Calcareous phytoplankton standing stocks, fluxes and accumulation in Holocene sediments off Bermuda (N. Atlantic

Calcareous phytoplankton standing stocks, fluxes and accumulation in Holocene sediments off Bermuda (N. Atlantic
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  * Corresponding author.  Present address: American University of Beirut, Department of Geology, P.O. Box 11-0236/26, Beirut,Lebanon.Deep-Sea Research II 47 (2000) 1907 } 1938 Calcareous phytoplankton standing stocks,  # uxesand accumulation in Holocene sediments o !  Bermuda (N. Atlantic) Ali T. Haidar   , Hans R. Thierstein   * , Werner G. Deuser   Geological Institute, ETH-Zentrum, CH-8092, Zu (  rich, Switzerland   Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA Received 24 September 1997; received in revised form 13 July 1998; accepted 24 July 1998 Abstract Standing stocks and taxonomic composition of dominant coccolithophore taxa in the photiczone at Bermuda Hydrostation ` S a  are compared to their # uxes measured in a sediment trap at3200 m water depth, located 1200 m above the sea  # oor, and to their accumulationin Holocenesediments on the sea  # oor. A pronounced monthly variability of coccolithophore cell densitiesin the photic zone over the three-year plankton sampling interval (1991 } 1994) was observed. Inthe sediment trap only an integrated seasonal signal is re # ected in the  # uxes of coccoliths andthe  " ne fraction ( ( 38   m) carbonate, which were determined for 12 biweekly samplingintervals during 1992. The average daily  # ux of all coccoliths during 1992 was 1.4  10  coccoliths m   d  , but it varied by at least a factor of   " ve, with highest  # uxes in late winterand late spring and low  # uxes in late fall. The  # uxes of individual coccoliths of the twodominant species,  E. huxleyi  and  F. profunda , accounted for 85% of the total coccolith  # uxes.The contribution of the other biogenic carbonate (mostly calcispheres of thoracosphaerids) tothe  " ne fraction carbonate  # uxes was minor compared to that of coccoliths. Fluxes of the  " nefraction carbonate to the deep Sargasso Sea, determined over 2-week intervals in 1992, werehigh in early spring (up to 23 mg CaCO   m   d  ) and low in late fall (minimum of 5 mgCaCO   m   d  ), with an annual mean of 12 mg CaCO   m   d  . The surface sedimentsaccumulating SE of Bermuda at 4300 m water depth consist of 11% (by weight) plankticforaminifera, 56%  " ne fraction ( ( 38   m) carbonate and 33% inorganic clay. The   C agedi !  erence between the planktic foraminifera carbonate (1.5 ka) and the  " ne fraction carbonate(6.7 ka) at 13 cm core depth, together with the fact that the accumulation rate of   " ne fractioncarbonate in the sediment is about an order of magnitude higher than the corresponding  # uxesto the trap, con " rms the previous reports of large-scale resuspension of   " ne-grained sediments 0967-0645/00/$-see front matter    2000 Elsevier Science Ltd. All rights reserved.PII: S0 96 7 -0 6 4 5 (0 0 ) 0 0 0 1 1 - 4  in this area. The taxonomic composition of the coccolith assemblages accumulating inHolocene sediments near Bermuda is very similar to the composition of coccoliths sampled inthe trap and to that of the average living assemblage, suggesting that transport and dissolutionprocesses in this area a !  ect all taxa in a similar way.    2000 Elsevier Science Ltd. All rightsreserved. 1. Introduction 1.1. Purpose and goals Particulate inorganic carbon (mostly calcium carbonate) is produced bypelagic calcifying organisms in the upper layers of the open ocean, and sinks sub-sequently to the deep sea, where it is partly dissolved and partly stored in thegeological archive (Westbroek et al., 1993). Approximately 80% of the carbon buriedin marine sediments each year is in the form of CaCO  , most of it bio-genic (Broecker and Peng, 1982). In the open ocean, coccolithophores are themajor producers of calcareous sediments. Their contribution to the total carbonate # uxis consideredbyHonjo (1996)to be about 60%,witha regionalvariabilityof 20 to ' 86% (Honjo et al., 1982; Samtleben and Bickert, 1990; Fabry and Deuser, 1991).The average estimate by Honjo (1996) is in good agreement with the averageproportion of the total carbonate found in the ( 38-  m fractions of 73 Holocenedeep-sea sediments from tropical to transitional latitudes (Paull et al., 1988, theirAppendix A).The production of coccoliths in the photic zone, their  # uxes through the watercolumn, and their alteration on the sea  # oor are still poorly understood, and severalongoing studies are focusing on this particular aspect. An assessment of the variationsin the seasonal  # ux of coccoliths is useful to understand the relationship betweenliving and dead assemblages on a short time scale. Furthermore, the alteration of coccolithophore assemblages on their way from the photic zone to the sea  # oor isof some importance to the global carbon cycle. The onging JGOFS activities atBermuda have provided a unique opportunity to compare the living, sinking, andburied coccolithophore assemblages in the same oceanic area and at several timescales. In addition, a comparison of standing stocks with  # uxes in the deep waterallows us to test models developed for estimations of the relative importance of vertical versus lateral transport.The purpose of this study is therefore coincident with the general goals of JGOFS: ` to determine and understand 2 the processes controlling the time-varying  # ux of carbon and associated biogenic elements in the ocean a  (Scienti " c Committee onOcean Research, 1987). Here we report on seasonal changes of the sinking coccolithsrecorded close to our contemporaneous plankton sampling station o !   Bermuda(Haidar, 1997). We compare the coccolith  # uxes to the traps with the averagetaxonomic composition of standing stocks and with the accumulation rates of coccoliths in Holocene sediments in the same area. 1908  A.T. Haidar et al.  /   Deep-Sea Research II 47 (2000) 1907  } 1938  Fig. 1. Geographic locations of sampling sites near Bermuda: The sediment trap samples and themeasurements of the environmental parameters are from the BATS/OFP (Bermuda Atlantic Time-seriesStation/Oceanic Flux Program), the plankton samples were taken at Hydrostation  ` S a , and the sedimentcores further to the south (in part from Michaels and Knap, 1996). 1.2. Bermuda :  long-term time series of deep ocean sediment trap collection in anoligotrophic region Bermuda, located in the subtropical gyre of the North Atlantic, is representative of large areas of the world oceans that are characterized by stable surface waterstrati " cation through most of the year, by low average primary production, and bylow to moderate seasonality of environmental conditions. The surroundingsea # oor is covered by a blanket of carbonate ooze. The available long record of deep-ocean sediment-trap materials (Deuser, 1986, 1996) is a key for calibration of qualitative and quantitative studies of the sedimentary archives. Deep-oceansediment traps can be used to assess the variability of sinking particulate # uxes, whichin turn can be compared to surface water assemblages and changing environmentalconditions.The Oceanic Flux Program (OFP) station, located 75 km southeast of Bermuda(Fig. 1), is the site of a nearly 20-year long deep-sea sediment-trap mooring(Deuser, 1986, 1996). Trap samples collected o !   Bermuda are known to containbiogenic carbonates (ca. 58%) and opal (ca. 13%), with the remainder consisting of mostly organic matter and a minor inorganic clay component (Deuser et al., 1981,1995). Fine fraction carbonates ( ( 38   m) consist mainly of coccoliths (Deuser et al.,1995).  A.T. Haidar et al.  /   Deep-Sea Research II 47 (2000) 1907  } 1938  1909  At Bermuda the seasonal  # uctuations in the phytoplankton production have beenfound to result in seasonal changes of particle  # uxes in traps at 500 } 3200 m depthwithin less than 30 days (Asper et al., 1992). Comparisons of seasonal pigmentdistributions observed in surface waters around Bermuda with the  # uxes in the deeptraps suggest that the main source area may be located about 140 km to the NE of thetrap (Fig. 1 in Deuser et al., 1990). More recently Siegel and Deuser (1997) modelledthe combined e !  ects of prevailing geostrophic currents, eddy dispersion, and variousparticle sinking speeds in this area. Assuming an average regional southwesterlygeostrophic  # ow with current velocities decreasing from about 3 km d   at 500mwater depth to about 1 km d   at 3200 m water depth, they found that an average of 85% of the particlescaught at 3200 m depth may have srcinatedfrom a surface waterarea upto 100 km in diameter (caused by the short-term variability of eddies) witha center about 85 km to the NE of the trap area.Holocene surface sediment samples were taken about 83 km to the southeast of Bermuda(Fig. 1). Theseprovidedthe opportunityto comparethe watercolumn # uxeswith the long-term accumulation in the sediments.At Hydrostation  ` S a , which lies about 60 km to the NW of the trap location,a study of coccolithophore standing stocks from 1 to 200 m water depth was carriedout between January 1991 and January 1994 in 32 pro " les, about 1 month apart. Ourdirect comparison of average standing stocks in the photic zone at Hydrostation  ` S a with trap  # uxes at the nearby OFP site presumes a regional extent of seasonalcoccolithophore successions on scales larger than about 100 km. Non-coherencebetween standing stocks and  # uxes may be due to patchiness on scales smaller thanthat.The general oceanographic setting is in the subtropical gyre of the North Atlantic,where the water temperatures in the top 200 m vary between 18.4 3 C (in winter, whenmixing occurs) and 29.6 3 C (near the surface in August) and minor (36.3 } 37.0  )salinity variations (Knap et al., 1994). In the three-year plankton sampling period themean primary productivity in the top 200 m was 3.2 mg C m   d  . The seasonalincreased every spring with a maximum value of 22.2 mg C m   d   in March 1991.Mean phosphate contents were only 0.02   mol kg  , with a maximum of 0.18   mol kg   at 150 m depth in July 1993, and mean nitrate plus nitrite contentswere 0.55   mol kg  , with a maximum of 5.0   mol kg   in February 1993 at200 m water depth (Knap et al., 1994). 1.3. Pre v ious studies on coccolithophore  y uxes The importance of coccolithophores for calcium carbonate deposition was  " rstemphasized by Bramlette (1958). A recent overview of the biology, transport andsedimentationof coccolithophoresis found in Winter and Siesser (1994). Several morerecent studies on coccolith distributions in plankton, sediment traps and surfacesediments have been carried out in the Norwegian } Greenland Seas (Samtleben andBickert, 1990; Samtleben and Schro  K der, 1992; Samtleben et al., 1995; Andruleit, 1997),the Mediterranean (Knappertsbusch, 1993), the North Atlantic (Knappertsbusch andBrummer, 1995), and the Southern California Bight (Ziveri et al., 1995). In most 1910  A.T. Haidar et al.  /   Deep-Sea Research II 47 (2000) 1907  } 1938  integrated of these studies (Samtleben et al., 1995; Andruleit, 1997) the coc-colithophore standing stocks in the upper 60 m of the Greenland and the NorwegianSeas were compared with the coccolith  # uxes at 500, 1000, and 2300 m (GreenlandSea) and 3000 m (Norwegian Sea), respectively. At both moorings, the lowermost trapwas 300 m above sea  # oor and showed higher coccolith  # uxes, which was interpretedas evidence for resuspension of bottom sediment. This nepheloid layer also wasconsidered responsible for the di !  erences in taxonomic composition between theupper traps and the sediment assemblages (Samtleben et al., 1995).In the present study, we compare the seasonal variability of coccolithophorestanding stocks in the photic zone near Bermuda with the coccolith  # uxes at 3.2 kmwater depth and the coccolith accumulation rates in the underlying Holocenesediments. 2. Materials and methods 2.1. Sample co v erage Standing stocks and taxonomic composition of coccolithophores in the photiczone at Bermuda Hydrostation  ` S a  were investigated at 7 water depths (1, 25, 50, 75,100, 150, and 200 m) about once a month between January 1991 and January 1994(Fig. 2a).Coccolithophore export production was studied in samples collected in the OFPsediment trap moored 75 km southeast of Bermuda (Fig. 1), at a depth of 3200 m $ 100 m (1200 m above the sea  # oor) in the Sargasso Sea near 31 3 50  N,64 3 10  W (Deuser, 1986). The conical trap has a collecting surface of 0.5 m  .Coccolith  # uxes were analysed for the period February 1992 to January 1993,because in that year the highest coccolithophore cell densities were observedwithin the three-year plankton study (January 1991 to January 1994, Fig. 2b).Trap samples were collected biweekly (Deuser, 1996). A selection of 12 trapsamples was chosen out of a larger collection of samples (see Appendix A for precisedates of samples). This subset covers about 58.7% of the total time elapsed betweenFebruary1, 1992 andJanuary 31,1993. Takinga mean settlingrate of 100 m d   intoaccount, which translates into a time delay of 30 days after export from the euphoticzone, the dates of the trap samples were selected about 1 } 1.5 months after thoseplankton samples, which showed the major  # uctuations in coccolithophore celldensities.Two Holocene surface sediment cores of 1-in diameter were also investigated(Fig. 1). The ROLAI  D (Robotically Operated Lander for the Analysis of Interfaceand Interstitial Diagenesis; Sayles and Dickinson, 1991) free vehicle benthic landerwas used to collect the cores. The sediment sampling site is at 31 3 45  N,64 3 21  W ( $ 1.5 km), about 83 km to the southeast of Bermuda and from a waterdepth of 4300 $ 100 m (Sayles and Martin, 1995). Sediment core RL-15/S, 8 cm long,was taken in December 1991 during benthic lander experiment RL 15, core RL-18/Bwas taken in November 1992 during benthic lander experiment RL 18 and was 13 cm  A.T. Haidar et al.  /   Deep-Sea Research II 47 (2000) 1907  } 1938  1911
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