA and STA) was accom-plished using excess 210Pb, 134Cs and 137Csradioisotopic determinations on samples takenat 0á5-cm, 1-cm and 2-cm intervals. The sampleswere ashed at 550 °C, and the <63-lm fractionwas sent to the Department of Physics, Universityof Bremen, Germany, where the samples weretreated and the concentrations of radioisotopes of134Cs, 137Cs, 210Pb and 214Pb determined byc-spectrometry using a Canberra 55% N-typeHigh-Purity Germanium detector. Before the mea-surements, the samples were sealed and storeduntil 226Ra was in equilibrium with its decaydaughter 214Pb. Ages were calculated from excess210Pb concentrations using the `constant initialconcentration model' (CIC model), which as-sumes a constant rate of 210Pb fallout and sedi-mentation rate (cf. Krishnaswamy et al., 1971).The `constant rate of supply model' (CRS model;Appleby & Old®eld, 1978), which allows for a

Fig. 4. Detailed site map of Streamstown showing thelocation of auger holes used to de®ne the lithostratig-raphy and the core locations where magnetic determi-nations were performed. Stippled area corresponds totidal ¯ats.

Fig. 5. North±south and east±west borehole transects revealing the lithostratigraphy at Streamstown. Boreholelocations are shown in Fig. 4.



variable sedimentation rate, was not applicable tothe STA pro®le and so was not adopted.

Additional samples were taken from BBA, BBB,BBC and STA at 0á5-cm intervals for magneticmeasurements. Samples were dried at 40 °C,disaggregated and subsamples packed into 10-mLpolystyrene sample holders before analysis formagnetic susceptibility (v), Anhysteretic reman-ent magnetization (ARM), here usually expressedas susceptibility of ARM (vARM), and `saturation'isothermal remanent magnetization (SIRM) weremeasured in a ®eld of 1 Tesla (T). Stepwise DCdemagnetization of SIRM, in reverse ®elds of±20 mT, ±40 mT, ±100 mT and ±300 mT, was alsocarried out. The DC demagnetization measure-ments allow construction of the reverse ®eld plotsin Figs 7±12. These show the demagnetization andsubsequent reverse magnetization steps as apercentage of the original SIRM. From thesereverse ®eld measurements, two further parame-ters have been de®ned: `hard' isothermal remanentmagnetization (HIRM) and `soft' isothermalremanent magnetization (IRMs). HIRM is thecomponent of SIRM that remains unreversed in aback-®eld of ±300 mT, and may be expressed bothin mass speci®c units (10±5 Am±2 kg±1) and as apercentage of SIRM. IRMs is the component ofSIRM that is lost in the low reverse ®eld of ±20 mT.

Magnetic susceptibility (v) is often used as anapproximate indicator of the changing concentra-tion of ferrimagnetic minerals such as magnetite,although it is also in¯uenced by variations inmagnetic grain size. In weakly magnetic samples,paramagnetic contributions to v may also besigni®cant. ARM, although also an expression ofmagnetic mineral concentrations, is much morestrongly in¯uenced by ferrimagnetic grain size,with peak values re¯ecting ®ne stable singledomain grains between » 0á02 and 0á05 lm (Ma-her, 1988). SIRM, a further indicator of magneticconcentrations, can include contributions fromall the remanence-carrying ferrimagnets and im-perfect antiferromagnets (haematite and goethite).`Hard' isothermal remanent magnetism (HIRM)will normally be dominated by remanence-carry-ing, imperfect antiferromagnets, such as haema-tite and goethite. `Soft' isothermal remanentmagnetism (IRMs) can be used to approximatethe concentration of remanence-carrying ferri-magnets, as it is unaffected by imperfect antifer-romagnets (cf. Thompson, 1986). A more detailedexplanation of magnetic properties is available inThompson & Old®eld (1986).

The standard procedures adopted for themeasurements and calculations can be found in

Old®eld et al. (1989) and Old®eld & Yu (1994).Susceptibility measurements were performed ona Bartington meter and dual-frequency sensor. Allremanence measurements were obtained from aportable Minispin slow-speed spinner Fluxgatemagnetometer. Anhysteretic remanences weregrown in a modi®ed Molspin AF demagnetizerusing a peak AF ®eld of 100 mT and a steady DCbias of 0á04 mT. Isothermal remanences weregrown in Molspin pulse magnetizers.

Four samples were split into the followingclasses on the basis of particle size: < 4 lm,4±16 lm, 16±32 lm, 32±63 lm, 63±125 lm, 125±250 lm and > 250 lm. All the magnetic measure-ments and calculations were repeated for thesesamples.

Percentage organic carbon data were generatedfrom loss-on-ignition at 550 °C (Dean, 1974).

RESULTS AND DISCUSSION

Chronostratigraphy

Cliffed-edge sequences from Bracky Bridge andStreamstown were dated using excess 210Pb, 137Csand 134Cs radioisotopes. Concentrations of 134Csand 137Cs in the pro®les re¯ect atmosphericnuclear fallout as a result of human activity andcan be interpreted to provide dates correspondingto the initiation of atomic weapons testing (1954),peak deposition before the test ban treaty (1963)and the 1986 Chernobyl accident (Penningtonet al., 1973; Ritchie et al., 1973; Ehlers et al.,1993). 210Pb (half-life � 22 years) is produced inthe atmosphere as a decay product of 222Rn and israpidly attached to particles in the atmosphere anddeposited on all exposed surfaces (Sugai, 1990). Inthe marine system, the isotope becomes bonded tosuspended sediment particles. By extrapolatingfrom the half-life curve of the isotope, it is possibleto produce an independent dated pro®le thatvalidates the caesium interpretations and allowsthe dating of older sediments (Sugai, 1990).

Pro®les of radioisotope concentrations are pre-sented for the two sites (Fig. 6). The peak incaesium emission in 1986 resulting from theChernobyl incident and the 1963 peak in 137Csare clearly visible in the Bracky Bridge andStreamstown pro®les. The independent datesderived from the excess 210Pb measurementscorrelate well with the 137Cs and 134Cs pro®les,although some offset is apparent, with the excess210Pb measurements producing apparentlyyounger dates.



Excess 210Pb dates for Bracky Bridge indicatethat the base of the section can be dated to1863 � 10á8 years, which is in accordance withmap evidence (see above). The average sedimen-tation rate for Bracky Bridge is 0á50 � 0á04 cmyear±1, which may be similar to sedimentationrates derived from other saltmarshes on the westcoast, e.g. `Grif®ns', Castlemaine Harbour:0á64 cm year±1; Timoleague: 0á78 cm year±1; andCorranroo: 0á28 cm year±1 (Duffy & Devoy, 1999).Assuming a constant sedimentation rate and noreworking of older material, the excess 210Pbvalues should decay logarithmically. Deviationsfrom this in the BBA pro®le are caused by avariable sedimentation rate and the possibilitythat reworked material also contributes to radio-isotopic values. This situation is apparent at1á45 m OD.

Excess 210Pb dates for Streamstown indicatethat the base of the pro®le can be dated at1955 � 16á4 years. The large error term is a resultof a diversity in excess 210Pb concentrations,suggesting a considerable degree of reworking.The steepness of a best-®t line to the excess 210Pbdata below 1á60 m OD implies rapid depositionbetween 1955 and 1962, with an average sedi-mentation rate for this part of the pro®le of7á25 � 11á53 cm year±1. This sedimentation rate isan order of magnitude higher than that recordedat other west coast Irish sites (see above),

although the large error term prohibits anymeaningful interpretation. Average sedimenta-tion rates after 1962 are 0á24 � 0á03 cm year±1

and are commensurate with the expected rate ofsedimentation and in accordance with the inde-pendent 137Cs and 134Cs pro®les.

Processes affecting magnetic properties

The three cores from the Bracky Bridge saltmarshform a series from the lower, outer edge of thesaltmarsh (BBA) to the supratidal zone (BBC).Percentage organic carbon values range from 7%to 22% in BBA (Fig. 7), 13±81% in BBB (Fig. 8)and 53±97% in BBC (Fig. 9), indicative of anincrease in organic content across the saltmarshfrom the saltmarsh edge to the supratidal limit.Comparison between the three pro®les revealsseveral similarities and some differences. Allthree pro®les have peak `magnetite' concentra-tions at or near the top, irrespective of whichconcentration indicator (v, SIRM, IRMs or vARM) isused. The origin of these peaks and the reducedconcentrations below them need to be consideredfor all three cores together.

In all the pro®les, the upper peaks in magnetiteconcentration include samples with vARM/SIRMvalues around or in excess of 1 ´ 10±3 mA±1

(cf. Maher, 1988). Such values, especially at siteswhere magnetically enhanced surface soil input

Fig. 6. Radioisotope concentrations with chronostratigraphic interpretations for Bracky Bridge and Streamstown.



is highly unlikely, point to a signi®cant contri-bution from stable single-domain (SSD) grains,suggestive of magnetotactic bacterial activity. Thesame samples are also characterized by higherremanence:susceptibility values (vARM/SIRM,SIRM/v and ARM/v) in all pro®les, ruling out asigni®cant contribution from in situ burning. Thismay have occurred at the supratidal site, although

this would have had the effect of reducing thesequotients and thus setting the record from BBCapart in this regard. Our initial inference, there-fore, is that bacterial magnetite probably forms asigni®cant component of the magnetite in thenear-surface samples from all three pro®les. Thisis consistent with the particle size-based mea-surements shown in Figs 11 and 12. Those from

Fig. 7. Downpro®le magnetic measurements for BBA. See text and Old®eld & Yu (1994) for explanation andinterpretation of magnetic parameters.

Fig. 8. Downpro®le magnetic measurements for BBB. See text and Old®eld & Yu (1994) for explanation andinterpretation of magnetic parameters.



the upper part of each pro®le, where the presenceof bacterial magnetite has been inferred (Figs 11and 12), show peak values for vARM, vARM/SIRMand vARM/v in the <4-lm fraction, which corre-sponds with the results reported by Yu & Old®eld(1993) from sites subsequently shown to be richin bacterial magnetite in the clay fractions.

In pro®les BBB and BBC, in which averageorganic carbon content exceeds 50% in the top5±10 cm, a decline in magnetite concentrationstakes place near the surface and is very steep(v, SIRM and vARM decrease by 70% in the top5 cm). It is accompanied by declines in thevARM/SIRM quotients and remanence±susceptibility

Fig. 9. Downpro®le magnetic measurements for BBC. See text and Old®eld & Yu (1994) for explanation andinterpretation of magnetic parameters.

Fig. 10. Downpro®le magnetic measurements for STA. See text and Old®eld & Yu (1994) for explanation andinterpretation of magnetic parameters.



ratios. These observations are compatible withreductive, dissolution diagenesis driven byorganic matter decomposition removing selec-tively the ®nest, probably biogenic, magneticgrains.

The decline is more gentle and takes place at alower level in BBA (decrease by 30% in the top10 cm). However, the declining vARM/SIRMquotients and remanence±susceptibility ratioscoincide with the reduction in concentrations,indicating that the same process may be inferred,although with an apparent delay, possibly relatedto the lower organic matter content of the core.The likelihood of a higher rate of accumulation atthis site may also be a contributing factor to thedifference in the pro®le. This core is the onlydated core from Bracky Bridge, from which wecan infer that the putative bacterial magnetite

begins to dissolve within some 20 years and hasmostly disappeared after 40 years.

In both BBB and, more especially, BBC, thereare lower peaks in vARM/SIRM than occur inhighly organic sediments. In the case of BBC, thevalues exceed those at the surface. Such featuresare absent from BBA. Selective, localized survivalseems unlikely, and the possibility that in situgrowth of magnetotactic bacteria is occurringmust be considered. Unfortunately, v values aretoo low at these depths to allow for reliableremanence±susceptibility ratio calculations.

In BBC, where organic carbon content is higher,a divergence between IRM-40 and IRM-10 per-centage reversal values occurs around 2á24±2á27 m OD. This type of feature re¯ects reman-ence behaviour typical of the ferrimagnetic ironsulphide, greigite. It is usually accompanied by

Fig. 11. Magnetic measurements on grain size fractions from BBA 1á40±1á50 m OD. See text and Old®eld & Yu (1994)for explanation and interpretation of magnetic parameters.

Fig. 12. Magnetic measurements on grain size fractions from BBA 1á10±1á20 m OD. See text and Old®eld & Yu (1994)for explanation and interpretation of magnetic parameters.



consistently high values (> 40) for SIRM/v but, inthe present case, as is indicated by the rapid¯uctuations in values from sample to sample, v istoo low for this quotient to be calculated con®-dently. The divergence in the reverse ®eld valuesis the only feature of its kind in any of the pro®lespresented here and, while not conclusive, sug-gests that some slight trace of authigenicallyformed greigite may be present (cf. Snowball &Thompson, 1988). Unpublished results show thatmuch less ambiguous indications of greigite arecommon in mid-Holocene coastal sediments fromnorth-west England, while similar indications ofgreigite in reverse-®eld remanence ratios havebeen noted in unpublished ombrotrophic peatpro®les, where they occur at depths indicative ofnineteenth century ages.

HIRM values are relatively high and consistentin BBA, somewhat lower and variable in BBBand, in BBC, near zero except in the top 10 cm.The particle size-based measurements (Figs 11and 12) show that the HIRM (indicating haematitein these samples) peaks in the coarse- to mid-siltsize range. Clearly, this is a potential indicator ofdetrital input to the sites, and the gradation fromlower to supratidal saltmarsh is consistent withthis proposition. In the BBA pro®le, the inferredeffects of magnetite dissolution are superimposedon a relatively constant HIRM pro®le. This meansthat, in mass speci®c terms, the `haematite'signature changes relatively little, but its propor-tion of the remanence-carrying minerals increasesdownsection over the top 20 cm. In the BBBpro®le, in the top 30 cm, there are small peaks inmass speci®c HIRM that mostly correspond withdips in the organic carbon curve. These resultssuggest that, above this level at least, the `hardremanence' component is surviving as a possibleindicator of changing detrital input. The negligi-ble HIRM values below »2á40 m OD in BBC maybe interpreted as a re¯ection of decreased marine,detrital input, the effects of advanced dissolutiondiagenesis or a combination of the two. If theformer process dominates, then the surface peakshould relate to an inundation event, althoughthere is no suggestion of this in the organic carbonpro®le. If the latter explanation applies, then thesurface peak may include a component of atmo-spheric deposition, as even peat sites remote fromindustrial activity retain surface peaks in mag-netic concentrations.

In addition to the above features, there areoccasional peaks in v and/or SIRM that are likelyto represent increased `magnetite' concentrationsin each core, which often coincide with minima

in organic matter. Notable are the spikes at 1á46 mOD in BBA, at 1á92 m OD in BBB and around2á44 m OD in BBC. Some of these peaks oftencoincide with laminae and/or the coarsest grainedsediment (Wheeler et al., 1996). In all cases, it isprobable that detrital magnetic minerals havebeen deposited in higher concentrations and,being the coarsest grains, have survived in thepro®le. The lower organic carbon values could be,at one and the same time, indications of miner-ogenic input and also the means whereby themagnetic minerals have survived moreeffectively.

The Streamstown (STA) pro®le (Fig. 10) issomewhat similar to BBA in that it is taken fromthe seaward edge of the saltmarsh and has organiccarbon values of between 2% and 20%. It sharesmany features with the Bracky Bridge pro®les,especially BBA and BBB. There is a comparabledecline in `magnetite' concentrations accompa-nied by falling vARM/SIRM and vARM/v valuesdownpro®le. HIRM values remain signi®cantthroughout the section, but there is a tendencyto lower values below » 1á30 m OD. There arenarrow peaks in magnetite concentration belowthe dissolution gradient, the most notable ofwhich, at 1á07 m OD, lies alongside a minimumin organic carbon. Persistence of some detritalmagnetic minerals throughout the pro®le is sug-gested, but this may not imply complete survival.

The age and deposition rate of the two datedpro®les are radically different: the base of BBA isdated at 1863 � 10á8 and STA at1955 � 16á4 years; the average sedimentation ratein BBA is 0á5 cm year±1, and in STA is2á7 cm year±1. This implies that, at Streamstown,reductive, dissolution diagenesis reaches com-pletion within 20 years after burial. However, thetiming of this change in magnetic properties mayalso be coincident with a signi®cant change indeposition rate (from 7á25 � 11á53 cm year±1 to0á24 � 0á03 cm year±1), suggesting a change insedimentation regime, although the large errorterm precludes any reliable inferences from thissite. These two observations may or may not beconnected.

CONCLUSIONS

The importance of organic carbon on diageneticprocesses on north-west coast saltmarshes hasalready been demonstrated in relation to heavymetal concentrations. Gallagher et al. (1996) stat-ed that, although north-west saltmarshes were



comparatively unpolluted by heavy metal con-taminants and represent an inventory for contem-porary background heavy metal concentrations, arelationship existed between heavy metal con-centration and organic carbon content. The dia-genetic controls on heavy metal content were notclear, probably because of the low concentrationsinvolved, but correlation with organic carboncontent and clay content was generally observedto be in accordance with the accepted concept ofthe binding properties of heavy metals within thesediments (De Groot et al., 1976; Fletcher et al.,1994; Williams et al., 1994). Gallagher et al.(1996) stated that the potential of north-westcoast saltmarshes to bind heavy metals (especial-ly in higher marsh areas) identi®es them aspotential pollution sinks.

Magnetic studies of recent coastal sediments inany way comparable with the present ones arerelatively rare. The results discussed above sug-gest that the magnetic properties of these sedi-ments have some features in common with therecent saltmarsh and mud¯at sediments from thenorth-east Irish Sea coast (Yu & Old®eld, 1993).These common features include the presence ofbimodal magnetic grain-size assemblages result-ing from a combination of detrital and biogenicmagnetite. On the other hand, there are somesigni®cant differences. The sites from the north-west coast of Ireland appear to have experiencedmore rapid and/or higher levels of diagenetictransformation of the magnetic mineral assem-blages, leading to declining magnetite concentra-tions downpro®le. This is thought to re¯ect thegenerally higher organic matter content of theIrish sites, as the diagenetic process is driven byorganic matter decomposition. One of the conse-quences of this is that some of the pro®les,especially those from the supratidal zone, havefeatures more characteristic of recent raised bogpeat pro®les (cf. Williams, 1992).

The in¯uence of high rainfall and freshwaterseepage in saltmarshes proves to be an importantcontrolling factor on numerous aspects of salt-marsh development in north-west Ireland. Thehigh water content in the saltmarsh system,coupled with a restricted sediment supply, leadsto a propensity for peat formation (Wheeler et al.,1996). As outlined above, it is this high organiccontent that is the dominant diagenetic controlaffecting both magnetic and heavy metal behav-iour. The in¯uence of a high freshwater contentaffects the competitive tolerance of plant speciesas expressed by poorly developed saltmarsh ¯oralzonation (Sheehy-Skef®ngton & Wymer, 1991)

and the presence of certain species in sitelocations unheard of in British and other Euro-pean sites, e.g. Juncus maritima as a pioneeringmud¯at species. Work on diatom assemblages onthe west coast of Ireland also notes the in¯uenceof fresh water on intraspeci®c competition andthe associated problems of using diatoms aspalaeosalinity indicators in this context (C. Bly-the, pers. comm., 1998). Furthermore, this workand other studies (e.g. Wheeler et al., 1999) showthat any consideration of the functioning anddevelopment of these saltmarshes must take intoaccount the strong climatic control in the form ofboth rainfall and the incidence of storms.

SUMMARY

Magnetic signatures in the saltmarsh sedimentsinvestigated in this study are related to clasticinput superimposed on the effects of biogenic,magnetotactic bacterial magnetite, the downcoredissolution of this component and the possibledissolution of even the detrital magnetic miner-als. Between-pro®le comparisons at BrackyBridge show a clear distinction in the nature ofthe detrital magnetic mineral record between theorganic, supratidal saltmarsh site and the lower,more minerogenic and more frequently inundatedsites. Further use of magnetic measurements asindicators of clastic input and potential clasticsource type would require much more completeevaluation of the origins and survival of both themagnetite and haematite components in theheavy mineral assemblages of which these detri-tal minerals form a part. There are differences inmagnetic `signature' between several apparent`spikes' of clastic input, but the evidence at thisstage does not allow for any con®dent discrimi-nation, let alone source ascription, on this basis.

Interpretations of magnetic properties fromsaltmarsh sequences on the north-west coast ofIreland shed light on depositional geochemicalprocesses. Similarities in magnetic propertiesbetween saltmarshes suggest that these conclu-sions are not of a site-speci®c nature but re¯ectquasi-regional controls.

ACKNOWLEDGEMENTS

The authors wish to thank the landowners forallowing access to the sites. Thanks go to JohnMeneely and Dr Liz Hide for their assistance inthe ®eld, and Bob Jude, who made a valuable



contribution to routine measurements. Radio-metric date measurements were provided by DrG. Kirchner, University of Bremen, for bothsaltmarsh sites. This paper is a contribution tothe EU research contract EV5V-CT94-0455. Fig-ure 1 was drafted by Michael Murphy, UniversityCollege Cork.

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Manuscript received4 27 October 1997; revision accept-ed 20 October 1998



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Depositional and post-depositional controls on magnetic signals from saltmarshes on the north-west...

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