Live foraminiferal faunas from the northern Arabian Sea

48 Pages·2013·4.79 MB·English

BiogeosciencesDiscuss.,10,15257–15304,2013 O D
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faunas from the
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northern Arabian Sea
Live foraminiferal faunas (Rose Bengal D
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c C.Caulleetal.
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stained) from the northern Arabian Sea: s
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links with bottom-water oxygenation
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1 2,3 1 2,3 1
C. Caulle , K. A. Koho , M. Mojtahid , G. J. Reichart , and F. J. Jorissen
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Conclusions References
1LPGN-BIAF,CNRSUMR6112,LaboratoryofRecentandFossilBio-Indicators,Angers D
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University,2BdLavoisier,49045AngersCedex01,France c Tables Figures
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2UtrechtUniversity,FacultyofGeosciences,DepartmentofEarthSciences,Budapestlaan4, ss
3584CDUtrecht,theNetherlands ion J I
3RoyalNetherlandInstituteforSeaResearch(RoyalNIOZ),Landsdiep4,1797SZ’tHorntje, P
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Texel,theNetherlands p J I
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Received:19August2013–Accepted:6September2013–Published:23September2013 Back Close
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Correspondenceto:C.Caulle([email protected]) D FullScreen/Esc
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PublishedbyCopernicusPublicationsonbehalfoftheEuropeanGeosciencesUnion. c
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Abstract D
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Live (Rose Bengal stained) benthic foraminifera from the Murray Ridge, within and be- s
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low the northern Arabian Sea Oxygen Minimum Zone (OMZ), were studied in order io 10,15257–15304,2013
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to determine the relationship between faunal composition, bottom-water oxygenation P
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5 (BWO), pore-water chemistry and organic matter (organic carbon and phytopigment) pe Live foraminiferal
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distribution. A series of multicores were recovered from a ten-station oxygen (BWO: faunas from the
2–78µM) and bathymetric (885–3010m depth) transect during the winter monsoon in | northern Arabian Sea
January 2009. Foraminifera were investigated from three different size fractions (63– D
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125µm, 125–150µm and >150µm). The larger foraminifera (>125µm) were strongly c C.Caulleetal.
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10 dominated by agglutinated species (e.g. Reophax spp.). In contrast, in the 63–125µm sio
fraction, calcareous taxa were more abundant, especially in the core of the OMZ, n
suggesting an opportunistic behaviour. On the basis of a Principal Component Analy- Pa TitlePage
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sis, three foraminiferal groups were identified, reflecting the environmental parameters e
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along the study transect. The faunas from the shallowest stations, in the core of the
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OMZ (BWO: 2µM), were composed of “low oxygen” species, typical of the Arabian Conclusions References
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Sea OMZ (e.g., Rotaliatinopsis semiinvoluta, Praeglobobulimina spp. , Bulimina ex- is
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ilis, Uvigerina peregrina type parva). These taxa are adapted to the very low BWO u
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conditions and to high phytodetritus supplies. The transitional group, typical for the s
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lower part of the OMZ (BWO: 5–16µM), is composed of more cosmopolitan taxa tol- n
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20 erant to low-oxygen concentrations (Globocassidulina subglobosa, Ehrenbergina trig- ap J I
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ona). Below the OMZ (BWO: 26–78µM), where food availability is more limited and r
becomes increasingly restricted to surficial sediments, more cosmopolitan calcareous Back Close
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taxa were present, such as Bulimina aculeata, Melonis barleeanus, Uvigerina pere-
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grina and Epistominella exigua. Miliolids were uniquely observed in this last group, is
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reflecting the higher BWO. At these deeper sites, the faunas exhibit a clear depth suc- u
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cession of superficial, intermediate and deep-infaunal microhabitats, because of the io
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deeper oxygen and nitrate penetration into the sediment. P InteractiveDiscussion
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1 Introduction D
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Oxygen minimum zones (OMZs) are present at intermediate water depths (250– s
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1500m) along the continental margin of the eastern Pacific, off southwest and north- io 10,15257–15304,2013
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west Africa, in the Bay of Bengal and in the northern Indian Ocean (Arabian Sea). P
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5 OMZs are defined as areas where the oxygen concentration in the water column is pe Live foraminiferal
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<22µM (Levin, 2003; Helly and Levin, 2004). Mostly, they are also areas of high pri- faunas from the
mary productivity where low-oxygen concentrations develop naturally and have per- | northern Arabian Sea
sisted over geological timescales (Reichart et al., 1998), due to enhanced organic D
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matter (OM) degradation. Where the OMZs intersect with the continental margin, the c C.Caulleetal.
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10 ocean floor is exposed to low-oxygen levels. Despite very low oxygen concentrations, sio
protozoan and metazoan assemblages thrive in these environments (e.g. Jannink et n
al.,1998;Levinetal.,2000;Levin,2003;Schumacheretal.,2007).Theselow-oxygen Pa TitlePage
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tolerant benthic assemblages can attain high densities, but have typically a low diver- e
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sity(e.g.denDulketal.,1998;Janninketal.,1998;Schumacheretal.,2007).Thepoor
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ventilation also impacts biogeochemical processes in the water column and within the Conclusions References
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sediment. This results in enhanced sequestration of organic material in the sediment. is
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Consequently, OMZs are considered as important organic carbon sinks (e.g. Cowie, u
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2005; van der Weijden et al., 1999; Koho et al., 2013). s
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The intensity of the Arabian Sea OMZ is closely related to the Indian monsoon sys- n
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20 tem(Wyrtki,1973).Duringthesummer(SW)monsoon(MaytoSeptember),anintense ap J I
coastal upwelling develops off the coast of Somalia, Oman and southwestern part of er
India. This upwelling triggers high biological productivity in the photic zone (e.g. Ry- Back Close
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ther and Menzel, 1965; Caron and Dennett, 1999; Rixen et al., 2000), leading to high
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export of organic carbon to intermediate water depths, where it is intensely recycled is
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andremineralized.Asecondhighbiologicalproductivityeventoccursduringthewinter u
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(NE) monsoon (e.g. Caron and Dennett, 1999; Rixen et al., 2000), due to convective io
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mixingofthesea-surfacewater(BanseandMcClain,1986;Madhupratapetal.,1996). P InteractiveDiscussion
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TheintensityoftheArabianSeaOMZisparticularlystrong,withoxygenconcentrations p
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<2µM in its core (Paulmier and Ruiz-Pino, 2009). This is not only due to high biologi- D
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calproductivity,butisalsorelatedtothesemi-enclosednatureofthenorthernArabian c BGD
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Sea(Wyrtki,1973;Shetyeetal.,1994),incombinationwiththerenewalofintermediate s
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waters by relatively oxygen-poor intermediate water-masses from the South and West n
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5 (Swallow, 1984; Olson et al., 1993). a
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To date, only few studies have dealt with living (Rose Bengal stained) benthic e Live foraminiferal
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foraminiferalfaunasfromtheArabianSeaOMZ.Thesestudieshavemostlyfocusedon faunas from the
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the Pakistan (Jannink et al., 1998; Maas, 2000; Erbacher and Nelskamp, 2006; Schu- northern Arabian Sea
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macher et al., 2007; Larkin and Gooday, 2009) and Oman margins (Stubbings, 1939;
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c C.Caulleetal.
10 Hermelin and Shimmield, 1990; Naidu and Malmgren, 1995; Gooday et al., 2000). In u
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addition, Kurbjeweit et al. (2000) and Heinz and Hemleben (2003, 2006) compared s
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theforaminiferaldistributionduringdifferentperiodsofthemonsoonalcycleinthecen- n
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tral, eastern and western parts of the Arabian Sea. Most of these studies described a a
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general zonation of benthic foraminiferal faunas across the OMZ. However, since geo- r Abstract Introduction
chemical measurements were generally not included in these studies, no clear, spe-
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cific correlations could be established with the controlling environmental parameters. Conclusions References
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Foraminiferal abundances and distributions are suggested to be controlled by organic is
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matter supply and oxygen content. Oxygen and organic matter content are inversely u
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correlated parameters which make their interpretation difficult. sio J I
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Here, we explore for the first time the distribution and abundance of living benthic
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foraminiferal faunas from the Murray Ridge, Northern Arabian Sea. The Murray Ridge p J I
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is a seamount with its top culminating in the core of the OMZ, and offers an ideal r
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naturallaboratoryforinvestigationofbiologicalprocessalongBWOandorganiccarbon |
gradients. Live benthic foraminifera were investigated from three size fractions (63–
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125µm, 125–150µm and >150µm). Further, foraminiferal data were correlated with is
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geochemical data obtained at the same stations from the studies of Kraal et al. (2012) s
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andKohoetal.(2013),includingBWOcontent,oxygenandnitrateporewaterprofiles, io
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phytopigmentsconcentrationsandorganiccarbon,inordertoexaminetheinfluenceof P InteractiveDiscussion
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environmental parameters on foraminiferal distribution along the study transect. p
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2 Material and methods D
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2.1 Study area s
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During the interdisciplinary PASOM cruise (Process study on the Arabian Sea Oxy- P
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gen Minimum Zone) in January 2009, a ten station transect was sampled along the p
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Murray Ridge, north Arabian Sea (Fig. 1), along a strong oxygen gradient. The cruise r
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wasdesignedtocoveratransectthroughtheOMZ(885–3010m),specificallytargeting |
northern Arabian Sea
the transition zone from low oxygen to more oxic conditions below the OMZ. All sed- D
iment samples were retrieved with a multicorer, allowing the recovery of undisturbed isc C.Caulleetal.
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surface sediment samples. The multicorer was equipped with eight cores measuring s
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6.6cm in diameter and four cores measuring 10cm in diameter. All cores collected for io
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foraminiferal (∅6.6cm) and geochemical analysis (∅6.6cm and 10cm) were retrieved P TitlePage
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fromthesamemulticorecast.Mostcoreswereslicedat0.5cmresolutiondownto2cm e
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sediment depth, at 1cm intervals between 2 and 6cm and then every 2cm intervals
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downto10cm.Onlythecoresusedforporewaterextractionandorganiccarbon(C )
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analyses were sliced differently: 0.5cm intervals in the top 2cm, 1cm intervals down D
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to 6cm and 2cm intervals down to 10cm. c Tables Figures
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2.2 Foraminiferal analyses io J I
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Onboard, samples were stained in Rose Bengal (Walton, 1952) in 95% ethanol solu- p J I
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tion (1gL ). They were gently shaken for several minutes and further stored in the
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Rose Bengal solution. In the laboratory, sediment samples were wet sieved into 150-, |
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125- and 63-µm fractions. To obtain a good staining, the samples were treated again D FullScreen/Esc
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with Rose Bengal for a minimum of 48h. All well-stained foraminifera were picked wet c
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(50%ethanol−50%water)from>150µmand125–150µmfractionsfromalllevelsun- ss Printer-friendlyVersion
til10cmdepthunderabinocularmicroscope.Foraminiferafromthesmallsizefraction ion
(63–125µm)wereonlyinvestigatedfromthefirstcentimetreofthesediment(0–0.5cm P InteractiveDiscussion
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and 0.5–1cm). e
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The use of the Rose Bengal technique is an inexpensive and easy method (Wal- D
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ton, 1952; Bernhard, 1988, 2000). However, it has been shown that especially in c BGD
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low-oxygen settings, Rose Bengal may stain protoplasm of recently dead foraminifera s
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(Corliss and Emerson, 1990; Bernhard, 2000). Therefore, to minimize the chance of n
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5 biasinourcountsoflivingforaminifera,verystrictstainingcriteriawerealwaysapplied. a
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Specimens were considered living only when all chambers expect the last one were e Live foraminiferal
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well stained. Furthermore, doubtful specimens were compared with perfectly stained faunas from the
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specimens of the same species and non-transparent agglutinated and miliolid taxa northern Arabian Sea
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werebrokentoinspecttheircontents.Fragmentsofbranchingandtubularforaminifera
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10 (e.g. Hyperammina, Rhizammina) were not included in the data analyses because of u
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their easily breakable tests, which makes it very difficult to quantify them correctly. s
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Foraminiferaltaxonomywasbasedoncommonlyusedtaxonomicreferenceworks(e.g. n
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LoeblichandTappan,1988;Jones,1994)andonsometaxonomicstudieswithempha- a
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sisonArabianSeaandanoxiczones(e.g.Mass,2000;Schumacheretal.,2007;Larkin r Abstract Introduction
and Gooday, 2009); see taxonomic appendix for more details.
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For all stations, diversity indices, including species richness (S; count of number of Conclusions References
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taxa in a sample), Shannon index (H(S)), and evenness (J) were calculated using the is
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free statistical software “PAST” (PAleontological STatistics; Version 2.14; Hammer et u
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al., 2001). io J I
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To better constrain the relationship between species variability and environmen-
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tal conditions, Principal Component Analysis (PCA) was conducted using the free p J I
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statistical software “R” (http://cran.r-project.org/). The PCA was run on geochemi- r
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cal/environmentaldataandonspeciesthatcontributedfor>2%tothetotalabundance |
of the entire database. Two PCA analyses were carried; the first one investigating the
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entire core (down to 10cm, >125µm fraction), and the second one the topcm of the is
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sediment (0–1cm, >63µm fraction). s
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2.3 Dissolved oxygen and nitrate D
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Inshort,BWOconcentrationswater-columnandbottom-waterdissolvedO concentra- s
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tionsweremeasuredwithanoxygensensor(Sea-BirdSBE43,accuracy2%;Sea-Bird io 10,15257–15304,2013
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Electronics Inc., 2011) that was built into the framework of a conductivity, temperature P
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5 anddepth(CTD)profiler(alldatapublishedinKohoetal.(2013);seeFig.2fordetails). pe Live foraminiferal
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The pore-water oxygen concentration was determined immediately following core faunas from the
recoveryinatemperaturecontrolledlaboratorysetupatinsitutemperature(somedata | northern Arabian Sea
publishedinKraaletal.,2012;seeFig.2fordetails).Allmeasurementsweredonewith D
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Unisenseoxygenmicroelectrodes(OXO100orOXO50).Themicroelectrodesweretwo c C.Caulleetal.
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10 pointcalibratedat100%O2saturatedseawater(bubbledwithoxygen)andinasolution sio
containing sodium ascorbate (0.1M) to obtain the 0% reading. n
The details of pore-water extraction for nitrate analyses are outlined in Kraal et Pa TitlePage
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al. (2012). In short, sediment slicing was carried out in a N -purged glovebox in a e
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temperature controlled laboratory set at in situ temperature (some data are published
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inKraaletal.,2012;seeFig.2fordetails).Sedimentsamplesweretransferredto50ml Conclusions References
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plastic centrifuge tubes under N2 atmosphere and centrifuged for 20min at 4500rpm is
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outsidetheglovebox.Thecentrifugetubeswerethentransferredbackintotheglovebox u
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where the supernatant pore-water was filtered over 0.45µm Teflon filters and frozen s
(−20◦C) until the analyses. Pore-water nitrate concentrations were measured by an ion J I
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20 autoanalyzer at the Royal Netherlands Institute for Sea Research (NIOZ) in Den Burg, ap J I
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2.4 Organic carbon and phytopigment analyses
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The details of organic carbon analyses are outlined in Koho et al. (2013) (all data c
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publishedinKohoetal.,2013;seeFig.2fordetails).Inshort,thesediment,fromwhich s Printer-friendlyVersion
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the pore waters were extracted, was further used for solid phase analyses and frozen n
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◦ P InteractiveDiscussion
at −20 C until analyses. In the laboratory, sediment was weighted, freeze-dried and a
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decalcified by reacting twice with 1molL−1 HCl (4h and 12h). After two subsequent e
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rinses with ultrapure water, the decalcified samples were freeze-dried and organic C D
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and N were measured with a CNS analyzer (Fisons Instruments NA 1500). c BGD
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The phytopigments were extracted from a different core than the one used for C ss
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analyses. However, the core came from the same multicore cast. The details of phy- n
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5 topigmentanalysesareoutlinedinKohoetal.(2013).Inshort,thesampleswerestored a
at −80◦C until analysed and freeze-dried prior to pigment extraction in 10mL of ace- pe Live foraminiferal
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tone:water(90:10).Thefullpigmentcompositionwasobtainedthroughapplicationof faunas from the
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high-performance liquid chromatography (HPLC) equipped with a C reverse phase northern Arabian Sea
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column at the Royal Netherlands Institute for Sea Research (NIOZ – YERSEKE).
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able compounds (DHI, Denmark). The pigment concentrations are reported per µg/g s
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3 Results
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Conclusions References
OnthebasisofBWOvalues,fourconsecutivezonescanbedistinguished:(1)thecore D
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15 of the OMZ, where BWO is around 2µM (Paulmier and Ruiz-Pino, 2009), (2) the lower cu Tables Figures
part of the OMZ with BWO <22µM (Helly and Levin, 2004; Middelburg and Levin, s
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2009), (3) the dysoxic zone where BWO is between 22 and 45µM (Bernhard and Sen ion J I
Gupta, 1999; Levin, 2003; Helly and Levin, 2004), and (4) relatively well-ventilated P
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deeper “oxic” sites where BWO is >45µM (Bernhard and Sen Gupta, 1999; Levin, e
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2003). According to these definitions, our 885m (PA1) and 1013m (PA2) stations are
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located in the core of the OMZ, stations between 1172 and 1379m (PA3, PA4 and
PA5) depths are in the lower part of the OMZ, the 1495m (PA6) station is located in D FullScreen/Esc
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the dysoxic zone, and stations from 1791m to 3010m (PA7, PA8, PA9 and PA10) are c
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in the oxic zone (Fig. 2). s Printer-friendlyVersion
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3.1 Environmental parameters D
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Details of the environmental parameters related to this study have been published in s
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Kraal et al. (2012) and Koho et al. (2013). Here, some new data together with a sum- io 10,15257–15304,2013
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mary of the existing data sets are presented in order to link them with the foraminiferal P
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Organic carbon (Corg) and chlorophyll a (Chl a) contents, for the top first centimetre, faunas from the
progressively change along the study tranect (Fig. 2; Koho et al., 2013). The high- | northern Arabian Sea
est concentrations (C =5.7wt%; Chl a=2.6µgg−1) are observed in the core of the D
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OMZ(885mand1013m).Inthedysoxic(1495m)andoxiczones(1791mto3010m), c C.Caulleetal.
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10 Corg and Chl a values are lower, <2wt% and <0.5µgg−1, respectively (Fig. 2). Min- ssio
imum concentrations (C = 0.82wt%; Chl a=0.1µgg−1) are found at the deepest n
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site(3010m).Generally,C andChlaconcentrationsdecreasewithincreasingwater a
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depth and BWO. r Abstract Introduction
The oxygen penetration depth into the sediment increases along the transect and is
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strongly related to the BWO content (r2=0.8) (Fig. 2). At the shallowest site (885m, Conclusions References
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BWO∼2µM),withintheOMZ,oxygenpenetrationisabout0.1cmwhereasatthedeep- is
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est site (3010m, BWO∼78.3µM) oxygen is available until 1.8cm depth. The nitrate u
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penetrationdepthshowsweakeryetrelativelystrongregressionwiththeBWOcontent io J I
(r2=0.4), increasing from the 1.2cm at 885m (OMZ site) to 7.5cm at 3010m (oxic n
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3.2 Foraminiferal assemblages of the 0-10cm interval (>125µm fraction) Back Close
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3.2.1 Total abundance is
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in three different size fractions: 125–150µm, >150µm and >125µm (Fig. 3a), the n
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>125µm fraction being the sum of the first two. All size fractions show a similar dis- a
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tribution of foraminiferal absolute abundance, and no clear trend is seen along the D
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transect (Fig. 3a). c BGD
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In the >125µm fraction, total live foraminiferal abundance varies from ∼90 to ∼970 ss
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ind/50cm , the two extremes being observed in the two sites from the core of the n
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5 OMZ (at 885m and 1013m, respectively). The lower OMZ stations (1172m, 1306m a
and 1379m) show values from 215ind/50cm2 to 450 ind/50cm2 , the dysoxic site pe Live foraminiferal
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(1495m) contains ∼970ind/50cm2 , whereas the oxic sites (1791m to 3010m) show faunas from the
densities varying from ∼400 ind/50cm2 to ∼870 ind/50cm2 (Fig. 3a). | northern Arabian Sea
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Agglutinated foraminifera are dominant at all stations. They always constitute more
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10 than half of the total assemblage (Fig. 3a). In the lower part of the OMZ (stations at u
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1172, 1306 and 1379m), they attain ∼70% on average. Abundance of hyaline taxa s
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varies along the transect without a clear pattern. Miliolids are only present at the oxic n
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sites (1791m to 3010m depth), constituting less than 14% of total standing stocks. a
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3.2.2 Diversity and dominance
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Atotalof191specieshavebeenidentified,ofwhich71areagglutinants,99arehyaline D
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and21aremiliolidspecies.Speciesrichness(S)differsconsiderablybetweentheOMZ c Tables Figures
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stations (core and lower part) and the dysoxic sites (1495m) (Fig. 3a). The lowest s
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number of species (17) is observed at 1013m depth, in the core of the OMZ. At the n
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dysoxic site (1495m) a maximum of 68 species is recorded. At the oxic sites (1791m a
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to 3010m), species richness is more constant, with about 50 species at all stations e
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(Fig. 3a). The species richness trend follows generally that of the total abundance Back Close
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(r2=0.5), suggesting that the number of foraminifera found is largely determined by
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the sample size. However, in general higher S coincides with a higher Shannon index is
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(H(S)) and a lower evenness (J) (Fig. 3a). Species are also distributed fairly evenly at u
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Shannon diversity (H(S)) is minimal in the core of the OMZ (2.3 to 2.5). It is higher n
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at the dysoxic and oxic sites, where values around 3.0 are observed. Maximum H(S) a
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