- Email: [email protected]
Sediment trap record of alkenones from the East Sea (Japan Sea)
Organic Geochemistry 42 (2011) 255–261
Contents lists available at ScienceDirect
Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Sediment trap record of alkenones from the East Sea (Japan Sea) Kyung Eun Lee a,⇑, Boo-Keun Khim b, Shigeyoshi Otosaka c, Shinichiro Noriki d a
Division of Marine Environment and Bioscience, Korea Maritime University, Busan 606-791, South Korea Division of Earth Environmental System, Pusan National University, Busan 609-735, South Korea c Japan Atomic Energy Agency, Tokai 319-1195, Japan d Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan b
a r t i c l e
i n f o
Article history: Received 8 March 2010 Received in revised form 2 December 2010 Accepted 28 December 2010 Available online 12 January 2011
a b s t r a c t Long chain (C37) alkenones in sinking particles from the East Sea were examined using time-series sediment trap records from the eastern Japan Basin (trap depth 1057 m and 3043 m) and the Yamato Basin (1057 m and 2100 m) for the period of October 2000 to August 2001. The 11month records of alkenone flux show that the flux to the upper trap appears to be higher during summer (June to August) and fall (October to November) than during the rest of the sample collection period at both stations. However, the seasonal trends in alkenone flux to the lower trap do not show such a strong seasonality. In general the alkenone sinking flux decreased with increasing depth. Despite the decomposition of the alkenones, 0 there was little difference in UK37 values and hence alkenone temperatures between traps. A comparison of alkenone-derived temperatures with satellite sea surface temperatures (SSTs), however, reveals that the alkenone temperatures were higher (7–11 °C) than SST during winter time and lower (2–5 °C) during summer time. This indicates that there is a time lag between the SST and trap records (about 3 months in winter and 1 month in summer). The reasons for the time lag could be (i) slow settling velocity and long residence time of alkenones in the upper water column especially during winter time when alkenone flux was low, (ii) biotic and abiotic degradation of alkenones and (iii) subsurface production of alkenones in summer. The flux-weighted alkenone temperatures are close to annual average SST at the sites, indicating that the sedimentary alkenone signal from the seafloor may represent the annual averaged temperature. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The use of alkenones as a sea surface temperature (SST) proxy is based on the observation that certain haptophyte microalgae produce long chain (C37) unsaturated ketones (alkenones) whose degree of unsaturation changes with seawater temperature. Despite the general application of the method to the reconstruction of past SSTs from marine sediments, there have been several arguments concerning (i) the season and water depth of alkenone production (e.g. Bijma et al., 2001), (ii) the compositional changes in alkenone producing algal species (e.g. Volkman et al., 1995), (iii) physiological factors other than temperature (e.g. Prahl et al., 2003), (iv) the effect of biochemical decomposition and degradation on the alkenone unsaturation index (e.g. Gong and Hollander, 1999; Grimalt et al., 2000) and (v) lateral transport of alkenones by oceanic currents (e.g. Ohkouchi et al., 2002; Rühlerman and Butzin, 2006). These are important issues to be examined in reconstructing accurate paleotemperature and climate change. One of the best ap-
proaches for studying these factors is to investigate time-series records from sediment trap deployment. Although alkenones are generally used as a proxy for SST, they can also be used as a tracer to identify water column processes in sinking particles in the ocean. Since alkenones keep the surface signal within them, the sediment trap record of alkenones actually provides important information on biogeochemical processes in the water column. In this study, we examined seasonal variations in the fluxes of alkenones and other biological components of sinking particles collected from sediment trap deployment (2000– 2001) at water depths of 1057, 2100 and 3043 m in the East Sea. The results make it possible to investigate the characteristics of production, export and degradation of alkenones in the water column and their influence on the alkenone unsaturation index. This will help evaluate the use of alkenones as a paleothermometer. Also, the results should provide important information on the biogeochemical characteristics of the East Sea.
2. Oceanographic setting ⇑ Corresponding author. Tel.: +82 (0)51 410 4759; fax: +82 (0)51 404 4750. E-mail address: [email protected] (K.E. Lee). 0146-6380/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2010.12.008
The East Sea, a semi-enclosed marginal sea in the northwestern Pacific, is characterized by the inflow of the Tsushima Warm
256
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
The East Sea waters below 200 or 300 m are considered to be formed offshore of Vladivostok by severe cooling of the surface water in winter (Kim et al., 2002). The waters are very cold (<1 °C) and highly oxygenated (300–250 lM) (Talley et al., 2004). At a depth of 1000 m, the dissolved oxygen concentration is 215 lM in the Yamato Basin and 230 lM in the Japan Basin. Near the bottom, it is 210 lM in the Yamato Basin and 215 lM in the Japan Basins. The vertical distribution pattern of nutrient concentration shows that nitrate concentration rapidly increases from zero at the surface to 24.6 lM at 1000 m and increases slightly to 25.8 lM near the bottom in the all basins. Other nutrient concentrations show a pattern similar to that of nitrate.
Current (TWC) through the Korea Strait and outflow through the Tsugaru and Soya Straits (Fig. 1). The TWC separates into two main branches. One flows along the Japanese coast and the other, East Korean Warm Current (EKWC), flows northward along the Korean coast and eastward at the shore near 38°N. In the north, the North Korean Cold Current (NKCC) or Liman Current flows southwards along the western boundary. In the central part of the East Sea, there is a thermal boundary in SST, the Subpolar Front (SPF). The SPF is located roughly along 38–40°N and separates warm water to the south from cold water to the north. The circulation system is driven by the inflow–outflow conditions and wind stress curl. Basically, the inflow–outflow conditions create an anti-cyclonic gyre like the EKWC in the south and a positive wind stress curl generates a cyclonic gyre like the NKCC in the north. The boundary between the EKWC and the NKCC is the SPF. At the surface, there is an evident seasonal variation in seawater temperature. In general, a basin-scale cold temperature dominates from December to May, with maximum cooling in February, and basin-scale warming from June to October, with the maximum warming in August. According to satellite SST datasets from 1990 to 1995, the temperature of the sea is in the range 4–13 °C in winter and 20–26 °C in summer (Park et al., 2004). In the south, SST is in the range 11–13 °C in winter and 25–26 °C in summer, while it is 4–7 °C in winter and 20–22 °C in summer in the north.
3. Materials and methods Sinking particles were collected using a sediment trap system deployed for 11 months in the eastern Japan Basin (station JN), and the Yamato Basin (JS-2) (Fig. 1). The deployment data are summarized in Table 1. Two traps were installed at each station. The upper and lower traps were at depths of 1057 m and 3043 m at station JN and 1057 m and 2100 m at station JS-2. The lower traps were installed about 500 m above the seafloor. Samples from JN and JS-2 were obtained between October 6, 2000 and August 14, 2001 at 26 day intervals. Sampling cups of the sediment trap were
TARTAR STRAIT (12 m)
1000 m 2000 m
SOYA STRAIT (55 m)
RUSSIA
45 O N
3000 m
Vladivostok
JN TSUGARU STRAIT (130 m)
Japan Basin SPF NKCC
40 O N Yamato Basin
JS-2 KOREA
Ulleung Basin EKWC
JAPAN
35 O N
TWC KOREA STRAIT (140 m)
130 O E
135 O E
140 O E
Fig. 1. Bathymetric map of the East Sea and sediment trap locations. TWC, EKWC, NKCC and SPF indicate Tsushima Warm Current, East Korea Warm Current, North Korea Cold Current and Subpolar Front, respectively. Black arrows indicates warm current and white arrows cold current.
257
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261 Table 1 Station data for mooring sediment trap system.
Table 2 C37 alkenone content and flux data for sediment traps.
Trap
Latitude (°N)
Longitude (°E)
Water depth (m)
Trap depth (m)
Collection period
Sample
JN
42.47
138.50
3642
JS-2
38.02
135.03
2900
1057, 3043 1057, 2100
October 2000– August 2001 October 2000– August 2001
filled with a neutral buffer (salinity controlled solution of 38 psu with 5% formalin to prevent biological degradation). Recovered samples were stored at 5 °C. The material in each cup was filtered through a 0.6 lm membrane filter after swimmers were removed. Particles on the filter were rinsed with Milli-Q water to remove salts and formalin and dried for 36 h at 60 °C (Otosaka et al., 2004). Particle samples (1–3 g) were taken for alkenone analysis. They were analyzed at JAMSTEC, Japan. Organic compounds were extracted using a Dionex accelerated solvent extractor (ASE 200) with a mixture of solvent (CH2Cl2:CH3OH, 99:1 v/v) at high temperature (100 °C) and pressure (1500 psi). The extracts were saponified at 80 °C for 2 h with 0.1 M KOH in 90/10 CH3OH/H2O. The neutral fraction was recovered by partitioning into hexane. After that, the extract was separated using an automatic solid preparation system (Rapid Trace SPE Workstation, Zymak, UK). The solvents used were, for fraction 1, 4 ml of hexane; for fraction 2, a mixture of 2 ml hexane/toluene (3:1 v/v), 2 ml hexane/toluene (1:1 v/v), and 2 ml hexane/ethyl acetate (95:5 v/v); for fraction 3, 2 ml hexane/ethyl acetate (9:1 v/v) and 2 ml hexane/ethyl acetate (85:15 v/ v); and for fraction 4, 4 ml hexane/ethyl acetate (4:1 v/v), followed by 2 ml ethyl acetate. The alkenones eluted in fraction 3. An aliquot of the alkenone fraction was gently concentrated under N2 and analyzed using gas chromatography with an Agilent 6890 N chromatograph equipped with a flame ionization detector and a HP5MS column (60 m 0.32 mm i.d.). Reproducibility of alkenone temperature assessment for replicate samples was <1 °C. Alkenone temperatures were calculated using the widely used 0 calibration equation of Prahl et al. (1988; UK37 = 0.034T + 0.039). The calibration equation was based upon analysis of Emiliania huxleyi collected from the northeast Pacific and grown in culture over the water temperature range from 8° to 25 °C. The culture calibration is almost identical to the global coretop calibration compiled 0 by Müller et al. (1998; UK37 = 0.033T + 0.044). A study of coccolithophorid distribution at coretop sediments from the East Sea shows that E. huxleyi is the dominant species in the area (Ishiwatari et al., 2001).
4. Results The 11 month records of alkenone concentration and flux were obtained from the upper and lower traps. The alkenone results are listed in Table 2. Four sample materials (JN3KA-1, 5, 6 and JSKA-1) were not available for measurement and their values are missing from the results. The alkenone concentration and flux for the upper trap at JN were relatively high (flux mean 9.8 ± 7.7 ng/m2/d) during October and November 2000 and remained low (6.4 ± 2.8 ng/m2/d) from December 2000 to May 2001 (Fig. 2a and c). The flux increased in June and remained high (16.2 ± 15.2 ng/m2/d) until August. It appears that the alkenone concentration and flux from the surface were relatively high during summer and fall and low during winter and spring. However, the lower trap records at JN showed no clear temporal variation. The mean value of alkenone flux at the lower trap was 0.9, 1.1 ± 0.6, and 1.6 ± 1.9 ng/m2/d for each interval. At JS-2, alkenone concentration and flux recorded a clear seasonality for the upper trap (Fig. 2b and d). The alkenone
0
0
Trap open date (yyyy.month.day)
Alk. content (lg/g)
Alk. flux (ng/m2/d)
UK37
UK37 T (°C)
1057 m JNKA-1 JNKA-2 JNKA-3 JNKA-4 JNKA-5 JNKA-6 JNKA-7 JNKA-8 JNKA-9 JNKA-10 JNKA-11 JNKA-12 JNKA-13
2000.10.6 2000.11.1 2000.11.27 2000.12.23 2001.1.18 2001.2.13 2001.3.11 2001.4.6 2001.5.2 2001.5.28 2001.6.23 2001.7.19 2001.8.14
8.1 10.9 6.0 3.4 2.6 2.5 2.4 3.7 3.9 5.7 20.2 11.9 7.6
4.4 15.3 8.1 4.1 2.7 3.2 6.7 7.3 8.5 10.5 33.9 10.1 4.8
0.749 0.675 0.598 0.587 0.577 0.494 0.342 0.266 0.298 0.352 0.440 0.522 0.599
20.9 18.7 16.4 16.1 15.8 13.4 8.9 6.7 7.6 9.2 11.8 14.2 16.5
3043 m JN3KA-1 JN3KA-2 JN3KA-3 JN3KA-4 JN3KA-5 JN3KA-6 JN3KA-7 JN3KA-8 JN3KA-9 JN3KA-10 JN3KA-11 JN3KA-12 JN3KA-13
2000.10.6 2000.11.1 2000.11.27 2000.12.23 2001.1.18 2001.2.13 2001.3.11 2001.4.6 2001.5.2 2001.5.28 2001.6.23 2001.7.19 2001.8.14
– 1.9 1.2 1.8 – – 1.4 2.1 1.4 0.8 5.4 2.1 1.4
–
1.8 1.6 1.3 0.6 3.8 0.6 0.3
– 0.641 0.547 0.514 – – 0.396 0.359 0.402 0.412 0.426 0.503 0.575
– 17.7 14.9 14.0 – – 10.5 9.4 10.7 11.0 11.4 13.6 15.8
1057 m JSKA-1 JSKA-2 JSKA-3 JSKA-4 JSKA-5 JSKA-6 JSKA-7 JSKA-8 JSKA-9 JSKA-10 JSKA-11 JSKA-12 JSKA-13
2000.10.6 2000.11.1 2000.11.27 2000.12.23 2001.1.18 2001.2.13 2001.3.11 2001.4.6 2001.5.2 2001.5.28 2001.6.23 2001.7.19 2001.8.14
– 24.4 10.2 8.8 2.6 2.2 5.1 1.6 8.1 6.5 17.1 21.1 33.6
– 53.9 18.5 17.5 9.0 8.5 31.5 5.9 19.2 10.2 20.5 40.1 56.8
– 0.743 0.735 0.666 0.612 0.552 0.348 0.337 0.417 0.449 0.516 0.734 0.623
– 20.7 20.5 18.4 16.8 15.1 9.1 8.8 11.1 12.1 14.0 20.4 17.2
2100 m JS3KA-1 JS3KA-2 JS3KA-3 JS3KA-4 JS3KA-5 JS3KA-6 JS3KA-7 JS3KA-8 JS3KA-9 JS3KA-10 JS3KA-11 JS3KA-12 JS3KA-13
2000.10.6 2000.11.1 2000.11.27 2000.12.23 2001.1.18 2001.2.13 2001.3.11 2001.4.6 2001.5.2 2001.5.28 2001.6.23 2001.7.19 2001.8.14
4.8 6.9 6.0 4.6 2.9 1.6 6.1 3.5 6.8 2.5 2.9 7.1 19.9
3.9 15.2 10.8 9.3 10.0 6.2 37.2 13.0 16.1 4.0 3.5 13.5 33.7
0.743 0.729 0.735 0.710 0.654 0.622 0.372 0.379 0.416 0.517 0.560 0.709 0.632
20.7 20.3 20.5 19.7 18.1 17.2 9.8 10.0 11.1 14.1 15.3 19.7 17.5
0.9 0.4 0.7 – –
flux was relatively high (53.9 ng/m2/d) in November 2000 and remained relatively low (15.0 ± 8.4 ng/m2/d) during December to May 2001. There was a relatively high flux of alkenones in March. After then, they increased during July and August 2001 (39.1 ± 18.2 ng/m2/d). The seasonal trends in the lower trap were not like those observed in the upper trap. The lower trap flux values were 9.5 ± 8.0, 13.3 ± 10.3, and 16.9 ± 15.3 ng/m2/d for each interval. At JN, the alkenone-derived temperature of the samples was 21 °C in October 2000 and decreased to 16 °C in December 2000 (Fig. 2e). It then dropped to 7–9 °C in March to June 2001. Subsequently, it increased gradually up to 16 °C in August. The lowest
258
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
Fig. 2. Variations in the content (a and b) and flux (c and d) of C37 alkenones and the alkenone-based temperature (e and f) at station JN and station JS-2 from October 2000 to August 2001. Closed circle indicates data from the upper trap. Open circle indicates data from the lower trap. Gray bar and dash line indicate the mean value of alkenone flux for each interval. Numbers are mean and standard deviation of alkenone flux for upper trap and numbers with parentheses are for lower trap. Open rectangle indicates satellite-derived sea surface temperature for the sampling period. Arrow indicates time lag between SST and trap alkenone temperatures.
alkenone temperature of 7 °C occurred in April 2001. The seasonal variation in the lower trap records shows similar patterns. At JS-2, the alkenone temperature was 20–21 °C in October and November 2000 and gradually decreased to 15 °C in January and February 2001 (Fig. 2f). Between March and April, the temperature was 9 °C, with the lowest temperature in April. After then, it increased to 20 °C in July 2001. A comparison of alkenone temperature between the upper and the lower traps illustrates that the difference between the traps was relatively small. At station JN, the difference was in the range of 3.1 to +2.1 °C through the sampling collection period. At JS-2, it was 2.1 to +0.8 °C. The alkenone temperatures were compared to actual SSTs for the station for the sample collection period. The SST data used were from the Satellite Image Data Base (NOAA Local Area Coverage data) of the Agriculture, Forestry and Fisheries Research Center, Japan (http://rms0.agsearch.agropedia.affrc.go.jp/sidab/). Seasonal variations in SST at stations JN and JS-2 are plotted in Fig. 2e and f. The minimum and maximum satellite temperatures were 4 °C in February and 21 °C in August at JN. At JS-2, they were 9 °C in February and 26 °C in August, respectively. A comparison of the sediment trap alkenone temperatures with the satellite SST shows that there are clear differences between them. The differences were large, up to 11 °C at JN and 7 °C at JS-2. The alkenone temperatures were higher than the SST during October 2000 to
March 2001, while they were lower during May to August 2001. It appears that there was a time lag between the SST and upper trap temperature (Fig. 2e and f). The time lag is 2–3 months in winter and about 1 month in summer. Although there was a significant time lag between the SST and trap alkenone temperatures, the flux-weighted alkenone temperatures were close to the 11 month averaged SST. At JS-2, the fluxweighted alkenone temperature for the lower trap was 15 °C, which is consistent with the averaged SST of 16 °C. At JN, the flux-weighted alkenone temperature of 12 °C was close to the averaged SST of 11 °C.
5. Discussion 5.1. Seasonality in alkenone flux Fluxes of total mass, biogenic opal and particulate organic carbon (POC) in the upper and lower traps at JN and JS-2 have been reported (Otosaka et al., 2004, 2008). According to the results, biogenic opal comprises ca. P30% of the total mass at both stations, indicating that it is the major component of the settling particles through the sample collection period. POC accounts for 9–10% of total mass in the upper trap of both stations, and 6–7% in the lower
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
2.0
80
400
1.5
60
300
1.0
40
200
0.5
20
100
0
0
0
80
400
0.6
60
300
0.4
40
200
0.2
20
100
0
0
0
0.8
2
2
JN
O N D J
F M A M J
2
JS -2 2
Chlorophyll a (mg/m 3 )
3
Chlorophyll a (mg/m )
traps. Records of seasonal variation in sinking particle fluxes illustrate that biogenic opal and POC fluxes were high during the period of March and April (Fig. 3). On the contrary, a higher flux of C37 alkenones for the upper trap occurred during summer and fall (July–August and October–November) than during the rest of the year at both stations in general. The time lag between biogenic opal flux and alkenone flux may be indicative of the ecological succession from the blooming of opal-secreting diatoms in spring to a high production of alkenone-producing coccolithophores in summer and fall. According to SEAWIFS data recovered during the sample collection period, the highest chlorophyll a concentration at surface occurred in spring, while it was lowest in summer, especially at JS-2 (Fig. 3). This is consistent with the sediment trap results. At JS-2, the highest concentration of chlorophyll a in spring was considered to be associated with an opal-secreting diatom bloom in the East Sea. There is a positive correlation coefficient between chlorophyll a concentration and opal and POC fluxes (correlation coefficient, 0.833 for opal and 0.825 for POC; the significance of the correlation coefficient was tested with confidence level of 0.5). The bloom may be related to the occurrence of the deep mixed layer and associated high nutrient concentration in spring, while a high production of coccolithophores seems to be related to the shallow mixed layer in summer (e.g. Lee et al., 2003). Diatoms are known to compete well in nutrient rich (especially with high silicate concentration) cold water systems and coccolithophores compete well in warm, low-nutrient water environments (Werne et al., 2000). At JN, there is no correlation between chlorophyll a concentration and opal and POC fluxes. A similar pattern in the time lag between opal and alkenone fluxes at JS-2 has been observed from sediment trap studies in the mid- and high-latitude NW Pacific (Sawada et al., 1998; Harada et al., 2006; Yamamoto et al., 2007). Although the upper trap records show clear seasonality in alkenone flux, the lower trap records were different. As far as transfer of surface signal to the seafloor is concerned, the lower trap record is more important than the upper trap results. Because alkenones
J A
Fig. 3. Variation in satellite-derived chlorophyll a concentration at sea surface (closed circle) and particulate organic carbon flux (gray line) and opal flux (dashed line) at the upper trap of station JN and JS-2 from October 2000 and August 2001. (Data from SEAWIFS and Otosaka et al. (2004, 2008).)
259
are used as a proxy for SST in order to reconstruct past SST from the sediments. In this case, it is important to know the season to which the alkenone temperature reconstruction refers. It most likely depends on seasonality of alkenone flux to the seafloor. The average flux of C37 alkenones to the lower trap was 1.2 ng/ m2/d at JN and 13.6 ng/m2/d at JS-2. There was no strong seasonality in the lower trap alkenone flux record at JN (Fig. 2c). At JS-2, the alkenone flux to the lower traps was relatively high in March and August. However, the mean value of each interval (Fig. 2d) indicates no strong seasonality. In addition, since the flux-weighted alkenone temperatures were close to the 11 month averaged SST, the sedimentary alkenone signal from the seafloor is considered to represent the annual averaged value. Alkenone analysis of sediment trap material from two different depths (1057 and 3043 m at JN, and 1057 and 2100 m at JS-2) showed that the alkenone sinking flux decreased with increasing depth. The average ratio of lower to upper trap values for alkenones flux was 12% at JN and 75% at JS-2, suggesting that decomposition was significant in the water column between traps at JN, but less effective at JS-2. Despite the decomposition of the alkenones, 0 there was little difference in UK37 values and hence alkenone temperatures between traps at both stations. The averaged value of 0 difference in UK37 -based temperatures between traps was 0.3 ± 1.8 °C for JN and 0.7 ± 0.9 °C for JS-2. T-test showed that the upper and the lower trap values were not different at JN, but they were different at JS-2 with 95–98% confidence range. However, if we consider reproducibility of alkenone temperature less than 1 °C, the difference is not significant.
5.2. Alkenone temperatures An important feature of our results is that there is almost no time lag between the upper and the lower trap alkenone temperatures, but there is a time lag between the SST and trap results. The time lag is relatively long (ca. 2–3 months) for the winter season but short (ca. 1 month) for the summer season. Four possibilities can be considered for this time lag between surface production and water column. First, alkenones were produced at the surface but, their settling velocity in the upper water column was slow and residence time was relatively long before sinking, especially in winter. Second, alkenones were synthesized in another area and transported by way of lateral advection. Third, biotic and abiotic degradation of alkenones affected the degree of unsaturation in the water column. Fourth, the depth of alkenone production may have been deeper than the surface. First of all, the time lag can be attributed to different settling velocity for the sinking particles of alkenones in the water column: (i) slow in winter and relatively fast in summer and (ii) slow in the upper water column and relatively fast in the deep water column. The 3 month time lag between the SST and the upper trap alkenone temperatures in winter indicates the settling velocity of 10 m/d for the alkenones in the upper water column between the surface and the upper trap. The 1 month time lag in summer indicates the settling velocity of 33 m/d in the upper water column. Since there is almost no time lag between the upper and the lower trap alkenone temperatures, the settling velocity of alkenones seems to become faster in the deep water than that in the upper water. In Fig. 3, the seasonal distribution pattern in chlorophyll a concentration at the East Sea surface is consistent with those of POC and opal fluxes from the upper traps at JS-2. This indicates that there might be no time lag between chlorophyll a and POC and opal, and POC and opal particles most likely settle down from the surface to a depth of 1057 m within the sampling interval of 26 days, suggesting that they rapidly sink with a settling velocity of ca. 80–117 m/d especially for spring (March–April–May). If this is true, the sinking
260
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
particles of alkenones settle down slowly compared to POC and opal. Another possibility is lateral advection effects on the alkenone signal (e.g. Ohkouchi et al., 2002; Harada et al., 2006; Rühlerman and Butzin, 2006). In the East Sea, the temperature gradient at the surface is small at a given time period. A difference in SST between the Korea Strait, the warmest part of the sea, and station JS2 is ca. only 1 °C in summer and 3 °C in winter (Park et al., 2004). This difference cannot explain the difference between the SST and trap alkenone temperatures. Also the alkenone temperatures were higher than the SST in winter and lower than the SST in summer. If lateral advection is an important factor, alkenone temperatures would be consistently higher than the SST regardless of season. Because main currents flow from the south to the north and SST in the south is always higher than that in the north. In addition, the flux-weighted alkenone temperatures were close to the 11 month averaged SST at both stations. These indicate that the possibility of lateral transport of alkenones in the East Sea is less likely. Previous studies suggest that degradation processes do not significantly alter the degree of unsaturation of alkenones in particles settling in water column (Bentaleb et al., 1999; Grimalt et al., 2000). However, it has been suggested by different authors that biotic degradation and abiotic degradation of alkenones affect the 0 UK37 values of particles in the water column and sediments (Rontani and Wakeham, 2008; Rontani et al., 2006, 2008). According to these studies, biotic and abiotic degradation of alkenones leads to a bias towards warm alkenone temperatures. In the East Sea, the warm alkenone signal in winter may be associated with such degradation in water column. There is a possibility that the degree of unsaturation of alkenones in the upper water column was influenced by such degradation during winter when the alkenone flux was low and the residence time was long. However, the magnitude 0 of the change in UK37 values resulting from degradation (2–3 °C) is not expected to be sufficiently large to explain the difference in SST and alkenone temperature (up to 11 °C). The use of alkenones as a SST proxy is based on the assumption that alkenone production occurs at or close to the sea surface, so that the surface signal is preserved within organic material. However, previous studies suggest that significant production of alkenones may occur below the surface mixed layer, and the alkenone-based temperatures could be biased toward a subsurface temperature significantly colder than the surface temperature (e.g. Prahl et al., 1993; Ternois et al., 1997; Bentaleb et al., 1999; Ohkouchi et al., 1999). A recent study of alkenone production depth in the Pacific Ocean (Lee and Schneider, 2005) shows that, in the western boundary current and equatorial regions, where the chl. maximum occurs near the surface, the alkenone concentration maximum is near the surface. In contrast, in the subtropical gyre, where the chl. maximum occurs in the subsurface, the alkenone concentration maximum occurs at the depth of the chl. maximum (ca. 100 m). In the East Sea, the vertical distribution pattern of chl. data from JODC database (web site: http://www.jodc.go.jp) shows that the subsurface chl. maximum occurs only during summer. Hence, there is a possibility that the alkenones were produced at the subsurface during summer. The low alkenone temperatures for summer may be related to this reason. However, this is not the case for winter. During winter, the surface mixed layer was deep in the East Sea and there was no significant chl. maximum at the subsurface water. Furthermore, in winter, alkenone temperatures were higher than the SST, which cannot be explained by subsurface production. A time lag between the SST and alkenone temperature was observed not only in the East Sea but also in the northwestern Pacific Ocean. Harada et al. (2006) investigated time-series sediment trap records from station KNOT (44°N, 155°E) and found higher alkenone temperatures than SST during winter (season for low export
of alkenones). They interpreted this phenomenon by way of contribution of allochthonous alkenones in particulate material transported from a subtropical area within a warm-core ring. However, based on time-series sediment trap records from a station (39°N, 147°E) in the mid-latitude northwestern Pacific, Yamamoto et al. (2007) documented that the time lag was attributed to seasonal variation in residence time rather than a lateral transport effect. They suggested that fresh and labile particles sank from spring to fall, while old and stable particles sank from fall to spring. A sediment trap alkenone record from a station of 34°100 N, 142°E (Sawada et al., 1998) corroborates the results of ours and Yamamoto et al. (2007). A time lag was also observed at station ALOHA in the subtropical North Pacific. Alkenone temperatures overestimate in situ winter temperature, while they underestimate in situ summer temperature in the surface mixed layer (Prahl et al., 2005; Popp et al., 2006). These authors suggest that nonthermal physiological factors such as light and nutrient concentration account for the discrepancies in measured and estimated temperatures. 6. Conclusions Sediment trap records of biogenic components including alkenones from the East Sea provide important information about the biogeochemical characteristics of the East Sea. The upper trap records of seasonal variation in sinking particles at JS-2 illustrate that biogenic opal and POC fluxes were high during spring (March and April), whereas C37 alkenone-producing coccolithophores were more abundant during summer and fall (July–August and October–November) than the rest of the year. This suggests that the phytoplankton community in the area may have changed seasonally from domination by opal-secreting diatoms in spring to domination by CaCO3-secreting coccolithophores in summer and fall. Although the alkenone flux shows that the flux to the upper trap appears to be higher during summer and fall, the flux to the lower trap does not show such a clear seasonality. In addition, flux-weighted alkenone temperatures are close to annual average SST at the sites, indicating that sedimentary alkenone signal from the seafloor may represent the annual averaged temperature. Our results indicate that the sinking flux decreased with increasing 0 depth in general, but there was little difference in UK37 values and hence alkenone temperatures between traps. However, a comparison of alkenone-derived temperatures with satellite sea surface temperatures (SSTs) reveals that the alkenone temperatures were higher (7–11 °C) than SST during winter time and lower (2–5 °C) during summer time. This indicates that there is a time lag between the SST and trap records, and the time lag is relatively long (ca. 2–3 months) for the winter season but short (ca. 1 month) for the summer season. This is probably because, during winter when alkenone flux is low, the particles with alkenones appear to sink slowly, leading to a relatively long residence time in the upper water column. Also there is a possibility that degradation processes influenced the degree of alkenone unsaturation of sinking particles. In summer when alkenone flux increased, alkenones appeared to rapidly sink. Also, there is a possibility that subsurface production caused the cold alkenone temperatures. Acknowledgments We thank N. Harada for providing the chance to use her laboratory facilities, J.-Y. Choi for laboratory assistance and P. Helmke for SEAWIFS chlorophyll data. We also thank two reviewers for constructive comments and suggestions. NOAA Local Area Coverage data (MCSST) was obtained from the Agriculture, Forestry and Fisheries Research Information Center, Ministry of Agriculture,
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
Forestry and Fisheries (MAFF), Tsukuba, Japan. The work was funded by Research Agency for Climate Science (RACS 20093001) and Global Joint R&D Program (Grant 2007-00206) through the Korea Foundation for International Cooperation of Science and Technology.
Associate Editor—E.A. Canuel
References Bentaleb, I., Grimalt, J.Q., Vidussi, F., Marty, J.-C., Martin, V., Denis, M., Hatté, C., Fontugne, M., 1999. The C37 alkenone record of seawater temperature during seasonal thermocline stratification. Marine Chemistry 64, 301–313. Bijma, J., Altabet, M., Conte, M., Kinkel, H., Versteegh, G.J.M., Volkman, J.K., Wakeham, S.G., Weaver, P.P., 2001. Primary signal: ecological and environmental factors-report from working group 2. Geochemistry Geophysics Geosystems 2. 2000GC000051. Gong, C., Hollander, D., 1999. Evidence for differential degradation of alkenones under contrasting bottom water oxygen conditions: implication for paleotemperature reconstruction. Geochimica et Cosmochimica Acta 63, 405– 411. } tter, J., Sicre, M.A., Summons, R., Farrington, J., Harvey, H.R., Grimalt, J.O., Rullko Goni, M., Sawada, K., 2000. Modifications of the C37 alkenone and alkenoate composition in the water column and sediment: possible implications for sea surface temperature estimates in paleoceanography. Geochemistry Geophysics Geosystems 1, GC000053. Harada, N., Sato, M., Shiraishi, A., Honda, M., 2006. Characteristics of alkenone distributions in suspended and sinking particles in the northwestern North Pacific. Geochimica et Cosmochimica Acta 70, 2045–2062. Ishiwatari, R., Houtatsu, M., Okada, H., 2001. Alkenone-sea surface temperatures in the Japan Sea over the past 36 kyr: warm temperatures at the last glacial maximum. Organic Geochemistry 32, 57–67. Kim, K.-R., Kim, G., Kim, K., Lobanov, V., Ponomarev, V., 2002. A sudden bottomwater formation during the severe winter 2000–2001: the case of the East/ Japan Sea. Geophysical Research Letters 29, 1234–1237. Lee, K.E., Schneider, R., 2005. Alkenone production in the upper 200 m of the Pacific Ocean. Deep-Sea Research I 52, 443–456. Lee, K.E., Bahk, J.J., Narita, H., 2003. Temporal variations in productivity and planktonic ecological structure in the East Sea (Japan Sea) since the last glaciation. Geo-Marine Letters 23, 125–129. Müller, P.J., Kirst, G., Ruhland, G., Storch, I.,0 Rosell-Melé, A., 1998. Calibration of the alkenone paleotemperature index UK37 based on core tops from the eastern South Atlantic and the global ocean (60°N–60°S). Geochimica et Cosmochimica Acta 62, 1757–1772. Ohkouchi, N., Kawamura, K., Kawahata, H., Okada, H., 1999. Depth ranges of alkenone production in the central Pacific Ocean. Global Biogeochemical Cycles 13, 695–704. Ohkouchi, N., Eglinton, T.I., Keigwin, L.D., Hayes, J.M., 2002. Spatial and temporal offsets between proxy records in a sediment drift. Science 298, 1224–1227.
261
Otosaka, S., Togawa, O., Baba, M., Karasev, E., Volkov, Y.N., Omata, N., Noriki, S., 2004. Lithogenic flux in the Japan Sea measured with sediment traps. Marine Chemistry 91, 143–163. Otosaka, S., Tanaka, T., Togawa, O., Amano, H., Karasev, E.V., Minakawa, M., Noriki, S., 2008. Deep sea circulation of particulated organic carbon in the Japan Sea. Journal of Oceanography 64, 911–923. Park, K., Chung, J.Y., Kim, K., 2004. Sea surface temperature fronts in the East (Japan) Sea and temporal variations. Geophysical Research Letters 31. doi:10.1029/ 2004GL019424. Popp, B.N., Prahl, F.G., Wallsgrove, R.J., Tanimoto, J., 2006. Seasonal patterns of alkenone production in the subtropical oligotrophic North Pacific. Paleoceanography 21, PA1004. doi:10.1029/2005PA001165. Prahl, F.G., Muehlhausen, L.A., Zahnle, D.L., 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochimica et Cosmochimica Acta 52, 2303–2310. Prahl, F.G., Collier, R.B., Dymond, J., Lyle, M., Sparrow, M.A., 1993. A biomarker perspective on prymnesiophyte productivity in the northeast Pacific Ocean. Deep-Sea Research I 40, 2061–2076. Prahl, F.G., Wolfe, G.V., Sparrow, M.A., 2003. Physiological impacts on alkenone paleothermometry. Paleoceanography 18. 2002PA000803. Prahl, F., Popp, B.N., Karl, D.M., Sparrow, M.A., 2005. Ecology and biogeochemistry of alkenone production at Station ALOHA. Deep-Sea Research I 52, 699–719. Rontani, J.-F., Wakeham, S., 2008. Alteration of alkenone unsaturation ratio with depth in the Black Sea: potential roles of stereomutation and aerobic biodegradation. Organic Geochemistry 39, 1259–1268. Rontani, J.-F., Marty, J.-C., Miquel, J.-C., Volkman, J.K., 2006. Free radical oxidation (autoxidation) of alkenones and other microalgal lipids in seawater. Organic Geochemistry 37, 354–368. Rontani, J.-F., Harji, R., Guasco, S., Prahl, F.G., Volkman, J.K., Bhosle, N.B., Bonin, P., 2008. Degradation of alkenones by alkenones by aerobic heterotrophic bacteria: selective or not? Organic Geochemistry 39, 34–51. Rühlerman, C., Butzin, M., 2006. Alkenone temperature anomalies in the BrazilMalvinas Confluence area caused by lateral advection of suspended particulate material. Geochemistry Geophysics Geosystem 7, Q10015. Sawada, K., Handa, N., Nakatsuka, T., 1998. Production and transport of long-chain alkenones and alkyl alkenones in a sea water column in the northwestern Pacific off central Japan. Marine Chemistry 59, 219–234. Talley, L.D., Tishchenko, P., Luchin, V., Nedashkovskiy, A., Sagalaev, S., Kang, D.-J., Warner, M., Min, D.-H., 2004. Atlas of Japan (East) Sea hydrographic properties in summer 1999. Progress in Oceanography 61, 277–348. Ternois, Y., Sicre, M.-A., Boireau, A., Conte, M.H., Eglinton, G., 1997. Evaluation of long-chain alkenones as paleotemperature indicators in the Mediterranean Sea. Deep Sea Research I 44, 271–286. Volkman, J.K., Barrett, S.M., Blankburn, S.I., Sikes, E.L., 1995. Alkenones in Gephyrocapsa oceanica: implications for studies of paleoclimate. Geochimica et Cosmochimica Acta 59, 513–520. Werne, J.P., Hollander, D.J., Lyons, T.W., Peterson, L.C., 2000. Climate-induced variations in productivity and planktonic ecosystem structure from the Younger Dryas to Holocene in the Cariaco Basin, Venezuela. Paleoceanography 15, 19– 29. Yamamoto, M., Shimamoto, A., Fukuhara, T., Naraoka, H., Tanaka, Y., Nishimura, A., 2007. Seasonal and depth variations in molecular and isotopic alkenone composition of sinking particles from the western North Pacific. Deep-Sea Research I 54, 1571–1592.
Contents lists available at ScienceDirect
Organic Geochemistry journal homepage: www.elsevier.com/locate/orggeochem
Sediment trap record of alkenones from the East Sea (Japan Sea) Kyung Eun Lee a,⇑, Boo-Keun Khim b, Shigeyoshi Otosaka c, Shinichiro Noriki d a
Division of Marine Environment and Bioscience, Korea Maritime University, Busan 606-791, South Korea Division of Earth Environmental System, Pusan National University, Busan 609-735, South Korea c Japan Atomic Energy Agency, Tokai 319-1195, Japan d Graduate School of Environmental Earth Science, Hokkaido University, Sapporo 060-0810, Japan b
a r t i c l e
i n f o
Article history: Received 8 March 2010 Received in revised form 2 December 2010 Accepted 28 December 2010 Available online 12 January 2011
a b s t r a c t Long chain (C37) alkenones in sinking particles from the East Sea were examined using time-series sediment trap records from the eastern Japan Basin (trap depth 1057 m and 3043 m) and the Yamato Basin (1057 m and 2100 m) for the period of October 2000 to August 2001. The 11month records of alkenone flux show that the flux to the upper trap appears to be higher during summer (June to August) and fall (October to November) than during the rest of the sample collection period at both stations. However, the seasonal trends in alkenone flux to the lower trap do not show such a strong seasonality. In general the alkenone sinking flux decreased with increasing depth. Despite the decomposition of the alkenones, 0 there was little difference in UK37 values and hence alkenone temperatures between traps. A comparison of alkenone-derived temperatures with satellite sea surface temperatures (SSTs), however, reveals that the alkenone temperatures were higher (7–11 °C) than SST during winter time and lower (2–5 °C) during summer time. This indicates that there is a time lag between the SST and trap records (about 3 months in winter and 1 month in summer). The reasons for the time lag could be (i) slow settling velocity and long residence time of alkenones in the upper water column especially during winter time when alkenone flux was low, (ii) biotic and abiotic degradation of alkenones and (iii) subsurface production of alkenones in summer. The flux-weighted alkenone temperatures are close to annual average SST at the sites, indicating that the sedimentary alkenone signal from the seafloor may represent the annual averaged temperature. Ó 2011 Elsevier Ltd. All rights reserved.
1. Introduction The use of alkenones as a sea surface temperature (SST) proxy is based on the observation that certain haptophyte microalgae produce long chain (C37) unsaturated ketones (alkenones) whose degree of unsaturation changes with seawater temperature. Despite the general application of the method to the reconstruction of past SSTs from marine sediments, there have been several arguments concerning (i) the season and water depth of alkenone production (e.g. Bijma et al., 2001), (ii) the compositional changes in alkenone producing algal species (e.g. Volkman et al., 1995), (iii) physiological factors other than temperature (e.g. Prahl et al., 2003), (iv) the effect of biochemical decomposition and degradation on the alkenone unsaturation index (e.g. Gong and Hollander, 1999; Grimalt et al., 2000) and (v) lateral transport of alkenones by oceanic currents (e.g. Ohkouchi et al., 2002; Rühlerman and Butzin, 2006). These are important issues to be examined in reconstructing accurate paleotemperature and climate change. One of the best ap-
proaches for studying these factors is to investigate time-series records from sediment trap deployment. Although alkenones are generally used as a proxy for SST, they can also be used as a tracer to identify water column processes in sinking particles in the ocean. Since alkenones keep the surface signal within them, the sediment trap record of alkenones actually provides important information on biogeochemical processes in the water column. In this study, we examined seasonal variations in the fluxes of alkenones and other biological components of sinking particles collected from sediment trap deployment (2000– 2001) at water depths of 1057, 2100 and 3043 m in the East Sea. The results make it possible to investigate the characteristics of production, export and degradation of alkenones in the water column and their influence on the alkenone unsaturation index. This will help evaluate the use of alkenones as a paleothermometer. Also, the results should provide important information on the biogeochemical characteristics of the East Sea.
2. Oceanographic setting ⇑ Corresponding author. Tel.: +82 (0)51 410 4759; fax: +82 (0)51 404 4750. E-mail address: [email protected] (K.E. Lee). 0146-6380/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2010.12.008
The East Sea, a semi-enclosed marginal sea in the northwestern Pacific, is characterized by the inflow of the Tsushima Warm
256
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
The East Sea waters below 200 or 300 m are considered to be formed offshore of Vladivostok by severe cooling of the surface water in winter (Kim et al., 2002). The waters are very cold (<1 °C) and highly oxygenated (300–250 lM) (Talley et al., 2004). At a depth of 1000 m, the dissolved oxygen concentration is 215 lM in the Yamato Basin and 230 lM in the Japan Basin. Near the bottom, it is 210 lM in the Yamato Basin and 215 lM in the Japan Basins. The vertical distribution pattern of nutrient concentration shows that nitrate concentration rapidly increases from zero at the surface to 24.6 lM at 1000 m and increases slightly to 25.8 lM near the bottom in the all basins. Other nutrient concentrations show a pattern similar to that of nitrate.
Current (TWC) through the Korea Strait and outflow through the Tsugaru and Soya Straits (Fig. 1). The TWC separates into two main branches. One flows along the Japanese coast and the other, East Korean Warm Current (EKWC), flows northward along the Korean coast and eastward at the shore near 38°N. In the north, the North Korean Cold Current (NKCC) or Liman Current flows southwards along the western boundary. In the central part of the East Sea, there is a thermal boundary in SST, the Subpolar Front (SPF). The SPF is located roughly along 38–40°N and separates warm water to the south from cold water to the north. The circulation system is driven by the inflow–outflow conditions and wind stress curl. Basically, the inflow–outflow conditions create an anti-cyclonic gyre like the EKWC in the south and a positive wind stress curl generates a cyclonic gyre like the NKCC in the north. The boundary between the EKWC and the NKCC is the SPF. At the surface, there is an evident seasonal variation in seawater temperature. In general, a basin-scale cold temperature dominates from December to May, with maximum cooling in February, and basin-scale warming from June to October, with the maximum warming in August. According to satellite SST datasets from 1990 to 1995, the temperature of the sea is in the range 4–13 °C in winter and 20–26 °C in summer (Park et al., 2004). In the south, SST is in the range 11–13 °C in winter and 25–26 °C in summer, while it is 4–7 °C in winter and 20–22 °C in summer in the north.
3. Materials and methods Sinking particles were collected using a sediment trap system deployed for 11 months in the eastern Japan Basin (station JN), and the Yamato Basin (JS-2) (Fig. 1). The deployment data are summarized in Table 1. Two traps were installed at each station. The upper and lower traps were at depths of 1057 m and 3043 m at station JN and 1057 m and 2100 m at station JS-2. The lower traps were installed about 500 m above the seafloor. Samples from JN and JS-2 were obtained between October 6, 2000 and August 14, 2001 at 26 day intervals. Sampling cups of the sediment trap were
TARTAR STRAIT (12 m)
1000 m 2000 m
SOYA STRAIT (55 m)
RUSSIA
45 O N
3000 m
Vladivostok
JN TSUGARU STRAIT (130 m)
Japan Basin SPF NKCC
40 O N Yamato Basin
JS-2 KOREA
Ulleung Basin EKWC
JAPAN
35 O N
TWC KOREA STRAIT (140 m)
130 O E
135 O E
140 O E
Fig. 1. Bathymetric map of the East Sea and sediment trap locations. TWC, EKWC, NKCC and SPF indicate Tsushima Warm Current, East Korea Warm Current, North Korea Cold Current and Subpolar Front, respectively. Black arrows indicates warm current and white arrows cold current.
257
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261 Table 1 Station data for mooring sediment trap system.
Table 2 C37 alkenone content and flux data for sediment traps.
Trap
Latitude (°N)
Longitude (°E)
Water depth (m)
Trap depth (m)
Collection period
Sample
JN
42.47
138.50
3642
JS-2
38.02
135.03
2900
1057, 3043 1057, 2100
October 2000– August 2001 October 2000– August 2001
filled with a neutral buffer (salinity controlled solution of 38 psu with 5% formalin to prevent biological degradation). Recovered samples were stored at 5 °C. The material in each cup was filtered through a 0.6 lm membrane filter after swimmers were removed. Particles on the filter were rinsed with Milli-Q water to remove salts and formalin and dried for 36 h at 60 °C (Otosaka et al., 2004). Particle samples (1–3 g) were taken for alkenone analysis. They were analyzed at JAMSTEC, Japan. Organic compounds were extracted using a Dionex accelerated solvent extractor (ASE 200) with a mixture of solvent (CH2Cl2:CH3OH, 99:1 v/v) at high temperature (100 °C) and pressure (1500 psi). The extracts were saponified at 80 °C for 2 h with 0.1 M KOH in 90/10 CH3OH/H2O. The neutral fraction was recovered by partitioning into hexane. After that, the extract was separated using an automatic solid preparation system (Rapid Trace SPE Workstation, Zymak, UK). The solvents used were, for fraction 1, 4 ml of hexane; for fraction 2, a mixture of 2 ml hexane/toluene (3:1 v/v), 2 ml hexane/toluene (1:1 v/v), and 2 ml hexane/ethyl acetate (95:5 v/v); for fraction 3, 2 ml hexane/ethyl acetate (9:1 v/v) and 2 ml hexane/ethyl acetate (85:15 v/ v); and for fraction 4, 4 ml hexane/ethyl acetate (4:1 v/v), followed by 2 ml ethyl acetate. The alkenones eluted in fraction 3. An aliquot of the alkenone fraction was gently concentrated under N2 and analyzed using gas chromatography with an Agilent 6890 N chromatograph equipped with a flame ionization detector and a HP5MS column (60 m 0.32 mm i.d.). Reproducibility of alkenone temperature assessment for replicate samples was <1 °C. Alkenone temperatures were calculated using the widely used 0 calibration equation of Prahl et al. (1988; UK37 = 0.034T + 0.039). The calibration equation was based upon analysis of Emiliania huxleyi collected from the northeast Pacific and grown in culture over the water temperature range from 8° to 25 °C. The culture calibration is almost identical to the global coretop calibration compiled 0 by Müller et al. (1998; UK37 = 0.033T + 0.044). A study of coccolithophorid distribution at coretop sediments from the East Sea shows that E. huxleyi is the dominant species in the area (Ishiwatari et al., 2001).
4. Results The 11 month records of alkenone concentration and flux were obtained from the upper and lower traps. The alkenone results are listed in Table 2. Four sample materials (JN3KA-1, 5, 6 and JSKA-1) were not available for measurement and their values are missing from the results. The alkenone concentration and flux for the upper trap at JN were relatively high (flux mean 9.8 ± 7.7 ng/m2/d) during October and November 2000 and remained low (6.4 ± 2.8 ng/m2/d) from December 2000 to May 2001 (Fig. 2a and c). The flux increased in June and remained high (16.2 ± 15.2 ng/m2/d) until August. It appears that the alkenone concentration and flux from the surface were relatively high during summer and fall and low during winter and spring. However, the lower trap records at JN showed no clear temporal variation. The mean value of alkenone flux at the lower trap was 0.9, 1.1 ± 0.6, and 1.6 ± 1.9 ng/m2/d for each interval. At JS-2, alkenone concentration and flux recorded a clear seasonality for the upper trap (Fig. 2b and d). The alkenone
0
0
Trap open date (yyyy.month.day)
Alk. content (lg/g)
Alk. flux (ng/m2/d)
UK37
UK37 T (°C)
1057 m JNKA-1 JNKA-2 JNKA-3 JNKA-4 JNKA-5 JNKA-6 JNKA-7 JNKA-8 JNKA-9 JNKA-10 JNKA-11 JNKA-12 JNKA-13
2000.10.6 2000.11.1 2000.11.27 2000.12.23 2001.1.18 2001.2.13 2001.3.11 2001.4.6 2001.5.2 2001.5.28 2001.6.23 2001.7.19 2001.8.14
8.1 10.9 6.0 3.4 2.6 2.5 2.4 3.7 3.9 5.7 20.2 11.9 7.6
4.4 15.3 8.1 4.1 2.7 3.2 6.7 7.3 8.5 10.5 33.9 10.1 4.8
0.749 0.675 0.598 0.587 0.577 0.494 0.342 0.266 0.298 0.352 0.440 0.522 0.599
20.9 18.7 16.4 16.1 15.8 13.4 8.9 6.7 7.6 9.2 11.8 14.2 16.5
3043 m JN3KA-1 JN3KA-2 JN3KA-3 JN3KA-4 JN3KA-5 JN3KA-6 JN3KA-7 JN3KA-8 JN3KA-9 JN3KA-10 JN3KA-11 JN3KA-12 JN3KA-13
2000.10.6 2000.11.1 2000.11.27 2000.12.23 2001.1.18 2001.2.13 2001.3.11 2001.4.6 2001.5.2 2001.5.28 2001.6.23 2001.7.19 2001.8.14
– 1.9 1.2 1.8 – – 1.4 2.1 1.4 0.8 5.4 2.1 1.4
–
1.8 1.6 1.3 0.6 3.8 0.6 0.3
– 0.641 0.547 0.514 – – 0.396 0.359 0.402 0.412 0.426 0.503 0.575
– 17.7 14.9 14.0 – – 10.5 9.4 10.7 11.0 11.4 13.6 15.8
1057 m JSKA-1 JSKA-2 JSKA-3 JSKA-4 JSKA-5 JSKA-6 JSKA-7 JSKA-8 JSKA-9 JSKA-10 JSKA-11 JSKA-12 JSKA-13
2000.10.6 2000.11.1 2000.11.27 2000.12.23 2001.1.18 2001.2.13 2001.3.11 2001.4.6 2001.5.2 2001.5.28 2001.6.23 2001.7.19 2001.8.14
– 24.4 10.2 8.8 2.6 2.2 5.1 1.6 8.1 6.5 17.1 21.1 33.6
– 53.9 18.5 17.5 9.0 8.5 31.5 5.9 19.2 10.2 20.5 40.1 56.8
– 0.743 0.735 0.666 0.612 0.552 0.348 0.337 0.417 0.449 0.516 0.734 0.623
– 20.7 20.5 18.4 16.8 15.1 9.1 8.8 11.1 12.1 14.0 20.4 17.2
2100 m JS3KA-1 JS3KA-2 JS3KA-3 JS3KA-4 JS3KA-5 JS3KA-6 JS3KA-7 JS3KA-8 JS3KA-9 JS3KA-10 JS3KA-11 JS3KA-12 JS3KA-13
2000.10.6 2000.11.1 2000.11.27 2000.12.23 2001.1.18 2001.2.13 2001.3.11 2001.4.6 2001.5.2 2001.5.28 2001.6.23 2001.7.19 2001.8.14
4.8 6.9 6.0 4.6 2.9 1.6 6.1 3.5 6.8 2.5 2.9 7.1 19.9
3.9 15.2 10.8 9.3 10.0 6.2 37.2 13.0 16.1 4.0 3.5 13.5 33.7
0.743 0.729 0.735 0.710 0.654 0.622 0.372 0.379 0.416 0.517 0.560 0.709 0.632
20.7 20.3 20.5 19.7 18.1 17.2 9.8 10.0 11.1 14.1 15.3 19.7 17.5
0.9 0.4 0.7 – –
flux was relatively high (53.9 ng/m2/d) in November 2000 and remained relatively low (15.0 ± 8.4 ng/m2/d) during December to May 2001. There was a relatively high flux of alkenones in March. After then, they increased during July and August 2001 (39.1 ± 18.2 ng/m2/d). The seasonal trends in the lower trap were not like those observed in the upper trap. The lower trap flux values were 9.5 ± 8.0, 13.3 ± 10.3, and 16.9 ± 15.3 ng/m2/d for each interval. At JN, the alkenone-derived temperature of the samples was 21 °C in October 2000 and decreased to 16 °C in December 2000 (Fig. 2e). It then dropped to 7–9 °C in March to June 2001. Subsequently, it increased gradually up to 16 °C in August. The lowest
258
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
Fig. 2. Variations in the content (a and b) and flux (c and d) of C37 alkenones and the alkenone-based temperature (e and f) at station JN and station JS-2 from October 2000 to August 2001. Closed circle indicates data from the upper trap. Open circle indicates data from the lower trap. Gray bar and dash line indicate the mean value of alkenone flux for each interval. Numbers are mean and standard deviation of alkenone flux for upper trap and numbers with parentheses are for lower trap. Open rectangle indicates satellite-derived sea surface temperature for the sampling period. Arrow indicates time lag between SST and trap alkenone temperatures.
alkenone temperature of 7 °C occurred in April 2001. The seasonal variation in the lower trap records shows similar patterns. At JS-2, the alkenone temperature was 20–21 °C in October and November 2000 and gradually decreased to 15 °C in January and February 2001 (Fig. 2f). Between March and April, the temperature was 9 °C, with the lowest temperature in April. After then, it increased to 20 °C in July 2001. A comparison of alkenone temperature between the upper and the lower traps illustrates that the difference between the traps was relatively small. At station JN, the difference was in the range of 3.1 to +2.1 °C through the sampling collection period. At JS-2, it was 2.1 to +0.8 °C. The alkenone temperatures were compared to actual SSTs for the station for the sample collection period. The SST data used were from the Satellite Image Data Base (NOAA Local Area Coverage data) of the Agriculture, Forestry and Fisheries Research Center, Japan (http://rms0.agsearch.agropedia.affrc.go.jp/sidab/). Seasonal variations in SST at stations JN and JS-2 are plotted in Fig. 2e and f. The minimum and maximum satellite temperatures were 4 °C in February and 21 °C in August at JN. At JS-2, they were 9 °C in February and 26 °C in August, respectively. A comparison of the sediment trap alkenone temperatures with the satellite SST shows that there are clear differences between them. The differences were large, up to 11 °C at JN and 7 °C at JS-2. The alkenone temperatures were higher than the SST during October 2000 to
March 2001, while they were lower during May to August 2001. It appears that there was a time lag between the SST and upper trap temperature (Fig. 2e and f). The time lag is 2–3 months in winter and about 1 month in summer. Although there was a significant time lag between the SST and trap alkenone temperatures, the flux-weighted alkenone temperatures were close to the 11 month averaged SST. At JS-2, the fluxweighted alkenone temperature for the lower trap was 15 °C, which is consistent with the averaged SST of 16 °C. At JN, the flux-weighted alkenone temperature of 12 °C was close to the averaged SST of 11 °C.
5. Discussion 5.1. Seasonality in alkenone flux Fluxes of total mass, biogenic opal and particulate organic carbon (POC) in the upper and lower traps at JN and JS-2 have been reported (Otosaka et al., 2004, 2008). According to the results, biogenic opal comprises ca. P30% of the total mass at both stations, indicating that it is the major component of the settling particles through the sample collection period. POC accounts for 9–10% of total mass in the upper trap of both stations, and 6–7% in the lower
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
2.0
80
400
1.5
60
300
1.0
40
200
0.5
20
100
0
0
0
80
400
0.6
60
300
0.4
40
200
0.2
20
100
0
0
0
0.8
2
2
JN
O N D J
F M A M J
2
JS -2 2
Chlorophyll a (mg/m 3 )
3
Chlorophyll a (mg/m )
traps. Records of seasonal variation in sinking particle fluxes illustrate that biogenic opal and POC fluxes were high during the period of March and April (Fig. 3). On the contrary, a higher flux of C37 alkenones for the upper trap occurred during summer and fall (July–August and October–November) than during the rest of the year at both stations in general. The time lag between biogenic opal flux and alkenone flux may be indicative of the ecological succession from the blooming of opal-secreting diatoms in spring to a high production of alkenone-producing coccolithophores in summer and fall. According to SEAWIFS data recovered during the sample collection period, the highest chlorophyll a concentration at surface occurred in spring, while it was lowest in summer, especially at JS-2 (Fig. 3). This is consistent with the sediment trap results. At JS-2, the highest concentration of chlorophyll a in spring was considered to be associated with an opal-secreting diatom bloom in the East Sea. There is a positive correlation coefficient between chlorophyll a concentration and opal and POC fluxes (correlation coefficient, 0.833 for opal and 0.825 for POC; the significance of the correlation coefficient was tested with confidence level of 0.5). The bloom may be related to the occurrence of the deep mixed layer and associated high nutrient concentration in spring, while a high production of coccolithophores seems to be related to the shallow mixed layer in summer (e.g. Lee et al., 2003). Diatoms are known to compete well in nutrient rich (especially with high silicate concentration) cold water systems and coccolithophores compete well in warm, low-nutrient water environments (Werne et al., 2000). At JN, there is no correlation between chlorophyll a concentration and opal and POC fluxes. A similar pattern in the time lag between opal and alkenone fluxes at JS-2 has been observed from sediment trap studies in the mid- and high-latitude NW Pacific (Sawada et al., 1998; Harada et al., 2006; Yamamoto et al., 2007). Although the upper trap records show clear seasonality in alkenone flux, the lower trap records were different. As far as transfer of surface signal to the seafloor is concerned, the lower trap record is more important than the upper trap results. Because alkenones
J A
Fig. 3. Variation in satellite-derived chlorophyll a concentration at sea surface (closed circle) and particulate organic carbon flux (gray line) and opal flux (dashed line) at the upper trap of station JN and JS-2 from October 2000 and August 2001. (Data from SEAWIFS and Otosaka et al. (2004, 2008).)
259
are used as a proxy for SST in order to reconstruct past SST from the sediments. In this case, it is important to know the season to which the alkenone temperature reconstruction refers. It most likely depends on seasonality of alkenone flux to the seafloor. The average flux of C37 alkenones to the lower trap was 1.2 ng/ m2/d at JN and 13.6 ng/m2/d at JS-2. There was no strong seasonality in the lower trap alkenone flux record at JN (Fig. 2c). At JS-2, the alkenone flux to the lower traps was relatively high in March and August. However, the mean value of each interval (Fig. 2d) indicates no strong seasonality. In addition, since the flux-weighted alkenone temperatures were close to the 11 month averaged SST, the sedimentary alkenone signal from the seafloor is considered to represent the annual averaged value. Alkenone analysis of sediment trap material from two different depths (1057 and 3043 m at JN, and 1057 and 2100 m at JS-2) showed that the alkenone sinking flux decreased with increasing depth. The average ratio of lower to upper trap values for alkenones flux was 12% at JN and 75% at JS-2, suggesting that decomposition was significant in the water column between traps at JN, but less effective at JS-2. Despite the decomposition of the alkenones, 0 there was little difference in UK37 values and hence alkenone temperatures between traps at both stations. The averaged value of 0 difference in UK37 -based temperatures between traps was 0.3 ± 1.8 °C for JN and 0.7 ± 0.9 °C for JS-2. T-test showed that the upper and the lower trap values were not different at JN, but they were different at JS-2 with 95–98% confidence range. However, if we consider reproducibility of alkenone temperature less than 1 °C, the difference is not significant.
5.2. Alkenone temperatures An important feature of our results is that there is almost no time lag between the upper and the lower trap alkenone temperatures, but there is a time lag between the SST and trap results. The time lag is relatively long (ca. 2–3 months) for the winter season but short (ca. 1 month) for the summer season. Four possibilities can be considered for this time lag between surface production and water column. First, alkenones were produced at the surface but, their settling velocity in the upper water column was slow and residence time was relatively long before sinking, especially in winter. Second, alkenones were synthesized in another area and transported by way of lateral advection. Third, biotic and abiotic degradation of alkenones affected the degree of unsaturation in the water column. Fourth, the depth of alkenone production may have been deeper than the surface. First of all, the time lag can be attributed to different settling velocity for the sinking particles of alkenones in the water column: (i) slow in winter and relatively fast in summer and (ii) slow in the upper water column and relatively fast in the deep water column. The 3 month time lag between the SST and the upper trap alkenone temperatures in winter indicates the settling velocity of 10 m/d for the alkenones in the upper water column between the surface and the upper trap. The 1 month time lag in summer indicates the settling velocity of 33 m/d in the upper water column. Since there is almost no time lag between the upper and the lower trap alkenone temperatures, the settling velocity of alkenones seems to become faster in the deep water than that in the upper water. In Fig. 3, the seasonal distribution pattern in chlorophyll a concentration at the East Sea surface is consistent with those of POC and opal fluxes from the upper traps at JS-2. This indicates that there might be no time lag between chlorophyll a and POC and opal, and POC and opal particles most likely settle down from the surface to a depth of 1057 m within the sampling interval of 26 days, suggesting that they rapidly sink with a settling velocity of ca. 80–117 m/d especially for spring (March–April–May). If this is true, the sinking
260
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
particles of alkenones settle down slowly compared to POC and opal. Another possibility is lateral advection effects on the alkenone signal (e.g. Ohkouchi et al., 2002; Harada et al., 2006; Rühlerman and Butzin, 2006). In the East Sea, the temperature gradient at the surface is small at a given time period. A difference in SST between the Korea Strait, the warmest part of the sea, and station JS2 is ca. only 1 °C in summer and 3 °C in winter (Park et al., 2004). This difference cannot explain the difference between the SST and trap alkenone temperatures. Also the alkenone temperatures were higher than the SST in winter and lower than the SST in summer. If lateral advection is an important factor, alkenone temperatures would be consistently higher than the SST regardless of season. Because main currents flow from the south to the north and SST in the south is always higher than that in the north. In addition, the flux-weighted alkenone temperatures were close to the 11 month averaged SST at both stations. These indicate that the possibility of lateral transport of alkenones in the East Sea is less likely. Previous studies suggest that degradation processes do not significantly alter the degree of unsaturation of alkenones in particles settling in water column (Bentaleb et al., 1999; Grimalt et al., 2000). However, it has been suggested by different authors that biotic degradation and abiotic degradation of alkenones affect the 0 UK37 values of particles in the water column and sediments (Rontani and Wakeham, 2008; Rontani et al., 2006, 2008). According to these studies, biotic and abiotic degradation of alkenones leads to a bias towards warm alkenone temperatures. In the East Sea, the warm alkenone signal in winter may be associated with such degradation in water column. There is a possibility that the degree of unsaturation of alkenones in the upper water column was influenced by such degradation during winter when the alkenone flux was low and the residence time was long. However, the magnitude 0 of the change in UK37 values resulting from degradation (2–3 °C) is not expected to be sufficiently large to explain the difference in SST and alkenone temperature (up to 11 °C). The use of alkenones as a SST proxy is based on the assumption that alkenone production occurs at or close to the sea surface, so that the surface signal is preserved within organic material. However, previous studies suggest that significant production of alkenones may occur below the surface mixed layer, and the alkenone-based temperatures could be biased toward a subsurface temperature significantly colder than the surface temperature (e.g. Prahl et al., 1993; Ternois et al., 1997; Bentaleb et al., 1999; Ohkouchi et al., 1999). A recent study of alkenone production depth in the Pacific Ocean (Lee and Schneider, 2005) shows that, in the western boundary current and equatorial regions, where the chl. maximum occurs near the surface, the alkenone concentration maximum is near the surface. In contrast, in the subtropical gyre, where the chl. maximum occurs in the subsurface, the alkenone concentration maximum occurs at the depth of the chl. maximum (ca. 100 m). In the East Sea, the vertical distribution pattern of chl. data from JODC database (web site: http://www.jodc.go.jp) shows that the subsurface chl. maximum occurs only during summer. Hence, there is a possibility that the alkenones were produced at the subsurface during summer. The low alkenone temperatures for summer may be related to this reason. However, this is not the case for winter. During winter, the surface mixed layer was deep in the East Sea and there was no significant chl. maximum at the subsurface water. Furthermore, in winter, alkenone temperatures were higher than the SST, which cannot be explained by subsurface production. A time lag between the SST and alkenone temperature was observed not only in the East Sea but also in the northwestern Pacific Ocean. Harada et al. (2006) investigated time-series sediment trap records from station KNOT (44°N, 155°E) and found higher alkenone temperatures than SST during winter (season for low export
of alkenones). They interpreted this phenomenon by way of contribution of allochthonous alkenones in particulate material transported from a subtropical area within a warm-core ring. However, based on time-series sediment trap records from a station (39°N, 147°E) in the mid-latitude northwestern Pacific, Yamamoto et al. (2007) documented that the time lag was attributed to seasonal variation in residence time rather than a lateral transport effect. They suggested that fresh and labile particles sank from spring to fall, while old and stable particles sank from fall to spring. A sediment trap alkenone record from a station of 34°100 N, 142°E (Sawada et al., 1998) corroborates the results of ours and Yamamoto et al. (2007). A time lag was also observed at station ALOHA in the subtropical North Pacific. Alkenone temperatures overestimate in situ winter temperature, while they underestimate in situ summer temperature in the surface mixed layer (Prahl et al., 2005; Popp et al., 2006). These authors suggest that nonthermal physiological factors such as light and nutrient concentration account for the discrepancies in measured and estimated temperatures. 6. Conclusions Sediment trap records of biogenic components including alkenones from the East Sea provide important information about the biogeochemical characteristics of the East Sea. The upper trap records of seasonal variation in sinking particles at JS-2 illustrate that biogenic opal and POC fluxes were high during spring (March and April), whereas C37 alkenone-producing coccolithophores were more abundant during summer and fall (July–August and October–November) than the rest of the year. This suggests that the phytoplankton community in the area may have changed seasonally from domination by opal-secreting diatoms in spring to domination by CaCO3-secreting coccolithophores in summer and fall. Although the alkenone flux shows that the flux to the upper trap appears to be higher during summer and fall, the flux to the lower trap does not show such a clear seasonality. In addition, flux-weighted alkenone temperatures are close to annual average SST at the sites, indicating that sedimentary alkenone signal from the seafloor may represent the annual averaged temperature. Our results indicate that the sinking flux decreased with increasing 0 depth in general, but there was little difference in UK37 values and hence alkenone temperatures between traps. However, a comparison of alkenone-derived temperatures with satellite sea surface temperatures (SSTs) reveals that the alkenone temperatures were higher (7–11 °C) than SST during winter time and lower (2–5 °C) during summer time. This indicates that there is a time lag between the SST and trap records, and the time lag is relatively long (ca. 2–3 months) for the winter season but short (ca. 1 month) for the summer season. This is probably because, during winter when alkenone flux is low, the particles with alkenones appear to sink slowly, leading to a relatively long residence time in the upper water column. Also there is a possibility that degradation processes influenced the degree of alkenone unsaturation of sinking particles. In summer when alkenone flux increased, alkenones appeared to rapidly sink. Also, there is a possibility that subsurface production caused the cold alkenone temperatures. Acknowledgments We thank N. Harada for providing the chance to use her laboratory facilities, J.-Y. Choi for laboratory assistance and P. Helmke for SEAWIFS chlorophyll data. We also thank two reviewers for constructive comments and suggestions. NOAA Local Area Coverage data (MCSST) was obtained from the Agriculture, Forestry and Fisheries Research Information Center, Ministry of Agriculture,
K.E. Lee et al. / Organic Geochemistry 42 (2011) 255–261
Forestry and Fisheries (MAFF), Tsukuba, Japan. The work was funded by Research Agency for Climate Science (RACS 20093001) and Global Joint R&D Program (Grant 2007-00206) through the Korea Foundation for International Cooperation of Science and Technology.
Associate Editor—E.A. Canuel
References Bentaleb, I., Grimalt, J.Q., Vidussi, F., Marty, J.-C., Martin, V., Denis, M., Hatté, C., Fontugne, M., 1999. The C37 alkenone record of seawater temperature during seasonal thermocline stratification. Marine Chemistry 64, 301–313. Bijma, J., Altabet, M., Conte, M., Kinkel, H., Versteegh, G.J.M., Volkman, J.K., Wakeham, S.G., Weaver, P.P., 2001. Primary signal: ecological and environmental factors-report from working group 2. Geochemistry Geophysics Geosystems 2. 2000GC000051. Gong, C., Hollander, D., 1999. Evidence for differential degradation of alkenones under contrasting bottom water oxygen conditions: implication for paleotemperature reconstruction. Geochimica et Cosmochimica Acta 63, 405– 411. } tter, J., Sicre, M.A., Summons, R., Farrington, J., Harvey, H.R., Grimalt, J.O., Rullko Goni, M., Sawada, K., 2000. Modifications of the C37 alkenone and alkenoate composition in the water column and sediment: possible implications for sea surface temperature estimates in paleoceanography. Geochemistry Geophysics Geosystems 1, GC000053. Harada, N., Sato, M., Shiraishi, A., Honda, M., 2006. Characteristics of alkenone distributions in suspended and sinking particles in the northwestern North Pacific. Geochimica et Cosmochimica Acta 70, 2045–2062. Ishiwatari, R., Houtatsu, M., Okada, H., 2001. Alkenone-sea surface temperatures in the Japan Sea over the past 36 kyr: warm temperatures at the last glacial maximum. Organic Geochemistry 32, 57–67. Kim, K.-R., Kim, G., Kim, K., Lobanov, V., Ponomarev, V., 2002. A sudden bottomwater formation during the severe winter 2000–2001: the case of the East/ Japan Sea. Geophysical Research Letters 29, 1234–1237. Lee, K.E., Schneider, R., 2005. Alkenone production in the upper 200 m of the Pacific Ocean. Deep-Sea Research I 52, 443–456. Lee, K.E., Bahk, J.J., Narita, H., 2003. Temporal variations in productivity and planktonic ecological structure in the East Sea (Japan Sea) since the last glaciation. Geo-Marine Letters 23, 125–129. Müller, P.J., Kirst, G., Ruhland, G., Storch, I.,0 Rosell-Melé, A., 1998. Calibration of the alkenone paleotemperature index UK37 based on core tops from the eastern South Atlantic and the global ocean (60°N–60°S). Geochimica et Cosmochimica Acta 62, 1757–1772. Ohkouchi, N., Kawamura, K., Kawahata, H., Okada, H., 1999. Depth ranges of alkenone production in the central Pacific Ocean. Global Biogeochemical Cycles 13, 695–704. Ohkouchi, N., Eglinton, T.I., Keigwin, L.D., Hayes, J.M., 2002. Spatial and temporal offsets between proxy records in a sediment drift. Science 298, 1224–1227.
261
Otosaka, S., Togawa, O., Baba, M., Karasev, E., Volkov, Y.N., Omata, N., Noriki, S., 2004. Lithogenic flux in the Japan Sea measured with sediment traps. Marine Chemistry 91, 143–163. Otosaka, S., Tanaka, T., Togawa, O., Amano, H., Karasev, E.V., Minakawa, M., Noriki, S., 2008. Deep sea circulation of particulated organic carbon in the Japan Sea. Journal of Oceanography 64, 911–923. Park, K., Chung, J.Y., Kim, K., 2004. Sea surface temperature fronts in the East (Japan) Sea and temporal variations. Geophysical Research Letters 31. doi:10.1029/ 2004GL019424. Popp, B.N., Prahl, F.G., Wallsgrove, R.J., Tanimoto, J., 2006. Seasonal patterns of alkenone production in the subtropical oligotrophic North Pacific. Paleoceanography 21, PA1004. doi:10.1029/2005PA001165. Prahl, F.G., Muehlhausen, L.A., Zahnle, D.L., 1988. Further evaluation of long-chain alkenones as indicators of paleoceanographic conditions. Geochimica et Cosmochimica Acta 52, 2303–2310. Prahl, F.G., Collier, R.B., Dymond, J., Lyle, M., Sparrow, M.A., 1993. A biomarker perspective on prymnesiophyte productivity in the northeast Pacific Ocean. Deep-Sea Research I 40, 2061–2076. Prahl, F.G., Wolfe, G.V., Sparrow, M.A., 2003. Physiological impacts on alkenone paleothermometry. Paleoceanography 18. 2002PA000803. Prahl, F., Popp, B.N., Karl, D.M., Sparrow, M.A., 2005. Ecology and biogeochemistry of alkenone production at Station ALOHA. Deep-Sea Research I 52, 699–719. Rontani, J.-F., Wakeham, S., 2008. Alteration of alkenone unsaturation ratio with depth in the Black Sea: potential roles of stereomutation and aerobic biodegradation. Organic Geochemistry 39, 1259–1268. Rontani, J.-F., Marty, J.-C., Miquel, J.-C., Volkman, J.K., 2006. Free radical oxidation (autoxidation) of alkenones and other microalgal lipids in seawater. Organic Geochemistry 37, 354–368. Rontani, J.-F., Harji, R., Guasco, S., Prahl, F.G., Volkman, J.K., Bhosle, N.B., Bonin, P., 2008. Degradation of alkenones by alkenones by aerobic heterotrophic bacteria: selective or not? Organic Geochemistry 39, 34–51. Rühlerman, C., Butzin, M., 2006. Alkenone temperature anomalies in the BrazilMalvinas Confluence area caused by lateral advection of suspended particulate material. Geochemistry Geophysics Geosystem 7, Q10015. Sawada, K., Handa, N., Nakatsuka, T., 1998. Production and transport of long-chain alkenones and alkyl alkenones in a sea water column in the northwestern Pacific off central Japan. Marine Chemistry 59, 219–234. Talley, L.D., Tishchenko, P., Luchin, V., Nedashkovskiy, A., Sagalaev, S., Kang, D.-J., Warner, M., Min, D.-H., 2004. Atlas of Japan (East) Sea hydrographic properties in summer 1999. Progress in Oceanography 61, 277–348. Ternois, Y., Sicre, M.-A., Boireau, A., Conte, M.H., Eglinton, G., 1997. Evaluation of long-chain alkenones as paleotemperature indicators in the Mediterranean Sea. Deep Sea Research I 44, 271–286. Volkman, J.K., Barrett, S.M., Blankburn, S.I., Sikes, E.L., 1995. Alkenones in Gephyrocapsa oceanica: implications for studies of paleoclimate. Geochimica et Cosmochimica Acta 59, 513–520. Werne, J.P., Hollander, D.J., Lyons, T.W., Peterson, L.C., 2000. Climate-induced variations in productivity and planktonic ecosystem structure from the Younger Dryas to Holocene in the Cariaco Basin, Venezuela. Paleoceanography 15, 19– 29. Yamamoto, M., Shimamoto, A., Fukuhara, T., Naraoka, H., Tanaka, Y., Nishimura, A., 2007. Seasonal and depth variations in molecular and isotopic alkenone composition of sinking particles from the western North Pacific. Deep-Sea Research I 54, 1571–1592.