Occurrence of C25 highly branched isoprenoids (HBIs

Organic Geochemistry Organic Geochemistry 37 (2006) 847–859 www.elsevier.com/locate/orggeochem

Occurrence of C25 highly branched isoprenoids (HBIs) in Florida Bay: Paleoenvironmental indicators of diatom-derived organic matter inputs Yunping Xu a, Rudolf Jaffe´

a,*

, Anna Wachnicka b, Evelyn E. Gaiser

c

a

Environmental Geochemistry Laboratory, Southeast Environmental Research Center (SERC) and Department of Chemistry and Biochemistry, Florida International University, University Park Campus OE-148, Miami, FL 33199, USA b Southeast Environmental Research Center (SERC) and Department of Earth Sciences, Florida International University, University Park Campus OE-148, Miami, FL 33199, USA c Southeast Environmental Research Center (SERC) and Department of Biological Sciences, Florida International University, Miami, FL 33199, USA Received 28 September 2005; received in revised form 1 February 2006; accepted 8 February 2006 Available online 22 May 2006

Abstract C25 highly branched isoprenoids (HBIs) are potentially valuable indicators of diatom-derived organic matter (OM) inputs to sediments and the C25 alkane has been used previously as a biomarker in paleoenvironmental studies. In this work, a suite of C25 HBI monoenes and dienes was detected in a sediment core from Russell Key, central Florida Bay, USA. Hydrogenation proved all these compounds to be acyclic alkenes with the parent structure of the C25 HBI alkane 2,6,10,14-tetramethyl-7-(3 0 -methylpentyl)pentadecane. The tentative double bond positions and geometry of three monoenes and one diene were also established on the basis of comparison of retention indices (RI) and mass spectra with those published for synthetic or isolated compounds. The abundance of individual alkenes showed significantly different depth profiles. The concentrations of dienes decreased rapidly with increasing depth, but this trend was not observed for the monoenes. The highest concentration of total C25 HBIs was observed at mid depth in the core, suggesting strong historical inputs of diatom-derived sedimentary OM during that period. In fact, the depth profile of C25 HBIs reflected quite well historical variations in diatom abundance and variations in diatom species composition in central Florida Bay, based on the results of fossil diatom species analysis using microscopy. This study provides further evidence that, with care, some C25 HBIs can be applied as biomarkers for diatom inputs in paleoenvironmental studies although our data did not allow the identification of one specific diatom group as the main HBI contributor. 2006 Elsevier Ltd. All rights reserved.

1. Introduction

* Corresponding author. Tel.: +1 305 348 3095/2456; fax: +1 305 348 4096. E-mail address: jaffer@fiu.edu (R. Jaffe´).

HBIs with 25 carbons (I: see Appendix A) have been reported over the last 30 years to occur widely in recent sediments (reviewed by Rowland and Robson, 1990; Belt et al., 2000a; Sinninghe

0146-6380/$ - see front matter 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.orggeochem.2006.02.001

848

Y. Xu et al. / Organic Geochemistry 37 (2006) 847–859

Damste´ et al., 2004). These compounds could be useful paleoenvironmental biomarkers for diatomderived organic matter contributions to sediments. Their sources remained largely unknown until 1994 when Volkman et al. (1994) first reported the identification of polyunsaturated C25 HBIs in laboratory-grown cultures of the diatom, Haslea ostrearia. Since then, the occurrences of C25 HBIs in diatoms have been observed in numerous studies, including in axenic diatom cultures (e.g. Allard et al., 2001; Belt et al., 2000b, 2001a,b; Grossi et al., 2004; Johns et al., 1999; Sinninghe Damste´ et al., 1999a,b, 2004; Wraige et al., 1998, 1999). Therefore, many diatom species, including benthic, planktonic, marine and freshwater species undoubtedly produce C25 HBIs and since these kinds of unusual, highly branched hydrocarbons have only been observed in diatoms to date, they are potentially valuable indicators for diatom inputs to sedimentary environments. However, the occurrence of C25 HBIs in diatoms is complex. Several studies (Rowland et al., 2001a; Sinninghe Damste´ et al., 1999b; Volkman et al., 1994) revealed the existence of different C25 HBIs in the same diatom species. These results suggested that the biosynthesis of C25 HBIs by diatoms may not be entirely species-specific, but is also controlled by environmental factors, such as temperature, salinity and light, as has subsequently been shown in laboratory cultures grown under well controlled conditions (e.g. Wraige et al., 1998; Rowland et al., 2001a,b). Therefore, the effect of environmental factors must also be considered when determining correlations of diatoms with specific biomarkers. The C25 HBIs in diatoms are composed of a series of polyunsaturated acyclic alkenes with up to four degrees of unsaturation. These unsaturated compounds may be reduced in sediments (Dunlop and Jefferies, 1985; Requejo and Quinn, 1983), or may easily be biodegraded (Wakeham et al., 2002), limiting their application as paleoenvironmental indicators. However, some studies reported the alkane (I) and some C25 monoenes to be relatively resistant to degradation (Robson and Rowland, 1988); hence, C25 HBIs are found in sediments up to the middle Cretaceous (see Sinninghe Damste´ et al., 2004). The objective of the present work was to investigate the molecular distributions, abundances and sources of C25 HBIs in a sediment core from central Florida Bay and assess their utility as paleoindicators of diatom inputs. For this purpose, the relation

between molecular distribution and abundance of C25 HBIs and fossil-based diatom composition throughout the core was determined. 2. Experimental 2.1. Study area and sampling Florida Bay is a shallow, subtropical marine ecosystem located at the southern tip of the Florida Peninsula. The present surface area of the bay is about 2000 km2, with a mean depth of less than 3 m (Fourqurean and Robbins, 1999). During the past 100 years, the ecosystem of the bay has undergone dynamic changes, such as salinity variations, a decrease in freshwater input, increasing nutrient availability and massive seagrass die-offs (Orem et al., 1999; Zieman et al., 1999); many of these changes have been attributed to increasing human activity (Wardlaw, 2001). In order to better understand historical variations in the ecosystem, several piston cores were collected in the summer of 2002. These coring sites represent the transition from the mangrove fringe in NE Florida Bay through the banks of the central bay, out to the western boundary. Piston coring was completed using a pontoon boat-mounted tripod and acrylic core barrels (14 cm diameter). Cores were sectioned into 2 cm intervals. The outer edge of the samples was discarded in order to eliminate contamination before they were wrapped in clean aluminum foil. The samples were placed on ice until they were transported to the laboratory where they were kept in a freezer at 20 C. We focused on one core from Russell Key in the central Florida Bay (Fig. 1) and determined the historical variation of C25 HBIs in order to assess changes in diatom inputs to the sediment. An average accumulation rate of 1.1 cm/year based on the 210 Pb activity was determined. Therefore, the entire core (166 cm) represents about 150 years accumulation. It was sampled at 2 cm intervals, which provided an approximately two year resolution for paleoenvironmental interpretation. 2.2. Extraction and fractionation The detailed analytical method has been described by Jaffe´ et al. (2001). Briefly, samples were freeze-dried and Soxhlet extracted with pure CH2Cl2 (Optima, Fisher, USA) for 24 h. Activated copper was used to remove elemental S from the


849

Fig. 1. Geographic location of sampling site.

extracts. Total extractable organic matter was saponified with 0.5 N KOH and separated into neutral and acid fractions. The former was further separated using silica gel adsorption chromatography (Jaffe´ et al., 1995). The aliphatic hydrocarbon fraction was eluted with pure hexane (Optima, Fisher, USA) and a known amount of squalane was added for quantitative analysis. The % error in compound quantification was of the order of 5%. Concentrations are reported normalized to TOC (total organic matter) content. The TOC values (data not shown) showed no significant changes from the surface to 140 cm depth (2.15 ± 0.15%), but were significantly lower for the bottom part of the core (140+ cm; 1.09 ± 0.25%).

som, CA). The GC oven was programmed from 60 to 180 C at a rate of 6 C/min after 1 min at the initial temperature; then increased to 315 C at a rate of 3.0 C/min and kept for 15 min. The identification of individual C25 HBIs was performed by comparison with reported retention indices (RIs), mass spectra and hydrogenation products. 2.4. Hydrogenation of C25 HBIs Hydrogenation of the aliphatic fraction containing the C25 HBIs was carried out in hexane under the mild pressure of H2 using palladium as catalyst for 12 h. Resulting products were determined using GC-MS and applying GC RIs by comparison with published data (Robson and Rowland, 1986).

2.3. Gas chromatography–mass spectrometry (GC– MS)

2.5. Diatom analysis

A Hewlett-Packard 5973 GC–MS system was operated in the electron ionization (EI) mode at 70 eV, while the source temperature was kept at 200 C. The column was a DB 1 capillary (30 m, 0.25 mm i.d., 0.25 lm film thickness; Agilent; Fol-

Approximately 1 g of wet sediment was removed from each 2 cm interval, dried at 50 C for 24 h, weighed, placed in a 500 ml glass beakers and oxidized according to the method developed by Hasle and Fryxell (1970). Permanent diatom slides were

850


prepared using the high resolution mounting medium Naphrax (refractive index 1.7). At least 500 diatom valves were counted and identified from each slide on randomly chosen transects at 1008· magnification using a Carl Zeiss light microscope. Diatom taxa were identified using standard and local literature. Because the flora has been poorly explored taxonomically, several taxa were unidentifiable to the species level and were, therefore, assigned numerical designations until they can be properly described. Photomicrographs and taxonomic information for all diatoms mentioned, including unknown taxa, can be found at http:// serc.fiu.edu/periphyton/.

3. Results and discussion 3.1. Identification of C25 HBIs A typical GC-MS total ion current (TIC) chromatogram for C25 HBIs in Russell Key core is presented in Fig. 2. Mass spectra showed that all those identified had a molecular weight of 348 (C25H48) or 350 (C25H50), indicating one or two degrees of unsaturation (Table 1). The C25 alkenes with >two double bonds were not observed, suggesting early diagenetic removal of polyunsaturated compounds or that environmental conditions were not adequate for their biosynthetic production. Fig. 2(a) and (b)

Fig. 2. Partial gas chromatograms of hydrocarbon fraction in Bay (a) 4–6 cm (b) before and (c) after hydrogenation at 90–92 cm depth.

Y. Xu et al. / Organic Geochemistry 37 (2006) 847–859 Table 1 Chromatographic data for identified C25 HBIs C25 HBI

No. double bonds

RIa

Characteristic ions (m/z)

C25:2 C25:1 C25:2 C25:2 C25:1 C25:1 C25:1 C25:1 C25:1

2 1 2 2 1 1 1 1 1

2059 2072 2076 2082 2101 2107 2110 2115 2124

348, 350, 348, 348, 350, 350, 350, 350, 350,

a

319, 266, 319, 320, 266, 266, 266, 280, 280,

266, 235 210 266, 235, 207 291, 266, 207 210 210 210 224 224

RI = retention index on DB-1.

show partial chromatograms of aliphatic hydrocarbon fractions at 4–6 cm and 90–92 cm (1920), while Fig. 2(c) shows the partial chromatogram of the 90–92 cm aliphatic hydrocarbon fraction after hydrogenation. Hydrogenation of the aliphatic hydrocarbon fraction from surface samples was also performed, providing similar results (data not shown). Comparing chromatograms before and after hydrogenation between Fig. 2(b) and (c), all the peaks for C25 HBIs in Fig. 2(b) disappeared after hydrogenation and a new single peak with molecular weight 352 appeared. The compound (C25H52) had an identical mass spectrum to that of 2,6,10,14-tetramethyl-7-(3 0 -methylpentyl)pentadecane (I) and the RIDB1 value of 2107 reported previously (Robson and Rowland, 1986); this showed that all C25 HBIs in the Russell Key core were acyclic compounds with one or two double bonds, although a previous study reported the occurrence of cyclic C25 compounds in Florida Bay (Jaffe´ et al., 2001). The structures and biosynthesis of those compounds have recently been elucidated after isolation from the diatom Rhizosolenia setigera (Masse´ et al., 2004a,b). The structures of many polyunsaturated (>two double bonds) C25 HBIs have also been reported (e.g. Belt et al., 2000b, 2002; Wraige et al., 1998, 1999) and the mechanisms of their biosynthesis firmly established (Masse´ et al., 2004c). However, only a few studies have determined the positions of double bonds in C25 HBI monoenes and dienes (Belt et al., 1994; Dunlop and Jefferies, 1985; Hird et al., 1992; Johns et al., 1999; Yruela et al., 1990). In the present work, the structures of four C25 compounds were tentatively established on the basis of hydrogenation and a comparison of RI and mass spectra with those reported previously. Using the reported (RIs) on a DB1 capillary column, three monoenes, namely br25:1DB1 2110, 2115 and 2124

851

were characterized as structures II, III and IV (Hird et al., 1992), respectively. The molecular ion (m/z 350) was present, in addition to reported principal fragments. Characteristic peaks at m/z 210 and 266 of br25:1DB1 2110 are thought due to b-cleavage in unsaturated compounds, which provided valuable information on the position of the double bond. The other two identified monoenes, br25:1DB1 2115 and 2124, were assigned as E and Z isomers at the C6 (7) position (III and IV), which also have relatively abundant mass fragments at m/z 224 and 280 due to b-cleavage (Hird et al., 1992). Besides the monoenes, the positions of double bonds in one diene (br25:2DB1 2082) were also tentatively identified as those in V (see Appendix A); this alkene had an identical mass spectrum to that reported by Johns et al. (1999) for the diene isolated from the diatom, H. ostrearia and characterized using nuclear magnetic resonance spectroscopy [i.e. D6(17) and D23(24)]. Alternatively, although reported at a slightly lower retention index (2079), this compound could be the E isomer of the 5,23 diene (Compound VI; Belt et al., 1994), which would agree with the presence of the D5 monoene (Compound II) further downcore. However, the precise structures of the other five C25 HBIs, including three monoenes (br25:1DB1 2072, 2101 and 2107) and two dienes (br25:2DB1 2059 and 2076), have not been completely determined in the current study and are the subject of future work. 3.2. Depth profiles of C25 HBIs Fig. 3(a) presents the depth profile of total C25 HBI abundance in the core expressed as lg/g TOC. The profile expressed as ng/g dw (dry weight; not shown) was very similar to that shown in Fig. 3(a). Based on the abundance of total C25 HBIs, the whole core was divided into three zones, namely Z-1 (upper; 0–40 cm), Z-2 (middle; 40– 140 cm) and Z-3 (bottom; 140–166 cm). In the upper zone (Z-1), the concentration of total C25 HBIs was relatively low ( 0.60; p < 0.01) were observed between some of the monoenes. For instance, the correlation coefficient was 0.97 between br25:1DB1 2072 and 2101 and 0.86 between br25:1DB1 2101 and 2115. This observation reflects a strong coupling between different monoenes, indicating that all of br25:1 alkenes were produced by the same organisms or by different organisms but with a strong coupling among these organisms. Like the monoenes, the three dienes (br25:2DB1 2059, 2076 and 2082) also showed strong correlations with each other; the correlation coefficients were 0.89 between br25:2DB1 2059 and 2082, 0.81 between br25:2DB1 2076 and 2082 and 0.64 between br25:2DB1 2059 and 2076. However, no significant correlation was observed between the monoenes and the dienes, which can be explained by the preferential degradation of the latter. 3.4. C25 HBIs as paleoenvironmental biomarkers of diatom inputs Polyunsaturated C25 HBIs are well known to be biosynthesized by diatoms such as H. ostrearia and R. setigera, but no monoenes have been observed in laboratory-grown cultures. Considering the identical parent structure of the monoenes in this study and that of the polyunsaturated C25 alkenes in the literature (Johns et al., 1999; Sinninghe Damste´ et al., 1999a; Wraige et al., 1997, 1999), the monoenes in the Russell Key sediments seemed to originate from the early diagenesis of polyunsaturated HBIs and dienes through reduction of dou-

853

ble bonds, a typical pathway during early diagenesis of unsaturated compounds (de Leeuw et al., 1989), or through direct biosynthesis by some diatom species. Dunlop and Jefferies (1985) reported the occurrence of high abundances of C25 monoenes in sediment from the hypersaline Shark Bay, Western Australia. Salinity in central Florida Bay has been reported as high as 50& (Fourqurean et al., 1992) during drought conditions. However, Wraige et al. (1998) observed no monoene production when H. ostrearia was cultured at different salinities; nor did Rowland et al. (2001a) report production of monoenes when R. setigera was cultured at varying salinity. In order to investigate further the relation between C25 HBIs and diatoms, the composition and habitat affinities of diatoms in the core was determined. Fig. 4 shows the depth profiles of total diatom abundance and the density of freshwater and planktonic, benthic and epiphytic marine diatom. Unlike the distribution of total C25 HBIs, total diatom abundance did not change greatly except between zones Z-2 and Z-3, when freshwater diatoms, which dominated basal sediments in the core, were replaced by marine taxa. This transition zone coincided with a dramatic increase in total C25 HBIs, corresponding to previous studies which have shown that marine, rather than freshwater diatoms produce C25 HBIs, although at least one freshwater source has been identified (Belt et al., 2001b). Together, these makers indicate an abrupt transition from freshwater to marine conditions in the Bay between 140 and 160 cm. This transition has been inferred from other biological proxies (ostracods, molluscs, pollen and foraminifera) from other sediment cores in the region (Ishman et al., 2001) and has been associated with a culminating effect of rising Holocene sea-level and wetter conditions

Table 2 Correlation coefficients (r) for different C25 HBIs in Russell Key core (bold indicates a strong correlation (p < 0.01) between two individual C25 HBIs)

br25:2 br25:1 br25:2 br25:2 br25:1 br25:1 br25:1 br25:1 br25:1

RI

br25:2 2059

br25:1 2072

br25:2 2076

br25:2 2082

br25:1 2101

br25:1 2107

br25:1 2110

br25:1 2115

br25:1 2124

2059 2072 2076 2082 2101 2107 2110 2115 2124

1.00 0.30 0.64 0.89 0.30 0.28 0.19 0.21 0.35

1.00 0.31 0.31 0.97 0.78 0.59 0.80 0.89

1.00 0.81 0.27 0.29 0.14 0.28 0.36

1.00 0.30 0.29 0.16 0.26 0.36

1.00 0.76 0.56 0.86 0.93

1.00 0.85 0.56 0.79

1.00 0.39 0.57

1.00 0.88

1.00

854


Fig. 4. Depth profiles of abundance (valves counted per gram) for diatoms in Russell Key, (a) freshwater; (b) planktonic marine; (c) benthic marine; (d) epiphytic marine; (e) total.

of the warm period in the mid 19th Century (Winkler et al., 2001). At the end of this period, the relative abundance of benthic marine diatoms increased concomitantly with total C25 HBIs, suggesting gradual flooding of shallow coastal sediments with marine water. Thereafter, epiphytic diatoms dominated until the mid-20th Century (Zone Z-2) until they were replaced by a dominance of planktonic diatoms in the 1960s. The rapid decrease in C25 HBIs in Z-1 appears associated with the increased contribution of epiphytic taxa, although the total abundance of marine diatoms did not change greatly over this time period. This suggests that C25 HBIs may be more associated with epiphytic or periphytic algal accumulations than phytoplanktonic communities. The depth profiles of individual diatom species provided more detailed information on the potential sources of C25 HBIs. Fig. 5 shows the changes in abundance of several diatom species in the core. Some of these data agreed well with the distribution of C25 HBIs; Z-1 (0–40 cm) was dominated by Cyclotella cf. distinguenda, Cyclotella litoralis, Tryb-

lionella granulata, Grammatophora macilenta, Grammatophora oceanica and Synedra sp. 01FB; Z-2 (40– 140 cm) shows abrupt changes in diatom species composition. Several taxa increased in abundance and became dominant (Mastogloia elegans, Mastogloia rostellata, Amphora floridiana, Mastogloia sp. 14FB, Mastogloia discontinua, Mastogloia erythraea, Mastogloia frickea, Parlibellus panduriformis and Navicula palestinae), whereas others rapidly declined in number (C. cf. distinguenda, T. granulata, G. macilenta). This transition in diatom species is thought to account for the increase in the abundance of the C25 HBIs from Z-1 to Z-2, suggesting that the diatoms in Z-1 did not produce significant amounts of C25 HBI monoenes. The cause for this change in diatom assemblages may relate to increased nutrient level in Florida Bay in the latter part of the 20th century, which encouraged the proliferation of planktonic algal, at the expense of benthic, light-limited periphytic communities that, under the natural, oligotrophic conditions, rely on sediment (rather than water borne) sources of nutrients (Orem et al., 1999).


855

Fig. 5. Depth profile of diatom species abundance (valves counted per gram).

The second drastic fluctuation in total C25 HBIs occurred at the interface between Z-2 and Z-3, consistent with the second transition of diatom species. From 140 cm to 144 cm, the dominant diatom species in Z-2 rapidly disappeared and some freshwater diatoms, which were not detected in Z-2, became relatively abundant (Mastogloia smithii, Encyonema evergladianum, Nitzschia semirobusta and Fragilaria synegrotesca). Corresponding to the transition in diatoms, the concentration of total C25 HBIs sharply dropped to 10 lg/g TOC from 50 lg/g TOC (Fig. 3a). These results again suggest that only certain diatom species are significant sources of C25 HBIs and that C25 HBIs are not commonly produced by the freshwater diatoms that inhabited Florida Bay in the late 19th Century. Sinninghe Damste´ et al. (2004) investigated over 120 marine diatoms and found that the capability to biosynthesize HBIs seems to be present in two specific phylogenetic clusters, which independently evolved into centric and pennate diatoms including Rhizosolenia spp. (centric) and Haslea spp. (pennate). Since fossil remains of none of these species were detected in our core, two further hypotheses may explain our observations:

(1) Diatoms other than Rhizosolenia spp. or Haslea spp. produced the C25 HBIs. (2) The known poor preservation of Rhizosolenia and Haslea spp. in sediments could account for the absence of these diatom species from the Russell Key sediment core, and the presence of the C25 HBIs could be an organic geochemical proxy for the paleohistory of this species in central Florida Bay. However, while R. setigera is a planktonic species, the high abundance of benthic diatoms in Z-2 is consistent with the possible (unrecorded using microscopy) occurrence of H. ostrearia. Therefore, both explanations are possible at this point. The presence of Navicula and Gyrosigma species in our core may also contribute to the presence of C25 HBIs since these organisms have also been reported to produce C25 HBIs (Sinninghe Damste´ et al., 2004). Table 3 shows correlation coefficients between diatom species and C25 HBI compounds based on the data for the entire core. Only certain diatom species, including three benthic diatoms (M. elegans, M. sp. 12 FB and A. floridiana) and five epiphytic diatoms (M. erythraea, M. sp.14FB, M. frickea, N.

856

Table 3 Correlation coefficients (r) between individual diatom species and C25 HBIs (bold indicates strong correlation (p < 0.05) between individual C25 HBIs and diatom species)

Mastogloia smithii Encyonema evergladianum Nitzschia semirobusta Fragilaria synegrotesca Cyclotella cf. distinguenda Cyclotella litoralis Tryblionella granulata Mastogloia elegans Mastogloia sp. 12FB Amphora corpulenta var.capitata Mastogloia rostellata Amphora floridiana Mastogloia erythraea Mastogloia sp. 14FB Mastogloia frickea Navicula palestinae Gyrosigma balticum Mastogloia discontinua Parlibellus panduriformis Synedra sp. 01 FB Grammatophora macilenta Grammatophora oceanica

Br25:2 RI2059

Br25:2 RI2076

Br25:2 RI2082

Br25:1 RI2072

Br25:1 RI2101

Br25:1 RI2107

Br25:1 RI2110

Br25:1 RI2115

Br25:1 RI2124

0.00 0.02 0.02 0.11 0.32 0.40 0.61 0.43 0.72 0.31 0.23 0.73 0.57 0.73 0.80 0.64 0.23 0.65 0.31 0.35 0.45 0.50

0.04 0.08 0.00 0.10 0.62 0.57 0.17 0.24 0.36 0.43 0.24 0.31 0.27 0.26 0.26 0.29 0.05 0.25 0.03 0.47 0.12 0.05

0.06 0.03 0.05 0.11 0.94 0.42 0.14 0.24 0.37 0.34 0.17 0.32 0.27 0.27 0.26 0.30 0.06 0.24 0.12 0.20 0.05 0.02

0.07 0.02 0.03 0.11 0.81 0.54 0.18 0.25 0.38 0.42 0.22 0.33 0.29 0.28 0.28 0.30 0.07 0.27 0.09 0.44 0.08 0.03

0.06 0.03 0.10 0.07 0.39 0.40 0.55 0.39 0.63 0.31 0.25 0.62 0.57 0.73 0.67 0.61 0.15 0.58 0.21 0.31 0.41 0.43

0.04 0.07 0.17 0.11 0.35 0.40 0.58 0.44 0.67 0.30 0.29 0.68 0.62 0.74 0.67 0.62 0.27 0.64 0.23 0.34 0.43 0.47

0.02 0.06 0.07 0.09 0.40 0.46 0.58 0.42 0.75 0.33 0.20 0.73 0.53 0.67 0.86 0.62 0.17 0.62 0.36 0.39 0.42 0.48

0.26 0.26 0.30 0.15 0.19 0.17 0.31 0.14 0.49 0.15 0.00 0.48 0.24 0.45 0.61 0.32 0.11 0.42 0.15 0.21 0.21 0.06

0.08 0.04 0.04 0.03 0.25 0.23 0.50 0.33 0.49 0.27 0.20 0.52 0.41 0.58 0.47 0.52 0.17 0.51 0.09 0.18 0.31 0.30

0.24 0.19 0.13 0.05 0.43 0.46 0.64 0.36 0.59 0.42 0.17 0.59 0.45 0.64 0.59 0.55 0.13 0.53 0.12 0.45 0.47 . 0.46


Freshwater Freshwater Freshwater Freshwater Planktonic marine Planktonic marine Benthic marine Benthic marine Benthic marine Benthic marine Benthic marine Benthic marine Epiphytic arine Epiphytic arine Epiphytic marine Epiphytic marine Epiphytic marine Epiphytic marine Epiphytic marine Epiphytic marine Epiphytic marine Epiphytic marine

Total C25 HBIs


palestinae and M. discontinua), have strong couplings with the total C25 HBI concentrations, especially with the C25 monoenes. In Z-1, C25 dienes showed high correlation coefficients with two planktonic diatoms, namely C. cf. distinguenda and C. litoralis, although diatom species of the genus Cycotella have been suggested not to produce C25 HBIs (Sinninghe Damste´ et al., 2004). However, it is important to point out that positive correlations between some diatom species and the C25 HBI abundance may also be due to co-occurrence of some species due to specific environmental conditions, regardless if the correlating species actually produces HBIs or not.

857

search was supported by the National Science Foundation through the FCE-LTER (DEB9910514) and the Division of Earth Sciences Geology and Paleontology Program (#0345812). Y.X. thanks FIU for a presidential fellowship during his Ph.D. study. This is SERC contribution #322. Appendix A 16

17

18

4 7

6

2 1

3

19 12

8

14

10

5

9

11

13

15

20 21 22

4. Summary

25

23

I 24

A suite of C25 HBIs has been found in a sediment core from central Florida Bay. Hydrogenation proved all to be acyclic dienes and monoenes with an identical parent structure, namely 2,6,10,14-tetramethyl-7-(3 0 -methylpentyl)pentadecane. The different depth profiles suggested that monoenes were more resistant to degradation or diagenetic transformations than the dienes. The concentration of total C25 HBIs was as high as 62 lg/g TOC in the mid zone (1875–1966 A.D.), much higher than that in the upper and bottom zones. A strong correlation was observed between C25 HBIs and some fossil diatom species, suggesting that C25 HBIs are only produced by certain diatom species although no specific diatom group could be identified as the main producer. Our results provide evidence that C25 HBIs can be a valuable indicator of diatom inputs in paleoenvironmental studies. As the principal sources of C25 HBIs, Rhizosolenia spp. and Haslea spp., were not observed in the sediment, so the C25 HBI depth profiles may reflect the paleohistory of other, as yet unknown, HBI-producing diatom species. Alternatively, the C25 HBI producers may have a low level of morphological fossil preservation in the marine environment, as suggested previously. Acknowledgements The authors thank W. Anderson, J. Fourqurean and L. Collins for assistance with sample collection and especially S.J. Rowland and two anonymous reviewers for helpful comments on the original manuscript. Our co-workers from the USGS are also thanked for providing sampling logistics. The re-

II

III

IV

V

VI

Associate Editor—S. Schouten References Allard, W.G., Belt, S.T., Masse´, G., Naumann, R., Robert, J.M., Rowland, S.J., 2001. Tetra-unsaturated sesterterpenoids (Haslenes) from Haslea ostrearia and related species. Phytochemistry 56, 795–800. Belt, S.T., Cooke, D.A., Hird, S.J., Rowland, S.J., 1994. Structural determination of a highly branched C25 sedimentary isoprenoid biomarker by NMR spectroscopy and mass spectrometry. Chemical Communications 18, 2077–2078. Belt, S.T., Allard, W.G., Masse´, G., Robert, J.-M., Rowland, S.J., 2000a. Highly branched isoprenoids (HBIs): identification of the most common and abundant sedimentary isomers. Geochimica et Cosmochimica Acta 64, 3839–3851. Belt, S.T., Allard, W.G., Masse, G., Robert, J.-M., Rowland, S., 2000b. Important sedimentary sesterterpenoids from the diatom Pleurosigma intermedium. Chemical Communications, 501–502.

858


Belt, S.T., Allard, W.G., Rintatalo, J., Johns, L.A., van Duin, A.C.T., Rowland, S.J., 2000c. Clay and acid catalysed isomerisation and cyclisation reactions of highly branched isoprenoid (HBI) alkenes: implications for sedimentary reactions and distributions. Geochimica et Cosmochimica Acta 64, 3337–3345. Belt, S.T., Masse´, G., Allard, W.G., Robert, J.-M., Rowland, S.J., 2001a. C25 highly branched isoprenoid alkenes in planktonic diatoms of the Pleurosigma genus. Organic Geochemistry 32, 1271–1275. Belt, S.T., Masse, G., Allard, W.G., Robert, J.-M., Rowland, S.J., 2001b. Identification of a C25 highly branched isoprenoid triene in the freshwater diatom Navicula sclesvicensis. Organic Geochemistry 32, 1169–1172. Belt, S.T., Masse´, G., Allard, W.G., Robert, J.-M., Rowland, S.J., 2002. Effects of auxosporulation on distributions of C25 and C30 isoprenoid alkenes in Rhizosolenia setigera. Phytochemistry 59, 141–148. de Leeuw, J.W., Cox, H.C., van Graas, G., van de Meer, F.W., Peakman, T.M., Baas, J.M.A., van de Graaf, B., 1989. Limited double bond isomerisation and selective hydrogenation of sterenes during early diagenesis-1. Geochimica et Cosmochimica Acta 53, 903–909. Dunlop, R.W., Jefferies, P.R., 1985. Hydrocarbons of the hypersaline basins of Shark Bay, western Australia. Organic Geochemistry 8, 313–320. Fourqurean, J.W., Robbins, M.B., 1999. Florida Bay: a history of recent ecological changes. Estuaries 22, 345–357. Fourqurean, J.W., Zieman, J.C., Powell, G.V.N., 1992. Phosphorus limitation of primary production in Florida Bay: evidence from the C:N:P ratios of the dominant seagrass Thalassia testudinum. Limnology and Oceanography 37, 162– 171. Grossi, V., Beker, B., Geenevasen, J.A.J., Schouten, S., Raphel, D., Fontaine, M.-F., Sinninghe Damste´, J.S., 2004. C25 highly branched isoprenoid alkenes from the marine benthic diatom Pleurosigma stigosum. Phytochemistry 65, 3049–3055. Hartgers, W.A., Lopez, J.F., Sinninghe Damste´, J.S., Riess, C., Maxwell, J.R., Grimalt, J.O., 1997. Sulfur-binding in recent environments: II. Speciation of sulfur and iron and implications for the occurrence of organo-sulfur compounds. Geochimica et Cosmochimica Acta 61, 4769–4788. Hasle, G., Fryxell, G., 1970. Diatoms: cleaning and mounting for light and electron microscopy. Transactions of the American Microscopical Society 89, 469–474. Hird, S.J., Evens, R., Rowland, S.J., 1992. Isolation and characterization of sedimentary and synthetic highly branched C20 and C25 monoenes. Marine Chemistry 37, 117–129. Ishman, S.E., Cronin, T.M., Brewster-Wingard, G.L., Willard, D.A., Verardo, D.J., 2001. A record of ecosystem change, Manatee Bay, Barnes Sound, Florida. Bulletins of American Paleontology 361, 125–138. Jaffe´, R., Wolff, G.A., Cabrera, A., Carvajal Chitty, H., 1995. The biogeochemistry of lipids in rivers of the Orinoco Basin. Geochimica et Cosmochimica Acta 59, 4507–4522. Jaffe´, R., Mead, R., Hernandez, M.E., Peralba, M.C., DiGuida, O.A., 2001. Origin and transport of sedimentary organic matter in two subtropical estuaries: a comparative, biomarker-based study. Organic Geochemistry 32, 507–526. Johns, L., Wraige, E.J., Belt, S.T., Lewis, C.A., Masse´, G., Robert, J.-M., Rowland, S.J., 1999. Identification of a C25

highly branched isoprenoid (HBI) diene in Antarctic sediments, Antarctic sea-ice diatoms and cultured diatoms. Organic Geochemistry 30, 1471–1475. Masse´, G., Belt, S.T., Allard, W.G., Anthony Lewis, C., Wakeham, S.G., Rowland, S.J., 2004a. Occurrence of novel monocyclic alkenes from diatoms in marine particulate matter and sediments. Organic Geochemistry 35, 813–822. Masse´, G., Belt, S.T., Rowland, S.J., 2004b. Biosynthesis of unusual monocyclic alkenes by the diatom Rhizosolenia setigera (Brightwell). Phytochemistry 65, 1101–1106. Masse´, G., Belt, S.T., Rowland, S.J.., Rohmer, M., 2004c. Isoprenoid biosynthesis in the diatoms Rhizosolenia setigera (Brightwell) and Haslea ostrearia (Simonsen). Proceedings of the National Academy of Sciences of the United States of America 101, 4413–4418. Orem, W.H., Holmes, C.W., Kendall, C., Lerch, H.E., Bates, A.L., Silva, S.R., Boylan, A., Corum, M., Marot, M., Hedgman, C., 1999. Geochemistry of Florida sediments: I. nutrient history at five sites in eastern and central Florida Bay. Journal of Coastal Research 15, 1055–1071. Requejo, A.G., Quinn, J.G., 1983. Geochemistry of C25 and C30 biogenic alkenes in sediments of the Narragansett Bay estuary. Geochimica et Cosmochimica Acta 47, 1075– 1090. Robson, J.N., Rowland, S.J., 1986. Identification of novel widely distributed sedimentary acyclic sesterterpenoids. Nature 324, 561–563. Robson, J.N., Rowland, S.J., 1988. Synthesis of a highly branched C30 sedimentary hydrocarbon. Tetrahedron Letters 29, 3837–3840. Rowland, S.J., Robson, J.N., 1990. The widespread occurrence of highly branched acyclic C20, C25 and C30 hydrocarbons in recent sediments and biota – A review. Marine Environmental Research 30, 191–216. Rowland, S.J., Allard, W.G., Belt, S.T., Masse´, G., Robert, J.M., Blackburn, S., Frampton, D., Revill, A.T., Volkman, J.K., 2001a. Factors influencing the distributions of polyunsaturated terpenoids in the diatom, Rhizosolenia setigera. Phytochemistry 58, 717–728. Rowland, S.J., Belt, S.T., Wraige, E.J., Masse´, G., Roussakis, C., Robert, J.-M., 2001b. Effects of temperature on polyunsaturation in cytostatic lipids of Haslea ostrearia. Phytochemistry 56, 597–602. Sinninghe Damste´, J.S., Muyzer, G., Abbas, B., Rampen, S.W., Masse´, G., Allard, W.G., Belt, S.T., Robert, J.-M., Rowland, S.J., Moldowan, J.M., Barbanti, S.M., Fago, F.J., Denisevich, P., Dahl, J., Trinidade, L.A.F., Schouten, S., 2004. The rise of rhizosolenid diatoms. Science 304, 584–587. Sinninghe Damste´, J.S., Rijpstra, W.I.C., Schouten, S., Peletier, H., van der Maarel, M.J.E.C., Gieskes, W.W.C., 1999a. A C25 highly branched isoprenoid alkene and C25 and C27 npolyenes in the marine diatom Rhizosolenia setigera. Organic Geochemistry 30, 95–100. Sinninghe Damste´, J.S., Schouten, S., Rijpstra, W.I.C., Hopmans, E.C., Peletier, H., Gieskes, W.W.C., Geenevasen, J.A.J., 1999b. Structural identification of the C25 highly branched isoprenoid pentaene in the marine diatom Rhizosolenia setigera. Organic Geochemistry 30, 1581–1583. Volkman, J.K., Barrett, S.M., Dunstan, G.A., 1994. C25 and C30 highly branched isoprenoid alkenes in laboratory cultures of two marine diatoms. Organic Geochemistry 21, 407–414.

Y. Xu et al. / Organic Geochemistry 37 (2006) 847–859 Wakeham, S.G., Peterson, M.L., Hedges, J.I., Lee, C., 2002. Lipid biomarker fluxes in the Arabian Sea, with a comparison to the equatorial Pacific Ocean. Deep Sea Research Part II: Topical Studies in Oceanography 49, 2265–2301. Wardlaw, B.R., 2001. Introduction to paleoecological studies of South Florida and the implications for land management decisions. Bulletins of American Paleontology 361, 5–15. Winkler, M.G., Sanford, P.R., Kaplan, S.W., 2001. Hydrology, vegetation, and climate change in the Southern Everglades during the Holocene. Bulletins of American Paleontology 361, 57–99. Wraige, E.J., Belt, S.T., Lewis, C.A., Cooke, D.A., Robert, J.-M., Masse´, G., Rowland, S.J., 1997. Variations in structures and distributions of C25 highly branched isoprenoid (HBI) alkenes in cultures of the diatom, Haslea ostrearia (Simonsen). Organic Geochemistry 27, 497–505.

859

Wraige, E.J., Belt, S.T., Masse´, G., Robert, J.-M., Rowland, S.J., 1998. Variations in distributions of C25 highly branched isoprenoid (HBI) alkenes in the diatom, Haslea ostrearia: influence of salinity. Organic Geochemistry 28, 855–859. Wraige, E.J., Johns, L., Belt, S.T., Masse´, G., Robert, J.-M., Rowland, S., 1999. Highly branched C25 isoprenoids in axenic cultures of Haslea ostrearia. Phytochemistry 51, 69–73. Yruela, I., Barbe, A., Grimalt, J.O., 1990. Determination of double bond position and geometry in linear and highly branched hydrocarbons and fatty acids from gas chromatography-mass spectrometry of epoxides and diols generated by stereospecific resin hydration. Journal of Chromatographic Science 28, 421–427. Zieman, J.C., Fourqurean, J.W., Frankovich, T.A., 1999. Seagrass die-off in Florida Bay: long-term trends in abundance and growth of turtle grass, Thalassia testudinum. Estuaries 22, 460–470.