Sedimentary sterols as biogeochemical indicators in the Southern Ocean

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Organic Geochemistry Organic Geochemistry 39 (2008) 567–588 www.elsevier.com/locate/orggeochem

Sedimentary sterols as biogeochemical indicators in the Southern Ocean Jennifer C. Villinski a,*, John M. Hayes b, Simon C. Brassell a, Virginia L. Riggert c, Robert B. Dunbar d a

Biogeochemical Laboratories, Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA b Department of Geology and Geophysics, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA c Carbon Capture and Storage Technology, BP AlternativEnergy, Houston, TX 77079, USA d Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305-2115, USA Received 30 March 2007; received in revised form 4 January 2008; accepted 8 January 2008 Available online 26 January 2008

Abstract Abundances and isotopic compositions of sterols and of total organic carbon in surface sediments were measured at 18 stations in the Ross Sea, Antarctica. Ten sterols, 5a-cholestan-3b-ol (cholestanol), cholest-5-en-3b-ol (cholesterol), cholest5,22E-dien-3b-ol, 24-methyl-5a-cholest-22E-en-3b-ol (brassicastanol), 24-methyl-cholest-5,22E-dien-3b-ol (brassicasterol), 24-ethyl-5a-cholestan-3b-ol (sitostanol), 24-ethyl-cholest-5-en-3b-ol (sitosterol), 24-ethyl-cholest-5,22E-dien-3b-ol (stigmasterol), 4a,23,24-trimethyl-5a-cholestan-3b-ol (dinostanol), and 4a,23,24-trimethyl-5a-cholest-22E-en-3b-ol (dinosterol), are most widely distributed. Polytopic vector analysis of the variations in abundance resolved four sources for these compounds: an assemblage of phytoplankton characteristic of the Ross Sea Polynya, diatoms and associated consumers, zooplankton, and processes associated with heterotrophic dinoflagellates. Concentrations of stanols were strongly correlated with those of dinosterol and dinostanol. Concentrations of total organic carbon (TOC) ranged from 0.1% to 1.2% and were lowest on crests and banks and higher in basins. The mole fraction of organic carbon occurring as sterols ranged from 3 to 1100 ppm. Values were lowest at stations with anomalously old TOC (estimated from regional variations in the radiocarbon age of acid-insoluble organic carbon), thus pointing to weathering and redistribution of surface sediments as important factors in the differential degradation of sterols and TOC. The difference in first order rate constants for the degradation of these materials is ca. 0.002 yr1. Stigmasterol and the C27 sterols were significantly enriched in 13C relative to other sterols. The abundance of 13C in TOC at four western stations was 4‰ higher than elsewhere. Abundances of 13C in all sterols at these stations is also 4‰ higher than elsewhere, indicating enrichment of 13C in the entire biological community. Independent observations of P CO2 in surface waters, together with known relationships between isotopic fractionation and the concentration of dissolved CO2, show that the isotopic zonation in organic carbon is due entirely to dynamic drawdown of CO2 in western surface waters. At those locations, late melting of ice produces salinity gradients that inhibit mixing of CO2 from deeper waters. Ó 2008 Elsevier Ltd. All rights reserved.

* Corresponding author. Present address: BP Egypt, P.O. Box 4381, Houston, TX 77210, USA. Tel.: +2 02 2706 2421; fax: +2 02 2706 2460. E-mail address: [email protected] (J.C. Villinski).

0146-6380/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2008.01.009

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J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

1. Introduction The Ross Sea has some of the highest observed productivity in the Antarctic coastal region (Arrigo and McClain, 1994) with implications for drawdown and sequestration of CO2 from the atmosphere (Arrigo et al., 1999). Knowledge of the assemblages of organisms and relative rates of primary production for those assemblages is important for understanding the overall impact of this area on the Southern Ocean carbon cycle. The sediments of the Ross Sea are largely made up of biogenic oozes mixed with lithic material derived both from the continent and from the local volcanic archipelago (Dunbar et al., 1985). The bathymetry consists of a series of ridges and troughs that, together with oceanographic current regimes, greatly favor or hinder sedimentation due to winnowing of sediments on highs and deposition in troughs (Dunbar et al., 1985). The surface sediments examined here have depositional rates ranging from 16 cm kyr1 in the southwestern study area to 4.5 cm kyr1 in the north central Ross Sea and as low as 1.2 cm kyr1 in the eastern Ross Sea (DeMaster et al., 1996). Accumulation rates of organic carbon range from 0.2 g cm2 kyr1 in the southwestern Ross Sea to 0.02 g cm2 kyr1 in the northern part of the study area (DeMaster et al., 1996). The box cores used in this study were bioturbated (Grebmeier, unpublished), containing no apparent lamination. Therefore, the geochemical data reported here derive from organic material accumulated over the last 1000–4000 years, depending on sample location. In the Ross Sea, biomarkers preserved in sediments should yield clues to persistent biological communities over time. The isotopic compositions of these biomarkers in turn will yield evidence for physical and biological processes driving the carbon isotopic composition of the bulk organic matter in the sediments. Of particular interest is identification of the factor, or factors, responsible for the sharp isotopic zonation, in which sedimentary organic material along the western margin of the Ross Sea is enriched in 13C by approximately four permil relative to that elsewhere in the Ross Sea (Villinski et al., 2000; Grebmeier et al., 2003). Similar contrasts have been noted in Prydz Bay (Kopczynska et al., 1995) and in Antarctic near shore waters in the region of the Princess Elizabeth Trough (Popp et al., 1999). In these cases, variations in growth rate, species specific effects, inputs from sea ice communities, increased heterotrophic recycling (with

consequent enrichment of 13C), and drawdown of CO2 have been mentioned as possible causes. To achieve these objectives, this study focuses on sterol biomarkers, chosen because of their abundance in sediments and proven utility as source diagnostic biomarkers (e.g. Volkman, 1986, 2003). Sterols share common biosynthetic pathways that lead to minimal differences in isotopic fractionation during their production, yet biota variously modify positions of unsaturation, and alkylation in sterols, especially in their side chain configurations, to yield a variety of discrete structures (e.g. De Leeuw and Baas, 1986; Brassell, 1994; Volkman, 2005). Most sterols derive from multiple biological sources; some structures occur widely, whereas others appear restricted to specific organisms (e.g. Brassell, 1994; Volkman, 2005). The extensive literature on sterols as constituents of marine eukaryotes grown in cultures (e.g. Gladu et al., 1991; Volkman et al., 1993, 1999; Barrett et al., 1995; Mansour et al., 2003; Leblond and Chapman, 2004) is complemented by analyses of marine organisms collected from natural habitats (e.g. Serrazanetti et al., 1989; Muhlebach et al., 1999). The potential environmental fate of sterols is well understood from investigations of water column particulates (e.g. Wakeham and Beier, 1991; Wakeham, 1995; Colombo et al., 1996; Ternois et al., 1998; Hudson et al., 2001) and a diverse range of sediments (e.g. Mackenzie et al., 1982; De Leeuw and Baas, 1986; Brassell, 1992), supported by studies of the pathways for degradative processes associated with herbivory and predation (e.g. Bradshaw et al., 1989; Harvey et al., 1989; Grice et al., 1998; Hamm et al., 2001; Nelson et al., 2001), and diagenetic alteration (e.g. Nishimura, 1978; Sun and Wakeham, 1999). Thus, assessment of correlations between observed distributions of species and the abundances of sterols in particulate or sedimentary organic material benefits from this substantive background information, further aided by evidence from prior investigations of sterols in the Southern Ocean (e.g. Venkatesan, 1988; Nichols et al., 1991; Skerratt et al., 1995; Muhlebach and Weber, 1998; Phleger et al., 2002; Ju and Harvey, 2004; Ju et al., 2004) that help identify biological communities likely to be important sources of sterols in the Ross Sea. We applied molecular identification and compound specific isotopic methods to organic matter from surface sediments of the Ross Sea to determine average spatial variability in composition of biological communities including both primary producers

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

also displays groupings based on benthic habitat, date of removal of sea ice (‘‘Polynya Groups”) and productivity (Barry et al., 2003). Benthic habitats are defined in terms of current speed as well as physiographic setting. Both crest and bank stations are at relatively shallow depths, but the former are distinguished by higher currents (on average, 54 vs. 25 cm/s). The published report in which benthic habitats were initially described (Barry et al., 2003) displays maps on which symbols indicate the classifications of numerous ROAVERRS sites. To be certain that we were correctly associating those symbols with the stations examined in this work, we referred to site specific data in the ROAVERRS database (J.P. Barry, private communication). This revealed that the published classification of benthic habitats

and consumers. We then used compound specific isotopic evidence to determine the root causes of variations in the isotopic composition of bulk organic carbon. 2. Methods 2.1. Samples and settings Sampling sites are shown in Fig. 1. Exact locations, water depths, and other descriptive information are listed in Table 1. The samples were collected as part of the ROAVERRA program (Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea), which encompassed three annual cruises (NBP96-6, NBP97-9, and NBP98-7) and observations at 334 stations. Table 1

ast

56

nd Co

74 S

0

Ba nk

57

Cr ary

Victo

50

0

ria La

70

500

300

39

nn Pe

ell

B

k an 75 S

30

0

31 35

75 S

180 E

175 E

170 E

165 E

74 S

569

00

5

60

0

500

50

29

50

0

300

61 0

26

76 S

62

70

76 S

75

90 9

n 73 Ba ss o R

k

71

92 77 S

66 -500

-500

0

700

0 -70

-

Ro ss Ice Sh elf

70

0

170 E

160 E

180 E

500

77 S

70 300

-300

Fig. 1. Southwestern portion of the Ross Sea, showing locations at which surface sediments were sampled. Symbols indicate classifications of benthic habitat at each site: circle, basin; square, bank; triangle up, crest; triangle down, slope. Details are provided in text and in Table 1.

570

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Table 1 Stations and their characteristicsa Station

Latitude

Longitude

Depth m

Current, cm/s

Descriptive groups

Site

Polynya

Productivity

Location

n

Benthic habitat

4 4 5 8 9 6 8 7 2 1 8 3 0 4 5 8 7 4

Basin Basin Slope Basin Basin Crest Bank Slope Crest Bank Slope Slope Basin Crest Bank Slope Slope Slope

Late Late Late Early Late Mid Mid Mid Mid Mid Early Early Mid Mid Mid Early Mid Early

High Low Low High Low Medium Low Low Medium High High Low Low Low Medium High High Medium

South West West West West North North North North Central Central South Central Central Central Central South South

Regional Average

9 26 29 31 35 39 56 57 60 61 62 66 70 71 73 75 90 92

76.8313°S 76.0080°S 75.5182°S 75.1200°S 75.0050°S 75.0000°S 73.8500°S 74.1683°S 75.3883°S 75.8717°S 76.3400°S 77.2967°S 76.8983°S 76.5217°S 76.4967°S 76.5017°S 76.5000°S 76.9817°S

167.3631°E 164.8591°E 165.8641°E 164.5250°E 166.5538°E 170.0833°E 173.5300°E 175.0683°E 174.7650°E 173.8633°E 172.9083°E 177.0150°E 179.2300°W 179.9767°W 176.8317°E 173.7117°E 170.5100°E 171.9617°E

743 850 757 1073 858 347 384 572 295 400 643 582 710 380 400 615 652 698

Std. dev.

40 17

13 16 18 13 20 56 26 20 42 22 37 17

2 0 3 5 3 8 9 2 8 0 5 0

57 10 40 19 26

40 14 39 21 23

22 7 3 3 4

16

22 39 18

a

20 December 1997–10 January 1998, cruise 97-09, R/V Nathaniel B Palmer, a portion of the ROAVERRS program (Research on Ocean-Atmosphere Variability and Ecosystem Response in the Ross Sea). As outlined in the text, station descriptions and current data derive from reports of that program.

covered only a subset of stations at which megafauna had been studied (Barry et al., 2003). To extend that classification to all sites examined here, we used physiographic and current data from the ROAVERRS database. Current speeds were available for the stations at which an entry appears in the CurrentjSite column of Table 1. To supplement those data, regional average currents were calculated. These are based on the ten ROAVERRS stations closest to each of the sites included in this study. The average between-station distance for this population was 30 km. Within each group of ten near neighbors, outliers (in terms of either depth or current) were excluded. The benthic habitats listed in Table 1 are based on the criteria described by Barry et al. (2003). Box cores, which frequently included undisturbed, in situ organisms and plants verifying recovery of surface sediment, were subdivided aboard the R/V Nathaniel B Palmer. Samples were shipped frozen to the University of Tennessee, Knoxville. Dr. J. Grebmeier provided samples for organic chemical and isotopic analyses. 2.2. Extraction and separation of compound classes Sediments (0–2 cm depth) were kept frozen until just prior to extraction. Lipids were extracted with

99:1 (v/v) dichloromethane:methanol at 100 °C using a Dionex Accelerated Solvent Extraction (ASE) system at Woods Hole Oceanographic Institution. The resulting total lipid extracts (TLEs) were separated into four fractions using 3 ml, Varian aminopropyl bond-elut cartridges. Hydrocarbons were eluted with 5 ml hexane, ketones with 5 ml 9:1 (v:v) hexane:dichloromethane, alcohols and sterols with 5 ml 9:1 dichloromethane:acetone, and fatty acids with 5 ml 2% formic acid in dichloromethane. This study focused on the alcohol and sterol fractions. 2.3. Organic chemical analysis All fractions were analyzed using a HewlettPackard 6980 GC-FID. Compounds were identified from mass spectra obtained using a Finnigan TSQ 700. The column used in all instruments was a 60 m Restek RTX-1 (100% dimethyl polysiloxane) with 0.32 mm i.d. and 0.5 lm film thickness. The same temperature program was used on all instruments. The column temperature was programmed from 60 to 320 °C at 4 °C per minute, then held at 320 °C for 60 min. The sensitivity of the flame ionization detector was calibrated regularly using nalkane standards.

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

Abundances of individual compounds are reported as carbon mole fractions. The numerator is the amount of carbon represented by the compound and is derived from the gas chromatographic analysis. The denominator is the total organic carbon in the sample and is derived from elemental analysis of the decalcified sediment sample. The reported mole fractions have been corrected for carbon contributed to the gas chromatographic signal by trimethylsilyl (TMS) derivatives. Because each extractable compound comprises only a small portion of the total organic matter, the mole fractions are expressed as parts per million. 2.4. Isotopic analysis The isotopic composition and concentration of total organic carbon (TOC) were determined using a Carlo Erba NA1500 elemental analyzer coupled to a Finnigan Delta Plus mass spectrometer. All results are reported as d values relative to the Vienna PDB standard. Sediments were acidified with 6 N HCl to remove carbonate and then freeze dried prior to analysis. Standard deviations of isotopic analyses were 0.1‰ for standards and 0.2‰ for replicates of samples. Standard deviations of weight percent analyses were strongly correlated with %TOC (r2 = 0.999). For samples with 1 wt% TOC, the standard deviation is 0.08 wt% C. For samples with 0.2 wt% TOC, the standard deviation is 0.04 wt%C. Specific values are reported below. Carbon isotopic compositions of individual compounds were determined by isotope-ratio-monitoring gas chromatography–mass spectrometry, or irmGCMS (Hayes et al., 1989). Compounds were separated using the chromatographic conditions described above and were oxidized to CO2 using a continuous flow combustion interface. The CO2, after passing through a Nafion dryer to remove water (Leckrone and Hayes, 1997), was introduced to a Finnigan MAT252 mass spectrometer for continuous measurement of ratios of masses m/z 44, 45, and 46. Deuterated n-alkanes with known isotopic compositions were co-injected with each sample and used to establish the d scale. Results were also checked against peaks of CO2 gas with a known isotopic composition that were analyzed at the start and end of each run. Performance of the system was monitored by analyzing a mixture of standard n-alkanes at the beginning of each day. Results were accepted when d values obtained for these test standards were accurate to within 0.2‰. Reported d val-

571

ues have been corrected for the presence of carbon contributed by derivatizing reagents. The reproducibilities of d values are expressed in terms of pooled standard deviations. Because of varying abundances and occurrences of minor, coeluting species, these vary between compounds. For each P sterol, the pooled standard deviation is given by [ d2/(D.F.)]0.5. An example follows: the isotopic composition of cholesterol was measured in extracts of samples from 17 stations, yielding 17 mean values based on 15 pairs of duplicates and two sets of triplicates. For each observation, it was possible to calculate d, the difference between the observed d value and the corresponding mean. For cholesterol, this yielded 36 values of d. These pertain to differences from 17 different means, but in all cases provide information about the scatter of results associated with analyses of cholesterol. P The sum of the squares of these deviations ( d2) is then divided by (D.F.), the number of degrees of freedom. In calculations of pooled standard deviations, the number of degrees of freedom is always given by (total number of observations)  (number of means). In this example, D.F. = 36  17 = 19. 3. Sedimentary organic carbon 3.1. Concentrations Concentrations of TOC range between 0.2% and 1.2% (Table 2). The distribution of TOC among stations is summarized graphically in the top panels of Table 2 Concentrations and isotopic compositions of TOC 13

C, ‰

Station

TOC, %

d

09 26 29 31 35 39 56 57 60 61 62 66 70 71 73 75 90 92

1.0 1.2 0.3 0.5 0.8 0.4 0.2 0.7 0.1 0.4 0.5 0.8 0.4 0.4 0.5 0.9 0.9 0.3

26.0 22.8 23.0 22.9 23.3 25.6 26.9 27.2 27.4 27.1 27.5 27.2 26.8 27.5 28.4 27.8 27.4 26.7

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

Fig. 2. Concentrations of TOC are not systematically related to location, Polynya Group, or productivity. Benthic habitats are more important. Concentrations of TOC at crest and bank stations are consistently low. The six highest concentrations of TOC are found at basin and slope sites. Similar patterns were reported by Barry et al. (2003), although the relationship between concentrations of TOC and habitat groups was not as pronounced as that observed here. Earlier, Dunbar et al. (1985) observed that coarse grained sediments are substantially more abundant on the Crary, Pennell, and Ross Banks (Fig. 1), which would lead to an expectation of lower concentrations of TOC at those sites.

4. Sterols 4.1. Quantitative variations

h

ed M

Lo w

te

M

La

id

ly Ea r

th W es t C re st Ba nk Sl op e Ba si n

So u

th or

N

-25

-25

-27

-27

-29

-29 H ig

ed M

So

or

en

C

h

-23

iu m

-23

Lo w

0.0

La te

0.0

M id

0.4

Ea rly

0.4

W es t C re st Ba nk Sl op e Ba si n

0.8

ut h

0.8

th

1.2

tra l

1.2

N

13 δ CTOC, ‰

Organic Carbon, wt%

C

en

tra

l

Isotopic compositions of total organic carbon in surface sediments (Table 2) are summarized graphically in the lower panels of Fig. 2. Sedimentary organic carbon at western stations, near the coast of Victoria Land, is enriched in 13C relative to that at all other stations, confirming results of earlier studies (Villinski et al., 2000; Grebmeier et al., 2003). Exclusion of the four, high d stations removes 86% of the variance in dTOC . Those which remain are not related systematically to productivity, habitat, or Polynya Group.

ig

3.2. Isotopic compositions

iu m

Ten compounds occur in most samples and account for the bulk of the sterols and stanols in the sediments analyzed. Structures, abbreviated, common, and systematic names are shown in Fig. 3. The abbreviated names indicate total numbers of carbon atoms and positions of double bonds (a superscript zero indicates no double bonds). Unless otherwise specified, any alkylation required to increase the carbon number beyond 27 occurs at C-24. Concentrations of the ten major compounds are reported in Table 3. Two measures are of interest: the absolute concentrations of sterols per gram of sediment and concentrations of sterols relative to total organic carbon. The latter is not affected by dilution with inorganic debris and is expressed here as the mole fraction of organic carbon represented by each sterol. The concentration of sterols relative to total organic carbon is a measure of the quality of the organic matter. Concentrations near 1200 ppmC have been reported for fresh debris accumulating in sediments from McMurdo Sound (Venkatasen, 1988), the Chukchi and Beaufort Seas (Belicka

H

572

Location

Habitat

Polynya

Productivity

Fig. 2. Concentration and isotopic composition of organic carbon at each station plotted as a function of the descriptive groups to which each station is assigned. Notably, concentrations of organic carbon are low at crest and bank habitats and organic carbon is enriched in 13 C at stations along the western margin of the Ross Sea. Numbers of points in each frame vary because some points are coincident.

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

HO

HO

HO

27 0, Cholestanol 5 -cholestan-3 -ol

573

27 5, Cholesterol cholest-5-en-3 -ol

27

5,22

, "trans-22-dehydrocholesterol" cholest-5,22E-dien-3 -ol

HO

HO

28 22, Brassicastanol 28 5,22, Brassicasterol 24-methyl-5 -cholest-22E-en-3 -ol 24-methyl-cholest-5,22E-dien-3 -ol

HO

HO

HO

29 5, Sitosterol 29 0, Sitostanol 24-ethyl-5 -cholestan-3 -ol 24-ethyl-cholest-5-en-3 -ol

29 5,22, Stigmasterol 24-ethyl-cholest-5,22E-dien-3 -ol

HO

HO 0

30(4 ,23,24) , Dinostanol 4 ,23,24-trimethyl-5 -cholestan-3 -ol

30(4 ,23,24) 22, Dinosterol 4 ,23,24-trimethyl-5 -cholest-22E-en-3 -ol

Fig. 3. Structures, short hand designations, common, and systematic names of the ten most abundant sterols found in surface sediments of the Ross Sea.

et al., 2004), and the Black Sea (Wakeham, 1995). Station 9 approximates that level (Table 3: XSterols = 1120 ppmC). Relative concentrations of sterols at the other stations are highly variable and lower – more than ten fold lower in about half of the cases. In shelf-to-basin transects, Belicka et al. (2004) found that XSterols decreased by three fold or less in the Beaufort Sea, with concentrations of sterols >300 ppmC persisting to depths of 30 cm in sediment cores. Concentrations of sterols dropped below 200 ppmC only at a depth of 2000 m in the Canada Basin of the Chukchi Sea. Concentrations in most samples from the Ross Sea are much lower, indicating that primary, autochthonous inputs have been significantly degraded, or have been diluted by allochthonous, sterol-depleted organic material, at all sites except station 9.

4.2. Compositional variations The distributions of sterols at each station were examined by means of polytopic vector analysis (PVA), a statistical approach that combines principal component and factor analyses (Evans et al., 1992). Like factor analysis, PVA resolves multiply correlated variables into subsets that vary coherently and independently (e.g., Johnson et al., 2002). The analysis generates (1) the number of end members contributing to the mixture; (2) a composition for each end member; and (3) the proportion of each end member in each sample. The method involves mathematically fitting an n-dimensional polytope (polyhedron) around a cloud of normalized data. The vertices of the polytope represent the end members. Sample locations within the polytope

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J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

Table 3 Concentrations of ten major sterols and of total sterols Station

Carbon mole fraction, ppmC 27

28

5,22

9 26 29 31 35 39 56 57 60 61 62 66 70 71 73 75 90 92 a b c

5

0

29

5,22

D

D

D

140 34 32 5.4 74 20 18 2.9 61 5.6 23 0.6 0.2 25 4.4 0.7 3.7 3.1

260 54 50 4.8 140 23 56 3.6 120 7.8 57 0.6 0.3 82 4.0 0.1 6.1 5.0

23 14 23 8.1 13 1.7 0.0 0.0 7.2 1.6 4.9 0.2 0.4 8.3 1.6 0.0 1.0 2.1

22

5,22

D

D

D

120 33 24 3.5 41 12 14 1.7 7.8 5.6 2.3 0.4 0.4 46 2.2 1.0 3.0 4.0

34 12 9.7 4.3 17 0.2 0.0 0.0 2.7 0.4 0.0 0.5 0.3 4.3 1.6 0.0 0.8 1.9

44 14 20 0.0 14 3.2 4.5 0.0 17 0.2 0.0 0.0 0.1 5.4 3.0 0.1 0.7 2.8

30(4a,23,24) 5

0

D

D

99 47 40 3.3 72 22 6.4 1.3 26 2.4 12 0.2 0.4 17 2.5 0.4 2.6 3.6

81 25 28 19 38 4.2 0.0 0.0 2.6 0.1 1.3 0.3 0.4 4.5 1.9 0.2 0.6 2.6

22

Xalgalb

Xsterols

660 200 230 71 280 72 72 5.9 220 18 110 2.4 2.3 130 18 3.0 14 19

1120 357 383 108 591 99 128 10 353 27 173 3 3 227 25 3 22 31

c

Total sterolsa lg/g sed.

0

D

D

55 18 22 5.9 29 1.6 0.0 0.0 3.5 0.1 0.0 0.0 0.2 1.3 0.5 0.0 0.6 1.2

26 9.1 12 7.0 8.8 0.2 0.0 0.0 0.0 0.1 0.0 0.2 0.1 0.0 0.0 0.0 0.0 1.2

380 150 52 16 160 2.3 1.2 0.2 2.2 0.9 17 0.9 0.8 22 3.0 0.9 4.4 3.9

Total sterols = all sterols found in sediment sample (not limited to the ten specific compounds listed here). Xalgal = carbon mole fraction for all sterols other than 27D5, 27D0, 30(4a,23,24)D22, 30(4a,23,24)D0, and their degradation products. Xsterols = carbon mole fraction for all sterols found in sediment sample (not limited to the ten specific compounds listed here).

correspond to the proportions of each end member in each sample. End member proportions sum to one in each sample. A system with only two end members explained at least 60% of the variance in each sample. One end member was rich in cholesterol and apparently represented consumers. The other appeared to represent all other sources and processes. Fig. 4 shows

Number of End Members 2

4

6

8

10

0.8 Predicted

Minimum Commonality

1.0

29Δ5

that consideration of larger numbers of end members yielded improving results until, in the trivial extreme, a 10 end member system could perfectly explain the variations of ten compounds. Large incremental improvements are associated with the steps from two to three and from five to six end members. The three end member system accounted for at least 90% of the variance in each sample. As shown by the inset graphs in Fig. 4, addition of a fourth end member significantly improved the correlation between predicted and observed concentrations of 29D5. A similar improvement was found for 29D5,22. Increasing the number of end members to six yielded a further increase in the explained variation but did so by reducing overall scatter rather than by improving correlations for any specific variable. Accordingly, the four end member system was selected for study. 4.3. Sources of sterols

0.6 Observed

Fig. 4. Minimum commonality, a measure of goodness of fit in polytopic vector analysis, vs. number of end members. The inset graphs show the three and four end member cases for the 29D5 sterol. A similar improvement was observed for 29D5,22.

The compositions of the end members defined by the polytopic vector analysis, each designated by a letter relating to its likely source, are shown in Table 4. The simplest is that of C, which is dominated by 27D5 (cholesterol). Cholesterol is produced by some algae (Volkman, 1986, 2003; Volkman et al., 1993)

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588 Table 4 Results of polytopic vector analysis: compositions of end membersa Compounds

End members D

C

P

M

27D5,22 27D5 27D0 28D5,22 28D22 29D5,22 29D5 29D0 30(4a,23,24)D22 30(4a,23,24)D0

0.23 0.28 0.02 0.06 0 0.12 0.26 0 0.04 0

0.18 0.74 0.01 0.02 0.01 0 0.02 0 0 0.01

0.28 0 0 0.52 0.02 0.03 0.14 0.02 0 0

0 0 0.18 0.03 0.14 0.02 0.02 0.35 0.12 0.13

a

Entries indicate fractional abundance of each compound in each end member. For example, 18% of the carbon in the first end member occurs as 27D5,22. Boldface entries indicate fractions >0.1.

but is commonly accepted as a biomarker for consumer organisms and as a proxy for zooplanktonic herbivory (Grice et al., 1998). The only other significant component of C is 27D5,22, sometimes called trans-dehydrocholesterol. It is the principal sterol in some diatoms, including Antarctic species (Barrett et al., 1995; Skerratt et al., 1995). It has also been reported as more abundant than cholesterol in some Antarctic salps (Phleger et al., 2000), is the second most abundant sterol in Adriatic zooplankton (Serrazanetti et al., 1989) and Antarctic amphipods (Nelson et al., 2001), and has been classified as a zooplanktonic product in a systematic survey of sterols at Trinity Bay, Newfoundland (Hudson et al., 2001). It is absent in Phaeocystis pouchetii (Nichols et al., 1991) and nearly all dinoflagellates (Volkman et al., 1999; Mansour et al., 2003; Leblond and Chapman, 2004), these being the other algae reported as abundant in the Ross Sea. On balance, C can be decisively associated with heterotrophy (C: consumers). The inclusion of 27D5,22 echoes the earlier, animal source reports cited above. This compound differs from cholesterol only by the presence of an additional double bond in the side chain, where its effects on molecular shape will be minimal. When heterotrophs encounter it in their diets, it may be preferentially preserved for inclusion in biomass, thus providing a mechanism for the association between 27D5 and 27D5,22 observed here. Only three sterols contribute significantly to P (Table 4). Of these, 28D5,22 accounts for more than

575

half of the total. This compound often comprises 100% of the sterols in Phaeocystis (Nichols et al., 1991). The second component is 27D5,22, noted above as a product of diatoms and a frequent constituent of zooplankton. During a bloom dominated by Nitzschia spp. near Magnetic Island in Prydz Bay, 27D5,22 accounted for 62% of total sterols in water column particulates (Skerratt et al., 1995). The same bloom did not produce the third component of P, namely 29D5, nor has that compound been reported as a product of Phaeocystis. It has been reported as abundant in some pennate diatoms, namely Haslea and Amphiphora (Barrett et al., 1995). Seasonal blooms in the Ross Sea Polynya, often described as dominated by Phaeocystis, usually also include abundant diatoms (Garrison et al., 2003). In some years, diatoms are favored (Worthen and Arrigo, 2003). Noting that 28D5,22, the Phaeocystis marker, occurs uniquely in this end member and that the other components have been found in diatoms, we tentatively associate P with the assemblage responsible for algal blooms in the Ross Sea Polynya (P: Polynya). End member D (Table 4), includes significant concentrations of 29D5,22, the dominant sterol in at least two species of diatoms of the genus Amphora (Gladu et al., 1991; Barrett et al., 1995). The other three constituents have already been discussed. Cholesterol is produced by some diatoms (Gladu et al., 1991; Barrett et al., 1995; Volkman, 2003) and diatoms have already been mentioned as likely sources for 27D5,22 and 29D5. Accordingly, D appears mainly to represent inputs from diatoms, with the substantial abundance of 27D5 probably indicating some additional contributions from heterotrophs (a point discussed in more detail after introduction of the isotopic evidence). The M end member (M: microbial, referring to dinoflagellates and to processes associated with those microbial heterotrophs) is comprised mainly of 30(4a,23,24)D22 (dinosterol) and 30(4a,23,24)D0 (25% of the total) and 5a(H) stanols (67%). For stations with total stanols >10 ppmC, the abundances of dinoflagellate products and stanols are, as shown in Fig. 5, very well correlated. Accordingly, placement of both groups in M is not an artifact of the polytopic vector analysis. A similar association was noted much earlier by Robinson et al. (1984), who found that 5a(H) stanols were abundant in the autotrophic, freshwater dinoflagellate Peridinium lomnickii. More recently, Hudson et al. (2001) have provided evidence that 27D22 can be produced

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J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

4.4. Geographic variations

30(4α,23,24)Δ22 + 30(4δ ,23,24)Δ0

100

80

9

60

40 35 29 26

20 31 39

0 0

r2 = 0.99

92 60 71

40

80 0

27Δ + 28Δ

120 22

160

0

+ 29Δ

Fig. 5. Observed correlation between summed concentrations (X, ppmC) of dinoflagellate related sterols and of stanols. Points are numbered to indicate stations which they represent.

by microbial hydrogenation of 27D5,22 and that 28D24(28) can similarly derive from 28D5,24(28). Like many previous reports, however, theirs mainly attributes 5b(H) stanols to this process, which they term ‘‘biohydrogenation” ( saturation of the double bond in the course of microbial alteration of the parent material). Here, the 5a(H) stanols 27D0, 28D22, and 29D0 share carbon skeletons with D5 stenols that are prominent in this suite of samples, namely 27D5 (heterotrophs), 28D5,22 (Phaeocystis), and 29D5 (diatoms). Those stenols have disparate origins, but their hydrogenated counterparts occur together in M. In the Ross Sea, dinoflagellates are dominantly heterotrophic. At multiple stations along 76°300 S, 75% of the dinoflagellate biomass observed during the 1996–1997 austral summer was heterotrophic, the balance autotrophic (Dennett et al., 2001). In coastal waters, dinoflagellates accounted for most of the non-algal dark respiration during the 1993– 1994 austral summer (Robinson et al., 1999). At face value, the association of 27D0, 28D22, and 29D0 with 30(4a,23,24)D22 and 30(4a,23,24)D0 indicates either that the dinoflagellates are producing the 5a(H) stanols by de novo biosynthesis or that the dinoflagellates are somehow associated with the biohydrogenation of stenols to produce 5a(H) stanols. We will return to these alternatives in our discussion of carbon isotopic compositions.

Relative abundances of the end members at each station are shown in Table 5. No end member varies more strongly than the others. For all, relative abundances range between about 0.7 and 0.0 with a standard deviation of about 0.2. In accord with observed distributions of phytoplankton in surface waters (DiTullio et al., 2003; Garrison et al., 2003), contributions from D are highest at western and northern sites and those from P are highest at central sites (compare Tables 1 and 5). Similarly, contributions from P are relatively high at Early and Mid Polynya sites whereas those from D maximize at the Late Polynya sites. Higher populations of dinoflagellates are observed in coastal and southern regions of the Ross Sea (Gowing et al., 2001). In the present samples, contributions from M (which includes dinosterol and dinostanol) maximize at western and southern sites. That produces a coincidentally strong correlation between M and depth (r = 0.754, P = 0.0003) which results from the geographic distribution of deep sites in the present study. Finally, contributions from C and M are inversely correlated (r = 0.597, P = 0.009), suggesting that microbial and zooplanktonic heterotrophs are competing for the same resources and that, when one flourishes, the other must decline. Table 5 Results of polytopic vector analysis: contributions of each end member at each station Stationa

9 26 29 31 35 39 56 57 60 61 62 66 70 71 73 75 90 92

End Members D

C

P

M

0.28 0.49 0.54 0.08 0.41 0.69 0.09 0.33 0.47 0.13 0.35 0 0.33 0 0.56 0.19 0.35 0.39

0.29 0.09 0.06 0.08 0.26 0.08 0.73 0.40 0.50 0.40 0.62 0.59 0 0.57 0.05 0.03 0.30 0.10

0.20 0.14 0.07 0.08 0.11 0.16 0.20 0.34 0.02 0.43 0.01 0.18 0.18 0.37 0.14 0.73 0.24 0.18

0.24 0.29 0.34 0.77 0.22 0.07 0 0 0.01 0.04 0.02 0.24 0.49 0.09 0.25 0.10 0.11 0.33

a Stations at which the total carbon mole fraction for sterols exceeds 100 ppm are indicated by boldface. Those at which it is less than 10 are italicized.

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

577

tion about relationships – or the lack of a relationship – between TOC and autochthonous biological debris. The lowest rows of the table summarize the variability of the isotopic compositions. Entries in the row marked ‘‘pooled s.d.” indicate the pooled standard deviation of replicate analyses of each sterol, calculated as described in Section 2.4. The degrees of freedom associated with these estimates vary widely because abundances of some sterols were low enough that replication was infrequent. In all cases, F tests show that isotopic variations between stations are highly significant relative to the analytical precision. The variations fall into three subsets. Values of d are most highly variable for 30(4a,23,24)D22 and 29D5,22 (range 11‰, standard deviations >3‰). The standard deviations of d for 28D5,22 and 29D5 (the principal components of P and D) and their products of biohydrogenation, 28D22 and 29D0, are smaller. The isotopically-leastvariable sterols are the three C27 compounds and 30(4a,23,24)D0.

4.5. Isotopic variations Values of d for the ten major sterols are shown in Table 6. In that same table, the column headed ‘‘Alg” reports the mass weighted average d for all algal sterols that yielded a quantity of CO2 adequate for analysis (the value of dAlg, which always refers to the isotopic composition of the sterols, not that of the biomass of the organisms that produced the sterols, thus includes contributions ranging from 0% to 37% from components in addition to those represented in the individual sterol columns of the table). For comparison, the table includes the d value of total organic carbon in each sample as well as the isotopic differences between algal sterols and cholesterol and between algal sterols and TOC. The former provides an estimate of the isotopic difference between primary products and consumer biomass. Because cholesterol is also produced by algae, the estimate is surely imperfect. The isotopic difference between algal sterols and TOC can provide informaTable 6 Isotopic compositions and relationships Station

Isotopic compositions, d, ‰ 27

9 26 29 31 35 39 56 57 60 61 62 66 70 71 73 75 90 92 Pooled s.d. D.F.c Avg. Range Std. dev.

Isotopic differences, ‰

28

D5,22

D5

30.0 28.6 28.9 27.3 29.7 29.7 30.7 30.0 31.2 32.0 29.1 29.8 29.3 30.9 30.5 31.1 31.6 30.0

27.4 27.2 32.4 29.9 31.1 30.7 29.1 31.9 32.2 29.4 29.7 31.3 31.2 31.4 31.3 31.2 32.4

0.37 19 30.0 4.7 1.2

0.37 19 30.6 5.2 1.6

29

D0

D5,22

D22

30.7 27.5 28.1 29.7 32.5 30.3 32.5

34.7 27.3 29.0 29.8 33.6 35.1 34.8 31.9 36.2 34.4 33.7 31.1 30.9 34.0 34.4 32.9 33.2 33.4

36.0 29.7 32.1 32.1 36.4 34.3

32.0 32.1 31.8 28.8 32.5 31.4 29.3 30.6 0.48 10 30.7 5.0 1.7

0.54 17 32.8 8.9 2.4

30(4a,23,24)

D5,22 25.1 26.9 28.7 30.5 29.0

32.0 32.8 33.7

35.9 27.8

29.9 34.6 32.7 30.9 30.2 31.1

25.1 33.1 30.0 24.5 33.4 32.5

0.37 14 32.6 6.7 2.1

0.54 10 29.4 11.4 3.6

D5

D0

D22

D0

32.6 27.1 30.1 30.1 30.4 35.6 35.5 27.1 35.6 33.4 34.5 30.6 30.3 32.5 29.8 32.8 30.7 32.2

31.8 28.3 31.1 30.2 29.9 33.2

32.1 27.6 34.5 27.9 30.6 39.1

32.8 31.9 32.4 29.6 31.7 30.3

34.5 33.8 33.5 32.0 32.2 31.4 32.5 31.7 25.4 33.3

35.6 31.1

0.47 17 31.7 8.5 2.6

0.47 16 31.6 9.1 2.3

27.5 30.7 30.1 33.2 31.1 0.59 13 31.6 11.6 3.3

a

Alg

b

TOC

27D5 – Alg TOC – Alg

29.0 32.3 31.7 34.4

30.8

32.7 27.9 30.6 30.5 30.8 33.0 31.4 29.9 33.0 33.5 31.5 30.7 30.0 32.7 31.3 31.6 31.2 32.0

26.0 22.8 23.0 22.9 23.3 25.6 26.9 27.2 27.4 27.1 27.5 27.2 26.8 27.5 28.4 27.8 27.4 26.7

0.5 3.4 1.9 0.9 1.9 0.7 0.8 1.1 1.3 2.1 1.0 1.3 1.5 0.1 0.3 0.0 0.5

6.7 5.1 7.6 7.6 7.5 7.4 4.5 2.7 5.6 6.4 4.0 3.5 3.2 5.2 2.9 3.8 3.8 5.2

31.4 26.2 5.6 5.6 1.4 1.9

0.7 5.3 1.3

5.2 4.9 1.7

0.66 7 31.5 5.4 1.5

a Weighted-average isotopic composition of all sterols other than 27D5, 27D0, 30(4a,23,24)D22, 30(4a,23,24)D0 and their degradation products. b TOC = total organic carbon. c Number of degrees of freedom on which the pooled standard deviation is based.

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J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

The contrasting levels of variability can be attributed to different degrees of averaging. For instance, the C27 sterols are most widely distributed among both autotrophic (Volkman et al., 1993; Barrett et al., 1995; Volkman, 1986, 2003) and heterotrophic (Bradshaw et al., 1989; Serrazanetti et al., 1989; Phleger et al., 2000; Grice et al., 1998; Nelson et al., 2001) organisms. At any given station, the isotopic compositions of heterotrophic products reflect two phenomena: (1) the averaging of contributions from diverse consumers, and (2) the averaging, by those consumers, of the carbon isotopic compositions of their food sources. Together, these can yield smaller variations in d values. Restriction of the number of sources and/or production in diverse settings – apparently the case for 30(4a,23,24)D22 and 29D5,22 – would decrease chances for averaging and increase the isotopic range. 5. Discussion 5.1. Correlations between isotopic compositions Table 7 summarizes correlations between isotopic compositions of sterols and related products which (a) are significant at the 95% confidence level or better and (b) explain at least 25% of the observed isotopic variations. Processes likely to explain the observed correlations are noted in the column headed ‘‘Relationship between 1 and 2.” The correlations ranked 7, 12, and 13 are between algal stenols and the related stanols. Such correla-

tions are expected if the stanols are related to the stenols by biohydrogenation. In each case, again as expected because the biohydrogenation reaction affects at most two carbon positions among the 27– 30 in each sterol, there is no significant isotopic offset between the precursor and the product [jd1  d2j < 2(s.d.)]. Saturation of the D5 double bond has been recognized as a common step in sterol diagenesis (Gaskell and Eglinton, 1975; Nishimura, 1978). In anaerobic environments, the saturation of stenols can consume reducing power and thus extend biodegradation. The process is dramatically complete at the sediment water interface in the Black Sea (Beier et al., 1991). Stanol/stenol ratios at the dysaerobic sediment water interface in Walvis Bay range from 0.34 to 1.0 (Gagosian et al., 1980). Before ascribing all of these observations to biohydrogenation, we should examine particularly the isotopic relationships at the five, ‘‘high-stanol” stations identified in Fig. 5 (i.e., stations 9, 35, 29, 26, and 31). When isotopic compositions of 29D5 and 29D0 are compared, the correlation improves to r = 0.925 and isotopic fractionation remains insignificant. The correlation between 28D5,22 and 28D22 is even tighter (r = 0.978) but d1  d2 = 2.4 ± 0.7 (i.e., isotopic fractionation, expected to be zero, is instead significant). The correlation between 27D5,22 and 27D0 degrades, but that between the weighted average of 27D5,22 + 27D5 and 27D0 is strong (r = 0.869) and isotopic fractionation is insignificant. Finally, the isotopic compositions of 27D0 and 28D22 are not significantly correlated with those

Table 7 Correlations between isotopically analyzed components Components

Correlation

d1  d2

s.d.

Relationship between 1 and 2

Consumption of Phaeocystis by animals, biohydrogenation of 27D5 Shared conditions of growth Production, accumulation Shared conditions of growth Consumption of diatoms by dinoflagellates Production, accumulation Biohydrogenation Consumption of Phaeocystis by animals Shared precursor, presumably 28D5,22 Consumption, biohydrogenation of 27D5 Shared conditions of growth Biohydrogenation Biohydrogenation Production, consumption, biohydrog, burial Consumption of Phaeocystis by dinoflagellates

1

2

r

Pairs

Probability

1

28D5,22

27D0

0.824

14

0.0001

2.3

0.4

2 3 4 5 6 7 8 9 10 11 12 13 14 15

28D5,22 27D5,22 27D5,22 29D5 28D5,22 29D5 28D5,22 28D22 29D5 28D5,22 28D5,22 27D5,22 27D0 28D5,22

29D5 TOC 28D5,22 30D22 TOC 29D0 27D5 27D0 27D0 29D5,22 28D22 27D0 TOC 30D22

0.754 0.713 0.675 0.743 0.631 0.650 0.610 0.672 0.647 0.650 0.517 0.530 0.524 0.534

18 18 18 13 18 16 17 13 14 13 15 14 14 13

0.0002 0.0004 0.001 0.002 0.002 0.003 0.005 0.006 0.006 0.008 0.02 0.03 0.03 0.03

1.1 3.8 2.8 0.1 6.6 0.2 2.1 2.6 1.7 3.6 0.3 0.7 4.7 1.1

0.4 0.3 0.4 0.6 0.4 0.5 0.5 0.6 0.6 0.8 0.6 0.4 0.5 0.8

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

6

TOC

4

δx - δAlg, ‰

of dinosterol and the correlation between 29D0 and dinosterol is much weaker than that between 29D5 and 29D0. With the exception of the significant fractionation between 28D5,22 and 28D22, these observations favor biohydrogenation and tend to exclude de novo biosynthesis by dinoflagellates as the source for 5a(H) stanols. But why should this process, normally characteristic of dysaerobic environments, flourish in the well oxygenated waters of the Ross Sea? For these samples, the average stanol/stenol ratio is 0.42, well above the ratio characteristic of unaltered phytoplanktonic debris (60.1; Gagosian et al., 1980). Moreover, dinoflagellates are planktonic and biohydrogenation typically becomes important only at the sediment water interface (Gagosian et al., 1980; Beier et al., 1991). Dinoflagellates produce membrane-bound fecal pellets that are exceptionally abundant in some Antarctic settings (Buck et al., 1990). If these provide anaerobic environments in which bacteria catalyze biohydrogenation, it may explain the observed association. Correlations 2, 4, and 11 associate two primary products (in the first case, for example, 28D5,22 and 29D5, the most prominent sterols from Phaeocystis and from diatoms, respectively). In all cases, the sterol derived from Phaeocystis is significantly depleted in 13C relative to the sterol with which its isotopic composition is correlated. Because the isotopic fractionation associated with carbon fixation depends on specific growth rate (Laws et al., 1995), correlated variations in d are expected if the sterols represent producers that were exposed to the same varying concentrations of nutrients. Correlations 1, 5, 8, 9, 10, and 15 reflect trophic relationships. Where animals are involved (rather than microbial heterotrophs), the sterol produced by the consumer is enriched in 13 C relative to the primary product. The remaining correlations (2, 6, 14) associate the isotopic composition of sedimentary total organic carbon with that of biosynthetic products in the overlying water column. In all cases, the TOC is significantly enriched in 13C relative to the lipid biomarkers. Correlations between dAlg and the isotopic compositions of individual sterols have been omitted from Table 7 because they derive from the functional relationships between dAlg and dSterol (i.e., computation of the weighted average). The differences between the weighted average and each of the individual sterols are summarized graphically in Fig. 6. Notably, the organism or organisms producing 27D5,22 and 29D5,22 must be systematically

579

29Δ5,22

2

27Δ

5,22 0 5 27Δ 27Δ

0 0 5 29Δ 29Δ 30Δ22 30Δ0

-2

28Δ

5,22

28Δ

22

Fig. 6. Isotopic differences, averaged across all stations, between individual sterols or total organic carbon and the weighted average isotopic composition of sterols produced by phytoplankton (values of d from Table 6).

enriched in 13C relative to other primary producers. Additionally, the expected trophic enrichment of 13 C is visible in 27D5 (d27D5 > dAlg) and in the product of its biohydrogenation (d27D0 > dAlg). 5.2. Diagenesis, redistribution, and dilution of biomarker lipids The observed, low concentrations of sterols relative to total organic carbon can be related to another unusual feature of Ross Sea sediments, namely their high radiocarbon ages. The radiocarbon content of organic matter in surface sediments has been measured at 39 sites within the present study area (Andrews et al., 1999; Domack et al., 1999; Licht and Andrews, 2002; Ohkouchi et al., 2003). Even if the younger age is selected wherever duplicates occur (e.g., trigger and piston cores at a single site), the average is 5375 radiocarbon years. In contrast, the age of organic material produced in Ross Sea surface waters unaffected by ‘‘bomb carbon” (the excess 14C resulting from atmospheric testing of nuclear weapons) is 1300 radiocarbon years (Andrews et al., 1999). Surface sediments are expected to record this ‘‘reservoir age” plus an increment due to bioturbation, which brings older carbon into surface sediments. The magnitude of the increment due to bioturbation (Berger and Heath, 1968) is related to the depth of bioturbation (greater depths will yield larger increments) and to the rate of sedimentation (larger increments are possible if the rate of sedimentation is low). Rates of sedimentation in the Ross Sea are typically 2–20 cm/1000 yrs (DeMaster et al., 1996; Licht et al., 1998; Ohkouchi et al., 2003). In that case, the depth of bioturbation required to

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J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

explain a sediment surface age of 2500 radiocarbon years (n.b., half the observed average) is 20-200 cm. This exceeds depth ranges over which linear age trends are observed in Antarctic sediments (Licht et al., 1998) and thus excludes bioturbation as the cause of the anomalous surface ages. Dilution of autochthonous inputs by older organic material is a possible explanation.

Xsterols, ppmC

9 26 29 31 35 39 56 57 60 61 62 66 70 71 73 75 90 92

1120 357 383 108 591 99 128 10 353 27 173 3 3 227 25 3 22 31

C

D14C, ‰ Average

Std. dev.

n

Avg. dist., km

348 318 309 317 317 387 447 385 365 405 433 511 593 549 466 433 366 395

98 59 66 66 66 115 125 40 36 74 111 195 237 244 209 111 102 119

9 9 9 7 8 9 8 7 9 7 3 9 8 8 7 3 6 4

109 89 92 80 85 108 69 59 96 109 32 115 79 82 106 35 100 61

1000

A

B

Fresh Debris

1000

565

1100

100

100 1600

2100

-700‰ Mixing

XSterols, ppmC

Fo

l ssi

g xin Mi

loid D ebris

Station

14

10

2700

3200

Nephe

Table 8 Regional abundances of

Given the possibility that the high ages are due to mixing of older and younger organic material, we have converted all ages to values of D14C (Stuiver and Polach, 1977) and have searched for correlations between dilution (lower values of D14C) and abundances of sterols. To begin, we computed distances between each of our stations and the 39 sites with radiocarbon analyses. We then selected subsets of the radiocarbon results in order to obtain regional abundances of 14C pertinent to each of our stations. Results are summarized in Table 8. Values of XSterols are plotted against the regional average values of D14C in Fig. 7A. The correlation between log XSterols and D14C is significant at the 99% confidence level (r = 0.577), but the results are scattered and the relationship is weak. The broken line in Fig. 7A has been placed so that half of the points lie on either side (n b., it is not a regression line). Points representing five out of the six bank and crest stations are to the left of this median line. The depletion of 14C (greater apparent age) at these stations suggests either that they are drifts at which pre-aged debris has mixed with modern inputs or that they have been scoured so that older debris is now at the surface. Such alternatives can be considered systematically. A point on the graph in Fig. 7A will be moved upward by the degradation of TOC (excluding sterols), downward by the degradation of sterols

10

Age, Yrs

1 -600

-500

-400 14

-300

Regional Δ CTOC, ‰

-1000 -800

-600

-400

-200

0

14

Regional Δ CTOC, ‰

Fig. 7. (A) XSterol (Table 3) vs. regional D14C (Table 8). Symbols indicate basin, slope, bank, and crest stations as in Fig. 1. The broken line is not a regression line. Instead, it has been placed so that half of the points lie on each side and (B) XSterol vs. regional D14C, but with stations represented by black dots and with axes expanded to accommodate mixing lines. Values of XSterol and D14C for biological products formed in surface waters prior to 1960 will lie in the area designated ‘‘Fresh Debris.” As explained in the text, the line labeled ‘‘Nepheloid Debris” represents the locus of D14C, XSterol points for materials exposed to continuous, maximally effective biodegradation and weathering.

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

relative to TOC, and to the left by radiocarbon decay or dilution with older material. Two specific cases can be modeled. The first is the relationship between XSterols and regional D14C if all variations are due simply to mixing of fresh debris with radiocarbon dead material that contains no sterols. If f is the fractional abundance of fresh debris, we have X Sterols ¼ f ð1200Þ þ ð1  f Þð0:1Þ

ð1Þ

where XSterols is the carbon mole fraction of sterols in the mixed sample, 1200 is the carbon mole fraction of sterols in the fresh debris, and 0.1 is the carbon mole fraction of sterols in the diluent (0.1 is the noise level in the analyses and is chosen instead of 0 to facilitate the use of a logarithmic scale). Similarly, for the abundance of radiocarbon, we can write D14 C ¼ f ð0:154Þ þ ð1  f Þð1:000Þ

ð2Þ

where D14C is the abundance of radiocarbon in the mixed sample relative to modern carbon, 0.154 (which corresponds to an age of 1300 radiocarbon years) is the D14C value of fresh debris, and 1.000 is the D14C value of the diluent (expressed in permil units, these values would be 154 and 1000‰). The path in Fig. 7B marked ‘‘Fossil Mixing” plots solutions of Eqs. (1) and (2) for 0.001 6 f 6 0.85. It is of course possible that the older, diluting component is not radiocarbon dead. In such cases, the asymptote of the mixing line would not lie at 1000‰ but at some higher value. Fig. 7B includes an example for a diluent with D14C = 700‰, corresponding to an age of 8600 years. A second path of interest represents material constantly exposed to oxidants so that, within limits imposed by temperature and the composition of the local microbial community, TOC and sterols are degraded at maximal rates (Hedges et al., 1999; Hoefs et al., 2002). This would pertain to debris suspended in the nepheloid layer near the seafloor or exposed at the sediment surface. In this case, changes in XSterols and D14C are due to three kinetically controlled processes, namely the degradation of TOC and of sterols and the decay of radiocarbon. For the decay of 14C, we can write lnðD14 C þ 1Þ  lnð0:154 þ 1Þ ¼ k C t

ð3Þ

where D14C is the radiocarbon abundance at time t, 0.154 is the initial radiocarbon abundance, and kC is the radiocarbon decay constant (1.21  104 yr1, corresponding to the physical half life of 5730 calen-

581

dar years). For the degradation of sterols and TOC, we can write lnðS t =S i Þ ¼ k S t

and

lnðRt =Ri Þ ¼ k R t ð4a; 4bÞ

where S and R are concentrations of sterols and of TOC, the subscripts t and i represent concentrations at time t and initially, and kS and kR are the rate constants for degradation of sterols and TOC, respectively. The initial carbon mole fraction of sterols is XSi = Si/Ri and the carbon mole fraction at time = t is XSt = St/Rt. It follows that lnðX St Þ  lnðX Si Þ ¼ ðk R  k S Þt

ð5Þ

Values for kR and kS under conditions prevailing in the Ross Sea are not known. They can, however, be constrained based on the present observations. The line marked ‘‘Nepheloid Debris” in Fig. 7B represents the simultaneous solution of Eqs. (3) and (5) as a function of t with kR  kS = 2.02  103 yr1. This value was chosen to yield a line passing to the right of all points on the graph and thus embodies an assumption that all of the samples represent materials that were shielded from degradation for at least some portion of their history. If kR = 0 (TOC is inert), the corresponding half life of sterols is 344 yrs. If the half life of TOC is 1000 yrs, the corresponding half life of sterols is 256 yrs. Canuel and Martens (1996) report kS  0.01 day1 for materials accumulating at the surface of Cape Lookout Bight, North Carolina, USA. Sun and Wakeham (1994) find kS = 0.033 yr1 in surface sediments of the Black Sea. Studying buried materials, Arzayus and Canuel (2004), computed first order rate constants based on down core concentration profiles. In bioturbated sediments from the lower York estuary, Chesapeake Bay, USA, they found kR = 0.0022 yr1 and kS = 0.013 yr1, corresponding to half lives of 315 and 53 yrs, respectively. At an upstream site where episodic disturbances of the sediment introduced electron acceptors at a greater depths, the observed half lives were 121 and 38 yrs. The finding that sterols are degraded more rapidly than TOC duplicates the present result. The high rates of degradation reported by Canuel and Martens (1996) and Sun and Wakeham (1994) suggest that the rates associated here with ‘‘Nepheloid Debris” pertain to materials that have spent most of their time shielded from optimal biodegradation. Given the very low temperatures prevailing in the Ross Sea, the half life for TOC is likely to exceed that in the lower York estuary. If it is 700 years, the

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magnitude of kR  kS indicates that the half life for sterols is approximately 3 fold shorter. If it is 3200 years, the sterol half life is approximately 10 fold shorter. These contrasts between the half lives of sterols and TOC are similar to the 6- and 3-fold differences found by Arzayus and Canuel (2004). Points will be displaced to the left of the nepheloid debris line because: (1) they represent organic matter that has been more strongly shielded from oxidants and thus XSterols has not declined at the maximum rate, and/or (2) they have been affected by mixing. The point at D14C = 549‰, XSterol = 227 ppmC (station 71; the point lies at the intersection of the broken lines in Fig. 7B) can serve as an example. If it was not affected by mixing, its true age is 5200 years (from solution of Eq. (3)). To account for the relatively good preservation of sterols, its organic constituents must have been exposed to oxidants for only a small portion of that time (solution of Eq. (5) with kR  kS = 2.02  103 yr1 provides an estimate of 820 years). If the sediment at this station is a mixture of components with differing histories, multiple pathways are possible. Two are shown in Fig. 7B. In one case, the organic material could have been exposed to oxidants for 560 yrs and then mixed with radiocarbon dead, sterol free material (57% recent debris + 43% diluent). The corresponding mixing line is shown by a broken line roughly paralleling the ‘‘Fossil Mixing” line. In another case, the organic material could have been exposed to oxidants for 205 years and then mixed with 700‰, sterol free material (29% recent debris + 71% diluent; see broken line roughly paralleling the ‘‘700‰ Mixing” line). These are only examples. A range of end members is possible, as are episodic processes that can be resolved to yield components like those indicated. In all cases involving mixing, the true age of the sediment horizon is less than that indicated by the radiocarbon content of the organic material. Points like that representing station 75, at the lower right corner of the distribution (D14C = 433‰, XSterols = 3 ppmC) cannot be explained by mixing, which moves points dominantly leftward rather than downward on a graph like that in Fig. 7. Instead, aggressive degradation of sterol biomarkers, resulting from nearly constant exposure to oxidants, is required. Overall, the distribution of points in Fig. 7 suggests a system dominated by enhanced degradation of biomarkers. Mixing is likely also to play a role in a system where material is frequently resuspended. If so, the width of the

distribution of points (i.e., parallel to the D14C axis) suggests that the diluent is not radiocarbon dead but instead contains appreciable 14C (e.g., D14C  700‰). 5.3. Causes of the isotopic zonation in Ross Sea organic material The lower left panel of Fig. 2 shows that sedimentary TOC at the western sites is enriched in 13C. In fact, the distribution of 13C is strongly bimodal. The four western sites have 22.8 P dTOC P 23.3‰. Values of d at the other sites range from 25.6 to 28.4‰. There is a 2.3‰ gap between the lightest western TOC and the heaviest TOC elsewhere. The remaining lower panels of Fig. 2 show that the isotopic bifurcation is not associated with habitat, Polynya group, or productivity. The finding is not new, nor is the phenomenon uncommon in Antarctic environments. The most recent and detailed investigations in the Ross Sea (Villinski et al., 2000; Grebmeier et al., 2003) found a similar geographic distribution and attributed it to ‘‘high growth rates, species specific fractionation, input from sea ice communities enriched in 13C, increased heterotrophic recycling (with consequent trophic enrichment of 13C), or high rates of bloom related CO2(aq) drawdown in the upper water column”. The last of these causes was considered most important but the others could not be eliminated. Earlier, similar isotopic dispersion was observed in Prydz Bay (Kopczynska et al., 1995). As in the Ross Sea (Villinski et al., 2000), the abundance of 13C in suspended particulate organic matter (SPOM) was strongly correlated with the concentration of SPOM. At Prydz Bay, detailed cell counts and morphometric studies showed that isotopic enrichment was associated with pennate diatoms and large, heterotrophic dinoflagellates. The isotopically lighter populations included Phaeocystis, centric diatoms, and autotrophic dinoflagellates. Finally, a third example of isotopic bimodality can be found in a much broader study of Southern Ocean phytoplankton (Popp et al., 1999). In that case, SPOM at six stations on the southern end of a 17 station transect across the Princess Elizabeth Trough (59°S, 82°E to 66°S, 85°E) is enriched in 13C by 4–5‰ relative to that at northward, open water stations. The associated water mass is designated as ‘‘ice melt” and is of markedly lower salinity. Concentrations of SPOM are not reported but concentrations of chlorophyll and of sterols were among the highest observed during this

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

This could be because those samples happen to derive from organisms with unprecedented, small biosynthetic isotope effects, or it could be because isotopic relationships in these samples have been reset by secondary processes (admixture of sterol-free, heavily degraded material; very extensive biodegradation of autochthonous sterols). Given that such processes have obviously affected the concentrations of the sterols, decreasing them more than 100 fold, the latter explanation is more likely. Subject to reexamination, we will accept the higher-concentration sterols, but not the sterols present only in trace amounts, as likely guides to the isotopic compositions of their phytoplanktonic sources. Fig. 9A shows that the abundances of end members differ strongly between the isotopically enriched

A

0.6

Abundance

study. The results of the present, compound specific isotopic analyses bear strongly on the causes of these isotopic enrichments. As a first step, we must examine relationships between the isotopic compositions of the sedimentary sterols and the coexisting total organic carbon. These are summarized graphically in Fig. 8. In the absence of mixing or secondary isotopic fractionations, the isotopic difference between TOC and sterols will be controlled by biosynthetic isotope effects. This is surely the case for the SPOM and sterols collected from Southern Ocean surface waters (Popp et al., 1999). In the Princess Elizabeth Trough study (the best available guide to products expected to form in the Ross Sea), sterols in the southern, isotopically enriched samples were depleted in 13C by 4–7‰ relative to SPOM. Those from the open water stations were depleted by 6–9‰. In the present data set, for samples in which the carbon mole fraction of algal sterols exceeds 71 ppmC (grey and black symbols; Fig. 8), sterols are depleted relative to TOC by 4– 8‰. The TOC-Alg fractionation is smaller in samples with lower concentrations of sterols (open symbols).

All others, Isotopically Depleted

0.4

0.2

-22

West, Isotopically Enriched 0.0

D

8

P

C

M

+

End Members

Al g

δ

+

4

B

δ

Al g

-25

4.1‰

-26

δ, ‰

δ TOC, ‰

-24

583

-30 4.5‰

-28 -35

-30 -34

TOC -32

-30

δAlg, ‰

-2 8

Fig. 8. Isotopic composition of total organic carbon vs. the weighted-average isotopic composition of algal sterols. Shapes of symbols indicate basin, slope, bank, and crest stations as in Fig. 1. Black symbols, 660 P Xalgal P 200; grey, 130 P Xalgal P 70; white, 19 P Xalgal P 3 ppmC. The dotted lines indicate the loci of points representing preservation of unaltered primary products with sterol biosynthetic offsets of 8 and 4‰.

29Δ5,22 29Δ5 28Δ5,22

27Δ5

29Δ0 30Δ22

Sterols Fig. 9. Average properties of stations with isotopically enriched TOC (upright triangles) and with isotopically depleted TOC (inverted triangles): (A) abundances of end members (error bars, ±1 std. dev.; data from Tables 1 and 5) and (B) isotopic compositions of sterols representing each end member (example: 29D5,22 and 29D5 are isotopic proxies for sterols in the D end member) and of total organic carbon (error bars, ±1 std. error; data from Table 6).

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and depleted zones of the Ross Sea. The observed distributions are in good agreement with biological observations which report higher concentrations of diatoms and heterotrophic dinoflagellates in western waters (Smith et al., 1996; Garrison et al., 2003). A finer point deserves specific attention. Although the abundance of the D end member is slightly higher at the western stations, this is not due to a specific increase in the 29D5,22 biomarker. The pertinent data are in Table 3, which shows that, as a percentage of total algal sterols at stations with XAlg P 71 ppmC, 29D5,22 averages 5.2 ± 3.8% (mean and s.d.) at the western stations and 4.9 ± 2.5% elsewhere. On the basis of biomarker abundances, therefore, the present results would support an association between isotopic enrichment and high concentrations of diatoms (though not just those producing 29D5,22) and heterotrophic dinoflagellates. Fig. 9B shows, however, (1) that the D and P end members are not enriched in 13C relative to the C and M end members and (2) that sterols representative of all end members are isotopically enriched in samples from western locations and isotopically depleted elsewhere. Accordingly, species effects cannot underlie the observed isotopic contrasts. This observation also excludes two other potential causes of the isotopic zonation, namely increased heterotrophic fractionation and inputs from sea-ice algae. Details follow. All of the sterols at the isotopically enriched stations are enriched in 13C relative to those at the isotopically depleted stations (Fig. 9B). With the exception of 27D5, the magnitude of enrichment (roughly 4.5‰) does not differ significantly from the 4.1‰ difference observed in the TOC. Even the small difference between 4.5 and 4.1‰ may be explicable. Zooplanktonic products are more abundant at the isotopically depleted sites (note the abundances of the C end member, Fig. 9A). Moreover, those products are exceptionally enriched in 13C (note d27D5 at depleted sites, Fig. 9B). In combination, these factors could increase dTOC at the depleted sites so that 4.5‰ difference in the primary inputs was reduced to 4.1‰ in the TOC. In sum, species effects are either not present (given the roughly equal enrichments in all sterols and TOC) or, if the finest details of the results are accepted as significant, they are detectable in terms of the contrast between the 4.1 and 4.5‰ differences (TOC vs. sterols). Either way, the isotopic enrichment at the western sites is not due to species effects. It is not due to heterotrophy because the abundance of isotopically enriched

heterotrophic products is actually higher at the isotopically depleted sites. It is not due to inputs from ice algae because there is no evidence for such highly enriched products (d ? 10‰; see also Kopczynska et al., 1995; Popp et al., 1999) and because the enrichment available from aquatic products is already more than large enough to account for the isotopic enrichment in sedimentary TOC. As causes for the isotopic enrichment, we are left with growth rate effects and phenomena referred to by previous investigators as ‘‘drawdown” of CO2. The first of these can be discarded. Data now available show that, if anything, specific growth rates are higher in the central Ross Sea than in western regions (Smith and van Hilst, 2003; measured by uptake of 14C under in situ conditions, 24-h incubations). The opposite pattern would be required if the isotopic zonation were caused by differences in specific growth rates. So we are led to points not treated in earlier reports. What, exactly, is meant by drawdown? Is there evidence to support it? And could the effect be large enough to explain the isotopic zonation? Two modes of drawdown can be envisioned. In the first, isotopic enrichment results simply from conversion of dissolved inorganic carbon to organic carbon. In the second, isotopic enrichment results from a change in the isotopic fractionation associated with fixation of CO2. The first supposes that stratification inhibits upward mixing of dissolved inorganic carbon from deeper waters and that invasion of CO2 from the atmosphere is much slower than production of organic matter. But even if all additions of inorganic carbon were prevented (no CO2 from the atmosphere, no DIC from deeper waters or from adjoining surface waters), the effect would not be large enough. The isotopic difference between biomass and DIC in the Ross Sea is approximately 27‰. Even with such a large isotope effect, the accumulating organic carbon would not be enriched by four permil until 27% of the DIC had been converted to biomass. The highest conversion observed by Arrigo et al. (1999) is 9%. Moreover, the full isotopic enrichment appears in waters where the conversion is as low as 1.3% (at which point the enrichment due to this mechanism would be 0.2‰). In the second case, ‘‘drawdown” refers not to conversion of DIC to organic matter but to dynamic depression of steady state concentrations of dissolved CO2. This is important because ep, the fractionation factor associated with photosynthetic

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fixation of carbon, depends on the concentration of dissolved CO2 and on the fraction of fixed C that derives from assimilation of bicarbonate (Laws et al., 1995; Keller and Morel, 1999). Concentrations of dissolved CO2 are not normally reported for Ross Sea surface waters, but values of P CO2 , the partial pressure of CO2 in equilibrium with the seawater, are. Typical values for the central and western regions of our study area are respectively 250 and 170 latm (Bates et al., 1998). Values of P CO2 were also measured continuously during the ROAVERRS cruises. For the isotopically enriched, western sites examined here, observed values of P CO2 are concentrated in the range 180-210 latm. At other sites, P CO2 varies widely between 150 and 420 latm. Mean values for December and January east of 170°E vary with longitude between 260 and 300 latm. Accordingly, both the earlier, published values of P CO2 and the ROAVERRS database demonstrate drawdown of dissolved CO2 at the western sites. This variation would affect all C-fixing organisms and thus is consistent with the observation that biomarker sterols produced by all algal species are enriched in 13C at the western sites. The mechanisms underlying the isotopic enrichment remain to be elucidated. The apparent magnitude of the observed variation in ep, the isotopic fractionation between dissolved CO2 and biomass, could be consistent simply with an inverse dependence on the concentration of dissolved CO2 (Laws et al., 1995). However, Cassar et al. (2004) have shown that assimilation of bicarbonate is widespread in the Southern Ocean and that its isotopic consequences fit well with the model introduced by Keller and Morel (1999). If, as is likely, the assimilation of bicarbonate is stimulated by drawdown of dissolved CO2, that mechanism must also play a large role. Finally, it remains possible that variations in algal cell geometry, specifically a prevalence at western sites of cells with low ratios of surface area to volume, could also help to explain the isotopic signals (Popp et al., 1998). 6. Conclusions Concentrations and isotopic compositions of sterols preserved in surface sediments of the Ross Sea, Antarctica, reveal systematic linkages to different biological communities and mixing conditions within the water column. Despite low concentrations due to intense oxidation, we were able to define 10 major sterols of sufficient abundance to

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yield reliable isotopic data. Four end member biological assemblages, derived from statistical analysis of all sterol compositional and isotopic data, are dominated by diatoms, Phaeocystis species, heterotrophic organisms, and dinoflagellates/ bacteria, respectively. Assemblages dominated by microbial heterotrophs do not have many zooplanktonic heterotrophs and vice versa. Overall, a great deal of isotopic averaging occurs during the process of heterotrophy. Isotopic compositions of sterols related to autotrophs have much greater scatter in contrast to those from heterotrophs. The isotopic zonation observed previously in organic matter from the Ross Sea can now be ascribed to one dominant mechanism; ‘‘drawdown” of dissolved CO2 in the water column during high productivity and stratification or minimal mixing of the water column. Acknowledgements This work was supported by NASA grant NAGW-1940 and by NSF cooperative agreement OCE-0228996. We thank the ROAVERRS group for allowing our participation in this project. We thank J. Grebmeier for assistance in sampling, John Volkman for helpful discussions, Sean Sylva for assistance at WHOI, and Steve Studley and Jon Fong for assistance at the mass spectrometer facility, Biogeochemical Laboratories, Indiana University. Thanks also to the crew of the Nathaniel B. Palmer as well as the ASA crew. Associate Editor—Rich Pancost

References Andrews, J.T., Domack, E.W., Cunningham, W.L., Leventer, A., Licht, K.J., Jull, A.J.T., DeMaster, D.J., Jennings, A.E., 1999. Problems and possible solutions concerning radiocarbon dating of surface marine sediments, Ross Sea, Antarctica. Quaternary Research 52, 206–216. Arrigo, K.R., McClain, C.R., 1994. Spring phytoplankton production in the Western Ross Sea. Science 266, 261–263. Arrigo, K.R., Robinson, D.H., Worthen, D.L., Dunbar, R.B., DiTullio, G.R., VanWoert, M., Lizotte, M.P., 1999. Phytoplankton community structure and the drawdown of nutrients and CO2 in the Southern Ocean. Science 283, 365–367. Arzayus, K.M., Canuel, E.A., 2004. Organic matter degradation in sediments of the York River estuary: effects of biological vs. physical mixing. Geochimica et Cosmochimica Acta 69, 455–463. Barrett, S.M., Volkman, J.K., Dunstan, G.A., Leroi, J.M., 1995. Sterols of 14 species of marine diatoms (Bacillariophyta). Journal of Phycology 31, 360–369.

586

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

Barry, J.P., Grebmeier, J.M., Smith, J., Dunbar, R.B., 2003. Oceanographic versus seafloor-habitat control of benthic megafaunal communities in the S.W. Ross Sea, Antarctica. In: DiTullio, G.R., Dunbar, R.B. (Eds.), Biogeochemistry of the Ross Sea, Antarctic Research Series, vol. 78. American Geophysical Union, pp. 327–354. Bates, N.R., Hansell, D.A., Carlson, C.A., Gordon, L.I., 1998. Distribution of CO2 species, estimates of net community production, and air–sea CO2 exchange in the Ross Sea Polynya. Journal of Geophysical Research 103, 2883–2896. Beier, J.A., Wakeham, S.G., Pilskaln, C.H., Honjo, S., 1991. Enrichment in saturated compounds of Black Sea interfacial sediment. Nature 351, 642–644. Belicka, L.L., Macdonald, R.W., Yunker, M.B., Harvey, H.R., 2004. The role of depositional regime on carbon transport and preservation in Arctic Ocean sediments. Marine Chemistry 86, 65–88. Berger, W.H., Heath, G.R., 1968. Vertical mixing in pelagic sediments. Journal of Marine Research 26, 134–143. Bradshaw, S.A., O’Hara, S.C.M., Corner, E.D.S., Eglinton, G., 1989. Assimilation of dietary sterols and faecal contribution of lipids by the marine invertebrates Neomysis integer, Scrobicularia plana and Nereis diversicolor. Journal of the Marine Biology Association UK 63, 891–911. Brassell, S.C., 1992. Biomarkers in recent and ancient sediments: the importance of the diagenetic continuum. In: Whelan, J.K., Farrington, J.W. (Eds.), Productivity, Accumulation and Preservation of Organic Matter in Recent and Ancient Sediments. Columbia University, New York, pp. 339–367. Brassell, S.C., 1994. Isopentenoids and geochemistry. In: Nes, W.D. (Ed.), Isopentenoids and other Natural Products: Evolution and Function, American Chemical Society Symposium Series, vol. 562, pp. 2–30. Buck, K.R., Bolt, P.A., Garrison, D.L., 1990. Phagotrophy and fecal pellet production by an athecate dinoflagellate in Antarctic sea ice. Marine Ecology Progress Series 60, 75–84. Canuel, E.A., Martens, C.S., 1996. Reactivity of recently deposited organic matter: degradation of lipid compounds near the sediment–water interface. Geochimica et Cosmochimica Acta 60, 1793–1806. Cassar, N., Laws, E.A., Bidigare, R.B., Popp, B.N., 2004. Bicarbonate uptake by Southern Ocean phytoplankton. Global Biogeochemical Cycles 18, GB2003. doi:10.1029/ 2003GB002116. Colombo, J.C., Silverberg, N., Gearing, J.N., 1996. Lipid biogeochemistry in the Laurentian Trough. 1. Fatty acids, sterols and aliphatic hydrocarbons in rapidly settling particles. Organic Geochemistry 25, 211–225. De Leeuw, J.W., Baas, M., 1986. Early-stage diagenesis of steroids. In: Johns, R.B. (Ed.), Biological Markers in the Sedimentary Record. Elsevier, pp. 101–123. DeMaster, D.J., Ragueneau, O., Nittrouer, C.A., 1996. Preservation efficiencies and accumulation rates for biogenic silica and organic C, N, and P in high-latitude sediments: the Ross Sea. Journal of Geophysical Research 101, 18501–18518. Dennett, M.R., Mathot, S., Caron, D.A., Smith Jr., W.O., Lonsdale, D.J., 2001. Abundance and distribution of phototrophic and heterotrophic nano- and microplankton in the southern Ross Sea. Deep Sea Research Part II 48, 4019–4037. DiTullio, G.R., Geesey, M.E., Leventer, A., Lizotte, M.P., 2003. Algal pigment ratios in the Ross Sea: implications for CHEMTAX analysis of Southern Ocean data. In: DiTullio,

G.R., Dunbar, R.B. (Eds.), Biogeochemistry of the Ross Sea, Antarctic Research Series, vol. 78. American Geophysical Union, pp. 35–52. Domack, E.W., Taviani, M., Rodriguez, A., 1999. Recent sediment remolding on a deep shelf, Ross Sea: implications for radiocarbon dating of Antarctic marine sediments. Quaternary Science Reviews 18, 1445–1451. Dunbar, R.B., Anderson, J.B., Domack, E.W., 1985. Oceanographic influences on sedimentation along the Antarctic continental shelf. In: Jacobs, S.S. (Ed.), Oceanology of the Antarctic Continental Shelf, Antarctic Research Series, vol. 45. American Geophysical Union, pp. 291–312. Evans, J.C., Ehrlich, R., Krantz, D., Full, W., 1992. A comparison between polytopic vector analysis and empirical orthogonal function analysis for analyzing quasi-geostrophic potential vorticity. Journal of Geophysical Research 97, 2365–2378. Gagosian, R.B., Smith, S.O., Lee, C., Farrington, J.W., Frew, N.M., 1980. Steroid transformation in recent marine sediments. In: Douglas, A.G., Maxwell, J.R. (Eds.), Advances in Organic Chemistry 1979. Pergamon Press, Oxford, pp. 407– 419. Garrison, D.L., Gibson, A., Kunze, H., Gowing, M.M., Vickers, C.L., Mathot, S., Bayre, R.C., 2003. The Ross Sea polynya project: diatom and Phaeocystis-dominated phytoplankton assemblages in the Ross Sea, Antarctica, 1994–1996. In: DiTullio, G.R., Dunbar, R.B. (Eds.), Biogeochemistry of the Ross Sea, Antarctic Research Series, vol. 78. American Geophysical Union, pp. 53–76. Gaskell, S.J., Eglinton, G., 1975. Rapid hydrogenation of sterols in a contemporary lacustrine sediment. Nature 254, 209–211. Gladu, P.K., Patterson, G.W., Wikfors, G.H., Chitwood, D.J., Lusby, W., 1991. Sterols of some diatoms. Phytochemistry 30, 2301–2303. Gowing, M.M., Garrison, D.L., Kunze, H.B., Winchell, C.J., 2001. Biological components of Ross Sea short-term particle fluxes in the austral summer of 1995–1996. Deep-Sea Research I 48, 2645–2671. Grebmeier, J.M., DiTullio, G.R., Barry, J.P., Cooper, L.W., 2003. Benthic carbon cycling in the Ross Sea Polyna, Antarctica: benthic community metabolism and sediment tracers. In: DiTullio, G.R., Dunbar, R.B. (Eds.), Biogeochemistry of the Ross Sea, Antarctic Research Series, vol. 78. American Geophysical Union, pp. 313–326. Grice, K., Klein-Breteler, W.C.M., Schouten, S., Grossi, V., de Leeuw, J.W., Sinninghe Damste´, J.S., 1998. Effects of zooplankton herbivory on biomarker proxy records. Paleoceanography 13, 686–693. Hamm, C., Reigstad, M., Riser, C.W., Muhlebach, A., Wassmann, P., 2001. On the trophic fate of Phaeocystis pouchetii. VII. Sterols and fatty acids reveal sedimentation of Ppouchetii-derived organic matter via krill fecal strings. Marine Ecology – Progress Series 209, 55–69. Harvey, H.R., O’Hara, S.C.M., Eglinton, G., Corner, E.D.S., 1989. The comparative fate of dinosterol and cholesterol in copepod feeding – implications for a conservative molecular biomarker in the marine water column. Organic Geochemistry 14, 635–641. Hayes, J.M., Freeman, K.H., Popp, B.N., Hoham, C.H., 1989. Compound-specific isotopic analyses: a novel tool for reconstruction of ancient biogeochemical processes. Organic Geochemistry 16, 1115–1128.

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588 Hedges, J.I., Hu, F.S., Devol, A.H., Hartnett, H.E., Tsamakis, E., Keil, R.G., 1999. Sedimentary organic matter preservation: a test for selective degradation under oxic conditions. American Journal of Science 299, 529–555. Hoefs, M.J.L., Rijpstra, W.I.C., Sinninghe Damste´, J.S., 2002. The influence of oxic degradation on the sedimentary biomarker record I: evidence from Madeira Abyssal Plain turbidites. Geochimica et Cosmochimica Acta 66, 2719– 2735. Hudson, E.D., Parrish, C.C., Helleur, R.J., 2001. Biogeochemistry of sterols in plankton, settling particles and recent sediments in a cold ocean ecosystem (Trinity Bay, Newfoundland). Marine Chemistry 76, 253–270. Johnson, G.W., Ehrlich, R., Full, W., 2002. Principal components analysis and receptor models in environmental forensics. In: Murphy, B.L., Morrison, R.D. (Eds.), An Introduction to Environmental Forensics. Academic Press, San Diego, pp. 461–515. Ju, S.J., Harvey, H.R., 2004. Lipids as markers of nutritional condition and diet in the Antarctic krill Euphausia superba and Euphausia crystallorophias during austral winter. Deep Sea Research Part II: Topical Studies in Oceanography 51, 2199–2214. Ju, S.J., Scolardi, K., Daly, K.L., Harvey, H.R., 2004. Understanding the trophic role of the Antarctic ctenophore, Callianira antarctica, using lipid biomarkers. Polar Biology 27, 782–792. Keller, K., Morel, F.M.M., 1999. A model of carbon isotopic fractionation and active carbon uptake in phytoplankton. Marine Ecology Progress Series 182, 295–298. Kopczynska, E.E., Goeyens, L., Semeneh, M., Dehairs, F., 1995. Phytoplankton composition and cell carbon distribution in Prydz Bay, Antarctica: relation to organic particulate matter and its d13C values. Journal of Plankton Research 17, 685– 707. Laws, E.A., Popp, B.N., Bidigare, R.R., Kennicutt, M.C., Macko, S.A., 1995. Dependence of phytoplankton carbon isotopic composition on growth rate and [CO2](aq): theoretical considerations and experimental results. Geochimica et Cosmochimica Acta 59, 1131–1138. Leblond, J.D., Chapman, P.J., 2004. Sterols of the heterotrophic dinoflagellate, pfiesteria piscicida (dinophyceae): is there a lipid biomarker? Journal of Phycology 40, 104–111. Leckrone, K.J., Hayes, J.M., 1997. Efficiency and temperature dependence of water removal by membrane dryers. Analytical Chemistry 69, 911–918. Licht, K.J., Andrews, J.T., 2002. The 14C record of late pleistocene ice advance and retreat in the central Ross Sea, Antarctica. Arctic, Antarctic and Alpine Research 34, 324– 333. Licht, K.J., Cunningham, W.L., Andrews, J.T., Domack, E.W., Jennings, A.E., 1998. Establishing chronologies from acidinsoluble organic 14C dates on Antarctic (Ross Sea) and Arctic (North Atlantic) marine sediments. Polar Research 17, 203–216. Mackenzie, A.S., Brassell, S.C., Eglinton, G., Maxwell, J.R., 1982. Chemical fossils – the geological fate of steroids. Science 217, 491–504. Mansour, M.P., Volkman, J.K., Blackburn, S.I., 2003. The effect of growth phase on the lipid class, fatty acid and sterol composition in the marine dinoflagellate, Gymnodinium sp. in batch culture. Phytochemistry 63, 145–153.

587

Muhlebach, A., Weber, K., 1998. Origins and fate of dissolved sterols in the Weddell Sea, Antarctica. Organic Geochemistry 29, 1595–1607. Muhlebach, A., Albers, C., Kattner, G., 1999. Differences in the sterol composition of dominant Antarctic zooplankton. Lipids 34, 45–51. Nelson, M.M., Mooney, B.D., Nichols, P.D., Phleger, C.F., 2001. Lipids of Antarctic Ocean amphipods: food chain interactions and the occurrence of novel biomarkers. Marine Chemistry 73, 53–64. Nichols, P.D., Skerratt, J.H., Davidson, A., Burton, H., McMeekin, T.A., 1991. Lipids of cultured Phaeocystis pouchetii: signatures for food-web, biogeochemical and environmental studies in Antarctica and the Southern Ocean. Phytochemistry 30, 3209–3214. Nishimura, M., 1978. Geochemical characteristics of the high reduction zone of stenols in Suwa sediments and the environmental factors controlling the conversion of stenols to stanols. Geochimica et Cosmochimica Acta 42, 349–357. Ohkouchi, N., Eglinton, T.I., Hayes, J.M., 2003. Radiocarbon dating of individual fatty acids as a tool for refining Antarctic Margin sediment chronologies. Radiocarbon 45, 17–24. Phleger, C.F., Nelson, M.M., Mooney, B., Nichols, P.D., 2000. Lipids of Antarctic salps and their commensal hyperiid amphipods. Polar Biology 23, 329–337. Phleger, C.F., Nelson, M.M., Mooney, B.D., Nichols, P.D., 2002. Interannual and between species comparison of the lipids, fatty acids and sterols of Antarctic krill from the US AMLR Elephant Island survey area. Comparative Biochemistry and Physiology B – Biochemistry and Molecular Biology 131, 733–747. Popp, B.N., Laws, E.A., Bidigare, R.B., Dore, J.E., Hanson, K.L., Wakeham, S.G., 1998. Effect of phytoplankton cell geometry on carbon isotopic fractionation. Geochimica et Cosmochimica Acta 62, 69–77. Popp, B.N., Trull, T., Kenig, F., Wakeham, S.G., Rust, T.M., Tilbrook, B., Griffiths, F.B., Wright, S.W., Marchant, H.J., Bidigare, R.R., Laws, E.A., 1999. Controls on the isotopic composition of Southern Ocean phytoplankton. Global Biogeochemical Cycles 13, 827–843. Robinson, C., Archer, S.D., Williams, P.J., le, B., 1999. Microbial dynamics in coastal waters of East Antarctica: plankton production and respiration. Marine Ecology Progress Series 180, 23–36. Robinson, N., Eglinton, G., Brassell, S.C., Cranwell, P.A., 1984. Dinoflagellate origin for sedimentary 4a-methylsteroids and 5a(H)-stanols. Nature 308, 439–442. Serrazanetti, G.P., Conte, L.S., Cattani, O., 1989. Aliphatic hydrocarbon and sterol content of zooplankton of the EmiliaRomagna coast (Northern Adriatic). Comparative Biochemical Physiology Part B: Biochemistry and Molecular Biology 94, 143–148. Skerratt, J.H., Nichols, P.D., McMeekin, T.A., Burton, H., 1995. Seasonal and inter-annual changes in planktonic biomass and community structure in eastern Antarctica using signature lipids. Marine Chemistry 51, 93–113. Smith Jr., W.O., Nelson, D.M., DiTullio, G.R., Leventer, A.R., 1996. Temporal and spatial patterns in the Ross Sea: phytoplankton biomass, elemental composition, productivity and growth rates. Journal of Geophysical Research 101, 18455–18465. Smith Jr., W.O., van Hilst, C.M., 2003. Effects of assemblage composition on the temporal dynamics of carbon and

588

J.C. Villinski et al. / Organic Geochemistry 39 (2008) 567–588

nitrogen uptake in the Ross Sea. In: DiTullio, R.R., Dunbar, R.B. (Eds.), Biogeochemistry of the Ross Sea, Antarctic Research Series, vol. 78. American Geophysical Union, pp. 197–208. Stuiver, M., Polach, H.A., 1977. Discussion: reporting of C-14 data. Radiocarbon 19, 355–363. Sun, M.Y., Wakeham, S.G., 1994. Molecular evidence for degradation and preservation of organic matter in the anoxic Black Sea Basin. Geochimica et Cosmochimica Acta 58, 3395–3406. Sun, M.Y., Wakeham, S.G., 1999. Diagenesis of planktonic fatty acids and sterols in Long Island Sound sediments: influences of a phytoplankton bloom and bottom water oxygen content. Journal of Marine Research 57, 357–385. Ternois, Y., Sicre, M.A., Boireau, A., Beaufort, L., Miquel, J.-C., Jeandel, C., 1998. Hydrocarbons, sterols and alkenones in sinking particles in the Indian Ocean sector of the Southern Ocean. Organic Geochemistry 28, 489–501. Venkatesan, M.I., 1988. Organic geochemistry of marine sediments in Antarctic region: marine lipids in McMurdo Sound. Organic Geochemistry 12, 13–27. Villinski, J.C., Dunbar, R.B., Mucciarone, D.A., 2000. 13C/12C ratios of sedimentary organic matter from the Ross Sea, Antarctica: a record of phytoplankton bloom dynamics. Journal of Geophysical Research, Oceans 105, 14163–14172. Volkman, J.K., 1986. A review of sterol markers for marine and terrigenous organic matter. Organic Geochemistry 9, 83–99.

Volkman, J.K., 2003. Sterols in microorganisms. Applied Microbiology and Biotechnology 60, 495–506. Volkman, J.K., 2005. Sterols and other triterpenoids: source specificity and evolution of biosynthetic pathways. Organic Geochemistry 36, 139–159. Volkman, J.K., Barrett, S.M., Dunstan, G.A., Jeffrey, S.W., 1993. Geochemical significance of the occurrence of dinosterol and other 4-methyl sterols in a marine diatom. Organic Geochemistry 20, 7–15. Volkman, J.K., Rijpstra, W.I.C., de Leeuw, J.W., Mansour, M.P., Jackson, A.E., Blackburn, S.I., 1999. Sterols of four dinoflagellates from the genus Prorocentrum. Phytochemistry 52, 659–668. Wakeham, S.G., 1995. Lipid biomarkers for heterotrophic alteration of suspended particulate organic matter in oxygenated and anoxic water columns of the ocean. Deep Sea Research I 42, 1749–1771. Wakeham, S.G., Beier, J.A., 1991. Fatty-acid and sterol biomarkers as indicators of particulate matter source and alteration processes in the Black Sea. Deep Sea Research Part A: Oceanographic Research Papers 38, S943–S968. Worthen, D.L., Arrigo, K.R., 2003. A coupled ocean-ecosystem model of the Ross Sea. Part I: Interannual variability of primary production and phytoplankton community structure. In: DiTullio, G.R., Dunbar, R.B. (Eds.), Biogeochemistry of the Ross Sea, Antarctic Research Series, vol. 78. American Geophysical Union, pp. 93–106.