Latitudinal trends in morphological characteristics of Neogloboquadrina pachyderma (Ehrenberg) along a north–south transect in the south-western Indian Ocean

Latitudinal trends in morphological characteristics of Neogloboquadrina pachyderma (Ehrenberg) along a north–south transect in the south-western Indian Ocean
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  ORIGINAL Latitudinal trends in morphological characteristicsof  Neogloboquadrina pachyderma (Ehrenberg) alonga north  –  south transect in the south-western Indian Ocean Neloy Khare & Pawan Govil & Abhijit Mazumder Received: 20 October 2007 /Accepted: 2 October 2008 / Published online: 17 October 2008 # Springer-Verlag 2008 Abstract A total of 25 surficial sediment samples (Petersongrab, gravity and piston cores), collected during the Pilot Expedition to Southern Ocean (PESO) 2004 cruises 199Cand 200 onboard the ORV Sagar Kanya along a N  –  Stransect between 9.69°N and 55.01°S, and 80 and 40°E inthe Indian Ocean sector of the Southern Ocean (SW IndianOcean), have been investigated for various morphologicalfeatures  —  test size, mean proloculus size and coiling direc-tion (dextral/sinistral forms)  —  of the planktic indicator species Neogloboquadrina pachyderma (Ehrenberg). Theresults show that the coiling directions co-vary withtemperature and salinity, the abundances of sinistrally coiledforms increasing towards higher latitudes (south of 40°S),whereas dextrally coiled forms show a reverse trend.Similarly, overall test and proloculus sizes depend largelyon the physicochemical properties (salinity, temperature,nutrients, calcium saturation) of the ambient water masses.These observations suggest that, particularly at the bound-aries between different water masses, variations in morpho-logical features of  N. pachyderma can meaningfully be usedto reconstruct paleoceanographic conditions from IndianOcean sediments. Introduction Due to excellent preservation, global occurrence and highabundance, planktic foraminifers in deep-sea sedimentshave been extensively used for paleoceanographic and paleoclimatic reconstructions (Bolli et al.1985; Berggren et al.1995). Numerous field and laboratory culture studieshave demonstrated that physical and chemical properties of the ambient seawater, such as temperature, nutrient avail-ability, carbonate saturation and oxygen availability, not only influence the distribution and abundance of foramini-fers but also affect various morphological characteristics of their tests (Bé and Hamlin1967; Berger 1969; Bé and Tolderlund1971; Tolderlund and Bé1971; Bé et al.1977; Caron et al.1987a; Bijma et al.1990a,1992; Schiebel and Hemleben2000; Schmidt et al.2004). Accordingly, in modern marine environments, morphological features of  planktic foraminifers such as size, shape, number of chambers, mean proloculus size and coiling directiongenerally reflect water-mass conditions, temperature and primary productivity. To date, however, large-scale inves-tigations of this aspect have yet to be carried out in theIndian Ocean.To test the applicability of such general trends to thewaters of the Indian Ocean, we have investigated thelatitudinal variations in the coiling direction (sinistralforms/dextral forms), test size and mean proloculus size inthe indicator species Neogloboquadrina pachyderma (Ehrenberg) along a north  –  south transect between 9.69°Nand 55.01°S, in a corridor between 40 and 80°E. Thelocation of the N  –  S transect was chosen such as toencompass a variety of ecological, water-mass and climateconditions (cf. tropical, subtropical, transitional and sub-Antarctic), which are known to commonly play a dominant role in controlling planktic assemblages (Bradshaw1959;Bé and Tolderlund1971).The goal of the present study is to investigate therelationships between various physicochemical surfaceoceanic parameters and key morphological features of  Geo-Mar Lett (2009) 29:61  –  69DOI 10.1007/s00367-008-0124-4 N. Khare ( * ) : P. Govil : A. Mazumder  National Centre for Antarctic and Ocean Research,Ministry of Earth Sciences,Headland Sada,Vasco-da-Gama, Goa 403804, Indiae-mail:   planktic foraminifers occurring from the tropical IndianOcean to the subpolar south-western Indian Ocean, in order to document and quantify the influence of key factors inthis region as a means of reconstructing past environmentalconditions from the sediment record. Regional oceanographic setting The transect covered in the present study broadly fallswithin the greater Indian Ocean (Fig.1). Unlike the Pacificand Atlantic oceans, which communicate to both the Northand South poles, the Indian Ocean is closed to the north bythe Asian landmass. Hence, there is no source of cold polar water to generate deep convection in the basins of theIndian Ocean (Murty and Murty2001). The northern IndianOcean experiences seasonal reversals of the monsoonalcirculation (Wyrtki1973), characteristically associated withchanges in the position and strength of the equatorialcurrents. Between the westward-flowing South EquatorialCurrent, which occupies the region south of 8°S, and thewesterly current induced by the northeast winter monsoonruns the eastward-flowing Equatorial Counter Current. Bycontrast, between July and September when the southwest summer monsoon is fully established, the entire regionnorth of 5°S is dominated by an eastward-setting flow. Thetransition between the onset of the northeast monsoon isagain characterized by the Equatorial Jet. This change inthe circulation north of the equator is thus largely controlled by the prevailing winds.Similarly, the southern Indian Ocean circulation ischaracterized by a subtropical anticyclonic gyre (Wyrtki1971). The westward-flowing South Equatorial Current inthe 10  –  20°S latitudinal belt forms the northern section of this gyre. The poleward-flowing Agulhas Current, in turn,forms the western, the eastward-flowing Antarctic Circum- polar Current the southern, and the equatorward-flowingWest Australian Current the eastern section of the gyre.Stramma (1992) has identified the South Indian OceanCurrent at or near the Subtropical Front, at approx. 40°S inthe central south Indian Ocean.The Subtropical Front separates the warmer and saltier water mass of the subtropics from the cold, fresh, nutrient-rich sub-Antarctic water mass. The water masses within thezone between the Antarctic coast and the AntarcticConvergence have been termed as Antarctic waters, and between the Antarctic Convergence and the SubtropicalConvergence as sub-Antarctic water masses. The eastwardflow of the Antarctic Circumpolar Current occupies theregion south of the Subtropical Convergence, and is driven by the world ’ s strongest westerly wind system, approx. at 45  –  55°S. Because of the landmass distribution, the Antarc-tic Circumpolar Current forms a unique global link connecting all the major oceans. Specific deep water masses srcinating farther north shoal dramatically beneaththis current, thereby entering the subpolar regime to mixlaterally with Antarctic shelf waters (Orsi et al.1995).As the boundaries between these various water massesare know to have shifted in the course of glacial  –  interglacial climate fluctuations (Watson and NaveiraGarabato2005), it can be expected that these shifts would be reflected in the planktic foraminifer records preserved inthe bottom deep-sea sediments. Materials and methods In all, 25 surficial sediment samples (Peterson grab, gravityand piston cores) were collected during cruises 199C and200 aboard the ORV Sagar Kanya from January to March2004. The cruise track covers a north  –  south transect  between 9.69°N and 55.01°S in a belt between 80 and40°E in the Indian Ocean sector of the Southern Ocean(south-western Indian Ocean). The locations of the variousstations are listed in Table1and illustrated in Fig.1. The calcium carbonate compensation depth and thelysocline in and around the study area lie below 4,400  –  4,700 m water depth (Banakar et al.1998). All sampleswere collected well above this (Table1), to avoid anydissolution effects on the tests of planktic foraminifers.Immediately upon recovery, the sediment samples (top1 cm of sediment cores/grabs) were stained with RoseBengal and preserved in 10% formalin to distinguish between living and dead specimens of benthic foraminifers.In the absence of exact age datings for the sediment samples, the stained benthic foraminifers collected at thevarious stations are considered to reflect modern ambient conditions at the seabed. Accordingly, the hydrographiccharacteristics of the uppermost 200 m of the water columnwould be tied to the planktic foraminifer record of theseabed sediment.The sediment samples were processed according tostandard laboratory procedures (Krumbein and Pettijohn1938). After drying overnight at 45°C, the samples weresoaked in water and subsequently treated with Na(PO 3 ) 6 (sodium hexa-metaphosphate) in order to dissociate claylumps. The treated sediments were then sieved through a63-µm sieve and the >63 µm fractions were dry-sieved toisolate the >150 µm fractions, from which representativealiquots were taken by quartering and coning. Thesubsamples were weighed, and about 40  –  50 specimens of   N. pachyderma (Ehrenberg) were subsequently selectedfrom each sample. The test size, coiling direction and proloculus size of each specimen was measured under astereo-zoom microscope using a micrometer ocular havinga scale of 14-µm intervals. 62 Geo-Mar Lett (2009) 29:61  –  69  Fig. 1 Locality map showingvarious characteristics of thegeneral oceanographic circula-tion in the Indian Ocean, to-gether with the locations of thesample stationsGeo-Mar Lett (2009) 29:61  –  69 63  Average salinity, temperature and nutrient (phosphate/ nitrate) data in the uppermost 200 m of the water columnwere extracted for selected positions along the transect fromthe NOAA World Ocean Atlas (Levitus and Boyer 1994). Results The data reveal that  N. pachyderma specimens withsinistral coiling direction show minimum relative abun-dance at station SK 199C/17 (2.78%) in the central IndianOcean east of Madagascar, whereas maximum relativeabundance values are found at stations SK 200/27 and SK 200/33 (100%) in the Southern Ocean (Figs.1and2, curve a). Dextrally coiled forms show the exact opposite trend(Figs.1and2, curve c). Average test size increases from 245 µm at station SK 199C/12 to 543 µm at station SK 199C/3 (Figs.1and3, curve a). Similarly, the mean proloculus size increases from6.5 µm at station SK 199C/12 northwards to 15.6 µm at station SK 199C/3, located in the southern part of theArabian Sea (Figs.1and3, curve b). Average temperature varies from 1.14°C at station SK 200/33 to 24.23°C at station SK 199C/6 (Figs.2, curve band3, curve c). Average salinity values fluctuate from33.53 psu at station SK 199C/5 to 35.50 psu at stations SK 200/9 and SK 200/14 (Figs.2, curve d and3, curve d). Phosphate and nitrate values lie within the range 1.17 µmolat station SK 200/21 and 8.43 µmol at station SK 200/19,and between 0.3 µmol at stations SK 199C/7 and SK 199C/ 13 and 9.11 µmol at station SK 200/9 (Fig.3, curves e andf). It is interesting to note that nitrate ranges from 2.19 to15.56 µmol at 10°N to 40°S, and south of 40°S from 1.85to 2.80 µmol. Phosphate concentration varies from 1.63 to7.47 µmol at 10°N to 40°S, and thereafter from 1.30 to2.10 µmol.Closer examination of the various trends of the coilingdirection of  N. pachyderma shows a significant increase insinistral relative abundance (%) between 0  –  15 and 40  –  55°S, whereas sea surface temperature and sea surface salinityshow decreasing trends (Fig.2). By contrast, dextrallycoiled forms show a reverse pattern along the transect (Fig.2, curve c). Moreover, N. pachyderma average sizevariation and mean proloculus size, as well as nutrient concentrations show decreasing trends between 0  –  15 and40  –  55°S (Fig.3). These findings demonstrate that nutrient levels as well as average sea surface temperature andsalinity correlate positively with dextral coiling, test sizeand mean proloculus size but negatively with sinistralcoiling. Discussion Planktic foraminifers have been widely used not only instudies dealing with stable isotopes and biogeochemicalelement tracers but also for the evaluation of variations intest size, coiling directions and mean proloculus size, whichhave proven to be valuable tools for paleoclimatic and paleoceanographic reconstructions. More than 100 yearsago, Murray (1897) explored many foraminifer species interms of their latitudinal distribution in function of seasurface temperature, food availability, and productivity.Indeed, many physicochemical properties of ambient water masses, such as temperature, turbidity, salinity, currents andtides, organic content, pH, Eh, and CO 2 and oxygencontents, have been shown to be major factors in governingvarious morphological features along latitudinal transects invarious ocean basins (Pfuhl and Shackleton2004; Zaric et al.2005). In the present study, the same approach has beenapplied along a latitudinal transect in the Indian Ocean.Temperature and salinityThe results of the present study suggest that test size andmean proloculus size are influenced by average seawater temperature and salinity. Previous studies have also shownthat temperature- and salinity-related effects are the most important factors influencing size variations in tests of  Table 1 Locations of sample stations, and water depth (m)St. no. Sample no. Latitude Longitude Water depth (m)1 SK199C/03 9.50 75.51 1,030.002 SK199C/04 9.41 75.39 1,516.003 SK199C/05 8.99 75.82 2,738.004 SK199C/06 8.13 73.56 2,250.005 SK199C/07 5.51 69.35 3,944.006 SK199C/10 − 1.92 67.88 2,597.007 SK199C/12 − 4.69 67.10 3,320.008 SK199C/13 − 7.36 67.17 3,305.009 SK199C/14 − 9.18 65.96 3,373.0010 SK199C/15 − 11.42 67.40 3,513.0011 SK199C/16 − 12.59 67.14 3,722.0012 SK199C/17 − 15.28 66.01 3,368.0013 SK199C/19 − 16.27 63.46 4,003.0014 SK200/05 − 28.32 48.73 2,295.0015 SK200/09 − 30.91 44.86 2,227.0016 SK200/14 − 36.12 44.89 2,805.0017 SK200/15 − 37.00 44.98 2,984.0018 SK200/17 − 39.03 44.97 4,022.0019 SK200/19 − 40.98 45.06 2,532.0020 SK200/21 − 43.15 44.98 3,210.0021 SK200/22A − 43.69 45.07 2,723.0022 SK200/23 − 45.00 45.01 1,423.0023 SK200/25 − 47.10 45.33 3,285.0024 SK200/27 − 49.01 45.22 4,377.0025 SK200/33 − 55.01 45.01 4,185.0064 Geo-Mar Lett (2009) 29:61  –  69   planktic foraminifers, both at a species and at an assem- blage level (Funnell1967; Bé et al.1973; Hecht et al.1976; Schmidt et al.2004). Globally, the highest correlations between size variation and environmental parameters have been reported for mean annual sea surface temperature andthe 200-m depth limit (Schmidt et al.2004).The first chamber (proloculus) has a strong influence onfinal test size because of its direct relationship with thegeometry of the shell (Berger 1969; Wei et al.1992). It was thus logical to expect that mean proloculus size would vary proportionally to overall shell size along the present transect. That this is correct is clearly shown in the highcorrelation coefficient (r=0.8922) for the relationship between these two parameters (Fig.4).Coiling direction is the most commonly studied mor- phological variable in foraminifers (e.g. Ericson1959;Bandy1960; Bé and Hamlin1967; Reynolds and Thunell 1986; Schmidt 1992; Wefer et al.1996; Ufkes et al.2000),  being frequently proposed as a temperature indicator. Nevertheless, a number of contradictions have beenreported where particular species failed to consistentlycorrelate with cold or warm conditions (e.g. Thiede1971;Olsson1974). More recently, a relationship between coilingdirection and temperature has been proposed for  Truncatulinatrancatulinoides , involving a morphological interdependencewith temperature, productivity, water mass and upwelling(Collins1990; Pfuhl and Shackleton2004). Coiling direction can vary from species to species (Herman-Rosenberg1965)or within the same species, in response to different environmental conditions in geographically distinct locations(Bandy1960; Takayanagi et al.1968; Bé1969; Thiede 1971; Parker and Berger 1971). The contradictory observa- tions mentioned above necessitated to establish the responsein coiling direction of individual species in a specific region, Fig. 2 Latitudinal changes in a the relative abundance of thesinistral form of  N. pachyderma (%), b the average sea surfacetemperature ( SST  , °C), c therelative abundance of the dextralform of  N. pachyderma (%), and d  the average sea surface salinity( SSS  , psu) along the N  –  S tran-sect shown in Fig.1. As guide,the locations of four samplestations are also indicatedGeo-Mar Lett (2009) 29:61  –  69 65
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