Carbonate mound development in contrasting settings on theIrish margin

Cees van der Land a,n, Markus Eisele b, Furu Mienis a, Henk de Haas a, Dierk Hebbeln b,John J.G. Reijmer c, Tjeerd C.E. van Weering a,d

a Royal Netherlands Institute for Sea Research, Department of Marine Geology, P.O. Box 59, 1790 AB Den Burg, The Netherlandsb MARUM, Center for Marine Environmental Sciences, University of Bremen, Leobener Str., 28359 Bremen, Germanyc Faculty of Earth and Life Sciences, Department of Sedimentology, VU University Amsterdam, De Boelelaan 1085, 1081 HV, Amsterdam, The Netherlandsd Faculty of Earth and Life Sciences, Department of Paleoclimatology and Geo-morphology, VU University Amsterdam, De Boelelaan 1085,1081 HV, Amsterdam, The Netherlands

a r t i c l e i n f o

Available online 15 October 2013

Keywords:Cold-water coral moundRockall TroughPorcupine SeabightNorth-AtlanticDeep-sea coralsCarbonate contentCementation

a b s t r a c t

Cold-water coral carbonate mounds, formed by framework building cold-water corals, are found inseveral mound provinces on the Irish margin. Differences in cold-water coral mound development ratesand sediment composition between mounds at the southwest Rockall Trough margin and the GalwayMound in the Porcupine Seabight are investigated. Variations in sediment composition in the two moundprovinces are related to the local environmental conditions and sediment sources. Mound accumulationrates are possibly higher at the Galway Mound probably due to a higher influx of hemipelagic finegrained non-carbonate sediments. In both cold-water coral mound areas, mound growth has beencontinuous for the last ca 11,000 years, before this period several hiatuses and unconformities exist in themound record. The most recent unconformity can be correlated across multiple mounds and moundprovinces at the Irish margin on the basis of apparent age. On the southwest Rockall Trough margin thesehiatuses/unconformities are associated with post-depositional aragonite dissolution in, and lithificationof, certain intervals, while at Galway Mound no lithification occurs. This study revealed that the influxand types of material transported to cold-water coral mounds may have a direct impact on the carbonatemound accumulation rate and on post-depositional processes. Significantly, the Logachev Mounds on theSW Rockall Trough margin accumulate slower but, because they contain lithified layers, are lesssusceptible to erosion. This net effect may account for their larger size compared to the Belgica Mounds.

& 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Cold-water corals are widely distributed and thrive oncontinental shelves, continental margins, on seamounts in fjordsand on canyon walls around the world (Cairns, 2007; Freiwald andRoberts, 2005; Roberts et al., 2006; Roberts et al., 2009). Underspecific environmental conditions framework building cold-watercorals such as Lophelia pertusa and Madrepora oculata can formelevated structures, so called cold-water coral mounds. Vital forreef and mound growth is the presence of a dense coral frame-work, creating local low energy environments, promoting theaccumulation of hemipelagic and bedload sediment between thecoral branches (De Haas et al., 2009; Dorschel et al., 2007; Mieniset al., 2009; Roberts et al., 2006; Wheeler et al., 2005a).

Several carbonate mound provinces are reported from the Irishmargin (Dorschel et al., 2010; Wheeler et al., 2007), on both sidesof the Rockall Trough (RT) (Akhmetzhanov et al., 2003; De Haaset al., 2009; Kenyon et al., 2003; Van Weering et al., 2003a;Wheeler et al., 2005b), on the western Rockall Bank (RB)(Wienberg et al., 2008) and in the Porcupine Seabight (PS)(De Mol et al., 2002; Henriet et al., 1998; Hovland et al., 1994;Huvenne et al., 2002; Huvenne et al., 2003) (Fig. 1). Carbonatemounds in the PS and along the SW Rockall Trough (SW RT) marginoccur in a confined depth zone, where strong currents enhance thefood supply to the corals and prevent the living corals from gettingsmothered by sediment (Dorschel et al., 2007; Duineveld et al.,2007; Mienis et al., 2007; White et al., 2005). However, somesignificant differences exist between the two areas, in moundmorphology (Wheeler et al., 2007), local near-bed hydrodynamics(Huvenne et al., 2005; White and Dorschel, 2010), cold-water coralfacies distribution (De Haas et al., 2009; Dorschel et al., 2007;Rüggeberg et al., 2007) and the composition of mound sediments(Dorschel et al., 2005; Eisele et al., 2008; Mienis et al., 2009;Titschack et al., 2009). The latter is the focus of this paper.

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n Corresponding author. Current address: Newcastle University, School of CivilEngineering and Geosciences, Drummond Building, Newcastle upon Tyne NE1 7RU,UK. Tel.: þ44 191 208 6513.

E-mail address: cees.van.der.land@ncl.ac.uk (C. van der Land).

Deep-Sea Research II 99 (2014) 297–306

Most models put emphasis on variations in climate and theassociated changes in ocean circulation as the main driving factorsfor mound growth and variations in mound accumulation ratesover time (Dorschel et al., 2005; Kano et al., 2010; Kano et al.,2007; Mienis et al., 2009; Roberts et al., 2006; Rüggeberg et al.,2007; Sakai et al., 2009). Challenger Mound in the PS, which wascored to its base during IODP expedition 307, reveals a 2.6 Ma longdevelopment (Kano et al. 2007). Multiple environmental proxyrecords from the Challenger Mound sequence have highlighted theresponse of carbonate mounds over glacial–interglacial cyclesalternating between current-influenced and ice-rafted deposits(Thierens et al., 2013). However, hiatuses and post-depositionalaragonite dissolution (Frank et al., 2010) complicate the ChallengerMound sediment record. Similar post-depositional alterationsand erosion of carbonate mound sediments have been observedin shallow cores from other mounds along the Irish margin(Dorschel et al., 2005; Eisele et al., 2008; Foubert et al., 2007;Frank et al., 2005; Pirlet et al., 2010; Rüggeberg et al., 2007; Vander Land et al., 2010).

This raises the question whether the underlying processescontrolling the post-depositional alterations have a regionalor local mound specific origin. Comparative studies betweencarbonate mound provinces have the potential to differentiatebetween the effects of local and regional forcing processes onmound development. By comparing the sediment record ofGalway Mound (GM) in the Belgica Mound Province (BMP) inthe Porcupine Seabight to mound sediments from the LogachevMounds (LM) on the SW RT (Fig. 1) we aim to obtain and explainthis differentiation. At both sites mound summits are coveredby a dense live coral cover and mound growth has been contin-uous for the last 11,000 years (Eisele et al., 2008; Mienis et al.,2009; Van der Land et al., 2010). Before this period the sedimentrecords in both areas contain several hiatuses. We focus on themost recent hiatus, associated with the last glacial–interglacialtransition.

2. Regional and local setting

2.1. Logachev Mounds, SW Rockall Trough margin

Cold-water coral mounds are present along the margins of theRockall Bank which forms an elongated NE–SW trending topo-graphic high (100–300 m water depth) defined by the Hatton-Rockall Basin to the west. It is isolated from the British and Irishmainland by the Rockall Trough which reaches water depths of upto 3000 m and opens into the Porcupine Abyssal Plain to the south(Fig. 1). At the SW RT margin carbonate mounds have formedkilometres long and wide clusters up to 380 m high, which occurbetween 600 and 1000 m water depth (Mienis et al., 2006), theirsummits are confined to a depth range between 500 and 600 mwater depth (Mienis et al., 2007) and are embedded in the EasternNorth Atlantic water mass (ENAW) (Holliday et al., 2000; New andSmythe-Wright, 2001; van Aken and Becker, 1996). Mound slopeinclinations are 251 on average, their flanks are covered withpatches of living corals alternating with dead colonies, while thesummits of the mounds are characterised by a dense cold-watercoral cover (De Haas et al., 2009; Van Weering et al., 2003b).Strong currents, related to internal waves, transport fresh foodparticles to the mounds (Mienis et al., 2007).

2.2. Galway Mound, Belgica Mound Province, Porcupine Seabight

The BMP is located on the eastern slope of the PS and ischaracterised by individual mounds that rise up to 150 m abovethe seafloor (Beyer et al., 2003; De Mol et al., 2007; Wheeler et al.,2007) (Fig. 1). The hydrography of the PS is mainly influenced byENAW and Mediterranean Outflow Water (MOW). The warm andsaline ENAW constitutes the uppermost layer of the water columnreaching down to a water depth of 800 m, whereas MOW becomespredominant below this depth (Rice et al., 1991). Galway Moundis part of a mound chain situated in 780 to 900 m water depth.

Fig. 1. (A) Overview of mound areas along the Irish margin. LM¼Logachev Mounds, PM¼ Pelagia Mounds, HMP¼Hovland Mound Province, BMP¼Belgica Mound Province.(B) Multibeam map of part of the carbonate mound area at the SW RT margin. M2001 and M2003 are stations of cruises 64PE182 and 64PE251 respectively. Multibeam datacourtesy of Gerard Duineveld (NIOZ) (Cruise 64PE249). (C) Multibeam map of the Belgica Mound Province. Multibeam data from Beyer et al. (2003) via PANGAEA. Multibeamimages are three times vertical exaggerated and view to the north.

C. van der Land et al. / Deep-Sea Research II 99 (2014) 297–306298

All mounds have a dense living coral cover at their summits (DeMol et al., 2002; Foubert et al., 2005; Huvenne et al., 2005;Wheeler et al., 2005a). In contrast, mound summits in a shallowermound chain (700–800 m water depth) at the BMP are coveredwith fossil corals (Foubert et al., 2005; Wheeler et al., 2005a).Galway Mound has its base at 870 m and its top at 790 m depth,which corresponds to the depth of the boundary of the MOW andthe overlying ENAW (Dorschel et al., 2007; Rice et al., 1991). Themound is 1 km wide and 2 km long, with the longest axisperpendicular to the dip of the slope (De Mol et al., 2002). Themound summit, where highest bottom current speeds were found,accommodates the highest density of living coral colonies(Dorschel et al., 2007).

3. Material and methods

3.1. Collection of material

Cores M2001-28 and M2001-43 were recovered during R.V.Pelagia cruise 64PE182 (De Stigter and De Haas, 2001). Theyoriginate from two elongated mound clusters (LM) oriented withtheir longest axis perpendicular to the slope of the SW RT margin.Core M2003-23 comes from an elongated slope-parallel moundcluster and was obtained on the SW RT margin during cruise64PE251 (De Haas and Mienis, 2003) (Fig. 1B).

During R/V Meteor cruise M61/3 in June 2004 cores GeoB 9213,GeoB 9214 and GeoB 9223 were retrieved from the summit andflanks of GM (Ratmeyer and Cruise Participants, 2006) (Fig. 1C).To keep the coral structure intact, cores were cut with a diamondsaw while frozen. The split cores were described, one half wasused for subsequent sampling and the other half was used for non-destructive measurements.

3.2. Age dating

Details on the method for AMS 14C age determinations for coreM2003-23 can be found in Mienis et al. (2009) and for cores GeoB9213, GeoB 9214 and GeoB 9223 in Eisele et al. (2008) (Table 1).Additional 230Th/U coral ages from six locations in the GM coreswere determined using a Thermo Fisher Scientific X seriesII ICP-QMS CCT installed at the Laboratory for Climate and EnvironmentSciences, Gif-sur-Yvette, France and follow the method fromDouville et al. (2010).

3.3. Calcium carbonate and organic carbon

The relative contribution of carbonate, non-carbonate andorganic material to the mound sediments was quantified for coresM2003-23, GeoB 9214 and GeoB 9223. For core M2003-23 thetotal CaCO3 content was determined by adding the weights of theCaCO3 material for all the analysed sieve fractions divided by thetotal sample weight, see Mienis et al. (2009) for details.

In cores GeoB 9214 and 9223 the total carbon, organic carbon andCaCO3 content were determined with a Heraeus CHN-O rapidelement analyser (Müller et al., 1994) at 5 cm sampling intervalsfrom matrix samples obtained with a syringe. Although total organiccarbon percentages were not determined on the same fractions forcores M2003-23 and GeoB 9214, down core patterns are comparable.

3.4. X-ray fluorescence core scanning and calibration

Following the approach of Van der Land et al. (2011) down coreelement intensities of calcium (Ca), titanium (Ti), iron (Fe) andstrontium (Sr) for cores GeoB 9213, GeoB 9214 and GeoB 9223were determined with the NIOZ-CORTEX XRF core scanner (Jansenet al., 1998; Richter et al., 2006). Element intensities are reportedin counts per second (cps) and converted to log-ratios of inten-sities for iron–calcium, titanium–calcium and strontium–calcium.Down core variability of the relative iron content is shown as a 5-point running average of the log ratio of Fe/Ca.

The carbonate content in weight percentage of total sampleweight (CaCO3%, see Section 3.3) was converted to a log ratiobetween carbonate and non-carbonate components (log(CaCO3%/non-CaCO3%)) and compared with the log(Ca/Ti) ratio at the samedepth for cores M2003-23 (n¼41, R2¼0.6973), GeoB 9214 (n¼70,R2¼0.6880) and GeoB 9223 (n¼81, R2¼0.5612) (Fig. 2A and B).Subsequently, this correlation was used to convert high-resolutionlog(Ca/Ti) records to high-resolution CaCO3% records. This calibra-tion assumes that variations in the amount of non-carbonatematerial in the mound sediments are proportionally related tovariations in XRF intensities of the Ca/Ti ratio.

A 5-point running average of the log ratio of Sr/Ca was comparedwith the relative aragonite content at the same core depth, obtainedat a 10 cm resolution (higher across lithostratigraphic transitions)for the LM cores by XRD (see Van der Land et al., 2011 for details)and is shown for core M2003-23 (n¼47, R2¼0.7432) (Fig. 2C andD). This correlation between log(Sr/Ca) and log Ara%/non-Ara% wasused to convert high-resolution log(Sr/Ca) records to high-resolution records of aragonite content.

4. Results

4.1. Down core mound sediment records at the SW RT margin

4.1.1. Sediment compositionPiston cores obtained from the summits of coral topped carbonate

mounds at the SW RT margin displayed an alternation betweenunlithified and lithified intervals (Fig. 3). Unlithified sedimentscontained cold-water coral fragments of mainly L. pertusa andM. oculata embedded in a fine grained matrix of which the micriticfraction mainly consisted of coccoliths. Other bioclasts in the matrixwere planktonic and benthic foraminifera, ostracods, brachiopods,bivalves, gastropods and echinoids in variable amounts, with a minorcontribution of sponges (only in the Holocene sections). In somelayers of unlithified sediments, benthic and planktonic foraminiferawere the dominant constituents (Mienis et al., 2009), prevalentforaminifera species were Globigerina bulloides, Globigerina glutinata,Neogloboquadrina pachyderma and Turborotalia quinqueloba. In thelithified sediments, coccoliths displayed thickening of their skeletalelements by calcite precipitation; this calcite cementation lithifiedthe sediment and resulted in reduced porosity.

The average log iron–calcium ratios are comparable for al SW RTcores with average values of �1.9 for core M2003-23, �1.8 in coreM2001-43 and �1.7 in core M2001-28 (Fig. 3). The widest rangeof values is found in core M2003-23 with a minimum log(Fe/Ca) ratioof �2.6 at 340 cm and a maximum ratio of �1.4 at 85 cm. �2.6 isalso the lowest ratio found among the three cores, the highest ratio of

Table 1Details of the U/Th age determinations carried out on three sediment cores fromthe Galway Mound. Fig. 3 displays these ages and previously reported ages fromMienis et al. (2009), Van der Land et al. (2010) and Eisele et al. (2008).

Core Depth (cmbs) Analysed fraction 230Th/U age (years)

GeoB 9213 377.5 Argonitic coral 223,00076200GeoB 9213 385 Argonitic coral 276,00075700GeoB 9214 270 Argonitic coral 10,4007100GeoB 9214 281 Argonitic coral 129,00071300GeoB 9223 427 Argonitic coral 10,00071110GeoB 9223 448 Argonitic coral 126,0007910

C. van der Land et al. / Deep-Sea Research II 99 (2014) 297–306 299

�1.05 is found in core M2001-28 at 186 cm. In all three SW RT coresthe uppermost lithified interval contains lower iron concentrationcompared to the overlying unlithified interval. However, in coreM2003-23 the lowest iron concentrations are associated withunlithified intervals further down core.

4.1.2. MineralogyTernary plots of high-resolution XRF core scanning calibrated

against discrete low resolution samples are shown in Fig. 4. Table 2distinguishes the relative average contribution of non-carbonate andcarbonate material. The carbonates were further divided into therelative contributions of aragonite and non-aragonite mineralogy.The average sediment of the SW RT cores comprised 10% of non-carbonate material, 30% is material with aragonite mineralogy and60% consisted of calcite. Earlier studies (Van der Land et al. 2011)revealed the contribution of dolomite and high-Mg calcite to benegligible. Non-carbonate components were most abundant in coreM2001-28 where they comprise 35% of the sediments (Table 2).Some discrete centimetre-thick layers displayed a higher content upof to 72% of non-carbonate components (i.e. lithoclasts), which wasresolved with the high-resolution XRF data.

The largest variety occurs in the aragonite content, whichvaried from below detection limit in the lithified interval ofM2001-43 to 93% in the unlithified coral dominated sedimentsof core M2003-23. The unlithified sediments of core M2003-23contained 43% aragonite on average, for cores M2001-28 andM2001-43 this was 34%. Lithified layers contained around9% aragonite material, with highest contents in core M2001-28with an average of 36%. Holocene sediments contained 34%aragonite average, sediment of older age only 26%. Due to the

high concentration of non-carbonate components at the base ofthe Holocene section, the average weight percentage for thiscomponent was 12% in the Holocene compared to 8% for oldersediments.

4.2. Down core mound sediment records at the Galway Mound

4.2.1. Sediment compositionThe sediments recovered from the cold-water coral topped GM

were entirely composed of a cold-water coral fragments embedded inunlithified matrix sediments. Bioclasts consisted of small coral frag-ments, planktonic and benthic foraminifera, bivalves and echinoids.Coral fragments are present throughout the entire length of the cores.Large lithoclasts/dropstones were a minor component, but finegrained non-carbonate lithoclasts and clay minerals were abundant.

A colour change of the matrix sediments around 280 cm coredepth was observed with lighter sediments (greyish brown 2.5Y 5/2 following the Munsell Colour chart) below, overlain by darker(dark greyish brown 2.5Y 4/2) sediments. A comparable colourchange, marking the erosional base of the inferred Holocene (seeSection 5), was observed in core GeoB 9213 at 375 cm, in coreGeoB 9214 at 280 cm and at 430 cm depth in core GeoB 9223.

Highest average Iron content is found in core GeoB 9213 withan average iron–calcium ratio of �0.34, compared to �0.94 forcore GeoB 9214 and �0.38 for core GeoB 9223 (Fig. 3). Lowest ironcontent is found at 384 cm in core GeoB 9214 with a ratio of �1.4,the highest ratio of �0.03 is at 145 cm in core GeoB 9213. Theaverage iron content of the Holocene sections of the GM cores ishigher compared to older sediments. However, all cores displaya decline in iron–calcium ratios in the upper metre, indicative ofa lower iron content.

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Fig. 2. (A) Correlation between the log ratio of Ca and Ti (log Ca/Ti) and the log of the ratio between CaCO3 content and non-CaCO3 (log(CaCO3%/non-CaCO3%)) for samplesfrom the same depth in core GeoB 9214. (B) Down-core variation of log(Ca/Ti) (triangles) and log(CaCO3%/non-CaCO3%) (grey diamonds) for core GeoB 9214. Note similaritiesin down core pattern for both datasets. (C) Correlation between the log ratio of Sr and Ca (log(Sr/Ca)) and the log of the ratio between aragonite carbonate content and non-aragonite carbonate content (log(Ara%/non-Ara%)) for samples from the same depth in core M2003-23. (D) Down-core variation of log(Sr/Ca) (triangles) and log(Ara%/non-Ara%) (grey diamonds) for core M2003-23. Note similarities in pattern for both datasets.

C. van der Land et al. / Deep-Sea Research II 99 (2014) 297–306300

4.2.2. MineralogyCaCO3 and XRD measurements combined with high-resolution

XRF scanning reveal that the GM cores contained on average about60% non-carbonate material, 27% of aragonite mineralogy and 13%with calcite mineralogy (Table 2, Fig. 3). Compositional changes weremainly caused by shifts between the proportions of non-carbonatecomponents and aragonite carbonates, whereas the amount ofcalcitic carbonates varied the least. The largest variationwas observedin core GeoB 9214 with non-carbonate components varying between13 and 73 wt%. The average weight percentage of carbonates witharagonite mineralogy was 24% for core GeoB 9213, 33% for core GeoB9214 and 23% for core GeoB 9223. The major difference betweenHolocene and pre-Holocene sediments was a shift from 51% non-carbonate components and 34% aragonite carbonates in the oldersediments to 64% and 24% in the younger sediments, respectively.

5. Discussion

5.1. Correlation of the down core records

For all three cores from the SW RT, coral skeletons at the base ofthe uppermost-unlithified dated by the 230Th/U method all showages around 11,000 years (Fig. 3). Combined with on-going recentsedimentation at the mound tops (De Haas et al., 2009), this

implies average mound growth rates of 11–14.4 cm kyr�1 for thisperiod (Table 2) (Mienis et al., 2009; Van der Land et al. 2010).Below the uppermost lithified interval three coral ages from coreM2003-23 of 43.570.1 kyr (169 cm, dated with 14 C AMS),24675 kyr (226 cm) and 363712 kyr (317 cm) indicated lowsedimentation rates or periods of extensive non-deposition orerosion during the late Pleistocene (Fig. 3).

Due to the presence of corals over the entire core length,a more detailed age model, with some of the U/Th ages previouslyreported by Eisele et al. (2008), could be obtained for the GM cores(GeoB 9213, GeoB 9214, GeoB 9223) compared to the SW RT cores(Fig. 3). Eisele et al. (2008) identified hiatuses in each core,determined from shifts in the δ18O signal and the Ca/(CaþFe)ratios, at 278 cm in core GeoB 9214 and at 375 cm core depth incores GeoB 9213 and GeoB 9223. Additional sampling for U/Thdating focussed on corals directly above and below this hiatus(Table 1). Above this hiatus, at 278 cm in core GeoB 9214 and at375 cm core depth in cores GeoB 9213 and GeoB 9223, thesediments are inferred as Holocene age. Combined with theobservation of a live coral cover on top of the mound (Dorschelet al., 2007), this implied an average mound growth rate of26.0 cm kyr-1 over the last 10 kyr for core GeoB 9214 (Table 2).If we assume sediments of similar age just above the hiatus incores GeoB 9213 and GeoB 9223, this results in Holocene moundgrowth rates of around 36 cm kyr-1. The coral ages obtained from

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Fig. 3. Five-point moving average of iron–calcium log ratios and stratigraphic profiles of SW RT margin mounds and Galway Mound cores. *¼AMS 14C age determination,other ages obtained by U-series dating.

C. van der Land et al. / Deep-Sea Research II 99 (2014) 297–306 301

the sediments underneath the hiatus indicate low sedimentationrates or periods of extensive non-deposition or erosion before thistime interval. For core GeoB 9214 corals just below the base of theHolocene were dated as 129.0 712.7 kyr and as 100.071.1 kyr forcore GeoB 9223; however, in core GeoB 9213 coral ages were223.0 76.2 just below the Holocene deposits. Furthermore, coralages from deeper core depth in the GM cores (Fig. 3) suggest thatmore hiatuses exist in the pre-Holocene deposits as also proposedfor the nearby Challenger mound (Kano et al., 2007).

5.2. Comparison of the Holocene records

The Holocene sediments at the Galway Mound and LogachevMounds consist of a cold-water coral framework embedded inunlithified fine-grained sediments. The average thickness of Holo-cene deposits at the GM is 364 cm versus 147 cm at the SW RTmounds. The ages of corals at the base of the Holocene are around11 kyr at both sites, which results in average mound growth ratesof �33 cm kyr�1 at GM and �13 cm kyr�1 at the SW RT moundsfor the Holocene, assuming mound growth starts at the verybeginning of the Holocene. The difference in mound growth ratescan be explained by the difference in input of non-carbonatesediments or the density of the coral cover, essential in promotingsediment deposition and preservation. This pattern was also

observed in cores from other mounds at the Irish margin(Dorschel et al., 2005; Foubert et al., 2007; Frank et al., 2009;Pirlet et al., 2010; Rüggeberg et al., 2007). At GM, approximately60% of the sediment (by weight) consists of non-carbonatecomponents, compared to only 2–20% for most of the sedimentsat the SW RT (Table 2, Fig. 4). As on-mound production andbiological production of non-carbonate components are minor atthe carbonate mounds at both locations (Dorschel et al., 2007;Mienis et al., 2009), we infer that these non-carbonate sedimentshave a dominantly off mound origin, either hemipelagic, terrige-nous or reworked sediments with a high non-carbonate (silici-clastic) content. If we deduct this input from the accumulatedHolocene deposits, the mound growth rates from carbonate(hemipelagic and benthic) input only are comparable with�13 cm kyr�1 at GM and �12 cm kyr�1 at the SW RT mounds.

The influx of fine grained siliciclastic particles at the SW RT islimited as the Rockall Bank is cut off from continental derivedsediments by the RT (Holliday et al., 2000; Manighetti andMcCave, 1995). Input from reworked sediments from the upperpart of the Rockall Bank is minimal as they are captured in theshallower parts of the mound ridges (Mienis et al., 2007). How-ever, high values of up to 73% siliciclastic material per cm arefound in the cores of the SW RT, but the siliciclastic material is inthe form of poorly sorted gravel-sized lithoclasts (Fig. 3) up to

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Fig. 4. Ternary plots of components in cold-water coral mound cores from the SW RT margin (upper part) and Galway Mound (lower part). The three major constituents ofsediments are non-carbonate material and carbonates with an aragonite or other calcite mineralogy. Large symbols denote data from discrete samples, small symbols arefrom calibrated XRF data.

C. van der Land et al. / Deep-Sea Research II 99 (2014) 297–306302

20 mm in diameter (e.g. Fig. 6 in Van der Land et al., 2011). Theselithoclast-rich layers are often found directly above the erosionalupper boundary of lithified intervals (Mienis et al., 2009; Van derLand et al., 2010, 2011), in particular at the base of the Holocenesection just above the most recent unconformity (Fig. 3). Thesource for such distinct layers of poorly sorted gravel-sizedparticles is generally attributed to non-size-selective ice-raftingprocesses, particularly in this part of the NE Atlantic (Bond et al.,1992; Peck et al., 2007; Prins et al., 2002). Gravel sized siliciclasticparticles were likely transported to the SW RT mounds by icebergrafting, but their appearance as distinct cm-thick layers associatedwith unconformities in the sediment record suggests that post-depositional winnowing was also important.

At the GM, a large portion of the fine-grained sediments have anon-carbonate mineralogy and distinct layers enriched in gravel sizedparticles are absent (Eisele et al., 2008). In the nearby ChallengerMound, a large portion of the fine-grained siliciclastic matrix sedi-ments in interglacial deposits have a glacial siliciclastic origin (Pirletet al., 2011). These glacial sediments were deposited between thecoral framework by reworking of glacial sediment during interglacialbedload transport. A similar glacial siliciclastic origin for the fine-grained non-carbonate Holocene matrix sediments of the nearby GMis possible. Contour currents, whose sedimentary products arediscussed by Van Rooij et al. (2007), in the area are probablysignificant in transporting non-carbonate/siliciclastic material to themound (Wheeler et al., 2005a). At present, the NNE directed residualcurrent speed at the summit of the GM is 3.0 cm s�1, with amaximum current speeds of up to 16.0 cm s�1 (Dorschel et al.,2007). Current speeds measured at five mound tops at the SW RT(Mienis et al., 2007) display a narrow range from 34.9 to 35.7 m s�1,residual currents from two of the mounds are 10.4 and 9.3 m s�1.The difference in current speeds between the two areas implieswinnowing of sediments is more extensive at the SW RT and could(partly) explain the relative lower accumulation rates of thosemounds. However, it is not certain this difference in current speedsbetween the two areas also occurred in the geological past.

5.3. Pre-Holocene sediments and diagenesis

In contrast to the Holocene section, older sediments from GMand the SW RT mounds differ considerably. Pre-Holocene GMsediments contain corals throughout, whereas sediments from theSW RT have several intervals that lack aragonitic coral fragments.These coral-barren intervals are lithified by calcite precipitation oncoccoliths during diagenesis (Noé et al., 2006; Van der Land et al.,2010). No lithification or significant dissolution of aragonitic coralsis observed in the GM sediments. As demonstrated before (Pirletet al. 2011; Wehrmann et al., 2009) the non-carbonate/siliciclasticfraction impacts diagenetic processes in carbonate mound sedi-ments. In the carbonate rich mound sediments of the SW RT,organic matter oxidation in the upper sediment column leads todissolution of aragonitic components and lithification of certainintervals (Van der Land et al., 2010): A process that was alsoobserved in other settings (James et al., 2005; Melim et al., 2002;Reuning et al., 2006). Below these lithified intervals, intervalsenriched in iron contain preserved corals (Van der Land et al.,2011); the iron enrichment is reflected by higher iron–calciumratios for these intervals (Fig. 3). As inferred from the XRF recordsof log iron–calcium ratios, the GM sediments contain a largerfraction of iron compared to the SW RT cores. The average logiron–calcium ratio for the SW RT is �1.8; the average ratio at theGM is a factor 2.2 higher at �0.55. In the sediments from GM, thepresence of reactive iron from siliciclastic material probablyhinders the lowering of the carbonate saturation by bufferingthe solution, thus preserving aragonitic corals. A similar processwas observed in coral bearing sediments from the Norwegianmargin (Wehrmann et al., 2009).

The carbonate mound sediments at the SW RT and GM eachcontain non-carbonate/siliciclastic components, aragonite andcalcite in specific ratios (Fig. 4). SW RT carbonate mound sedi-ments contain a relative constant volume of non-carbonatecomponents; with the amounts of calcite and aragonite varyingproportionally. This variation is mainly driven by post-depositional

Table 2Comparison of SW RT mounds and Galway Mound characteristics. Average values, for mineral composition, between brackets.

Location

SW Rockall Trough margin mounds Galway Mound, Belgica Mound Province

M2001-43 M2001-28 M2003-23 GeoB 9214 GeoB 9213 GeoB 9223

Mound height �150 m �150 m 380 m �100 mMound top (water depth) 634 m 671 m 673 m 515 mHolocene Unlithified coral pack- to rudstone, with foraminiferal pack-

to grainstone intervalsUnlithified coral pack- to rudstone, with foraminiferal pack-to grainstone intervalsSediment composition

Grain size (wt%) o63 μm 24-56 (37) 29–68 (51) o63 μm63–150 μm 10–28 (19) 12–38 (27) 63–150 μm150μm-2 mm 9–35 (21) 6–51 (22) 4150 μm42 mm 4–46 (23)

Aragonite (wt%) 13–77 (35) 10–79 (32) 11–74 (35) 17–56 (29) 18–32 (22) 12–35 (23)Calcite (wt%) 18–79 (55) 18–75 (56) 16–77 (52) 6–26 (13) 8–16 (10) 7–24 (13)Non-carb. (wt%) 4–33 (10) 2–37 (13) 6–32 (13) 22–73 (58) 56–73 (68) 47–76 (64)Mound accumulation rate (cm kyr�1) 13.970.1 14.470.1 11.070.1 26.070.2 16.470.3 43.274.8Carbonate accumulation rate (cm kyr�1) 12.670.1 12.670.1 9.670.1 16.170.2 5.270.1 19.972.2Pre-Holocene Unlithified coral pack- to rudstone alternating with lithified

echinoid floatstoneUnlithified coral pack- to rudstone, with foraminiferal pack- tograinstone intervalsSediment composition

Grain size (wt%) o63 μm 8–70 (30) 13–77 (59) o63 μm63–150 μm 2–35 (12) 4–27 (16) 63–150 μm150 μm–2 mm 15–61 (38) 10–74 (25) 4150 μm42 mm 1–48 (20)

Aragonite (wt%) 0–9 (3) 3–80 (38) 0–94 (26) 19–57 (37) 22–34 (29) 19–57 (37)Calcite (wt%) 70–94 (90) 14–93(51) 4–97 (66) 8–33 (17) 10–15 (12) 8–33 (17)Non-carb. (wt%) 3–26 (7) 2–63 (11) 0–73 (8) 13–69 (46) 53–66(59) 13–69 (46)Mound accumulation rate (cm kyr�1) – – 0.4570.0 0.7070.1 1.270.3 –

Total minimum acc. rate (cm kyr�1)a – – 0.870.0 1.470.0 1.570.1 3.670.0

a Mound sediments accumulated since the oldest dated sediments for each core, minimum rate since multiple hiatuses exist.

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diagenesis leading to aragonite dissolution and calcite precipita-tion in certain intervals. Compared to sediments from the SW RT,the ratio between the main minerals in the sediments at theGM is relative constant. Cores display a relative constant amountof aragonite with larger variations observed in the calcite and non-carbonate content. The amount of aragonite primarily reflects theamount of aragonite coral framework in the sediment; thereforefluctuations in the amount of calcite and non-carbonate compo-nents probably reflect changes in the composition of the matrixsediments. These fluctuations in the composition of matrix sedi-ments are driven by variations in the influx of these componentsto the mound and the intensity of sediment reworking. We suggestthat fluctuations in the mineralogy of sediments at the SW RT areprimarily driven by diagenetic/secondary processes, while at theGM, comparable variations are dominated by fluctuations inprimary input of each component to the sediment (Figs. 3 and 4).

The most recent unconformity associated with a lithifiedinterval has been recovered in all SW RT cores and this horizoncan be correlated with a similar unconformity or hiatus horizon inthe GM, based on age. This hiatus or unconformity is alsorecognised in other nearby located mounds at the SW RT (Franket al., 2005), in the BMP (Frank et al., 2005; Pirlet et al., 2010) andat a mound in the Hovland Mound Province (HMP) (Dorschel et al.,2005; Rüggeberg et al., 2007) also in the Porcupine Seabight.However, below this hiatus, absolute ages are scarce. Coral agesdirectly below the hiatus in the GM are 129.0 kyr, 233.0 kyr and100.0 kyr (Fig. 3). In the Challenger Mound just below the hiatuscoral ages are 78.8 kyr and 109.2 kyr (Frank et al., 2005). Theyoungest coral age below the most recent hiatus in PropellerMound is 93.1 kyr (Rüggeberg et al., 2007). The only coral age fromthe SW RT directly below the most recent hiatus is 43.5 kyr in coreM2003-23. This age, obtained directly below a lithified interval, ismuch younger than those in cores which only contain unlithifiedsediments. We could argue that erosional processes forming theunconformity halted on the lithified sediments, leading to morepreserved carbonate mound sediments. However, the next datedcoral down core in core M2003-23 is already 246.6 kyr old, whichimplies the presence of additional hiatuses or unconformities. Asno lithified sediments are preserved between these ages, we couldargue that no lithification took place during this period, maybedue to a lower carbonate content (similar to GM) or that mounderosion was more intensive during this period, also removing(partly) lithified material. As lithification took place before andafter this period, intensified erosion seems more plausible.

Erosion of mound sediments most likely occurs during periodswith enhanced bottom currents when a coral cover was alsoabsent from the mound tops. It is generally accepted that coralgrowth at the Irish margin was retarded during glacial periods(Frank et al., 2011; Roberts et al., 2006; Sakai et al., 2009), whichimplies a glacial origin of easily erodible sediments not beingsupported by a coral framework. Dorschel et al. (2005) pinpointedto the onset of a vigorous interglacial bottom current systemeroding these unsupported sediments as the cause for the forma-tion of unconformities in the Propeller Mound sediment record inthe Hovland Mound Province. Most likely, the most recent hiatus/unconformity has been – at least partly – caused during the lastdeglaciation when bottom currents picked up again after a periodof rather sluggish bottom water circulation during the last glacial(Manighetti and McCave, 1995). Sediments from the GM display alarger age gap for the most recent hiatus, compared to moundsediment from the SW RT. The lack of lithified intervals mightmake the GM more susceptible to extensive erosion leading to alower net accumulation rate and consequently smaller mound sizecompared to the mounds at the SW RT margin. However, it couldalso be possible that GM lacked coral growth for an extensiveperiod before the Holocene.

Another factor which might limit the height of mounds in theBMP might be a fluctuation in the depth of the optimal habitat forcold-water corals. The mound chains of the BMP show conspic-uous differences concerning coral coverage and vitality, with themounds of the deeper chain (including GM) covered by a vital livecold-water coral cover at present, while the mounds of theshallower chain are covered in fossil corals (Foubert et al., 2005;Wheeler et al., 2005a) with an early Holocene age. The recentdistribution of live and fossil corals on the mound chains is linkedto the water depth and thus to the water mass structure favouringcoral growth in the deeper habitats. Small-scale changes of theregional water mass structure can cause a shift of the optimalhabitat for cold-water corals from the shallower to the deepermound chain leading to the observed distribution of fossil and livecoral covered mounds.

6. Conclusions

Differences and similarities in down-core records from twodistinct mound provinces along the Irish margin can be explainedby their proximity to the margin and their specific local environ-mental setting. Mound sediments at the SW RT margin and GMboth consist primarily of a cold-water coral framework embeddedin unlithified fine-grained sediments. Due to its position along theIrish continental margin, GM contains a larger fraction of terrige-nous sediments. In the Holocene, the only period that can beassessed explicitly, the higher sediment input resulted in highermound growth rates compared to those at mounds on the SW RTmargin.

Terrigenous sediments are present in relative high proportions(60%) throughout the entire core at GM while in the SW RTmounds such high percentages only occur in discrete centimetre-thick layers which contain mm-sized lithoclasts. Iceberg raftingtransported these coarse grained non-carbonate particles to theSW RT mounds, subsequent winnowing processes concentratedthese coarse particles in discrete layers.

Variations in the composition of the GM sediments canprimarily be explained by fluctuations in the input, both external(hemipelagic/terrigenous) and internal (density of coral cover onmound). Based on the current hydrodynamic regime, winnowingprobably also played a role. For the SW RT mounds, the maincompositional variations are caused by secondary processes, e.g.diagenesis leads to the dissolution of aragonite in certain intervalsand subsequent calcitic cementation. The shift in the carbonatesolubility equilibrium, which caused this dissolution, is prominentin the carbonate rich sediments of the SW RT, but is absent in theGM sediments. In the latter mound, the presence of reactive iron,available from the siliciclastic material probably hindered thelowering of the carbonate saturation by buffering the solution.This might have led to the well-preserved corals in the GM cores.

At both locations, the sediments deposited before the Holocenemound accumulation showmultiple hiatuses in the mound record.The most recent of these hiatuses/unconformities can be observedat both locations and other mounds along the Irish margin andprobably was formed during the last deglaciation, when glacialsediments were lacking a stabilizing coral framework and wereeroded by current intensification.

The influx and types of material transported to carbonatemounds has a direct impact on the mound accumulation rateand on post-depositional processes. A higher influx of non-carbonate components to the mounds can lead to higher moundaccumulation rates, but can also hinder the formation of lithifiedintervals. Lithification might reduce mound erosion leading tohigher and more steeply flanked mounds.

C. van der Land et al. / Deep-Sea Research II 99 (2014) 297–306304

Acknowledgments

The officers, crew and shipboard parties of the Dutch R.V.Pelagia and German R.V. Meteor are thanked for their supportduring cruise preparations and at sea. Claudia Wienberg isthanked for sharing grain size data and samples from the GMcores. Norbert Frank is acknowledged for 230Th/U coral age datingat the Laboratory for Climate and Environment Sciences, Gif-sur-Yvette. The carbonate mound studies were supported financiallyby the European Science Foundation (ESF)/Netherlands Organisa-tion for Scientific Research (NWO) under projects Moundforce,Microsystems and Carbonate. FM is supported financially by theInnovational Research Incentives Scheme of NWO (NWO-VENI).The first author was funded by NWO through NEBROC (Nether-lands-Bremen Cooperation in Oceanography) Grant 800.08.290.

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