G 3. AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society

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1 Geosystems G 3 AN ELECTRONIC JOURNAL OF THE EARTH SCIENCES Published by AGU and the Geochemical Society Article Volume 9, Number October 2008 Q10019, doi: /2008gc ISSN: A study of the alkenone, TEX 86, and planktonic foraminifera in the Benguela Upwelling System: Implications for past sea surface temperature estimates yung Eun Lee Division of Marine Environment and Bioscience, orea Maritime University, Busan , South orea (kyung@hhu.ac.kr) Jung-Hyun im Department of Marine Organic Biogeochemistry, Royal Netherlands Institute for Sea Research, P. O. Box 59, NL-1790 AB Den Burg, Texel, Netherlands Iris Wilke Ocean Sciences Department, University of California, 1156 High Street, Santa Cruz, California 95064, USA Peer Helmke School of Life Sciences, Arizona State University, P. O. Box , Tempe, Arizona , USA Now at Federal Institute of Hydrology, Am Mainzer Tor 1, D oblenz, Germany Stefan Schouten Department of Marine Organic Biogeochemistry, Royal Netherlands Institute for Sea Research, P. O. Box 59, NL-1790 AB Den Burg, Texel, Netherlands [1] Suspended particulate matter from seawater and core top sediments were collected during 2003 Meteor cruise M57/1 in January February from the continental margin off western South Africa for analysis of alkenones, glycerol dibiphytanyl glycerol tetraether (GDGT) lipids, and planktonic foraminifera. Alkenone analysis of suspended particulates in seawater and core top sediments indicates that U 0 37 temperatures were representative of annual mean sea surface temperature. In contrast, GDGT analysis suggests that TEX 86 temperatures are cold-biased due to upward transports of GDGTs produced below the mixed layer. The analysis of plankton tow samples revealed that the d 18 OofGloborotalia inflata in core top sediments could be biased toward lower temperatures due to subsurface calcification. Accordingly, our study shows that each paleotemperature proxy may record different temperature signals in the Benguela upwelling system emphasizing the general need to constrain potential biases in each proxy for better interpreting paleoclimate records. Components: 9413 words, 8 figures, 2 tables. eywords: alkenones; TEX 86 ; sea surface temperature. Index Terms: 4924 Paleoceanography: Geochemical tracers; 4954 Paleoceanography: Sea surface temperature; 4944 Paleoceanography: Micropaleontology (0459, 3030). Received 12 April 2008; Revised 10 September 2008; Accepted 15 September 2008; Published 31 October Copyright 2008 by the American Geophysical Union 1 of 19

2 Lee,. E., J.-H. im, I. Wilke, P. Helmke, and S. Schouten (2008), A study of the alkenone, TEX 86, and planktonic foraminifera in the Benguela Upwelling System: Implications for past sea surface temperature estimates, Geochem. Geophys. Geosyst., 9, Q10019, doi: /2008gc Introduction [2] The reliable estimation of sea surface temperature (SST) for past oceans is one of the main goals for paleoceanographers since SST is an important boundary condition for both running and evaluating general circulation models of the atmosphere and ocean as well as for interpreting the processes and mechanisms of past climate changes [e.g., CLIMAP Project Members, 1976]. Paleoceanographers have developed a range of tools and analytical techniques to extract SST information from marine sediments. These tools include, for example, planktonic foraminiferal assemblages, geochemical properties of planktonic foraminifera (d 18 O and Mg/Ca ratio), and the degree of unsaturation of C 37 alkenones (U 0 37 index). Recently, a new temperature proxy based on archaeal tetraether lipids, the TEX 86 proxy (tetraether index of tetraethers consisting of 86 carbon atoms), has been introduced and is thought to reflect mostly annual mean temperatures of the upper mixed layer [Schouten et al., 2002; im et al., 2008]. [3] Although there have been successful applications of several SST proxies in a range of marine sediments, it has been frequently found that each proxy has certain limitations. In general, SST proxies are associated with marine organisms once alive in the upper water column and ecological and physiological responses of the organism to the environmental conditions other than SST in the upper water column must be carefully considered for each technique [e.g., Rosell-Melé, 1998; Prahl et al., 2003; Lee and Schneider, 2005]. Also, effects of biogeochemical processes such as dissolution, degradation, and diagenesis in the water column must be examined for sinking particles [e.g., Bé, 1977; Brown and Elderfield, 1996; Gong and Hollander, 1999]. In addition, the influences of physical oceanic processes such as oceanic currents on the resuspension, transport, and redeposition of surface sediments must also be considered for the reliable reconstruction of past SST [e.g., Ohkouchi et al., 2002; Mollenhauer et al., 2003]. These problems are exemplified in a recent comprehensive multiproxy study for the Last Glacial Maximum (LGM) global ocean which has been conducted under the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) project [ucera et al., 2005a]. These studies often observe discrepancies between different SST proxies, especially in upwelling regions. For example, in the upwelling area off South Africa, alkenone-based SSTs were warmer (7 C) than those derived from planktonic foraminifera assemblages [Niebler et al., 2003]. [4] To investigate the causes for the different behavior of each proxy in upwelling areas, we measured alkenones and glycerol dibiphytanyl glycerol tetraethers (GDGTs) in surface and subsurface (100 m) seawaters as well as core top sediments in the Benguela Upwelling system, southeastern Atlantic Ocean. In addition, planktonic foraminiferal assemblages and their oxygen isotopic compositions were investigated by analyzing plankton tow samples from the upper water column in the region. The results from our study will help to evaluate the use of each SST proxy and to constrain discrepancies between different temperature proxies in the Benguela upwelling system. 2. Hydrography of Benguela Upwelling System [5] The Benguela Current, the eastern boundary current of the South Atlantic subtropical gyre, flows northward along the west coast of southern Africa [Peterson and Stramma, 1991] (Figure 1). The current originates from the South Atlantic Current and the Agulhas Current off the Cape of Good Hope, and it runs northward. Along the coast, the prevailing southerly and southwesterly winds produce intense upwelling of subsurface water during September to March [Andrews and Hutchings, 1980]. Recurring patches of intense upwelling in the study area are situated at the Cape Peninsular (34 S), Cape Columbine (33 S), and Namaqua (29 S). These sites extend offshore to km roughly coinciding with the edge of the continental shelf. There is a strong thermal front between the cold upwelled coastal waters and the Benguela Current. Underneath the surface water of the Benguela Current offshore, the South Atlantic Central Water comprises the thermocline 2of19

3 Geosystems G 3 lee et al.: sst proxies in benguela upwelling /2008GC Figure 1. Location of study area. Arrow indicates ocean current (BC, Benguela Current; SAC, South Atlantic Current; AC, Agulhas Current). Black dot indicates the location of stations. Red double circle indicates station where subsurface waters were collected. Green rectangle indicates station where plankton tow samples were collected. NM, CC, and CP indicate Namaqua, Cape Columbine, and Cape Peninsula, respectively. water. The surface waters are in the temperature range of C and in the salinity range of The temperature and salinity of the thermocline waters are 6 15 C and , respectively. The thermocline waters extend to the bottom of the continental shelf and are uplifted to the surface along the coast. In the offshore area, the thermocline water is underlain by the Antarctic 3of19

4 Intermediate Water, which is in turn underlain by the North Atlantic Deep Water. 3. Material and Methods 3.1. Seawater and Surface Sediment Sampling [6] Samples were collected during 2003 Meteor cruise M57/1 (20 January to 8 February) from the continental margin off the western South Africa (Figure 1). Suspended particulate matter from surface waters (water depth, 0 5 m) for alkenone and GDGTs analysis were retrieved by membrane pump. They were obtained from 24 stations covering from the continental shelf and slope (Table 1). The amount of seawater filtered for analysis is indicated in Table 1. After filtration, sample filters were immediately stored at 20 C until analysis. [7] Subsurface waters were collected at the depth of 10, 30, and 70 m using Niskin bottles of a Rosette sampler at three stations (stations GeoB8337, GeoB8340, and GeoB8333) for alkenones and GDGTs. About 20 L of seawater from each water depth was filtered for stations GeoB8337 and GeoB8333, and 100 L was filtered for station GeoB8340. Sample filters were immediately stored at 20 C afterthe filtration. [8] Plankton tow samples were collected from stations GeoB8305, GeoB8318, GeoB8333, GeoB8336, and GeoB8338 by the multiple opening and closing net and environmental sampling system (MOCNESS). The nets of 63 mm mesh size were towed vertically from 500 to 300 m, and at 100 m interval from 100 to 300 m, and at 20 m intervals for the upper 100 m water column. Samples are conserved with a saturated HgCl 2 solution and stored at 4 C. Planktonic foraminifera species (size fraction >150 mm) were picked from the wet samples and identified for fauna and stable isotopic analysis. This study did not calculate the seawater temperature from the assemblage data. Because our data represent a snapshot for a very specific time slice in a year, it is very difficult to directly compare this to core top data. Furthermore, the uncertainties of foraminiferal sampling problems such as collection efficiency of nets, timing for tow, preservation, split accuracy, and the extent of subsampling make it difficult to do provide a quantitative temperature calculation. [9] Surface sediments were collected by multicore from 33 stations for alkenone and GDGT analysis (Figure 1). The water depths at the core sites range from 70 to 3620 m. Multicore samples were sectioned by 1 cm slices and stored at 20 C. The upper 1 cm of sediment samples was used in this study. [10] During the cruise, surface temperature and salinity data were obtained from thermosalinograph every 10 min. For all stations, conductivitytemperature-depth (CTD) recorder measurements were conducted to retrieve seawater temperature and salinity Alkenone Analysis and Temperature Estimation [11] All samples for alkenones were analyzed at the University of Bremen. Briefly, long-chain alkenones were extracted from filters by ultrasonication using a UP 200H sonic disruptor probe (200W, amplitude 0.5, pulse 0.5) with successively less polar mixtures of methanol and dichloromethane (MeOH, MeOH/DCM 1:1 v/v, DCM), each for 3 min. The combined extracts were concentrated and separated into DCM and DCM:MeOH (1:1 v/v) fractions through a commercial silica cartridge. The DCM fractions were used for alkenones, while the DCM:MeOH fractions were used for GDGTs. For alkenones, the DCM fractions were further saponified at 80 C for 2 h with 0.1 M OH in 90/10 MeOH/H 2 O. The alkenone fraction was obtained by partitioning into hexane. The final extracts were analyzed by capillary gas chromatography with a HP 5890A, a 60 m fused silica column (0.32 mm 0.1 mm), and flame ionization detection. The oven temperature was programmed from 50 to 150 Cat30 Cmin 1, from 150 to 230 C at 8 Cmin 1, and from 230 to 320 Cat6 Cmin 1, and the final temperature was maintained for 45 min. Quantification of C 37 alkenones was achieved with squalane as internal standard and the relative response factors of the C 38 and C 39 n-alkanes. We calculated the alkenone unsaturation index as defined by Prahl et al. [1988]: U 0 37 ¼ ½ C 37:2Š= ð½c 37:2 Šþ½C 37:3 ŠÞ ð1þ where C 37:2 and C 37:3 represent the diunsaturated and triunsaturated C 37 alkenones, respectively. Then, the U 0 37 values were converted into temperature values by applying the culture calibration of Prahl et al. [1988], which is nearly identical to that obtained by sediment core tops [Müller et al., 1998]: T ¼ U :039 =0:034 ð2þ 4of19

5 Table 1. Results of Alkenone and Glycerol Dibiphytanyl Glycerol Tetraether Analysis for Seawater Samples Station Latitude (S) Longitude (E) Water Depth (m) Water Volume (L) C 37:3 (ng/l) C 37:2 (ng/l) Alkenone Total C 37 Temperature a (ng/l) U 0 37 ( C) Temperature in Situ ( C) TEX 86 (m) TEX86 Temperature b ( C) GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB a Temperature was calculated from U 37 0 value using the published calibration equation of Prahl et al. [1988]. b Temperature was calculated from TEX 86 values using the calibration equation of im et al. [2008]. 5of19

6 The reproducibility of the analysis is smaller than U 0 37 unit (or 0.5 C) based on multiple extractions of a marine bulk sediment standard from the Southeast Atlantic GDGT Analysis and Temperature Estimation [12] Aliquots of the DCM:MeOH (1:1 v/v) fractions obtained after separation on the silica cartridge were blown down under a stream of nitrogen and redissolved by sonication (5 min) in hexane: propanol (99:1 v/v). After filtration through 0.45 mm PTFE filters, the GDGT fractions were analyzed by high performance liquid chromatography/atmospheric pressure positive ion chemical ionization mass spectrometry (HPLC/APCI-MS) as described previously [Schouten et al., 2007]. Briefly, analyses were performed on an Agilent (Palo Alto, California, USA) 1100 series HPLC/ MS. Separation was achieved on a Prevail Cyano column ( mm, 3 mm; Alltech, Deerfield, Illinois, USA), maintained at 30 C. Flow rate of the hexane:propanol (99:1 v/v) was 0.2 ml/min for the first 5 min, thereafter with a linear gradient to 1.8% propanol over 45 min. After each analysis, the column was cleaned by back-flushing hexane/ propanol (90:10, v/v) at 0.2 ml min 1 for 10 min. Detection was achieved using APCI-MS. Conditions for APCI-MS were as follows: nebulizer pressure of 60 psi, vaporizer temperature of 400 C, drying gas (N 2 ) flow rate of 6 l min 1 and temperature of 200 C, capillary voltage of 3 kv, corona 5 ma (3.2 kv). GDGTs were detected by single ion monitoring of their [M + H] + ions. The TEX 86 ratio was calculated as follows [Schouten et al., 2002]: TEX 86 ¼ ðgdgt 2 þ GDGT 3 þ GDGT 4 0Þ = ðgdgt 1 þ GDGT 2 þ GDGT 3 þ GDGT 4 0Þ ð3þ where GDGT 1 + GDGT 2 + GDGT 3 and GDGT 4 0 indicate GDGTs containing 1, 2, and 3 cyclopentane moieties and crenarchaeol-isomer, respectively. The TEX 86 was converted into temperature by using a recently proposed sediment core top calibration [im et al., 2008]: T ¼ 10:78 þ 56:2*TEX 86 The reproducibility of the analysis is smaller than (or 0.5 C), based on the duplicate measurements of each sample Stable Oxygen Isotope Analysis [14] Stable oxygen isotopic analysis of Globorotalia inflata in two size fractions ( mm and ð4þ mm) and Neogloboquadrina pachyderma (sin) ( mm) were conducted at the University of Bremen using a Finnigan Mat 252 mass spectrometer equipped with an automatic carbonate preparation device. Results are presented with respect to Vienna Pee Dee Belemnite (V-PDB) standard. The precision of the measurements at 1 sigma based on replicates of an internal standard (Solnhofen limestone) was ±0.05%. 4. Results 4.1. Temperature and Salinity [15] The SST data obtained from the ship thermosalinograph show that SST at the time of sampling range from 19.7 to 21.2 C at stations in the continental slope, whereas the temperatures are C and C at stations near the coast in the continental shelf (Figure 2c). The low temperature near the coast was caused by upwelled cold deepwater. The SST data in Figure 2c shows the upwelling sites of Namaqua (29 S) and Cape Columbine (33 S), but the Cape Peninsula site (34 S) is not evident from these data. The vertical profile of seawater temperature along the east-west transect between 29 S and 30 S [Schneider and cruise participants, 2003] illustrates the cool upwelling water near the coast with temperatures as low as 10 C, and the upwelling front situated at approximately 15.5 E roughly coinciding with the shelf break. At the upwelling front, the SST changes rapidly from 12 C in the upwelling area to 17 C toward further offshore. Water of 10 C at the Namaqua site is indicative of active upwelling during the sampling period, while the SST of C at the Cape Columbine is not. In this study, the surface mixed layer varied from 10 to 40 m. The salinity of the surface waters is in the range of [16] The seasonal variations in SST of the study area were retrieved from the NSIPP AVHRR Pathfinder and Erosion Global 9 km SST Climatology data set for the period of 1985 to 1995 [Casey and Cornillon, 1999]. The monthly averaged SST data show that average austral summer (December, January, February) temperature is 20 C, while the austral winter (June, July, August) temperature is 16.4 C at the stations in the continental slope. The annual average temperature is 18.1 C. At stations in the continental shelf, the temperatures are 17.6 C, 13.7 C, and 15.6 C, respectively. A comparison of ship thermosalinograph SST with the satellite data indicates that the measured SST is 6of19

7 Figure 2. (a) The distribution of total C 37 alkenone concentration in surface water, (b) total C 37 alkenone concentration in core top, (c) the thermosalinograph sea surface temperature (SST), (d) the alkenone temperature of surface water, (e) the satellite-derived annual average sea surface temperature, and (f) the alkenone temperature of core tops. NM, CC, and CP indicate Namaqua, Cape Columbine, and Cape Peninsula respectively. Green line indicates the water depth of 200 m. 7of19

8 Table 2. Results of Alkenone and Glycerol Dibiphytanyl Glycerol Tetraether Analysis for Core Top Sediments Station Latitude (S) Longitude (E) Water Depth (m) C37:3 (mg/g) C37:2 (mg/g) Alkenone Total C37 Temperature a (mg/g) U 0 37 ( C) Satellite Temperature ( C) TEX 86 TEX86 Temperature b ( C) GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB GeoB a Temperature was calculated from U 37 0 value using the published calibration equation of Prahl et al. [1988]. b Temperature was calculated from TEX 86 values using the calibration equation of im et al. [2008]. 8of19

9 Figure 3. The vertical distributions of total C 37 alkenone concentration (black dot), conductivity-temperature-depth recorder (CTD)-measured temperature (thick dash line), and alkenone-based temperature (closed rectangle) in stations (a) GeoB8337 and (b) GeoB8340. Arrows indicate the surface mixed layer. consistent with the satellite austral summer temperature for the stations in the continental slope, while the measured SST at the continental shelf is lower than that obtained from the satellite data. The low SST near the coast is due to active upwelling during the austral summer. The SST at the Cape Columbine is still lower than that obtained from the satellite data Alkenone-Based Temperatures [17] The alkenone concentration in the surface waters is in the range of ngl 1 at stations on the continental slope, ng L 1 at the sites of Cape Columbine, and ngl 1 at the Namaqua sites (Figure 2a and Table 1). In surface sediments, the concentrations of total C 37 alkenones range from 0.5 to 11 mg g 1 (Figure 2b and Table 2). The alkenone concentration does not vary systematically between the continental slope and shelf sites in both surface water and core top sediments. [18] The vertical distribution of alkenone concentration in the upper water column is shown in Figure 3. At station GeoB8337, the concentration of total C 37 alkenones is high in the surface mixed layer and decreases with depth (Figure 3). At station GeoB8340, the concentration is low at the depth of 10 m, and the highest alkenone concentration occurs at the depth of 30 m, and it decreases at the depth of 75 m. Thus, the alkenone concentration maxima occur in the surface mixed layer. The alkenone-based temperatures of surface water samples range from 18.2 to 22.9 C at stations in the continental slope and from 10.0 to 14.6 C at the upwelling sites (Figure 2d). These alkenonebased temperatures agree in general well with thermosalinograph-derived SSTs except at the upwelling site of Cape Columbine where they are lower by up to 5 C compared to in situ temperatures. [19] The alkenone-based temperatures of surface sediments range from 16.8 to 19.1 C at stations in the continental slope and from 14.3 to 15.5 C at the upwelling sites (Figure 2f). The difference between alkenone-based temperatures (3 C) of surface sediments at stations in the continental slope and upwelling sites is relatively small compared to the difference observed for the surface waters (8 C). The core top alkenone temperatures were then compared with the monthly averaged satellite SST data rather than averaged climate data as the resolution of COADS and LEVITUS data set is too low to detect the temperature signal of local upwelling sites. This showed that the alkenone-based temperatures of the surface sediments are similar to the annual average SST for both continental slope and shelf sites. [20] At station GeoB8337, alkenone-based temperatures measured from suspended material at the depths of 10 m, 30 m, and 75 m are 20.4 C, 21.0 C, and 16.7 C, respectively, which are similar to CTD-measured temperatures (Figure 3). At station GeoB8340, the temperature estimated from the U 0 37 is 19.5 C at the depth of 10 m, 21.1 C at the depth of 30 m, and 17.7 C at the depth of 75 m. The alkenone-based temperatures at the surface 9of19

10 Figure 4. (a) The thermosalinograph SST, (b) the TEX 86 temperature of surface water, (c) the satellite-derived annual average sea surface temperature, and (d) the TEX 86 temperature of core tops. Green line indicates the water depth of 200 m. mixed layer are similar to CTD-measured temperatures, but there is discrepancy (3.6 C) between CTD-measured temperature and alkenone-based temperature at the depth of 75 m TEX 86 -Based Temperatures [21] The TEX 86 -based temperatures derived from surface water particulate organic matter range from 13.3 to 22.8 C at stations in the continental slope and from 7.3 to 12.1 C at upwelling sites (Figure 4b). Comparisons of thermosalinograph SSTs with TEX 86 -based temperatures of surface water show that the reconstructed temperatures are colder (up to 7 C) than in situ temperatures. The TEX 86 -based temperatures of surface sediments vary between 12.8 C and 18.5 C at stations in the continental slope and between 11.7 C and 13.1 C at upwelling sites (Figure 4d). A comparison of TEX 86 -based temperatures with the satellite-derived SSTs suggests that the TEX 86 - based temperatures of the surface sediments are cooler by up to 4 C. [22] At station GeoB8333, the upwelling site, TEX 86 -based temperatures measured from suspended material are very similar, i.e., 9.1 C, 9.5 C, and 9.4 C at the depths of 10 m, 30 m, and 75 m, respectively (Figure 5). The TEX 86 - based temperature at 10 m is cooler compared to the CTD-measured temperature, but TEX 86 -based temperatures correspond well to CTD-measured measurements below the mixed layer. At station GeoB8340, TEX 86 -based temperatures measured from suspended material are also very similar, i.e., 15.6 C, 15.4 C, and 15.6 C at the depths of 10 of 19

11 Figure 5. The vertical distributions of CTD-measured temperature and TEX 86 -based temperature. Rectangle indicates TEX 86 -based temperature. Arrows indicate the surface mixed layer. 10 m, 30 m, and 75 m, respectively. Similar to station GeoB8333, all TEX 86 -based temperatures compare well with CTD-measured temperatures from below the mixed layer. At station GeoB8337 in the continental slope, TEX 86 -based temperatures measured from suspended material, i.e., 16.3 C, 17.6 C, and 17.9 C at the depths of 10 m, 30 m, and 75 m, respectively (Figure 5) also agree best with CTD-measured temperatures below the mixed layer. In general, TEX 86 -based temperatures increase from the upwelling station (GeoB8333) to the continental slope station (GeoB8337) similar to the CTD-measured temperatures Foraminiferal Assemblages [23] The distribution patterns of planktonic foraminifera were analyzed at stations located in the continental slope (GeoB8305, 8336, and 8338), in the boundary between slope and upwelling areas (GeoB8318), and in the upwelling area (GeoB8333). In general, planktonic foraminifera are more abundant at the upwelling station (GeoB8333) than at the continental slope stations (GeoB8305, 8336, and 8338). During the cruise period of the warm season, N. pachyderma (sin) clearly dominates the assemblage at the upwelling station GeoB8333. On the other hand, G. inflata is the most dominant species (70 89%) in the water column of the slope and transitional stations. The percentage of N. pachyderma (sin) is relatively higher at station GeoB8318 than at the slope stations, while N. pachyderma (dex) is relatively abundant at station GeoB8338. Globigerinoides ruber and Globigerinella calida are present in the southern part (GeoB8305 and GeoB8318), but this is not the case in the northern part. The vertical distribution pattern of planktonic foraminifera show that G. inflata, N. pachyderma (sin), and N. pachyderma (dex) are abundant in the upper part of water column (0 100 m), whereas Globorotalia scitula is mostly present below the water depth of 100 m. The maximum number of G. inflata occurs at the depth of m in the continental slope stations, and the maximum number of N. pachyderma (sin) occurs at the depth of m in the upwelling station GeoB Foraminiferal Stable Oxygen Isotopic Data [24] The vertical distribution pattern of the oxygen isotopic values of G. inflata and N. pachyderma (sin) is shown in Figure 6. Variations in d 18 O 11 of 19

12 values of G. inflata duplicates ( mm) are in the range of 0 0.3%, while differences between the size fraction of mm and mm range from 0 to 0.5%. The average d 18 O value of G. inflata in surface water is 0.7% at stations GeoB8305, 8336, and 8338 and 0.4% at station GeoB8318. The vertical profiles of G. inflata d 18 O values show an increase with depth in the water column. At the depth of 100 m, the values are 0 to 0.3% at four stations while at a depth of 300 m, values are 0.6 to 0.7% at GeoB8336 and 8338 and 1.0% at GeoB8305. At station GeoB8333, in the upwelling region, the d 18 O value of N. pachyderma (sin) at the water depth of 30 m is 0.8% and increases to 1.2% at a depth of 50 m and remains constant below. 5. Discussion 5.1. Alkenone-Based Temperatures [25] In Figure 7a, the U 0 37 index values are compared to in situ measured temperatures for surface water samples. The dashed line indicates the calibration equation of Prahl et al. [1988]. In general, a good correspondence between U 0 37 index and measured temperature is observed. The deviations from the alkenone-temperature calibration line are illustrated in Figure 7b. The deviations (alkenonebased temperature minus in situ temperature) are mostly in the range of ±2 C, except for the Cape Columbine area (see below). For core tops, U derived temperature estimates agree well (i.e., within 1 C) with annual average SST estimated from satellite data (Figure 7c). Hence, it seems that alkenone derived temperatures in the Benguela Upwelling system mostly represents an annual average signal from the upper mixed layer. This agrees with the findings of Mitchell-Innes and Winter [1987] who showed that E. huxleyi, the most likely source of long chain alkenones, is the most abundant species in living coccolithophore assemblages in the region and that the highest abundance occurred at the surface (less than the Figure 6. The vertical distribution of the oxygen isotopic values of G. inflata and N. pachyderma (sin). Open circle indicates G. inflata (the size fraction, mm). Closed rectangle indicates G. inflata (the size fraction, mm). Closed circle indicates N. pachyderma (the size fraction, mm). Solid line indicates CTD temperature. Gray line indicates CTD salinity. Dashed line indicates the predicted oxygen isotopic equilibrium values of calcium carbonate (see text for detailed explanation). 12 of 19

13 Figure 7. Graphs showing the relation between U 0 37 and satellite derived SST. (a) Comparison between the alkenone unsaturation index and in situ temperature for surface water samples. (b) Difference between alkenone-based temperature and in situ temperature for surface water samples. (c) Comparison between the values of alkenone unsaturation index and the satellite-derived annual average SST for core top sediments. (d) Difference between alkenone-based temperature and the satellite-derived annual average SST for core top sediments. Dash line represents the calibration equation of Prahl et al. [1988]. water depth of m) in the open ocean as well as in the upwelling area. Furthermore, studies show that that there is not a strong seasonality in occurrence of E. huxleyi. Austin [1980] showed that coccolithophores formed up to 65.5% of the total population during July and August On the other hand, in January 1984, ruger (refer to Mitchell-Innes and Winter [1987]) found a widespread bloom of coccolithophores extending between 8 and 56 km offshore from 21 to 24 S with maximum concentrations of cells l 1 and forming up to 96% of the phytoplankton population. In March 1983, Mitchell-Innes and Winter [1987] found an abundant population of coccolithophores in the upwelling water off the Cape Peninsula. [26] For surface waters, a large discrepancy (3 5 C) is found between U derived and measured temperatures in the Cape Columbine area where the alkenone temperature estimates are lower than in situ measured ones. There may be several explanations for this apparent discrepancy. First, the suspended particulates may be transferred from other areas as the water upwelled along the coast most likely originated from thermocline waters offshore. However, alkenone concentrations offshore are very low even at the depth of 70 m and are likely even lower in the thermocline waters. Thus, transfer of alkenones by upwelling from deeper waters is unlikely to have had a major impact. Instead, there is a possibility that the suspended particulates were actually produced when seawater temperature was low, which may have been the case if upwelling was more intense just before the time of sampling. If it is the case, a bloom might occur during active upwelling encoding U 0 37 signature of colder upwelled water. A high concentration of alkenones observed at the site fits well with this. As the bloom run out of nutrients in seawater, the chilling effect of nutrient depletion on the U 0 37 index [e.g., Prahl et al., 2006] might cause the temperature offset between alkenone and measured ones as well TEX 86 -Based Temperatures [27] In general, TEX 86 temperatures in core top sediments and surface waters are colder than in situ and satellite-derived temperatures. The deviations 13 of 19

14 Figure 8. Graphs showing the relation between TEX 86 and satellite derived SST. (a) Comparison between the values of TEX 86 and in situ temperature for surface water samples. (b) Difference between TEX 86 -based temperature and in situ temperature for surface water samples. (c) Comparison between the values of TEX 86 and the satellitederived annual average SST for core top sediments. (d) Difference between TEX 86 -based temperature and the satellite-derived annual average SST for core top sediments. Solid lines represent the calibration equation of im et al. [2008] and dashed line represents 1 sigma error range. (TEX 86 -based temperature minus satellite-derived SSTs) are illustrated in Figure 8. The differences are small but some values are out of the standard deviation range of the TEX 86 -SST calibration line (±1.7 C) [im et al., 2008]. These values mostly originate from sites within the upwelling area. [28] One striking feature is that TEX 86 -based temperatures from two depth profiles of upwelling area show nearly identical values at different water depths, reflecting temperatures below the mixed layer (Figure 5). This implies that Crenarchaeota production was higher below the mixed layer and the crenarchaeotal GDGTs produced below the mixed layer were transported upwards through upwelling likely resulting in lower TEX 86 -based temperatures compared to in situ surface temperatures (see Figure 8). If upward advection is significant for GDGTs, why it is not for alkenones? It is because alkenone production seems to occur at or close to sea surface whereas the depth for GDGT synthesis is below that of alkenone production. [29] The water depth profile of GeoB8337 (Figure 5) showed the same trend compared to those of two profiles, showing that TEX 86 -based temperatures are colder than in situ temperatures in the mixed layer. The mixed layer samples showed even slightly lower temperatures than that from below the mixed layer. This implies that TEX 86 -based temperatures in the mixed layer of the continental slope might have been influenced by offshore streaming filaments of upwelled cold waters in the coastal zone. Such phenomena have been observed in other upwelling areas off NW Africa [Helmke et al., 2005]. [30] The TEX 86 -based temperatures of the core top sediments are also lower than satellite-derived SST, albeit to a lesser extent. Lower TEX 86 -based temperatures relative to real annual mean temperatures were also observed in surface sediments off NW Africa, another upwelling area [im et al., 2008]. This suggests that the higher depth production of Crenarchaeota may cause cold-biased TEX 86 -based temperatures in upwelling areas as has, for exam- 14 of 19

15 ple, also been observed for the Santa Barbara basin [Huguet et al., 2007]. [31] Alternatively, cold-biased TEX 86 -based temperatures in upwelling areas may be due to higher Crenarchaeota production during upwelling seasons which are characterized by cold, nutrient-rich waters. It has been frequently documented that Crenarchaeota are more abundant in certain times of the season [e.g., Wuchter et al., 2005; Herfort et al., 2006]. Thus, if the seasonal temperatures at times of growth of Crenarchaeota are substantially different from the annual mean SST, then there will likely be a bias toward seasonal temperatures in the TEX 86 proxy. Indeed, Herfort et al. [2006] showed that the TEX 86 values in North Sea sediments primarily reflected winter temperatures in agreement with the high abundance of Crenarchaeota in winter. However, it is impossible at this stage to estimate the seasonal bias in our core top sediments due to the lack of data on the seasonal abundance of Crenarchaeota in the study area Foraminiferal Assemblages [32] Studies on the distribution of planktonic foraminifera in the surface sediment offshore South Africa and Namibia [Giraudeau, 1993] showed that G. inflata was the dominant species in the continental slope area. The next dominant species was N. pachyderma (dex). On the contrary, in the upwelling area, N. pachyderma (sin) dominates the surface sediments. Similarly, according to ucera et al. [2005b], the most abundant species in the Benguela offshore was G. inflata (35 56%) and the next abundant one was N. pachyderma (dex) (12 29%). While fauna data from core top sediments have been published so far, studies on faunal data from water column are rare. In this study, we present limited faunal records from the water column corresponding to warm season, which makes it difficult to compare them directly to core top data. However, our plankton tow results nicely confirm what was found at the surface sediments, although they represent a distinct season, i.e., austral summer. It seems that during the warm season, the export flux of G. inflata from the upper part of water column to the bottom is significant for all stations including both continental slope and transitional areas. On the contrary, at station GeoB8333 in the upwelling area, N. pachyderma (sin) is a dominant species. At station GeoB8318, the transitional area, the percentage of N. pachyderma (sin) is relatively higher than at other stations more offshore, but it is lower than at the station in intensive upwelling area during the warm season. [33] In general, SST estimates reconstructed from faunal counts of core top sediments in the study area (faunal count source from ucera et al. [2005b]) agree well with satellite-derived SST values (15 16 C for cold season and C for warm season). However, it has been reported that a remarkable discrepancy between foraminiferal-based and alkenone-based temperatures exist in the Namibia upwelling system [Niebler et al., 2003]. Planktonic foraminiferal assemblages from two core top samples produce cold-biased SST estimates (9 12 C for cold season and C for warm season). These are cores RC (28.08S, 13.02E) and GeoB1719 (28.93S, 14.17E) located close to the Namibia upwelling system. The compositional change of planktonic foraminifera in these cores shows that N. pachyderma (dex) is the most dominant species for core RC (43%), and G. bulloides for core GeoB1719 (74%). It seems that the anomalously high percentage of N. pachyderma (dex) or G. bulloides cause the low SST estimates. The reason why N. pachyderma (dex) and G. bulloides are abundant in the core top sediment is not clear. However, considering our faunal data set, it is exceptional. There is a possibility that not only the seawater temperature but also other factors such as food availability could affect the assemblages in the specific environment [e.g., Giraudeau, 1993] Foraminiferal Stable Oxygen Isotopes [34] For the d 18 O values of G. inflata, variation between stations for surface water (0.3%) is relatively small compared to that for vertical distribution (1 1.5%). Since the d 18 O of foraminiferal calcite is known to increase with decreasing temperature [im and O Neil, 1997], the difference of 0.3% in G. inflata d 18 O values between the continental slope stations (GeoB8305, 8336, and 8338) and the transitional station (GeoB8318) at surface water is likely due to the different SSTs, i.e., 21 C at the slope stations and 18 C at station GeoB8318. According to Erez and Luz [1983], a difference of 0.3% corresponds to a temperature difference of 2 C. Salinity measurements in surface water show a small range of difference (0.37) between stations. Thus, it is likely that the heavier foraminiferal d 18 O value in the surface waters of station GeoB8318 corresponds to the lower temperature at this station compared to other 15 of 19

16 stations. Beside, the d 18 O of planktic foraminiferal calcite may be affected by the carbonate chemistry of the ambient seawater [Spero et al., 1997]. However, Wilke et al. [2006] found no significant relationship between the d 18 O of the shell of G. inflata and the carbonate ion concentration of the ambient seawater in the Cape Basin. [35] Foraminiferal d 18 O values become heavier with increasing water depth. To obtain continuous profiles of the oxygen isotopic composition of seawater (d 18 O w ), we used a linear relationship between salinity and d 18 O w established for Southern Ocean by Duplessy et al. [1991]. Then, the expected equilibrium value for the d 18 O of calcite was calculated using the equation of im and O Neil [1997]. The results are illustrated in Figure 6. While the d 18 O of G. inflata is generally close to the calculated isotopic equilibrium values within the surface mixed layer, the d 18 O values are lower than the equilibrium values by about 0.5% to 1.1% in deeper water depth. It seems that shell growth of G. inflata is not restricted to a constant depth level along the SW African continental margin, but continues with increasing water depth at lower temperature [Wilke et al., 2006]. [36] The vertical distribution pattern of G. inflata shows that they are abundant in the upper 100 m water column with a maximum occurrence at the depth of m. This indicates that this species lives at the surface water as well as subsurface water, and subsurface-dwelling species could be of importance for the flux from the surface to the bottom. On the basis of this it is highly likely that the core top G. inflata d 18 O could be biased toward lower temperatures. Unfortunately, we do not have any core top d 18 O data. However, our d 18 O values of G. inflata from the deepest depth interval at the southernmost station GeoB8305 (1%) are identical to the core top value (1%) published by Niebler et al. [1999] for a station (34.75 S and E) very close to our site. The d 18 O value of 0.6% found in the deepest nets at stations GeoB8336 and 8338 is similar to the value (0.5%) found by Lončarić et al. [2006] at the Walvis Ridge. 6. Implications [37] The ecological and physiological responses of organisms to environmental conditions other than temperature are one of the main issues in the reliable application of paleotemperature proxies. Our study shows that alkenones appear to be produced in the surface mixed layer and thus subsurface production is not a significant factor for alkenones in the study area. In contrast to alkenones, the results for GDGTs and foraminferal oxygen isotopes suggest subsurface production and calcification leading to a cold bias in temperature estimates based on these proxies. [38] Physiological impacts on alkenone unsaturation index are controversial. Previous studies suggest that physiological factors such as nutrient and light stress could contribute to variability in the alkenone unsaturation index [e.g., Prahl et al., 2003]. Recently published studies [Popp et al., 2006; Prahl et al., 2006] confirm that nutrient depletion causes a decrease of the unsaturation degree of alkenones. In contrast, Prahl et al. [2003] show that dark stress results in an increase of the alkenone unsaturation index, and thus has an opposite effect on U 0 37, compared to nutrient stress. In our study area, alkenone temperatures are in general consistent with in situ temperatures, even in the dynamic upwelling area, indicating that seawater temperature is a major control factor of the degree of unsaturation of alkenones, and other physiological factors such as nutrient and light conditions are less important and counteract each other in the Benguela upwelling system. [39] The influence of nutrient on the TEX 86 is unclear. Recently, it has been suggested that community changes along with nutrient availability affect TEX 86 -based temperature estimation. Turich et al. [2007] investigated archaeal lipids distributions from water column and surface sediments from various places, and suggest that changes of GDGT producer in surface water during nutrientrich episodes, such as upwelling, would result in elevated TEX 86 ratios and warmer reconstructed temperatures. However, this conclusion is currently under debate [Schouten et al., 2008; Turich et al., 2008]. Our data now allows to evaluate this hypothesis. If there was a community shift like Turich et al. [2007] suggested, the TEX 86 temperature would be overestimating in situ temperatures at the upwelling sites. In contrast we observe that TEX 86 systematically underestimates SST at sites in the upwelling area. This suggests that nutrientrelated compositional change is not a significant factor for TEX 86 temperature estimates and is also consistent with TEX 86 analysis from other upwelling areas which indicate underestimations of SST [Huguet et al., 2007; im et al., 2008]. [40] Another potential confounding factor in the application of organic proxies is the influence of physical oceanic processes such as oceanic currents 16 of 19

17 on the redeposition of organic matter. Mollenhauer et al. [2003] investigated radiocarbon age of alkenone and foraminifera in the continental slope and shelf off Namibia, near our study area, and found an age difference of more than 1000 years between them for surface sediments in the continental slope. They suggest that organic matter on the inner shelf area appear to be autochthonous, while finegrained organic matter, including alkenones, is mostly allochthonous in the continental slope. They suggest that tidal water movement near the bottom of the upper continental slope off Namibia is responsible for the resuspension and net seaward transport of organic aggregates. Thus, bottom water currents may have affected the composition of alkenones as well as GDGTs in our core tops. However, comparisons of alkenone temperature of surface water with in situ SST and core top alkenone temperature with annual average satellite SST suggest that influence of bottom water currents is relatively minor. If the organic matter was mostly derived from the continental shelf, alkenone temperature estimates at stations in the continental slope would be biased toward low temperatures which we do not observe here. TEX 86 estimates in core top sediments are lower than SST but this phenomenon is also observed in the water column and thus it does not necessarily indicate a large input of allochtonous organic matter. Furthermore, GDGTs seem to be less affected by input of long distance transported organic matter than alkenones [Mollenhauer et al., 2008]. 7. Summary [41] Our study of alkenone, GDGTs, and planktonic foraminifera in seawater and surface sediments from the continental margin off the western South Africa shows a varying picture for the different paleothermometers. Alkenone-based temperatures in surface waters compare in general well with in situ temperatures and those from core top sediments agree well with the annual average SSTs derived from satellite data. This, together with the vertical distributions of C 37 alkenone concentrations, shows that the U 0 37 index records annual mean SSTs in the eastern boundary current system of the South Atlantic. In contrast, results of GDGT analysis indicate that, for surface waters, the TEX 86 -based temperatures are in general colder than in situ SST, similar to core top sediments where the TEX 86 -based temperatures are also coldbiased compared to satellite-derived annual average SSTs. This may be due to upward transport of GDGTs produced below the mixed layer and offshore streaming filaments of upwelled cold waters in the coastal zone. [42] Foraminiferal abundance data from plankton tow samples show that during the warm season of the cruise period, G. inflata is the most dominant species in the water column of the offshore stations, while N. pachyderma (sin) dominates the upwelling area. The vertical distribution patterns of planktonic foraminifera show that G. inflata, N. pachyderma (sin), and N. pachyderma (dex) inhabit in the upper part of water column (0 100 m) with a maximum occurrence at the depth of m, whereas G. scitula is mostly present below the water depth of 100 m. Finally, a comparison of the average d 18 O values of G. inflata with those predicted by oxygen isotopic equilibrium values shows that shell growth is not restricted to a fixed depth but occurs over a depth range. This indicates that G. inflata d 18 O from the core top sediments could be biased toward low temperature due to subsurface calcification at lower temperature. [43] In summary, our study highlights that each paleothermometer may record different temperature signals (e.g., different season and water depth) and hence for correct interpretation of reconstructed temperature signals it is important to constrain differences between paleoproxies on regional scales. Acknowledgments [44] We thank R. Schneider and other participants in the cruise Meteor 57/1 and two reviewers for constructive comments. The AVHRR Oceans Pathfinder SST data were obtained from the Physical Oceanography Distributed Active Archive Center at the NASA Jet Propulsion Laboratory, Pasadena, California. This research was partly supported by the MA grant CATER References Andrews, W. R. H., and L. Hutchings (1980), Upwelling in the southern Benguela Current, Prog. Oceanogr., 9, 1 81, doi: / (80) Austin, N. E. H. (1980), Plankton of the Benguela Current: A preliminary survey, 127 pp., M.Sc. thesis, Univ. of Natal, Natal, South Africa. Bé, A. W. H. (1977), An ecological, zoogeographic and taxonomic review of recent planktonic foraminifera, in Oceanic Micropaleontology, edited by A. T. S. Ramsay pp , Academic, London. Brown, S. J., and H. Elderfield (1996), Variations in Mg/Ca and Sr/Ca ratios of planktonic foraminifera caused by postdepositional dissolution: Evidence of shallow Mg-dependent dissolution, Paleoceanography, 11, , doi: / 96PA of 19